Development of wavelength shifter coated reflectors for the ArDM argon dark matter detector

Abstract

To optimise the design of the light readout in the ArDM 1-ton liquid argon dark matter detector, a range of reflector and WLS coating combinations were investigated in several small setups, where argon scintillation light was generated by radioactive sources in gas at normal temperature and pressure and shifted into the blue region by tetraphenyl butadiene (TPB). Various thicknesses of TPB were deposited by spraying and vacuum evaporation onto specular 3MTM-foil and diffuse Tetratex® (TTX) substrates. Light yields of each reflector and TPB coating combination were compared. Reflection coefficients of TPB coated reflectors were independently measured using a spectroradiometer in a wavelength range between 200 and 650 nm. WLS coating on the PMT window was also studied. These measurements were used to define the parameters of the light reflectors of the ArDM experiment. Fifteen large 120 × 25 cm2 TTX sheets were coated and assembled in the detector. Measurements in argon gas are reported providing good evidence of fulfilling the light collection requirements of the experiment.

Liquid noble gas detectors for low energy particle physics

“…For example, p-terphenyl emits mostly between 280 nm and 350 nm, diphenyloxazolyl-benzene (POPOP) shifts the light to the region 340-420 nm, diphenyloxazole (PPO) emits between 305 nm and 365 nm [229]. Tetraphenyl butadiene (TPB), which emits in the blue wavelength region from 400 to 470 nm [230,231,232], is a popular choice for large liquid argon detectors [233,234]. A more complete list of wavelength shifting materials can be found in [9, p.76].…”

“…The active volume is viewed by 14 hemispherical 8-inch photomultipliers placed at the bottom (Hamamatsu R5912-02MOD bialkali tubes with platinum underlay to operate at LAr temperature). TPB is used to wavelength-shift the argon light; a thin transparent layer is deposited over the PMT windows, while all other surfaces are covered with a PTFE fabric (Tetratex) coated with an opaque TPB layer, which works also as a reflector [234]. A light yield of 1 photoelectron per keV at zero electric field was predicted for the completed chamber based on tests with 7 out of 14 PMTs [398], which would enable a nuclear recoil energy threshold of 30 keV as originally planned.…”

Wavelength Shifters for Applications in Liquid Argon Detectors

“…Liquid argon (LAr) is used as a scintillator in a number of fundamental physics experiments. These include direct dark matter searches (WArP [1], DEAP [2], Mini-CLEAN [3], ArDM [4], DarkSide [5]), prototype and full-scale neutrino oscillation experiments (ICARUS [6], MicroBooNE [7], LArIAT [8], SBND [9], ProtoDUNE [10], DUNE [11]), experiments searching for neutrinoless double beta decay (0νββ-decay) (GERDA [12], LEG-END [13]) and measuring the coherent elastic neutrino-nucleon scattering (CENNS-10 [14]). Other applications of LAr-based detectors include medical physics (PET) [15] and nuclear or homeland security [16,17].…”

Study of the material photon and electron background and the liquid argon detector veto efficiency of the CDEX-10 experiment

“… Reflections The light reflection on the TPB coating is almost completely (>97%) diffuse, irrespective of the TPB coating thickness [15]. The reflection coefficient in the TPB emission spectral region was measured to be is about 95% [10,16].…”