Measurement of Ge detector by New cooling system @ KEK Apr 25th K.Shirotori
Purpose • Present Ge detectors are cooled by LN2.But with this method, the large LN2 dewar makes Hyperball array geometry difficult to be flexible and filling LN2 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 LN2 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)
Present status of LN2 cooling and energy resolution of 60Co 1.3MeVγ-ray LN2 Dewar • PopTop type detector • Relative efficiency @ 1MeV γ-ray →30% (compared with NaI 3Φ) • P-type semiconductor • Size 5cmΦ×5cm Ge Ge Easily removable Cooling machine Ge
Temperature measurements @ LN2 • Temperature after LN2 filling. • It is measured at thermo couple. • It takes 12 hours to reach the final (equalibullium) temperature. • The operating temperature is 97[K].
Energy Resolution @ Tohoku Univ. Set up of measurement • HV +2500 (P-type semi-con) • 60Co 1.33MeV • 6μs shaping time @ 671 Amp • DAQ Amptek MCA8000A (maru-chan) 1.72[keV] (3μsis 1.80[keV])
Cooling Mechanism TH TC TH • The cooling mechanism is simply a heat cycle. 1 2 1→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) 3 4 TH TC
What to check in KEK • Can new cooling system reach the LN2 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 ?
Temperature @ Cooling machine • The final (steady states) temperature is 77[K]. • The goal that temperature is below 85[K] isachieved. • 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 resolution @ 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.