RADIOACTIVE ISOTOPES OF IODINE

B. G. Dzantiiev and M. B. Neiman

INTRODUCTION

The use of artificially radioactive elements in various fields of chemistry, physics, and biology requires familiarity with the physical characteristics of the radioactive isotopes employed.

Depending on the conditions of the problem for whose solution radioactive isotopes are used, it may be necessary to impose on them requirements not only of a chemical or physiological nature, but also of a physical one. In a number of cases it is necessary that an element with a definite chemical behavior and physiological action should at the same time be the carrier of definite physical properties, i.e., should have a definite half-life, emit β-particles and γ-quanta of definite energies, etc.

Within the limits of a given chemical element, these requirements can in a number of cases be satisfied by using different radioactive isotopes. This applies to a considerable extent to radioactive iodine, which has long since found wide application both as an indicator in various processes and as a therapeutic agent. With the aid of this element, Soviet investigators have solved a number of problems in the field of the mechanism of chemical reactions, the chemistry of complex compounds, the structure of solutions, the mobility of atoms in the crystal lattice, and so on. In most of these works the iodine isotope \(J^{128}\), with a half-life of 25 minutes, was used.

At the present time as many as 14 radioactive isotopes of iodine are known, with different half-lives ranging from several seconds to several months and with different types and energies of radiation. Rational use of these isotopes, taking into account their physical characteristics, makes it possible to considerably broaden the range of problems that can be solved with the aid of radioactive iodine.

Some isotopes of iodine have been discovered only very recently, and acquaintance with them is of independent interest.

PRODUCTION AND SEPARATION OF IODINE ISOTOPES

The isotopes of iodine have mass numbers in the interval \(M = 124—137\), and only one of the isotopes is stable[^1].

A considerable fraction of these isotopes has been separated from the products of the fission of uranium and thorium under the action of neutrons. Some radioactive isotopes of iodine are obtained by bombarding stable isotopes of \(J\), Te, Sb, Cs with neutrons, protons, deuterons, and \(\alpha\)-particles. Evidently, iodine can also be obtained by neutron bombardment of xenon according to the reaction \(\mathrm{Xe}_{54}(n,p)\mathrm{J}_{53}\), but, as far as we know, such a reaction has not been carried out. A substantial advantage of radioactive iodine, in comparison with many other artificially radioactive elements, is the possibility of separating it relatively simply from the bombarded target and from the side radioactive products of the nuclear reaction. Since iodine is most completely separated in elemental form[^2], the chemical methods usually employed for separating radioiodine from the target consist in operations of oxidizing iodine to the elemental state and isolating it from the target solution by using either the high volatility of elemental iodine vapor or its solubility in a number of organic solvents. The carrier is usually added to the irradiated target either in the form of KJ or in the form of an equivalent mixture \(KJ + KJO_3\). In the first case, “mild” oxidants of the type \(KNO_2\), \(Fe_2(SO_4)_3\), which do not act on other halides, are used for oxidizing iodine; this is especially important in separating iodine from fission products of heavy nuclei. In the second case, elemental iodine is separated upon acidification of the solution according to the reaction:

\[ 5KJ + KJO_3 + 3H_2SO_4 = 3J_2 + 3K_2SO_4 + 3H_2O. \]

There is reason to believe that this route achieves a more complete quantitative separation of iodine, since in this case all atoms of radioactive iodine, in whatever valence state they may have been obtained as a result of the nuclear reaction, must participate in the process of transition to the elemental state[^3].

In those cases where there is some probability of simultaneous oxidation of bromine, a certain amount of KBr is added to the solution in addition to KJ (in order to reduce the probability of separation of active bromine).

If stable iodine serves as the target, then, according to the Szilard and Chalmers method[^4], widely developed by a number of authors[^5,^6], the concentration of weighless amounts of the radioactive isotope and its separation from stable iodine are carried out by using the change in the valence state and chemical behavior of the atom obtained as a result of the nuclear reaction. The elemental iodine isolated in one way or another, containing active isotopes, is separated from the target solution either by shaking with the corresponding organic solvents \((CCl_4, CHCl_3)\), or by steam distillation from the boiling solution.

As a result of these operations it is possible to avoid the necessity, usual in most other cases, of precipitating the required element directly in the target solution. Thus in the present case the danger of coprecipitation and adsorption of other radioactive elements, which may be formed together with iodine in the nuclear reaction, is eliminated. In addition to the indicated chemical methods, in some cases electrolytic separation of active iodine directly during the irradiation process has been used.^{5,7}

IODINE ISOTOPES AND THEIR PHYSICAL CHARACTERISTICS

In considering the individual isotopes of iodine we shall touch upon the following questions: a) nuclear reactions leading to the formation of the given isotope, b) type of radioactive decay, c) half-life, d) mass number corresponding to the given activity, e) β-spectrum, γ-rays, decay scheme.

Fig. 1. Decay curves of \(J^{124}\) and \(J^{126}\).
1. Decay of radioactive isotopes of iodine formed by the reaction \(Sb(\alpha,n)J\). 2. Decay of \(J^{126}\), formed upon irradiation of iodine with fast neutrons.

\[ J^{124}. \quad T = 4 \text{ days.} \]

The isotope with \(M=124\) has the smallest mass number of all known iodine isotopes. It was obtained by Livingood and Seaborg^8 by bombarding antimony with α-particles of \(E=16\) MeV. In measurements of the activity of iodine separated from antimony irradiated with α-particles, the decay curve shown in Fig. 1 was obtained; it can be represented as the result of the superposition of two curves with periods \(T=4.0 \pm 0.3\) days and \(T=13.0 \pm 0.3\) days. The presence of two periods is in agreement with the fact that antimony has two stable isotopes—\(Sb_{51}^{121}\) (56%) and \(Sb_{51}^{123}\) (44%), from which, under the action of α-particles, two iodine isotopes are formed by the reaction \(Sb_{51}(\alpha,n)J_{53}\): \(J^{124}\) and \(J^{126}\). Since as a result of other nuclear reactions it has been shown that the 13-day period must belong to \(J^{126}\), the activity with \(T=4\) days corresponds to \(J^{124}\). According to Livingood and Seaborg, this isotope is obtained by the reaction

\[ Sb_{51}^{121}(\alpha,n)J_{53}^{124} \]

with a yield of 1 active atom per \(2\cdot 10^{7}\) bombarding α-particles having energy \(E=16\) MeV. \(J^{124}\) emits positrons, with stable tellurium being formed:

\[ J^{124} \rightarrow Te^{124} + \beta^{+}. \]

The spectrum of positrons emitted by \(J^{124}\), and the presence of \(\gamma\)-rays in \(\beta\)-decay, have not been investigated.

There are reports in the literature\(^{9}\) that the same iodine isotope was obtained by Dobridge in the bombardment of tellurium with protons in the reaction \(\mathrm{Te}(p,n)\mathrm{J}\).

It is curious that, in the bombardment of tellurium with deuterons of \(E=8\ \mathrm{MeV}\) and \(E=11.5\ \mathrm{MeV}\), the appearance of a 4-day positron activity was not detected\(^{8,10,11}\), although its formation could have been expected from the reactions \((d,n)\) and \((d,2n)\).

\(J^{125}. \ T=56\) days.

Until recently, among the radioactive isotopes of iodine there was none to which the mass number \(M=125\) could be assigned. In Seaborg’s table of isotopes\(^{9}\), the isotope \(J^{125}\) is absent. In 1946 Reid and Keston\(^{12}\) reported that, on bombarding tellurium for 40 hours with deuterons of \(E=14.5\ \mathrm{MeV}\), they obtained a long-lived activity which did not disappear over the course of 6 months. This activity follows iodine when AgJ is precipitated and when extracted with \(\mathrm{CCl}_4\); when introduced into rats it concentrates in the thyroid gland. The half-life of this activity, which must be recognized as belonging to iodine, proved to be \(T=56\) days.

In investigating the radiation emitted by this radioactive isotope, by absorption in aluminum, copper, and silver foils, characteristic x-radiation with \(E=27.0\ \mathrm{KeV}\) was detected, corresponding to the \(K\alpha\) line of Te. No \(\gamma\)-rays of higher energy were found. From this the authors concluded that the radioactive transformation of the new iodine isotope consists chiefly in \(K\)-capture with transition to Te. At the same time they put forward an insufficiently substantiated supposition that the 56-day period belongs to \(J^{129}\), and proposed a complex transformation scheme including the unique case of an isomeric transition consisting of successive \(\beta\)-decay and \(K\)-capture (scheme).

In 1947 Glendenin and Edwards\(^{13}\) confirmed the existence of an iodine isotope with \(T=56\) days, but assigned this period to \(J^{125}\). In this case the 56-day activity was separated from tellurium that had been subjected to deuteron bombardment about a year earlier. The chemical identity of this activity with iodine was demonstrated by a series of successive

...separate operations of oxidizing iodine with sodium nitrite, extracting it with CCl₄, and reducing it with a solution of sodium sulfite; moreover, it turned out that the 56-day activity in all operations follows the stable iodine. As a result of three cycles of such operations, the initial activity remained unchanged within the limits of experimental error.

In studying the absorption of the radiation emitted by 56-day iodine in Ag, Cd, In, Sn, Al, and polystyrene, only X-radiation with \(E=27.5\ \mathrm{KeV}\) and \(E=3.8\ \mathrm{KeV}\) was found, which corresponds to the \(K\alpha\) and \(L\alpha\) lines of tellurium. No \(\gamma\)-rays with \(E>30\ \mathrm{KeV}\) and no particles were found in the radiation of 56-day iodine. Thus, it may evidently be considered proven that a radioactive isotope of iodine with \(T=56\) days exists, and that its radioactive decay consists in pure \(K\)-capture.

As regards the mass number of 56-day iodine, this period should probably be assigned to \(J^{125}\).

The supposition of Reid and Keston should be recognized as erroneous, both on the basis of general considerations[^13] concerning the probable character of the radioactive transformation of a nucleus with \(Z=53\) and \(M=129\), and on the basis of experimental data which will be discussed later and from which it may be concluded that \(J^{129}\) must be assigned another period.

On the other hand, \(J^{125}\) can be formed in the bombardment of tellurium by deuterons from the stable isotopes \(\mathrm{Te}^{124}\) (4.5%) and \(\mathrm{Te}^{125}\) (6%) by the reactions: \({}_{52}\mathrm{Te}^{124}(d,n){}_{53}J^{125}\) and \({}_{52}\mathrm{Te}^{125}(d,2n){}_{53}J^{125}\). Radioactive decay by \(K\)-capture in \(J^{125}\) is not unexpected (cf. the neighboring \(J^{124}\), which undergoes the same process of nucleon transformation \(p\to n\)) and leads directly to stable \(\mathrm{Te}^{125}\).

In this case, the artificial assumptions which Reid and Keston were compelled to introduce are not required.

All these considerations, though indirect, nevertheless quite convincingly indicate that the 56-day period must belong to the isotope \(J^{125}\). However, direct proof of the identity of the iodine isotope with \(T=56\) days with \(J^{125}\) is not yet available.

\(J^{126}. T=13\) days.

Thein and Cork[^14], in bombarding stable iodine with fast neutrons, discovered a radioactive isotope of iodine with a half-life \(T=13\) days, to which they assigned the mass number \(M=126\). Observations in a Wilson chamber established that 13-day iodine emits electrons during radioactive decay.

The existence of electron-active 13-day iodine was confirmed by Livingood and Seaborg[^8,^15], who obtained it by irradiating stable iodine with fast neutrons, and also isolated it from antimony bombarded with \(\alpha\)-particles of \(E=16\ \mathrm{MeV}\), and from tellurium activated by deuterons with \(E=8\ \mathrm{MeV}\).

Decay curves of 13-day iodine, separated from antimony and from stable iodine, are shown in Fig. 1.

Because tellurium has a large number of stable isotopes, the decay curve of radioactive iodine separated from tellurium consists of a series of periods. Analysis of the end of the integral curve, taking into account the abundance of the various stable tellurium isotopes, makes it possible to conclude that the mixture of radioactive iodine isotopes contains a component with a 13-day half-life.

According to Seaborg,^9 this same isotope of iodine was obtained by DuBridge in the bombardment of tellurium with protons via the reaction \((p,n)\).

The formation of 13-day iodine is possible by four independent routes, in the reactions:

\[ {}_{53}\mathrm{I}^{127}(n,2n){}_{53}\mathrm{I}^{126}; \qquad {}_{51}\mathrm{Sb}^{121}(\alpha,n){}_{53}\mathrm{I}^{126}; \]

\[ {}_{52}\mathrm{Te}^{125}(d,n){}_{53}\mathrm{I}^{126}; \qquad {}_{52}\mathrm{Te}^{126}(p,n){}_{53}\mathrm{I}^{126}. \]

This convincingly proves that the period \(T=13\) days must be assigned the mass number \(M=126\).

The radiation emitted by \(\mathrm{I}^{126}\) was studied by Livingood and Seaborg^8 by absorption in Al and Pb, and by Tayn^16 with the aid of a Wilson chamber in a magnetic field. In the absorption measurements, \(\beta\)-particles with a range \(R=0.44\ \mathrm{g/cm^2}\) Al and \(\gamma\)-rays absorbed by half in \(4.5\ \mathrm{g/cm^2}\) Pb were found. Hence the maximum energy of the \(\beta\)-particles, calculated from Fezer’s formula^17

\[ R=0.543E_m-0.160 \]

is \(E_m=1.1\ \mathrm{MeV}\). The energy of the \(\gamma\)-rays is \(E_\gamma=0.5\ \mathrm{MeV}\).

Tayn, using a Wilson chamber in a magnetic field, investigated the \(\beta\)-spectrum of radioiodine obtained by bombarding \(\mathrm{NaJO}_3\) with fast \((\mathrm{Li}+\mathrm{D})\) neutrons. On the basis of measurements of 1060 tracks, a simple \(\beta\)-spectrum with \(E_m=1.20\pm0.03\ \mathrm{MeV}\) was obtained. Extrapolation of the F. Curie graph,^18 constructed for Fermi’s theory of \(\beta\)-decay in the coordinates \(\left(\frac{N}{f}\right)^{1/2}\) and \(E\), gives the value of the maximum energy of the \(\beta\)-spectrum \(E_m=1.22\ \mathrm{MeV}\).

Fig. 2. Decay scheme of \(\mathrm{J}^{126}\to \mathrm{Xe}^{126}\).

Thus, within the accuracy of the available data, the decay scheme

\[ \mathrm{J}^{126}\to \mathrm{Xe}^{126} \]

can be represented in the form of the diagram shown in Fig. 2.

In view of the fact that there are indications of the presence of X-radiation in the long-lived iodine,^11 it is possible that for \(\mathrm{J}^{126}\) there also exists some probability of another decay scheme, consisting in \(K\)-capture with transition to \(\mathrm{Te}^{126}\).

\(J^{127}\)

From Nier’s mass-spectrographic measurements\(^1\) it follows that \(J^{127}\) is the only stable isotope of iodine. The presence in natural iodine of other stable iodine isotopes is possible—in relation to \(J^{127}\)—in the amounts:

\[ \begin{gathered} J^{123} \leq 1/50\,000;\qquad J^{124} \leq 1/50\,000;\qquad J^{125} \leq 1/50\,000,\\ J^{126} \leq 1/25\,000;\qquad J^{128} \leq 1/15\,000;\qquad J^{129} \leq 1/40\,000,\\ J^{130} \leq 1/120\,000;\qquad J^{131} \leq 1/250\,000; \end{gathered} \]

\[ J^{128}. \ T=25\ \text{min.} \]

In the first works on obtaining artificially radioactive elements under the action of neutrons, Fermi, Amaldi, and others\(^5,19\), upon irradiating iodine with slow neutrons, discovered an activity with \(T=25\) min., following iodine when it was precipitated in the form of AgJ in the presence of Te and Sb. This activity was assigned to \(J^{128}\), formed during radiative capture of a neutron by stable iodine according to the reaction:

\[ {}^{127}_{53}J(n,\gamma){}^{128}_{53}J. \]

Subsequently radioactive iodine was obtained and studied in a number of laboratories.

The investigation and use of \(J^{128}\) were greatly facilitated thanks to the possibility, discovered by Szilard and Chalmers\(^4\), of separating small amounts of the active isotope from the mass of inactive material. The method of chemical separation of isotopes, which subsequently found wide application, was first tested precisely on the reaction \(J^{127}(n,\gamma)J^{128}\), with separation of active \(J^{128}\) from the mass of inactive ethyl iodide \(C_2H_5J^{127}\).

It is interesting that this process, based on the use of the recoil experienced by an atom upon emission of a \(\gamma\)-quantum, admits a kind of reversal. The recoil occurring as a result of radiative capture of a neutron can be used not only to tear the active atom out of a molecule, but also to introduce the active atom into the molecule of some substance. Thus, Reid\(^20\), irradiating with slow neutrons a solution of iodine in normal pentane, found that about 40% of the activity proves to be bound with \(n\)-amyl iodide. Evidently the process takes place:

\[ {}^{127}_{53}J + {}^{1}_{0}n \longrightarrow {}^{128}_{53}J + \gamma \]

\[ C_5H_{12}+J^{128}_{\text{upon recoil}} \longrightarrow C_5H_{11}J^{128}+H. \]

Processes of this kind may possibly be applied for the synthesis of chemical compounds containing active atoms, both for purposes of chemical research and for obtaining highly radioactive preparations with a definite physiological action.

The yield of radioactive iodine in the reaction \(J^{127}(n,\gamma)J^{128}\) does not depend on the valence state of the stable iodine irradiated with neutrons.

RADIOACTIVE ISOTOPES OF IODINE

According to Knauer’s data[^21], when iodine is irradiated in the form of \(J'\), \(JO_3'\), \(J_2'\), the yield of radioactive iodine is unchanged within the limits of 2–3%.

The effective cross section of the nuclear reaction

\[ {}^{127}_{53}J + {}^{1}_{0}n \to {}^{128}_{53}J + \gamma \]

and its dependence on neutron energy have been investigated by a number of authors[^22–^29]. For fast \((\mathrm{Rn}-\mathrm{Re})\) neutrons, V. S. Dementii and D. V. Timoshchuk[^29] determined \(\sigma = 6.57 \cdot 10^{-26}\ \mathrm{cm}^2\). For slow neutrons the effective cross section is at least 100 times larger. Fig. 3 presents the dependence, obtained by Jones[^27], of the total

Fig. 3. Cross section of \(J\) as a function of neutron energy.

cross section of \(J\) on neutron energy in the energy interval from 0.0026 eV to 1000 eV. For thermal neutrons \((E = 0.025\ \mathrm{eV})\) the cross section of the \((n,\gamma)\) reaction, allowing for the scattering cross section, is \(\sigma_c = 6.7 \cdot 10^{-24}\ \mathrm{cm}^2\). At \(E < 3\ \mathrm{eV}\), the interaction of iodine with neutrons follows the \(1/v\) law; at \(E = 20.3\ \mathrm{eV}\) there is a resonance line, and in the interval \(E = 25\text{–}50\ \mathrm{eV}\) there is a resonance band containing at least three resonance maxima.

Radioactive iodine with period \(T = 25\) min can be obtained not only by the nuclear reaction considered above, but also by other routes. Livingood and Seaborg[^8] isolated iodine with \(T = 25 \pm 1\) min from tellurium bombarded with deuterons of \(E = 8\ \mathrm{MeV}\). In this case \(J^{128}\) is formed from the stable isotope \(\mathrm{Te}^{128}\) (32.8%) by the reaction \({}^{128}_{52}\mathrm{Te}(d,2n){}^{128}_{53}J\), with a yield of 1 active atom per \(2 \cdot 10^7\) deuterons with \(E = 8\ \mathrm{MeV}\). The formation of \(J^{128}\) in this way was confirmed by Tain[^16], who also obtained it by bombarding lead iodide with 7 MeV deuterons, evidently by the reaction \({}^{127}_{53}J(d,p){}^{128}_{53}J\). In addition,

therefore, this isotope of iodine is formed when tellurium is bombarded with protons[^9] via a \((p,n)\) reaction.

The half-life of \(J^{128}\) has been determined very accurately; as a result this isotope can be used for calibrating instruments used to measure radioactivity. According to Hall and Seelig[^30], the half-life of \(J^{128}\) is

\[ T = 24.99 \pm 0.02 \text{ min.} \]

The radiation emitted by \(J^{128}\) in \(\beta\)-decay has been studied by a number of authors. Fermi, Amaldi, and others[^3], having isolated 25-minute iodine, proved the emission by this isotope of electrons and \(\gamma\)-rays and determined the absorption of \(\beta\)-particles in aluminum. The \(\beta\)-spectrum of \(J^{128}\) was first obtained by Soviet scientists Alikhanov, Alikhanyan, and Dzhelepov[^31,^32] with the aid of an original apparatus with two coincidence counters in a magnetic field. The \(\beta\)-spectrum obtained in this way is shown in Fig. 4 and has \(E_m = 2.1\) MeV. The largest number of electrons has \(E \simeq 0.5\) MeV.

Fig. 4. \(\beta\)-spectrum of \(J^{128}\).

M. I. Korsunskii, N. N. Nikolaevskaya, and M. A. Bak[^7] studied the \(\beta\)-spectrum of \(J^{128}\) in the energy interval \(0.6\)—\(2.0\) MeV by the same method of magnetic analysis with two coincidence counters. Using a more powerful source, they studied in greater detail the \(\beta\)-spectrum of 25-minute \(J\) within the indicated energy limits.

Subsequently the \(\beta\)-spectrum of \(J^{128}\) was investigated in a number of laboratories by various methods, and the results of individual determinations do not always agree well with one another. Livingood and Seaborg[^8] determined the absorbing layer for the \(\beta\)-rays of \(J^{128}\) to be \(1.05 \text{ g}/\text{cm}^2\) Al, which, recalculated by Feather’s formula, corresponds to \(E_m = 2.23\) MeV. Tèhn[^16], in measurements in a Wilson chamber with a magnetic field, obtained, from measuring the radii of curvature of 1330 tracks obtained from several preparations, a \(\beta\)-spectrum with \(E_m = 2.4\) MeV. He also reports[^33] another value, \(E_m = 2.2\) MeV. Construction of a F. Curie plot in the coordinates

\[ \left(\frac{N}{f}\right)^{1/k} - E \]

for \(K = 2\) and \(K = 4\) gives, on extrapolation, \(E_m = 2.2\) MeV in the first case and \(E_m = 2.94\) MeV in the second. This leads one to believe that the \(\beta\)-spectrum of \(J^{128}\) should be described rather by Fermi’s original theory than by the Konopinski–Uhlenbeck theory.

On the other hand, Beghon, Grasewood, and van der Merwe[^34,^35] also investigated the spectrum of \(\beta\)-particles emitted by \(J^{128}\), using a Wilson chamber in a magnetic field, and analyzed the Curie plot for \(K = 4\), proceeding from the assumption that the Konopinski–Uhlenbeck theory is applicable to the \(\beta\)-spectrum of 25-minute iodine. Under this assumption

RADIOACTIVE ISOTOPES OF IODINE

...the break in the Konopinski–Uhlenbeck plot (Fig. 5) was interpreted by the authors as indicating the presence in \(J^{128}\) of a complex spectrum. When the Konopinski–Uhlenbeck plot is decomposed into two straight lines and these are extrapolated to their intersections with the abscissa axis, the values of the limiting energies of two groups of \(\beta\)-particles are obtained: \(E'_m = 1.05\ \mathrm{MeV}\), \(E''_m = 2.10\ \mathrm{MeV}\). These data are cited in the well-known isotope tables of Seaborg and Mattauch. However, these values are open to doubt both because they were obtained under the questionable assumption that the Konopinski–Uhlenbeck theory is applicable to the \(\beta\)-decay of \(J^{128}\), and because they do not agree with other experimental data.

Indeed, according to the data of other authors \(^{7,16}\), the Konopinski–Uhlenbeck theory does not agree sufficiently well with the experimental values of the maximum energy of the \(\beta\)-spectrum of \(J^{128}\), giving overestimated results. Incidentally, the authors themselves \(^{35}\) do not draw definite conclusions about the applicability of the Konopinski–Uhlenbeck theory.

Fig. 5. Fermi and Konopinski–Uhlenbeck plots for the \(\beta\)-spectrum of \(J^{128}\) (according to Bakon, Graisevaid, and van der Merwe).

On the other hand, for deciding the question of the presence in \(J^{128}\) of two groups of \(\beta\)-particles with \(E'_m = 1.05\ \mathrm{MeV}\) and \(E''_m = 2.10\ \mathrm{MeV}\), the character and energy of the \(\gamma\)-rays emitted by the xenon nucleus \(Xe^{128}\) during the radioactive decay \(J^{128} \to Xe^{128}\) are very essential. It is obvious that if the \(E_{\max}\) values proposed by Bakon, Graisevaid, and van der Merwe are correct, then in \(\beta\)-decay fairly intense \(\gamma\)-rays with \(E_\gamma = 1.05\ \mathrm{MeV}\) should be observed.

However, according to data obtained in a number of laboratories, it is known that although \(J^{128}\) does emit \(\gamma\)-rays in \(\beta\)-decay \(^{5,16,36}\), their intensity is very small, and their energy is considerably less than \(1\ \mathrm{MeV}\). Thus, Roberts and Irvine \(^{37}\), and Bakon, Graisevaid, and van der Merwe themselves \(^{35}\), were unable to detect \(\gamma\)-rays in an amount greater than 1 photon per 10 \(\beta\)-particles. Even if the energy of these photons corresponded to the difference in limiting energies of the two groups of \(\beta\)-particles, it is very doubtful that such an insignificant number of \(\beta\)-particles (10%), whose emission is followed by the emission of a \(\gamma\)-quantum, could have significantly affected the shape of the \(\beta\)-spectrum and the corresponding Kurie plot.

Moreover, absorption measurements of the energy of \(\gamma\)-rays, carried out...

conducted by M. A. Bak and N. N. Nikolaevskaya and by Livingood and Seaborg\(^8\), give for the energy of the \(\gamma\)-rays of 25-minute iodine the coincident value \(E_\gamma \simeq 0.4\) MeV, which is wholly inconsistent with the above-indicated values of the maximum energy of the two groups of \(\beta\)-particles.

A more satisfactory decay scheme for \(J^{128}\) was proposed by Siegbahn and Hole\(^ {39}\), who studied the \(\beta\)-radiation of this iodine isotope with the aid of a \(\beta\)-spectrograph and determined the \(\gamma\)-ray energy from the energies of photoelectrons ejected from lead. The preparation of radioactive iodine was placed in a copper capsule, in front of which there was

Fig. 6. Spectrum of secondary electrons formed under the action of \(\gamma\)-rays of \(J^{128}\).

Fig. 7. Fermi plot for the \(\beta\)-spectrum of \(J^{128}\).

a lead foil \(0.1\) mm thick. The \(\beta\)-particles emitted by \(J^{128}\) were completely absorbed in the walls of the capsule, while the secondary electrons were studied with the aid of a \(\beta\)-spectrograph. Figure 6 presents the spectrum of secondary electrons arising under the action of the radioiodine \(\gamma\)-rays on the copper capsule and the lead foil. Against the background of Compton electrons two peaks are visible, corresponding to photoelectrons from the \(K\) and \(L\) shells of lead. The value of the \(\gamma\)-ray energy, calculated from the photoelectron energy with allowance for the \(K\)- and \(L\)-binding energies of Pb, agree well with one another and are equal to \(E_\gamma = 0.428\) MeV. No \(\gamma\)-rays of higher energy were found. These results agree with the data obtained by absorption measurements. The \(\gamma\)-rays with \(E_\gamma = 0.428\) MeV have low intensity and, as shown by coincidence registration in \(\beta\)- and \(\gamma\)-counters, are emitted in the amount of 7 \(\gamma\)-quanta per 100 \(\beta\)-particles. Thus the emission of \(\gamma\)-quanta accompanies only 7% of the \(\beta\)-decays and, consequently, the \(\beta\)-spectrum of \(J^{128}\) must consist of two groups with a difference between the values of \(E_{\max}\) equal to 0.428 MeV.

Construction of the Fermi plot shown in Fig. 7 gives \(E = 2.02\) MeV, which agrees rather well with the value obtained by Alikhanov, Alikhanian, and Dzhelepov.

RADIOACTIVE ISOTOPES OF IODINE

It is clear that, owing to the low intensity of the second group of β-particles, their presence is not reflected in the Fermi plot. In accordance with what was set forth above, the decay \(J^{128}\to Xe^{128}\) may be represented, according to Zigbahn and Hole, by the scheme in Fig. 8. 93% of the β-particles are emitted with transition to the ground level of the \(Xe^{128}\) nucleus. 7% of the β-particles are emitted with transition to the excited level of \(Xe^{128}\), with an energy release of 1.59 MeV and subsequent emission of a γ-quantum with \(E_\gamma = 0.428\) MeV.

\[ J^{129}.\quad T \gg 10^8\ \text{years}. \]

An iodine isotope with mass number \(M=129\) for a long time could not be detected either among the stable or among the radioactive isotopes.

In studying the isomerism of \(Te^{129}\), Seaborg, Livingood, and Kennedy \(^{40}\) discovered an isomeric transition

\[ Te^{*129}\ (32\ \text{days}) \longrightarrow Te^{129}\ (72\ \text{min.}), \]

but found no radiation from the decay products of the 72-minute \(Te^{129}\). On the other hand, according to Nier \(^{1}\), \(J^{129}\) is not stable or, more precisely, is not contained in natural iodine in an amount greater than \(1/40\,000\). Thus \(J^{129}\) must be either a very rare stable isotope or have a very long half-life.

Fig. 8. Decay scheme of
\(J^{128}\to Xe^{128}\).

Indeed, from the published data on the plutonium project \(^{41}\) it follows that \(J^{129}\) is formed in the fission of \(U^{235}\), has a “very long” half-life, and emits electrons, transforming into stable \(Xe^{129}\). \(J^{129}\) is formed as a result of the chain of β-transformations of the primary fragments arising in uranium fission, according to the scheme:

\[ \begin{array}{cccccc} & & Te^{*129}_{52}\ (32\ \text{days}) & & & \\ & \nearrow & \downarrow & & & \\ Sb^{129}_{51}\ (4.2\ \text{hours}) & \longrightarrow & Te^{129}_{52}\ (72\ \text{min.}) & \longrightarrow & J^{129}_{53}\ (\text{very long}) & \longrightarrow Xe^{129}_{54}\ (\text{stable}). \end{array} \]

In the analysis of the integral decay curve of radioactive iodine isotopes separated from neutron-irradiated uranium, it is not possible to detect a period \(T>8\) days. Even when uranium is irradiated for 123 days in the Clinton pile \(^{42}\), the iodine separated from it gives a decay curve containing only the known periods and decays to background with \(T=8\) days, which corresponds to the well-known isotope \(J^{131}\). But this same sample of iodine, after decay to background, when subjected to irradiation with slow neutrons, detects

significant activity belonging to iodine with \(T=12.5\) hours. This activity, from its half-life period and from the results of absorption measurements in Al, has been identified as \(J^{130}\). Its formation should evidently be attributed to the radiative capture of neutrons by the isotope of iodine with \(M=129\). The effective cross section of the reaction \(J^{129}(n,\gamma)J^{130}\) may be roughly estimated as \(\sigma=8\cdot 10^{-24}\ \text{cm}^2\).

The very long half-life period of \(J^{129}\) (\(T\gg 10^8\) years) makes it possible to assume its presence in natural iodine in an amount approximately 80 times smaller than the limit indicated by Nier.

Fig. 9. Decay curve of radioactive iodine isotopes formed in the bombardment of Te with deuterons.

\(J^{130}\). \(T=12.6\) hours.

Livingood and Seaborg \(^{8,15}\) separated from tellurium bombarded with deuterons of \(E=8\) MeV a radioactive isotope of iodine with \(T=12.6\) hours, emitting electrons in its decay. This period was assigned to \(J^{130}\), formed from stable \(Te^{130}\) (33.1%) by the reaction: \(Te_{52}^{130}(d,2n)J_{53}^{130}\), with a yield of 1 atom per \(1\cdot 10^7\) deuterons with \(E=8\) MeV. When tellurium is bombarded with deuterons of \(E=14\) MeV, the yield of \(J^{130}\) in absolute disintegrations is \(32\cdot 10^6\) disintegrations/\(\mu\)A·hour, which corresponds to a yield of one active atom per \(1.1\cdot 10^4\) deuterons. In bombarding tellurium with deuterons, several iodine isotopes are formed. Fig. 9 presents the composite decay curve of iodine separated from deuteron-activated tellurium, and its analysis (according to Livingood and Seaborg).

Convincing proof that the period \(T=12.6\) hours belongs to \(J^{130}\) is the fact that iodine with \(T=12.6\) hours is also obtained in the bombardment of cesium by fast neutrons \(^{44}\). Since cesium has only one stable isotope, \(Cs_{55}^{133}\), the formation of radioactive iodine occurs by the reaction

\[ Cs_{55}^{133}(n,\alpha)J_{53}^{130}. \]

This same isotope was obtained by irradiating tellurium with protons \(^{9}\) by the evident reaction: \(Te_{52}^{130}(p,n)J_{53}^{130}\).

The reports in the literature that radioactive iodine with a period close to \(T=13\) hours can be separated from the fission products of uranium \(^{45}\) and thorium \(^{45,46}\) under the action of neutrons should be ...

...should be treated with known caution. The presence of several radioactive iodine isotopes formed in these cases, the decay of some isotopes with the formation of active products, and the change in the ratios between the intensities of the individual activities depending on the irradiation conditions make the analysis of the integral decay curve extremely difficult. It is very probable that, under certain irradiation conditions, the 22-hour period of \(J^{133}\) may be masked by the presence of the 6.6-hour period of \(J^{135}\) and the 9.4-hour period of \(Xe^{135}\) arising from it. Obviously, this circumstance should be attributed to the fact that in various laboratories, in the analysis of the integral decay curve of radioiodines separated from the products of fission of heavy nuclei, a number of intermediate periods were obtained: \(T = 12\) hours \(^{45}\), \(T = 15\text{–}16\) hours \(^{46,47}\), \(T = 18.5\) hours \(^{48}\).

Fig. 10. \(\beta\)-spectrum of \(J^{130}\).

Thus the period \(T = 15\text{–}16\) hours discovered by Polessitskii and Orbelii, formed in the fission of thorium, may belong both to \(J^{130}\) and to \(J^{133}\). This circumstance, however, is also noted by the authors themselves \(^{47}\).

As for the presence of \(J^{130}\) among the fission products of uranium, it is apparently excluded by later work \(^{41}\). In the fission of \(U^{235}\), as a primary fragment with mass 130, stable \(Te^{130}\) is formed, and thus the occurrence of a chain of \(\beta\)-transformations leading to \(J^{130}\) is impossible.

The radiation emitted by \(J^{130}\) during \(\beta\)-decay was studied by means of absorption \(^{8}\) in Al and Pb, in a Wilson chamber in a magnetic field \(^{16}\), and—most thoroughly—by Robertson, Downing, Elliott, and Deitch \(^{11,49,50}\) with the aid of a magnetic spectrometer and the coincidence method.

Figure 10 presents the \(\beta\)-spectrum of \(J^{130}\) obtained by these authors, on which 4 peaks are visible, corresponding to internal-conversion electrons. The energies of the corresponding \(\gamma\)-quanta, calculated with allowance for the Xe \(K\)-binding energy, are: \(E_1 = 0.416 \pm 0.005\) MeV, \(E_2 = 0.538 \pm 0.007\) MeV, \(E_3 = 0.665 \pm 0.008\) MeV, \(E_4 = 0.747 \pm 0.010\) MeV. Close values of \(E_\gamma\) are obtained in the analysis of the spectra of secondary electrons formed under the action of \(\gamma\)-rays of \(J^{130}\) on Au, Pb, Sn (0.417; 0.535; 0.670; 0.740 MeV).

For the investigation of the decay scheme of \(J^{130}\), the fact that these 4 types of \(\gamma\)-quanta are not equivalent is very important. The intensity of the line \(E_\gamma = 0.416\ \mathrm{MeV}\) is only \(0.3 \pm 0.1\) of the mean intensity of the other \(\gamma\)-lines. Consequently, the \(\gamma\)-quantum with \(E_\gamma = 0.416\ \mathrm{MeV}\) is not emitted in every act of \(\beta\)-decay. On the other hand, investigation of the change in the number of \(\beta-\gamma\) coincidences falling to one registered \(\beta\)-particle, as a function of the thickness of the absorber placed between the \(\beta\)-counter and the source,

Fig. 11. Fermi plot for the \(\beta\)-spectrum of \(J^{130}\).

Fig. 12. Decay scheme \(J^{130} \to Xe^{130}\).

makes it possible to establish that, for \(\beta\)-particle energies \(E\) greater than \(0.6\ \mathrm{MeV}\), the number of \(\beta-\gamma\) coincidences does not depend on the thickness of the absorber. From these experimental facts it follows that the \(\beta\)-spectrum of \(J^{130}\) must consist of two groups of \(\beta\)-particles, one of which, with \(E \simeq 0.6\ \mathrm{MeV}\), is accompanied by the emission of 4 \(\gamma\)-quanta, while the second is accompanied by the same \(\gamma\)-quanta, with the exception of the quantum with \(E_\gamma = 0.416\ \mathrm{MeV}\). These conclusions agree with the data obtained in the analysis of the Fermi plot constructed by the above-mentioned authors for the \(\beta\)-spectrum of \(J^{130}\) obtained by them (Fig. 11). In the coordinates \(\left(\dfrac{N}{f}\right)^{1/2} - E\), a broken line is obtained; when this is resolved into two straight lines, two values of the maximum \(\beta\)-particle energy are determined: \(E'_m = 0.61 \pm 0.02\ \mathrm{MeV}\) and \(E''_m = 1.03 \pm 0.02\ \mathrm{MeV}\). The value \(E'_m = 0.61\ \mathrm{MeV}\) agrees well with the results of the investigation of \(\beta-\gamma\) coincidences, while \(E''_m = 1.03\ \mathrm{MeV}\) practically coincides with \(E_m = 1.05\ \mathrm{MeV}\), obtained by Livingood and Seaborg in absorption measurements. The difference \(E''_m - E'_m = 0.42\ \mathrm{MeV}\), which precisely corresponds to \(E_\gamma = 0.416\ \mathrm{MeV}\).

Thus the decay \(J^{130} \to Xe^{130}\) should basically correspond to the scheme\(^ {11}\) given in Fig. 12. There is emission of two

groups of β-particles with \(E'_m = 0.61\) MeV and \(E''_m = 1.03\) MeV. In both cases there is a transition to excited levels of \(\mathrm{Xe}^{130}\). The energies of these levels differ by 0.416 MeV. Both transitions are accompanied by the emission of several \(\gamma\)-quanta in cascade, as a result of which the \(\mathrm{Xe}^{130}\) nucleus passes into the ground state. The order of emission of the \(\gamma\)-quanta, apart from the \(\gamma\)-quantum with \(E_\gamma = 0.416\) MeV, is not determined and is indicated arbitrarily in the scheme.

\[ \mathrm{J}^{131}. \quad T = 8 \ \text{days}. \]

Teichmann and Cork \(^{14}\), in bombarding tellurium with deuterons, obtained an activity with a period \(T = 8\) days, which they erroneously assigned to \(\mathrm{Te}^{131}\). Soon Livingood and Seaborg \(^{8,15}\) showed that the eight-day activity is chemically identical with iodine and is formed when tellurium is bombarded with deuterons and neutrons. Eight-day iodine is obtained both directly by the reaction \(\mathrm{Te}^{130}_{52}(d,n)\mathrm{J}^{131}_{53}\), and also in the β-decay of the radioactive isotope \(\mathrm{Te}^{131}\), which arises upon activation of tellurium by deuterons and neutrons:

\[ \mathrm{Te}^{130}(d,p)\mathrm{Te}^{131}; \qquad \mathrm{Te}^{130}(n,\gamma)\mathrm{Te}^{131}; \qquad \mathrm{Te}^{131}_{52} \longrightarrow \mathrm{J}^{131}_{53} + \beta^- . \]

The formation of \(\mathrm{J}^{131}\) directly in the nuclear reaction \(\mathrm{Te}^{130}_{52}(d,n)\mathrm{J}^{131}_{53}\) occurs with a yield: at a deuteron energy \(E = 8\) MeV—1 atom per \(5 \cdot 10^6\) deuterons \(^{8}\), at \(E = 14\) MeV—1 atom per \(0.7 \cdot 10^4\) deuterons \(^{43}\).

Since two isomeric states with \(T = 30\) hours and \(T = 25\) minutes are known for \(\mathrm{Te}^{131}\), it is of interest to clarify the question of which of the isomers is the immediate predecessor of \(\mathrm{J}^{131}\), when the latter is formed in the β-decay of \(\mathrm{Te}^{131}\). As Seaborg and Kennedy \(^{40,51}\) showed, \(\mathrm{J}^{131}\) is formed from the 25-minute isomer of \(\mathrm{Te}^{131}\), and the following chain of transformations takes place:

\[ \mathrm{Te}^{*131}_{52} \ \xrightarrow[\ ]{30\ \text{hours}}\ \mathrm{Te}^{131}_{52} \ \xrightarrow[\ ]{25\ \text{min.}}\ \mathrm{J}^{131}_{53} \ \xrightarrow[\ ]{8\ \text{days}}\ \mathrm{Xe}^{131}_{54}\,(\text{stable}). \]

The existence of precisely this mechanism of formation of \(\mathrm{J}^{131}\) in the β-decay of \(\mathrm{Te}^{131}\) was proved by means of the chemical method of separating isomers \(^{52}\), applicable in the case when the isomers are genetically related, i.e., when one isomeric state arises from another in the process of an isomeric transition. The idea of this method is that, as a result of the recoil experienced by the nucleus in highly converted isomeric \(\gamma\)-transitions, a change occurs in the valence state of the atom that has undergone the isomeric transition. As a result, atoms that have undergone the isomeric transition and are in the ground state can be separated from atoms that are in the metastable state.

It turns out that, if inactive tellurous acid is added to telluric acid irradiated with deuterons and after some time the latter is reduced in a \(3N\) HCl solution by passing \(SO_2\) to elemental Te, the precipitate contains only 25-minute tellurium, which decays with the formation of 8-day iodine. This is proof of the above scheme of transformations.

In connection with the question of the isomeric transformations of \(Te^{131}\) preceding the formation of \(J^{131}\), there is of interest the unverified, but also not refuted, report of Roberts and Irvine\({}^{10}\), who isolated from tellurium activated by deuterons with \(E = 11.5\) MeV, again over a period of several months, radioactive iodine whose half-life and absorption measurements differed very little from 8-day \(J^{131}\). If this activity is identified with \(J^{131}\), it becomes necessary to assume the existence in \(Te^{131}\) of a third long-lived isomer.

The formation of \(J^{131}\) takes place whenever, as a result of some process, \(Te^{131}\) arises. Thus, this isotope of iodine was discovered by Abelson\({}^{5),54,55}\) and by Hahn and Strassmann\({}^{48,56}\) in the fission products of uranium under the action of neutrons, while Fermi and Segrè\({}^{57}\) isolated it from uranium irradiated with \(\alpha\)-particles with \(E = 32\) MeV.

\(J^{131}\) is not a primary product of uranium fission, but arises as a result of the \(\beta\)-decay of \(Te^{131}\), the chain being

\[ {}_{52}Te^{*131}\ \xrightarrow[\,]{30\ \text{hr.}}\ {}_{52}Te^{131}\ \xrightarrow[\,]{25\ \text{min.}}\ {}_{53}J^{131}\ \xrightarrow[\,]{8\ \text{days}}\ {}_{54}Xe^{131}\ (\text{stable}). \]

The yield of this chain in the fission of uranium by both slow and fast neutrons has been studied by a number of authors. On the basis of the work of Encke\({}^{58,59}\), it may be assumed that there should be no significant difference in the yield coefficients for these two cases. Indeed, the yield coefficients of the chain with mass number \(M = 131\) in the fission of uranium by slow\({}^{41,60,61}\) and by fast\({}^{61}\) neutrons are very close and amount to \(2.2\)–\(2.8\%\) of the total number of fissions. Yaffe and Mackintosh\({}^{61}\), by determining the ratios of the activities of \(J^{131}\) and \(Ba^{140}\) isolated from uranium upon irradiation of the latter with slow and fast neutrons, obtained almost the same value for these coefficients. From the previously determined values of the yields of \(Ba^{140}\) in the fission of uranium by slow and by fast neutrons\({}^{62}\), the yield coefficients of \(J^{113}\) were determined to be, respectively, \(2.23 \pm 0.11\%\) and \(2.27 \pm 0.11\%\).

Radiation and decay scheme. The \(\beta\)-spectrum of 8-day iodine has been studied by absorption in Al\({}^{18,41,61}\), with a Wilson chamber in a magnetic field\({}^{16}\), and with a magnetic spectrometer\({}^{63,64}\). Among the rather varied data, the most reliable appear to be the results of Deutsch, Downing, and Roberts, who found, with the aid of a magnetic spectrometer and a study of \(\beta\)–\(\gamma\) coincidences, that \(J^{131}\)

has a simple spectrum with \(E_m = 0.595 \pm 0.01\ \mathrm{MeV}\). The value of the maximum energy of the \(\beta\)-spectrum, \(E_m = 0.595\ \mathrm{MeV}\), is obtained both from the \(\beta\)-spectrum itself (Fig. 13) and from the corresponding Fermi plot (Fig. 14). This value agrees with the value \(E_m = 0.6\ \mathrm{MeV}\) obtained from absorption measurements \(^{41}\).

The energy of the \(\gamma\)-rays of \(J^{131}\) was determined by absorption in Pb \(^{8,41}\), and from the energies of secondary \(^{16,49,64}\) and conversion \(^{49,63,64}\) electrons; the results of measurements by different methods agree rather well with one another. According to Dunning, Deutsch, and Roberts \(^{50,64}\), each \(\beta\)-particle is accompanied by the emission of two \(\gamma\)-quanta in cascade with \(E_\gamma = 0.357 \pm 0.007\ \mathrm{MeV}\) and \(E_\gamma = 0.080 \pm 0.001\ \mathrm{MeV}\). The order of emission of the \(\gamma\)-quanta has not been determined.

Fig. 13. \(\beta\)-spectrum of \(J^{131}\).

The probable decay scheme \(J^{131} \to Xe^{131}\) is shown in Fig. 15 and consists in the emission of \(\beta\)-particles with \(E_m = 0.595\ \mathrm{MeV}\), with transition to an excited level of \(Xe^{131}\), and in the subsequent emission of two \(\gamma\)-quanta in cascade, with transition to the ground state of \(Xe^{131}\).

Fig. 14. Fermi plot for the \(\beta\)-spectrum of \(J^{131}\).

Fig. 15. Decay scheme
\(J^{131} \to Xe^{131}\).

\[ J^{(132)}.\ T = 2.4\ \text{hours}. \]

\(\beta\)-activity with a period \(T = 2.5\) hours was first discovered in 1937 by Meitner, Hahn, and Strassmann \(^{65}\) in uranium irradiated with neutrons. It was established at the same time that it does not belong to the primary product of the interaction of uranium with neutrons,

and arises as a result of a series of β-transformations. In accordance with the ideas about “transuranics” that existed at that time, the 2.5-hour activity was ascribed to EkaPt, arising according to the scheme:

\[ \mathrm{U}+n \longrightarrow \mathrm{U}_{92} \;\xrightarrow[\,]{10\ \mathrm{sec.}}\; \mathrm{EkaRe}_{93} \;\xrightarrow[\,]{2.2\ \mathrm{min.}}\; \mathrm{EkaOs}_{94} \;\xrightarrow[\,]{59\ \mathrm{min.}}\; \mathrm{EkaIr}_{95} \longrightarrow \]

\[ \xrightarrow[\,]{66\ \mathrm{hr.}}\; \mathrm{EkaPt}_{96} \;\xrightarrow[\,]{2.5\ \mathrm{hr.}}\; \mathrm{EkaAu}_{97}. \]

Soon after the discovery of the fact of uranium fission, Abelson^66 and Fezer and Bretscher^67 almost simultaneously showed that the 2.5-hour activity is chemically identical with iodine, and that its precursor, with a period of about 3 days (66–77 hr.), is tellurium. These results were obtained by analyzing the absorption, in a series of substances, of the radiation emitted by the “transuranic” elements, and by chemical separation of the activities with the corresponding carriers. They constituted new and independent proof of the fact of fission of the uranium nucleus.

It seems very strange that the typical metalloid iodine could previously have been identified as EkaPt. It is interesting that what seemed to be weighty evidence was cited for this. The 66-hour and 2.5-hour activities were precipitated from an acid solution of a uranium salt irradiated with neutrons by hydrogen sulfide, together with platinum. Then the 2.5-hour activity was separated during crystallization of ammonium chloroplatinate. In the light of the ideas then current about “transuranics,” all this seemed a sufficiently good proof of the chemical identity of the 2.5-hour activity with EkaPt. In reality, on the one hand, there was precipitation of Te, which on β-decay gave 2.5-hour iodine, and adsorption of I on surface-active sulfide precipitates; on the other hand, there was co-crystallization with ammonium chloroplatinate of the corresponding iodine complex.

The chain of β-decays arising from the primary fragment obtained in the fission of uranium under the action of neutrons and leading to radioiodine was studied in detail by Abelson^53,55, Acad. V. G. Khlopin with collaborators^68,69,70,71, and Hahn and Strassmann^3,48,56. As a result, all the elements of the chain were chemically identified and their periods refined. The primary fragment is Sb, decaying according to the scheme:

\[ \mathrm{Sb}_{51} \;\xrightarrow[\,]{\sim 5\ \mathrm{min.}}\; \mathrm{Te}_{52} \;\xrightarrow[\,]{77\ \mathrm{hr.}}\; \mathrm{J}_{53} \;\xrightarrow[\,]{2.4\ \mathrm{hr.}}\; \mathrm{Xe}_{54}\;(\mathrm{stable}). \]

The iodine isotope under consideration is formed not only in the fission of uranium but, as Hahn, Strassmann and Flügge^72 and Polesskii and Nemerovskii^73 showed, also in the fission of thorium under the action of neutrons. It was also isolated from uranium subjected to bombardment by α-particles with \(E=32\) MeV^57.

Since, as far as we know, radioactive iodine with \(T=2.4\) hours has been isolated only from products of fission of heavy nuclei and has not been obtained in other nuclear reactions, not all its characteristics can be indicated with sufficient definiteness. This in

first of all, to the mass number of the 2.4-hour isotope. It seems probable to assign to this radioiodine the mass number \(M=132\), but direct proof of this supposition is difficult.

The second characteristic—the half-life—can be determined sufficiently accurately, despite the complex character of the integral decay curve of the radioactive isotopes of iodine separated from the fission products of heavy nuclei. This is possible because the “precursor” of 2.4-hour iodine has a long half-life \(T=77\) hours, whereas all the other radioactive isotopes of iodine are formed from short-lived tellurium isotopes. As a result, the 2.4-hour activity can be isolated in practically pure form during the secondary extraction of iodine from the irradiated salt. By rationally choosing the times of the first and second extraction of iodine, one can obtain a sufficient amount of pure \(J^{132}\) and determine its half-life without resorting to analysis of the integral decay curve, which is associated with possible errors. From a series of determinations \(T=2.3—2.4\) hours.

The yield coefficient of 2.4-hour iodine in the fission of uranium under the action of neutrons is \(3.3—3.6\%\) \(^{41,61}\). The radiation emitted by 2.4-hour iodine has not been sufficiently investigated. The data available in the literature are few and do not agree with one another. According to the absorption measurements of Born and Seelmann-Eggebert \(^{74}\), this iodine isotope has a simple \(\beta\)-spectrum with \(E_m=1.35\) MeV and emits \(\gamma\)-rays with \(E_\gamma=0.85\) MeV. According to data cited in a published report on the plutonium project \(^{41}\), and also obtained by absorption in Al and Pb, the \(\beta\)-spectrum of 2.4-hour \(J^{(132)}\) consists of two groups with \(E'_m=1.0\) MeV (50%) and \(E''_m=2.1\) MeV (50%), and there is emission of two \(\gamma\)-quanta with \(E_\gamma=0.60\) MeV (50%) and \(E_\gamma=1.4\) MeV (50%).

Such contradictory results do not make it possible to construct a decay scheme for this iodine isotope.

\(J^{133}.\ T=22\) hours.

A radioactive isotope of iodine with \(T=22\) hours was first discovered by Abelson \(^{53,54}\) in the products of uranium fission under the action of neutrons. The formation of this isotope upon irradiation of uranium with neutrons was confirmed by Hahn and Strassmann \(^{48,56}\), V. G. Khlopin and co-workers \(^{69}\), Dodson and Fowler \(^{75}\), and others. This radioactive iodine is also obtained in the fission of uranium under the action of \(\alpha\)-particles \(^{57}\) and, apparently, upon irradiation of thorium with fast neutrons \(^{45,47,76,77}\). Investigation of this isotope presents certain difficulties. Since 22-hour iodine has been obtained only in the fission of heavy nuclei, it has always been studied in a mixture with other radioactive iodine isotopes formed in this case, and with the active products of their decay. In this case there is no possibility of isolating the activity of interest in pure form, as is possible in the case of \(J^{132}\),

Under such conditions, the fact of separating a 22-hour activity with inactive iodine is not yet proof that this activity belongs to iodine. It becomes necessary to set up special experiments in order to make sure that the activity of interest does indeed belong to iodine, and not to the decay product of some iodine isotope[^48]. Determination of the half-life is possible only by analyzing a complex decay curve, reflecting both the decay of several iodine isotopes with different yields, half-lives, and β-particle energies, and the complex character of the change in their decay products (Fig. 16). Since the character of the integral decay curve is determined by many factors, including the conditions of irradiation of the target, the nature of the counting device used, etc., the difficulties arising in determining the half-life are obvious. Of the many determinations[^41],[^48],[^53],[^55],[^69],[^77], the most reliable value is \(T = 22\) hours.

Fig. 16. Example of an integral decay curve of radioactive iodine isotopes.
\(a\) — 8 days, \(J^{131}\); \(b\) — 22 hours, \(J^{133}\);
\(c\) — 54 min., \(J^{134}\).

Despite the difficulties indicated, not only was the period of the 22-hour iodine established, but by successive separations it was also shown that this radioiodine is not a primary fragment, but is formed from tellurium with \(T = 60\) minutes, which in turn arises in the β-decay of Sb with \(T < 10\) minutes[^55]. Thus the following chain takes place:

\[ \underset{<10\ \text{min.}}{\mathrm{Sb}_{51}} \longrightarrow \underset{60\ \text{min.}}{\mathrm{Te}_{52}} \longrightarrow \underset{22\ \text{hours}}{\mathrm{J}_{53}} \longrightarrow \mathrm{Xe}_{54}. \]

However, this chain does not end with xenon. Still earlier, a number of authors[^7],[^78],[^79] had noted the fact of the formation of radioactive xenon from iodine separated from the fission products of uranium and thorium. By successive separations of xenon from iodine separated from uranium, Segrè and Wu[^77],[^80] and Dodson and Fowler[^75] showed that in the decay of 22-hour iodine radioactive xenon is formed with a period of about 5 days, which in turn decays with the formation of stable cesium.

This circumstance was used by Wu[^44],[^80] to determine the mass number of 22-hour \(J\) and of the entire chain. Cesium, which has one stable isotope \(Cs^{133}\), was bombarded with neutrons. In this way a radioactive gas was obtained which, by the half-life and by the absorption curve of the radiation it emitted, was identified with the 5-day Xe separated from 22-hour iodine.

Since, evidently, the nuclear reaction \( \mathrm{Cs}^{133}_{55}(n,p)\mathrm{Xe}^{133}_{54}\) takes place, the 5-day xenon, the 22-hour iodine, and all their “ancestors” must be assigned the mass number \(M=133\).

Thus, on the basis of all the above, the following chain exists:

\[ \mathrm{Sb}^{133}_{51} \ \xrightarrow[\,]{<10\ \mathrm{min.}}\ \mathrm{Te}^{133}_{52} \ \xrightarrow[\,]{60\ \mathrm{min.}}\ \mathrm{J}^{133}_{53} \ \xrightarrow[\,]{22\ \mathrm{hr.}}\ \mathrm{Xe}^{133}_{54} \ \xrightarrow[\,]{5.4\ \mathrm{d.}}\ \mathrm{Cs}^{133}_{55}\ (\text{stable}). \]

The yield of this chain in the fission of \( \mathrm{U}^{235}\) is \(4.5\%\) \(^{41}\).

The radiation emitted by \( \mathrm{J}^{133}\) in \(\beta\)-decay was investigated by means of a Wilson chamber in a magnetic field \(^{81}\) and by absorption in Al and Pb \(^{41}\). The values obtained by these methods for the maximum energy of the \(\beta\)-spectrum of \( \mathrm{J}^{133}\) are respectively: \(E_m=1.1\ \mathrm{MeV}\) and \(E_m=1.3\ \mathrm{MeV}\). The energy of the \(\gamma\)-rays, according to absorption measurements, is \(E_\gamma=0.55\ \mathrm{MeV}\). Despite the rather satisfactory agreement between the values of \(E_m\) obtained by two different methods, the reliability of the data on the \(\beta\)-spectrum of \( \mathrm{J}^{133}\), owing to the difficulties of separating it from other radioactive isotopes, should be assessed with some caution. In particular, the method used to obtain the \( \mathrm{J}^{133}\) sample in Perfilov’s experiments \(^{81}\) permits, in principle, an error in the determination of the \(\beta\)-spectrum of \( \mathrm{J}^{133}\). On the basis of data on the yield coefficients of the chains leading to the formation of radioactive iodine isotopes, and on the half-lives of the isotopes entering into these chains \(^{41}\), it is easy to show that, for the duration of irradiation used in the experiment under discussion and the subsequent holding of the iodine separated from uranium before its introduction into the Wilson chamber, considerable amounts of other iodine isotopes (\(\mathrm{J}^{131}\), \(\mathrm{J}^{135}\)) and xenon (\(\mathrm{Xe}^{133}\), \(\mathrm{Xe}^{135}\)) must inevitably be present in the mixture with \( \mathrm{J}^{133}\). Even under the quite arbitrary assumption that all the xenon formed in the AgJ crystals in the interval between the separation of iodine from uranium and the beginning of the measurements completely diffuses into the atmosphere, the sample contains far from pure \( \mathrm{J}^{133}\). Incidentally, the character of the Fermi plot constructed for the obtained \(\beta\)-spectrum \(^{81}\) indicates that more than one group of \(\beta\)-particles is present.

\[ \mathrm{J}^{134}.\quad T=54\ \mathrm{min.} \]

Like the preceding iodine isotope, the 54-minute \( \mathrm{J}^{134}\) has been isolated only from the fission products of heavy nuclei. This isotope was first discovered by Abelson \(^{53,54,55}\) in the analysis of the decay curve of radioactive iodine separated from neutron-irradiated uranium. By this method the 54-minute iodine was subsequently obtained in a number of laboratories \(^{48,56,60}\). This same iodine isotope is also produced in the splitting of uranium under the action of \(\alpha\)-particles \(^{57}\) and, as Dodson and Fowler \(^{45}\) and Polessitsky and Orbeli \(^{46,47}\) showed, can be isolated from thorium irradiated with neutrons. The 54-minute iodine is formed in the fi—

of heavy nuclei not in the form of a primary fragment, but is the product of a series of β-decays. The chain that includes the 54-minute iodine was investigated by a number of authors[^46][^47][^53][^54][^55][^70] and at present can be represented as:

\[ \underset{52}{\mathrm{Sb}^{134}} \ \xrightarrow[\ <10\ \mathrm{min.}\ ]{}\ \underset{52}{\mathrm{Te}^{134}} \ \xrightarrow[\ 43\ \mathrm{min.}\ ]{}\ \underset{53}{\mathrm{J}^{134}} \ \xrightarrow[\ 54\ \mathrm{min.}\ ]{}\ \underset{54}{\mathrm{Xe}^{134}}\ (\mathrm{stable}). \]

The periods of the elements entering the chain and preceding the 54-minute \(J\) are established by studying the change in activity of iodine (respectively tellurium) during successive separations from tellurium isolated from uranium (or from a uranium solution).

In the study of \(J^{134}\) the same difficulties arise as in the case of \(J^{133}\).

The yield coefficient of 54-minute \(J\) in the fission of uranium by slow neutrons is approximately \(5.7\%\)[^41].

The mass number of the 54-minute iodine has been determined only very recently. In Seaborg’s isotope tables[^9] the 54-minute \(J\) is designated as \(J^{131}\), while in the report of the plutonium project[^41] it is provisionally assigned mass 134 \((J^{134})\).

Verification of this assumption by bombarding elements with atomic numbers close to iodine with neutrons, protons, deuterons, and \(\alpha\)-particles is impossible. Sb, Te, and Cs lack suitable stable isotopes, and activation of Xe cannot give an unambiguous answer. However, another way of solving this problem is possible. In 1947, Yaffe and Mackintosh[^61], determining the ratio of the activities of 54-minute \(J\) and \(Ba^{139}\), from the known yield of \(Ba^{139}\) in uranium fission[^62] obtained for 54-minute \(J\) a fission-yield coefficient, for both slow and fast neutrons, equal on average to \(5.75\%\). On the other hand, from the mass-spectrographic measurements of Thode and Graham[^82] it is known that in the fission of uranium four stable xenon isotopes are formed: \(Xe^{131}\), \(Xe^{132}\), \(Xe^{134}\), \(Xe^{136}\), which are the end products of chains with the corresponding mass numbers. The ratios of the yields of xenon isotopes with \(M = 132, 134, 136\) to \(Xe^{131}\), and consequently the ratios of the yield coefficients of the corresponding chains, are equal: \(k_{132}/k_{131}=1.48\text{–}1.50\); \(k_{134}/k_{131}=2.63\text{–}2.67\); \(k_{136}/k_{131}=2.17\text{–}2.30\).

\(k_{131}\) was determined for 8-day \(J^{131}\) from the ratio of the activities of \(J^{131}\) and \(Ba^{140}\) and is equal to \(2.23\text{–}2.27\%\)[^61].

It follows from this that the yield coefficient of the chain with \(M = 134\) is \(5.85\%\), which within the error of the experiment corresponds to the yield of 54-minute iodine. Incidentally, the same method can be applied to determine the mass number of 2.4-hour iodine.

The yields of \(Ba^{139}\) and \(Ba^{140}\), from which the yield coefficients of 54-minute \(J\) and 8-day \(J\) were determined—for fission by slow neutrons—from the ratio of the activities

$U^{239}$ and $Ba^{139}$ (respectively $Ba^{140}$) from the ratio

\[ \frac{\beta\text{-activity } U^{239}}{\beta\text{-activity } Ba^{139}} = \frac{\sigma_c}{\sigma_f \cdot k_{139}}, \]

where $\sigma_c$ is the capture cross section of $U^{238}$, $\sigma_f$ is the fission cross section of $U^{235}$, $k_{139}$ is the yield coefficient of $Ba^{139}$ in fission of $U^{235}$ by slow neutrons, i.e., the ratio of the number of fissions leading to the formation of $Ba^{139}$ to the total number of fissions. The yields of $Ba^{139}$ and $Ba^{140}$ in fission by fast neutrons are determined from the ratio of the corresponding activities in fission of $U^{235}$ and $U^{238}$.

The radiation of $J^{134}$ has practically not been investigated. It is known only that, in the $\beta$-decay of $J^{134}$, $\gamma$ rays with $E_\gamma > 1 \mathrm{Mev}$ are emitted.

\[ J^{135}. \quad T = 6.6 \text{ hours.} \]

Segrè and Wu$^{77,80}$ found in the fission products of heavy nuclei another isotope of iodine with a period $T = 6.6$ hours. This radioactive iodine, whose presence was confirmed by other authors$^{47,60,75}$, is formed in the decay of short-lived tellurium$^{75,77}$, which arises directly in the fission of uranium and thorium.

The xenon formed in the $\beta$-decay of $J^{135}$ is radioactive$^{75,77}$, which made it possible to establish both the presence of 6.6-hour iodine among the fission products of heavy nuclei and its mass number.

The presence of the 6.6-hour period is difficult to determine directly from the integral decay curve of radioactive iodine separated from neutron-irradiated uranium and thorium. The large number of activities forming this curve masks the 6.6-hour period, and its determination in this way succeeds only in rare cases. Usually, in analyzing the integral curve, it is possible to note only the fact of an increase in activity belonging to the radioactive daughter product of 6.6-hour iodine—xenon, with a half-life $T = 9.2—9.4$ hours. In practice, the half-life of $J^{135}$ is established by a series of successive separations of xenon from iodine isolated from neutron-irradiated uranium and thorium.

The separation of xenon from iodine is carried out either by passing gas through a boiling alkaline solution of sodium iodide, or by the method of Langsdorf and Segrè$^{79}$, which consists in preparing an emanating sample from which xenon passes into an evacuated chamber. The emanating sample is prepared as follows. Silica gel is impregnated with silver nitrate by immersion for 15 minutes in a $0.1N$ solution of $AgNO_3$ and then dried for several hours at $T = 70^\circ C$. Several cubic centimeters of silica gel impregnated in this way are shaken with a solution of active iodine in $CCl_4$ until the solution is decolorized. The active xenon formed in the emanating sample passes either into an evacuated chamber surrounding the counter, or directly into the counter.

The xenon separated in one way or another from iodine contains two isotopes: \(Xe^{133}\) with \(T=5.4\) days and \(Xe^{135}\) with \(T=9.4\) hours. However, owing to the large difference in the periods and the insignificant electron energy emitted by \(Xe^{133}\) \((Xe^{133}—E_m \approx 0.3\ \mathrm{MeV};\ Xe^{135}—E_m \approx 0.95\ \mathrm{MeV})\), under certain experimental conditions xenon separated from iodine decays in fact with a single period \(T=9.4\) hours. In Fig. 17, in semilogarithmic coordinates, the decay of xenon is represented by points for four extractions following one another at intervals of 12 hours. In extrapolating the decay curves (straight lines in the coordinates of Fig. 17) to the moment of separation of xenon from iodine according to the initial points, denoted by circles, one obtains the decay curve of the “parent” of the 9.4-hour Xe—radioiodine with \(T=6.6\) hours. Proceeding from the fact that 9.4-hour Xe is formed in the \(\beta\)-decay of 6.6-hour J and that, consequently, both have one and the same mass number, it is possible to prove that the period \(T=6.6\) hours belongs to the iodine isotope with mass number \(M=135\). In the bombardment of Ba by neutrons \(^{44,80}\), two radioactive gases were obtained with periods \(T=5\) days and \(T=9.4\) hours; these, by their periods and absorption curves, were identified with radioactive xenon isotopes formed in the decay of iodine separated from the fission products of uranium and thorium. Although barium has 7 stable isotopes, the obvious reaction

\[ {}_{56}\mathrm{Ba}^{A}(n,\alpha){}_{54}\mathrm{Xe}^{A-3} \]

leads to radioactive xenon only for \(M=130, 136, 138\). Since \(Xe^{133}\) is formed from \(Ba^{136}\), which, as was shown earlier, has \(T \approx 5\) days, and the ratio \(Ba^{130}:Ba^{138}=1/700\), it may be concluded that 9.4-hour Xe is formed by the reaction:

\[ {}_{56}\mathrm{Ba}^{138}(n,\alpha){}_{54}\mathrm{Xe}^{135}. \]

Consequently, the 6.6-hour period belongs to \(J^{135}\).

Fig. 17. Decay curve of \(J^{135}\).

As concerns the decay scheme of \(J^{135}\), it has been established that the \(Xe^{135}\) arising in the \(\beta\)-decay of 6.6-hour iodine is formed in two isomeric states \(^{80,83,84}\): \(Xe^{*135}\) with \(T=10—13\) minutes and \(Xe^{135}\) with \(T=9.2—9.4\) hours.

Tette \(^{83}\), studying the radioactive xenon formed from iodine with the aid of a Seelmann-Eggebert apparatus \(^{85}\), was the first to discover 10-minute Xe and showed that the periods \(T=10\) minutes and \(T=9.4\) hours belong not to separate xenon isotopes formed from

of different iodine isotopes, and both have as their “precursor” 6.6-hour \(J\) and, consequently, belong to two isomers of one isotope \(Xe^{135}\).

Active iodine was isolated from uranium irradiated with neutrons, using the usual carrier technique. Radioxenon was carried away from a heated sodium iodide solution by a stream of hydrogen and was adsorbed on activated charcoal at the temperature of liquid air. Upon subsequent heating of the charcoal the desorbing xenon passed into an evacuated chamber surrounding a Geiger–Müller counter. The hydrogen was blown through at two-hour intervals. The decay curve of the radioactive xenon isotopes could each time be resolved into two straight lines with \(T=10\) minutes and \(T=9.4\) hours. Activity with \(T=5\) days, belonging to \(Xe^{133}\), was not observed, evidently both because of the short accumulation period and because the \(\beta\)-particles of \(Xe^{133}\) were to a considerable extent absorbed in the wall of the counter. Extrapolating both straight lines with \(T=10\) minutes and \(T=9.4\) hours to the initial time, Goette obtained, in both cases, a straight line with one and the same 6.6-hour slope from the initial activities.

Wu and Segré\({}^{80}\), isolating xenon from iodine after a short accumulation interval and analyzing the decay curves obtained from the counting of \(\beta\)-particles and \(\gamma\)-quanta, found that \(T=10\) minutes belongs to an excited isomeric state of \(Xe^{135}\). This isomer is produced in the \(\beta\)-decay of \(J^{135}\) in an amount of \(\sim 10\%\) and, with the emission of \(\gamma\)-quanta (and internal-conversion electrons), passes into the ground state of \(Xe^{135}\), which decays with \(T=9.4\) hours.

According to the latest data\({}^{41}\), the chain of transformations of the primary fragment with \(M=135\), including the formation and decay of \(J^{135}\), may be represented as:

\[ \begin{array}{ccccccccc} & & & \sim 10\% & \nearrow & Xe^{*135}_{54}\ (10\ \text{min.}) & & & \\ Te^{135}_{52} & \xrightarrow{<2\ \text{min.}} & J^{135}_{53}\ (6.7\ \text{hr.}) & \longrightarrow & Xe^{135}_{54} & \xrightarrow{9.4\ \text{hr.}} & Cs^{135}_{55} & \xrightarrow{>2.5\cdot 10^{4}\ \text{yr}} & Ba^{135}_{56}\ (\text{stable}). \\ & & & & & & & & \\ & & & & \downarrow & & & & \end{array} \]

The yield of this chain in the fission of uranium by slow neutrons is \(5.9\%\).

The energy transitions which take place in the \(\beta\)-decay of \(J^{135}\) have not been established precisely. According to the available data, obtained by the absorption method\({}^{41}\), the maximum energy of the \(\beta\)-particles emitted by \(J^{135}\) lies within the limits \(E_m=1.35—1.5\ \text{MeV}\), and the energy of the \(\gamma\)-quanta emitted in the \(\beta\)-decay lies within the limits \(E_\gamma=1.6—1.3\ \text{MeV}\).

The difference between the energy levels of the ground and metastable states of \(Xe^{135}\) is \(\sim 0.55\ \text{MeV}^{41,80,81}\).

Within the limits of accuracy of the available experimental data, the decay scheme of \(J^{135}\), on the basis of general regularities of the “de-excitation” of excited nuclei, may provisionally be represented as follows: in \(\beta\)-decay, \(100\%\) of the \(\beta\)-particles are emis-

…with \(E_{\pi}=1.35\text{--}1.5\ \mathrm{MeV}\), and a transition occurs to an excited level of \(\mathrm{Xe}^{135}\) with excitation energy \(E_{\mathrm{exc.}}=1.6\text{--}1.3\ \mathrm{MeV}\). This excited state, with the emission of \(\gamma\)-quanta, “lights up” to the ground or to the metastable level of \(\mathrm{Xe}^{135}\). Approximately 90% of the \(\gamma\)-quanta following the emission of the \(\beta\)-particle have an energy \(E_{\gamma}=1.6\text{--}1.3\ \mathrm{MeV}\), and thus in 90% of all cases a transition to the ground state takes place. 10% of the \(\gamma\)-quanta have energy \(E_{\gamma}=E_{\mathrm{exc.}}-0.55\simeq 0.9\ \mathrm{MeV}\), and in this case a transition occurs to the metastable state of \(\mathrm{Xe}^{135}\), which, with a period \(T=13\) minutes, discharges by means of converted \(\gamma\)-transitions with \(E\simeq0.55\ \mathrm{MeV}\). In absorption measurements the group of \(\gamma\)-quanta with \(E_{\gamma}\simeq0.9\ \mathrm{MeV}\) could not be detected because of its low intensity (10%).

Short-lived iodine isotopes:

\[ \mathrm{J}^{(136)},\quad T=1.8\ \mathrm{min.};\quad \mathrm{J}^{137},\quad T=30\ \mathrm{sec.};\quad \mathrm{J}^{(137)},\quad T=22\ \mathrm{sec.} \]

Strassmann and Hahn\({}^{86}\) succeeded in developing such a method for separating radioactive iodine from uranium irradiated with neutrons that measurement of the activity \(\mathrm{AgJ}^{*}\) could be begun 3 minutes after the end of irradiation. Thanks to this, they discovered in the uranium-fission products two short-lived iodine isotopes with periods: \(T=1.8\pm0.4\) minutes and \(T=30\pm6\) seconds.

The period \(T=1.8\) minutes is conditionally assigned\({}^{41}\) to \(\mathrm{J}^{136}\), formed as a primary fragment in uranium fission and decaying into stable \(\mathrm{Xe}^{136}\).

If the 1.8-minute period indeed belongs to \(\mathrm{J}^{136}\) and the process takes place:

\[ {}_{53}\mathrm{J}^{136} \ \xrightarrow[\ ]{1.8\ \mathrm{min.}}\ {}_{54}\mathrm{Xe}^{136}\;(\mathrm{stable}), \]

then the yield of 1.8-minute \(\mathrm{J}\) in uranium fission should be equal to\({}^{61}\) 4.85%.

Seelmann-Eggebert and Born\({}^{84,87}\) showed that 30-second iodine decays with the formation of \(\mathrm{Xe}^{137}\) with \(T=3.4\) minutes. According to American data\({}^{41}\), 30-second \(\mathrm{J}^{137}\) is formed directly in fission and decays according to the scheme:

\[ {}_{53}\mathrm{J}^{137} \ \xrightarrow[\ ]{30\ \mathrm{sec.}}\ {}_{54}\mathrm{Xe}^{137} \ \xrightarrow[\ ]{3.4\ \mathrm{min.}}\ {}_{55}\mathrm{Cs}^{137} \ \xrightarrow[\ ]{33\ \mathrm{years}}\ {}_{56}\mathrm{Ba}^{137}\;(\mathrm{stable}). \]

According to the same data\({}^{41,88}\), in uranium fission another short-lived iodine isotope is formed as a primary fragment, with \(T=22\text{--}23\) seconds, decaying with the emission of electrons and delayed neutrons.

Tables I and II give the characteristic constants of radioactive iodine isotopes and the nuclear reactions leading to the formation of these isotopes.

RADIOACTIVE ISOTOPES OF IODINE

Table I

Table II

Radioactive isotopes of iodine formed in the fission of uranium \(^{41}\)

CONCLUSION

In nuclear reactions leading to the formation of radioactive isotopes of iodine, several isotopes are usually formed simultaneously. This applies especially to the case of fission of heavy nuclei. In this process more than half of the iodine isotopes are formed, while the production of many of them by other routes is unknown. The fission reaction of heavy nuclei, as a source of radioiodine, has a number of advantages and disadvantages. On the one hand, it is the most accessible route for obtaining radioactive isotopes of iodine with a wide range of half-lives. This method is available to any laboratory possessing an \((\mathrm{Ra}—\mathrm{Be})\) or \((\mathrm{Rn}—\mathrm{Be})\) neutron source, and does not require complex high-voltage equipment. The iodine obtained by this method consists of a series of isotopes with periods from several seconds to several days, which makes it possible to solve a whole range of problems.

On the other hand, the simultaneous formation of a number of isotopes that are difficult to separate from one another is a certain shortcoming of the method. To some extent this shortcoming can be overcome by varying the irradiation time, aging the irradiated uranium, and aging the separated iodine. On the basis of the data in Table II, the conditions for obtaining radioiodine can be rationally selected in such a way as to yield optimal amounts of the isotope of interest. It is also possible to separate periods by using the different energies of the \(\beta\)-particles and by applying appropriate filters when measuring the activity.

An important circumstance is the fact that radioactive xenon isotopes are formed in the decay of \(J^{133}\) and \(J^{135}\), which causes distortion of the decay curve. When radioiodine containing these isotopes is used as an indicator, it is evidently necessary to take measures to ensure that all samples of radioactive iodine participating in the various processes remain, throughout the experiment, under equal conditions with respect to the possibility of diffusion of radioxenon out of the sample.

CITED LITERATURE

1. A. O. Nier, Phys. Rev. 52, 933 (1937).
2. C. G. Lu, S. Sugden, J. Chem. Soc. 1273 (1939).
3. O. Hahn, F. Strassmann, Naturwiss., 27, 451 (1939).
4. L. Szilard, T. A. Chalmers, Nature 134, 462 (1934).
5. E. Amaldi, O. D’Agostino, E. Fermi, B. Pontecorvo, F. Rasetti, E. Segrè, Proc. Roy. Soc. A 149, 522 (1935).
6. O. Erbacher, K. Philipp, Zeits. phys. Chemie, A 176, 169 (1936).
7. M. I. Korsunskii, N. N. Nikolaevskaya, M. A. Bak, ZhETF 9, 524 (1939).
8. J. J. Livingood, G. T. Seaborg, Phys. Rev. 54, 775 (1938).
9. G. T. Seaborg, Rev. Mod. Phys. 16, 1 (1944).
10. J. Roberts, J. Irvine, Phys. Rev. 59, 936 (1941).
11. A. Roberts, L. Elliott, J. Dawning, W. Peacock, M. Deutsch, Phys. Rev. 64, 268 (1943).
12. A. F. Reid, A. S. Keston, Phys. Rev. 70, 987 (1946).
13. L. E. Glendenin, R. R. Edwards, Phys. Rev. 71, 742 (1947).
14. G. F. Tape, J. M. Cork, Phys. Rev. 53, 676 (1938).
15. J. J. Livingood, G. T. Seaborg, Phys. Rev. 53, 1015 (1938).
16. G. F. Tape, Phys. Rev. 56, 965 (1939).
17. N. Feather, Proc. Cambr. Phil. Soc. 34, 599 (1938).
18. F. Kurie, J. Richardson, H. Paxton, Phys. Rev. 49, 368 (1936).
19. E. Fermi, E. Amaldi, O. D’Agostino, F. Rasetti, E. Segrè, Proc. Roy. Soc. A 146, 483 (1934).
20. A. F. Reid, Phys. Rev. 69, 530 (1946).
21. F. Knauer, Zeits. f. Physik, 120, 103 (1942).
22. C. Lapointe, F. Rasetti, Phys. Rev. 58, 554 (1940).
23. F. Rasetti, Phys. Rev. 58, 869 (1940).
24. K. Sinma, F. Yamasaki, Phys. Rev. 59, 402 (1941).
25. F. Houtermans, Zeits. f. Physik, 118, 424 (1941).
26. C. S. Wu, L. J. Rainwater, W. W. Havens, Phys. Rev. 71, 175 (1947).

1. W. B. Jones, Phys. Rev. 72, 362 (1947).
2. J. H. E. Griffiths, Proc. Roy. Soc. A 170, 513 (1939).
3. V. S. Dementii, D. V. Timoshchuk, DAN, 27, 926 (1940).
4. D. Hull, H. Seelig, Phys. Rev. 60, 553 (1941).
5. A. I. Alikhanov, A. I. Alikhanyan, B. S. Dzhelepov, Nature, 135, 393 (1935).
6. A. I. Alikhanov, A. I. Alikhanyan, B. S. Dzhelepov, Phys. Zeits. Sowjetun. 10, 78 (1936).
7. G. E. Tape, Phys. Rev. 55, 1135 (1939).
8. R. Bacon, E. Grisewood, C. van-der Merwe, Phys. Rev. 54, 315 (1938).
9. R. Bacon, F. Grisewood, C. van-der-Merwe, Phys. Rev. 59, 531 (1941).
10. E. Amaldi, Phys. Zeits. 38, 692 (1937).
11. A. Roberts, J. Irvine, Phys. Rev. 53, 609 (1938).
12. M. A. Bak, N. N. Nikolaevskaya, DAN, 22, 316 (1939).
13. K. Siegbahn, N. Hole, Phys. Rev. 70, 133 (1946).
14. G. T. Seaborg, J. J. Livingood, J. W. Kennedy, Phys. Rev. 57, 363 (1940).
15. “Nuclei formed in Fission,” J. Am. Chem. Soc. 68, 2411, (1946).
16. S. Katcoff, Phys. Rev. 71, 826 (1947).
17. E. T. Clarke, J. W. Irvine, Phys. Rev. 70, 893 (1946).
18. C. S. Wu, Phys. Rev. 58, 926 (1940).
19. R. Dodson, R. Fowler, Phys. Rev. 55, 880 (1939).
20. A. E. Polesitskii, M. L. Orbeli, DAN, 28, 216 (1940).
21. A. E. Polesitskii, N. Nemerovskii, M. L. Orbeli, N. M. Baranchik, Izv. AN SSSR, ser. fiz. 5, 603 (1941).
22. O. Hahn, F. Strassmann, Naturwiss. 27, 529 (1939).
23. M. Deutsch, A. Roberts, Phys. Rev. 60, 362 (1941).
24. J. Downing, M. Deutsch, A. Roberts, Phys. Rev. 61, 389 (1942).
25. G. T. Seaborg, J. W. Kennedy, Phys. Rev. 55, 410 (1939).
26. E. Segrè, R. S. Halford, G. T. Seaborg, Phys. Rev. 55, 321 (1939).
27. P. Abelson, Phys. Rev. 55, 670 (1939).
28. P. Abelson, Phys. Rev. 55, 876 (1939).
29. P. Abelson, Phys. Rev. 56, 1 (1939).
30. O. Hahn, F. Strassmann, Phys. Zeits. 40, 673 (1939).
31. E. Fermi, E. Segrè, Phys. Rev. 59, 680 (1941).
32. W. Jentschke, F. Prankl, Zeits. f. Physik. 119, 696 (1942).
33. W. Jentschke, Zeits. f. Physik, 120, 165 (1943).
34. H. Andersen, E. Fermi, A. Grosse, Phys. Rev. 59, 52 (1941).
35. L. Yaffe, C. Mackintosh, Canad. Journ. Res. B 25, 371 (1947).
36. W. Grummit, L. Guéron, G. Wilkinson, L. Yaffe, Canad. J. Res. B 25, 364 (1947).
37. M. Deutsch, Phys. Rev. 59, 940 (1941).
38. J. Downing, M. Deutsch, A. Roberts, Phys. Rev. 61, 686 (1942).
39. L. Meitner, O. Hahn, F. Strassmann, Zeits. f. Physik 106, 249 (1937).
40. P. Abelson, Phys. Rev. 55, 418 (1939).
41. N. Feather, E. Bretscher, Nature, 143, 516 (1939).
42. V. G. Khlopin, M. A. Pasvik-Khlopina, N. F. Volkov, DAN, 24, 117 (1939).
43. V. G. Khlopin, M. A. Pasvik-Khlopina, N. F. Volkov, DAN, 24, 847 (1939).
44. V. G. Khlopin, M. A. Pasvik-Khlopina, N. F. Volkov, DAN, 24, 851 (1939).
45. V. G. Khlopin, Izv. AN, ser. fiz. 4, 305 (1940).
46. O. Hahn, F. Strassmann, S. Flügge, Naturwiss. 27, 544 (1939).
47. A. E. Polesitskii, N. N. Nemerovskii, DAN, 28, 218 (1940).

1. H. J. Born, W. Seelmann-Eggebert, Naturwiss. 31, 201 (1943).
2. R. Dodson, R. Fowler, Phys. Rev. 57, 966 (1940).
3. A. Langsdorf, Phys. Rev. 56, 205 (1939).
4. E. Segrè, C. Wu, Phys. Rev. 57, 552 (1940).
5. V. G. Khlopin, M. A. Pasvik-Khlopina, N. F. Volkov, DAN, 24, 665 (1939).
6. A. Langsdorf, E. Segrè, Phys. Rev. 57, 105 (1940).
7. C. S. Wu, E. Segrè, Phys. Rev. 67, 142 (1945).
8. N. A. Perfilov, DAN, 33, 491 (1941).
9. H. G. Thode, R. L. Graham, Canad. J. Res. A 25, 1 (1947).
10. H. Götte, Naturwiss. 28, 449 (1940).
11. W. Seelmann-Eggebert, Naturwiss. 31, 491 (1943).
12. W. Seelmann-Eggebert, Naturwiss. 28, 451 (1940).
13. F. Strassmann, O. Hahn, Naturwiss. 28, 817 (1940).
14. W. Seelmann-Eggebert, H. J. Born, Naturwiss. 31, 59 (1943).
15. A. H. Snell, J. Levinger, Wilkinson, Meiners, Sampson, Phys. Rev. 70, 111 (1946).