Full Text
CORRELATION BETWEEN THE EMISSION ANGLES OF ELECTRONS AND RECOIL NUCLEI IN THE β-DECAY OF P³²
In the February issue of Physical Review the results were published of an experimental study of the angular distribution of electrons and recoil nuclei formed in the β-decay of P³², i.e., in the reaction
\[ \mathrm{P}^{32} \to e^- + \nu + \mathrm{S}^{32}. \]
Previous attempts to determine the angular distribution of electrons and recoil nuclei in the β-decay of a nucleus encountered great difficulties because of the impossibility of obtaining a pure β-active substance. Now, as
Fig. 1.
the author notes, in connection with the production of the phosphorus isotope (P³²), this difficulty is almost completely overcome. This material is so pure that it permits a correct monatomic layer of P³² to be deposited on a given surface and thereby avoids energy losses and scattering of the recoil nuclei in the β-active element itself.
The basic idea of the experiment consists in measuring the flight time of the recoil nuclei in a field-free space. If the distance over which the recoil nuclei fly freely is known, then, by measuring the time during which they traverse this distance, one can determine their momenta.
A diagram of the experimental setup is shown in Fig. 1.
A thin mica plate \(S\) (from 1 to 1.5 mg/cm²), on whose surface a monatomic layer of β-active P³² had been deposited, served as the source of electrons and recoil nuclei according to the following reaction
\[ {}_{15}\mathrm{P}^{32} \to \mathrm{S}^{32} + e^- + \nu . \]
The total β-activity of the source was from 1 to \(3 \cdot 10^4\) electrons per second. The size of the surface was \(1 \cdot 0.6\) cm. The pressure in the measuring chamber was of the order of \(10^{-7}\) mm Hg.
After preparation in an auxiliary chamber, the plate with the β-active substance was introduced into the measuring chamber.
Electrons passing through windows made of thin mica (9 mg/cm²) and situated at various angles to the source were recorded by a Geiger counter.
Recoil nuclei emitted in one of the directions inside a small cone with an aperture of \(\sim 6^\circ\) were registered by means of an electron multiplier.²
Since the velocity of the electrons is much greater than the velocity of the recoil nuclei and is close to the speed of light, it may be assumed that the electrons reach the Geiger counter instantaneously, whereas the recoil nuclei are registered with a certain delay. The time interval between the registration of the electron and the registration of the recoil nucleus is evidently equal to the interval of time during which the recoil nucleus passes the distance from the source to the electron multiplier (about \(6.5\ \mathrm{cm}\)). Measurement of this time interval, i.e., the interval from the appearance of the electron to the appearance of the recoil nucleus, determines the momentum of the recoil nucleus.
Fig. 2.
In Fig. 2 the spectra of recoil nuclei obtained by the author are presented. On the ordinate is plotted the number of recoil nuclei observed during \(1/2\) microsecond; on the abscissa, the time of flight in microseconds. Small flight times of the recoil nuclei correspond to their high momenta, as shown in the figure.
For each individual case the total number of electrons registered during the observation time is given. For example, in the first case, i.e., when the recoil nuclei and electrons are emitted in opposite directions, the total number of observed electrons is \(2.9 \cdot 10^5\).
The author then compares the results obtained (Fig. 3) with theory. As is known,³ in the theory of \(\beta\)-decay of the nucleus there exist five possible forms of interaction: scalar, vector, tensor, pseudotensor, and pseudo-
scalar. Each of these variants of the interaction gives a quite definite correlation between the directions of emission of the electron and the neutrino[^4]. For example, the probability of decay in which the directions of emergence of the electron and neutrino form an angle \(\vartheta\), in the scalar and pseudoscalar variants of the interaction, is proportional to \(1-\beta\cos\vartheta\), where \(\beta=\dfrac{v_{\mathrm{el}}}{c}\).
The author arrives at the following conclusion.
First, the experimental data lead to the conclusion that, between the electrons and recoil nuclei in the \(\beta\)-decay of \(\mathrm{P}^{32}\), the law of conservation of momentum does not hold. Consequently, in order for the law of conservation of momentum to be satisfied, we must necessarily assume the presence of the neutrino.
Second, the function \(1+\beta\cos\vartheta\) (\(\vartheta\) is the angle between the directions of the electron and the neutrino), predicted by the vector variant of the interaction, is in very strong disagreement with experiment.
Third, the function \(1-\beta\cos\vartheta\), predicted by both the scalar and pseudoscalar variants of the interaction, in the region of recoil-nucleus energies above 25 eV, agrees well with the experimental data.
Fig. 3.
The author assumes that \(\mathrm{P}^{32}\to e^-+\nu+\mathrm{S}^{32}\) is an allowed transition. However, in his opinion, a first forbidden transition cannot be definitively excluded.
As M. A. Markov noted, the comparison made by the author of the experimental data with the predictions of various variants of the theory of interaction is not entirely valid. Indeed, it is known that the process \(\mathrm{P}^{32}\to e^-+\nu+\mathrm{S}^{32}\) is doubly forbidden[^3]; therefore the results of the theory developed for allowed transitions cannot be extended to it.
Zh. S. Takibaev
References Cited
- Chalmers W. Sherwin, Phys. Rev., 73, 216 (1948).
- I. S. Allen, Rev. Sci. Inst., 12, 582 (1941).
- Bethe and Bacher, Nuclear Physics; Emil Jan Konopinsky, Reviews of Modern Physics, 15, 209 (1943).
- D. R. Hamilton, Phys. Rev., 71, 456 (1947).