homebrew multi-channel analyzer
Posted: Wed Aug 20, 2014 3:26 pm
Hi!
This is a small multi-channel analyzer I've been working on recently. It's to be used with scintillation probes that have a PMT as primary amplifying stage.
I only have a BC-412 plastic crystal (sensitive to >50keV gamma and fast neutrons) at the moment, which is pretty sensitive on the one hand, but pretty useless for gamma spectroscopy on the other. Still, it's sensitive to fast neutrons and I hope I can use the energy information to do some discrimination of real neutron hits against gamma / x-ray background and EMI.
This is how it looks inside with the shielding removed, all PCBs were etched at home.
These are the schematics of the 4 main boards (the ones inside the old TV-tuner box):
The detection of the impulse peak heights is done in hardware, the microcontroller (dsPIC33f) only has to sample the peak values afterwards. No pileup-rejection is implemented yet -- I saved this for revision #2, if necessary. I get the data out by means of I2C:
The HV for the PMT comes from a selfwound ferrite-type transformer used in a resonant Royer-oscillator (@ about 50kHz). The output voltage can be set to any value from 150V to 1500V, so it's a generic supply for geiger / PMT type projects (I would have posted a picture, had the board software let me add another attachment).
For a test, I fed pulses of well-defined height into the device using an arb-function generator. This is how the "spectrum" looks like:
Looks pretty gaussian to me, the total width of the peak corresponds to about 40mV "noise" on the A/D input. (This is one of the things I don't know yet -- is it "real" noise from outside, or is it just the S/H circuit jitter?)
A "real" spectrum looks more like this (sample was a small piece of Uranocircite, reads about 0.8uSv/h on my pocket geiger):
Of course, this is nowhere near being perfect; but I think it's a start and good for some measurements -- if I can get neutron discrimination right with the energy resolution of the crystal, it's served it's purpose.
(I also posted the circuit over at fusor.net and asked for improvements / comments; but got nothing yet -- so probably somebody here can comment on the EE-part and give me some hints what to do better next time).
-Philipp
This is a small multi-channel analyzer I've been working on recently. It's to be used with scintillation probes that have a PMT as primary amplifying stage.
I only have a BC-412 plastic crystal (sensitive to >50keV gamma and fast neutrons) at the moment, which is pretty sensitive on the one hand, but pretty useless for gamma spectroscopy on the other. Still, it's sensitive to fast neutrons and I hope I can use the energy information to do some discrimination of real neutron hits against gamma / x-ray background and EMI.
This is how it looks inside with the shielding removed, all PCBs were etched at home.
These are the schematics of the 4 main boards (the ones inside the old TV-tuner box):
The detection of the impulse peak heights is done in hardware, the microcontroller (dsPIC33f) only has to sample the peak values afterwards. No pileup-rejection is implemented yet -- I saved this for revision #2, if necessary. I get the data out by means of I2C:
The HV for the PMT comes from a selfwound ferrite-type transformer used in a resonant Royer-oscillator (@ about 50kHz). The output voltage can be set to any value from 150V to 1500V, so it's a generic supply for geiger / PMT type projects (I would have posted a picture, had the board software let me add another attachment).
For a test, I fed pulses of well-defined height into the device using an arb-function generator. This is how the "spectrum" looks like:
Looks pretty gaussian to me, the total width of the peak corresponds to about 40mV "noise" on the A/D input. (This is one of the things I don't know yet -- is it "real" noise from outside, or is it just the S/H circuit jitter?)
A "real" spectrum looks more like this (sample was a small piece of Uranocircite, reads about 0.8uSv/h on my pocket geiger):
Of course, this is nowhere near being perfect; but I think it's a start and good for some measurements -- if I can get neutron discrimination right with the energy resolution of the crystal, it's served it's purpose.
(I also posted the circuit over at fusor.net and asked for improvements / comments; but got nothing yet -- so probably somebody here can comment on the EE-part and give me some hints what to do better next time).
-Philipp