RADIOACTIVE ISOTOPE OF HYDROGEN—TRITIUM

B. V. Aivazov and M. B. Neiman

THE DISCOVERY OF TRITIUM AND METHODS FOR ITS PRODUCTION

The first suggestion of the possible existence of an isotope of hydrogen with mass 3 was made in 1933 by Gilbert, Lewis, and Frank^53, but it was not confirmed experimentally.

Latimer and Young^76, investigating solutions of HCl and HBr in heavy water by a magneto-optical method, found, in addition to two minima corresponding to H^1 and H^2, one more minimum, whose position gave the authors reason to ascribe it to a hydrogen isotope of mass 3.

However, this method proved to be erroneous, and the results of these researchers’ measurements cannot be regarded as reliable.

The discovery of the hydrogen isotope of mass 3 belongs to Oliphant, Harteck, and Rutherford^95,96, who in 1934 bombarded targets of ND₄Cl, (ND₄)₂SO₄, and D₃PO₄ with deuterons of energy from 20,000 to 100,000 volts. During the bombardment, a group of protons with a range of 14.3 cm and a group of other charged particles with a range of 1.6 cm were observed, in equal quantity with the first. The authors proposed the existence of a reaction as a result of which a proton and a hitherto unknown particle H^3 are formed:

\[ \mathrm{D}^{2}_{1}+\mathrm{D}^{2}_{1}=\mathrm{H}^{1}_{1}+\mathrm{H}^{3}_{1}. \tag{1} \]

Along with the charged particles, neutrons were also observed, which led the authors to propose a second, parallel reaction with the formation of a new helium isotope of mass 3:

\[ \mathrm{D}^{2}_{1}+\mathrm{D}^{2}_{1}=\mathrm{He}^{3}_{2}+\mathrm{n}^{1}_{0}. \tag{2} \]

From the range of 1.6 cm, the energy and mass of this particle were calculated. The energy proved to be equal to 1 MeV, and the mass of H^3—3.0151.

An article by Dee^41 is devoted to the study of this same reaction; on bombarding a target of (ND₄)₂SO₄ with deuterons, he obtained two groups of charged particles with ranges of 14.3 and 1.6 cm and neutrons with

with a maximum energy of about 3 MeV. It was also shown that the resulting proton and the particle \(H^3\) fly apart in opposite directions.

Subsequent work confirmed this discovery, and at present the isotope of hydrogen with mass 3 has come to be used as a tracer in the study of various processes in chemistry and biology \(^{69,137}\), along with other radioactive isotopes \(^{1,3,6,12}\), and also deuterium \(^{4,10}\). The literature contains a considerable number of works devoted to the study of the properties of this new isotope of hydrogen, as well as several reviews \(^{5,13}\).

In 1935 a nomenclature for the isotopes of hydrogen was proposed \(^{40}\), according to which hydrogen with mass 3 is called tritium and is denoted by the symbol T.

After the first studies on the production of tritium by reaction (1), a number of papers appeared on its production from other elements. F. Curie \(^{75}\), bombarding beryllium with neutrons, obtained in a Wilson chamber, along with the tracks of \(\alpha\)-particles, thinner and longer tracks, which he attributed to tritium formed according to the reaction:

\[ \mathrm{Be}^{9}_{4}+n^{1}_{0}=\mathrm{He}^{4}_{2}+\mathrm{H}^{3}_{1}+\mathrm{H}^{3}_{1}. \tag{3} \]

Chadwick and Goldhaber \(^{35,36}\) and Fermi \(^{11}\), with collaborators, obtained tritium by bombarding lithium according to the reaction:

\[ \mathrm{Li}^{6}_{3}+n^{1}_{0}=\mathrm{He}^{4}_{2}+\mathrm{H}^{3}_{1}, \tag{4} \]

and boron according to the reaction:

\[ \mathrm{B}^{10}_{5}+n^{1}_{0}=2\mathrm{He}^{4}_{2}+\mathrm{H}^{3}_{1}. \tag{5} \]

Both these reactions proceed with slow neutrons.

In contrast to Chadwick, who worked with an ionization chamber, Taylor and Goldhaber \(^{126}\) used photographic plates for the investigation; these were first impregnated with a lithium salt and then irradiated with slow neutrons. The observed tracks corresponded to particles produced in reaction (4).

The length of the tracks corresponded to the sum of the track lengths of both particles and was, in air, \(6.9 \pm 0.2\) cm.

For reaction (5), Taylor \(^{126}\) determined an energy value equal to 3.3 MeV.

The results obtained by Chadwick and Goldhaber were soon checked by Budnitskii, Kurchatov, and Latyshev \(^{33}\), who obtained particle ranges for reaction (4) equal, respectively, to 2.0 and 6.5 cm.

Williams with collaborators \(^{135}\) studied the reaction for producing tritium by bombarding a beryllium target with deuterons of energy 212 KeV:

\[ \mathrm{Be}^{9}_{4}+\mathrm{D}^{2}_{1}=\mathrm{Be}^{8}_{4}+\mathrm{H}^{3}_{1}. \tag{6} \]

Along with the reaction for obtaining tritium by bombarding boron with slow neutrons, Cornog and Libby \({}^{39}\) studied the reaction for obtaining tritium by bombarding nitrogen with neutrons. The authors give two possible reaction schemes:

\[ {}^{14}_{7}\mathrm{N}+{}^{1}_{0}n={}^{12}_{6}\mathrm{C}+{}^{3}_{1}\mathrm{H}-4.3\ \mathrm{MeV}, \tag{7} \]

\[ {}^{14}_{7}\mathrm{N}+{}^{1}_{0}n=3{}^{4}_{2}\mathrm{He}+{}^{3}_{1}\mathrm{H}-11.5\ \mathrm{MeV}. \tag{8} \]

The calculated cross section for the capture of slow neutrons by nitrogen is \(10^{-26}\ \mathrm{cm}^{2}\).

Borst \({}^{27}\) studied the reaction for obtaining tritium from nitrogen by bombarding it with deuterons

\[ {}^{14}_{7}\mathrm{N}+{}^{2}_{1}\mathrm{D}={}^{13}_{7}\mathrm{N}+{}^{3}_{1}\mathrm{H}. \tag{9} \]

The threshold of this reaction is \((6.8\pm0.1)\) MeV, and the energy is \(4.54\) MeV.

Krishnan \({}^{72}\) and Borst \({}^{28}\) studied the reaction for obtaining tritium from fluorine salts (\(\mathrm{NaF}\) was bombarded with deuterons of energy 9 MeV):

\[ {}^{19}_{9}\mathrm{F}+{}^{2}_{1}\mathrm{D}={}^{18}_{9}\mathrm{F}+{}^{3}_{1}\mathrm{H}. \tag{10} \]

The threshold of this reaction is \((6.6\pm0.2)\) MeV. The resulting \({}^{18}_{9}\mathrm{F}\) is radioactive, with positron emission and a half-life of \((112\pm2)\) min. The neutron-capture cross section with formation of \({}^{3}_{1}\mathrm{H}\) is \((3.9\pm0.4)\cdot10^{-27}\ \mathrm{cm}^{2}\).

Of the reactions for obtaining tritium by bombarding heavy elements, the following have been studied:

\[ {}^{63}_{29}\mathrm{Cu}+{}^{2}_{1}\mathrm{D}={}^{62}_{29}\mathrm{Cu}+{}^{3}_{1}\mathrm{H}\quad(\text{Meitner }{}^{87}) \tag{11} \]

and

\[ {}^{93}_{41}\mathrm{Nb}+{}^{2}_{1}\mathrm{D}={}^{92}_{41}\mathrm{Nb}+{}^{3}_{1}\mathrm{H}\quad(\text{Wiedenbeck }{}^{134}). \tag{12} \]

For the first reaction the threshold is below 6 MeV. The copper formed is radioactive, with a half-life of 10.5 min.

For the second reaction the threshold is 5–6 MeV. The niobium formed has two radioactive isomers with half-lives of 21.6 hours and 11 days.

Of the reactions given for obtaining tritium, reactions (1), (4), (5), and (6) are used in practice. These reactions are the simplest to carry out and have a larger neutron-capture cross section.

Table I gives a summary of the currently known reactions for obtaining tritium, with indication of the authors who investigated these reactions.

In practice, only some of the reactions listed in Table I are used. With the powerful sources of neutrons and deuterons now available to the experimenter, the process of obtaining tritium in quantities sufficient for experiment presents no difficulty. Usually metallic lithium, beryllium and

Table 1

Reactions for obtaining tritium

No.	Reactions	Target	Range of \(H^3\), in cm	Yield of \(H^3\)	Reaction energy, MeV	Reaction threshold, eV	Capture cross section, cm\(^2\)	Authors
1	\(D,(D,H^3)H\)	\(ND_4Cl, D_3PO_4\)	1.6	\(1:10^6\)	—	\(<2\cdot10^4\)	—	Rutherford and collaborators95,96; Roberts107
1	Same	\((ND_4)_2SO_4\)	1.6	—	—	—	—	Dee41
1	Same	\(D_2O\)	—	—	4.0	—	—	Bretscher and collaborators30
1	Same	\(D_2O\)	—	—	—	—	\(2\cdot10^{-28}\)	Borst25
1	Same	\(D_2\)	—	\(1:5000\,D\)	—	—	—	Harnwell and collaborators60
1	Same	\(D_2\)	—	—	—	—	—	Alvarez and collaborators15
1	Same	\(D_2\)	—	\(1:10^2\)	—	\(<10^4\)	—	Burhop34
1	Same	\(NaOD\)	—	\(1:10^9\)	—	—	—	Dopel43,44
1	Same	—	—	—	—	—	\(1.4—4.9\times10^{-26}\)	Van Allen and collaborators131
1	Same	—	2.39	—	—	—	—	Bauer29
1	Same	—	\(1.31\pm0.10\)	—	—	—	—	Bonner and collaborators65
1	Same	—	—	—	—	\(3\cdot10^4\)	\(10^{-26}\)	Flügge49
2	\(Li^6(n,H^3)\alpha\)	\(Li\)	5.5	—	\(\sim5\)	—	\(10^{-21}\)	Chadwick and Goldhaber35,36
2	Same	\(LiOH\)	—	—	—	—	—	Fermi and collaborators11
2	Same	\(LiCl\)	—	—	—	—	—	Taylor and Goldhaber126

Continuation of Table 1

No.	Reactions	Target	Range of H³ in cm	Yield of H³	Reaction energy in MeV	Reaction threshold in eV	Capture cross section in cm²	Authors
	Li⁶ (n, H³) α	—	6.5	—	—	—	2·10⁻²²	Kurchatov et al. ³³
	Same	—	5.9±0.06	—	4.86	—	—	Livingston and Hoffman ⁸³
	” ”	Li	6.1	—	4.97	—	—	Rumbo et al. ¹¹⁰
	” ”	—	5.36	—	—	—	—	Rotblat ¹⁰⁹
3	Be⁹ (n, 2H³) α	Be	—	—	—	—	—	F. Curie ⁷⁵
4	Be⁹ (D, H³) Be⁸	Be	8.0	—	—	—	—	O’Neal and Goldhaber ⁹⁸,⁹⁹
	Same	—	—	1:2·10¹⁰	4.32	<2·10⁵	—	Williams et al. ¹³⁵
5	B¹⁰ (n, H³) 2α	B	—	—	—	—	10⁻²¹	Chadwick ³⁶
	Same	boron	1.1±0.1	—	3.3	3.6·10⁶ ?	—	Taylor ¹²⁵
	” ”	H₃BO₃	—	—	—	—	10⁻²⁶	Cornog and Libby ³⁹
	” ”	—	1.08	—	—	—	—	Rotblat ¹⁰⁹
6	N¹⁴ (n, H³) C¹²	NaNO₃	—	—	—	—	10⁻²⁶	Cornog and Libby ³⁹
7	N¹⁴ (D, H³) N¹³	—	—	—	−4.54	(6.8±0.1)·10⁶	—	Borst ²⁷
8	F¹⁹ (D, H³) F¹⁸	—	—	—	4.1±1.1	(6.6±0.2)·10⁶	—	Borst ²⁸
	Same	NaF	—	—	—	~6·10⁶	(3.9±0.4)·10⁻²⁷	Krishnan ⁷²
9	Cu⁶³ (D, H³) Cu⁶²	—	—	—	—	<6·10⁶	—	Meitner ⁸⁷
10	Nb⁹³ (D, H³) Nb⁹²	—	—	—	—	(5,6)·10⁶	—	Wiedenbeck ¹³⁴
11	He³ (n, H³) H	—	—	—	—	—	—	Hughes and Eggler ⁶⁶

or, more rarely, boron is irradiated with slow neutrons in a cyclotron or in a pile for the production of plutonium from uranium.⁸ The irradiated lithium is either dissolved in water, thereby obtaining hydrogen containing tritium, or the tritium obtained is blown out of the molten lithium with hydrogen. In the first case, half of the tritium goes into the formation of LiOT and is not used as an active indicator. In the second case the losses of active substance should be smaller, although they are also considerable because of the formation of LiT.

After irradiation, beryllium and boron are most often treated with sulfuric acid, and hydrogen containing tritium is obtained.

When working with deuterons of high energy, heavy water is usually irradiated; after irradiation it is subjected to electrolysis or to the action of magnesium amalgam. The deuterium obtained contains tritium.

In all experiments an attempt is made to obtain tritium in the form of a gaseous mixture with hydrogen or deuterium, since the energy of tritium radiation is too small for particles to penetrate through the wall of the counter, and it has to be introduced into the counter as an impurity in the counting gas. In such cases, water vapor interferes with the normal operation, and therefore its introduction into the counter is avoided.

In some cases tritium is extracted from water T₂O or HTO (in which form it may be obtained in the course of its preparation) by isotopic exchange with vapors of ethyl alcohol, which is a necessary admixture to the counting gas. This method eliminates the need to introduce hydrogen or water vapor into the counter.

HALF-LIFE PERIOD AND ENERGY OF THE β-RADIATION OF TRITIUM

For some time after its discovery, tritium was considered a stable isotope, although its radioactivity had been theoretically predicted by Bonner²⁵ in 1938, before the experimental confirmation of its instability. This supposition was based on the fact that the precisely determined mass of tritium proved to be greater than the mass of the isotope He³ by more than the mass of one electron. Bleakney¹¹⁶ and co-workers somewhat later confirmed Bonner’s supposition.

Proceeding from the assumption of the stability of tritium, a number of investigators determined its content in ordinary hydrogen, deuterium, and heavy water. Tove¹³⁰ and co-workers determined the content of tritium in deuterium as 1:10⁶. Bleakney²³˒⁸⁴ and co-workers considered the ratio of tritium to hydrogen in natural hydrogen to be 1:10⁹ or less, and for heavy hydrogen—1:10⁵. Goldsmith⁵⁶, having found the presence of tritium in stars, also considered that tritium was stable. All these assumptions and measurements proved to be erroneous.

In 1940 Alvarez and Cornog¹⁶ were the first to measure the activity of the gas obtained by them in bombarding heavy water with deuterons. The yield of active atoms, determined with a Geiger–Müller counter—

... was approximately equal to the number of neutrons obtained in the D + D reaction.

Somewhat later, Alvarez85 and collaborators measured the half-life of tritium. According to their data it should have been more than 10 years.

At the same time as Alvarez, O’Neal and Goldhaber100 determined the half-life and the decay constant of tritium obtained by reaction (4). According to these authors,

\[ T_{50\%}=31\pm 8\ \text{years} \quad \text{and} \quad \lambda = 7\cdot 10^{-10}\ \text{s}^{-1}\pm 25\%. \]

The energy of the β-radiation proved to be very small; its maximum was equal to \(15\pm 3\ \mathrm{KeV}\). In this case the maximum energy of the β-radiation was measured from its absorption in aluminum.

Brown32 used, for determining the energy of the β-radiation of tritium, a specially constructed Geiger–Müller counter with a grid cathode. The scheme of such a counter is shown in Fig. 1. The source was a plate with phosphorus pentoxide deposited on it, which had previously absorbed water vapor containing tritium. The plate was placed inside the counter and could be moved, approaching or receding from the electrodes of the counter. The counter was filled with helium at atmospheric pressure.

Fig. 1. Scheme of a counter for detecting soft radiation:
\(1\) — movable source of β-particles, \(2\) — grid cathode.

Two series of measurements were made of the activity emitted by tritium: with aluminum foil \(0.5\ \mathrm{mm}\) thick placed around the cathode, and with the same foil lying directly on the activity source. The number of pulses per minute was measured as a function of the distance of the source from the counter grid. The results of these experiments are given in Fig. 2.

In this way the maximum range of the β-particles of tritium in helium was determined; the range proved to be \(13\pm 1\ \mathrm{mm}\). The counter had first been calibrated with respect to the α-radiation of polonium. The maximum energy of the β-radiation of tritium calculated from these experiments was equal to \(9.5\pm 2.0\ \mathrm{KeV}\).

These measurement results were criticized in the work of O’Neal \(^{101}\), who considered Goldhaber’s measurements more reliable.

Nilson \(^{91}\) determined the maximum energy of the \(\beta\)-radiation of tritium from measurements of photographs of tracks obtained in a Wilson chamber. From measurements of 108 photographs, the maximum energy was calculated to be \(14.5 \pm 1\) KeV.

An interesting method for measuring the energy of soft radiation was proposed by Watts and Williams \(^{138}\). They made a Geiger–Müller counter

Fig. 2. Dependence of counting intensity on the distance from the source to the cathode (absorption of \(\beta\)-particles by helium).

with a very thin \((0.07\ \text{mg}/\text{cm}^2)\) window, 12 mm in diameter, made of collodion film. In the experiment, the minimum voltage was measured at which electrons emitted by a tungsten filament and accelerated by a potential difference passed through the window and were collected on an anode placed behind the window. It turned out that the electrons penetrated at a potential difference of 12 KV. Next, an activity source containing tritium was prepared. A thin layer of aluminum oxide covering an aluminum disk was hydrolyzed with water containing HTO. The disk was placed in vacuum at a distance of 12 mm from a brass plate opposite the window, and an accelerating or retarding potential was applied to it. The potential was determined at which the penetration through the window of the electrons emitted by tritium ceased. From the obtained value of the potential necessary to stop the electrons, and from the thickness of the window, it was possible to obtain the value of the maximum energy, which proved to be \(11 \pm 2\) KeV. The curve of the dependence of the number of electrons penetrating through the window on the magnitude of the potential, obtained by Williams, is shown in Fig. 3.

At the end of 1947, two papers appeared in which, it seems to us, the most reliable values for the half-life of tritium are given. Novick\(^94\) gives the value \(12.1 \pm 0.5\) years. This value was determined from the accumulation of \(\mathrm{He}^3\) in a mixture of hydrogen with tritium.

It is known that tritium is an electron emitter and that its decay proceeds according to the reaction

\[ \mathrm{H}^{3}_{1} \to \mathrm{He}^{3}_{2} + \beta^{-}. \tag{13} \]

Incidentally, some authors (see, for example, Libby\(^81\)) attribute the existence of \(\mathrm{He}^3\) in the atmosphere to this reaction. It is assumed that

Fig. 3. Curve of the dependence of the number of counts on the potential.

neutrons of cosmic rays cause the formation of \(\mathrm{C}^{14}\) and \(\mathrm{H}^3\) by the reactions:

\[ \mathrm{N}^{14}_{7} + n^{1}_{0} = \mathrm{C}^{14}_{6} + \mathrm{H}^{1}_{1} \tag{14} \]

and

\[ \mathrm{N}^{14}_{7} + n^{1}_{0} = \mathrm{C}^{12}_{6} + \mathrm{H}^{3}_{1}. \tag{15} \]

Tritium, on decaying, gives helium of mass 3.

The half-life of tritium in the work of Goldblatt, Robinson, and Spence\(^54\) was determined in an ionization chamber having two concentric gold electrodes. The outer electrode was connected to the positive pole, and the inner one to the negative pole of a voltage source. Hydrogen containing tritium was introduced into the chamber, and the ionization current was determined. Measurements were made every 18 days. From the slope of the straight line \(\lg IR\)—time, where \(I\) is the measured current and \(R\) is the resistance of the instrument, obtained from the experimental data, the half-life of tritium was determined. Taking into account possible errors of 20%, the authors give the value of the half-life as \(10.7 \pm 2.0\) years.

For clarity, in Table II we give a summary of the values of the half-life and the maximum energy of the \(\beta\)-radiation of tritium.

Table II

No.	Half-life period	\(E_{\max}\) in KeV	Authors
1	—	\(13 \pm 5\)	O’Neal and Goldhaber \(^{99}\)
2	\(>10\) years	—	Lewis, Alvarez, and Cornog \(^{85}\)
3	—	\(10 \pm 1\)	Perlov \(^{103}\)
4	\(31 \pm 8\) years	\(15 \pm ?\)	O’Neal and Goldhaber \(^{100}\)
5	—	\(9.5 \pm 2\)	Brown \(^{32}\)
6	—	\(14.5 \pm 1\)	Nielsen \(^{92}\)
7	—	\(11 \pm 2\)	Yates and Williams \(^{133}\)
8	31 years	9.5	Libby \(^{82}\)
9	\(12.1 \pm 0.5\) years	—	Novik \(^{94}\)
10	\(10.7 \pm 2\) years	—	Goldblatt et al. \(^{54}\)

PROPERTIES OF TRITIUM

1. Elasticity of tritium vapor and equilibrium of the water—tritium system

The vapor pressure of liquid HT and DT was determined by Libby and Barter \(^{79}\) by distilling a very small volume of liquid hydrogen or deuterium containing tritium. After distillation, gas samples were taken and their activity was measured in a Geiger–Müller counter. Hydrogen or deuterium in a mixture with ethyl-alcohol vapors served as the counting gas. The hydrogen pressure was 15 mm, and that of the alcohol 2–3 mm.

Tritium was obtained from lithium in the Berkeley cyclotron, with subsequent treatment with \(\mathrm{H_2O}\) or \(\mathrm{D_2O}\), whereby HT or DT was evolved. The tritium concentration was approximately \(10^{-9}\) mole per mole of hydrogen.

In determining the specific activity of tritium during distillation, the authors calculated the molar fraction of tritium in hydrogen, and hence the vapor pressure of HT and DT.

According to the authors’ data, tritium is readily separated by distillation of liquid hydrogen isotopes, provided that the mixture of isotopes is passed through a catalyst in order to establish equilibrium according to the reactions:

\[ 2\mathrm{HT} \rightleftarrows \mathrm{T_2} + \mathrm{H_2}, \tag{16} \]

\[ 2\mathrm{DT} \rightleftarrows \mathrm{T_2} + \mathrm{D_2}. \tag{17} \]

The authors do not indicate what catalyst was used by them.

The results of measurements of the vapor pressures of HT and DT are given in Table III, where, in addition to these measurements, for comparison are given ...

...the data on measuring the vapor pressures of \(\mathrm{H_2}\), HD, and \(\mathrm{D_2}\), obtained by other investigators\(^{118,119}\). The vapor pressure for \(\mathrm{T_2}\) was obtained by extrapolating the data of Libby and Barter.

Table III

Dependence of certain constants of hydrogen molecules on their mass

Molecule: symbol	Molecule: mass	Vapor pressure at \(20.4^\circ\mathrm{K}\), mm Hg	Boiling temperature, degrees K	Latent heat, cal/mole: vaporization	Latent heat, cal/mole: melting
\(\mathrm{H_2}\)	2	700	20,4	215,9	—
HD	3	438	22,13	261	37 (at \(16.6^\circ\mathrm{K}\))
\(\mathrm{D_2}\)	4	256	23,5	355,4	52,3 (at \(18.6^\circ\mathrm{K}\))
HT	4	\(254 \pm 16\)	—	—	—
DT	5	\(123 \pm 6\)	—	—	—
\(\mathrm{T_2}\)	6	\(45 \pm 10\)	—	—	—

Theoretically, the equilibrium constants of the water—tritium system were calculated by Libby\(^{80}\), and the vapor pressure of HTO by Libby and Cornog\(^{78}\). The equilibrium constants are given in Table IV, where the equilibrium constants of the water—deuterium system are also given.

Table IV

Equilibrium constants

Equilibria	\(20^\circ\mathrm{C}\)	\(100^\circ\mathrm{C}\)	\(200^\circ\mathrm{C}\)	\(300^\circ\mathrm{C}\)	\(400^\circ\mathrm{C}\)	\(500^\circ\mathrm{C}\)
\(\dfrac{(\mathrm{HDO})(\mathrm{H_2})}{(\mathrm{H_2O})(\mathrm{HD})}\)	3,78	2,65	2,09	1,75	1,54	1,37
\(\dfrac{(\mathrm{HTO})(\mathrm{H_2})}{(\mathrm{H_2O})(\mathrm{HT})}\)	6,24	3,83	2,66	2,09	1,77	1,52
\(\dfrac{(\mathrm{DTO})(\mathrm{D_2})}{(\mathrm{D_2O})(\mathrm{DT})}\)	1,58	1,37	1,17	1,15	1,12	1,08
\(\dfrac{(\mathrm{D_2O})(\mathrm{HD})}{(\mathrm{HDO})(\mathrm{D_2})}\)	3,03	2,27	1,78	1,55	1,40	1,29
\(\dfrac{(\mathrm{HDO})^2}{(\mathrm{H_2O})(\mathrm{D_2O})}\)	4,10	4,13	4,16	4,17	4,17	4,18
\(\dfrac{(\mathrm{T_2O})(\mathrm{DT})}{(\mathrm{DTO})(\mathrm{T_2})}\)	1,77	1,45	1,36	1,24	1,19	1,18
\(\dfrac{(\mathrm{T_2O})(\mathrm{HT})}{(\mathrm{HTO})(\mathrm{T_2})}\)	5,00	3,10	2,27	1,84	1,62	1,52
\(\dfrac{(\mathrm{HTO})^2}{(\mathrm{H_2O})(\mathrm{T_2O})}\)	3,18	3,57	3,71	3,77	3,82	3,83
\(\dfrac{(\mathrm{DTO})^2}{(\mathrm{D_2O})(\mathrm{T_2O})}\)	3,32	3,66	3,74	3,78	3,78	3,79

The dependence of the equilibrium constant on temperature for the most practically interesting systems

\[ \frac{(\mathrm{HDO})(\mathrm{H}_2)}{(\mathrm{H}_2\mathrm{O})(\mathrm{HD})} \quad \text{and} \quad \frac{(\mathrm{HTO})(\mathrm{H}_2)}{(\mathrm{H}_2\mathrm{O})(\mathrm{HT})} \]

is given in Figs. 4 and 5.

An experimental investigation of the equilibrium of the water–tritium system was carried out by Black and Taylor\({}^{22}\). The equilibrium constant \(K\) was determined for the system

\[ \mathrm{HT} + \mathrm{H}_2\mathrm{O} \rightleftarrows \mathrm{HTO} + \mathrm{H}_2 . \]

The partial pressures of HTO and HT are proportional to the number of decays of the active substance; therefore the partial pressures were determined

Fig. 4. Dependence of the equilibrium constant \(K\) of the system
\[ \mathrm{H}_2\mathrm{O} + \mathrm{HD} \rightleftarrows \mathrm{HDO} + \mathrm{H}_2 \]
on temperature.

from the activity measured on a 1-liter Geiger–Müller counter. The counting gas consisted of water vapor (pressure 2 mm Hg), alcohol vapor (pressure 25 mm), hydrogen (pressure from 8 to 20 mm), and argon (pressure 20 mm). Such a mixture ensured normal operation of the counter.

The experiments were carried out in the system of apparatus shown schematically in Fig. 6. System 1 contained a vacuum apparatus, a counter tube, measuring instruments, and reserve reservoirs.

In system 2, which was fused to system 1, the equilibrium at low temperature was determined.

The catalyst in the study of the equilibrium was platinum on charcoal; therefore system 2 contained a tube filled with this catalyst. The tube had a water jacket for maintaining a constant temperature. Radioactive water was introduced into system 2 through stopcock 4. Through stopcock 6 there entered hydrogen that had been previously purified from кис-

of hydrogen chloride. Circulation was carried out by pump \(E\) and the system of mercury vessels \(F\).

To investigate equilibrium at high temperatures, system 3 was used, as was system 2, soldered to system 1. Hydrogen entered through filter \(A\); radioactive water was introduced through ground joint \(F\) and entered trap \(E\). The catalyst was located in tube \(G\), which was heated by a water or oil bath. Circulation was carried out by a pump not shown in the diagram.

After equilibrium had been established, the mixture was introduced into the counter together with the counting gas. The partial pressure of the components was determined from the activity. Since the true concentration of tritium in hydrogen or in water was of the order of \(10^{-12}\), the pressure of tritium could be taken as equal to zero, and the pressure of radioactive water or hydrogen equal to the pressure of the inactive component.

Fig. 5. Dependence of the equilibrium constant \(K\) of the system \(H_2O + HT \rightleftarrows HTO + H_2\) on temperature.

For calculating the equilibrium constant, hydrogen (both active and inactive) was separated from water (also active and inactive) and introduced into the counter, in which the pressure, volume, and number of pulses per minute were measured. The same was done for water vapor. The data obtained were sufficient for determining the equilibrium constant, since the ratio of the number of pulses per minute to the pressure of water vapor in mm Hg gives the ratio \([HTO]:[H_2O]\), while the ratio of the number of pulses per minute to the pressure of hydrogen in mm Hg gives the ratio \([HT]:[H_2]\).

The results of measurements at temperatures from 16 to 303° C are given in Table V, where the measured constants are compared with the values calculated theoretically by Libby^80. As is evident from the table, the agreement of the experimental data obtained by Black and Taylor with Libby’s theoretical values is satisfactory.

System 2

System 3

System 1

Fig. 6. Diagram of the apparatus for determining the equilibrium of the water–tritium system: System 1. A—Geiger–Müller counter; B—bath with a standard uranium salt solution; C—manometer; D—trap; E, F, G, H, I—vessels for storing gases, water, and alcohol; J—McLeod manometer; K—mercury pump; 1–15—stopcocks. System 2. A—tube with catalyst; B—device for admitting water into tube A; C—circulation pump; E—pump, 1–8—stopcocks. System 3. A–B—traps; E—trap with water containing tritium; G—tube with catalyst, 1–6—stopcocks.

Table V

Value of the equilibrium constants of the system \( \mathrm{HT}+\mathrm{H_2O}\rightleftarrows \mathrm{HTO}+\mathrm{H_2} \)

\(T\ ^\circ\mathrm{C}\)	\(K\), experimental	\(K\), theoretical
16.0	\(6.75\pm0.04\)	6.47
20.2	\(6.47\pm0.12\)	6.24
25.0	\(6.25\pm0.05\)	6.01
56.2	\(5.05\pm0.05\)	4.84
79.6	\(4.37\pm0.05\)	4.23
111.2	\(3.76\pm0.04\)	3.64
158.4	\(3.10\pm0.06\)	3.03
217.1	\(2.64\pm0.04\)	2.54
302.9	\(2.17\pm0.02\)	2.08

From the measurement data the authors calculated all the thermodynamic quantities of the system studied. The values of these quantities are given in Table VI.

Table VI

Thermodynamic quantities for the reaction \( \mathrm{HT}+\mathrm{H_2O}\rightleftarrows \mathrm{HTO}+\mathrm{H_2} \)

\(\lg K = 0.292 \lg T + 336.5/T - 1.055\)
\(\Delta F^\circ = +4.83\,T - 1.34\,T\lg T - 1540\)
\(\Delta H^\circ = 0.58\,T - 1540\)
\(\Delta S^\circ = 1.34\lg T - 4.25\)
\(\Delta C_p^\circ = 0.58 \pm 0.05\ \text{cal/deg mole}\)
\(\Delta H_0^\circ = -1540 \pm 160\ \text{cal/mole}\)
\(J = 1.055\)

2. Mass, magnetic moment, and spin of tritium

For the first time the mass of tritium, equal to 3.0151, was determined in 1934 by Rutherford and co-workers\(^{96,97,111}\). The same value is cited by Finberg\(^{47}\). Bethe\(^{21}\) gives the value of the mass of tritium as \(3.01610\pm0.00033\). The latest and, apparently, more accurate data date from 1941. In Berger’s work\(^{20}\) the value of the mass of tritium is given as \(3.01683\pm0.00009\). In the book by Pollard and Davidson\(^{7}\), without indicating the source, the value 3.01704 is given for the mass of tritium.

There are few works on the experimental determination of the magnetic moment of tritium\(^{17,24}\). Sachs\(^{112—115}\) in his works gives the obtained

...a theoretical value for the magnetic moment of tritium equal to 2.68 nuclear magnetons. In another paper, Zaks and Schwinger \(^{113}\) give the value 2.71. Villars \(^{127,132}\) calculates the magnetic moment of tritium as \(\mu_{\mathrm H^3}=\mu_{\mathrm H^1}+0.186=2.975\) nuclear magnetons, while the magnetic moment of the proton, according to Arnold and Roberts \(^{18,19}\), is 2.789.

Bloch \(^{24}\) and coworkers give the ratio of the magnetic moment of tritium to the magnetic moment of the proton as \(1.067 \pm 0.001\). Anderson and Novick \(^{17}\) determined the ratio of the nuclear factor \(g\) for tritium to the nuclear factor for the proton. It proved to be \(1.06666 \pm 0.00010\).

The spin of tritium is \(1/2\). The calculated data are given by Goldhaber \(^{55}\) and Bloch \(^{24}\) and coworkers.

3. Binding Energy of the \(\mathrm H^3\) Nucleus

A considerable number of papers have been devoted to the calculation of the binding energy of the tritium nucleus, especially in recent times (see, for example, the works of Thomas \(^{129}\), Present \(^{105}\), Wilson \(^{136}\), and others \(^{31,42,48,67,70,88,121,122}\)).

Massey and Mohr \(^{86}\) calculated the binding energy on the assumption that there is no interaction between neutrons. This assumption led to incorrect results. When the interaction between neutrons was introduced into the calculation, an energy equal to 8.1 MeV was obtained. Frohlich \(^{51}\) and coworkers also obtained from calculation a value close to 8.1 MeV.

Golovin \(^{2}\), on the basis of the works of other authors, calculated the binding energy of the tritium nucleus and obtained the value 8.46 MeV. From these calculations Golovin came to the conclusion that the depth of the potential well for the \(\mathrm H^3\) nucleus must be less than 40 MeV, and its width greater than \(1.8\cdot 10^{-13}\) cm.

Kundu and Pool \(^{74}\) suppose that in the tritium nucleus there are not two neutrons, but one particle—a dineutron. The authors propose the use of tritium as a particle for introducing two neutrons into the nuclei of other elements in order to obtain the desired isotopes. They assume the following mechanism for such a reaction: when the \(\mathrm H^3\) nucleus approaches the high potential barrier of the target, it becomes polarized and the charged proton is strongly repelled. The dineutron then penetrates into the target nucleus as a whole particle—the dineutron \(n_0^2\).

4. Angular Distribution of Nuclear Particles in Reactions Producing Tritium

Several papers have been devoted to this question, including the detailed work of Neher \(^{90,91}\). In that work the angular distribution of the products of reaction (1) was investigated and the dependence of the angular distribution of protons on voltage was given. A number of papers \(^{61,71,89,106,117}\) are devoted to theoretical calculations of the angular distribution of particles obtained mainly in reaction (1).

5. Tritium as a Projectile of Nuclear Artillery

Recently, papers have appeared in print describing the use of tritium for the bombardment of heavy targets in obtaining radioactive isotopes.

As early as 1939, Gamow[^52], studying nuclear reactions occurring in stars at very high temperatures, drew attention to the possibility of the reaction with tritium

\[ \mathrm{H}_1^3+\mathrm{H}_1^1=\mathrm{He}_2^4+h\nu \tag{18} \]

Colby and Little[^38] propose two variants of the reaction of tritium with deuterium. One of them,

\[ \mathrm{H}_1^3+\mathrm{H}_1^2=\mathrm{He}_2^4+n_0^1, \tag{19} \]

may serve as a source of neutrons of high energy. The mass difference gives an energy value equal to 17.6 MeV.

The second reaction:

\[ \mathrm{H}_1^3+\mathrm{H}_1^2=\mathrm{He}_2^3+2n_0^1. \tag{20} \]

Here the formation is possible not of two neutrons, but of a dineutron.

Kundu and Pool[^73],[^74] were the first to carry out experiments with bombardment by tritium nuclei. Tritium was obtained by reaction (6). A tube made of silver foil, serving as the target, was subjected to bombardment. Beryllium was placed in front of the tube and was irradiated with a beam of 10 MeV deuterons. The beryllium was taken in various thicknesses.

As a result of the reaction, radioactive palladium was obtained according to the equation

\[ \mathrm{Ag}_{47}^{109}+\mathrm{H}_1^3=\mathrm{Pd}_{46}^{109}+\mathrm{He}_2^3. \tag{21} \]

The threshold of the reaction is approximately equal to 1.1–1.5 MeV. The half-life period of palladium is 13.4 hours.

The same authors carried out other reactions according to the equations:

\[ \mathrm{Rh}_{45}^{103}+\mathrm{H}_1^3=\mathrm{Rh}_{45}^{105}+\mathrm{H}_1^1, \tag{22} \]

\[ \mathrm{Rh}_{45}^{103}+\mathrm{H}_1^3=\mathrm{Rh}_{45}^{102}+\mathrm{H}_1^1+3n_0^1. \tag{23} \]

In these reactions radioactive isotopes of rhodium were obtained: \(\mathrm{Rh}^{105}\), with a half-life period of 35 hours, and \(\mathrm{Rh}^{102}\)—210 days. The decay curves of these isotopes are shown in Fig. 7.

In addition to rhodium, cobalt was bombarded:

\[ \mathrm{Co}_{27}^{59}+\mathrm{H}_1^3=\mathrm{Co}_{27}^{61}+\mathrm{H}_1^1. \tag{24} \]

The active cobalt had a half-life period of 1.75 hours.

It is interesting to note that in these reactions two neutrons are simultaneously introduced into the nucleus and a new radioactive isotope is obtained. This once again confirms the possibility of the existence of the dineutron.

For such heavy nuclei as Rh and Co, the formation of a compound nucleus is unlikely. In the cited work of Kundu and Pool it was experimentally established that the yield of the 35-hour activity did not co-

Fig. 7. Decay curves of radioactive isotopes of rhodium obtained as a result of bombardment of rhodium with particles \(H^3\).

rrespond to the theoretical yield calculated for the case in which a compound nucleus would be formed. Such schemes as \((H^3, n)\), \((H^3, 2n)\), and \((H^3, 3n)\) are not suitable, since according to these schemes stable Rh and Co nuclei are obtained, whereas in the present case \(\beta\)-active nuclei were obtained.

On the basis of these experiments the authors propose a reaction of the type \((H^3, p)\) and assume the existence of a dineutron in the tritium nucleus.

METHODS OF DETERMINING TRITIUM

In most cases the radioactivity of the preparation under study is measured with a Geiger–Müller counter, which is a convenient and sufficiently accurate instrument for a whole series of measurements. In such measurements, usually, the radioactive preparation in the form of a solid precip-

ka, less often in liquid form, is placed in immediate proximity to the counter, and the energy of the β-radiator must be sufficient for the electrons to penetrate through the wall of the counter.

Because of the very low energy of the β-radiation of tritium, it cannot be prepared as a solid precipitate and its activity measured by placing the preparation outside the counter. To measure the activity of tritium, it is usually introduced into the counter together with the counting gas. Specially designed counters, which usually form a single unit with the apparatus used for the investigation, serve this purpose.

In this case serious attention must be paid to the composition of the counting gas, since not every gas, nor in all respects, can be used for the normal operation of the counter. It is not recommended to use pure hydrogen as the counting gas, although it would be convenient for measuring the activity of tritium. It is also not recommended to introduce water vapor into the counter. All this imposes stringent conditions for obtaining accurate results when working with tritium.

A number of authors have used various substances as the counting gas. Thus, for example, Allen and Ruben^14, in studying syntheses by the method of labeled atoms, used hydrogen as the counting gas, admitting it into the counter together with tritium. Under these conditions the counting accuracy did not exceed 10–15%.

The same authors, as well as Gould, Bleakney, and Taylor^57, used argon with the addition of alcohol vapor as the counting gas. At the present time such a gas mixture is considered the most suitable for Geiger–Müller counters.

Fontana^50 used propane for filling counters, and Pauly and Ried^104— butane.

Streijman^124 designed a counter with a small mica window, the arrangement of which is shown in Fig. 8. Owing to the thin window, which readily transmits soft radiation, the counter makes it possible to count active substances of low energy without introducing them inside. The author states that the proposed counter operates reliably. Libby and Lee^77 propose a counter with a mesh tube, filled with hydrogen. According to these investigators, such a counter makes it possible to count particles of low energy.

An interesting method for determining tritium was proposed by Henriques and Margnetti^62. This method, which permits the determination of a radioactive substance with an accuracy of up to 2%, was originally proposed by the same authors for the determination of radiocarbon (C^14) in the form of CO₂.

In the work cited, tritium was obtained by bombarding a beryllium target in a cyclotron with deuterons. The target was cooled with water. The tritium, formed in this case by the two reactions (6) and (1), apparently remained in the target in the form of TD gas. To extract this gas, the target was placed in a quartz tube, from which, after this, the gases were pumped out. Hydrogen was then introduced and the tube

was heated to red heat for 15 min. Then the hydrogen was oxidized over copper oxide at 300°C, giving water containing tritium. For the measurements, not water was used, but hydrogen containing tritium; for this purpose the water was acted upon by magnesium amalgam. The measurements were carried out in a quartz chamber filled with hydrogen to 1 atm, with a Lauritsen electrometer. The sample used contained about 1 millicurie of tritium in 2 millimoles of hydrogen.

Fig. 8. Diagram of a counter with a mica window: \(K\)—window; \(N\)—filament; \(M\)—cathode; \(G\)—protective jacket; \(R\)—leads.

The method was worked out on benzene, into which tritium was introduced by isotopic exchange according to the method of Polanyi \(^{64}\). The exchange was carried out on a nickel catalyst, ordinary benzene and hydrogen with an HT tritium content being taken. The experiments were conducted at different concentrations of active benzene in inactive benzene. After the exchange, the amount of active benzene was determined by combustion, the water formed being decomposed on magnesium amalgam and the evolved hydrogen, with an admixture of tritium, being introduced into the counter. From the activity of the hydrogen obtained, knowing the activity of the hydrogen before the exchange, the amount of active benzene was determined.

The most precise method for the quantitative determination of tritium was developed by Eidinoff \(^{45}\). The author proposes, for work with tritium,

radioactive isotope of hydrogen—tritium

a large Geiger–Müller counter. A diagram of such a counter is shown in Fig. 9. The counter was filled with hydrogen and alcohol vapor. The total gas pressure was 65–75 mm Hg.

Table VII gives, by way of example, counting data as a function of the partial pressure of hydrogen containing HT and of the total gas pressure. As is seen from the table, the counting remains proportional to the amount of gas containing tritium.

This proportionality to the pressure of the active gas makes it possible to determine tritium quantitatively within the indicated pressure ranges.

To check the reproducibility of the results, the author carried out measurements of identical samples containing tritium in several counters simultaneously. The results of some measurements are given in Table VIII. As is seen from the table, the deviations from the mean for a given sample and for one and the same sample measured in different counters do not exceed 3%, which for most measurements constitutes quite sufficient accuracy.

This method of determining tritium was used by the author\(^{134}\) in his work on determining the separation factor of hydrogen from tritium by electrolysis.

The method of enriching water with tritium by electrolysis was proposed by Taylor\(^{129}\) and co-workers. In Eidinoff’s work a description is given of the apparatus for electrolysis and values of the isotopic separation factor are presented. An aqueous NaOH solution containing a known amount of NaOT was subjected to electrolysis. The electrolysis was carried out in the apparatus shown schematically in Fig. 10. The NaOH solution,

Fig. 9. Diagram of a counter operating when the active substance is introduced together with the counting gas: A — filament; B — cathode; C — small baths with standard solution; D, E — stopcocks.

Table VII

Dependence of the counting rate on the partial pressure of HT in the counter

Pressure in mm Hg at 25°C: hydrogen containing HT	Pressure in mm Hg at 25°C: total H₂	Pressure in mm Hg at 25°C: alcohol	Pressure in mm Hg at 25°C: argon	Pressure in mm Hg at 25°C: total	Number of counts per minute	Number of counts per minute per 1 mm Hg pressure of H₂ containing HT
10.8	25.5	20.0	19.5	65.0	1140	106
17.0	24.5	21.0	22.0	67.5	1820	107
25.0	25.0	25.0	25.0	75.0	2660	106

Table VIII

Example of the accuracy of measurements of tritium activity in various counters

Counter No.	HT + H₂	Alcohol	Argon	Total	Number of counts per minute	Number of counts per minute per 1 mm Hg pressure of HT + H₂
Counter No. 2	23.4	22	23	68.4	2730	117
Counter No. 2	22.1	23	25	69.1	2620	118
Sample No. 1	23.7	23	22	69.7	2790	118
Sample No. 1	25.3	24	21	70.3	2990	118
Sample No. 1	23.3	25	22	70.3	2770	119
Counter No. 1	23.2	21	24	68.2	2660	115
Counter No. 1	21.4	23	25	69.4	2470	115
Sample No. 1	25.3	25	28	78.3	2920	115
Sample No. 1	21.2	24	23	68.2	2430	115

with tritium content was poured from vessel \(E\) into the electrolyzer \(A, A'\). The hydrogen containing tritium was collected in flask \(H'\), whence, through

Fig. 10. Diagram of the apparatus for enriching hydrogen and deuterium with tritium by electrolysis: \(A, A'\)—vessel for electrolysis; \(B, C, D\)—drying and combustion of the gas; \(E\)—trap with water containing tritium; \(G, H\)—vessels for collecting gas.

tubes for freezing out water and burning out oxygen, it entered a Geiger–Müller counter in a mixture with alcohol and argon. The data obtained are given in Table IX.

Yurish and Taylor^68, for measuring the activity of tritium, likewise used a Geiger–Müller counter with internal filling. As the counting gas they used a mixture of argon and alcohol vapor. Tritium was introduced into the alcohol molecule by isotopic exchange from water. The total gas pressure in the counter was 20 mm.

Table IX

Separation factor of hydrogen and tritium

Hydrogen source	Tritium activity per minute per 1 mm Hg pressure of H₂ at 25° (mean values)	Isotopic separation factor
Alkaline solution . . . . .	571	—
Hydrogen — Electrolysis No. 1 . . .	38.9	14.7
Hydrogen — ” No. 2 . . .	42.7	13.4

APPLICATION OF TRITIUM AS AN INDICATOR IN CHEMISTRY AND BIOLOGY

At the present time the number of works on the application of tritium as an indicator in the study of chemical and biochemical processes is still small.

There is no doubt, however, that in this respect tritium will occupy one of the foremost places, along with carbon, phosphorus, and sulfur.

If the nonradioactive isotope of hydrogen—deuterium—has already found broad application in the study of a number of problems, especially in chemistry, then the application of tritium will help elucidate the mechanism of a number of reactions, for example, the synthesis of chlorophyll.

1. Oxidation of fumaric acid

The beginning of the use of tritium as an indicator was laid by the work of Almen and Ruben^14 in 1942.

For the investigation of the mechanism of oxidation and synthesis of fumaric acid, Almen and Ruben used radioactive carbon C^11 and tritium.

The activity of tritium was determined in a Geiger–Müller counter, which was filled with hydrogen containing an admixture of tritium and an addition of ethyl alcohol at a pressure of 15 mm Hg, or else with a mixture of argon with 10% alcohol and an addition of water vapor containing tritium.

Fumaric acid, containing no activity, was dissolved in active sulfuric acid and oxidized with permanganate according to the equation

\[ \mathrm{COOH-CH{=}CH-COOH + 2KMnO_4 + 3H_2SO_4} \]
\[ = \mathrm{3CO_2 + HCOOH + 2MnSO_4 + K_2SO_4 + 4H_2O}. \tag{25} \]

The sulfate ion was precipitated with \(\mathrm{BaCl_2}\), and the formic acid formed, mixed with water, was distilled off in vacuum. By neutralization with \(\mathrm{Na_2CO_3}\) and drying of the resulting salt, dry sodium formate was prepared; it was burned to \(\mathrm{CO_2}\) and \(\mathrm{H_2O}\). The water obtained in this process was collected, and hydrogen was isolated from it with magnesium at \(620^\circ\mathrm{C}\). The hydrogen was admitted into a counter, where its activity was determined. It turned out that the activity of such hydrogen was very small.

From this the authors conclude that during the oxidation of fumaric acid, despite the profound destruction of its molecule, the carbon–hydrogen bond in the methine group \(\mathrm{CH}\) is not broken.

2. Synthesis of methyl iodide and study of the Menshutkin reaction

Harman, Stewart, and Ruben\({}^{59}\) synthesized methyl iodide containing radioactive carbon \((\mathrm{C^{11}H_3I})\) and tritium \((\mathrm{CTH_2I})\). The latter was obtained by the reactions:

\[ \mathrm{CH_3NO_2 + HTO \to TCH_2NO_2 \to TCH_2NH_2 \to TCH_2I} \tag{26} \]

or

\[ \mathrm{CH_2N_2 + TI \to TCH_2I}, \tag{27} \]

or

\[ \mathrm{CH_2N_2 + TOH \to TCH_2OH \to TCH_2I}. \tag{28} \]

About \(2\ \mathrm{g}\) of active methyl iodide was obtained. The synthesis was carried out with the aim of using active methyl iodide to study exchange reactions in amines.

The reaction between amines and organic halides

\[ \mathrm{R_3N + RX \to R_4N^{+} + X^{-}} \tag{29} \]

had long attracted researchers and had given rise to disputes concerning its mechanism.

Harman, Stewart, and Ruben\({}^{59}\) attempted to study the mechanism of this reaction with the aid of methyl iodide prepared by them, containing tritium. They proceeded from the assumption that, if in the intermediate product formed as the result of a reversible reaction all alkyl groups are equivalent with respect to the halide, then, when carrying out the alkylation reaction with an alkyl containing an active substance, for example tritium, the activity as a result of ob—

exchange should be detected in the unreacted amine as well as in the alkyl halide.

The reaction may be represented by the scheme:

\[ \mathrm{R_3N + R^*X \rightleftarrows \left[ \begin{array}{c} \mathrm{R}\\ |\\ \mathrm{R-N-R^*}\\ |\\ \mathrm{R} \end{array} \right]^+ + X^- \rightleftarrows R_2R^*N + RX,} \tag{30} \]

where the asterisk denotes the active radical.

To test this assumption, the reaction between \(\mathrm{TCH_2I}\) and \((\mathrm{CH_3})_3\mathrm{N}\) was carried out in alcoholic and benzene solutions.

For two hours in 95% alcohol there were 2 equivalents of \((\mathrm{CH_3})_3\mathrm{N}\) per 1 equivalent of \(\mathrm{TCH_2I}\). After this, the activity of the amine was determined. It turned out that its activity was quite insignificant (less than 1% of the total activity). All the activity was found in \((\mathrm{CH_3})_4\mathrm{NI}\), formed as a result of the reaction.

A reaction was carried out between \(\mathrm{C_6H_5(CH_3)_2N}\) and \(\mathrm{TCH_2I}\) in alcoholic and benzene solutions. After 3 hours no activity was found in the amine in either case.

An alcoholic solution of tetramethylammonium chloride and \(\mathrm{TCH_2I}\) was prepared in order to check for the presence of exchange between the radical bound to chlorine in the salt and the radical from RI. After 8 hours no exchange was detected.

From this the authors conclude that, in the intermediate compound of the type \([\mathrm{R_4N}]^+\) that is formed, the four radicals are not equivalent to one another, and that one of them, upon decomposition of the molecule, binds with the halide and does not exchange with the remaining radicals.

3. Hydrogen exchange in aromatic amines

Fontana \(^{50}\) studied hydrogen exchange in complex aromatic amines. For this purpose he used deuterium and tritium. The exchange between the hydrogen of water and the following amines was studied: crystal violet, methylene blue, methyl orange, Congo red, benzidine, benzidine-1-HCl, and benzidine-2-HCl.

It was shown that at room temperature the exchange reaction proceeds very slowly. When the temperature is raised to \(100^\circ\mathrm{C}\) and sulfuric acid is added as a catalyst, the exchange is markedly accelerated.

Thus, for example, crystal violet containing tritium lost only 2% of its initial activity in 3 months at room temperature. At \(100^\circ\) with sulfuric acid as catalyst, crystal violet lost about 60% of its activity in 7 days.

4. Some experiments on photosynthesis and the role of chlorophyll

To test the theory of the role of chlorophyll as a hydrogen donor in photosynthesis, Norris, Ruben, and Allen\({}^{103}\) used tritium. In the experiments water containing tritium (HTO) was used. Chlorella pyrenoidosa was taken as the test plant.

In the process of photosynthesis no chlorophyll containing tritium was detected. The synthesis was carried out in parallel in HTO and H\(_2\)O.

Less than 5% of the expected exchange was also found in the reaction between pure chlorophyll and 80% alcohol containing HTO.

After this work, the question of the role of chlorophyll in photosynthesis remained open.

5. Investigation of the mechanism of butane isomerization

To establish the mechanism of isomerization of butane into isobutane, Tauzin and Reid\({}^{104}\) used tritium obtained by them on the cyclotron according to reaction (1) in the form of water. Hydrogen was liberated from the water by electrolysis. TCl was prepared by the reaction of HT with chlorine on activated charcoal at 300° C. Normal butane was prepared by hydrolysis of CH\(_3\)CH\(_2\)CH\(_2\)CH\(_2\)MgBr in an acid solution containing HTO. Isobutane was likewise prepared by hydrolysis of (CH\(_3\))\(_2\)CHCH\(_2\)MgBr. For measuring the activity, butane was used in a mixture with hydrogen containing HT. This mixture is regarded by the authors as quite suitable for good counter operation. The pressure in the counter was about 40 mm, the voltage—1800 volts. Two types of counters were used—one with a volume of 482 cm\(^3\), the other—41.4 cm\(^3\).

The exchange between butane and hydrogen from HT and TCl was investigated. The results of the experiments make it possible to conclude that exchange from hydrogen does not proceed directly, but that exchange first occurs between HT and HCl, always formed on the AlCl\(_3\) catalyst, and then between TCl and the hydrocarbon according to the scheme

\[ \mathrm{HT} + \mathrm{HCl} \rightarrow \mathrm{H_2} + \mathrm{TCl}, \tag{31} \]

\[ \mathrm{TCl} + \mathrm{C_4H_{10}} \rightarrow \mathrm{HCl} + \mathrm{C_4H_9T}. \tag{32} \]

Next, the exchange between C\(_4\)H\(_9\)T and H\(_2\) was investigated. The results proved interesting and somewhat unexpected.

The rate of exchange of tritium bound to the primary carbon in isobutane proved to be greater than the rate of exchange of tritium bound to the tertiary carbon. Conversely, in normal butane the rate of exchange of tritium bound to the secondary carbon proved to be greater than the rate of exchange of tritium bound to the primary carbon.

Experiments on the isomerization of n-butane into isobutane were carried out on an AlCl catalyst in the presence of HCl and TCl.

RADIOACTIVE ISOTOPE OF HYDROGEN—TRITIUM

The authors propose the following mechanism of isomerization, confirmed by experiments using tritium. Butane comes into contact with the catalyst, so that the hydrogen of hydrochloric acid in combination with \(\mathrm{AlCl_3}\) approaches the hydrogen located at the secondary carbon in butane:

\[ \begin{array}{ccccc} & & \mathrm{Cl} & & \\[-2pt] & & \mathrm{ClAlCl} & & \\[-2pt] & & \mathrm{Cl} & & \\[-2pt] & & \mathrm{T} & & \\[-2pt] \mathrm{H} & \mathrm{H} & \mathrm{H} & \mathrm{H} & \\[-2pt] \mathrm{HC} & {-}\mathrm{C} & {-}\mathrm{C} & {-}\mathrm{CH} & \\[-2pt] \mathrm{H} & \mathrm{H} & \mathrm{H} & \mathrm{H} & \end{array} \quad \longrightarrow \quad \begin{array}{ccccc} & & \mathrm{H} & & \\[-2pt] & & \mathrm{HCH} & & \\[-2pt] \mathrm{H} & & | & & \mathrm{H} \\[-2pt] \mathrm{TC} & {-} & \mathrm{C} & {-} & \mathrm{CH} \\[-2pt] \mathrm{H} & & \mathrm{H} & & \mathrm{H} \end{array} \]

The weakening of the bond between the primary and secondary carbons and its rupture occur because of the presence of three hydrogen atoms and two carbon atoms at one carbon. \(\mathrm{AlCl_4}\), existing without hydrogen, tends to take a position between two secondary carbons and to tear hydrogen away from the second secondary carbon, which leads to a free bond between the two secondary carbons.

After this, the methyl group \(\mathrm{CH_3}\) can either combine with the second secondary carbon, forming isobutane, or return to its original position, forming normal butane.

If the scheme is correct, then tritium from \(\mathrm{TCl}\) should pass into isobutane, which is indeed observed experimentally when the reaction is carried out with \(\mathrm{TCl}\).

6. Study of the isomerization reaction of butylene

On the basis of experiments on the isomerization of butylene to pseudobutylene, carried out on phosphoric acid containing tritium, Turkevich and Smith \(^{19}\) propose an isomerization mechanism likewise connected with transfer of hydrogen by the catalyst, as was proposed by Paul and Reid in the work just considered.

In general outline the mechanism of isomerization of butylene reduces to the following. When the butylene molecule approaches the phosphoric acid molecule, one of the hydrogen atoms of the acid occupies a position close to the terminal carbon with the double bond,

\[ \begin{array}{cccc} \mathrm{H} & \mathrm{H} & \mathrm{H} & \\[-2pt] \mathrm{HC}=\mathrm{C} & {-}\mathrm{C} & {-}\mathrm{CH_3} & \\[-2pt] \mathrm{T} & \vdots & \mathrm{H} & \\[-2pt] \mathrm{O} & {-}\mathrm{P}=\mathrm{O} & & \\[-2pt] & / \ \backslash & & \\[-2pt] \mathrm{HO} & & \mathrm{OH} & \end{array} \quad \longrightarrow \quad \begin{array}{cccccc} \mathrm{H} & \mathrm{H} & \mathrm{H} & & & \\[-2pt] \mathrm{HC} & {-}\mathrm{C}=\mathrm{C} & {-}\mathrm{CH_3} & + & \mathrm{H_3PO_4} \\[-2pt] \mathrm{T} & & & & & \end{array} \]

The oxygen in the acid that has no hydrogen then occupies a position close to the secondary carbon.

Upon decomposition of this complex, the acid gives up its hydrogen to the terminal carbon and removes hydrogen from the secondary carbon, in

as a result of which the double bond shifts with the formation of pseudobutylene.

If such a scheme is correct, then tritium, contained in the phosphoric acid, should be detected as a result of isomerization in the hydrocarbon. In the experiments carried out by the authors, activity was found in the pseudobutylene obtained upon isomerization.

On the basis of their experiments the authors believe that reactions connected with the presence of a carbon double bond—such as migration, alkylation, polymerization, cracking, and isomerization—proceed by a common mechanism with transfer of hydrogen by the catalyst.

Thus, polymerization, for example, proceeds, in their opinion, by the following mechanism: hydrogen is transferred by one of the two olefins, and the catalyst participates in the transfer. In alkylation there is transfer of hydrogen from a paraffin to an olefin.

There is no doubt that a detailed study of these processes, with the use of tritium, will make it possible to understand most fully the mechanism of action of the catalyst in the reactions taking place.

7. Exchange of tritium and hydrogen ions in alkylation

The study of the alkylation process, also connected with the transfer of hydrogen by the catalyst, is the subject of the work of Stewart and Harman¹²³.

The authors attempted to elucidate the mechanism of alkylation of isobutane by pseudobutylene in the presence of tritium-sulfuric acid.

The experiments showed that hydrogen exchange in pseudobutylene proceeds very rapidly and that there is a hydrogen equilibrium between pseudobutylene and sulfuric acid. In addition, by determining the activity in the various hydrocarbon fractions obtained as a result of alkylation, a uniform distribution of tritium among all the atoms in pseudobutylene was established.

8. Measurement of the amount of water in living organisms

The first work on the measurement of tritium as an indicator in biochemical processes appeared in 1947. This work by Pace¹⁰² and co-workers was devoted to determining the amount of water in living organisms with the aid of tritium.

Radioactive water, HTO, was prepared by dissolving a beryllium target, after bombarding it with deuterons, in 6 N hydrochloric acid in vacuo. The activity was measured in the vapors by introducing them into a Geiger–Müller counter.

Two rabbits received an internal infusion of a concentrated HTO solution—the first 5.09 ml and the second 1.96 ml. Then, by determining the activity of various extracts, the total amount of water contained in the organism was determined.

This quantity was determined as 73.2% of the total weight of the rabbit, which agrees well with the results obtained by other analyses. The distribution time of active water in the rabbit organism is less than 30 min.

After testing on rabbits, the experiment was repeated on humans. The amount of water in the human organism proved, by this method, to be 64.7%. The calculated amount for a person of average weight is 65.2%. The distribution time of the activity throughout the human organism is 1 hour.

9. Application of Tritium for Determining the Solubility of Water

Yurish and Taylor68 investigated the solubility of water in benzene by the radioactive-indicator method and developed a method for measuring the activity of tritium.

Fig. 11. Diagram of an apparatus for saturating a solvent with a small amount of water: A — saturation vessel; A′ — vessel with a calibrated capillary for transferring the sample; S — capillary for passing air saturated with water vapor.

Tritium was prepared in a cyclotron by bombardment of D₂O. The experimental method consisted of: 1) saturating a hydrocarbon with vapors of heavy water containing tritium (TDO), 2) taking a sample of the saturated solution, and 3) measuring the activity of the sample.

The stream of air saturated with vapors of TDO and benzene was maintained in a circulating apparatus, the diagram of which is shown in Fig. 11.

The circulation was effected by a mercury pump and two vessels with mercury, \(V\) and \(V'\). The removable vessel \(A\) contained benzene, the temperature of which was kept constant by immersion in a Dewar vessel with water. The air entered a trap, likewise immersed in water. To avoid condensation of benzene vapors in the tubes, the entire apparatus was placed in a heated cabinet, the temperature of which was higher than the temperature of the trap.

Fig. 12. Diagram of the apparatus for determining tritium contained in water dissolved in benzene: \(A\) — vessel with the sample containing tritium; \(B\) — counter; \(C\) — flasks; \(E\) — U-shaped tube with CaO.

Radioactive water in an amount of \(0.1\) ml was introduced into trap \(C\), after which the air was saturated with it. Benzene was poured into vessel \(A\), and circulation of the air saturated with water was begun until its benzene was saturated. This operation was carried out for 4 hours.

After saturation, part of the radioactive sample was drawn off into the thin tube \(S\) by a pump by opening tap 2. Vessel \(A\) was removed, and in its place a calibrated vessel \(A'\), shown at the side in the same figure, was substituted. The sample was transferred into this vessel, and its volume was measured.

To measure the activity, vessel \(A'\) was transferred to another apparatus equipped with a counter. The diagram of this apparatus is shown in Fig. 12.

The contents of vessel \(A\) were frozen with liquid air and pumped out; after thawing, the benzene evaporated and entered the U-shaped tube \(E\), containing freshly prepared CaO. In this tube the water from the benzene was absorbed, after which the benzene was removed by vacuum distillation and collected again in vessel \(A\).

Tritium was removed from the Ca(OT)\(_2\) formed as a result of the reaction by isotope exchange with alcohol, which was then introduced into the counter. The entire system was flushed with alcohol vapor. In addition, argon was introduced into the counter (3 parts to 1 part alcohol). The total pressure of the mixture was 20 mm Hg; the diameter of the counter was 5 cm, and its length 25 cm.

In one of the experiments, 1.10 cm\(^3\) of benzene was taken for measurement; it had been saturated with TDO at \(20^\circ\)C. The results of this experiment are given in Table X.

Table X

Results of measuring the activity of the water saturating benzene

Object of measurement	Number of counts per minute
1. Background	383
2. Flushing the tube with the sample with alcohol vapor (without background)	360
3. Activity volatilizing from CaO: 1st portion (without background)	1 532
3. Activity volatilizing from CaO: 2nd portion (without background)	460
4. Activity after flushing CaO with alcohol vapor (without background): 1st flushing	7 977
4. Activity after flushing CaO with alcohol vapor (without background): 2nd flushing	3 657
4. Activity after flushing CaO with alcohol vapor (without background): 3rd flushing	2 387
4. Activity after flushing CaO with alcohol vapor (without background): 4th flushing	789
4. Activity after flushing CaO with alcohol vapor (without background): 5th flushing	517
5. Residual activity	520
Total activity of the entire sample	18 200
Activity per 1 cm\(^3\) of sample	16 550

The calculation of the solubility was carried out under the assumption that the solubilities of ordinary water and of heavy water containing tritium are equal.

The molecular concentration of TDO in the sample of benzene vapor taken for measurement (volume 33.4 cm\(^3\), \(\rho = 7\) cm, \(t = 22^\circ\)C) is equal to:

\[ \frac{33.4 \cdot 7 \cdot 273 \cdot p_1}{22400 \cdot 76 \cdot 295 \cdot 760} = 1.685 \cdot 10^{-7} p_1 \frac{\text{moles TDO}}{\text{mole}}, \tag{33} \]

where \(p_1\) is the pressure of TDO, in mm Hg, in the gas sample used for the measurement. This is the pressure of TDO in the gas phase in equilibrium with the solution, when the benzene is saturated with water.

Let \(A\) be the number of pulses per minute per mole of TDO. The number of moles is determined from equation (33). The observed activity of the sample was 2314 counts per minute, whence

\[ \frac{2314}{A}=1.685\cdot 10^{-7}p_1, \]

or

\[ p_1=\frac{2314}{1.685A}\cdot 10^7=\frac{1.375}{A}10^{10}\ \text{mm Hg}. \]

In \(1\ \text{cm}^3\) of benzene at \(20^\circ\text{C}\), 16550 counts per minute were found. Therefore the molecular concentration of TDO in the aqueous solution in benzene is

\[ \frac{16550\cdot 78.05}{A\cdot 0.885}=\frac{1.445}{A}10^6 \]

molecules of TDO per molecule of benzene.

Hence the constant \(K\) for TDO will be:

\[ K=\frac{p_1}{n_1}= \frac{1.375A^{-1}\cdot 10^{10}}{1.445A^{-1}\cdot 10^6} =0.952\cdot 10^4. \]

Consequently, the solubility of \(\mathrm{H_2O}\) in benzene at \(20^\circ\text{C}\) is equal to

\[ n_2=\frac{p_2}{0.952}10^{-4}, \]

where \(p_2\) is the vapor pressure of \(\mathrm{H_2O}\). At \(20^\circ\text{C}\), \(p_2=17.535\ \text{mm Hg}\), and

\[ n_2=18.4\cdot 10^{-4}\frac{\text{mol } \mathrm{H_2O}}{\text{mol } \mathrm{C_6H_6}}. \]

Accordingly, in other units the solubility is expressed as:

1) in \(100\ \text{cm}^3\) of benzene, \(2.105\cdot 10^{-3}\) mol of \(\mathrm{H_2O}\), or \(3.80\cdot 10^{-2}\) g of \(\mathrm{H_2O}\), dissolves;

2) in 100 g of benzene, \(4.25\cdot 10^{-2}\) g of \(\mathrm{H_2O}\) dissolves.

The results obtained by the authors at three temperatures are compared in Table XI with the data of other investigators.

Table XI

Solubility of water in benzene, determined by various methods
(in g \(\mathrm{H_2O}\) per 100 g \(\mathrm{C_6H_6}\))

Temperature, °C	Data of Moritz and Taylor	Data of other authors	Data of other authors	Data of other authors	Data of other authors	Data of other authors
		58	9	63	87	108
10	0.030	0.040	0.035	0.043	0.032	0.045
20	0.0425	0.056	0.055	0.057	0.044	0.057
	0.0445
26	0.054	0.070	0.069	0.070	0.057	0.067

The results of the measurements by Eyring and Taylor agree best of all with Clifford’s data, who measured the solubility of water by its absorption by CaO. Other methods give higher values. Apparently, the agreement with Clifford’s data should be attributed to incomplete absorption of water by calcium oxide.

The possible error of the measurements is estimated by the authors at several percent.

It seems to us that this method deserves attention and may be applied in a number of other cases.

The method is applicable, however, only to those cases in which there is no isotopic exchange of hydrogen between the water and the solvent.

CITED LITERATURE

1. N. Brezhneva, S. Roginskii, Usp. Khim. 7, 1503 (1937).
2. I. Golovin, Zh. E. T. F. 8, 658 (1938).
3. G. Grinberg, Usp. Khim. 9, 771 (1940).
4. G. Landsberg, U. F. N. 12, 343 (1932).
5. G. Lyubarskii, Usp. Khim. 16, 422 (1947).
6. M. Neiman, An. Nesmeyanov, Usp. Khim. 17, 401 (1948).
7. E. Pollard and W. Davidson, Applied Nuclear Physics, OGIZ (1947).
8. G. Smith, Atomic Energy for Military Purposes, Goszheldorizdat (1946).
9. N. Tarasenkov, E. Polozhintseva, Zh. O. Kh. 1, 71 (1931).
10. L. Farkas, Usp. Khim. 4, 22 (1935).
11. E. Fermi, E. Amaldi, O. D’Agostino, B. Pontecorvo, F. Rasetti, E. Segrè, U. F. N. 14, 993 (1934).
12. G. Frank, U. F. N. 25, 179 (1941).
13. V. Chernyaev, U. F. N. 21, 466 (1939).
14. M. Allen, S. Ruben, J. Am. Chem. Soc. 64, 948 (1942).
15. L. Alvarez, R. Cornog, Phys. Rev. 56, 613 (1939).
16. L. Alvarez, R. Cornog, Phys. Rev. 57, 248 (1940).
17. H. Anderson, A. Novick, Phys. Rev. 71, 372 (1947).
18. W. Arnold, A. Roberts, Phys. Rev. 70, 766 (1946).
19. W. Arnold, A. Roberts, Phys. Rev. 71, 878 (1947).
20. A. Berger, Zeits. Ges. Naturwissensch. 7, 297 (1941).
21. H. Bethe, Phys. Rev. 47, 633 (1935).
22. J. Black, H. Taylor, Journ. Chem. Phys. 11, 395 (1943).
23. W. Bleakney, A. Gould, Phys. Rev. 45, 281 (1934).
24. F. Bloch, A. Graves, M. Packard, R. Spence, Phys. Rev. 71, 373, 551 (1947).
25. T. Bonner, Phys. Rev. 53, 711 (1938).
26. L. Borst, Phys. Rev. 57, 659 (1940).
27. L. Borst, Phys. Rev. 59, 941 (1941).

1. L. Borst, Phys. Rev., 61, 106 (1942).
2. J. Bower, Proc. Cambr. Phil. Soc., 34, 450 (1938).
3. E. Bretscher, A. French, F. Seidl, Phys. Rev. 73, 815 (1948).
4. F. Brown, Phys. Rev. 56, 1107 (1939).
5. S. Brown, Phys. Rev. 59, 954 (1941).

6. D. Budnizkii, I. Kurchatov, G. Latuischev, Phys. Zeits. Sow., 7, 474 (1935).

7. E. Burhop, Proc. Cambr. Phil. Soc., 32, 643 (1936).

8. J. Chadwick, M. Goldhaber, Nature, 135, 65 (1935).
9. J. Chadwick, M. Goldhaber, Proc. Cambr. Phil. Soc., 31, 612 (1935).
10. C. Clifford, Ind. Eng. Chem. Anal. Ed., 13, 631 (1921).
11. M. Colby, R. Little, Phys. Rev., 70, 437 (1946).
12. R. Cornog, W. Libby, Phys. Rev., 59, 1046 (1941).
13. E. Crane, Ind. Eng. Chem. News Ed., 13, 200 (1935).
14. P. Dee, Nature, 133, 564 (1934).
15. H. Dolch, Zeits. f. Phys., 100, 401 (1936).
16. R. Döpel, Naturwiss., 24, 237 (1936).
17. R. Döpel, Ann. Phys. 28, 87 (1937).
18. M. Ejdinoff, J. Am. Chem. Soc., 69, 2504 (1947).
19. M. Ejdinoff, J. Am. Chem. Soc., 69, 977, 2507 (1947).
20. E. Feenberg, Phys. Rev., 47, 850 (1935).
21. H. Feschbach, Phys. Rev., 61, 544 (1942).
22. S. Flügge, Zeits. f. Phys. 108, 545 (1938).
23. B. Fontana, J. Am. Chem. Soc., 64, 2503 (1942).

24. H. Fröhlich, Kun Huang, I. Sneddon, Proc. Roy. Soc., 191 A, 61 (1947).

25. G. Gamow, Nature, 144, 575, 620 (1939).

26. Gilbert Lewis, F. Spedding, Phys. Rev., 43, 964 (1933).
27. M. Goldblatt, E. Robinson, R. Spence, Phys. Rev. 72, 973 (1947).
28. M. Goldhaber, Proc. Cambr. Phil. Soc., 30, 561 (1934).
29. H. Goldsmith, Phys. Rev., 46, 78 (1934).

30. A. Gould, W. Bleakney, H. Taylor, Journ. Chem. Phys., 2, 362 (1934).

31. E. Groschuff, Zeits. Elektrochem., 17, 348 (1911).

32. D. Harman, T. Stewart, S. Ruben, J. Am. Chem. Soc., 64, 2293, 2294 (1942).

33. G. Harnwell, H. Smyth, S. van Voorhis, J. Kuper, Phys. Rev., 45, 655 (1934).

34. R. Haxby, J. Allen, J. Williams, Phys. Rev., 55, 140 (1939).
35. F. Henriques, Ch. Margnetti, Ind. Eng. Chem. Anal. Ed. 18, 417, 420 (1946).
36. A. Hill, Journ. Am. Chem. Soc., 45, 1143 (1923).
37. J. Horinch, G. Ogden, M. Polany, Trans. Far. Soc., 30, 663 (1934).
38. E. Hudspeth, T. Bonner, Phys. Rev., 54, 308 (1938).
39. D. Hughes, C. Eggler, Phys. Rev., 73, 809 (1948).

1. E. Hylleraas, Zeits. f. Phys., 107, 258 (1937).
2. G. Joris, H. Taylor, J. Chem. Phys., 16, 45 (1948).
3. M. Kamen, Radioactive Tracers in Biologie, N. Y. (1947).
4. E. Konopinski, Phys. Rev., 72, 518 (1947).
5. E. Konopinski, Phys. Rev., 73, 822 (1948).
6. R. Krishman, Nature, 148, 407 (1941).
7. D. Kundu, M. Pool, Phys. Rev., 72, 101 (1947).
8. D. Kundu, M. Pool, Phys. Rev., 73, 22 (1948).
9. F. Kurie, Phys. Rev., 45, 904 (1934).
10. W. Latimer, H. Young, Phys. Rev., 44, 690 (1933).
11. W. Libby, D. Lee, Phys. Rev., 55, 245 (1939).
12. W. Libby, R. Cornog, Phys. Rev., 60, 171 (1941).
13. W. Libby, C. Barter, Journ. Chem. Phys., 10, 184 (1942).
14. W. Libby, Journ. Chem. Phys., 11, 101 (1943).
15. W. Libby, Phys. Rev., 69, 671 (1946).
16. W. Libby, Analyt. Chem., 19, 2 (1947).
17. M. Livingston, J. Hoffman, Phys. Rev., 53, 227 (1938).
18. W. Lozier, P. Smith, W. Bleakney, Phys. Rev., 45, 655 (1934).
19. W. Luis, L. Alvarez, R. Cornog, Phys. Rev., 58, 197 (1940).
20. H. Massey, C. Mohr, Proc. Roy. Soc., 152, 693 (1935).
21. L. Meitner, Arkiv Mat. Astron. Phys., A 33, 12 (1945).
22. F. Myers, L. Langer, Phys. Rev., 54, 90 (1938).
23. R. Myers, Phys. Rev., 54, 361 (1938).
24. H. Neuert, Phys. Zeits., 39, 890 (1938).
25. H. Neuert, Naturwiss., 26, 429 (1938).
26. G. Nielsen, Phys. Rev., 60, 160 (1941).
27. T. Norris, S. Ruben, M. Allen, J. Am. Chem. Soc., 64, 3037 (1942).
28. A. Novick, Phys. Rev., 72, 972 (1947).
29. M. Oliphant, T. Harteck, L. Rutherford, Nature, 133, 413 (1934).
30. M. Oliphant, T. Harteck, L. Rutherford, Proc. Roy. Soc., A 144, 692 (1934).
31. M. Oliphant, A. Kempton, L. Rutherford, Proc. Roy. Soc., A 149, 406 (1935).
32. R. O’Neal, M. Goldhaber, Trans. Illin. State Acad. Sci., 33, 187 (1940).
33. R. O’Neal, M. Goldhaber, Phys. Rev. 57, 1086 (1940).
34. R. O’Neal, M. Goldhaber, Phys. Rev., 58, 574 (1940).
35. R. O’Neal, Phys. Rev., 60, 359 (1941).
36. N. Pace, L. Kline, H. Schachman, M. Harfenist, J. Biol. Chem., 168, 459 (1947).
37. G. Perlow, Phys. Rev., 58, 218 (1940).
38. T. Powell, E. Reid, Journ. Am. Chem. Soc., 67, 1020 (1945).
39. R. Present, Phys. Rev., 50, 635 (1936).
40. C. Reinsberg, Zeits. Phys., 108, 189 (1938).
41. A. Roberts, Phys. Rev., 51, 810 (1937).
42. C. Rosenbaum, J. Walton, Journ. Am. Chem. Soc., 52, 3568 (1930).

1. J. Rotblat, Nature, 133, 202 (1936).

2. L. Rumbaugh, R. Roberts, L. Hafstad, Phys. Rev., 54, 657 (1938).

3. L. Rutherford, Nature, 140, 303 (1937).

4. R. Sachs, J. Schwinger, Phys. Rev., 61, 732 (1942).

5. R. Sachs, J. Schwinger, Phys. Rev., 70, 41 (1946).

6. R. Sachs, Phys. Rev., 71, 457 (1947).

7. R. Sachs, Phys. Rev., 72, 312 (1947).

8. R. Scherr, L. Smith, W. Bleakney, Phys. Rev., 54, 388 (1938).

9. L. Schiff, Phys. Rev., 51, 783 (1937).

10. R. Scott, F. Brickwedde, Phys. Rev., 48, 483 (1935).

11. R. Scott, F. Brickwedde, H. Urey, M. Wahl, Journ. Chem. Phys., 2, 454 (1939).

12. P. Selwood, H. Taylor, W. Lozier, W. Bleakney, Journ. Am. Chem. Soc., 57, 780 (1935).

13. S. Share, Phys. Rev., 53, 875 (1938).

14. W. Stephens, Phys. Rev. 57, 938 (1940).

15. T. Steward, D. Harman, Journ. Am. Chem. Soc., 68, 1185 (1946).

16. E. Strajman, Rev. Scient. Instr., 17, 232 (1946).

17. H. Taylor, M. Goldhaber, Nature 135, 341 (1935).

18. H. Taylor, Proc. Phys. Soc. Lond., 47, 873 (1935).

19. A. Thellung, F. Villars, Phys. Rev., 73, 924 (1948).

20. L. Thomas, Phys. Rev., 47, 903 (1935).

21. J. Turkewich, R. Smith, Nature, 157, 874 (1946).

22. M. Tuve, L. Hafstad, O. Dahl, Phys. Rev. 45, 840 (1934).

23. J. van Allen, A. Ellett, D. Bayley, Phys. Rev., 56, 381 (1939).

24. F. Villars, Phys. Rev., 72, 256 (1947).

25. R. Watts, D. Williams, Phys. Rev., 70, 640 (1946).

26. M. Wiedenbeck, Phys. Rev. 70, 435 (1946).

27. J. Williams, R. Haxly, W. Shepherd, Phys. Rev., 52, 1031 (1937).

28. A. Wilson, Proc. Cambr. Phil. Soc., 34, 365 (1938).

29. D. Wilson, A. Nier, S. Reimann, Ann. Arbor. Michigan Edwards (1947).