Full Text
From Current Literature
DETERMINATION OF THE MASSES OF FRAGMENT NUCLEI EMITTING DELAYED NEUTRONS
According to the hypothesis of Bohr and Wheeler, the delayed neutrons emitted after the fission of uranium are born in fragment nuclei. The emission of delayed neutrons is preceded by β-decay to an excited level whose energy is higher than the neutron binding energy. Thus, the observed decay of the neutron activity after irradiation of \(U^{235}\) should occur with a period equal to the period of β-decay.
In confirmation of this hypothesis, uranium fission products with which the emission of delayed neutrons is associated were separated\(^{1,2}\). A solution of uranyl nitrate was irradiated by a flux of thermal neutrons from a reactor or a cyclotron. After the irradiation was stopped, the solution was subjected to the action of various reagents in order to separate one or another element, and the neutron activity of the precipitate and filtrate was measured.
After a number of unsuccessful attempts, Snell and co-workers\(^{1}\) separated, with bromine and iodine, neutron activities with periods, respectively, of \(54 \pm 1\) and \(23.8 \pm 0.7\) sec. The decrease in the half-life of bromine was caused by imperfect chemical separation with rapid filtration.
These periods were identified with the β-activities \(J^{137}\), found by Hahn, with a period of \(30 \pm 6\) sec, and \(Br^{87}\), with a period of \(50 \pm 10\) sec. \(Kr^{87}\) and \(Xe^{137}\), which accumulate, respectively, from \(J\) and \(Br\), are probable emitters of delayed neutrons. The difference between the radiochemical yields of masses 87 and 137, determined respectively as 2.5 and 6.2%, and the yields obtained by emission of delayed neutrons and equal to 0.045–0.13% for Br and 0.32–0.96% for J, is explained by branching in the decay (see scheme).
The task of chemically separating, in a short time, one element from 31 uranium fission products is a very difficult one.
Shurman\(^{2}\) carried out additional studies in order to limit the choice of elements with which the emission of delayed neutrons may be associated. The ranges in aluminum of fragment nuclei with 56-, 22-, 4.5-, and 1.52-sec neutron activities were measured.
A thin layer of uranium enriched in \(U^{235}\) was covered with aluminum foils and a Bakelite fragment collector and moved pneumatically into the reactor for irradiation and then outward for measurement. After irradiation the collector was separated from the sample and the neutron activity was measured. For the activity of each period, absorption curves were obtained in the form:
\[ A_t = k(R - t), \]
where \(A_t\) is the activity of a fragment nucleus that has passed through an absorber of thickness \(t\), \(k\) is a constant, and \(R\) is the range of the fragments.
The values of the maximum ranges are given in the table on p. 590.
Corrected for the source thickness, the ranges of fragments leading to the emission of delayed neutrons
| Period in sec . . . | 4.51 | 55.6 | 1.52 | 22.0 |
| Range in $\dfrac{mg}{cm^2}$ Al . . . | $4.05 \pm 0.3$ | $3.98 \pm 0.06$ | $3.68 \pm 0.12$ | $3.21 \pm 0.3$ |
From the curve* expressing the dependence of the range of fragment nuclei on mass, an estimate was made of the masses corresponding to the known ranges of the nuclei. In doing so, masses whose yield is less than the neutron yield were excluded.
were excluded. The mass of the fragment with a period of 4.51 sec was found to be 86–90, and the mass with a period of 1.52 sec, 129–135. From consideration of the possible decay chains it follows that the first period may be associated with Se or with Br, and the second with In, Sn, or Sb.
An attempt at chemical separation of the 4.51-sec period with Se was unsuccessful. Chemical separation of Br led to separation of this activity together with the 55.6-sec one, which indicates the chemical identity of the elements associated with these activities. The mass of the 4.51-sec bromine was determined as 86–90 mass units. The maximum energy of the $\beta$ decay of $\mathrm{Br}^{88}$, calculated according to Bohr and Wheeler, is 1.5 MeV, which is less than the energy of evaporation of neutrons from $\mathrm{Kr}^{88}$, the product of this decay. Consequently, $\mathrm{Br}^{88} \to \mathrm{Kr}^{88}$ cannot be a source of delayed neutrons, and mass 86 is excluded.
Thus, the mass of the nuclei emitting delayed neutrons of the 4.51-sec period may have one of the values from 87 to 90.
Separation of the 1.52-sec activity by chemical means was not carried out.
The phenomenon of delayed-neutron emission should lead to jumps in the smooth curve of the yield of uranium-fission products as a function of mass. Indeed, if the 4.51-sec neutron activity of Br has a yield of 0.5%, then the final yield of bromine because of neutron emission will be less
* In an unpublished work by Shugarman.
by 0.5%, which amounts to about 10% of the total fraction of bromine in the fission products, while the yield of the element with mass one unit smaller will increase by 10%. This was used to estimate the mass of 22.0-sec iodine.
Todd and Graham³, in determining the relative yields of the noble gases formed in uranium fission, found that the yield of Xe¹³⁶ is 15% lower than Xe¹³⁷, whereas the opposite should be the case as a consequence of the smooth rise of the yield curve in this region and as a consequence of the additional yield due to delayed neutrons, which according to Snell are emitted by mass 137. It is therefore more probable to assume that the mass of 22.0-sec iodine is 136. This introduces certain changes into Snell’s scheme.
O. I. Kozinets
References
- A. Snell, J. S. Levinger et al., Phys. Rev. 72, No. 7 (1947).
- N. Sugarman, J. Chem. Phys. 15, No. 8 (1947).
- H. G. Thode, R. L. Graham, Can. J. Research 25, 1 (1947).