Analysis of the composition of Halo's 252CF neutron source

Abstract

The Helium and Lead Observatory (HALO) is a supernova neutrino detector located un- derground at SNOLAB. HALO consists of 128 3He neutron counters and 79 tonnes of lead. In the event of a supernova, neutrinos will interact with the lead, releasing neutrons which will be detected by the 3He neutron counters. Each neutron counter in HALO has a different neutron capture efficiency by virtue of its position within the array. These efficiencies were found by previous work using Monte Carlo simulations and verified with a californium neu- tron source. A previous analysis using this source has assumed that it was purely comprised of 252Cf, but recently it has been recognized that several isotopes are present and contribute to the source’s neutron output. A better knowledge of the isotopic composition of the cali- bration source is necessary to more accurately determine HALO’s neutron capture efficiency in different locations. In addition to the 252Cf present in the source, other neutron emitting isotopes of various half-lives and multiplicity distributions are present, such as some 250Cf, which was present at the source’s creation, and 248Cm which is created as an alpha decay product of 252Cf. This demands a more complex model of the source, which is undertaken in this thesis. New data of the 252Cf source was taken with the source in HALO’s central tube. This data, as well as data taken over the past few years, was analyzed with maximum likelihood methods with hopes of learning more about the isotopic composition of the source. It was found that by analyzing the past five years of runs taken with the source in HALO, the decay pattern does not match any expected set of initial conditions. Assuming a central neutron capture efficiency of 48%, the strength of the source appears to be 24.8 ± 0.2 neutrons per second greater than the 7.7 ± 0.1 neutrons per second expected. If the ICP-MS report made at the beginning of the source’s lifetime is correct pertaining to the source’s isotopic composition and the curium was removed from the source on the date provided, then some other phenomena must account for the discrepancy experienced. Various explanations for this discrepancy were tested, including gamma-induced fission, alpha-induced fission, and spontaneous fission of isotopes in the nearby rocks. No method examined appeared to give a strong explanation for this discrepancy.