What do you do when you’ve built a radiation detector? Why, you build another one, of course! I wrote a while ago about how my son and I had built a cheap electron (β-particle to you) detector using a low-cost design from CERN. Well, CERN actually have designs for two cheap detectors, the other one being an alpha particle detector. So, we decided to build that too.
The alpha detector uses many of the same components as the electron detector, which is good. We had discovered that you cannot buy single quantity numbers of many components. You can only buy resistors in multiple of 10, for instance. Even the circuit board for the detector had a minimum order of three, so building another detector was a good way to use up surplus components.
Building the alpha detector is only slightly more difficult than building the electron detector, but there were other factors that made a two hour job drag on for 8 weeks. My son had returned to school and had homework to contend with, and now seems to have a wide circle of on-line game-playing school mates competing for his attention. Also, I had returned to study myself by starting an MSc. course in Astrophysics. It was only when the schools and universities closed down for the Christmas break that we were able to complete the assembly.
Snags and niggles
Unlike with the electron detector, there were a few minor problems in building the circuit board. First of all, one of the components in the KitSpace parts manifest was wrong and we were sent the wrong resistor for one component. (Don’t panic! This is now fixed on the site.) Then we discovered that one of the holes on our circuit board hadn’t been drilled out properly. This was easy to fix with a large darning needle. The biggest problem was the surface-mounted 40MΩ resistor: this thing is tiny! If you sneezed or breathed too hard, it would vanish into the distance and take forever to find. (I recommend you work over a sheet of white paper when positioning this part.) Despite all this, we soon had all the parts bar one successfully attached to the board and continuity checked.
The final part to be added was the silicon detector diode itself. This is a tiny, flat silicon chip in a metal can with a glass cover. As even this small glass barrier will block alpha radiation, it has to be removed – carefully, as the devices cost around £10 each. We used small cutting pliers to make 4 small dents in the case around the edge of the metal can. Each time required a gentle, slowly increasing pressure on the plier handles until the glass cracked. After four such cracks, I turned the diode upside down and flicked it with my finger. The pieces of the glass cover fell out easily, and the board was soon complete.
As previously, we mounted the board and its various connectors and switches in an old metal watch box provided by my wife. We connected it to a web oscilloscope, powered it up and I flicked the metal case with my finger. A waveform on the ‘scope meant that everything was looking good.
Finding your alpha source
You can’t test an alpha detector without a source of alpha particles. This is more difficult than finding an electron source, since pretty much everything containing potassium (e.g. a banana) gives off β particles. Uranium is a good source of alpha and surprisingly common. There seemed to be two reasonable options: uranium glass or Fiestaware.
Uranium glass is a decorative glass that was popular from the mid 1800s right through to the mid 1900s and, as the name suggests, it contains uranium. It has a characteristic greenish colour and fluoresces nicely under UV light. I’m pretty sure my parents had a glass bowl made of the stuff when I was a kid. However, the more I read about uranium glass the less confident I was that it was a reliable alpha source. Alpha particles originating in a lump of uranium glass are most likely to be absorbed within the glass itself, and the density of uranium near the edges of the glass is very low. The stuff was just too safe and inactive.
Vintage Fiestaware looked a better bet. It’s a type of domestic crockery popular in the US in the 1930s-1970s because of its deep orange-red colour. That colour is caused by a pottery glaze laced with uranium, so all the radiation comes from a thin layer on the outside of the material. We found a supplier on eBay and sent off for a small sample. Had I known exactly how small that sample would be for £20, we would have been better off buying a whole Fiestaware cup (£29.99 on eBay), taking a hammer to it, and selling on what we didn’t need!
Anyway, we now had a reliable alpha particle source and a mildly amused wife and mother.
Using the detector
As we discovered when building the electron detector, the CERN software for it actually is optimized for the alpha detector version. There’s a web oscilloscope, and a python script for recording pulses, plus another python script to process the data files. The platform that we chose to run the software was different to what we’d used before. My 10-year-old Asus mini laptop simply wasn’t up to the job, but a Raspberry Pi 4 model B performed excellently. We equipped it with the same cheap soundcard that we’d used previously. (There’s an important step with this, which is to use the “alsamixer” utility to set the recording level to max otherwise you’ll be underrecording the energy levels of the pulses.)
First step: check the detector works with the web oscillosope. We actually used the detector upside down so that a shard of Fiestaware could easily rest on the diode detector. Here’s what we saw:
It doesn’t look much at this stage, but it confirms both that the detector is working and that our Fiesta ware really is Fiesta ware!
The python data recorder is much more reliable and is suited for collecting large amounts of data, but there are limits. If you’re going to do any sort of series analysis, you’ll probably need to collect data overnight. Many of the pulses will be electron pulses so we’d decided to collect at least 100,000 pulses, expecting about 60,000 of these to be alpha pulses. Our pottery shard gave us around 4 counts per second, so that’s 25,000 seconds or roughly 7 hours of collection time. We left the detector running for about 15 hours and could see that it had collected about 145,000 pulses. Unfortunately, the recorder holds the pulse data for all of these pulses in memory and, when I tried to save the data, the Raspberry Pi (4GB) ran out of memory. The whole collection run was lost! I tinkered with the collector script settings (setting the parameter THL = -1000) to eliminate most of the electron pulses and began another long collection run.
Interpreting the output
Recording the data is only half the story; displaying and interpreting it is a journey of its own. It took us quite a few attempts, with a lot of input from the ever-helpful Oliver to understand the energy spectrum from my pottery shards. There is a data analysis python script on the project wiki and this can be used to produce some really great graphs, though you may have to tinker with the settings a little.
First, let’s look at the first data plot that we produced:
Given that our detector seemed to be working perfectly, I was a little disappointed that our graph (the blue line above) didn’t look anything like I was expecting. I wasn’t expecting it to look like the red line as this is a simulation for a completely different radiation source. What I was expecting was something like the output from the glazed pendant that’s described in the wiki. Instead of a broad peak around 4MeV, we seem to have the tail end of the electron peak (below 1MeV) and then just a broad plateau with no discernible features. What had gone wrong? The answer, as it turned out, was “nothing”. Manipulating the output from the sound card is a just a bit of a dark art. So here is what we learned.
The electron peak at low energies is H-U-G-E. If you use the analysis software without modification the it will scale the y axis for this peak, and the detail for the alpha pulses will be almost washed out at the noise level. For our data, I had to set a parameter in the script called “max_y_level” (look around line 490) to a setting of 400. If you try this for yourself, you’ll need to adjust it to whatever works for your data.
Not all sound cards based on the same chipset behave the same. Our sound card was based on the same CM108 chip that’s mentioned in the wiki. However, ours seems to have some additional amplification factor in it that was amplifying the alpha particle pulses, and hence their apparent energies, by a factor of almost double compared to the reference ones. There’s another parameter in the script that extends the display plot to the right to show [supposedly] higher energies. I had to set “max_bin_kev” (around line 480) to 16,500.
After these adjustments, we re-ran the analysis and we got this:
Now, that’s more like it! We have a working detector and working software. You can follow the discussion on getting to this point here.
So, this is where we are at the time of writing. There’s still a bit of work to do to calibrate the detector properly. It may be that’s some way to adjust the amplification of my sound card back to the same levels as the reference cards and we’ll continue to look for that. Plus, my son and I will look for some more radiation sources like the radon balloon or one of the many igneous rocks that I have acquired over the years as part of a small mineral and fossil collection. I’ll keep you posted.
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