Plateau and Efficiency of a 44-2

Scintillation crystals, PMTs, voltage dividers etc...
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glhansen
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Plateau and Efficiency of a 44-2

Post by glhansen » 06 Apr 2022, 22:43

I just got a Ludlum Model 3 with 44-2, and my plateau voltage is 1025 (with 58 kcpm and background 0.5 kcpm graph below) and Cs-137 sensitivity at 1 mR/hr is around 300,000 cpm/mR/hr. The "as found" voltage (teed off to pulser and probe) was 815, and with the Am-241 source in contact the meter beeped lazily and erratically, so I'm sure the as-found voltage was way too low. But 1025? And the nominal sensitivity is 175,000, so I'm way above that.

Does that have a likely explanation? Can the plateau voltage increase over time? And I can think of ways a scintillator might lose sensitivity, but not overcount, unless the voltage is way too high, but this background is only 1%.

"The other guy", whose meters I am getting now, seems to have routinely set his plateau voltages lower than I would. I don't know why. But I hate being at odds with other calibrators because I feel like one of us must be wrong, and I'm not sure that it's not me. I have one of his plateaus for a different instrument, and if I had gotten the numbers he recorded, I would have set my plateau where he did, and it says he used 1 uCi Am-241, so that part looks fine.
screenshot_231603_20220404_pleateau.png
screenshot_231603_20220404_pleateau.png (13.44 KiB) Viewed 148 times
Greg Hansen

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Re: Plateau and Efficiency of a 44-2

Post by Sesselmann » 07 Apr 2022, 13:56

Greg,

Without seeing the complete S curve it is difficult to verify your claim, 1025V does seem high for most PMT's.

The objective of finding a plateau is to identify where the count rate flattens off before rising exponentially, so starting at very low voltages 500V you would get little of no counts, then you will see the count rate rise, flatten off and eventually rise again.

At some point the count rate will exceed the capability of the electronics and the count rate flattens off again.

I would like to see how your curve looks below 850 V.

Steven

glhansen
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Re: Plateau and Efficiency of a 44-2

Post by glhansen » 08 Apr 2022, 04:34

Unfortunately, I don't have that instrument any more. But before I started the curve I assumed it was set somewhere on the plateau, and I started by turning the voltage down to see where it began to drop. And there just wasn't far for it to go. With that Am-241 source it should have been screaming, but as found there was hardly any signal, that was immediately obvious just by listening to it.

Ludlum can install a resistor pack so a scintillator will operate at 900V. Like one that is to be used on a 14C, which has the internal probe whose voltage is derived from the 900 volts for the external probe. Obviously 900 < 1025, but I thought that might be a reason the operating voltage was quite a bit high rather than just a bit high. I didn't open it up to look.

I know it's hard to diagnose something if you don't have it in front of you to work with it, but I always hope someone will say Oh, I see this all the time, it's because of this or that.

How far down and up do you generally do your curves? Since I'm looking for the flat section I start at a point where the signal seems clearly small compared with that region, like 10% of it or so, but never thought there was a reason to go lower than that, I just want to be able to see the knee.
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Re: Plateau and Efficiency of a 44-2

Post by iRad » 08 Apr 2022, 04:59

Oh, I see that all the time... Bad crystal. Used Ludlum 44-2 are often found with bad 1X1 crystals, and one way to quickly diagnose is to see the response to a low energy source like Am241. With a good crystal (and good PMT), you should have gotten plenty of counts at less than 800 volts using an Am241 source. Depending on when your 44-2 was made, the crystal could be an easy swap, but sometimes they are epoxied to the PMT and Ludlum wants to sell you the whole assembly (ouch).
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glhansen
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Re: Plateau and Efficiency of a 44-2

Post by glhansen » 08 Apr 2022, 22:50

Ah, thank you! If a crystal is changing over time, going bad, that would be a nice reason that I saw something different than the other guy did.

I recognize a bad crystal when I don't see a plateau, just a sloping line. Otherwise, I have to feel pretty sure before I tell the owner to spend a few hundred dollars because I said so.
Greg Hansen

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Re: Plateau and Efficiency of a 44-2

Post by glhansen » 08 Apr 2022, 22:56

I got another one. Ludlum 3 with 44-2 again. This time the as-found voltage was 1008, so it was pretty high when it came to me. And this time I went down far enough to get the S. And, from 1000V to 1025V my signal went from 1.2 to 2.2 uR/hr, hard to see on the attached graph. I didn't want to take it over 1200V, and I think this time I'm going to have to give the owner some bad news. But he's an internal customer, so I don't feel as bad about that.
screenshot_108061_20220408_plateau.png
screenshot_108061_20220408_plateau.png (10.57 KiB) Viewed 121 times
Greg Hansen

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Re: Plateau and Efficiency of a 44-2

Post by Sparky » 10 Apr 2022, 11:53

The attached plot is just for reference. While not a 44-2, it is a of very similar 1" NaI probe that has worked very well for me.
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1in Victoreen NaI Voltage Reponse.jpg
1in Victoreen NaI Voltage Reponse.jpg (60.75 KiB) Viewed 102 times
Michael Loughlin

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