Periodic Reporting for period 2 - vPESS (Neutrino physics at ESS using coherent elastic neutrino-nucleus scattering.)
Berichtszeitraum: 2023-03-01 bis 2024-02-29
Coherent Elastic Neutrino-Nucleus Scattering (CEνNS) is a recently demonstrated novel process of neutrino interaction. It provides numerous avenues to advance our sensitivity to new nuclear and particle physics beyond the Standard Model while simultaneously allowing a dramatic miniaturization of otherwise massive neutrino detectors, opening up the possibility of technological applications.
Both the supervisor and the partner organization host, along a number of collaborators, recently proposed to use the neutrino flux from the upcoming European Spallation Source (ESS) for a definitive exploration of all phenomenological opportunities provided by CEνNS. The proposal includes a variety of detection techniques to evaluate such a process including the use high-pressure gaseous xenon (HPGXe) chambers and cryogenic undoped CsI scintillator crystals. For reasons of nuclear structure, CsI and Xe detectors are identical in their response to CEνNS. However, the technologies and their expected systematics are fully different. Simultaneous use at the ESS will therefore provide robust confirmation for any possible signatures of new physics.
The goal of the action is to develop and characterize the two different approaches for deployment at the ESS, with a strong focus on the low-energy calibration of the techniques. Concretely, the quenching factor of pure CsI will be determined in energies below existing measurements. While HPGXe detectors are a mature technology, their response to nuclear recoils in the energy range of CEνNS is unknown. Characterizing the response in such range is one of the main goals of this proposal. In addition, the researcher will evaluate the neutron background for CEνNS searches at ESS. Such work will be of interest to other users of the facilities.
Both the supervisor and the partner organization host, along a number of collaborators, recently proposed to use the neutrino flux from the upcoming European Spallation Source (ESS) for a definitive exploration of all phenomenological opportunities provided by CEνNS. The proposal includes a variety of detection techniques to evaluate such a process including the use high-pressure gaseous xenon (HPGXe) chambers and cryogenic undoped CsI scintillator crystals. For reasons of nuclear structure, CsI and Xe detectors are identical in their response to CEνNS. However, the technologies and their expected systematics are fully different. Simultaneous use at the ESS will therefore provide robust confirmation for any possible signatures of new physics.
The goal of the action is to develop and characterize the two different approaches for deployment at the ESS, with a strong focus on the low-energy calibration of the techniques. Concretely, the quenching factor of pure CsI will be determined in energies below existing measurements. While HPGXe detectors are a mature technology, their response to nuclear recoils in the energy range of CEνNS is unknown. Characterizing the response in such range is one of the main goals of this proposal. In addition, the researcher will evaluate the neutron background for CEνNS searches at ESS. Such work will be of interest to other users of the facilities.
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.
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.
In the first months of this year, the pure CsI quenching factor analysis will be finished and published, as the publication is already being prepared. This will be the first reported quenching factor for CsI at such energies. In parallel, a second production attempt of LAAPDs in University of Chicago will start shortly. The researcher will guide and coordinate a young PhD student to implement the procedure he developed during his stay at University of Chicago and will, if needed, travel to University of Chicago and support the development. Finally, the simulation of the neutronic backgrounds is expected to be completed before summer, with CEνNS simulations to follow to fully understand the expected significance of the experiments and assert if additional background may be needed.
On DIPC, a small time projection chamber will be mounted in March. The chamber will be used to characterize the low-energy response of gaseous argon, xenon and krypton. The study will focus first on characterizing electron signals in argon using various gamma sources (83mKr, 55Fe, 133Ba, 22Na and 241Am). This selection gives a broad energy range, ideal for linearity studies. The same sources will later be used in gaseous Xe. The head start with argon is due to safety issues, as xenon is much more expensive than argon, therefore the first calibration stage will also be dedicated to certificate the gas and recirculation system. By the end of the year neutron sources will be used. While the exact sources may change due to market availability and safety procedures of the institution, the sources that are currently being considered are 252Cf, a DD neutron generator and the same photoneutron sources that were used during the first phase of the project. By comparing the electron and neutron recoil response the quenching factor of the different noble gases will be measured for the first time in their gaseous form.
On DIPC, a small time projection chamber will be mounted in March. The chamber will be used to characterize the low-energy response of gaseous argon, xenon and krypton. The study will focus first on characterizing electron signals in argon using various gamma sources (83mKr, 55Fe, 133Ba, 22Na and 241Am). This selection gives a broad energy range, ideal for linearity studies. The same sources will later be used in gaseous Xe. The head start with argon is due to safety issues, as xenon is much more expensive than argon, therefore the first calibration stage will also be dedicated to certificate the gas and recirculation system. By the end of the year neutron sources will be used. While the exact sources may change due to market availability and safety procedures of the institution, the sources that are currently being considered are 252Cf, a DD neutron generator and the same photoneutron sources that were used during the first phase of the project. By comparing the electron and neutron recoil response the quenching factor of the different noble gases will be measured for the first time in their gaseous form.