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DOUBLE DECAY OF Re \(^{186}\)
The paper reviewed\(^{1}\) was undertaken in connection with the existence of ambiguous data\(^{2,3}\) concerning the character of the \(\beta\)-decay and the subsequent \(\gamma\)-radiation of Re \(^{186}\). In particular, it had not been reliably established whether the spectrum of Re \(^{186}\) is simple or complex.
For the measurements, various compounds of the radioactive isotope Re \(^{186}\) (rhenium oxide and \(\mathrm{Re}_3\mathrm{Si}_2\)) were used; this isotope was obtained by neutron irradiation of the principal isotope Re \(^{187}\). The distribution of electrons by momentum was investigated by means of a magnetic spectrometer; it is shown in Fig. 1.
The continuous part of the electron spectrum, which is a spectrum of \(\beta\)-electrons, can, on the basis of the theory of \(\beta\)-decay, be represented as the superposition of two different \(\beta\)-spectra corresponding to maximum electron energies of \(1.02 \pm 0.005\) MeV and \(0.942 \pm 0.008\) MeV, with intensities \(74 \pm 5\) and \(27 \pm 5\%\), respectively.
Analysis of the three most intense peaks (see Fig. 1) of the electron spectrum shows that they are due to \(\gamma\)-rays with an energy of \(0.136\) MeV, which caused conversion of electrons in the \(K\)-, \(L\)-, and \(M\)-shells of osmium (Os \(^{186}\)). The energy of these \(\gamma\)-rays is, within the limits of experimental errors, equal to the difference between the upper energy limits of the two observed \(\beta\)-spectra and corresponds, therefore, to the energy of the excited state of the Os \(^{186}\) nucleus.
Two other peaks of smaller magnitude, found in the electron spectrum, correspond, in all probability, to electrons converted from the \(K\)- and \(L\)-shells of W \(^{186}\) by \(\gamma\)-rays with an energy of \(0.122\) MeV, emitted from an excited state of tungsten—the daughter product of the decay of Re \(^{186}\). This is also confirmed by the existence of exact equality between the difference of the electron energies corresponding to the small peaks (\(0.0525\) and \(0.110\) MeV) and the difference of the electron binding energies in the \(K\)- and \(L\)-shells of tungsten \(\left(E_K^W = 69.2\ \text{keV};\ E_L^W = 11.8\ \text{keV}\right)\). Thus, along with the \(\beta^{-}\)-decay of Re \(^{186}\) to Os \(^{186}\) with electron emission, there also occurs a radioactive transition of Re \(^{186}\) to W \(^{186}\), caused by \(K\)-capture.
On the electron spectrum one can see one more broadened line (in the region of 0.046 MeV), apparently due to Auger electrons associated with \(K\)-capture.
Fig. 1. Electron spectrum of \(\mathrm{Re}^{186}\). The energy values of the conversion lines and of the corresponding \(\gamma\)-rays are given in MeV.
The interpretation of the data obtained from the electron spectrum is given in Table I.
Table I
| Energy of electron lines (MeV) | Origin of the electrons | Energy of the corresponding \(\gamma\)-rays | Relative intensity |
|---|---|---|---|
| 0.1330 | \(M\)-shell Os | 0.1360 | 0.0154 |
| 0.1235 | \(L\)-shell Os | 0.1362 | 0.0800 |
| 0.0620 | \(K\)-shell Os | 0.1357 | 0.0485 |
| 0.1103 | \(L\)-shell W | 0.1223 | 0.010 |
| 0.0525 | \(K\)-shell W | 0.1212 | 0.0059 |
| 0.046 | Auger electrons | 0.005 |
The study of the \(\gamma\)-spectrum of \(\mathrm{Re}^{186}\) was carried out by observing photoelectrons knocked out by \(\gamma\)-rays from lead, gold, and uranium detectors of thickness \(19\), \(5\), and \(0.3\ \mathrm{mg/cm^2}\), respectively.
The results of the study of the electron spectra thus found are systematized in Table II.
Fig. 2. Decay scheme of \(\mathrm{Re}^{186}\).
According to the above data, the decay scheme has the form shown in Fig. 2. Final confirmation of the existence of the left
Table II
\(\gamma\)-rays emitted by \(\mathrm{Re}^{186}\)
| Detector | Energy of photoelectron peak | Shell | Corresponding \(\gamma\)-ray energy | Relative intensity of \(\gamma\)-rays |
|---|---|---|---|---|
| Pb \((19\ \mathrm{mg/cm^2})\) | 0,048 | \(K\) | 0,136 | 1,0 |
| Pb \((19\ \mathrm{mg/cm^2})\) | 0,120 | \(L\) | 0,136 | 1,0 |
| Pb \((19\ \mathrm{mg/cm^2})\) | 0,132 | \(M\) | 0,136 | 1,0 |
| Pb \((19\ \mathrm{mg/cm^2})\) | 0,035 | \(K\) | 0,123 | \(0,1 \pm 0,02\) |
| Pb \((19\ \mathrm{mg/cm^2})\) | 0,106 | \(L\) | 0,122 | \(0,1 \pm 0,02\) |
| Au \((5\ \mathrm{mg/cm^2})\) | 0,055 | \(K\) | 0,135 | 1,0 |
| Au \((5\ \mathrm{mg/cm^2})\) | 0,122 | \(L\) | 0,136 | 1,0 |
| Au \((5\ \mathrm{mg/cm^2})\) | 0,133 | \(M\) | 0,136 | 1,0 |
| Au \((5\ \mathrm{mg/cm^2})\) | 0,042 | \(K\) | 0,1225 | \(0,1 \pm 0,02\) |
| Au \((5\ \mathrm{mg/cm^2})\) | 0,108 | \(L\) | 0,122 | \(0,1 \pm 0,02\) |
| U \((0,3\ \mathrm{mg/cm^2})\) | 0,020 | \(K\) | 0,136 | 1 |
| U \((0,3\ \mathrm{mg/cm^2})\) | 0,115 | \(L_{\mathrm{I}}, L_{\mathrm{II}}\) | 0,136 | 1 |
| U \((0,3\ \mathrm{mg/cm^2})\) | 0,119 | \(L_{\mathrm{III}}\) | 0,136 | \(0,11 \pm 0,03\) |
| U \((0,3\ \mathrm{mg/cm^2})\) | 0,100 | \(L\) | 0,122 | \(0,11 \pm 0,03\) |
branch of the decay of Re\(^{186}\), associated with \(K\)-capture, was obtained by means of experiments in which the number of coincidences of \(\beta\)-electrons with \(\gamma\)-rays of energy 0.122 MeV and with conversion electrons was observed. Determination of the conversion coefficients from the intensity ratios of the conversion lines made it possible to characterize the \(\gamma\)-rays as electric quadrupole transitions of the nuclei Os\(^{186}\) and W\(^{186}\).
The measurements of the decay of Re\(^{186}\) carried out independently by other authors\(^4\) are in good agreement with the decay scheme of this isotope proposed here.
V. F.
CITED LITERATURE
- R. M. Steffen, Phys. Rev. 82, 827 (1951).
- Beach, Peacock and Wilkinson, Phys. Rev. 76, 1585 (1949).
- P. J. Grant and R. Richmond, Proc. Phys. Soc. 62, 575 (1949).
- F. R. Metrgev and P. D. Hill, Phys. Rev. 81, 300 (1951).