Needing more than a spark test?

homebrewed said:
I've been debating over the 3x3mm or 6x6mm SiPM -- see this list on Digikey (please let me know if the link doesn't work). The 3x3 is a little too small for the LYSO scintillators I got here, which are 4x4mm on the SiPM face. If I want to avoid the complication of a light pipe I guess that means the more expensive 6x6, at about $100USD. The bare chips are darn near the same price, something to think about if you're going to lay out a PCB for an ADC anyway.
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Yes - $100 - $68 = $32. You are saying the hassle of getting the perspex in, and making a polished taper exceeds $32, and on the face of it, I agree. I also get it why you went for LYSO. It's non hygroscopic! No extra waterproofs and X-ray transparent foils, etc.

A calculation
I was thinking about it entirely another way. Previously, @RJSakowski showed us that for the energies we want to capture, scintillator need only be 1 or 2mm thick to stop 98% of the X-rays.

The cost of a scintillator is very much about the amount of crystal, and the hassle of dealing with hygroscopic, but my considerations were all about not wasting the area, starting with the shower of X-Rays coming from the sample. I put a lot of value on the input area of the scintillator. I spent a while trawling the QST Photonics site looking for thin versions with area, unfortunately mostly among the hygroscopic ones.

For your scintillator.
For the size of the scintillator 4 x 4 x 22 mm, then using it against a 3x 3 photodiode gathers 56% of the flashes.
If the original area delivering the X-Rays was (reasonably) about 1" diameter, that was 506.2mm2. (Units!)
Those X-rays photons would be headed all directions, including deeper into the sample, and only half headed approximately in the hemisphere toward the scintillator (say about 25mm diameter, at about 40mm range), and only some of those aimed well enough to hit it.

In a crude volume estimate, what I am saying is that there is a huge waste of photons unless the scintillator area presents a bigger truncated cone capture region. If the end of the scintillator is 16mm2 compared to 506mm2, we get 56% of only 3.1%. I was hoping for better than this 1.8%.

If you cut your scintillator into 4 pieces with 3 cuts, using a metal disc and wet abrasive, losing (say) 1mm per cut, and then stacked them together to yield a 4.75mm thick scintillator of 64mm2 area, you get a massive increase in efficiency. If you are willing to use up both your scintillators like this, you get 128mm2.

I set this against getting hold of a chunk of this acryllic stuff.

eBayAcrylic.png

Then turning a taper onto it, and rubbing up the surface with abrasive, finishing with toothpaste or whatever folk use to polish plastic. Even if the S-(PM) diode is only 1mm2, it captures the lot, compared to a horrifically expensive area array 314mm2.

The "bang for your buck" is much amplified by a 11 quid piece of plastic and the HM machining style effort.
I am sure there are many HM members who can offer good advice on how to make this thing.

The taper needs to be narrow enough that most rays starting from the scintillator end hit the inside at more than 42° to the normal.
Looking at the Theremino approach, it appears to me that the main thrust of their approach is twofold. The first is noise management using the passive filters. I'm thinking that a Bessel low pass filter instead of a crappy passive "whatever" filter might be beneficial, because it will do a better job of preserving the pulse shape. This said, an active filter that can handle sub-microsecond input pulses suggests a pretty fast amplifier will be needed in order to really achieve the filter performance you want. The equivalent of a 741 op-amp will NOT cut it. This is where LTC's Spice package can really come in handy, since it includes models for a number of their amplifiers. I have Wine, guess I need to download LTC Spice.
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Sometime around 1985 I had already abandoned any kind of 741 OP-Amp. To lose the DC drift, I would use chopper-type AC versions, or Instrumentation types. By around 2002, there was every kind of video speed op-amp including current feedback types. It is not too hard to find Op-Amps with >50MHz bandwidth. You can get a OPA2300AIDGSR from mouser.com for £2.03. It is a low noise 16-bit accurate 150MHz bandwidth (which would come down to about 10MHz actual operation with feedback). It's a very old device. LTC devices come with Spice models. We have nothing to fear here. It's a dual. Use one for a zero-loss peak detector.

The second Theremino approach is pulse management, where they cull out all the doubtful pulses . They also claim some improvements using their custom deconvolution S/W but that's where I would want to use either Octave or SciPy and leverage stuff that's already out there.
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This is one area I have not thought through, but I think there is massive potential in the PyMCA and all that free CERN software from the LHC, and the NRLXRF (Fortran) software to get somewhere.

Before even we go there, I think there is mileage in side-stepping a whole lot of it, and just do what you would do if you were just looking at it. Your original suggestion, similar to @RJSakowski 's switched capacitor, of a one-shot monostable timed capture-lockout-reset does a lot of the hard stuff in one hit.

Really fancy stuff, like capturing the whole scintillation, and/or parts thereof, subjecting to FFT analysis via octave, and all that clever stuff only has a point if there really is new information in the pulse to spot overlaps, etc. If the main information is the pulse amplitude, that is all we need, we can skip much of the rest.

The really clever sorting will be the analysis of accumulated counts of pulses meeting the criteria to be in the quantized energy "buckets". Assigning some kind of hash value, as a signature of the set of weighted buckets might allow a fast lookup on alloy probability, and display the conclusion on the plot. Maybe that is already done for us in PyMCA

Theremino
The biggest, nicest chunk of Theremino is the app display - a general purpose screen of graph that can be used on phones, etc. None of the XRF stuff that fed it need be replicated by us.
For me, the Raspberry Pi already comes with 2 full HDMI displays. Initially, I don't need a Theremino.

For folk who might want to use a smartphone, I think there are very likely apps that will do a USB link, or use Theremino. This is completely another branch to this job, something to be tacked on at the end.

Re: PCBs etc.
Most folk here would not have too much trouble hanging together evaluation boards. More difficult when it comes to soldering SOIC components. All one can fit on 1sq ft of PCB cost about £40 as a prototyping quantity, which would make about 10 lumps of electronics. I was hoping to use something like THIS

Raspberry Proto Hat1.png

The "plug-in" prototyping breakout, ribbon cable extension, breadboard, and set of connector wires as seen in post #249 cost £8.49.

I will be trying out a S-(PM) like you are, and it will be one of the smallest cheapest. I am convinced that a light-pipe is a good idea. More problematic is a "big enough" scintillator, though it need not be very thick.
 
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Sawing a thick scintillator into multiple thinner scintilators seems like a good plan if you can do it. Perhaps a lapidary saw? That adds a whole new discipline!
Robert
 
I think most scintillator crystals are >2mm thick because they're designed for higher energy X-rays. Unfortunately, the thinner ones don't seem to show up on ebay. I found a supplier that sells these. Based on info in this paper, CsI(Tl) .1mm thick scintillators are OK for x-rays up to 10Kev, and you'd get more than 75mm^2 in area. LYSO is denser than CsI (7.4g/cm^3 vs 4.57g/cm^3) but the NIST absorptivity tables indicate that Cs is twice as absorptive as Lutetium -- so overall the crystals probably are similar.

The idea of making a larger composite scintillator does have its attractions. But I'm a little concerned about cutting and polishing the LYSO. It's a big unknown w/regard to its brittleness. I have done a LOT of polishing, both on glass and silicon so I do have some experience with polishing hard, brittle materials -- but some, like GaAs, are a real SOB no matter what you do. Also, according to Wikipedia Lutetium Oxide is toxic if inhaled so some care would be needed during the cutting/polishing process.

My thoughts on collection efficiency are a bit different anyway. If I have to accumulate counts over an hour to get decent statistics that's OK in my book. I'm not gong to be doing this for a living.

I've assembled SMT PCBs without much difficulty, but I used a stereomicroscope to do it. Leadless packages might seem to be a nonstarter but if you pre-tin the package pins AND the PCB footprint for it, it can be done on a hot plate (it also is advisable to tin the pins where they are exposed on the SIDES of the package, too) . When the solder melts its surface tension automatically aligns the package to the PCB footprint. If your board has a solder mask layer, anyway. You also want to use small-diameter solder and a fine-tipped soldering iron, and so-called "smt tweezers" are very handy for holding resistors and capacitors in place while you solder them down. They have wider flat tips to better grip small rectangular parts. Oh, and water-soluble flux really helps when soldering down packages.
 
"Really fancy stuff, like capturing the whole scintillation, and/or parts thereof, subjecting to FFT analysis via octave, and all that clever stuff only has a point if there really is new information in the pulse to spot overlaps, etc. If the main information is the pulse amplitude, that is all we need, we can skip much of the rest. "
Yep. Good point.
"My thoughts on collection efficiency are a bit different anyway. If I have to accumulate counts over an hour to get decent statistics that's OK in my book. I'm not gong to be doing this for a living. "
YES- agree strongly. Would using a lesser collection efficiency or a lower activity source also be helpful by preventing simultaneous or temporally overlapping signals?
Those crystals are getting pricey but if that is the only major expense it still looks cheap compared to a Bruker.
Robert
 
I realized there is a good reason to prevent Americium's gamma rays from hitting a thin scintillator. Due to the greater penetration depth of the 59.54Kev photon, a thin scintillator will generate SOME photons, just not the full complement -- so the pulse will "look" like some other element. Fortunately, based on RJ's input, it won't take too much lead shielding to accomplish this, but the physical layout of the americium, shielding material and scintillator is more important than just getting rid of the 59.54Kev background signal.
 
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Hi Graham,

I had looked at the KETEK site as well. There are plusses and minuses with their offerings compared to the ON Semiconductor SiPMs. ON's pricing for the bare IC is nearly the same for 1x1 or 3x3, a bit less than $60USD -- unlike KETEK, which is asking a lot less ($17.90-$22.36 for the 1x1 and 3x3 respectively). But if you want to buy a demo board with the SiPM pre-installed both are asking nearly the same price (not including shipping).

If you have the ability to solder WLP packages you definitely could save some money by going with a KETEK product. However, I have no confidence in my ability to solder a WLP down to a PCB. At work we had a technician who had figured out how to do it, but it took awhile to get it down -- and she had access to the right solder flux, a temperature-controlled hotplate, and lots of parts to play with. Just for an example of all the variables at play, she found that the parts moved around too much when heated up, because the solder flux would start to bubble up as steam was released. She solved that problem by pre-baking the fluxed boards in an oven we always kept heated to 80C. It also made the flux tackier, which made it easier to nudge the parts into place.

BTW, unless you immediately solder surface mount IC's after opening their original packaging, you need to bake them for a few hours at 80C to remove moisture that enters the package. That's why we had that lab oven always turned on. Otherwise the absorbed water will turn into steam and can potentially break bond wires due to the pressure buildup when heated to soldering temperature (we called it the popcorn effect). We once had a horrendous failure rate on an assembly line due to this problem, which literally shut that product line down for awhile. It is particularly insidious because the bond wires can temporarily reconnect once the package cools down, but eventually the part would fail -- and by then the products were in customer hands!! This is something to keep in mind for those DIYers among us who may wish to roll their own PCB and populate it.

H
 
After looking a bit more at the KETEK SiPM modules, I like theirs better than the ON Semiconductor demo boards. The ON versions have a built-in pulse transformer so the design options are more limited. The KETEK versions just bring out the anode and cathode of the device, so it's more like a standard break out board.
 
Hi @homebrewed :
The Ukrainian PMT finally arrived. The Soviet version of cold-war surplus. Clearly new old stock. When I cut past the labels and un-stapled the box, it was nestling in sponge that clearly had be given 3 decades to deteriorate. I am having to put some time into decoding the Cyrillic :)
Titov's 84-5b-fragment.png
Who knows why on Earth the data sheet document is a "passport"?
This one would work with almost any scintillator put up against it, but initially I am going to try an unblemished clear Na(Tl) that fits right on it (I think). Ordered from the same seller.

I am, of course, going for the Si(PM), but I did order this one quite a while back. If it works, it will still be a bargain.
Even so, for any who may check on this thread, I think the stuff in the Hamamatsu PMT handbook Third Edition should be here for them to find.
 

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graham-xrf said:
Hi @homebrewed :
The Ukrainian PMT finally arrived. The Soviet version of cold-war surplus. Clearly new old stock. When I cut past the labels and un-stapled the box, it was nestling in sponge that clearly had be given 3 decades to deteriorate. I am having to put some time into decoding the Cyrillic :)
View attachment 326291
Who knows why on Earth the data sheet document is a "passport"?
This one would work with almost any scintillator put up against it, but initially I am going to try an unblemished clear Na(Tl) that fits right on it (I think). Ordered from the same seller.

I am, of course, going for the Si(PM), but I did order this one quite a while back. If it works, it will still be a bargain.
Even so, for any who may check on this thread, I think the stuff in the Hamamatsu PMT handbook Third Edition should be here for them to find.
Click to expand...
Passport, eh? It's gotta be the output of one of those amusing automated translator programs. Some of the Theremino publications are hard to figure out as well, probably for the same reason.

Now "all" you need is the HV supply, resistor divider chain, fast A/D and MCA S/W :grin: . The python-based stuff you found may be helpful in the latter case. Just guessing here, but I suspect you have a decent 'scope to look at waveforms. I got spoiled at work so my tastes run on the expensive side there; but, really, a 1GSPS Hantek, Rigol or Siglent should be plenty good for stuff like this.

H


On a slightly different subject I downloaded the LTspice package and tried it with my version of Wine. It seems to work just fine.
 
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