Comparison HPGe-CsI-NaI-Radiacode 120

[Marco75]

Hi Forum,
It all started with the search for and measurement of Cs-137 in 90 g of ash obtained from firewood. In some areas of north-eastern Italy, it is still possible to detect the presence of Cs-137 originating from the Chernobyl accident.
This is a very low activity, measured with my HPGe at about 0.019 cps of Cs-137, corresponding to approximately 665 pCi (equivalent to about 8.31 nCi/kg). Ra-226 and its progeny were also found, a small amount of Th-232, and a large amount of K-40.
The sample was measured for 24 hours inside the shielding.

Blue spectrum: HPGe GC1018;
green spectrum: background (Ra and progeny all indicated under the Ra-226 label).

Encouraged by some friends, I wanted to test what the spectrum would look like using other instruments, mainly to understand the minimum identification level.
I therefore used Steven’s excellent GS20202-CsI probe, a Scionix NaI probe 1 × 1.5" (old and battered—$100, 10% FWHM, I don’t use it for spectrometry), and a Radiacode 102. Everything was, of course, measured inside the shielding and always for 24 hours, except for the Radiacode, which was left for 48 hours (otherwise absolutely nothing could be seen)!!
This resulted in an interesting comparison, highlighting the strong limitations of a small crystal like the one in the Radiacode for anyone wanting to look for contamination in food, ash, or other low-activity samples. In this case, even a battered old probe with any MCA performs better, at a lower cost.
Obviously, it depends on how the instrument is used, field work, travel, or home use, but I hope this comparison helps to visualize the differences that exist.

Yellow > CsI probe, 24 hours
Red > NaI probe, 24 hours
Purple > Radiacode 102, 48 hours
Also worth noting is the excellent result obtained with the GS2020-CsI.

[ColoRad-o]

Thanks, Marco!
Could you say more about how you calibrated the CsI and NaI data? What bias voltages, etc?

[Marco75]

HI,
the bias voltage depends of course by the type of the detector, you should take the best value, inside the allowed bias values of the photomultiplier, that gives you the best linear attitude and the best sensibility. The two scintillators have been set at 650V for the Scionix and 800V for the GS2020-CSI. For the energy calibration I typically use a multi-source with Eu152, Ba133, Cs137. This gives really lot of peaks from very low energies to 1408kev for a perfect energy calibration of the scintillation detectors. This was not the situation, but in case I want to have a high energy calibration, I use some Th232 with its peak at 2614kev
The energy calibrations were done before taking the spectrums and this is normally sufficient to have a good final result in the spectrum, but you can always refine later. In this case I used InterSpec for fine adjustments and show the spectrums together
Hope to have answered to your questions

[ColoRad-o]

Marco,
Thanks for the response!The NaI:Tl GS2020 kit with a GS-USB-PRO box I have has been a problem for me. You can see the thread "Problems calibrating between 1500-2600 keV" . I find that bias voltages LOW ENOUGH to give a fairly linear calibration curve also have a low count rate. (I also find drift is annoying) Any suggestions?

[Marco75]

With NaI and CsI it is impossible to have an exact linear approach , you always need a polynomial calibration and a bias that makes sense , not too high (bad linearity), not too low (low counts).
Considering what already said my the other members, I don't have other suggestions sorry.