Discussion Category:  Scintillation Counters

Won't calibrate

Counter won't calibrate with C14. Tried replacing high voltage board and main board without sucess. Any suggestions?

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Asked by

Guri
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dpkleessr

Guri, What happens when you try to count in only the CPM mode?  This is important because it will tell us whether or not the PMT's and front end electronics are functional without worrying about the 137Cs source being in play.  Perform an efficiency check in the CPM mode.  You take your CPM value and divide it by the DPM value on the standards label.  This will tell me a lot about how the LS counter is functioning.  Try counting that way and get back to me and we'll go from there.

Don

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Guri

It looks like the efficiency is normal. C14=95% and H3=65%. but the background is thru the roof at over 800K CPM. so it looks like it could be contaminated. This could be why it's not calibrating? Also seing that the CS137 has dropped a halve-life could that also effect it.

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dpkleessr

Guri, Yes, the 800K background can certainly cause your problem. Whether or not it's contaminated is another issue. The next thing that I would like you to try is see if anyone else has another set of standards so that you can borrow their background standard and recount in a wide open window just their standard, but before you even do that, try wiping all of your standards off with a paper towel or wipe that has 95% ethanol on it. This has been shown to eliminate any static that may have accumulated on the vial itself. That static can cause your high backgrounds.

The significance of your 137Cs having gone through its first half life may or may not be an issue. Based upon you asking the question, can I assume that you are working on an LS6000 and not an LS6500 because the half life of 137Cs is 30.7 years and even that doesn't track with the oldest LS6000 counters which were first manufactured in 1989. I know because I was in the first training class on them. The other possibility is that your 137Cs source may have somehow been freed from its retainer in the source spring assembly. We can go into this in more detail after you do a retest of the backgrounds after wiping them with the ETHANOL. NO METHANOL as it leaves a fog on the glass of the standards.

Don

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Guri

I have decontaminated the counter and the backgroung is down to less than 50 for a wide window. I am getting good counts from my standards and the efficiency is still ok, but if I include the H# parameter I get a message saying "invalid count rate too low". I have tried different standards giving the same results. The counter is actually an LS6000, 23 years of age. I should of said getting close to a halve-life.

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dpkleessr

If you are getting an "invalid count rate too low" message then more than likely you have a problem with the 137Cs source drive or the source itself.  Do you know whether or not the 137Cs source was ever replaced?  It should have been under a mandatory program that Beckman had in place over the last 10 years.  The original source itself was encapsulated in an epoxy based slug whereas the new source is inside a stainless steel slug.  The originals have been known to either break apart or even become loose within the source spring assembly itself.  Hopefully it is the later because if the source has come apart inside of the spring then you have a MAJOR contamination issue that must be dealt with and Beckman engineers are the best people to take care of it as it requires replacing the contaminated pieces and returning them to Beckman for disposal.  This kind of repair is nothing to fool around with as it is very easy to spread the 137Cs pieces around.

The easist way to figure out what's going on is to take the right side panel off and using a survey meter configured for gamma detection, scan around the right side of the frame.  Then, manually rotate the gray plastic source pulley clockwise in order to move the spring up into position behind the source assembly.  If it has come free inside of the spring then you should suddenly get a high count rate as it passes by the survey meter.  You may actually be able to see the source inside of the plastic tube but the survey meter will be your best source of info.  Get back to me with the results of this check.

Don

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Guri

Hi Don, well I'm confirming that the source is actually loose inside the transport tube. Therefore I will call Beckman to rectify this problem. Thank you for your much appreciated help.

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dpkleessr

Guri, Well, at least you now know what the problem is.  The fix, assuming that the 137Cs pea is intact, is actually pretty simple if you want to try to fix it yourself.  I can probably guide you through it.

Don

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Guri

Thanks again Don but the users actually prefer to have Beckman replace the source.

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dpkleessr

Guri, Hopefully it only needs to be reinstalled and not replaced.  If they tell you that it needs to be replaced they should do it for no charge.  Beckamn, as the original owner of the source and manufacturer of the LS counter is ultimately responsible for the source so don't eat the cost of the source if they tell you that it needs to be replaced.

Don

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Guri

Thanks again for this valuable information. Your the best.

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