ON THE DECAY SCHEME OF IONIUM AND RADIOTHORIUM

Ionium and radiothorium are isotopes of thorium and have even-even nuclei. As a result of $\alpha$-decay these nuclei are transformed into nuclei of even-even isotopes of radium

\[ {}^{230}_{90}\mathrm{Th}(\mathrm{Io}) \xrightarrow{\alpha} {}^{226}_{88}\mathrm{Ra}, \qquad {}^{228}_{90}\mathrm{Th}(\mathrm{RdTh}) \xrightarrow{\alpha} {}^{224}_{88}\mathrm{Ra}(\mathrm{ThX}). \]

Even-even nuclei possess zero angular momentum. Therefore an $\alpha$-transition of such a nucleus leading to the ground state of the daughter nucleus will occur without a change in angular momentum ($\Delta I = 0$), whereas an $\alpha$-transition as a result of which a nucleus is formed in an excited state is associated with a change in angular momentum ($\Delta I \ne 0$). The probability of $\alpha$-decay decreases with increasing $\Delta I$ and depends substantially on the energy of the $\alpha$-transition. As a result of this, even-even nuclei, in accordance with the theory of $\alpha$-decay, as a rule, do not

Fig. 1.

have an intensive fine structure of the α-spectrum. In this respect the cases considered are exceptions.

In recent years, with the aid of an α-spectrograph with a large magnet, new measurements have been made of the α-spectra of radiothorium\(^1\) and ionium\(^3\). The use of thick photographic plates considerably improved the technique of the experiment. In processing the α-spectra with a microscope, the number of tracks of α-particles was counted at various points of the photoemulsion. This method of registering α-particles made it possible to use thin-layer sources, and also to improve the technique of photometry of the plate.

Figure 1 shows the α-spectrum of radiothorium\(^1\). Two intense α-lines (\(a_0\) and \(a_1\)) are visible, differing in energy by \(86.7\) kev. The small peaks visible in the figure belong to ThX. The γ-radiation of RdTh with energies \(83.4\) kev and \(86.8\) kev is known; α-lines differing in energy by the sum of the energies of these γ-quanta have not been found. Therefore it is concluded that one of the observed α-lines is double, and the difference of \(3.4\) kev is not resolved by the apparatus\(^1\). A broadening of the line \(a_0\) was noticed.

In studying the α-spectrum of ionium\(^3\), an intensive fine structure was also found. Table I gives the data on the α-spectra of RdTh and Io:

Table I

α-spectra of RdTh and Io

the energies of the α-lines, the relative intensity of the lines, and the probable change of angular momentum (\(\Delta I\)) in the transformation. In studying an α-particle with energy \(a_0\), the nucleus formed is in the ground state. Consequently, the transition \(a_0\) corresponds to \(\Delta I = 0\). If an α-particle with energy \(a_1\) is emitted, then the resulting nucleus remains in an excited state. The change \(\Delta I\) for this case has been found from the theory of α-decay from the measured intensity of the fine-structure lines of the α-spectrum.

In the cases considered, the angular momentum of the ground state is \(I = 0\), and, according to fine-structure data, in the excited state it is also \(I = 0\). Therefore one may suppose that in the luminescence of Ra and ThX there occurs a case of the so-called “0—0” transition.

For “0—0” transitions of nuclei the usual theory of radiative transitions\(^ {4,5}\) is inapplicable, since the conservation laws lead to an absolute prohibition of the emission of one photon by the nucleus. Of the few cases of “0—0” nuclear transitions known at present, the “0—0” transition in O\(^{16*}\) has been studied, and interesting theoretical results were obtained by Soviet physicists\(^6\). Below are considered works in which research...

the radiation emitted in the discharge of the excited levels of Ra and ThX.

For the investigation of electron radiation in work\(^{7}\), ionium was introduced into the center of a Wilson chamber at reduced pressure. The intensity of the source was estimated from the number of α-particle tracks. The electron spectrum found in the work is shown in Fig. 2. Electrons of such energies appear as a result of the conversion of γ-quanta with an energy of 68 kev in the \(L\)-, \(M\)-, and \(N\)-shells of the radium atom. On average, \(10 \pm 1\) electrons per 100 ionium decays were found. In another work\(^{8}\), the electron radiation of ionium was investigated by means of photographic plates with a thick emulsion (200 μ), sensitive to electrons (Ilford G5 plates). The plates were impregnated with a solution containing ionium, and after development

Fig. 2.

tracks of α-particles of Io were selected on them. Electron tracks beginning from the initial point of the α-track were studied. The results of this work are in complete agreement with the results of work\(^{7}\).

The spectrum of radio-thorium electrons was also studied by the method of photographic plates sensitive to electrons\(^{9}\). In all, 1850 tracks of RdTh α-particles (range 29 μ) were selected. From the beginning of 141 tracks of these α-particles an electron track was observed. Thus, 7.6 electrons per 100 RdTh decays were found. The maximum in the energy distribution of the electrons was obtained at an energy of 65 kev. This electron radiation appears as a result of internal conversion of γ-radiation with an energy of 86.8 kev on the atomic electrons of ThX.

In works\(^{10,11,12}\) the intensity of the γ-radiation of Io and RdTh was studied. By measuring the absorption of γ-radiation in Al, Cu, Ta, and Pb, the separation of the constituent components of this radiation was carried out. The calculated efficiency of the counter used in the work for registering γ-radiation makes it possible, from these measurements, to determine the number of γ-quanta of individual γ-lines. In work\(^{10}\) an ordinary thin-walled aluminum counter filled with argon was used. In measurements of hard γ-radiation this counter was surrounded by lead foil, which increased its efficiency for hard γ-radiation. In works\(^{11,12}\) an end-window counter filled with xenon (9–18 mm Hg) and alcohol (1–2 mm Hg) was used. The aluminum window of the counter had a thickness of 20 μ. Such a counter has increased efficiency for X-ray and soft γ-radiation. (The efficiency for registering photons with an energy of 14.5 kev by such a counter is 25%.)

In the cited works it was shown that the preparation Io and RdTh emits characteristic X-ray radiation of radium and nuclear $\gamma$-radiation of low intensity (Table II). In internal conversion of $\gamma$-radiation on the atomic electrons, the atom formed in the decay of Io and RdTh remains in an excited state and can pass to the ground state by emitting characteristic X-ray radiation. On the basis of the observed $L$-radiation it is possible to determine the intensity of the $L$-radiation. The author of work$^{12}$, estimating the intensity of $M$- and $N$-radiation and taking for the fluorescence yield of the $L$-radiation the value $\omega_L = 0.37$, obtains the number of atomic excitations. In this way agreement is obtained with the intensity of the fine structure of the $\alpha$-spectrum of Io and RdTh. However, a direct count of the number of emitted $\gamma$-quanta and conversion electrons does not give such agreement.

Table II

Intensity of $\gamma$-radiation (number of $\gamma$-quanta per 100 decays)

The material presented on the radiations of Io and RdTh makes it possible to draw up the decay scheme of these nuclei$^{12}$ (Fig. 3). Table III gives data on the level Ra C with excitation energy 68 kev and the level ThX with excitation energy 86.8 kev, obtained by counting the number of conversion electrons ($N_e$) and emitted $\gamma$-quanta ($N_\gamma$). The quantity $N_e + N_\gamma$, referred to the number of decays of Io or RdTh, according to the data of Table III is almost two times smaller than the intensity of the fine structure of the $\alpha$-spectrum (Table I).

From the ratio $\dfrac{N_e}{N_\gamma}$ one can estimate the multipolarity of the corresponding transition. The values of $\Delta I$ thus found, equal to 4 or 3, are in sharp contradiction with the conclusions obtained from the intensity of the fine structure of the $\alpha$-spectrum. However, if a case of a “0—0” transition occurs, then the theory of internal conversion, on the basis of which $\Delta I$ is determined in Table III, is not applicable.

For a “0—0” transition with a transition energy of 60–90 keV, one could expect complete conversion of the radiation^13. The presence of γ-radiation with an excitation energy allows the possibility of another path for the “0—0” transition, for example, the simultaneous emission of two photons or of a conversion electron

Fig. 3.

and a photon. In this case the radiation should have a continuous energy spectrum. The experimental material does not contradict the assumption of a continuous spectrum of electrons and photons, if this energy distribution has a sharp maximum^12. The maximum observed

Table III

in the experiments in the distribution of electrons and photons may be taken as a maximum in the region of high energies. A maximum at small electron and photon energies is practically impossible to observe. Several competing discharge processes are also possible in the considered cases of “0—0” transitions.

I. Estulin

CITED LITERATURE

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3. S. Rosenblum, M. Valaders, M. Perey et I. Vial, Comptes Rendus 229, 1009 (1949).

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