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< End section for Naming Czars - thank you for your help! >
How to set the HV
From: Larry King <king@jlab.org>
Sent: Wednesday, March 9, 2022 12:49
To: Nelson, Janice L. <jnelson@slac.stanford.edu>
Subject: RE: decarad hv settings
Hi there, Janice,
No bother at all, I’m happy to tell you what I know about the HV setting.
I’m assuming you’re using the same G-M tube as the original design. The data sheet for the LND714 tube says the operating range is 450-650V, and they’ll work as low as 380V. Setting the HV is really just a balance between longevity and sensitivity. The tubes are only good for a limited number of interactions, so the higher the voltage, the more interactions and the shorter the lifespan.
When we first rolled the DecaRads out in 2010, we picked 650V for max sensitivity. After a few years of replacing tubes, we opted for a lower bias of 500V. While the cost of the tubes is not exorbitant, the downtime for tube replacement can add up. Since the 10 tubes share a common HVPS, when one tube dies it often affects the response of the remaining tubes. Sometimes the dead tubes just stop responding, but other times it may also present a significant load on the small HVPS and drag down the HV for all.
The HV for each chassis is set with a small pot on the front panel, and a DMM on the 1000:1 BNC connector. There’s no feedback in the bias, so when the heads are loaded down by radiation or tube death, the HV droops. Also, be extremely careful tweaking the pot. It’s a very small, surface mounted potentiometer that is easily popped off of the PCB with too much insertion force with the tweaker, OR just by rotating the tweaker too forcefully, if the pot and the hole in the chassis are off-axis. Please pass this message along to whomever might be doing this adjustment. I’d recommend taping up the hole after adjustment, just to discourage the casual tweaker. You know the type.
Good to hear from you. Just let me know if I can help.
Regards,
Larry
To Do:
- Order from
TECO - 80/20 - Get db9 cables from Doug
- Test digis in tunnel
- if cables ok, roll up other cable on reel
- Test a system in the tunnel
- See what values HVSTAT gets
- Test software: displays, save/restore, archiving
- Put caps on frames
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