A spectroscopic study of liquid argon from the vacuum ultraviolet at 110 nm to 1000 nm is presented. Excitation was performed using continuous and pulsed 12 keV electron beams. The emission is dominated by the analogue of the so called 2 nd excimer continuum. Various additional emission features were found. The time structure of the light emission has been measured for a set of well defined wavelength positions. The results help to interpret literature data in the context of liquid rare gas detectors in which the wavelength information is lost due to the use of wavelength shifters.

Abstract. The spectral and temporal light emission properties of liquid argon have been studied in the context of its use in large liquid rare-gas detectors for detecting Dark Matter particles in astronomy. A table-top setup has been developed. Continuous and pulsed low energy electron beam excitation is used to stimulate light emission. A spectral range from 110 to 1000 nm in wavelength is covered by the detection system with a time resolution on the order of 1 ns.

The transmission of liquid argon has been measured, wavelength resolved, for a wavelength interval from 118 to 250 nm. The wavelength dependent attenuation length is presented for pure argon. It is shown that no universal wavelength independent attenuation length can be assigned to liquid argon for its own fluorescence light due to the interplay between the wavelength dependent emission and absorption. A decreasing transmission is observed below 130 nm in both chemically cleaned and distilled liquid argon and assigned to absorption by the analogue of the first argon excimer continuum. For not perfectly cleaned argon a strong influence of impurities on the transmission is observed. Two strong absorption bands at 126.5 and 141.0 nm with approximately 2 and 4 nm width, respectively, are assigned to traces of xenon in argon. A broad absorption region below 180 nm is found for unpurified argon and tentatively attributed to the presence of water in the argon sample.

Vacuum ultraviolet light emission from xenon-doped liquid argon is described in the context of liquid noble gas particle detectors. Xenon concentrations in liquid argon from 0.1 ppm to 1000 ppm were studied. The energy transfer from the second excimer continuum of argon (∼ 127 nm) to the second excimer continuum of xenon (∼ 174 nm) is observed by recording optical emission spectra. The transfer almost saturates at a xenon concentration of ∼10 ppm for which, in addition, an intense emission in the infrared at a peak wavelength of 1.17 µm with 13000 ± 4000 photons per MeV deposited by electrons had been found. The corresponding value for the VUV emission at a peak wavelength of 174 nm (second excimer continuum of xenon) is determined to be 20000±6000 photons per MeV electron energy deposited. Under these excitation conditions pure liquid argon emits 22000 ± 3000 photons per MeV electron energy deposited at a peak wavelength of 127 nm. An electron-beam induced emission spectrum for the 10 ppm Ar-Xe liquid mixture ranging from 115 nm to 3.5 µm is presented. VUV emission spectra from xenon-doped liquid argon with exponentially varied xenon concentrations from 0.1 ppm to 1000 ppm are also shown. Time structure measurements of the light emissions at well-defined wavelength positions in the vacuum ultraviolet as well as in the near-infrared are presented. PACS. 29.40.Mc Scintillation detectors -33.20.Ni Vacuum ultraviolet spectra -61.25.Bi Liquid noble gases arXiv:1511.07723v1 [physics.ins-det]

The scintillation light of liquid argon has been recorded wavelength and time resolved with very good statistics in a wavelength interval ranging from 118 nm through 970 nm. Three different ion beams, protons, sulfur ions and gold ions, were used to excite liquid argon. Only minor differences were observed in the wavelengthspectra obtained with the different incident particles. Light emission in the wavelength range of the third excimer continuum was found to be strongly suppressed in the liquid phase. In time-resolved measurements, the time structure of the scintillation light can be directly attributed to wavelength in our studies, as no wavelength shifter has been used. These measurements confirm that the singlet-to-triplet intensity ratio in the second excimer continuum range is a useful parameter for particle discrimination, which can also be employed in wavelength-integrated measurements as long as the sensitivity of the detector system does not rise steeply for wavelengths longer than 190 nm. Using our values for the singlet-to-triplet ratio down to low energies deposited a discrimination threshold between incident protons and sulfur ions as low as ∼2.5 keV seems possible, which represents the principle limit for the discrimination of these two species in liquid argon.