High baseline "noise" with Theremino MCA. How to improve?

[stelios70]

I have two relevant devices. A Radiacode 103 and an Sohtech Orion IIIA PRO Gamma Spectrometer with an Alpha Spectra scintillation probe, 1.15" x 1.15" x 3" Na(Tl) crystal, FWHM resolution at 661 keV: 13.6% +/- 1%. The Orion with the probe came pre calibrated to Cs-137 31keV and 661keV peaks. The first problem that I had was that I was getting a very low cps in ambient light condition. It seemed that there was light leakage to the probe and after I covered the probe with aluminum foil, this problem was solved. The main issue now is that I am getting a lot of "noise" as the background cps tend to be around 160 and they go up to 250 when using a known source. That makes peaks very difficult to identify. Please see the background spectrum and the spectrums for Lu-176 (15 gramm of Lutetium metal) and Am-241 in Theremino MCA. For comparison I also have the spectrums of the same sources as recorded by the Radiacode software.
Any advise on how to improve the Theremino outputs and/or reduce background "noise" are highly appreciated.

[Sesselmann]

Stelios,

Thanks for registering and writing your first post, but please go and introduce yourself in the Introductions forum as well.

Frankly I had never heard of a Sohtech Orion IIIA PRO, so I looked it up and it looks like a nice copy of my GS-PRO.

I have never seen or tested this spectrometer, but the chances are that your problem lies with the detector, any NaI crystal with a resolution of 14% is pretty useless, most likely it has taken in some moisture and has a wet crystal.

If the problem was electronic noise you would see it as a tall peak in the first couple of bins on the left hand side of your spectrum.

When it comes to gamma spectrometry, the detector is always the weakest link, so you should spend most of your budget on a good detector.

Steven