Gamma Ray Spectrometer Circuit Peer Review - Page 1
Forum section: electronics / beginners

author: Rad_to_Curie
author_rank: Newbie
timestamp: April 21, 2025, 02:03:17 am
content: Hello!I'm a first year Nuclear Engineering student brand new to circuitry and decided to torture myself by choosing an ambitious project. I'm very good at the theoretical aspect (dealing with the math and physics) but the hardware side is fresh. I happened to have access to some CsI(Tl) [a scintillation crystal, Cesium Iodide doped with Thallium. It glows dimly when radiation hits it] crystals and an Adafruit feather RP2040, and figured why not?Attached is a screenshot of several days of datasheet searching and research worth of a schematic and PCB, if someone would be willing to give it a once over that would be great!Any feedback is appreciated!A synopsis of the circuit is below:- 3.7V 5500mAh LiPo battery powers the entire circuit.- Analog ground is kept separate from digital ground, meeting only at the common ground point.- An analog ground plane is used (see attached for PCB)- The capacitors are all C0G/NP0, except for the decoupling capacitors that are X7R's1) LT3467 boosts a 30V rail from the 3.7V input2) The 30V rail feeds into a Hamamatsu S2386-44K (reverse biased) and a LM317T.3) The LM317T converts the 30V to 5V to power the analog chips4) The photodiode connects to an OPA656 TIA5) The output of the TIA goes through an AC filter to cut off 15.9 kHz and reduce noise6) The output of the filter leads to a SPDT switch, the outputs of the switch lead to a faster low-res mode and slower high-res mode.7) The two modes shape the pulse using an RC filter.The outputs are passed through a AD8307 for precision and higher dynamic range.9) The output of the log amp feeds into an AD8606 buffer.10) The buffer outputs into an ADS8326 (the onboard Analog to Digital converter is too slow for my application).11) Digital processing of the signal.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 21, 2025, 03:54:44 am
content: Can't really comment on the schematic as I have little knowledge around gamma spectrometry, but if you haven't already, you may be able to take some inspiration and information from the popular Radiacode devices.

author: calzap
author_rank: Frequent Contributor
timestamp: April 21, 2025, 05:32:10 am
content: Looks like a neat project. Can’t really comment on circuit … not that familiar with some of the components and too lazy to learn. I find it a little strange to have conduction paths on a schematic passing through components. Maybe others do it, but I avoid it. A layout diagram is different. I’ve used commercial gamma specs in the past; they used a PM tube optically coupled to the crystal. Have you checked that your crystal/photodiode combo is sensitive enough? Could do it with a PSU, scope and calibrated gamma sources. If you have two crystals and photodetectors available, you could consider constructing a positron counter.Mike

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: April 21, 2025, 06:44:34 am
content: Some problems:1. Diodes are necessary to prevent the LM317 from breaking during power off (see image). If you care about noise get a better LDO like the TPS7A91. It is only a few bucks and will beat the LM317 at noise.2. You do not have enough capacitance on the output of the boost converter. You probably need at least 1uF of capacitance on the output node. The 220pF capacitor is completely useless here. If you don't know what to do just ask TI Webench and you'll usually get a reasonable answer.3. You will waste a ridiculous amount of power with the LM317. It is going from 30V to 5V. Drawing 100mA would cause it to dissipate 2.5W of heat, and also cause your boost converter to work harder for no reason.4. Your buck converter switching noise will go through the diode (via its capacitance) and it will show up on your ADC. This will completely destroy your performance if you're dealing with extremely small signals.5. Your schottky diode should have at least 2x voltage margin to account for switching spikes (D1)6. The ferrite bead filter needs to be simulated, since ferrite beads will resonate with decoupling capacitors and actually introduce more noise. Get LTSpice, and download the filter bead model and do an AC sweep to make sure it has no resonances. You will need to add a damping capacitor probably.7. The op amp is NOT rail to rail, and even if it were, it can't generate negative voltage. If charge is injected into pin 4 of the op amp, the output would have to go negative, which it can't do in your current circuit.8. 100k is way too small to see anything most likely, considering currents are going to be in the nanoamps range. Use a much higher resistance. You should look here:https://physicsopenlab.org/2017/06/22/x-ray-spectroscopy-with-pin-photodiode/A 1nA signal would only be boosted to 0.1mV, which is way too small to see with the rest of your circuit.9. You need decoupling capacitors for all the ICs here10. You can't reuse the SPI clock for clocking the ADC, at least not without insane amounts of programming effort.11. Your op amp buffer circuit is incorrect (you shorted the inputs)12. The AD8307 is for RF applications where the signal is usually repetitive. You are trying to find the peak of a signal which is not repetitive. This chip can't be used and will give you bad results. You have no idea whether the output is X because of many repetitive small pulses or one big pulse that doesn't repeat as much. I'm assuming that you want to be able to tell isotopes apart, which requires knowing how high the pulse is. This chip just does the average so you actually don't know how high the pulse is.There's probably more mistakes here, those are all that I can see off the top of my head. Powering the device with a switching converter is going to be difficult since we are dealing with nanoamp currents. You want to use a boost converter and a lot of filtering (like the capacitance multiplier or LDOs like the TPS7A47). Not going to be easy.Some style tips:1. Always put grounds facing downwards and positive voltage sources facing upwards (to make your schematic easy to read). Ideally the top of the schematic should have the highest voltages, while the bottom should have the lowest. The schematic should make sense when read left to right. Putting grounds in random orientations makes the schematic hard to read. With power converters always put the input on the left side and the output on the right side.2. Do not run wires through parts in your schematic.3. Make sure the signal is on the left, and the ADCs and other devices which are at the end of the signal path are on the right. Put the photodiode on the left and put the ADC on the right. Don't make everything go around the schematic, otherwise it makes everything hard to read.My suggestion is to start simple. Use a lab bench power supply to generate the high voltages, and try to get something to show up on the photodiode + amplifier first. Make sure everything works and nothing oscillates (transimpedance amplifier designs are quite difficult). The layout of the transimpedance amplifier is critical too. Use an oscilloscope instead of an ADC to measure the signal. Once you can actually see a pulse, you can worry about pulse shaping and other things. Ideally you can just use a computer audio jack to digitize the signal, and use software to do all the digital signal processing. Try to get everything working before you touch the microcontroller (since if you can't get it to work on a computer there is no way you can get it to work on a MCU).

author: Kleinstein
author_rank: Super Contributor
timestamp: April 21, 2025, 06:45:16 am
content: I have some doubt whether the photodiode is sensitive enough to really do spectroscopy. I have build a somewhat similar gamma (initially alpha with a diode as detector) spectrometer, however this was with photo-muliplier. The individual peaks are rather weak and the photodiode may not be sensitive enough. It would be at least tricky and need a lower noise TIA - the OPA656 is super fast but not super low noise. I would really look for a photo-mulitplier instead of the photodiode. If photo-diode(s) chances would be better with a few smaller ones with optics. This gives better S/N ratio.The boost converter would definitely like more capacitance for filtering. This is both at the input and output. The 317 regulator is not really good in suppressing the high frequency ripple. The 317 also needs a lot of minimum load and there would be better alternatives. For the regulator part and filtering the layout is also critical, not just the schematics.Given the low current ( low µA range) and very low noise needed for the biasing voltage, I would consider just some extra batteries (e.g. multiple CR2032, 9 V block(s)). Not sure about the voltage needed - it is balance between the added noise and reduced capacitance.The circuit is an odd mix between using an odd peak detector (still with flaws) and the digital way with a fast ADC and doing the peak detection digital. With the correct gain, the digital way could get away with lower resolution. Even with a PMT the signal quality is normally so that a 12 bit ADC would be plenty (the RP2040 internal one has however a bug and is thus not that great - otherwise it could be sufficient). Even the analog peak detector way is usually OK with less resolution and can use a slower ADC. The peak detector should however be a more precision type with 2 OP-amps. The detector crystalls usually don't give more than some 10 bit resolution - though maybe nonlinear spaced, so that a bit more ADC resolution could still be usefull.The easy way for learing may be a alpha spectometer with an opened photo-diode. I used a relatively affordable BPW24 with the lense removed. The signal is similar shaped, but quite a lot (mayby 100x) stronger than for the gamma case.Of cause the scintillator is more fun as it can also see the background radiation quite well.

author: 5U4GB
author_rank: Super Contributor
timestamp: April 21, 2025, 07:51:52 am
content: You may also want to look atGammaspectacular, which is a specialised forum for just this kind of thing.

author: jwet
author_rank: Frequent Contributor
timestamp: April 22, 2025, 01:16:22 pm
content: I've designed several circuits like this in my past- its a challenging task. Most of mine were portable instruments which made it more challenging.The biggest thing that I see missing is a real peak detector. Spectrometers are pulse height analyzers. The signal chain is a charge amp. pulse shaping amp(s) and then discrimination, peak detection and pulse height capture followed by an ADC. The peak detector block is very key. A classic circuit used by Ortec and Canberra is a pulse differentiator that has a parallel path of an all pass (delay). The signal is run through the all pass in parallel with the differentiator. The differentiator feeds a zero crossing detector (zero slope = peak). A comparator monitors the zero crossing, fires a track and hold to grab the peak sample. The ADC is then triggered, the peak is digitized and binned. Without capturing individual peaks, you'll just get a muddy smear rather than a spectrum.The old reference for such topics was "Radiation Detection and Measurement" by Glenn Knoll. It mainly covers every type of detector in some detail but has a good couple of chapters about processing signals mainly from a NIM Bin level.If you haven't already, I would build up your charge amp and shaping and feed it to a lab mca to get a handle on your detector resolution etc as Kleinstein mentioned. Silicon photodiodes (PIN's especially) make just passable detectors if they can be biased to give enough depletion to give some detection volume. I have experience using very large (10 mm x 10 mm) Hammamatsu Diodes biased at 30v to reduce capacitance and get a good depletion volume. These had something like 15% FWHM type resolutions at low energies/X Rays. At CS-137 (662 KeV), they were around 20%.I don't know what your goals but its might be better to narrow your scope a bit and build a good subsystem. I think an MCA core with a peak detector, digitiizer and memory would be a worthy goal. The rest of the system is just a power supply, detector and a microcontroller system which is pretty straight forward these days.One of my instruments I still see on Ebay for sale is made by SAIC (Leidos) as the AP-1. It is an handheld Alpha Probe with isotopic identification and spectrum display built for weapons labs and other spooks. I designed it in the early 90's and it became the core of other Spectrometers after I left. It used a 2" Silicon Detector to detect and identify Alpha's. The MCA used a 12 bit ADC but the displayed resolution was 256 channels to enhance DNL, etc. Alpha resolution in air is a bit smeary and more resolution dign't make sense. I've built 8000 channel digitizers in the lab.\Have fun. I worked in the Nucleonics field for the first 10 years of my carreer- I loved the problems but Three Mile Island here in the US just stopped pursuit of Nuclear in the US until recently.Take care and good luck.