aa r X i v : . [ a s t r o - ph ] O c t th International Conference on High Energy Physics, Philadelphia, 2008
Development of a large area gas photomultiplier with GEM/ µ PIC
H. Sekiya
Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo456 Higashi-Mozumi, Kamioka, Hida, Gifu, 506-1205 JAPAN
We are developing a new photon detector with micro pattern gaseous detectors. A semitransparent CsI photocathodeis combined with 10cm × µ PIC for the first prototype which is aimed for the large liquid Xe detectors.Using Ar+C H (10%) gas, we achieved the gas gain of 10 which is enough to detect single photoelectron. We, then,irradiated UV photons from a newly developed solid scintillator, LaF (Nd), to the detector and successfully detectedsingle photoelectron.
1. INTRODUCTION
In the last two decades, many gaseous photomultipliers with CsI photocathodes operated in both flushed gas modeand sealed gas mode have been developed and tested[1, 2, 3]. Moreover, recently, large area micro pattern gaseousdetectors with avalanche multiplication structures, such as Micromegas[4], GEM[5], and µ PIC[6] have been developedand successfully operated. These devices with photocathodes can realize a low cost large area photon detector withposition sensitivity and can be applied to large astroparticle detectors. In particular, the quantum efficiency of theCsI photocathode matches the liquid Xe scintillator, thus dark matter search via Xe is one of the first targets of thisphoton detector.In this work, for the first step, we investigated the feasibility of manufacturing a large size gaseous photon detectorwith CsI photocathode and evaluated its performance.
2. THE DETECTOR
The configuration of the prototype detector is shown schematically in Fig. 1. 2 GEMs and a µ PIC were used forthe charge amplification, which allows to suppress the avalanche-induced photon and ion feedback and provide thehigh gain operation.The GEM which was manufactured by SciEnergy Co., Ltd. is 10cm × µ m φ holes of 140 µ m pitch[7],and the insulator is Liquid Crystal Polymer of 100 µ m thick. The µ PIC is the standard type of 10cm × µ PIC has the very fine positionsensitivity, for this prototype detector, the cathode strips and anode strips were summed into 4+4 channels to reducethe number of the readout circuit.The MgF window is 54mm φ and the thickness is 5mm. The CsI photocathode was evaporated to the window byHamamatsu Photonics and the effective area is 34mm φ . In order to apply high voltage (-HV) to the photocathode anAl electrode was also evaporated at the edge of the MgF window. The -HV is supplied to the electrode via contactCu ring. Between the photocatode and the first GEM, a guard ring is placed in order to make a uniform electricfield and to drift photoelectrons to the first GEM.These devices are put in a stainless steal chamber with a gas/vacuum port, therefore, when the gas spoils, we canreplace the gas and we can also exchange the other components. 1 atm of Ar+10%C H mixture gas was loaded andthe chamber was sealed during this measurement.All the components given above are assembled in a nitrogen-sealed glove box to avoid the deliquescence of the CsIphotocathode. 14 th International Conference on High Energy Physics, Philadelphia, 2008
GEM2GEM1 Vacuum portMgFAlCsICu guard ring µ PIC
Figure 1: Schematic drawing of the prototype detector. The size of GEMs and µ PIC is 10cm ×
3. UV PHOTON SCINTILLATOR
The fluoride single crystals have a wide transparent range from the vacuum ultraviolet(VUV) to the infraredregion, therefore, they are used in various applications such as the window material, the host material for the shortwavelength laser and so on. Recently, several studies have been made on fluoride crystals with dopant as scintillatorsthat might find their applications in γ /x-ray detection[8]. Although light yields of these crystals are rather limitedat this point, because of their small light yields and easiness of handling, these crystals can be light sources for theprototype detector instead of liquid xenon.It is known that the scintillation spectrum of the LaF (Nd) is agree with that of liquid Xe (the central wavelengthis 172nm), however the absolute light yield of LaF (Nd) had not been measured yet.Accordingly, we measured the light yields of 15mm × × (Nd) for the first step. The Am(5.5MeV α source) was attached directly to one face of the crystal and the VUV sensitive PMT (Hamamatsu R8778)was attached to the opposite face. Other four sides were covered with GORE-TEX R (cid:13) . The obtained spectrum of5.5MeV α through the LaF (Nd) is shown in Fig. 2. Figure 2: Obtained spectrum of 5.5MeV α through the LaF (Nd). The R8778 was operated with the gain of 2.2 × . As the quantum efficiency of the R8878 at 172nm is 30%, the photon yield of the LaF (Nd) was found to be 100photons/5.5MeV α . We use this LaF (Nd)+ Am as a test light source for the prototype detector. 24 th International Conference on High Energy Physics, Philadelphia, 2008
4. RESPONSE TO THE VUV LIGHT SOURCE
Next, we attached the light source to the prototype detector. The detector was operated with the gas gain of 2.6 × stably without any discharges more than one hour. The typical output signal after the 0.1V/pC charge amplifierand the obtained spectrum are shown in Fig. 3. Figure 3: Left: Typical output signal after the 0.1V/pC charge amplifier. Right: The obtained spectrum. The detector wasoperated with the gain of 2.6 × . It is confirmed that this detector has enough sensitivity to single photo-electron. The quantum efficiency of thisCsI photocathode is about 1% which is consistent with that in Hamamatsu’s data sheet of the semitransparent CsIphotocathode[9].
5. CONCLUSION AND PROSPECTS
A UV photon detector based a semitransparent CsI photocathode combined with large area GEMs and a µ PIC hasbeen reported. It is demonstrated that the detector has the ability of detecting single photoelectron. The quantumefficiency is about 1% as expected.In order to increase the quantum efficiency, a reflective type CsI photocathode (200nm-thick CsI evaporated toone side of the GEM) is being developed and tested now.