Gamma Spectroscopy is the identification of radioactive isotopes based on the gamma radiation energies emitted by a source. As a home gamma spectrometry hobbyist, I use radioactive sources such as vaseline glass (colored using Uranium), orange Fiestaware (colored with Uranium oxide), Radium-painted watches and clocks, Thorium lantern mantles, and even Uranium ore samples. Most of these are examples of common household items that emit radiation, and I find many of these items at antique stores (most of these things made today do not contain radioactive elements).
How it works: A photomultiplier tube (PMT) is inside a probe with a Thallium activated Sodium Iodide crystal, or NaI(Tl). The probe is sealed off so that no light can get to the photomultiplier; when gamma rays pass through the crystal, it emits its own light which is them picked up by the PMT. The probe is connected to a Multi-channel Analyzer, which sends info through an audio cable into the microphone jack of a computer or tablet. Then software analyzes and graphs the signal.
Here is the hardware and software of my setup:
HP G71 Notebook laptop, 64 bit, Intel Core 2 Duo CPU T6600 @ 2.20GHz;
soundcard: IDT High Definition Audio CODEC, mic set at 2 channel 16 bit, 44100 Hz. This is all stock, I have not upgraded any parts of this laptop.
PMT is homemade with a 30x70mm NaI(Tl) crystal. I use Gamma Spectular’s GS1100A MCA and Theremino’s free gamma spec software.
Theremino settings: I’m not sure what the default settings are, but they definitely need to be adjusted for gamma spec. Also, they will be a little different for every different PMT used. Here are the settings I used yesterday that gave me a pretty accurate reading.
Bins: x2
Samp.: 192000
Pulses: Auto
Audio Gain: 2.0
Zero Trim: 0
[External Mic]
Baseline Test ☑
Position (uS) 100
Size (uS) 200
[Max Slope %] 20
Max Noise % 20
Integr. Time (sec) 120
Draw Speed (fps) 4
Min energy (KeV) 10
[IIR Filter %] 10
With these parameters, I ran a background (bg) check by letting the probe run for one hour with no sources nearby. This gives us the readings from natural sources, etc. so that we can discern them from the sources that we’re checking, to minimize interference. I ran the bg check at 7.5 power (750Amps).
Next, I chose this watch face that was painted with radium paint to make it glow in the dark. It is one of the hottest alpha radiation sources I have, based on readings from a Radalert 50.
The green line on the graph is the 10 minute check on the radium source, and the gray line is the bg. Radium spec was taken at 8.5 power. Moving forward I will be more consistent with run times and power settings, but for last night I think this turned out alright!
After this, I subtracted the bg by clicking the Use BKG button on Theremino (because I loaded the results of the bg run into the software beforehand). This is the result:
From this point I use the Isotope Identifier to show me what isotopes line up with my energy peaks. I know what elements I am looking for, so I will have to make some minor adjustments:
Now, I already know what isotopes I should expect to find: Radium and it’s decay daughters (other isotopes that are the product of Radium decay). They are Radium-226, Lead-214, and Bismuth-214. Now, for this last pic I have something in my settings that I need to fix; I had to adjust the Energy Trimmer (right side of graph) to 555 for my setup to show an accurate reading. This allowed me to ensure that all my peaks lined up correctly, and is a normal adjustment to my knowledge as all probes are going to be slightly different and will need tweaked accordingly. Also, the graph should look the same as the last pic but with the elements identified as they are. Somehow I have it showing only isotopes I select and not the rest of the graph. Here’s what it looks like:
Success! I have Ra226 at 186KeV; Pb124 at 242KeV, 295KeV, and 352KeV; and Bi214 at 609KeV. Again, the next time I run this, I will have a better final graph, but this experiment was definitely a good start!
I hope this was educational, and if you use Theremino definitely feel free to try these parameters if you have a similar probe. If you have ideas of different parameters to try or know some tips that I should learn and use, definitely leave a comment!
Mikl Moose 