Notes trying to get a gamma spectrometer working. No real goal, just educational
This is for people trying to figure out if they want a GS and for Bee Research on suggestions on how to improve
Overall evaluation: if you don't know anything about gamma spectroscopy this is a good place to start. Its relatively low cost and comes with quick start documentation. The documentation could use some polishing but I was able to figure it out without too much trouble.
For my evaluation I used:
Higher activity (order 10 uC estimated):
Hooked up PMT assembly to GS but couldn't get any usable readings. As I turn the GS voltage knob I can hear audio crackle and the frequency output changes slightly
Found that 800V dial 740V actual. Tried cranking up a little to see if that made a difference.
Still using the GS to power the PMT assembly, I removed the signal cable from the GS and instead fed that output into my scope. Yipee! I get some pulses
Unfortunately, the pulses don't seem to be correlated to radiation levels. That is, I tried putting some sources (Am-210, U, Th) near it and pulses seem to come in at about the same (random) rate.
TODO: add scope shots
Magically started working while sitting on the desk. Lightly sanded PMT contacts and hasn't had problems since
Also noted that while both the Ortec and the GS seem to be able to reach required voltage levels, it runs much better on the Ortec supply. I'm guessing thta the GS simply can't supply enough power. Will look into this more
Tests also show that some of the long cables I have right are causing unnecessary interference. I need to get some shorter cables.
Scope shots may show that the GS supplier is much nosier than the Ortec. Need to collect better data to verify this but it looks like GS may be switch mode and Ortec is linear
The U and Th sources seem to have a bunch of different energy levels. I'd like to start with something simpler like Cs-137. I have an old Cs-137 spark gap but I don't know enough about it to trust it as a calibration standard.
Looked into acquiring a Cs-137 check source. Looked at three suppliers:
Originally I placed with United Nuclear but some combination of the lag on the order actually going through (PayPal is not actually integrated into their order flow) and the lower price I ordered with Images, SI instead
Back from business trip
Got Cs-137 check source in mail
Got SAINT-GOBAIN 1600-2265 in mail
Has a single signal/voltage port. Need to better understand dual vs single BNC topologies
Gamma Spectacular is stereo jack with one half connected
Running 1600-2265 at 900V. Responds to Cs-137
Can't get audio to record. Tried:
Earlier I had some limited success using the large tube but got only very week audio signals (were they maybe the built in mic?)
Borrowed W520 (windows) and was able to easily record spectra. Indicates the GS does in fact work and I just need to solve my recording problems
Issues were caused by bad audio cable. Replaced
Borrowed some isotopes for testing to augment what I have. Net list:
|Sample||uCi (orig)||Date||Half life|| |
|Cd-109||10,000||~10 uC||XRF gun|
|Cd-109||10,000||~10 uC||XRF gun|
|Cs-137||1||Spectrum Tech (BK)|
|Cs-137||10||Spectrum Tech (JM)|
|Co-57||1||Spectrum Tech (BK)|
|Co-60||1||Spectrum Tech (BK)|
|Eu-152||1||Spectrum Tech (BK)|
|Tl-204||1||Spectrum Tech (BK)|
Above: 2 major peaks and 3 minor peaks visible
Idea was that this was going to serve as the master spectrum for calibration and so wanted to get it as accurate as possible. Unfortunately things have proven to be somewhat non-linear
Each collected for 3 minutes. AU / no energy correction
Tl-204 is not a gamma emitter yet there is one major peak and two smaller peaks. Is this background? Beta sensitivity?
I started trying to use the Cs-137 spectra for calibration (two peaks).
This gave the Co-57 peak around 210 keV when it should be around 125.
Co-60 should have been around 1250 but was far off (above has AU, I didn't record what the keV was).
I let the Co-60 settle for a bit and then took a more accurate Co-57 (above) to produce the calibration table above. Because my calibration is non-linear it must be set to interpolate mode.
Following above Eu-152 (my last gamma emitter) was evaluated to see how the calibration applied. It has a number of peaks which is good in that it gives me more to test against but is bad in that if things go awry it will be difficult to match expected vs actual peaks.
Once again, I'm using the Spectrum Techniques (ST) table as source of truth. For peaks that are close together, I measure against a weighted average. Note that only gammas that occur at least 3% of the time are included in these calculations
|Meas keV||Meas keV range||ST keV||ST %||ST weighted keV||% error||Notes|
|41.2||27.7 - 57.1|| |
|86.7||81.1 - 93.1||What is this?|
|125||111 - 154||121||28.4||121||3.31||Cal at 122 keV means this isn't a shifted 244|
|7.6||244.7||N/A||Why didn't this show up?|
|314||287 - 343||344.3||26.5||344.3||9.65|
|429||381 - 480|| |
|1084||628 - 1750|| |
A slightly longer running test shows that there might be a peak around 205
I expected higher energies inaccurate and lower energies accurate but its actually the inverse.
Not getting a clean signal. I had problems with this tube when I first got it not producing signals at all due to dirty contacts or something. Think this needs an overhaul before it can be trusted.
The tube portion is epoxied shut and is probably okay. The base was custom built by an individual (not familiar with their workmanship). Needs to be opened to examine solder joints, review schematic, etc.
Above: my reading. Peaks at roughly 99.3 keV and 621 keV. The right peak looks like it might be several peaks…going to let it sit for a while longer to see what longer integration reveals. 1,280,000 counts in 37,500 sec ⇒ 34.1 CPS ⇒ 2048 CPM
Above (source): radon spectra. Peaks roughly at 75 keV and 575 keV. Not a perfect match, but seems reasonable given possible calibration errors on both sides
Over about an hour (TODO: add detailed count information) in a lead castle. The immediate takeaway is that one of the peaks in the unshielded background is clearly visible while the other is gone. CPM has gone down dramatically to around 2 CPS ⇒ 120 CPM ⇒ 5.9% of unshielded level
Used proton beam / cyclotron at UC Davis to test some PCBs. Ran some measurements on the lingering radiation for kicks
Above: test setup. One geiger counter is inside the lead shield (not visible) and another is outside.
Above: gamma spectrum using Gamma Spectacular w/ Saint Gobain sensor
Above: half life is about 8 hours from internal MyGeiger2