Nuclear recoils within a high-purity germanium detector (HPGe) will be produced via a three-step sequence: thermal neutron capture on Ge-72, excitation to a metastable nuclear state of Ge-73m, and decay back to the ground state via gamma emission. Low energy, <1 keV, nuclear recoils will be produced from the emission of high energy, 5.8 MeV gamma rays. A University of Michigan (UM)-owned Ortec GLP low-energy germanium detector will serve as both the nuclear target for irradiation as well as the measurement instrument for the induced nuclear recoils. The detector will be irradiated with 3.4x10^6 n/cm^2/s, while a large BGO detector will be used for tagging the 5.8 MeV gamma ray. All waveforms will be digitized and saved using modern small-footprint CAEN data acquisition systems for offline analysis.

A one-week campaign at the Ohio State University Thermal Neutron Beam Facility is envisioned. The first day will be reserved for set-up and calibration. Three and a half days will be reserved for data taking of the experiment. The last half-day will be used for tear down and packing up. This experiment only requires access to the thermal neutron beam and would not interfere with any experiments at other research facilities at the OSU reactor.

The ionization and/or scintillation produced by nuclear recoils is a key methodology of neutron (and neutral particle) measurement. The ratio of ionization/scintillation as a function of nucleon kinetic energy (denoted the quenching factor) is well understood at higher energies above ~100 keV; however, experimental measurements under ~10keVnr vary considerably both for individual detector materials as well as from detector material to detector material.

This experiment aims at the precision measurement of the ionization produced via ultra-low energy nuclear recoils utilizing modern digitizer and data analysis techniques for better interpretation of the underlying physics. Saving the raw digitized waveforms, as opposed to only saving pulse height data via multi-channel analyzers, will also allow for the reconciliation of several conflicting measurements of low-energy nuclear recoils in germanium specifically.

A better understanding of the micro-physics and detector response of low-energy nuclear recoils will inform both the expected response of next-generation neutral particle detectors. It will also enable the evaluation of the viability of reactor monitoring via coherent elastic neutrino-nucleus scattering (CEvNS), an interaction directly analogous to neutrons scattering off detector nuclei.

In efforts to enhance domestic as well as international nuclear safeguards capabilities, it has recently been proposed to monitor nuclear reactors via their antineutrino emissions. Reactor neutrinos have been observed in multiple experimental efforts at distances of 10's of meters to kilometers from both commercial and research reactor cores However, an effort is still required to produce a viable monitoring system. As discussed in the recent report “NuTools: Nu Tools: Exploring Practical Roles for Neutrinos in Nuclear Energy and Security,” there is a need and opportunity to develop novel antineutrino detectors to monitor advanced nuclear reactors and small modular reactor sites, which have the potential to address both the lack of viable material control and accounting for bulk fuel as well as improvement of economic performance by the reduced need for in-person inspections.



Antineutrino detection is typically achieved using hydrogen-rich detectors and the Inverse Beta Decay (IBD) process where an anti-neutrino interacts with a hydrogen nucleus to produce a neutron and a positron. For necessary statistics, IBD-based detectors are typically large with masses in metric tons. An alternative avenue exists for reactor monitoring based on Coherent Elastic Neutrino Nucleus Scattering (CEvNS: pronounced as ''sevens") where the direct nuclear recoil from the anti-neutrino is observed. The scattering physics of CEvNS is nearly identical to that of the elastic scattering of neutrons off nuclei. This is due to the Z-boson being a common mediator in both interactions. While the ionization (and/or scintillation) signal from nuclear recoils is well understood at higher energies, understanding breaks down at lower energies below ~ 10 keV.

CEvNS was first detected in 2017 with a 14 kg detector. This offers the potential to drastically reduce the mass scale of anti-neutrino detectors thanks to its three orders of magnitude higher cross-section compared to IBD. More recently, High Purity Germanium Detectors (HPGe), have been utilized in various CEvNS experiments and are envisioned as a potential reactor monitoring technology in large part thanks to their high resolution, low thresholds, and the commercial viability of detector arrays with a mass of 5-10 kg. The primary obstacle to CEvNS as a viable reactor monitoring technology is a lack of understanding of the detector response to low-energy nuclear recoils both for germanium as well as other detector media.



This work aims at resolving existing tension between various nuclear recoil experiments and at establishing a more precise Quenching Factor value for germanium at 0.254 keV as an effort to both resolve tension between experimental measurements, evaluate the effectiveness of an HPGe-based CEvNS detector for reactor monitoring, and enhance the general understanding of the underlying physics of low energy nuclear recoils produced by any neutral particle. A potential high impact of this study is that it could provide a pathway for the construction of considerably smaller detectors capable of monitoring nuclear reactors and supporting the safeguards needs of next-generation reactors.