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From Current Literature
Determination of the Upper Bound of the Emission Time of Fission Neutrons
The generally accepted idea of the “evaporation” of fast neutrons from moving fragments of the nucleus during fission (with the exception of \(\sim 0.8\%\) of delayed neutrons associated with \(\beta\)-decay of fission products) was confirmed in work 1 on the investigation of the angular correlation of fission fragments and neutrons. The small statistics did not make it possible in this case to make any quantitative estimates. The paper under review 2 describes an original method for determining the upper limit for the time interval between nuclear fission and the emission of fast neutrons (\(T_0\)).
A thin \(U^{235}\) sample is deposited on a cylindrical proportional counter (see figure). The counter, whose collecting electrode is coated with paraffin, records neutrons (by recoil protons) with energies above 300 keV. The counter and the sample are placed in a vacuum chamber, which is irradiated by a flux of thermal neutrons. In the evacuated chamber, some of the fragments remain at the wall for \(5 \cdot 10^{-9}\) sec, whereas when the chamber is filled with gas these fragments remain near the counter wall. Therefore, if \(T_0 > 5 \cdot 10^{-9}\) sec, a difference should be observed in the number of pulses in the counter when the chamber is evacuated and when it is filled. The difference in the counting rate should depend on the fraction of fragments flying out
outward, and from the ratio of the solid angles. The solid angle of the chamber wall found by placing the sample near it proved to be about 35%. The authors introduced a correction for the absorption of slow neutrons by hydrogen in the gas filling (propane at a pressure of several atm was used as the filling). As a result of the measurements, the average percentage of neutrons delayed by a time exceeding \(8 \cdot 10^{-9}\) sec was found to be \(3.6 \pm 2.8\%\). Taking into account that this includes neutrons delayed for a longer time (0.8%), the authors come to the conclusion that, with an accuracy of up to 3%, neutrons emerging later than \(8 \cdot 10^{-9}\) sec after fission of the nucleus are absent.
B. R.
CITED LITERATURE
- R. R. Wilson, Phys. Rev. 72, 189 (1947).
- T. M. Snyder and R. W. Williams, Phys. Rev. 81, 171 (1951).