Gamma spectroscopy with an SCA

[ChristofferBraestrup]

While waiting for the software for my MCA ISA card, I've been playing around with using a single-channel analyzer for energy spectroscopy.

While slow and tedious, and low resolution, I did actually get something out of it that looks like a spectrum.

My setup is: about 200 gram potassium carbonate as source (chosen as it has a single gamma energy, approx. in the middle of the normal spectroscopic range).

[HV BIAS SUPPLY]--->[NaI(Tl) scint. det.]-->[Preamp]-->[spectroscopy amp]-->[SCA]--->[Counter]

Count time was 1 minute, SCA ΔE was 400 mV, which gives 25 bins over a 10 V range. I would then just start counting at the SCA E lower limit, and move it up one window size or 'bin' and repeat.

The resolution isn't great, but the spectrum looks similar to what I would expect it to, and using the SCA to count the entire supposed 40K photo peak, I saw a drastic drop in counts when the potassium (carbonate) sample was removed.

The SCA (Tennelec TC441) has an analog input to set E, and I have an Ortec 879 RS232 counter interface, so I'm considering computerizing the thing, having a DAC move a tiny window up for each count cycle and then recording the counts.

Might not be super practical but I find it pretty interesting. Hope you do as well!

I made a little video on the setup as well: https://www.youtube.com/watch?v=HVlnT7g0P_A

[Richard Hull]

I watched the video. Nice little piece of work and great use of the NIM in such a fashion. Tedious, yes, but teaches the entire concept of gamma spectroscopy better than reading it in a book as you own the concept at the core level.

While I have had my old large Canberra Gamma spec (1980's vintage), for about 18 years now, It is all self contained TTL, no software needed. It will spit the data points out of its old 25 pin RS232 port. A simple program written years ago collects the data and I can download it to an excel file for graphing. Crude but it works great.

Note: I was faced with getting the most potassium possible per unit mass and volume, I came to use only Potassium Chloride (KCl) to keep the atomic percent of K as high as possible in a sample for a quick signal grab. It is easy to buy. Many water iron traps us it by the ton. I obtained a 25lb bag years ago for under $20.00. I have a small poly zip lock bag full of the ice cube sized lumps.

I love doing the demo's for folks who think all radiation kills. I don't tell them what it is. I put the baggie of white lumps in front of a 2" pancake GM detector and show it is radioactive well above background. I then put it in front of the 3" NaI:Tl scintillator hooked to my gamma spec. They watch the two >1mev gamma peaks grow on the glowing CRT screen. I tell them those are two fierce gamma rays of over 1 million electron volt energy, each. They then gasp as I take a large lump out of the baggie and lick it all over and around like a lolli-pop. They typically freak out and say I am crazy. I say, no! I ask, "Does anyone use or know of a person using a salt substitute or know of those taking, by prescription, potassium supplements?" Most say yes. I mention they are gulping down this very substance, daily. I lick this stuff only in demonstration maybe once every two months! Folks just do not know radiation. My first ever Potassium source was an expensive set of ten "No Salt" shakers from the grocery. Then, a friend told me about the water filter bags of the stuff for almost nothing. I still have all of the No Salt, but the pure lumps work much better for gamma spec work.

Richard Hull

[ChristofferBraestrup]

Interesting with the potassium demo! You're right, density and elemental ratio of potassium is higher in KCl, I just happened to have the carbonate on hand.

K-40 has the main disadvantage in spectroscopy that it needs a long counting period to yield really good data, but the main advantage is that it has only one peak, and since it's about in the middle of the normal gamma spectroscopy range (0-3 MeV),, calibrating PMT bias and amplifier gains to put the K-40 peak around 5V pulse height is a good starting point, i found.

Full disclosure, I tried this with a sample of uranium glass and the resulting "spectrum" was not encouraging. Nothing that even looked like peaks was found, but given the lower activity, and the counts dispersed on more energies that shouldn't be a surprise.

[Richard Hull]

I'm sorry I was thinking of the double peaks beyond 1 MeV of Co60. There is just one gamma peak in K40. Sadly, it is only 11% of the emission. The rather fierce 1.3 MeV beta particle is seen as 89% of the radiation from K40 and is easily detected well over background in even a crude GM counter.

I did just run a nearly 1500 second collect and 1500 second subtraction count on my Canberra 40, MCA in PHA mode. I used a 3 lb bag of KCL. I attach images of the KCL and then a screen capture. the energy of 1454 keV is seen in the lower left and the cursor tick is in the middle of the peak. I set my system to 1 keV for each of the 2048 channels. Fairly close. The actual recorded run was on the order of 1500 seconds and the collection was stopped and then resumed in subtract mode after removing the KCL. You see the background did not fully subtract at the low end. No big deal, of course. The peak at 1500 seconds was 438 counts and we see very few subtractions were seen from this peak. The source was moved over 12 feet away. Could a few gammas have reached the 3X3 NaI crystal from there is subtract mode? I think I bought this model 40 from my good friend Bill Kolb in 2003 for about $400. It has been a good MCA and not given much trouble save for a replaced video CRT sync driver IC some years back.

Richard Hull

[ChristofferBraestrup]

Ah that looks almost like a textbook gamma spectrum! Beautiful!

I've been looking at the different canberra MCA units on ebay as well, they are nice, and there's no proprietary software as with the PC based ones. However freight is a bit unforgiving, and "does not power on" could be a good many issues.

Background potassium-40 is almost impossible to get rid of completely, I've heard somewhere that even in high-end HPGE spectrometers, there's usually a small peak from K-40.

I've just received a few new NIM-modules, including an Ortec 771 preset counter-timer, so I can do two-channel counting in a set time period,and then reset and re-count by a single button push. Works a treat!

new nim.jpg (56.94 KiB) Viewed 3759 times

[Richard Hull]

Nice, clean, old Ortec counters. I have a 771 in my collection. I have never seen the 778 dual unit. You got some good stuff there. NIM is the nuke guy's erector set. Crude, old, but still useful building blocks.

I have 6 volt supplies and just drop a 6 amp silicon diode or two in series with the +6 v buss within any module I hand craft to be the 5 volt supply. (I prefer TTL levels to NIM levels) Sometimes I hand select and test many pairs of diodes and can wind up between 5.1 and 4.9 volts on my 5 v internal cards. I hate zeners and I could probably get close with 1 6 amp silicon and a small 3 amp string of schottky diodes, but 5 volt stuff doesn't really care nor do I.

Some will say use the 12 volt buss with a 7805. A lot of stuff in a bin uses +12V and I do not want to load that buss needlessly or throw away the 7 volts in waste heat. The + 6 v supply that I have is rated 10 amps and almost nothing in my bin uses 6 volts.

Richard Hull

[ChristofferBraestrup]

Yeah I got the 778 from an antiquities dealer in Germany, strangely enough, I can't find any info on it online either. It was sold with manual, which they of course forgot to ship me, but I'm told it's underway.

My bin PSU is the older model that doesn't have the +-6V module - however 6V is furnished in the backplane connectors. I'm considering either tapping the 12V directly and making a high current regulator (7806/7906 driving a 2N3055 as in this article: https://inis.iaea.org/collection/NCLCol ... 015538.pdf

-Alternatively just add local regulation in each module, disconnecting it from the +-6V rail. Right now my only 6V module is a strange single-unit scaler, "Emetron ESA-2000"

I redid the K-40 experiment with twice the resolution, and twice the count time, and got something that looks reasonable. Why the K-40 peak is cut in two is beyond me but both peak signals was significantly reduced upon removing the source. I suspect the two middle counts between the peaks are just unnaturally unlucky low counts.

k40 spectrum hires.jpg (35.85 KiB) Viewed 3703 times

[Richard Hull]

I have never heard of a NIM bin with a built in, powered, +/- 6 volt supply in the rear appended supply. However, I have also never seen or encountered a single NIM bin that was not already fully wired for a +/- 6 volt buss. I have seen a number of 6 volt modules that use the mains, 120 volt AC buss wired to every module on the buss to power up the 6 volt multi-amp supply within it to feed and power the 6 volt pre-wired buss. Thus, all bins will need a plug-in 6 volt power supply module plugged into the bin to activate the buss. Such 6 volt plug-in supplies are rare and relatively expensive if found surplus.

Richard Hull

[ChristofferBraestrup]

Strange, you seem to be right. I have the feeling that all the major manufacturers (ND+Canberra, Ortec, Tennelec) somehow de-facto decided to stick to the 12/24V system.
The NIM standard predates the 5V logic standard (I think), so maybe the original intention with the 6V rail was for tube based module's filament supplies?

Most of the 6V supplied modules I've seen has been from smaller companies noone's heard of, or stuff that looks like it was manufactured at laboratories/universities in very low volume.

By the way, I've settled for using short 50 ohm cables on all signals, as opposed to the proper 93 ohm coax for pulse signals. I believe the impedance mismatch is almost nonexistent if it's a short cable, and it's terminated in the proper load.

Hopefully I'll get the software for my PC MCA soon, so I can start optimizing my spectroscopy setup properly.

[Richard Hull]

I use only short 75 ohm video cables myself. I have checked with a storage scope and have found no real issues in significantly altered waveforms or reflected pulses. Of course in fusor work 1khz rates are rare and 1-2 microsecond shaping is well handled in the 75 ohm cables. The average amateur environment is not classically EMI or RF noisy.

The +/- 6 buss might have also been for older germanium transistor amps as a lot of Ge circuitry could be run efficiently at low signal currents. In the 70's and after, the 6 volt buss became a bit redundant. I have never seen a vacuum tube based NIM Module or pre-amp. However, DC on filaments would be perfect in low level measurements killing 60hz induced noise. NIM was codified at the cusp of tube to transistor technology and that technology was mostly PNP germanium. Silicon changed the entire complexion and power requirements for all instrumentation. IC's compatible with NIM would be CMOS and is indeed used extensively in the most modern NIM modules. NIM is being made obsolete with complete systems the size of a pack of cigarettes, (with expensive and special software to link it to a computer, of course).

Richard Hull

[ChristofferBraestrup]

I try avoiding 75 ohm BNC cables, since the low-end ones are a whole dimension worse than the low-end 58 ohm ones, and the fit of the connectors are so bad I'm worrying about damaging the instrument connectors. If I'm in doubt, I use a 'sacrificial' T-adaptor of decent quality inbetween.

I've done some troubleshooting on my SCA. Here's what I found:

Tennelec TC441 (mid-1970's vintage):

Delta-E 0-10V range:

panel set: actual window size:
0 mV 3.84-4.0 7= 230 mV
100 mV 3.76-4.07 = 310 mV
200 mV 3.65-4.07 = 420 mV
300 mV 3.54-4.07 = 530 mV
400 mV 3.45-4.07 = 620 mV

Delta-E 0-1V range:

panel set: actual window size:
0 mV 0 - 0 = 0 mV
100 mV 3.96-4.07 = 110 mV
200 mV 3.86-4.07 = 210 mV
300 mV 3.75-4.07 = 320 mV
400 mV 3.65-4.07 = 420 mV
500 mV 3.55-4.07 = 520 mV


Keep in mind, voltages are set by dial alone, so the lat two significant digits are dubious.

This looks as if the 0-1V range is spot on, and the 0-10V one is offset 200 mV. Strange, but I guess it isn't critical, as long as one knows. Maybe someone modified it to suit a certain experiment ( a fixed window will always be more precise than variable) or maybe it's just supposed to be like that (I don't have the manual).

Also an interesting note is that the upper window limit is unchanged, it's really a -delta-E window, since it moves the window below E. Not sure if this is standard.



A neat trick I found with the Ortec 771 timer: connecting its STOP to RESET connection on the back of the unit, the INTERVAL connector will give a short positive 5V pulse every time the preset is met.
I used this as a digital pulse generator for my experiment, feeding it to my preamp, to my spec. amp, to a gate stretcher and to the SCA.



The window size was determined by feeding this fixed height (ca. 4V) signal into the SCA, then setting some window size, then moving the window up above the voltage and then down slightly, noting the voltage at which it appeared, then disappeared.

I must say, the absolute best tool for troubleshooting NIM modules is having multiple modules that does the same, or are identical. If two modules behave the same it's less likely a fault and more likely the user's understanding/something else in the signal chain that's causing the odd result.

[Richard Hull]

I have several modules that have been purchased at hamfests that are untested. I work on them easily. I took a 1 slot worthless module, gutted it and ran a straight through NIM plug on a cable. This allows power-up from a regulation bin and the module can lay flat on a table in front of the bin with full access to scope and meters. It can be flipped over as needed to access both sides for repair and soldering. Re-calibration is a snap.

I often find out much about the module that is interesting during test. With no manual, I do a quick little write up of how its functions on the more complex modules for later reference. I guess I have owned, used, repaired and sold a total of over 100 NIM modules and about 10 Bins. I might have about 40 modules left floating around. Most all were purchased from Don Orie of O.E. technologies when that wonderful edifice existed back in the 1990's. I have manufactured from scratch, using worthless modules, about 5 different custom modules.

Richard Hull