Re: gamma spectrogram

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Carl Willis
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Re: gamma spectrogram

Post by Carl Willis » Wed Sep 26, 2001 8:54 pm

Nice spectrogram Richard. I'm considering the options of either purchasing or building a PMT and NaI detector, and was wondering what you are using. I have read that the big ones have trouble with energy resolution unless very carefully assembled, and these are extremely expensive (they abrade the crystal, pack it in a reflective layer of magnesia, polish the "viewing" surface, use optical coupling gel, and of course must seal it against water vapor). Are there homebrew techniques for NaI detectors? Good practical rules for insuring that the finished product will have fine resolution?
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Richard Hull
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Re: gamma spectrogram

Post by Richard Hull » Thu Sep 27, 2001 2:30 pm

Most of my NaI (Tl) setups, I got off E-bay or at hamfests.
The absolute best spectrum work is done with GeLi, (pronounced "jelly" in th' biz), detectors. These are ultra expensive (10 kilobucks or more). They require a 'forever' supply of LN2. You cannot allow them to reach room temperature, ever!! They are shipped with integral 50 liter dewar fully loaded with LN2 which from that point on, you are responsible for. The spectrograms from these super detectors are spikes with almost no width!! Needle points. These babies pick up every thing, especially in the low end with extreme resolution!

NaI (Tl), by comparison, is crap and doesn't even rate honorable mention, but it was the first extremely useful gamma spectrograph and still does yeoman duty throughout the world. Vast numbers of new systems based on the sodium iodide detectors are still being manufactured. They are cheap by comparison and serve most scientific and industrial gamma ray spectrum needs. They suffer from wide peaks and crowded gamma energies in the spectrum often merge into a single mis-shaped peak. The key to operating a sodium Iodide detector is a rock steady HV supply. For even a volt or two shift, (0.5% regulation) is totally unacceptable. Using a simple HV supply is a no-no. Unheeded, this admonition will see your spectrum drift all over the place, smear and broaden peaks, etc.

I suggest trying to get a preassembled, matched NaI detector and PMT surplus. A warranted, used item might run $400 or more in 2" size and $600.00 in a used 3" size. A brand new Bicron matched unit in 3" is only $1380.00 so don't pay too much on the used market.

After the detector, you still need a multichannel analyizer.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

davidtrimmell
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Re: gamma spectrogram

Post by davidtrimmell » Fri Jan 11, 2002 8:56 pm

Hi guys, Actually most Geli's are going by the wayside as HPGe's (High Purity Germanium) detectors have much better resolution and can sit at room temperature (still operate under LN2). See: http://www.ortec-online.com/detectors/gedet.htm
But finding a decent MCA (inexpensive) is the hard part. I can still kick myself for not jumping on a surplus Canberra MCA that was going to be auctioned off at DOE Grand Junction when I was working there in '94.
Something like this would be nice : http://ww2.canberra.com/pcatalog.nsf/PC ... -+Portable

Geli's and HPGe's are great improvement over the NaI (Ti) detectors ( I have used those while working at the count lab at Surry), but are not very portable. E-Bay does seem to be a good source for these things, if you live in a area near any DOE Nuke sites or a Utillity that runs some Nuclear Power plants sometimes older equipment gets auctioned off...

Regards,

David Trimmell



Richard Hull wrote:
> Most of my NaI (Tl) setups, I got off E-bay or at hamfests.
> The absolute best spectrum work is done with GeLi, (pronounced "jelly" in th' biz), detectors. These are ultra expensive (10 kilobucks or more). They require a 'forever' supply of LN2. You cannot allow them to reach room temperature, ever!! They are shipped with integral 50 liter dewar fully loaded with LN2 which from that point on, you are responsible for. The spectrograms from these super detectors are spikes with almost no width!! Needle points. These babies pick up every thing, especially in the low end with extreme resolution!
>
> NaI (Tl), by comparison, is crap and doesn't even rate honorable mention, but it was the first extremely useful gamma spectrograph and still does yeoman duty throughout the world. Vast numbers of new systems based on the sodium iodide detectors are still being manufactured. They are cheap by comparison and serve most scientific and industrial gamma ray spectrum needs. They suffer from wide peaks and crowded gamma energies in the spectrum often merge into a single mis-shaped peak. The key to operating a sodium Iodide detector is a rock steady HV supply. For even a volt or two shift, (0.5% regulation) is totally unacceptable. Using a simple HV supply is a no-no. Unheeded, this admonition will see your spectrum drift all over the place, smear and broaden peaks, etc.
>
> I suggest trying to get a preassembled, matched NaI detector and PMT surplus. A warranted, used item might run $400 or more in 2" size and $600.00 in a used 3" size. A brand new Bicron matched unit in 3" is only $1380.00 so don't pay too much on the used market.
>
> After the detector, you still need a multichannel analyizer.
>
> Richard Hull

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