ABSOLUTE YIELD OF PHOTONEUTRONS FROM VARIOUS ELEMENTS

Investigations of the nuclear photoeffect carried out up to now have been limited mainly to the determination of threshold energies

Fig. 1. Dependence of the absolute yield of photoneutrons on atomic number for a maximum energy in the spectrum of 18 MeV and 22 MeV.

and did not make it possible to find the dependence of the neutron yield in the reaction \((\gamma, n)\)

from the atomic number of the element. In the referenced works,^1,2 performed on 22-MeV and 50-MeV betatrons, this dependence was found. In the first of them, 53 elements of the periodic system were investigated, the samples of which, as a rule, were ordinary isotopic mixtures. Photoneutrons were recorded by means of a rhodium detector placed in a paraffin block with \(B_4C\) spacers. The activity with a period of 44 sec induced in the detector was measured with Geiger counters. A second rhodium foil, placed in the concrete shielding of the accelerator, served to monitor the beam intensity. The calibration of the absolute neutron yield was carried out by means of a standard (Ra-Be) neutron source. A correction was introduced for the absorption of \(\gamma\)-rays in the sample, using the data for the absorption coefficient of \(\gamma\)-rays with energy 13.73 MeV.^3 It was found that the dependence of the absolute yield of photoneutrons \((N)\) on the \(Z\) of the element is well approximated by the function \(aZ^b\) (Fig. 1). Very close to it is the curve constructed using the theory of Goldhaber and Teller,^4 who consider the occurrence of collective oscillations of a dipole type upon excitation of the nucleus by a \(\gamma\)-quantum.

Similar results were also obtained in work carried out by an analogous method on a 50-MeV betatron (Fig. 2). Here \(N = 1860 Z^2\). Deviations from the experimental curve obtained for thorium and uranium are explained by the presence of photofission. Assuming that in fission 2 or 3 neutrons arise with equal probability, one obtains:

![Figure 2 and Figure 3]

Fig. 2. Neutron yield per mole·roentgen as a function of atomic number (50-MeV betatron).

Fig. 3. Angular distribution of photoneutrons at \(E_m = 19.7\) MeV for D, Be, Fe, and Pb.

\[ 2.5K + (1 - K) = R, \]

where \(K\) is the fraction of nuclei undergoing fission, and \(R\) is the ratio of the neutron yield for Th (or U) to the yield extrapolated from the curve for these elements. For uranium \(K\) turned out to be equal to 0.51 at \(E_m = 22\) MeV and 0.43 at \(E_m = 18\) MeV. For thorium, respectively, 0.24 and 0.17.

The angular distribution of photoneutrons from D, Be, Fe, and Pb at \(E = 19.7\) MeV was also investigated (see Fig. 3). For Fe and Pb the distribution proved to be isotropic. In the case of D and Be there is a maximum in the neutron intensity in the direction perpendicular to the \(\gamma\)-beam.

B. R.

References

1. G. A. Price and D. W. Kerst, Phys. Rev. 76, 182A (1949).
2. G. C. Baldwin and F. R. Elder, Phys. Rev. 78, 76 (1950).
3. Adams, Phys. Rev. 74, 1707 (1948).
4. Goldgaber and Teller, Phys. Rev. 74, 1046 (1948).