Abstract
We have measured the ionization efficiency of silicon nuclear recoils with kinetic energy between 1.8 and 20 keV . We bombarded a silicon-drift diode with a neutron beam to perform an elastic-scattering experiment. A broad-energy neutron spectrum was used and the nuclear recoil energy was reconstructed using a measurement of the time of flight and scattering angle of the scattered neutron. The overall trend of the results of this work is well described by the theory of Lindhard et al. above 4 keV of recoil energy. Below this energy, the presented data shows a deviation from the model. The data indicates a faster drop than the theory prediction at low energies.
Highlights
The development of technologies for detecting low energy nuclear recoils has been a very active field in recent years, mainly driven by dark matter searches and coherent neutrino nucleus scattering (CENNS) experiments
Silicon target (SiDet) neutron detector θr electron equivalent, so-named because an electron recoil, because an electron recoil transforms all its kinetic energy in ionization), and energy in the SiDet (ENR) is quantified in eVNR
The nuclear recoils deposit their kinetic energy in the silicon detector (SiDet) producing an ionization signal (Ei), while the neutrons, scattered off the target, are detected again by a secondary neutron detector
Summary
The development of technologies for detecting low energy nuclear recoils has been a very active field in recent years, mainly driven by dark matter searches and coherent neutrino nucleus scattering (CENNS) experiments. In order to calibrate the response of the semiconductor detectors used for these searches, a series of experiments measured ε between 4 and 20 keVNR in silicon during the early 1990’s These measurements showed relatively good agreement with the Lindhard model [4,5,6] in this energy range. Current experiments using semiconductor detectors that measure nuclear recoils in the sub-keV range include COGENT [10], DAMIC [11], EDELWEISS [12] and SuperCDMS [13] These low-threshold experiments demand a new effort in nuclear recoil calibration at lower energies. Ionization production by nuclear recoils in silicon has recently been measured in the energy range [0.7, 2] keVNR using a photoneutron source [16] This result indicates for the first time a significant deviation from the Lindhard model. The presented result maps the transition between low-energy measurements [16] and previous measurements, consistent with the Lindhard model at higher energies [4,5,6]
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