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Abstract P-type point contact (PPC) HPGe detectors are a leading technology for rare event searches due to their excellent energy resolution, low thresholds, and multi-site event rejection capabilities. We have characterized a PPC detector’s response to $$\alpha $$ α particles incident on the sensitive passivated and p $$^+$$ + surfaces, a previously poorly-understood source of background. The detector studied is identical to those in the Majorana Demonstrator experiment, a search for neutrinoless double-beta decay ( $$0\nu \beta \beta $$ 0 ν β β ) in $$^{76}$$ 76 Ge. $$\alpha $$ α decays on most of the passivated surface exhibit significant energy loss due to charge trapping, with waveforms exhibiting a delayed charge recovery (DCR) signature caused by the slow collection of a fraction of the trapped charge. The DCR is found to be complementary to existing methods of $$\alpha $$ α identification, reliably identifying $$\alpha $$ α background events on the passivated surface of the detector. We demonstrate effective rejection of all surface $$\alpha $$ α events (to within statistical uncertainty) with a loss of only 0.2% of bulk events by combining the DCR discriminator with previously-used methods. The DCR discriminator has been used to reduce the background rate in the $$0\nu \beta \beta $$ 0 ν β β region of interest window by an order of magnitude in the Majorana Demonstrator and will be used in the upcoming LEGEND-200 experiment.
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Deformation of solid earth by surface pressure: equivalence between Ben-Menahem and Singh’s formula and Sorrells’ formula
SUMMARY Atmospheric pressure changes on Earth’s surface can deform the solid Earth. Sorrells derived analytical formulae for displacement in a homogeneous, elastic half-space, generated by a moving surface pressure source with speed $$c$$. Ben-Menahem and Singh derived formulae when an atmospheric P wave impinges on Earth’s surface. For a P wave with an incident angle close to the grazing angle, which essentially meant a slow apparent velocity $$c_a$$ in comparison to P- ($$\alpha ^{\prime }$$) and S-wave velocities ($$\beta ^{\prime }$$) in the Earth ($$c_a \ll \beta ^{\prime } \lt \alpha ^{\prime }$$), they showed that their formulae for solid-Earth deformations become identical with Sorrells’ formulae if $$c_a$$ is replaced by $$c$$. But this agreement was only for the asymptotic cases ($$c_a \ll \beta ^{\prime }$$). The first point of this paper is that the agreement of the two solutions extends to non-asymptotic cases, or when $$c_a /\beta ^{\prime }$$ is not small. The second point is that the angle of incidence in Ben-Menahem and Singh’s problem does not have to be the grazing angle. As long as the incident angle exceeds the critical angle of refraction from the P wave in the atmosphere to the S wave in the solid Earth, the formulae for Ben-Menahem and Singh’s solution become identical to Sorrell’s formulae. The third point is that this solution has two different domains depending on the speed $$c$$ (or $$c_a$$) on the surface. When $$c/\beta ^{\prime }$$ is small, deformations consist of the evanescent waves. When $$c$$ approaches Rayleigh-wave phase velocity, the driven oscillation in the solid Earth turns into a free oscillation due to resonance and dominates the wavefield. The non-asymptotic analytical solutions may be useful for the initial modelling of seismic deformations by fast-moving sources, such as those generated by shock waves from meteoroids and volcanic eruptions because the condition $$c / \beta ^{\prime } \ll 1$$ may be violated for such fast-moving sources.
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- Award ID(s):
- 2225216
- PAR ID:
- 10610306
- Publisher / Repository:
- Geophysical Journal International
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 238
- Issue:
- 2
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- 820 to 826
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/gji/ggae185
