8π Ge Detector Testing

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Resolution and DC Offset Test

The resolution and DC offset of an 8pi detector should be measured whenever the detector is pumped on or adjustments are made to it. The test is performed with the MultiChannel Analyzer (MCA) and radioactive sources of 60Co and 241Am. A test sheet can be found here Media:8PI_GE_TEST_SHEET.pdf

Preamp Adjustments

Three things can be adjusted on the preamp, the Bias Shutdown, charge-loop and the pole-zero. To access the preamp:

  • First remove the white plastic end cap
  • Remove the metal tube which covers and holds the BGOs in place.
  • Remove the screws which hold the preamp cover in place and slide it open.
  • The positions of the adjustment resistors are shown in these photos.
Preamp setup adjustments
  • On the new square preamps, adjust the pole-zero and bias shutdown on the resistors under the circuit board, on the left.

Bias Shutdown Adjust

Resistor connected to thermal resistor connectors during BSD adjustment
Resistance vs Temperature

When the detector is cold the bias shutdown signal gives a signal of -24V, when the detector is warm this signal changes to +5V. To measure the signal apply preamp power and connect a cable to the BSD cable (It has a blue label on the connector) and an oscilloscope with a 1MΩ termination. Alternatively use a Digital VoltMeter (DVM) to probe between the BSD signal and ground.

  • The BSD should be adjusted so that the signal changes around -150 degrees centigrade which is when the thermal resistor measures 196Ω.
  • See the relationship between temperature and resistance in the list to the right
  • To simulate this temperature remove the green and black cables from the left-hand side of the preamp and connect them by a 196Ω resistor as shown in the diagram where one 100Ω and two 48Ω resistors are connected in series to make 196Ω.
  • Connect either an oscilloscope or DVM to the BSD signal as described above.
  • Next adjust the BSD by turning the screw on the BSD Adjust resistor (R11) on the right-hand side of the preamp.
  • The BSD should be set to the point at which it just changes between -24V and +5V.
  • Remove the 150Ω resistor and reconnect the thermal resistor cables to the correct cryostat pins.
  • Replace the preamp cover ensuring the grounding cable is seated correctly in the groove.



Charge-Loop Adjust

  • Open the preamp cover as described above.
  • Use a Digital VoltMeter (DVM) to measure the DC voltage between the ground (Black probe) and TP3 (Red Probe). TP3 is either end of the red wire on the left-hand side of the preamp as shown in the photo above. To get a ground signal rest the black probe in the screw hole of the preamp cover.
  • Now turn the screw on the charge-loop adjust resistor on the right-hand side of the preamp. Adjustment should be made so that the voltage is as close to 0mV as possible.
  • Replace the preamp cover ensuring the grounding cable is seated correctly in the groove.



Pole-Zero Adjust

The detector must be at liquid nitrogen temperature to perform this sdjustment.

  • Remove the preamp cover as described above.
  • Rotate the detector so the Pole-Zero (P/Z) adjust resistor is easily accessible without touching the detector.
  • Connect the output signal to an oscilloscope.
  • Apply preamp power for 15 minutes.
  • Apply the rated Bias voltage to the detector. Leave the detector for 30 minutes.
  • Place a 137Cs source in sight of the detector.
  • The oscilloscope should have 50Ω termination and trigger on a negative slope at a level around 5mV below the baseline. The x and y ranges should be around 400μs and 10mV respectively.
  • It can help to change to an AC signal on the oscilloscope to compensate for a large DC offset induced by the preamp whilst viewing the signal on a fine y scale.
  • Use a plastic screwdriver (cable ties work pretty well) to change the P/Z adjust resistor.
  • Power down the detector.
  • Replace the preamp cover ensuring the grounding cable is seated correctly in the groove.

Replacing the HV Filter

  • Remove the preamp cover.
  • The HV Filter is the large black box found in the preamp that the HV supply cable goes into.
  • Terminate the HV supply cable as shown below.
  • Remove the top preamp plate by taking out the screws shown in the first photo below.
  • Remove the screws in the top of the HV filter and remove it.
  • Insert the new HV filter ensuring the central pin goes into the hole and screw into place.
  • Replace the preamp top panel ensuring the black rubber seals are in place around each cable, the LED is inserted into the hole.
  • Replace the preamp cover ensuring the HV filter grounding cable is correctly seated in the groove.


Replacing the Preamp

Installing a new square preamp:

  • Remove the two screws in the preamp cover sleeve and slide the cover down the detector to remove.
  • Remove the preamp plate on the side of the preamp circuit board by taking out the two screws. (A 3/32" hex driver or allen key should fit.)
  • Remove the two screws holding the old preamp in place.
  • If you are replacing the preamp with the same type, make a note of the orientation of the connections. Take care not to bend the pins when removing the preamp.
  • When attaching the new preamp, the orientation of the round white teflon connectors should be such that the flat side is facing the HV filter. Don't bend the pins.
  • Screw the preamp into position through the small brackets.
  • Replace the preamp plate. Make sure the cables are fed cleanly through the hole, through the black rubber seal. Check that the grounding cable is seated in the groove.
  • Tape a sheet of insulating plastic over the preamp before replacing the preamp cover.
  • If changing the preamp cover, make sure the new cover has a label including, most importantly, the serial number of the detector and its bias voltage and polarity.

Pumping and Toasting

8pi Detector pumping/toasting setup in the Tigress lab (Room 156)
Vacuum gauges and controls
The detector valve
Attaching the valve opening device
Detector valve is open
Screw B is engaged with detector valve thread but detector valve is closed
Heating rod control box
Spinning down the turbo whilst venting with nitrogen gas

Many of the common problems observed in these Ge detectors can be solved by regenerating the vacuum. This is done with the vacuum system setup in the Tigress lab (Room 156). The proceedure is described below.

Setting up and Opening the Detector Valve

  • Slide the holding plate into the pumping stand
  • Place the detector in the holding plate, resting the nose on a piece of foam on top of the turbo punp
  • Rotate the detector so that the detector valve is facing you
  • Tighten the bolts around the dewar DO NOT TIGHTEN THE BOLTS SO MUCH AS TO DENT THE DEWAR!
  • Remove the detector valve cover
  • Slide the valve-opening device over the detector valve and tighten the two small bolts with a 3/32 allen key
  • Ensure all vent valves on the vacuum system are closed
  • Switch on the black fan by plugging it in and point it towards the turbo pump. This is to ensure that the turbo pump stays cool
  • Open the gate valve between the turbo and the roughing pump by plugging in the lead
  • Plug in the roughing pump
  • Observe the pressure change on the roughing pump gauge (Granville-Phillips 375 convectron) and TC2 on the main gauge box (Varian UHV2C)
  • Wait until the roughing pump valve reads less than about 60 millitorr (It will get down to around 6 millitorr if left for a long time) NEVER START THE TURBO BEFORE THE PRESSURE IS LESS THAN 100 millitor
  • Switch on the turbo by pressing 'start' on the control box (Varian, Turbo - V 301). The turbo pump takes about one minute to get up to normal operating speed of 56 krpm
  • Once the turbo is up to speed switch on the EMIS IG by pressing 'EMIS' on the main gauge box
  • The pressure should now be displayed on the 'IG' panel of the main gauge box
  • Wait for the pressure to get down to the mid 10-7 torr range
  • Open the valve to the detector slowly, observing the pressure change
    • To open the valve turn screw A anti-clockwise to allow screw B to slide towards the detector valve
    • When screw B is touching the detector valve turn screw B anti-clockwise until you feel the it engage with the thread of the detector valve (This is to ensure the screw is not cross-threaded)
    • Turn screw B clockwise until tight to fully engage with the detector valve
    • Slowly turn screw A clockwise to pull screw B and the detector valve open - You will see the pressure change
    • Screw A clockwise as far as possible to fully open the detector valve
  • Let the system pump on the detector

Toasting

At this point if the detector is also going to be toasted then follow these instructions:

  • Insert the heating rod into the dewar and tighten the locking nut
  • Set the heating rod control box to 80 degrees
  • Switch on the heating rod - The light on the front of the control box will come on
  • Heat for 24 hours
  • Switch off the heating rod at the control box
  • When the rod has cooled remove it from the dewar
  • Allow the detector to fully cool down to room temperature before closing the valve to the detector, overnight is good

Closing the Detector Valve and Venting the System

  • Close the valve to the detector - Basically the reverse of opening it
    • Screw A anti-clockwise so screw B and detector valve move towards detector
    • Screw B anti-clockwise to disengage with detector valve threads
    • Pull screw B away from detector and hold it in that position
    • Screw A clockwise until tight to hold screw B away from detector
  • Switch off turbo by pressing 'stop' on the control box (Varian, Turbo - V 301)
  • Switch off roughing pump by unplugging the lead
  • The safest way to spin down the turbo is by the vent valve on the turbo itself indicated in the photo to the right. Dont use the green valve of the vacuum system
    • Hold a supply of nitrogen close to the turbo vent valve so that nitrogen populates the vacuum system instead of air. This will make pumping down quicker next time
    • Carefully and slowly open the turbo vent valve whilst monitoring the pressure with the emis gauge.
    • Switch off the EMIS ion gauge when it reads 10-4 and use the roughing pump gauge and TC2 instead
    • Let the pressure increase slowly up to atmosphere
  • Switch off the black fan by unplugging it
  • Undo the bolts of the detector valve opening device with a 3/32 allen key and remove it from the detector
  • Cover the end with aluminium foil to prevent dust and dirt getting in
  • Replace the cover of the detector valve
  • Undo the dewar holding bolts and remove the detector from the holding plate