TIGRESS Clover Initial Testing
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[edit] Initial Testing of Clovers
[edit] Biasing of Crystals
- There is a set of HV cables for biasing. Connect the shut down and the HV cables between the clover and the HV supply CAEN SY2527 tighv03
- Signal cables from the preamps are colour coded. (Blue, Green, Red and White)
- Full volume energy signals #10 (FV) are connected from each crystal over to the scope.
- To control bias
- ssh midtig03, account tigress, normal password
- telnet tighv03 1527
- Move around shell with tab, arrows.
- The voltages for all four crystals are arranged as a group so that four voltages are increased together.
- Bias is increased at 3 volts/sec. Step up in 100 Volt steps to 500V and then in 500V steps up to recommended operating voltage (written on clover dewar)
- After a few hundred volts inspect the signals to see that (negative) rise time is of order 50-100 ns and fall time ~ 50 us
- If sparking occurs , lower voltage below the sparking voltage and let sit. (condition)
- After full bias is reached, add sources and let sit for a day.
[edit] Preamp gain
- Preamp output should be 200 mV/ MeV.
- With 50 ohm input impedance on scope,then 137Cs line will be about 65 mV.
[edit] Resolution testing
- use 60Co and 241Am sources
- Each of the channels is then fed, in turn, through amplifier to MCA.
- Shaping time 4 us (Canberra uses 6us but 4us appears to give better resolution with this amplifier).
- Adjust the pole-zero.
- calibrate spectrum.
- for FV signals accumulate ~10,000 counts.
- for sectors about 1,600 with sources somewhat closer.
- for back sector must move Am241 around (won’t be seen for sector 7)
- store spectra.
[edit] Preamp Risetime Measurement
It is not necessary to make the four incorrect measurements that I did so they won’t be listed here.
- Feed pulses from HP 8082A pulse generator to test input of clover.
- 1 kHz rate and 1 ns transition time.
- FV output through short cable to scope. Trigger scope from pulse generator.
- select measure on scope to set an automatic fall time measurement.
[edit] Efficiency measurement
Standard Co60 source is placed 25 cm from front of Clover. (There is a modified ruler which is 25 cm long.)
- Accumulate spectra for all four crystals noting live times.
- Calculate the yield for the 1332 keV line in c/s for each crystal.
- Source used was 38.48 kBq on June 1, 2007. Determine time since then in days.
- Half life is 5.2714 years.
- Therefore source strength is now 38480 exp (0.693 * days/365.25/ 5.2714).
Germanium detector efficiencies are measured relative to that of a 3" x 3" NaI crystal which has a relative efficiency of 0.0012 at 25 cm for 1332 keV.
Then crystal efficiency expressed as a percentage = yield / source strength / 0.12 .
