Neutron detection via long-range events resulting from fission in thick 238U disks

Abstract

In our previous work, a 1 mm thick 238U (uranium) disk backed by a charged-particle detector, when exposed to the neutron flux from a 252Cf (californium) source (mean neutron energy of 2.2 MeV), showed a count-rate increase that was higher than can be accounted for by the detection of short-range primary fission fragments. Electrons resulting from fission product decays were seen as a more plausible reaction mechanism. The current study extends the investigation of this phenomenon by using six 238U disks ranging in thickness from 100 μm to 1 mm in order to discern the relationship between disk thickness and neutron detection efficiency. Also considered is the effect of using thin foils of nonfissionable material in order to suppress the α-particle background from the 238U disks. The exact isotopic composition of these 238U samples is known, which eliminates a source of ambiguity in the previous study. With these samples we find that the excess neutrons found in the previous study are not seen. This forces us to conclude that the results reported in our previous work were not due to the detection of electrons resulting from β-decay of the fission products of 238U, but were most likely the result of the interaction of a neutron with an impurity in the previous samples.