Characterization of large area photomultipliers and its application to dark matter search with noble liquid detectors

There is growing interest in the use of noble liquid detectors to study particle properties and search for new phenomena. In particular, they are extremely suitable for performing direct searches for dark matter. In this kind of experiments, the light produced after an interaction within the sensitive volume is usually read-out by photomultipliers. The need to go to masses in the tonne scale to explore deeper regions of the parameter space, calls for the use of large area photomultipliers. In this paper we address the need to perform laboratory calibration measurements of these large photomultipliers, in particular to characterize its behaviour at cryogenic temperatures where no reference from the manufacturer is available. We present comparative tests of phototubes from two companies. The tests are performed in conditions similar to those of operation in a real experiment. Measurements of the most relevant phototube parameters (quantum efficiency, gain, linearity, etc.) both at room and liquid Argon temperatures are reported. The results show that the studied phototubes comply with the stringent requirements posed by current dark matter searches performed with noble-liquid detectors.This work has been supported by CICYT Grants FPA-2006-00684, FPA-2002-01835 and FPA- 2005-07605-C02-01

Study of infrared scintillations in gaseous and liquid argon - Part II: light yield and possible applications

We present here a comprehensive study of the light yield of primary and secondary scintillations produced in gaseous and liquid Ar in the near infrared (NIR) and visible region, at cryogenic temperatures. The measurements were performed using Geiger-mode avalanche photodiodes (GAPDs) and pulsed X-ray irradiation. The primary scintillation yield of the fast emission component in gaseous Ar was found to be independent of temperature in the range of 87-160 K; it amounted to 17000+/-3000 photon/MeV in the NIR in the range of 690-1000 nm. In liquid Ar at 87 K, the primary scintillation yield of the fast component was considerably reduced, amounting to 510+/-90 photon/MeV, in the range of 400-1000 nm. Proportional NIR scintillations (electroluminescence) in gaseous Ar were also observed; their amplification parameter at 160 K was measured to be 13 photons per drifting electron per kV. No proportional scintillations were observed in liquid Ar up to the electric fields of 30 kV/cm. The applications of NIR scintillations in dark matter search and coherent neutrino-nucleus scattering experiments and in ion beam radiotherapy are considered.Comment: 20 pages, 11 figures. Submitted to JINS

Design of a pulsed x-ray system for fluorescent lifetime measurements with a timing accuracy of 109 ps

We describe the design of a table-top pulsed x-ray system for measuring fluorescent lifetime and wavelength spectra of samples in both crystal and powdered form. The novel element of the system is a light-excited x-ray tube with a tungsten anode at +30 kV potential. The S-20 photocathode is excited by a laser diode with a maximum rate of 10 MHz, each pulse having [lt]100 ps fwhm (full-width at half-maximum) and [gt]107 photons. In a collimated 2 mm [times] 2 mm beam spot 40 mm from the anode we expect [gt]1 x-ray per pulse. A sample is exposed to these x-rays and fluorescent photons are detected by a microchannel PMT with a photoelectron transit time spread of 60 ps fwhm, a sapphire window, and a bialkali photocathode (wavelength range 180--600 nm). The combined time spread of a laser diode, the x-ray tube, and a microchannel tube has been measured to be 109 ps fwhm. To measure wavelength spectra, a reflection grating monochromator is placed between the sample and the PMT