Measurement of the scintillation yield of low-energy electrons in liquid xenon

Abstract

We have measured the energy dependence of the liquid xenon (LXe) scintillation yield of electrons with energies between 2.1 and 120.2 keV, using the Compton coincidence technique. A LXe scintillation detector with a very high light detection efficiency was irradiated with $^{137}\mathrm{Cs}$ $\ensuremath{\gamma}$ rays, and the energy of the Compton-scattered $\ensuremath{\gamma}$ rays was measured with a high-purity germanium detector placed at different scattering angles. The excellent energy resolution of the high-purity germanium detector allows the selection of events with Compton electrons of known energy in the LXe detector. We find that the scintillation yield initially increases as the electron energy decreases from 120 to about 60 keV but then decreases by about 30% from 60 to 2 keV. The scintillation yield was also measured with conversion electrons from the 32.1 and 9.4 keV transitions of the $^{83m}\mathrm{Kr}$ isomer, used as an internal calibration source. We find that the scintillation yield of the 32.1 keV transition is compatible with that obtained from the Compton coincidence measurement. On the other hand, the yield for the 9.4 keV transition is much higher than that measured for a Compton electron of the same energy. We interpret the enhancement in the scintillation yield as due to the enhanced recombination rate in the presence of Xe ions left from the 32.1 keV transition, which precedes the 9.4 keV one by 220 ns, on average.