Investigation of wavelength shifter properties of p-terphenyl and TPB

Wavelength shifter materials like pTerphenyl and Tetra Phenyl Butadiene (TPB) of thicknesses between 65 and 250 μg/cm have been studied in order to improve the quantum efficiency (QE) of a photomultiplier in the UV range. The best result has been obtained for p-Terphenyl of 100 μg/cm thickness. In this case the quantum efficiency integrated over a broad range of photon energies up to 6.2 eV shows a gain of a factor 1.6 compared to an uncovered photomultiplier. Introduction The rate of photons from Cherenkov radiation per energy bin is constant (dN/dE = const) therefore the bulk of the photons is produced in the UV and far UV region. The quantum efficiency of standard photomultipliers with glass windows reaches only 25% around 400 nm and drops down essentially to 0% below 250 nm. The aim of this work Photon Energy [eV] 2 3 4 5 6 Q E 0 0.05 0.1 0.15 0.2 0.25 0.3 No WLS 95 ug/cm2 pTer 92 ug/cm2 TPB Absolute QE of PMs coated with TPB and pTerphenyl Figure 1: Quantum efficiency versus photon energy for the uncoated PMT (Photonis XP3102) and with TPB and pTerphenyl coating. was to reinvestigate the potential of the quantum efficiency improvement in the UV region using wavelength shifting substances on top of the glass window. Technical details Two of such wavelength shifter substances have been evaporated in vacuum with well controlled thickness on the entrance windows of several Photonis XP3102 photomultipliers. The quantum efficiency of those photomultipliers was measured in a wavelength range from 200 to 650 nm in a monochromator by comparison to an absolutely calibrated photodiode [1]. Photon Energy [eV] 2 3 4 5 6 R u n n in g In te g ra l [ a. u .] 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 No WLS 95 ug/cm2 pTer 92 ug/cm2 TPB Gain factor for PMs coated w/ TPB and pTerphenyl Figure 2: Gain of quantum efficiency obtained by integration of the QE curves from Fig. 1. ResultsFig. 1 shows the improvement of the quantum efficiencyas a function of photon energy for 95 μg/cm p-Terphenyland 100 μg/cm TPB evaporated on the entrance windowof the PMT compared to the quantum efficiency of an un-treated PMT. TPB improves the efficiency for highest en-ergies (around 5 eV=246 nm) moderately but deterioratesit in the visible region by a factor of 2. For p-Terphenylwe observe a clear improvement of QE in the UV and nochange in the visible range. In order to quantify the over-all QE improvement all curves have been integrated overthe whole energy range. Running integrals are presented inFig.2 and compared to the integral of the pure PMT (whichis normalised to 1 at 6 eV). The value of the running inte-gral for TPB lies below the reference PMT between 3.2 and5.3 eV and finally reaches a gain of 30% only. The resultfor p-Terphenyl clearly shows a gain of 60 % above the un-covered PMT. Extrapolating to the far UV one can expectstill higher gain values.In order to investigate possible ageing processes of thewavelength shifter film in the radiator gas of the futureRICH detector of CBM at FAIR the photomultipliers withan evaporated p-Terphenyl layer have been stored in CO2atmosphere and will be remeasured to monitor their perfor-mance every 6 12 months. References[1] A. Braem et al., NIM A 504 (2003) Pages 19-23GSI SCIENTIFIC REPORT 2008FAIR-EXPERIMENTS-16