Development of a photon detector using gas-multiplication process

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We started a R & D work to establish a Gas PMT which has potential advantages such as immunity for high magnetic field and easiness to enlarge its size with reasonable cost. In order to detect photons of visible light, we introduce a bi-alkali photocathode into a tip glass tubes, inside of which is filled with Ar/CH4 gas mixture. Then we occasionally measured the quantum efficiency (QE) of the Gas PMT for 600days. We found that the QE is stable for 600days even in such a gas environment. We also confirm that this kind of Gas PMT can detect photons in a high magnetic field of 1.5 Tesla. During the production stage of this kind of Gas PMTs, we noticed that there is incompatibility between bi-alkali photocathode and GEM which is usually made of Kapton. A Gas PMT made with Kapton GEM lost its sensitivity to visible light immediately after activation of photocathode. Same phenomenon is observed for LCD GEM. We recognized that organic GEM adsorb vapor of alkali metals easily. Hence we decide … More d to use Pyrex glass for the material of GEM instead of Kapton and LCD. In order to make Pyrex glass GEMs we introduced sand-blasting (micro-blasting) method, since it is very cheap compared with other methods such as a laser processing. We found that thus produced micro-blasting Pyrex glass GEM (MB-GEM) has almost same performance as Kapton and LCD GEMs. We constructed a Gas PMT with MB-GEM and bi-alkali photocathode operated in a Ar/CH4 gas mixture. Although visible light signals are well seen by this Gas PMT but the gain(G) doesn't obey the equation G∝AV^α, where A and α is a constant and V is the applied voltage to the GEM. The deviation from the equation is understood as feedback phenomenon ; ion-feedback. Ions produced during a gas multiplication process hit the photocathode and create extra electrons. In order to make an operation of Gas PMT more stable, such ion feedback should be minimized. To understand the mechanism of ion-feedback, we made a simulation study by using programs of Maxwell and Garfield, which can provide 3-dimensional trace lines of electrons and ions in a gas mixture. From this simulation study we noticed that Micromegas (a thin metal mesh) can highly suppress ion-feedback than GEM. For instance the suppression factor of Micromegas is 3% while GEM is 20%. Then we tried to produce several types of GEM with a combination of MB-GEM and Micromegas. Though it is not satisfactory, we could have a good sample of Gas PMT by using a double MB-GEM and a Micromegas. In a middle term test of this Gas PMT we found that QE of bi-alkali photocathode degrade substantially due to ion-feedback. Hence further reduction of ion-feedback is vital for a construction of really usable Gas-PMT. Less