These months have been dedicated to calibrate and understand the CsI response in the low-energy regime. 2 photoneutron sources, 88YBe and 124SbBe, have been used. Photoneutron sources involve the gamma-induced disintegration of a light nucleus with modest neutron binding energy, such as 9Be. This mechanism is exploited to obtain ~153 keV (~24 keV) neutrons from 88Y (~124Sb) gammas interacting in 9Be. These neutrons can produce a nuclear recoil in CsI of up to ~4.7 keV and ~0.7 keV respectively, allowing to determine the quenching factor at such energies. Those sources were studied at University of Chicago using a dedicated setup with a cryogenically operated (80K) small CsI crystal coupled to a large area avalanche photodiode (LAAPD) to read-out CsI light. The gamma sources (88Y, 124Sb) were encapsulated in identical Be and Al holders with data-taking alternating between the Be holder and the Al, to subtract background. Lead shielding was used to minimize the gamma background from the sources. While analysis is still undergoing, the quenching factor at 4.7 keV appears to be between 4 and 6%, in line with the trend observed by the host in previous experiments at higher energies. The observed distribution for recoils at 0.7 keV point towards a quenching factor below 2%, slightly higher than anticipated.
Nuclear recoils of ~0.18 keV have also been studied by exploiting the gamma emission which follows thermal neutron capture in CsI. While such de-excitation usually involves a gamma cascade, there is a small probability of single gamma emission which has enough energy (~6.8 MeV) to produce a significant nuclear recoil in either Cs or I. By tagging that high energy gamma with an external detector and evaluating the response in coincidence in CsI, one can study such low-energy recoils. This study was done in Ohio State University Research Reactor, using their thermal neutron beam to shoot thermal neutrons onto a CsI crystal and using a BGO crystal to tag interesting events. As before, analysis is still underway, however, given the small energy range and the low quenching factor that can be expected, it's highly likely that only an upper limit will be reported.
The researcher has also undertaken the task of building LAAPDs from scratch to be used for reading CsI light. These detectors were previously commercially available but production was shut down recently leaving self-production as the only way to proceed. The work has been done in Pritzker Nanofabrication Facility (PNF), a world class nanofabrication facility located at University of Chicago. The researcher has designed and tested by himself the full fabrication procedure. The methodology involves several nanofabrication techniques such as lithography or etching among others. The first attempt at a full production was recently performed and, while, no functional sensors were produced, a series of improvements have been proposed to further polish the procedure in near-future attempts.
The researcher has been remotely coordinating and supervising the neutronic background simulations team in Spain while acting as a direct link with the team at University of Chicago. The goal is to characterize the neutron flux in the different regions of the ESS to evaluate their suitability to accommodate the final detector as well as study the impact of additional external shielding to reduce the flux. The Spanish team is developing a Geant4 simulation that will be compared with University of Chicago simulations, based on MCNP. The simulations are on a final stage of minimal safety checks before running a full production. Such simulations will provide a detailed description of neutronic backgrounds around the facility.
