Gamma Ray Spectrometer Peer Review (Pt. 2, electric boogaloo!) - Page 1
Forum section: electronics / projects

author: Rad_to_Curie
author_rank: Newbie
timestamp: May 19, 2025, 08:32:20 pm
content: Hello again!Originally, I posted in the beginner forum, but I think the projects forum might be a better fit now!In case you missed my last post, here is (an updated) review of me: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 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? Also, “First Year” is kind of misleading, as I’ve taken up to physics 3 and applied Diff-Eq 1, as well as some basic engineering classes already. So I do have experience in math and electronics, just book experience and not practical knowledge.I completely redesigned the circuit to be a more traditional setup with a Transimpedance Amplifier, Differential amplifier, and Integrator amplifier, followed by a voltage follower and Analog to Digital Converter to be processed by an RP2040 feather.To address some concerts from the previous post:@calzap Thanks for the comment! I thought about a PMT, however more recently SiPM are being used in place of traditional PMT’s. This is because of their increased efficiency, far lower power demands, and comparable sensitivity. I found the BPW-21 which produces a current of ~5.73nA per pulse of CsI(Tl) when reacting with a Cs 137 sample. With a dark current of <150pA and capacitance of <23pF, this is adequate for my application.—@EggertEnjoyer123 I fixed all the schematic issues, hopefully making it easier to read. In my inexperienced ADHD mind, it all made sense, but it’s perfectly reasonable that it looked awful to everybody else!- I changed from a 30V reverse bias to a 2.5V with a different diode that better matched the spectral output of CsI(Tl), removing the need for the LM31T and opting for a TPS65130 to produce regulated +2.5V and -2.5V with an LC filter on each to limit noise.- I removed the ferrite bead because I’m not smart enough to run more simulations and don’t have the best knowledge at interpreting and correcting the results.- Every IC has a 10u and .1u capacitor for decoupling- I isolated the SPI clock to just the ADC- The ADC has a max input voltage of .3V. The signal will be further amplified digitally, as modifying the time constant for more gain causes too much pileup.- Removed the log converterI also completely redesigned the circuit and simulated it with LTSpice. With the listed values on the schematic, I got the following values with a 5.73nA exponential pulse with dark current and capacitance modeled:- TIA: 1.25mV peak with .15mV resting state (dark current), and a settling time of around 20us- Active Differentiator: -3.7mV peak with a 1.5mV second peak, settling after 30us- Active Integrator: 22mV peak with a -10mV second peak, settling after 35usThank you for your input! It was integral to designing the circuit.—@Kleinstein Thanks for your comment as well! See the first section to address sensitivity issues. Because the ADC clock rate is much higher than the pulse width, I can capture the entire waveform in plenty of detail to do all the pulse analysis from a MCA digitally.—To the rest: Thank you for your encouragement/constructive criticism. It really has been helpful.Let me know if anything looks wrong or if you have any corrections. It’s a learning experience and has been a fun journey!I attached photos of the schematics for each section as well as the LTSpice sim file for the amplifying stages.Also, ask any and all questions!Best!P.S. Bonus point goes towards whoever can name the show and character who seems to use her explanation points a little too "haphazardly!"

author: jwet
author_rank: Frequent Contributor
timestamp: May 22, 2025, 10:09:39 pm
content: Good progress in many areas. The signal chain is much improved.I still don't see a discriminator (comparator) and a peak detector. The discriminator is basically a comparator that rejects pulses less than a certain amplitude. This discriminator will also arm a peak pulse sampler that I suggested to your first design. The pulses coming out your detector will have a wide range of amplitude and are asynchronous. You need to capture peaks and hold them long enough for the ADC to convert them to digitial. If you just run shaped pulses into an ADC even one sampling at a high rate, you won't get a Pulse Height Spectrum, you'll get a smear of amplitudes including lots of noise and then every amplitude in the signal. The spectrum peaks that you want to recover are buried in a smear. Your chance of capturing any peaks will depend on the count rate. Your probabililty of detecting noise is close to 1 and your probability of digitizing a peak is near zero. I hope this makes sense.A while back, there were "MCA's" that used a sound card sampling at 48 Khz, or so. They suffered from the problem I'm describing here. You can probably google some of those old designs. As a Nuc Engineer, you should understand the issue.Good luck with your project.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 22, 2025, 10:13:13 pm
content: Just so you know, there is a "Move topic" button, at the bottom of the page.The forum section doesn't matter much, but duplicated topics are discouraged.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 23, 2025, 03:41:41 am
content: You can't power a 3.3V reference generator with 3.3V. (If your 3.3V is actually 3.2V, then the reference IC can't magically pull it up to 3.3V)

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2025, 03:42:25 am
content: The ADC looks just fast enough to resolve the peaks. So the discrimination could be done in software. It is still not super fast and the maximum usually rate would be a little limtied, but not too bad.For the amplifiers I would avoid AZ type amplifiers, as they produce chopper artifacts in the frequency range of interest.The BPW21 is not really such a good choice. It includes a filter to adapt to the eye's sensitivity and this filter reduces the sensitity. The Scintilator signal is very weak to start with. A good silicon photo diode may barely work - so no room for compromises here. From the sensitivity the BPW21 is quite a bit lower than modern SiPMs. If it works at all one would need really good photodiodes and some optics.I doubt that the photocurrent would reach 6 nA - at least not for a significant time. The diode is nice for brightness measurements, but not really suited for sensitive measurements. From the datasheet I have the diode capacitance is quite a bit larger - more like 300 pF.

author: jwet
author_rank: Frequent Contributor
timestamp: May 23, 2025, 06:07:45 am
content: My experience is designing portable instruments that were power efficient. The AP-1 probe that I mentioned and is still sold in many variations has a power consumption of about 60 mW total with uP and display- this is a variation with 4096 displayed channels. Running a fast ADC flat out and then sorting out everything in software is pretty barbaric and inelegant to me. It can be done for about 1/100th of the power and 1/100th of CPU horsepower. Capturing peaks in the analog domain and then triggering a conversion is the right way to do this IMO and is still the way high performance MCA's are made.Also, the chosen detector is limited by its area mostly. I guess this isn't that important since you're tying to demonstrate the MCA, etc. Hammamatsu makes some large PIN's that are much more suitable this application- look at their 10 mm square detectors. Even the best Si only detector will be generally limited to low energies, Xrays basically- their QE declines rapidly at energies over a couple hundred KeV- they can't stop the photons. Detectors using a scintillator offer much better energy response beyond X-Rays. There are solid state detectors that make reasonable detectors without a scintillator like CdTe and other compound materials. This doesn't matter much as I said.

author: voltsandjolts
author_rank: Unknown
timestamp: May 23, 2025, 07:40:42 am
content: Quote from: jwet on May 23, 2025, 06:07:45 amYup. And this can be achieved quite simply; I've seenindustrial spectral gamma detectorsthat used the discriminator output to trigger a short time delay monostable which then triggered the sample/hold for the ADC. The theory being, that over the range of interest the pulse widths were quite consistent, so the a few hundred ns delay after the discriminator got you to the middle (max amplitude) of the pulse.

author: voltsandjolts
author_rank: Unknown
timestamp: May 23, 2025, 07:47:49 am
content: Quote from: Rad_to_Curie on May 19, 2025, 08:32:20 pmHmm, it doesn't visibly glow, it emits photons. A PMT then converts the photons to electrons, which are then multiplied by several million as they cascade through the PMT plates. The photodiode used by the OP has much less capture area and much less gain than typical PMT detector arrangements.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2025, 08:13:23 am
content: The output from the scinillator are short pulses of light. So with enough radiation one could see it glow and a dark adaped eye could see individual pulses.Depending on the energy the pulses would be some 10000 photons, so not really that much intensity. For an ideal photodiode that would be a pulse of some 1.6 fC or as an example 1.6 nA for 1 µs (if assuming square shape for simplicity of the calculation). Stretching the pulse to some 3 µs resuces the pulse hight correspondingly. With so little light one can not really affort to use only a small fraction of the dedector area.For just testing the pulse detection and getting a simple system running, one could use a bare photodiode to dedect alpha particles from a near source. It still need complete darkness from a good cover over the experiment. These usually have more energy and can give a bit larger (e.g. 10-100x) pulses already in a smaller diode. I have build such a system with a BPW24 with the lense removed. For initial test without a radioactive source and adjustable stable peaks a LED with RC drive from a square wave works OK. AFAIR I used some 50 pF and 100 ohm parallel to the LED.

author: voltsandjolts
author_rank: Unknown
timestamp: May 23, 2025, 08:28:29 am
content: Quote from: Kleinstein on May 23, 2025, 08:13:23 amWell, OK, I guess 'glow' is a loosely defined term. It's certainly not like looking into a torch!Apparentlythe human eye can detect single photons.

author: iMo
author_rank: Super Contributor
timestamp: May 23, 2025, 09:12:23 am
content: FYI - long time back I made a detector with the SFH205 IR diode (because I got it cheap in a local store), while amplifying the pulses by 3 transistors. It detected pulses from my WWII dials, but way much less frequently compared to Geiger tube. On the other hand the pulse's height may indicate the energy, so perhaps I would fire up it again and log the pulses with my DHO and post process the data then..

author: RoGeorge
author_rank: Super Contributor
timestamp: May 23, 2025, 09:39:02 am
content: Quote from: Rad_to_Curie on May 19, 2025, 08:32:20 pmIt is good engineering practice to always write the leading zero for values as 0.3V or 0.15mV.Or maybe write 300mV and 150uV, your call..3V or .15mV is prone to misinterpretation errors, and harder to read.I think there is a typo in the decoupling capacitors of the first first 2 opamps. C16, C17, C22, C23 are all 0.1uF. I guess two of these has to be 10uF, similar with the decoupling capacitors from the positive rail.Another mention about the decoupling capacitors, put the 0.1uF as closest as possible to the opamp pins, then the 10uF next. In practice, the 10uF may not be needed at all. Not saying to remove them, the 10uF are (probably*) safe to have.* Some voltage regulator topologies may become unstable (may oscillate), or may have poorer voltage regulation when too much capacitance is added at their output. Same for heaving too little filtering capacitance at the output of a regulator. This last paragraph is a generic note, you have to read the datasheet for the exact voltage regulator in your design, and follow the datasheet recommendations. If not specified in the datasheet, then the missing info is usually to be found in theApplication Notesfor the given regulator model/manufacturer. Again, this is generic best practices info, I didn't check the datasheet for your +/-2.5V regulator, to see if it can withstand or not the additional 4x10uF (from the opamps).Oh, and very nice project as a first step into practical implementation of theoretical knowledge!

author: iMo
author_rank: Super Contributor
timestamp: May 23, 2025, 10:34:11 am
content: Quote from: iMo on May 23, 2025, 09:12:23 amHere you are.. Actually only 2 transistors, the first one being the BF245 jfet..The white partially naked wire is the grounding for the aluminum foil cover sheet.The time base is 10sec/div. Some pulses are over 200mV from time to time..

author: Rad_to_Curie
author_rank: Newbie
timestamp: May 23, 2025, 02:00:42 pm
content: Wow! Lots of good info to unpack here. Let's get started.---Quote from: jwet on May 22, 2025, 10:09:39 pmThank you for your feedback! I'll begin implementing a comparator/true analog peak detector. For reference, the ADC I have is 300kHz (ideal) sampling frequency. While you would think Nukes would know something about circuit design, but no class I have taken or in my future curriculum utilizes circuitry knowledge aside from 3 phase generation and transmission, and even this is elementary. More the macro level, as almost all of are instruments are direct analog measurements with as little complexity as possible to expedite repair and for safety purposes. Some people are surprised to hear the tech we use in plants is still from the 60s. In short, as of now unless I chose another major/minor, I won't know too much aside from my own research in circuitry.---Quote from: EggertEnjoyer123 on May 23, 2025, 03:41:41 amThanks! I'm thinking I'll just using a direct battery connection with a voltage regulator at 3.5V-3.7V---Quote from: Kleinstein on May 23, 2025, 03:42:25 amThanks for reviewing the SiPM. I didn't register "optimized to human eye" as being the sensitivity of the human eye but rather the spectral sensitivity (as a percentage). I'm not sure where the 300 pF came from on the datasheet, as the graphs in the appendix show a far lower capacitance, and even the typ. capacitance with 0 Vr is 43 pF.---Quote from: jwet on May 23, 2025, 06:07:45 amI am implementing the pulse hold and trigger system. The crystal I have is also limited by its area, with a 4mm L x 4mm W x 16mm H area, so the area of the sensor would be limited by the critical angle of refraction of CsI(Tl). This can be achieved by Snell's Law:θ=arcsin(N2/N1)CsI(Tl) has a refractive index of 1.79, and the Si optical gel's is 1.43. This yields a critical angle of ~53.1 degrees. Using trig, we can estimate the maximum length of a sensor chip. Assuming the distance from the glass to the sensor is about 2.5mm, this means any sensor wider than about 8mm (assuming centered perfectly) will not be significantly more efficient than the 8mm sensor (theoretically). Thanks for the input!---Quote from: voltsandjolts on May 23, 2025, 07:47:49 amClose. Emitting photons is exactly what glowing objects do, whether they glow to the point a human eye can see it is a different question. However, CsI(Tl) is able to glow visibly (especially if close to a radioactive source) if the eye is adapted correctly to the dark. You are right in that it's not a good reading lamp!---Quote from: iMo on May 23, 2025, 10:34:11 amWow! Interesting. Be careful with the radium though, I feel bad for the poor women who had to make those dials...---Quote from: RoGeorge on May 23, 2025, 09:39:02 amThanks for your knowledge! You were correct about the decoupling caps, and with the schematic I didn't worry about their exact position. In practice, they will be extremely close.---Thank you all again! You've been very helpful as always. Expect another update in a few weeks!I'll be implementing a comparator pulse-holding system and revising the photodiode. In case you wondered how I modeled the radiation event, I used an exponential function with an initial amplitude of the bias current, a peak amplitude of the typical pulse, and rise and fall taus to model the rise and fall of a pulse (1 tau rise and 3 tau fall followed by a 10 tau low amplitude tail).Edit: My equation didn't turn out right.... Sad

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2025, 02:39:38 pm
content: There seem to be different version of the BPW21: the DS from Siemens (old), Vishay and Osram give quite different capacitance ( 580 pF,1000 pF , 48 pF at zero voltage) and dark current.Not sure if they changed the part that much or if there are mistakes.4x4x16 mm³ is a really small detector crystal. I had something much larger in mind. The one I build a unit with was some 38 mm diameter x maybe 30 mm.The BPW21 is still quite a bit smaller (2.75 mm square) than the detector, so roughly half the area and the glass adds some distance.The filter at the BPW is to approximate the eye spectral curve. For the main wavelegth (415 nm) from NaI(Tl) this may loose some 30-40% of the light. So better get a diode without filter.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 23, 2025, 02:53:32 pm
content: Quote from: Rad_to_Curie on May 23, 2025, 02:00:42 pmUse backslash dollar for embedded MathJax, or backslash double dollar for centered formula:https://www.eevblog.com/forum/beginners/how-does-posting-math-notation-work/msg3994178/#msg3994178If you forget the syntax, press the last emoticon sign for ohms,. In editing mode, it will divulge the syntax.

author: Rad_to_Curie
author_rank: Newbie
timestamp: May 23, 2025, 03:11:21 pm
content: Quote from: RoGeorge on May 23, 2025, 02:53:32 pmAhhhh the / $. I even read that in the forum and didn't do it with the MathJac

author: Rad_to_Curie
author_rank: Newbie
timestamp: May 23, 2025, 03:15:27 pm
content: Quote from: Kleinstein on May 23, 2025, 02:39:38 pmYeah, when I looked into it I also found several models of the BPW21. The spectrum of CsI(Tl) is around 530-540nm. The crystal is small, but thats really because thats all I can afford at the moment... CsI(Tl) crystals are very expensive nowI plan on getting a larger diode as well.

author: iMo
author_rank: Super Contributor
timestamp: May 23, 2025, 04:06:06 pm
content: fyi - this is alowmid cost spectrometer project (well, under $600 in 2018)..https://www.reed.edu/physics/332/pdf/Gamma%20Ray%20Spectroscopy%20-%20Arduino.pdfPS: I repeated the measurement with twice the voltage at the amplifier and diode (16V), better shielding with the foil, better o'scope setting (50MPts at 2us each, 20MHz BW) and closer to the dial. Below 100secs of the measurement..Added a Radium energy spectra fromhttps://www.gammaspectacular.com/blue/ra226-spectrumIt looks like with my $3 detector I can see similarities with the "peaks" position, can I??

author: jwet
author_rank: Frequent Contributor
timestamp: May 24, 2025, 04:33:25 pm
content: Attached is a block diagram of the type of peak detector used in a Pulse Height Analyzer. The very simplest peak detector is to just take a rising discriminator output and fire a delay to capture the peak. The problem with that approach is the peak isn't a fixed time from the first rise depending on pulse height. This creates an issue called "pulse walking" where peak capture depends on peak height a bit.This block is a high performance pulse height capture circuit and is used extensively. It is suitable for medium performance systems. It can be implemented with simple comparators, 5 Mhz op-amps, HC MOS Logic, a 10 uS data converter (with a convert now pin) and a microcontroller with good interrupt latency. The figure is annotated with test points and traces at the bottom. Minor details aren't shown. A pulse arriving goes through and all pass/delay to maintain timing between the differentiator and the input signal. The differentiator has a delay along with its d/dt function. Any pulse over a noise level, arms a SR flip flop that controls a track and hold. The zero crossing of the differentiator captures the peak of the pulse, stops the track and hold and starts a conversion. At end of conversion, an interrupt is generated to pick up the ADC value. With a moderate quality ADC (10us), count rates up to about 20k/s can be processed. Typically, a SAR with 4 greater bits than bins than required is used to control bin width and DNL effects of raw ADC's. I hope this makes sense.Higher performance PHA's do several things beyond this to improve specific areas- you can read up on these in the literature or the schematics from Canberra or Ortec, etc.1. A short FIFO with or without DMA may follow the ADC to alleviate getting clobbered by interrupts. This can also be implemented in FPGA and pulses can be pre-binned in the FPGA logic.2. A "slider" circuit adds random voltage to the input and then subtracts this in the digital domain. This can dramatically increase DNL(Bin width) with a minor decrease in dynamic range. The idea is the width of individual ADC code widths are averaged over the width of the slider (usually 10%) which is the DR reduction.3. "Automatic Gain Control" can be implemented by putting a small source into the scintillator and by using closed loop gain control based on a single channel analyzer (window comparator) that tracks the dopant. Similar things can be done in software with a suitable scintillator.4. Post processing after RAW PHA capture can be used to enhance a lot of leftover non linearities before display.Good luck with your project. I worked in the field at a high level for about 10 years and I see so many amateurish circuits and techniques on the net.NB- these functions can be accomplished in the digital domain in an FPGA or very fast processors after a fast ADC digitizes everything that comes in. This seems to be the trend in some commercial equipment. Its akin to software defined radio which is completely legitimate and has some real benefits. The difference in my mind is radiation detector outputs are asynchronous, pulses appear according to counting statistics. Applying a fast enough ADC to overcome these corners and then processing in an FPGA that is also pushed to a very high speed for similar reasons is a heavy hammer and a power hungry way to accomplish this. Similar to SDR, they do offer flexibility but the PHA function isn't something that is likely to change. I think to have decent MCA SDR chain, you'd probably want to sample at over 1 MSPS. I've learned a bit in this exercise, which I was I'm here. I hope I wasn't too harsh in my criticism of your designs. Its a great topic and can be a good career at the right company. Good luck.jwet

author: Rad_to_Curie
author_rank: Newbie
timestamp: May 27, 2025, 12:01:03 am
content: Quote from: jwet on May 24, 2025, 04:33:25 pmWow! Thanks for this. Don't worry at all about criticism, this is literally the first ever electrical design I have ever attempted. I'm a lot more comfortable in the lab dealing with the physics and chemistry than this completely unfamiliar field. Your input is extremely helpful, and I'm glad you've learned things alongside me!

author: jwet
author_rank: Frequent Contributor
timestamp: May 27, 2025, 02:59:22 am
content: Rad_to_Curie- you're very welcome. Its encouraging to see a new grad embracing a field and taking on an ambitious project. If you are going to be interviewing or sending out feelers, try some of the Nucleonics companies in Oakridge/Knoxville Area. Some of the best are there- Ortec, Tennelec and a lot of smaller start ups. There are more in the Albuquerque/Los Alamos area- Eberline and spin offs. Cleveland, Ohio has some too- Victoreen is a respected company that I think is in the Fluke/Tek/Danaher family. There is work at the national labs too. The problems range from equipment to calibrate and maintain X-Ray and CAT Scan type equipment to Area and Effluent monitors for Nuclear facilities. Make sure you bring some documentation of your MCA- it would be very impressive to an interviewer.iMo- that's really cool. Better than I would expect at those conversion rates. What is the source- it looks like Cs-137 maybe?Carry on guys.

author: Rad_to_Curie
author_rank: Newbie
timestamp: May 27, 2025, 03:06:06 am
content: Quote from: jwet on May 27, 2025, 02:59:22 amThanks! But just to clarify, I'm a "First year" (actually a junior with the amount of college credit I came in with from high school), and I'll graduate in 2027. Currently, I'm also training as a reactor operator.Also, I believe the source iMo used were radium painted watch faces and hands, which is Ra 226.As a general update, I have been working to find a properly suited photodiode to begin implementing the aforementioned suggestions. So if anyone has any recommendations that would be great!

author: jwet
author_rank: Frequent Contributor
timestamp: May 27, 2025, 03:21:22 am
content: I have some large area Hammamatsu photodiodes- 100 mm^2. This was the core of an Army Pocket Radiac that I worked on in the mid 80's. This was a very ambitious project mainly because it was designed for a tactical nuclear battlefield. Our Rad Hardness specs were the same as the M1 Abrams tank- ridiculous prompt neutron fluxes and gamma rates to 1000 R/Hr. It also read out dosimeters that would be worn by individual soldier. This was crazy cold war stuff where the Army was preparing for lobbing battlefied nukes around in Europe. We shipped 10,000 units but I don't think they ever got of the dry pack in depot storage. Around 1990, after Reagan and the fall of the Soviet Union/Berlin Wall- cooler heads prevailed.The Pocket Radiac has crude MCA that had 32 channels IIRC. It didn't capture all the pulses but captured enough to get a "spectrum" that we could use for energy compensation to give tissue equivalent readings in Grays, etc.PM me a snail mail address and I'll put at least one in an envelope- I have to find them. They were made with high resistivity material for very low leakage. Reverse leakage generates shot noise and limits your lower detection limit/noise floor. I think they were OK on bias up to 18v.Take care.

author: Rad_to_Curie
author_rank: Newbie
timestamp: May 27, 2025, 03:26:10 am
content: Quote from: jwet on May 27, 2025, 03:21:22 amWow, thanks! You've been an enormous help.Best

author: jwet
author_rank: Frequent Contributor
timestamp: May 27, 2025, 04:10:23 am
content: I got your PM and responded. I couldn't attach a Photo so did it here.

author: Someone
author_rank: Super Contributor
timestamp: May 27, 2025, 04:27:51 am
content: Quote from: jwet on May 27, 2025, 04:10:23 amHamamatsu are still making those in the same packaging, pricey but a wide range of options to match the exact application.

author: jwet
author_rank: Frequent Contributor
timestamp: May 27, 2025, 04:45:33 am
content: Cool- I think for his app, he should shoot for something with lower leakage/shot noise.I do the calc to see what your noise level will be with these. These might have been from a different project that was an optical power meter.

author: Someone
author_rank: Super Contributor
timestamp: May 27, 2025, 09:16:28 am
content: Quote from: jwet on May 27, 2025, 04:45:33 amWith a well defined and small dynamic range but a desire for speed I feel SPM is probably more appropriate (as the OP already considered)https://www.gmp.ch/htmlarea/pdf/SPMMicro_Datasheet_V203.pdf

author: EC8010
author_rank: Frequent Contributor
timestamp: July 11, 2025, 07:46:46 am
content: Don't know if you're still working on this project, but the AD8034 data sheet includes a very useful peak detector circuit. As shown, it uses three ordinary junction diodes, but for radiometric work they should be replaced by Schottky types to avoid charge storage problems. If I remember correctly (over fifteen years ago) 1N5711 worked. You still need a means of discharging the storage capacitor after you've measured each pulse, otherwise you just see a gently rising voltage. I think I used DG211 analogue switch.

author: Rad_to_Curie
author_rank: Newbie
timestamp: August 25, 2025, 06:06:22 pm
content: Hello all,Thank you all for your replies. Unfortunately, I haven't had too much time to work on this project as of late because of my class schedule, research, and reactor operator training. Thanks to all who have replied since my last update.What I have decided on is to use the BROADCOM AFBR-S4N44P014M SiPM. After reviewing photodiodes, they are not effective due to the cost-to-performance ratio required (they get extremely expensive when the specs needed are present).That is all I have been able to accomplish as of late; I have implemented the new chip into the circuit and added the necessary power supply. I will continue with updates as they come!Best,

author: KE5FX
author_rank: Super Contributor
timestamp: August 25, 2025, 10:47:32 pm
content: It looks like the integrated PMT-based scintillator-detector combos thatI played with a while agoare still available, if not quite as cheap as they used to be. It might be interesting to compare your SiPM-based detector with one of those units as part of your project.The PMT-based units are very sensitive -- more so than a Geiger counter (or at least, more sensitive thanmyGeiger counter.)

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2025, 12:31:46 am
content: Quote from: KE5FX on August 25, 2025, 10:47:32 pmI have an old GR-135 which has a 4 cubic inch NaI detector and PM, (1) and it picks up background radiation just fine, although I was disappointed that it could not detect a bunch of bananas at contact range. It will easily detect and identify the Americium-241 inside of a smoke detector at contact range without disassembling the smoke detector.(1) At least I think it uses a photomultiplier, but maybe the detector is a photodiode.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 26, 2025, 08:24:34 am
content: Potassium as in bananas is a relatively weak source. AFAIR much of the radiation is beta and would not leave the banana. One can still see the posassium from some potasium salt (e.g. pot-ash).A scinillator detector should be quite a bit more sensitive than a GM counter. It is the larger mass of the active medium (solid vs gas) that makes the difference. Using a PMT or avalance diode detector for the light mainly effects to extra electronic noise, not how many actual counts from the background come in. It can be indirectly as the electronic noise looks like low energy pulses and thus require a higher cut off.I had a 1.5 inch NaI detection and AFAIR I got background counts of around 20 to 100 per second, depending on the low energy cut off. A GM counter is more like 10 per minute, but it depends on the size and region of cause.There are usually quite a lot lower energy events.