I am starting a new project for my university using an arduino board. The goal is to measure the energy of alpha particles (radioactive decay) using an alpha particle energy detector. I already have the detector, its sending pulses about 50 mikroseconds long (picture 1) and has a peak voltage of max. 4-5 volt (the absolute height does not matter, I only need relative heights to calculate the relative energies).
So my plan is on the one hand to measure this peak voltage using a capacitor and a transistor (picture 2) and on the other hand do stuff with the data like evaluate it, sync it with a cloud server etc. So the first question is: Does my circuit layout look okay? The idea is to store the peak voltage of the alpha detector using a capacitor, read it out using the ADC and then use the transistor to decharge the capacitor again. There will be less than 20k peaks during a half hour period, so the ADC read out frequency should be fast enough I think (?). According to google, the Arduino Uno can use its ADC about 10k times per seconds.
The measurement of the peaks is very time critical, because I dont want to miss them (get all peaks during a time period). That's why I was wondering, what board or board combination I could use for the simultaneous measurement and evalutaion stuff? The Arduino yun seems suitable for that because of its co processor: it could handle the synchronizing and evaluation reading the SD card, while the arduino itself keeps measuring and writing the energies to the SD card. Or am I misunderstanding something?
Sorry for the long text I am very interested for your suggestions. If you have questions, feel free to ask.
Actually measuring the total ev energy of an alpha particle will have a lot of assumptions. The numbers you supplied indicate that the pulse rate will average about 17 pulses per second. I assume you alpha detector out put pulse voltage is an indicator of the alpha particle energy. A capacitor in the detection circuit might just suppress the voltage you are measuring unless you are going with the voltage charge on the capacitor after it reaches a max charge. Maybe more info on your thoughts on the project operation.
I would use a faster ADC and just sample the detector output directly.
I use the ADC in the Teensy 3.2 to sample data at 100K SPS, 12 bits, and it works very well, with plenty of time left over for peak detection. The ADC could probably be pushed to 5x that speed.
This is discussed in various threads on the PJRC forum, for example this one.
alpha detector out put pulse voltage is an indicator of the alpha particle energy
yes exactly
A capacitor in the detection circuit might just suppress the voltage
I am not sure if I understood you correctly. My idea is that the sampling rate of the Arduino ADC is too low, therefore I want to "store" the peak voltage of the pulse in the capacitor. The arduino is constantly reading out the capacitor and if it reads out the same voltage for two times in a row (or maybe just once a voltage above 0,1V) it saves the value and decharges the capacitor. Then the capacitor is ready to "store" the next peak. You mean the charging and discharging of the capacitor will take too long?
So the time resolution would be 1s/(100K*5)=2us (maybe higher with the teensy 4.0) which would be fast enough to measure the peak voltage. That would also make the circuit redundant and a master Arduino (or a Raspberry Pi then) could process the detected peaks further. The connection between the the two should be asynchronous, allowing the master to to his stuff while only reacting to detected peak voltages. Also the message sending shouldn't be too long, causing the sampling rate to slow down and not measuring the correct peak voltage.
Maybe we coud also combine these two stategies: Use a teensy (or Arduino) just to handle the measurement circuit and the capacitor (for me this is still the more reliable way to measure the peak voltage - maybe more robust and requiring less hardware ressources) and send the peak voltage to a raspberry pi which will process it. What do you think?
I am not sure, is it relevant? (I am "only" a physician and computer scientist, be patient pls )
Yes it is relevant, and required to know for proper design and function your circuit. The output impedance "R" of the alpha detector and the capacitor have an RC time constant, which must be correctly selected.
R can be estimated by placing a variable resistance across the output terminals and monitoring the output voltage. When that voltage drops by half, the variable resistance value is equal to the internal impedance.
You are more likely to destroy the microcontroller output pin, if you omit the base resistor.
@MorganS
Just writing the data on a SD card is not good enough, since I want to integrate some comfort functions I already mentioned (especially a backup function and remote data access). But you are right, having a Microcontroller for direct measurement and sending the peak voltages to a second device asychronously (e.g. like this) would be the ideal solution. But even the teensy barely does 500k SPS; when it needs to calculate the peak voltage and send it, the rate will drop and peaks might be missed if I judge it correctly.
Do you have suggestions of other devices that would be capable of a sampling rate of >500k SPS and simultaneously calculate the peak and sending the value?
Peak calculation would be like compare the last x measured values and see if there is a maximum in between (I can post some code if you like). This needs to be done every x/2 measurements or something like that.
@jremington
ok thanks I will have a look into that and measure the impedance asap
Thank you guys again for your answers! Appreciate it a lot
I am very interested for your suggestions. If you have questions, feel free to ask.
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