Photoneutron Thresholds of 13 Isotopes

A. K. Berzin

(Presented by Prof. A. A. Vorob'ev, Doctor of Physical and Mathematical Sciences)

The measurement of photoneutron thresholds was carried out by two methods: 1) by the method of “direct registration” of neutrons, described earlier by us [1], with the only difference that the flash produced in the detector was registered not by one but by two photoelectron multipliers connected in a coincidence circuit; 2) by the method of “residual activity” [2].

Control of the maximum energy of the bremsstrahlung $\gamma$-radiation was performed using the circuit [3], which ensured an accuracy of $\pm 10$ keV. The pulse from the mixing circuit, arriving at the sectorial expansion windings, had a duration of 4 $\mu$sec. The mixing circuit was assembled on the GGI-400/16 thyratron.

The investigation was carried out after the thermal regime of the betatron electromagnet had become established. The energy scale was calibrated by the threshold energies of the isotopes: ${}^{9}_{4}\mathrm{Be}$ (1.666 MeV); ${}^{7}_{3}\mathrm{Li}$ (7.15 MeV) and ${}^{63}_{29}\mathrm{Cu}$ (10.73 MeV).

In the work the threshold energies of the $(\gamma,n)$ reaction were measured for thirteen isotopes. Figures 1 and 2 show neutron yields as a function of the bremsstrahlung $\gamma$-radiation energy for La, Ce, Pr, Nd, Sm, and Ba, obtained by the method of “direct registration.” The dashed curve corresponds to the background increased by a factor of 5. Powders of oxides of the indicated elements, weighing from 40 to 70 g, loaded into thin-walled aluminum ampoules, were irradiated.

The determination of the yields belonging to individual isotopes was carried out by a graphical method. It turned out that all the data obtained, as in a number of previous works [4, 5], can be described with sufficient accuracy by the relation: $N = K(E_{max} - E_0)^m$. The threshold energy $E_0$ and the constant $m$ (characteristic of the given isotope) were selected graphically. The coefficient $K$ depends on the sample weight and on the irradiation dose received by the specimen. In Fig. 3, as an example, the graphical separation of yields for the isotopes $\mathrm{Ce}^{140}$ and $\mathrm{Ce}^{142}$ is shown. In the case where the isotope—the reaction product—had a half-life convenient for measurement, the residual-activity method was also used. Samples prepared from oxides, with a small addition of paraffin, in the form of tablets, had a diameter of 15 mm and a thickness of 2 mm. The induced activity was measured in a lead housing (wall thickness 30 mm) using an MST-17 end-window counter and a PS-64 scaling device. The counter was powered from a stabilized high-voltage rectifier of the VSE-2500 type.

Fig. 1

Graph of the dependence of neutron yield on the energy of bremsstrahlung radiation for La, Ce, and Sm.

Fig. 2

Graph of the dependence of neutron yield on the energy of bremsstrahlung radiation for Ba, Nd, and Pr.

Table 1 gives the results of the measurements carried out. The total measurement error was composed of the sum of the errors introduced by the monitoring scheme and by the statistical uncertainty of the data obtained. It follows from the table that the previously measured threshold energies refer to isotopes that occur in the natural mixture in small amounts and in large percentage abundance, i.e., are very convenient for study.

Fig. 3

Graphical separation of the yields for the isotopes
$\mathrm{Ce}^{140}$ and $\mathrm{Ce}^{142}$.

The isotopes $\mathrm{Ba}^{138}$, $\mathrm{La}^{139}$, $\mathrm{Ce}^{140}$, $\mathrm{Pr}^{141}$, $\mathrm{Nd}^{142}$, and $\mathrm{Sm}^{144}$, belonging to isotopes with the “magic” number 82, possess an increased neutron binding energy. Isotopes of the same element containing $(82 \pm 2)$ neutrons have, on the contrary, a reduced neutron binding energy. This circumstance speaks in favor of the shell model of nuclear structure. The lowered values of the threshold energies for nuclei with $(82 + 2)$ neutrons did not make it possible to say with certainty to which of the neodymium isotopes, $\mathrm{Nd}^{143}$ or $\mathrm{Nd}^{145}$, the measured threshold of $6.69 \pm 0.08$ MeV belongs. It also remained unclear to which of the barium isotopes the threshold value $6.69 \pm 0.08$ MeV can be assigned, since $\mathrm{Ba}^{137}$, having $(82 - 1)$ neutrons, should apparently possess an increased energy.

Table 1

LITERATURE

1. A. K. Berzin, Threshold energies of \((\gamma,n)\)-reactions. Proceedings of the Tomsk Polytechnic Institute, vol. 87, 1957.
2. Butement, Proc. Phys. Soc. 64, 395 (1951).
3. A. K. Berzin, R. P. Meshcheryakov, R. G. Nemkov, A new scheme for monitoring the maximum energy of X-rays. Proceedings of the Tomsk Polytechnic Institute, vol. 87, 1957.
4. R. Sher, J. Halpern, A. Mann. Phys. Rev. 84, 387 (1951).
5. J. MeElhinney, A. Mann. Phys. Rev. 75, 542 (1949).