From tigwiki

Author: EFZ on July 19, 2005

[edit] Notes on PACES

• The preamps can be powered up when warm, but NOT if light is available to the Si(Li). You should connect/disconnect preamp power at the distribution box which is cable tied to the Dependix Flange.

• It takes 6 hours to cool. You CANNOT apply HV bias before about 6 hours. Full equilibrium will take a few more hours.

• HV bias is NEGATIVE 500 volts.

• When filling the dewar, use the funnel and black delrin insert provided. Stick for measuring depth is nearby. Insert stick vertically so as not to stick it into the discharge hole in the center of the dewar. Liquid will come out when it is full.

• A full dewar will register about 8 inches on the stick. Top off the dewar after 6 hours following the first filling because so much LN2 wil be lost to "cool down". After that, the rate of useage is about 1/4 inch per hour. BE SURE TO PUT THE CAP BACK ON THE DEWAR.

• A fill in the morning and one in the evening and one in the evening (about 12 hours apart) is recommended. Record date and time for each fill on the log book provided.
  • Note: Should LN2 run out with the HV bias still on, the FETs will "pop".

• Aluminum foil is required over the delrin hemispheres to provide electronic shielding when the array is open. The foil should be in contact with the PACES side of the beamline, but not in contact with the MTC side of the beamline. Use Saran wrap on the MTC side between the beamline and the aluminum foil, or just stop the foil short of the aluminum beam pipe on the MTC side.

• Major errors:
  • Applying HV bias with the system warm
  • Warming the system with the HV bias still on
  • Opening the vaccum before system is at room temperature
    • Note: A catastrophic vacuum failure on a cold system with a HV bias on will take out all crystals and all FETs.

[edit] PACES START UP

• System has been cooling for at least 6 hours.

• Connect preamp power (connect cable to PACES and turn on preamp power supplies).

• Monitor output of preamp on scope. Use 20 millivolt scale and 2 microsecond sweep.

• With no HV bias you should see white noise. Sometime you need to change the scope scales to "look" things over. You will see microphonics at lower frequencies and depending upon the laboratory and what else is going on, you may see some other "pick up". Some of that will disappear or be reduced when bias is applied, and much of the rest will be taken care of by the 572 amplifier. Each set-up seems to be unique.

• Apply 20 volts. The white noise should collapse to the 5-10 mV range (sometimes better). This means that things are cabled properly and working properly. You should already see preamp signals. These are negative with a sharp rise and a long tail. They will get better with more bias.

• SLOWLY (1-2 minutes) bring up the bias to 500 volts - keeping an eye on the signals. Increase the voltage in small steps and wait for it to equilibrate before increasing again. If something looks strange, stop and measure leaking current. Note, you must let the system equilibrate before measuring leaking since changing the voltage generates its own leaking reading.

• Repeat for each channel. Cable to the Amplifiers and proceed.

• Since we will not want to reinsert the 207Bi source, the desired amplifier gain and "rough" calibration should be done NOW. The spectra will return to within a few channels after the system is shut down. The precise calibration can be done later using gamma-ray energies and the tabulated electron binding energies.

[edit] PACES SHUT DOWN

• SLOWLY (1-2 minutes) reduce HV bias to zero. Wait at least 2 minutes, then switch HV to "off". (It is not necessary to "monitor" this procedure on a scope, but if you did you would see that you need the extra 2 minutes to wait for the bias to "really" go to zero as indicated by the return of the white noise.)

• Carefully remove the dewar and take care not to spill LN2 on your feet as it runs out the tube at the bottom. Pour out the contents from the top and place the dewar in its "slot" in the PACES storage box. Later, when "warm and dry", screw on the "fill-tube protector stub".

• You may consider removing the preamp power - depending on what you have planned next.

• For a a quick warm up (6-7 hours)
  • Blow air into the reservoir. The first "blow" will eject the remaining LN2.

• For a slow warm up (overnight)
  • Use your mouth and a small tube to eject the remaining LN2. Let warm overnight.
    • Note: If you use the "slow warm up", then you MUST dry out the reservoir before replacing the dewar for the next cool-down. Use a KimWipe.

• For a very slow warm up (up to 44 hours)
  • Allow enough time for the remaining LN2 to be used up (1 hour per 1/4 inch) and for PACES to warm up (12 hours)

• NOTE NOTE NOTE: After the LN2 has been removed, the vacuum will, during the next hour, go up to the 10-5 range and even possibly "spike" in the 10-4 range if PACES has been cold a long time. Therefore, it would be best to have the operator close the valve to the beamline when you warm PACES. After the "spike" the pressure will drop back quickly to the low 10-6 region.

[edit] PACES 207Bi SOURCE PROCEEDURES

• The 207Bi source is an OPEN source. If the activity area is touched, material will be removed. It is also not super stable and should be handled with great care. It is a 1 microcurie source.

• When employing the source, be sure to check the surfaces it has passed over and your hands after you have either inserted it into the chamber, or after you returned it to the safe. Wash hands thoroughly.

• Whenever you remove it from the system, be sure to check both halves of the delrin sphere for possible contamination. Take care, however, not to touch the detectors with the probe.

• The small aluminum disk that is in the source "container box" covers the active area which is recessed. The sponge goes on top of the aluminum disk so that you can read the information about the source through the other side of the "container box".