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• Education & Outreach

  mihai.cuciuc • 08/04/2026 at 05:06 • 0 comments

  For the past year I've been busy improving the hardware but also running a small-ish educational project funded by the Romanian Ministry of Education and Research. I'm going to high-schools to discuss ionizing radiation, and to take measurements with the students in the classroom using this hardware. It's been a great opportunity to get feedback on the hardware and to try to get students interested in the many topics of radiation and its measurement.

  One of the deliverables for this project is a series of YouTube videos. While I'm definitely not made to be on camera, I had a blast making them! I'll post them here. Any feedback is extremely welcome!

• Pomelo on Crowd Supply

  mihai.cuciuc • 01/30/2025 at 05:33 • 0 comments

  Pomelo will be launching on Crowd Supply where you can subscribe to be notified when it's ready: https://www.crowdsupply.com/pomelo-instrumentation/pomelo
  

  The business side of things is a very new topic to me, and it's full of challenges. But what I did not expect was that, since I'm super passionate about building this detector and sharing it with the community, I seem to enjoy most of these challenges.
  

• Theremino MCA

  mihai.cuciuc • 08/02/2024 at 21:16 • 0 comments

  Pomelo is all about interfacing with as many things as possible. Theremino MCA is a popular open-source gamma spectroscopy software designed to work with DIY detectors. It relies on the PC’s sound card for digitizing pulses in order to build a spectrum.

  Pomelo Core makes internal analog signals available on a pin header, mostly for debugging. Among these there is a low-pass filtered version of the SiPM output. Unfortunately, it’s still a bit too fast to be properly digitized by a PC sound card and while it does work, the results are a bit wonky. But if we slow it down a bit more using an RC low-pass filter, it becomes just right. I used 10 kOhm and 10 nF.

  To get a feel for how well the pulse amplitudes match between the Pomelo Core output and the signal that is read by the sound card I grabbed some pulses both with a Saleae logic analyzer and with Audacity and manually aligned the time scales to look at the same pulses.
  

  Pulses from a Th-232 source at different stages in the analog chain. The time scale is manually aligned to look at the same pulses in both Saleae Logic and Audacity.

  Same plot as above but zoomed in to see the shapes of individual pulses

  The sound card is detecting pulses as being negative, despite the Pomelo output being positive. I inverted the waveform in Audacity to make things easier to see.
  

  One thing to notice is that the RC filtered signal rides on a DC baseline of ~2.5V. This is due to the PC’s sound card providing bias for the microphone -- it’s not a huge problem for Pomelo, as its output is fed directly from a stiff source, an op-amp’s output. The 10 kOhm resistor nicely isolates these two.

  The spectrum of a Th-232 source looks very good both in the Theremino MCA software, as well as exported and displayed alongside the spectrum recorded by Pomelo Core.
  

  Th-232 spectrum and individual pulse in the Theremino MCA software
  

  Spectra taken with both Pomelo Core as well as with Theremino MCA, with uncalibrated x axes scaled to cover the same energy range
  

• Pomelo core in coincidence mode

  mihai.cuciuc • 07/15/2024 at 04:18 • 0 comments

  I used a pair of modules are run in coincidence mode to detect and measure cosmic muons.
  

• Pomelo Core new use case

  mihai.cuciuc • 06/21/2024 at 04:28 • 0 comments

  I added a new project describing an experiment that uses Pomelo to measure gamma attenuation in aluminum. The result is surprisingly close to the literature value.

• Updated scintillator and wide range energy linearity

  mihai.cuciuc • 06/17/2024 at 16:15 • 0 comments

  Pomelo sales rep showing off the latest prototype sporting a 10mm x 10mm x 20mm CsI(Tl) crystal

  I designed the device dimensions with a 2cc (10mm x 10mm x 20mm) CsI(Tl) scintillator in mind, but only had 3cc ones to test with. It’s why the detector element extended beyond the device’s edge, but now that the 2cc crystals arrived, I can share the final design and some preliminary performance plots.

  The scintillator is not yet potted in silicone, which does decrease light output a little (and thus worsens resolution). I took spectra with several lab sources and computed the resolutions for every gamma line and checked the detector linearity.

  Spectra for Am-241, Na-22, Cs-137, Mn-54, Co-60, and Th-232 on the same plot in logarithmic scale. Energy resolutions for each energy shown in the last column

  Detector linearity plot. There is a small quadratic term that is typical of SiPMs

• Pomelo Core use cases

  mihai.cuciuc • 06/01/2024 at 09:44 • 0 comments

  I updated the project description to include use cases for the Pomelo. One is a complete hand-held instrument with battery, user interface, and lots of connectivity, and others are physics experiments that are enabled by the Pomelo:

  • Gamma ray inverse square law
  • Compton scattering

• Pomelo Core v1

  mihai.cuciuc • 05/23/2024 at 04:54 • 0 comments

  This contains all the functionality to do gamma spectroscopy and nothing more
  

  I'm planning on producing something that looks more similar to other gamma detectors, like the excellent products that are already out there: the Radiacode, the Kromek Sigma25, or the CapeScint CsI-MacroPixel-MCA. To shift towards that I merged the digital and analog electronics into a single PCB, removing the battery and charging circuit, as these are not core to the functionality of the detector. The detector now consists of two parts:

  Pomelo Physics:

  • CsI(Tl) scintillator, 3cm^3
  • Silicon photomultiplier
  • 3D printed dark box to house scintillator and keep stray light away from SiPM
  • Temperature sensor

  Pomelo Core:

  • Digitally controlled SiPM bias supply ~32V - ~48V
  • Low power analog front-end
  • Microchip SAML21 (ARM Cortex-M0+) microcontroller
  • USB connectivity
  • UART connectivity with integrated level shifter

  Pomelo Physics. Scintillator and SiPM detachable module

  Back of Pomelo Core PCB
  

• Scintillator potting

  mihai.cuciuc • 05/13/2024 at 04:48 • 0 comments

  CsI(Tl) scintillators are slightly hygroscopic – they absorb some moisture and this affects their crystal structure, worsening their performances. However, the magnitude of this worsening is very small as shown by this paper and this paper.

  In any case, my plan is to limit the amount of moisture that can reach the scintillator by adding silicone in between the Teflon-wrapped scintillator and the 3D printed detector case that serves as a dark box. Using a somewhat rubbery silicone will also work to provide some cozy mechanical support for the scintillator. The resulting internal structure of the detector assembly should look like the following diagram:

  Detector assembly structure. The scintillator is attached to the SiPM (not shown) that is soldered to the PCB. I plan on filling the gap between scintillator and dark box with silicone

  Room-temperature vulcanizing silicone curing test

  As I’m using a one-component RTV silicone I wanted to make sure that curing still happens inside the 3D printed box, as the exposure to outside air is quite limited in that case. To test this I printed a fake “scintillator” block that I wrapped in Teflon and I made a few test runs that I could then break and look inside for how the curing went.

  Test pieces for testing potting and curing. No expensive components (SiPM or CsI(Tl) crystals) were hurt in these tests

  Loading some silicone into the dark box and then squishing the assembly in
  

  Having assembled these units I let them cure for ~1 week. I then broke a few and extracted the plastic fake “scintillator” and its Teflon wrapping to find this very nicely cured shell around the scintillator block.

  Broken test assembly. The silicone cured nicely everywhere, but I did not manage to get it completely centred
  

  One thing I did not consider is that the fake "scintillator" I 3D printed with ~30% infill was full of air. While it was covered in Teflon, I'm not sure this was an air-tight barrier, so some of the curing might have happened because of that empty space. In any case I assume that it will still cure just through the shell, even if it will take longer.
  

  With this test successful I went on to pot an actual scintillator and see how this affects its performance.

  Detector performance after curing

  I potted a real detector and took Na-22 spectra before and afterwards to obtain the following spectra.
  

  Na-22 spectrum and resolution for the 511 keV gamma line before silicone potting (red) and after potting (blue)
  

  The peaks are shifted a little to the left after potting, so the pulse height is somewhat lower, suggesting that light collection is not as good as before potting. This is also indicated by the slight worsening of the energy resolution. While there is some loss in light that gets collected on the SiPM, the added resilience of the detector assembly is worth it.

• Digital temperature compensation

  mihai.cuciuc • 05/09/2024 at 05:13 • 0 comments

  The gain of silicon photomultipliers (SiPMs) increases with applied voltage and decreases with temperature. Hamamatsu has an informative write-up on this topic.

  This presents an opportunity to compensate for temperature changes by tuning the bias voltage, with some circuits doing this directly in hardware, by having a thermistor in the voltage feedback loop of the bias supply. An example of this is in Figure 8 in the MAX1932 datasheet.

  I decided to go with a digital temperature sensor and tweak the bias voltage in firmware. The temperature sensor is placed directly underneath the SiPM on the detector PCB to get a reading as close as possible to the sensor.
  

  A TMP126 chip in the middle of the detector PCB provides very accurate temperature readings. The SiPM is directly underneath it, on the other side of the PCB.
  

  I ran a few tests to see how temperature and bias voltage influence the spectra from a Na-22 source, with the following animated results.

  Increasing temperature of the SiPM decreases its gain, so the gamma peaks shift to lower values in the spectrum. This can really mess up the long-term stability of the detector and make gamma peaks disappear into blobs. Temperature is in degrees Celsius.

  Higher bias voltage increases the SiPM gain.

  From this data I derived a voltage compensation coefficient and managed to keep the gain fairly stable, with a loop that runs every 5 seconds and updates the bias voltage. While it’s not great, it’s much better than without it.

  Relative gain shift with fixed bias voltage (orange) and with a varying bias voltage calculated from the temperature value.
  

  The difference that this makes is in keeping an overall better energy resolution for the system at varying temperatures.
  

  Left plot has data taken between 29 and 40 degrees Celsius, without temperature compensation loop running. Plot on the right is taken between 24 and 40 degrees Celsius, but with the temperature compensation enabled and results in a better energy resolution.
  

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