Help with detector project - Page 1
Forum section: electronics / projects

author: snx
author_rank: Contributor
timestamp: January 11, 2018, 05:32:22 pm
content: Hello everyoneI'm new here and are have a medium to good experience on some part of electronics, mostly radiation detection and nixe tube circuits. I'm planning a new project and need your help.I want to build my very own gamma spectroscopy system with silicon detectors for mainly two reasons. Commercial detectors are beyond hobbyists budget, and current free systems are comming to their limits - or are to unpractical.Basic Info:For gamma spectrometry you need to measure radiation, better the radiation energy. For this you can use a photomultiplier (a silicon one in my case) with a special crystal. When the crystal gets hit by radiation, it puts out a amount of light in relation to the radiation energy. This light is measured by the photomultiplier.The silicon PMT is connected to a 28V Source and coupled by a capacitor. In case of an event, the capacitor produces a negative spike on its output. The shape of the signal roughly looks like this (but in negative):http://aemstatic-ww1.azureedge.net/content/dam/lfw/print-articles/2014/11/1411LFW04f2.jpgThe parameter for my signal is as following:Smallest signal: Amplitude ~5mV, lenght 2 uSLargest Signal: Amplitude ~1.7V, lenght 40uSThe task is to measure the amplitude, there have to be atleast 4096 different amplitudes, but the more the better. The only relevant information of the pulse is its height, and for an option its lenghtI have little to no experience with ditialization of analog signals. So i would like to kindly ask if someone here could provide help on how to do this with a mircocontroller. I'm familiar with STM32Thank you very much for your help

author: massivephoton
author_rank: Contributor
timestamp: January 11, 2018, 08:34:21 pm
content: Hi. Welcome.Quote from: snx on January 11, 2018, 05:32:22 pmStarting from a muon detector might be half way into the job:http://cosmicwatch.lns.mit.edu/detector#section4The link circuit uses an amplifier and a peak detector.The circuit uses the internal microcontroller ADC, but it shouldn't be that difficult to interface with an external ADC should you need more time and/or amplitude resolution.Regards.

author: dmills
author_rank: Super Contributor
timestamp: January 11, 2018, 10:50:35 pm
content: There is a design for a seriously old school pulse height analyser in AOE second edition, the processory bits are comically out of date, but the front end stuff may give you ideas....regards, Dan.

author: snx
author_rank: Contributor
timestamp: January 12, 2018, 06:35:34 pm
content: Quote from: massivephoton on January 11, 2018, 08:34:21 pmFor a very slow MCU like an Atmega328 (16Mhz) a peak detector might work. But i've read that peak detectors have very bad performance at high speeds.And as far as i understand peak detectors work on diodes, but wich diode will conduct at 5mV!, the Diodes i know conduct at >350mVI will provide a image of the pulses soon so you can better understand my question

author: snx
author_rank: Contributor
timestamp: January 12, 2018, 07:04:37 pm
content: Find attached two signals"small.png" represents one of the small pulses, but not the smallest! Amplitude of around 50mV, lenght 19uS"big.png" represents a medium large pulse. Amplitude of around 500mV, lenght 55uSMy current test setup lacks a few options, this is why its impossible to capture the smallest and largest signals. The smallest can't be detected because of the 10mV pk-pk noise, and the largest can't be detected because of the small area crystal.But for the general task, to digitze the amplitude, the signals could be used in a test application. Since a SIPM + final size crystal will be around 300$, i would like to have a working circuit before ordering the crystal...I hope this gives you a better idea of the signals

author: vindoline
author_rank: Unknown
timestamp: January 13, 2018, 01:20:33 am
content: Hi and and welcome to the Eevblog! However, I would recommend that you ask your questions over at the gamma spectroscopy group:https://groups.io/g/GammaSpectroscopy/There are many of us over there who have built very respectable systems on a hobby budget.Also, if you haven't yet, please take a look at the theremino gamma spectroscopy project:http://www.theremino.com/en/blog/gamma-spectrometryThat said, almost all of us amateur gamma spectroscopists use traditional hardware involving photomultiplier tubes coupled to thallium doped sodium iodide crystals NaI(Tl). Excellent parts are available on the secondary market at reasonable prices.My understanding of silicon PMT's is that their advantages are small size and low bias voltage requirements - however fairly expensive. Also, I'm not sure that their spectral sensitivity matches well with commonly available scintillation crystals. Exotic crystals can be difficult to source and also expensive.In any event, the basic details of photodiode amplifiers and spectroscopy pulse shaping will be the same regardless of the detector and are widely available on the web.

author: snx
author_rank: Contributor
timestamp: January 13, 2018, 06:41:17 pm
content: Hello,I currently own a commercial grade NaI(TI) detector. For the current setup, i constructed a pcb that has the HV supply and a sound-card ic on board.Also i know the Theremino Software and the PRA Software which both are free and operate on sound cards. I really like it and use it in my lab. But i'm currently look for a smaller, portable version.However, these setup is rather very large and is more parts. A standard Soundcard (96khZ) sample time is around 10uS, therefor its not suitable for measuring very small pules.If we look at a very small pulse from a SiliconPMT with CsI(Ti) its going to be 5uS long, if we want at 50 Samples of that signal, we would need 100nS sampling time which would be a 10Msps ADC.Maybe my question was explaned enough, i currently don't see the problem in hardware part. I would have no problem to feed a signal to a ADC which is read out by a STM32 (for example STM32F @ 180MhZ with a 10msps, 16bit ADC). But i currently have not the knowledge to calculate the peak height in software.

author: vindoline
author_rank: Unknown
timestamp: January 14, 2018, 07:43:02 pm
content: Quote from: snx on January 13, 2018, 06:41:17 pmThe answer is that you don't try and sample the raw signal pulse from your detector. You need hardware pulse filtering and shaping to give a gaussian shaped pulse where the pulse height is proportional to the particle energy and the pulse width is able to be sampled by your system. This is explained in "Techniques of signal conditioning for Gamma Spectrometry" from the Theremino people. The circuit in the Theremino PMT adapter is very simple and works well. You can just leave off the HV supply portion and utilize whatever bias circuitry your silicon PMT needs.

author: snx
author_rank: Contributor
timestamp: January 16, 2018, 06:02:30 pm
content: But sampling with only 96kHz will limit your count rate to around 48kHz, since the pulses either get piled up or ignored, depending on the circuit.Still, i want to point out my problem:Its not about how fast or slow, i'm looking to digitize the signal in a MCU, not a pc software solution. For the conversion it doesn't matter how fast / slow the signal is, because the shape will always be the same!And i don't think its a good idea to just use one MAX value on the pulse. As far as i know, most commercial solutions are using somekind of math function/dsp.For the signal shaping and amplification amptek has provided a pretty good schematic from its pulse processor:http://experimentationlab.berkeley.edu/sites/default/files/images/DP5_User_Manual_A1.pdfPage 29,But obviously, they don't share their code, because they want to sell it