Development of low cost school array device for observing wide area cosmic ray air showers

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Low cost school array device consists of gas sealed type resistive plate chamber for detecting cosmic rays, an electronics circuit box which has trigger circuit for cosmic rays and circuits to measure pulse height, arrival time, etc, and a personal computer which perform the overall control. Among these components, the most difficult one to develop was gas sealed type resistive plate chamber. Resistive plate chamber has two types of operation modes. One is streamer mode and the other is avalanche mode. We built test chambers for both types and test the sealed type operation and how long do the chambers work. For the streamer mode chamber, we used argon, iso-butane, and tetrafluoroethane, mixture. As the result of the test, the streamer mode chamber could maintain 95% detection efficiency and no large difference of pulse height for one year. Aging effect, however, was observed since then and about 600 days of operation, glue came out. Avalanche mode signal has two orders of smaller signals, hence the effect to the electrodes and gas is small. Since the avalanche mode test chamber was built later than the streamer mode chamber, test period was shorter. The chamber keeps its performance for more than one year. Since the avalanche mode signals are smaller than the streamer mode signals and avalanche mode chamber is expected to last longer, we built the full size chamber with avalanche mode. The full size chamber is about four times larger. Since the area of the pickup electrode is much larger in the full size chamber than the test chamber, pickup noise was also much larger. To reduce the noise to an acceptable level, we had to divide the pickup electrode in to six. We have made six full size chambers so far. To complete the detector, we put those chambers into the stainless case and put connections, which we have not done yet. Electronics had been complete, which uses low cost parts to reduce the cost.