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Alan E. Robinson

Publications and source records attributed to Alan E. Robinson.

6 recordsLinked to original sources
arXiv2020

Electrovolt-scale backgrounds from surfaces

Recent results from the SENSEI experiment show that a cut on event clustering can reduce low-energy excesses in their eV-sensitive calorimeter. This hints at the role of surrounding uninstrumented surfaces in producing backgrounds. Charged particles crossing dielectric boundaries are well known to produce low-energy radiation. In particular, transition radiation, secondary electron emission, and sputtering may contribute to the spectrum, morphology, and rate of events observed in eV-sensitive detectors. The rich phenomenology and high yields of these surface processes will complicate comparisons of low-threshold dark matter detectors both to each other and to background models.

astro-ph.IM↗
arXiv2020

XENON1T observes tritium

XENON1T recently reported an excess of low-energy electron recoil events that may be attributable to either new physics or to the radioactive decay of tritium. It is likely that hydrogen is not be effectively removed by the hot zirconium getters deployed in the detector. Cosmogenic activation of the xenon underground is found to be insufficient to describe the observed excess, although gases diffusing out of detector materials from cosmogenic activation on surface may contribute. Changes in the operation of gas purification systems for XENON1T and other liquid nobel gas detectors could both confirm the tritium hypothesis and remove it from the detector.

hep-ex↗
arXiv2017

The coherent photon scattering background in sub-GeV/$c^2$ direct dark matter searches

Proposed dark matter detectors with eV-scale sensitivities will detect a large background of atomic (nuclear) recoils from coherent photon scattering. This background climbs steeply below $\sim10$~eVnr, far exceeding the declining rate of low-energy Compton recoils. The upcoming generation of dark matter detectors will not be limited by this background, but further development of eV-scale and sub-eV detectors will require the use of low-$Z$ target materials, such as helium, to avoid a large rate of coherent photon scattering, or highly efficient methods to reject photons when they scatter.

astro-ph.IM↗
arXiv2016

A reanalysis of radioisotope measurements of the $^9$Be$(γ,n)^8$Be cross-section

The $^9$Be$(γ,n)^8$Be reaction is enhanced by a near threshold $1/2^+$ state. Contradictions between existing measurements of this reaction cross-section affect calculations of astrophysical r-process yields, dark matter detector calibrations, and the theory of the nuclear structure of $^9$Be. Select well-documented radioisotope $^9$Be$(γ,n)$ source yield measurements have been reanalyzed, providing a set of high-accuracy independently measured cross sections. A Breit-Wigner fit of these corrected measurements yields $E_R=1738.8\pm1.9$ keV, $Γ_γ=0.771\pm0.021$ eV, and $Γ_n=268\pm15$ keV for the $1/2^+$ state. A virtual $1/2^+$ state is excluded with 99.3\% confidence.

nucl-ex↗
arXiv2014

New libraries for simulating neutron scattering in dark matter detector calibrations

Dark matter detectors require calibrations of their energy scale and efficiency to detect nuclear recoils in the $1-50$ keV range. Most calibrations use neutron scattering and require MCNP or Geant4 simulations of neutron propagation through the detector. For most nuclei heavier than $^{16}$O, these simulations' libraries ignore the contribution of resolved resonances to the neutron elastic differential cross-section. For many isotopes and neutron energies of importance to dark matter detection, this invalid assumption can severely distort simulated nuclear recoil spectra. The correct angular distributions can be calculated from the resonance parameters using R-matrix formalism. A set of neutron scattering libraries with high resolution angular distributions for MCNP and Geant4 of $^{19}$F, $^{40}$Ar, $^{50,52}$Cr, $^{56}$Fe, $^{136}$Xe, and $^{206,207,208}$Pb is presented. An MCNPX library for simulating the production of low-energy neutrons in the $^9$Be$(γ,n)^8$Be reaction is also presented. Example dark matter detector calibrations are simulated with the new libraries showing how detector sensitivity could be overestimated by factors of two by relying on existing MCNP and Geant4 libraries.

nucl-ex↗