selfmade MCA / gamma spectroscopy setup

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Philipp W
Posts: 19
Joined: Sat Mar 23, 2013 11:03 am
Real name: Philipp Windischhofer
Location: Grein, Austria

selfmade MCA / gamma spectroscopy setup

Post by Philipp W »

Hello!

Recently, I've been working on a 12-bit multi-channel analyzer to be used with scintillation detectors that use a PMT as the primary amplifying stage (as opposed to a photodiode). Since I only have a BC-412 plastic scintillator (Datasheet) at this point, the main aim was not primarily to do serious gamma spectroscopy, but to provide a way to discriminate fast neutrons (the BC-412 is sensitive to >100keV gammas and fast neutrons) against gamma background and EMI.
The MCA was designed in a way to make it easily adaptable to other crystals (Na:I for 'real' spectroscopy) as well - all you have to do is adapt the gain of the amplifiers (by means of potentiometers) and probably the time constants of some filters.

Some pictures:
this is how it looks inside with the shielding removed
this is how it looks inside with the shielding removed
The MCA consists of 5 independent PCBs (etched at home), see below for schematics.
the finished device
the finished device
Everything is placed inside a smallish box, detector and high voltage supply have BNC-type connectors on the rear side, front connections are logic power in (12V DC) and data out (I2C).
high voltage (140-1500V) supply
high voltage (140-1500V) supply
This box houses the high voltage supply, a self-wound ferrite transformer operated in a resonant Royer topology. This guarantees an almost sinusoidal output voltage (almost no higher harmonics are present).
BC-412 crystal with small sample of uranocircite
BC-412 crystal with small sample of uranocircite
The BC-412 crystal together with a small sample of Uranocircite (reads about 0.7 - 0.8 uSv/h on my geiger counter) for a first test.

Here are the schematics of the PCBs:
Preamp.pdf
Preamplifier for PMT signals
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The preamplifier is just a conventional charge amplifier followed by a noninverting amplifier for more flexible total gain (important if you use the MCA with different probes).
RippleStripper.pdf
Ripple stripper for clean HV
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This is a capacitance multiplier (aka "Ripple Stripper") that actively compensates for AC components on the HV supply and brings them down to the ~10mV level (mandatory for detecting low-energy events).
PeakDetector.pdf
peak detector for analog "preprocessing" of input signals
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The peak detector is a central part of the MCA. It detects (and holds) the peak voltage of the pulses from the PMT to allow for digitalization by the processor.
Logic.pdf
logic part
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This is the logic part of the MCA. It uses a small DSP (dsPIC33F) which samples the peak values (= impulse heights) and generates the spectrum. Data can be retrieved from the MCA by means of the I2C bus. Software for the PIC is written in C.

For a first test, I tried to get a spectrum of the Uranocircite sample (see picture above), background was subtracted:
spectrum of uranocircite sample
spectrum of uranocircite sample
Of course, one can't compare the energy resolution of a plastic scintillator to that of a more professional Na:I probe! Nevertheless, I think my setup is principally functioning and I hope it proves to be useful for neutron detection.

Any comments, suggestions for improvements, questions, information on how to best process a spectrum etc. are highly appreciated!

Best regards,
Philipp
Philipp W
Posts: 19
Joined: Sat Mar 23, 2013 11:03 am
Real name: Philipp Windischhofer
Location: Grein, Austria

Re: selfmade MCA / gamma spectroscopy setup

Post by Philipp W »

Recently Doug Coulter sent me two Na:I detectors. Using those and the MCA described in the previous post, the spectra look much more interesting / useful.
uranocircite sample, 1hr live time
uranocircite sample, 1hr live time
small pitchblende sample, 1hr sample time
small pitchblende sample, 1hr sample time
Cs-137 check source, about 20 minutes sample time
Cs-137 check source, about 20 minutes sample time
radium watch hands, about 1hr live time
radium watch hands, about 1hr live time
Of course, since most of the active nuclids being members of the Uranium-Radium decay chain, the spectra (apart from Cs-137) look more or less identical.

Philipp
Philipp W
Posts: 19
Joined: Sat Mar 23, 2013 11:03 am
Real name: Philipp Windischhofer
Location: Grein, Austria

Re: selfmade MCA / gamma spectroscopy setup

Post by Philipp W »

Out of curiosity, I did another analysis - this time of naturally occuring spring water with significant radon content. The sample I used is from the "Hedwigsquelle" in Bad Zell, which is a nearby health resort (that also uses the radon-containing water for some treatments).

I obtained two canisters of about 5 litres each directly from the source. As soon as I got home, I put the Na:I probe between the two plastic canisters. The activity of the water was easily measurable (about 75cps) and about twice the natural background (about 25-30cps).

After energy-calibrating the probe with small check sources of Am-241 and Cs-137, I obtained a three-hour spectrum of the water sample and subtracted a background spectrum of equal life-time.

This is the result of the procedure:
gamma spectrum of natural spring water
gamma spectrum of natural spring water
The higher energy lines of e.g. Bi-214 show up not nearly as significant as the lower energy photopeaks do. (I put this to the fact that the EM-shower started by the incident photon is no longer contained in the scintillator crystal and thus only a fraction of the energy is deposited and measured.)

Philipp
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