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Scintillation Probe (Photomultiplier/NaI)

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radhoo

Sun Mar 11 2012, 04:35PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Now that would be a crazy project: building my own crystal. If you remember I did the test for determining fluorescence with the few minerals I had under x-rays. Since them I got a few more stones.

Proud Mary

Sun Mar 11 2012, 06:03PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radhoo wrote ...

Now that would be a crazy project: building my own crystal. If you remember I did the test for determining fluorescence with the few minerals I had under x-rays. Since them I got a few more stones.

Not such a crazy idea at all!

Fluorite (CaF2) is one of the earliest scintillators to have been used with PMTs, and synthetic CaF2 is still very much in use today.

Lead carbonate (PbCO3) - found naturally as the mineral cerussite - (including in the Carpathians!

) is one of the new generation of 'heavy scintillators' that everyone is talking about. Exciting stuff, old friend!

Making a scintillation tester for minerals using a PMT would be an excellent project, since there certainly will be natural scintillators not yet discovered in the unique combinations of elements with trace elements - impurities - dopants - found in nature.

See: Stephen E. Derenzo and William W. Moses EXPERIMENTAL EFFORTS AND RESULTS IN FINDING NEW HEAVY SCINTILLATORS

radhoo

Sun Mar 11 2012, 09:51PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Thank you for the great reference. Heavy scintillators seem perfect for absorbing all those fast photons.

The problem with scintillation crystals is that they need to be perfectly polished, and cracks or impurities, such as those found in natural crystals are intolerable, at least in a quality scintillator. The crystal including its edges, must be transparent, and perfectly polished, so it would offer a good transition environment for the light produced inside.

I know that my PMT would record something, but that might not be enough.

Proud Mary

Sun Mar 11 2012, 11:05PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Yes, you are right that the flaws in a natural crystal will be a disadvantage, but I have an idea to try a cut crystal of CaF2 to see how it compares to BGO. I am thinking of using this miniature Hamamatsu R647 PMT so I can couple a gem-cut piece of fluorite jewelry directly onto the window with optical grease. (The dynode resistor chain is on the board sprayed with black conformal coating, and the pre-amp is inside the metal box)

I'll give it a try when I've finished my underwater plasma experiment, and we can compare notes then.

R647 datasheet is here:

Just look at the quantum efficiency at 400 nm!

radhoo

Mon May 21 2012, 03:42PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Here is a photo showing three NaI(Tl) scintillation crystals with quality varying from left to right. The yellowish nuances indicate humidity damage:

All three crystals are functional.

Proud Mary

Mon May 21 2012, 03:53PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radhoo wrote ...

Here is a photo showing three NaI(Tl) scintillation crystals with quality varying from left to right. The yellowish nuances indicate humidity damage:

All three crystals are functional.

The one on the top left looks very nice!

Good pictures too!

radhoo

Mon May 21 2012, 04:52PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

A gamma spectrometer can be easily built with a photomultiplier tube coupled to a NaI scintillator crystal. The counting of the number of peaks corresponding to each energy to get the gamma spectrum can be done very quickly and easily, without additional hardware, using the freeware software PRA (and InTUNE as well) developed by Marek Dolleiser, both available online.

A short guide on how to use PRA is available on Youtube:

I have attached an audio recording representing the output of my NaI Scintillator. It lasts for 1 min, where the first half contains background radiation, while for the last 30 seconds a Cs137 source has been placed near the scintillator. The file is encoded as MP3 so it is only ment for hearing it in your headsets, if anyone needs the original uncompressed data, I can provide it as well:
]30sec_background_30sec_cs137_2.mp3[/file]

Here is the signal representation as amplitude/time, where the Cs137 presence after the first half is clearly visible:

Using PRA, it is possible to generate the pulse Height histogram, that we can use to view the radiation energy spectrum, after the proper calibration.

Here is the spectrum for the 30sec Cs137 recording (attached here as mp3):

And here is another one for a longer recording, 10minutes with the Cs137 sample placed on the scintillator:

Proud Mary

Mon May 21 2012, 05:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Good work, Radu!

radhoo

Mon May 21 2012, 08:25PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Proud Mary wrote ...

Good work, Radu!

Thank you. I need to see if I can get the Am241 in the game.
Using headsets, I hear it making considerable counts in the scintillator, but I do have a lot of noise in that area of the spectrum.
There is some signal contamination from the HV inverter, and some other sources. This is an industrial probe, I didn't make it myself, so I'll try to come with a few improvements. I already tested a different PMT preamplifier that comes with lower noise and better amplification. There's a lot to do here.

The final goal is to have a portable unit, with a nice lcd showing the spectrum , to easily identify various nuclides.

BTW, there's the FitzPeaks Gamma Analysis software that seems to bring a lot of joy to this field. Still need to learn how to use it with my scintillator, however the demo files and analysis it comes with look great.

Proud Mary

Mon May 21 2012, 09:28PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

There are some good practical tips on improving PMT SNR in this application note, including details of a simple co-axial cable snubber to improve output pulse shape:

Signal Recovery with PMTs

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