Absolute number of scintillation photons emitted by alpha particles in rare gases

To determine absolute scintillation yields due to alpha particles in high-pressure rare gases, the number of scintillation photons N_p was measured using a vacuum ultraviolet (VUV) sensitive photodiode with spectral quantum efficiency q_e(λ) measured as a function of wavelength λ. The absolute number of photoelectrons from the photocathode N_pe was measured using a charge-sensitive preamplifier calibrated with respect to charge number. The collection efficiency for scintillation photons F_ce at the photocathode was determined from the solid angle subtended by the photocathode at a scintillation point under the condition that there are no photons reflected off the surrounding walls. NP was determined from N_p = N_pe/(Q_eF_ce), where Q_e is the average quantum efficiency calculated from q_e(λ) and a relative intensity I(λ) of scintillation in rare gases. We measured the luminescence spectra using a VUV monochromator of known efficiency in order to obtain I(λ). Measurements were performed in gaseous argon, krypton, and xenon in the pressure range from 1.01 × 10⁵ Pa to 1.01 × 10⁶ Pa. N_p in xenon at 1.01 × 10⁵ Pa is 1.60 × 10⁵ for 5.49-MeV alpha particles.