Measurement of Ge detector by New cooling KEK Apr 25 th K.Shirotori

Purpose Present Ge detectors are cooled by LN 2. But with this method, the large LN 2 dewar makes Hyperball array geometry difficult to be flexible and filling LN 2 every 12h is cumbersome and difficult to operate in the counter hall in J-PARC. Automatic, Compact, Easy to operate, (+Maintenance free) New cooling system is needed.

Requirements of new cooling system The operating temperature should be the same as LN 2 cooling or better. → ～ 90[K] Ge detector which has sever radiation damage shows resolution degradation for more than 90[K] (energy resolution >10keV). It works stably for several months. Our beam time is 1~2 month long. During beam time, once Ge detectors are warmed up, their resolution will deteriorate because of radiation damage. In such a case they need to be annealed! when achieved Make it as compact, cheap, and easy operation as possible while low maintenance and so on. (Next development)

PopTop type detector Present status of LN 2 cooling and energy resolution of 60 Co 1.3MeV γ-ray Ge Easily removable Relative 1MeV γ-ray →30% (compared with NaI 3Φ) P-type semiconductor Size 5cmΦ×5cm Ge Cooling machine LN2 Dewar

Temperature LN2 Temperature after LN 2 filling. It is measured at thermo couple. It takes 12 hours to reach the final (equalibullium) temperature. The operating temperature is 97[K].

Energy Tohoku Univ. HV (P-type semi-con) 60Co 1.33MeV 6μs shaping 671 Amp DAQ Amptek MCA8000A (maru-chan) Set up of measurement 1.72[keV] (3μs is 1.80[keV])

New cooling system “ Pulse tube ”

Cooling Mechanism The cooling mechanism is simply a heat cycle →2 : Isothermal absorption ( 等温吸熱 ) 2→3 :Adiabatic compression ( 断熱圧縮 ) 3→4 :Isothermal heat rejection ( 等温放熱 ) 4→1 :Adiabatic expansion ( 断熱膨張 ) TCTC THTH THTH THTH TCTC

What to check in KEK Can new cooling system reach the LN 2 cooling temperature ～ 90[k] or under the threshold temperature where radiation damage appears, ～ 85[k] ? Dose the oscillation of compressor for pulse tube cooler make energy resolution worse ? Are there any problems when operating new cooling system ?

Set KEK

Cooling machine The final (steady states) temperature is 77[K]. The goal that temperature is below 85[K] is achieved. But, after switching on the preamp of Ge detector, the heat from amp makes temperature increased to 84[K]. The temperature of Ge is sensitive to the increase from external environment. The connection and electrical insulation between Ge and the cooling machine are not optimized.

Energy KEK Energy resolution is very bad because of a lot of noise, such as motor of vacuum pump, power supply of the cooling machine and …. The oscillation of cooling machine and noise from power supply are large factors which make energy resolution worse. Present resolution is 12[keV]. When the cooling machine is switched off, energy resolution is 6.5[keV] (still a lot of noise).

Problems and things to do The heat leak around the connection between Ge detector and the cooling machine. → improvement on this connection. The oscillation of cooling system. Noise. But….. They were almost expected before this measurement. We gained some information whether or not these problems can be solved.

Next step The first thing is to construct new connection parts between Ge detector and the cooling machine. This improvement will reduce the heat leak, the oscillation and noise. The second is to cool and measure the resolution again and to test the long time stability.

Summary The new cooling system has comparable or better performance than LN2 cooling. There are a lot of problems that we identified, but these problems are expected and we think they are manageable. The next point of development is clear.

一目瞭然