NSUF 25-5405: Investigation of Anomalously High Ionization Produced by 254 eV Nuclear Recoils in Germanium
Detailed characterization of detector response to nuclear recoils is crucial for the detection of neutrinos via coherent elastic neutrino-nucleus scattering (CEvNS). In high-purity germanium (HPGe) detectors, the amount of recoil kinetic energy yielded as ionization is measured. The ionization yield is well understood at energies above ~100 keV; however, measurements for energies under ~10 keV vary considerably. Determining ionization yield requires the production of nuclear recoils of known energy, which is difficult at the low energies relevant for CEvNS. With germanium, the thermal neutron capture reaction 72Ge(n, γ)73Ge produces the excited metastable state 73mGe, which decays to the ground state of 73Ge via a gamma-ray cascade. The gamma-ray emission induces 73Ge nuclear recoils of known energy due to conservation of momentum. Previous measurements at the Ohio State University (OSU) Thermal Neutron Beam Facility with RTE support utilized this process to measure the germanium ionization yield at 254 eV. These experiments measured an ionization yield significantly higher than both theoretical predictions, but suffer from possible systematic error due to reliance on a decades-old nuclear data value for the energy of the final ~68.75 keV gamma ray in the decay cascade. We propose to perform a campaign at the OSU Thermal Neutron Beam Facility to remeasure this gamma-ray energy using a germanium sample and a CdZnTe (CZT) gamma-ray detector. CZT is well suited for this measurement because it provides energy resolution as close as possible to HPGe without the risk of systematic effects associated with 72Ge(n, γ)73Ge reactions in the detector. Continuous calibration in the region of interest will be established via the 241Am 59.5 keV gamma ray as well as the 75.0 keV and 72.8 keV Pb X rays. This experiment will provide a definitive ionization yield value for germanium at 254 eV when paired with our previous results.
Additional Info
| Field | Value |
|---|---|
| Awarded Institution | University of Michigan |
| DOI | 10.46936/NSUF/60015379 |
| Embargo End Date | 2027-09-03 |
| Facility Tech Lead | Raymond Cao |
| Irradiation Facilities | Ohio State University Research Reactor |
| NSUF Call | FY 2025 RTE 2nd Call |
| PI | Igor Jovanovic |
| Project Member | Professor Igor Jovanovic, Professor - University of Michigan (https://orcid.org/0000-0003-0573-3150) |
| Project Type | RTE |