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  6. Unstable C14 counts after heatwave and power outage

Discussion Category:  Scintillation Counters

Unstable C14 counts after heatwave and power outage

I'm attempting to long-distance diagnose an LS6500 located at a remote research facility. During the recent they experienced multiple power outages and since then have been experiencing wildly unstable C14 counts even after calibration. Here is an excerpt from the email they sent me; "The Tritium counts are coming back stable and within the range we expect, but the C14 counts are bouncing around wildly. The same sample, run several times is giving counts that vary between 10,000 - 40,000 cpm." Would this be indicative of a failing PMT?
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Gyuji
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dpkleessr

Just came upon your post so sorry for the delay.  I find it interesting that the 14C counts would be wildly unstable since that is the primary calibration standard for the LS Counter but the 3H counts are not.  Depending on the options, both 14C and 3H may be needed for the calibration but without knowing the installed options that is impossible to determine.  Typically if the PMT's are failing then the 3H counts would also be affected and probably more than the 14C counts.  Since the 3H cunts are stable it appears that the hardware related to the chamber in which the samples are counted appear to be intact.  Power failures are something that the LSC is designed to handle whether samples are counting or not so unless it got hit by a spike in the line voltage I don't believe that is cause for concern.  If possible, please let me know what options are installed on the counter and also get copies of the calibration record along with counting results for both standards and the background standard and attach them to this site.  For the record, I worked on these series of LSC from their inception back in 1989 up to when I retired in 2012 so I had quite a lot of experience with them.

Don

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