Ionization yield measurement in a germanium CDMSlite detector using photo-neutron sources

Abstract

Two photo-neutron sources, 88 Y 9 Be and 124 Sb 9 Be, have been used to investigate the ionization yield of nuclear recoils in the CDMSlite germanium detectors by the SuperCDMS collaboration. This work evaluates the yield for nuclear recoil energies between 1 and 7 keV at a temperature of ∼ 50 mK. We use a GEANT4 simulation to model the neutron spectrum assuming a charge yield model that is a generalization of the standard Lindhard model and consists of two energy dependent parameters. We perform a likelihood analysis using the simulated neutron spectrum, modeled background, and experimental data to obtain the best fit values of the yield model. The ionization yield between recoil energies of 1 and 7 keV is shown to be significantly lower than predicted by the standard Lindhard model for germanium. There is a general lack of agreement among different experiments using a variety of techniques studying the low energy range of the nuclear recoil yield, which is most critical for interpretation of direct dark matter searches. This suggests complexity in the physical process that many direct detection experiments use to model their primary signal detection mechanism and highlights the need for further studies to clarify underlying systematic effects that have not been well understood up to this point.

Direct measurement of the ionization quenching factor of nuclear recoils in germanium in the keV energy range

“…The data follow the Lindhard theory and are in agreement with several previous measurements that were performed at liquid nitrogen temperature in the 0.5-10 keV nr range [20][21][22][23]. However, this result is found to fully disagree with a recent precision measurement at 50 mK [19] in which a significant drop of the quenching factor below 7 keV nr was found. In this case the measure-ment technique and experimental conditions (temperature, electric field) were nevertheless significantly different from the ones reported in this article.…”

“…The neutron beams used for this experiment were produced by bombarding metallic lithium targets with proton beams from the 2 MV Tandetron accelerator of PIAF. The advantage of metallic lithium target compared to the commonly used lithium fluoride targets is the increased neutron yield per unit target thickness and the reduced yield of parasitic highenergy photons from 19 F(p,αγ ) 16 O reactions which could cause an unwanted background increase. The proton beams were produced in pulsed mode with a repetition frequency of 1.25 MHz using the chopper/buncher system in the injection beam line.…”

“…At these energies, precise experimental measurements of the ionization quenching factor are still lacking and the overall validity of the Lindhard theory has been questioned [12][13][14][15]. Moreover, recent measurements have shown a deviation with respect to the prediction in silicon [16,17] and germanium [18,19], although previous measurements [20][21][22][23][24] below 10 keV nr were in agreement within uncertainties.…”

Neutron capture-induced silicon nuclear recoils for dark matter and CE$ν$NS

“…Calibrations using thermal neutron induced captures are superior to other styles of calibrations that have been used: direct elastic neutron scattering [15], photoneutron sources [16,17], and 252 Cf sources [18]. Figure 7 shows that the spectrum has sharp mono-energetic features that are lacking in wide-band photoneutron or 252 Cf sources.…”

Snowmass 2021 Scintillating Bubble Chambers: Liquid-noble Bubble Chambers for Dark Matter and CE$ν$NS Detection

“…This approach has two problems. First, it measures only a fraction of the available energy: in the case of electron-recoil detection the kinematics of dark matter-electron scattering limit the allowed energy transfer, and in the case of nuclear recoil detection only a small fraction (in some cases <10% [14]) of the transferred energy goes towards ionization due to the Lindhard effect [15]. Second, with only a single sensitive energy deposition channel it is impossible to distinguish electron-and nuclear-recoil signals, putting quasi-background-free operation well out of reach for both current and next-generation experiments.…”