Gamma Spectroscopy

This page shows the spectra I'm getting from tests I was doing years ago. Note that a professional HV supply was used for the first sections of this page and the Geiger Kit is not involved. Only the last section uses the HV supply from the Geiger Kit. Also note that I have abandoned this work some time ago so it is only provided here as historical information. Click the images for full size.

Status:

I think I'm just beginning to get into the ballpark for making viable spectra. There is definitely room for improvement, and the information and images below are subject to change as they improve. At this point, I'm not saying any of these are any good, so if you have comments about them I would appreciate hearing them.

Setup:

HV: Tennelec TC 952A HV supply. (Geiger Kit with v4.0.2 PCB or HV Mod used in last section.)

Scint Probe: "Bicron" = Saint Gobain - Bicron 1.12X1 12M3/1.12L (1" x 3" NaI(TI) crystal). The pot on the probe is adj full CW for 11MΩ (minimum setting)

RAP47 - a low energy probe (1″ X 1mm thick CsI(Tl) crystal and a thin aluminum window.)

Signal Splitter: RL (anode resistor) is currently 4.7MΩ.

Currently using the circuit below. Scope shot of pulse train and ~30uS pulse shape from Bicron.

Sound Card: Laptop with "MIC" input configured to "LINE" via driver. Mixer set to about 25% for Bicron and left unchanged whenever possible.

Software: Theremino 2.20 and PRA 7.0.0.1 (newest)

Physical: No shielding on scint probe (yet). Strong samples set at a distance from the front of the probe - see notes on images.

Settings:

Theremino: Energy Compensation = off. Min. KeV=1 (whenever possible). Calibrated right and left Trim for Cs137 sample and left it that way for other samples. This causes inaccuracies for other samples.

PRA: Here is a link to the current Settings screen I'm using, Set audio to 192kHz sample rate, 16 bits. Ran Pulse Shape Acquisition on background and saved. Used same setting for other samples. Calibrated KeV scale just for 32 and 662KeV peaks for Cs137. Left it that way for other samples. This causes inaccuracies for both peaks in other samples.

Bicron probe

Cs-137 - 5uCI sample disk at 10cm with Bicron probe:

After shielding my splitter circuit better, I found I could use a lower voltage (700V) and lower volume (25%).

Am-241 - sample on Bicron probe:

Other casual tests with Bircon probe:

Thorite - This is a pretty hot rock ~500k CPM right on Bicron probe. I will use it's peaks calibrate the PRA from this point on.

The peaks for thorium are roughly 98, 238, 338, 583, 911 keV. Enter them in Energy Calibration screen and choose "Interpolate".

Also spectra using the new Theremino 4.3.

FYI - In the beginning I was struggling with my sound-card on my laptop (it had only MIC-IN). So I tried a USB soundcard with LINE-IN. (Later with new drivers I was able to get LINE-IN with the existing built-in soundcard and have been using that since.)

At left is a comparison with the PRA Thorite spectrum just above. The same settings were used, only the PRA was getting input from a USB soundcard. (The pulse shape was also re-acquired.)

Note the lack of the 98keV peak and it's offset to higher energy levels. Otherwise the spectrum is basically there and might be improved by tweaking other settings.

Ra-226 - A calibration disk from an old PRI 111B - believed to be Ra228. And as a sanity check, old radium watch hands.

I was happy to see the similarity between the two different sources of radium. (Thanks to J. Bravo for the peak references I used.)

RAP-47 probe

I didn't think I was getting good spectra with this probe until I was told "The RAP47 is best between 8 and 100 Kev, above 100 the observed counts are much attenuated." I take this to mean that I will never see the full spectra that I get with the Bicron, but instead I am zooming in on what's happening on the low end of the spectra. After trying lower voltages, I feel I need to run this probe at 900V and 100% volume to get the best definition. I only trust PRA with this probe.

Cs-137 - 5uCI sample disk and Ra-226 - on RAP-47 probe:

The Cs-137 PRA image below was from a long (1 hour) run to look for the 662KeV peak. No sign of it. It looks like 200keV is about the max keV I'll get.

Am-241 and Thorite - on RAP-47 probe:

Am241 being low energy gives me a similar (and maybe a better) spectrum as when using the Bicron probe. However I see that the energy calibration is off when calibrated with a different source. (Off by 7.2keV in this case.

Experiments with the Geiger Kit power supply and Bicron probe

The newer version of the Geiger Kit (or the older kits with the HV mod) provides about 950V max open circuit. I've started testing using the kits HV supply instead of the "pro" supply I was using for the spectra above.

I found that "as is" there was too much ripple - 120mV. However using a simple RC filter has brought the ripple down to 14mV and I am now beginning to get decent spectra with PRA while using the kit's HV supply.

This is the current filter I am using . . .

Being battery operated, the max voltage depends on the load of the scint:

RAP47 (122MΩ) ~ 950V max - very little sag
Bicron (11MΩ) ~ 850V max - some sag but still OK
DIY Probe (4.2MΩ) ~ 550V max - too low to use

From the voltage drops, I'm guessing that the Kit is capable of putting out ~100uA. If that's true the max load would be ~9MΩ.

Below are the PRA spectra I'm getting using the Bicron scint, Geiger Kit HV and the RC filter . . .

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