Small radiation detector

This is bound to get mixed up with things in Electronics, check both. Physics-specific stuff here, mostly.

Re: Small radiation detector- more data

Postby Doug Coulter » Thu Aug 12, 2010 10:43 am

Here's another table and related text on this topic. Note that plastic isn't all that numb -- makes plenty photons for the radiation it actually absorbs. It does have a much longer radiation length than say, BGO, but unless you're making a spectrometer, all it takes is enough photons to trip a threshold in the back end electronics.

ScintDat.gif
Scints and text
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Re: Small radiation detector

Postby lutzhoffman » Fri Aug 13, 2010 9:59 pm

So, the search is on for a "batch" of micro-PMT tubes? This part has been figured out, one question I have on this subject is what about PMT tubes with a somewhat extended sensitivity photocathode, like multialkali, instead of bialkali? The blue sensitivity and gain on these is the same for both, except the multialkali will go good all the way to 650-700nm before it really drops off to near nothing at 900nm. This way we could include some of the newer longer wavelength scintillators like ZnSe(Te), with no down side that I know of.

I am currently working on getting some ZnSe(Te) to grind up for our test use, with enough extra for 6-10 units, if it works. It is to early for me to count chickens here though. This material could be combined with either plastic, or BGO, to boost the low energy gamma and x-ray sensitivity of both of these materials as discussed before, it is also highly sensitive to even low energy beta. If I can get enough ZnSe(Te), and if it works well with either plastic scintillator material, or BGO, then the detector response curve will be more flat, in addition to it providing a nice boost in overall sensitivity.

My feeling is: What if we assemble a micro-PMT, with a VM power supply as Doug suggested, and then try some things in real life on the bench. The scintillator selection debate could go on for a long time, with good rational theory on all sides. Now bring some of these ideas to "life" on the bench, and see what happens? I bet that this will narrow the scintillation material selection down pretty fast. We will all have our pet great ideas here, and I am willing to bet that most of these will be good, so why not test the top 6 or so, and pick the best 2 as a goal. It will be very hard to nail it down to just one design, since we all may have different things that are important to us, but narrowing it down to 2, or maybe even 3 designs at the most, is possible. Before I start searching, should I also look for some other small PMT variations like MCP PMT's etc? Thanks...
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Re: Small radiation detector

Postby Doug Coulter » Fri Aug 13, 2010 10:41 pm

The downside of long wavelength sensitivity is necessarily more noise from random electron emissions at room temperature, and making it harder to be effectively light tight but not radiation tight. In my experience so far, this eliminates any advantages of the longer wave output scintillators. I've not tried them all, but a few all day work sessions just trying to get the noise and light leaks down convinced me not to pursue that path here anymore. I have a junk box full of poorly working long wave stuff, and a nice bunch of working-daily stuff that uses the bi-alkalies with plastics, BGO, and NaI:Tl. I'd be interested in how the newer stuff works, maybe, but I know the bi-alkalies work great, no problems, easy, and big outputs (directly drive logic levels). Just my take -- it's hard to get me to do work on something I just don't believe in, having spent endless time and money trying to make it work already, with fairly lousy results. This new blue only stuff is a nice dream by comparison.
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Re: Small radiation detector

Postby Alex Funk » Tue Aug 23, 2011 1:49 pm

Here are some initial thoughts, from a starting point of true ignorance in radiation metrology. Please beat it up as necessary, and I will morph the block diagram.

1) In the 90's, all electronic instrumentation became a computer peripheral. Now more so, except connected by wi-fi or bluetooth to either a computer, tablet or smart phone; modules to do so are commonly available. The 'Altoid can' (actually a plastic box so the RF link would not be shielded) would contain this module, a uP, lithium battery, battery charge controller, probe HV PSU, preamp PSU.

RadMonitorStrawMan.gif
RadMonitorStrawMan.gif (11.51 KiB) Viewed 8431 times

2) Make the sensor/preamp remote on a cable, so that it is pluggable into the head unit. Different probes for evaluating different environments can then be used, and the cable length helps isolate the preamp from the HV PSU. If the interface between the 'Altoid can' and the probe includes ID lines (say 3 inputs with pullup into the uP that are selectively grounded by the probe), the uP can tailor the HV and preamp power to one of up to 8 probe types automatically.
3) I agree with Doug on not modding a CCF PSU -- I think they are designed for much more amps out than we need. I have had good results getting +5V--> +100V with a 470uH coil (CDRH74NP-471MC from DigiKey). It is about as small as they get: http://parts.digikey.com/1/parts/642131 ... 471mc.html.
4) As a team, come up with a specification and feature list. I favor putting a hardware platform in place that can accomodate more analysis features (like spectroscopy) than just a count further on. It seems like the Russian-made (http://www.google.com/search?q=Soeks+Radiation+Detector) is setting the price/performance bar right now in handheld devices, largely due to its software.
5) As a team, determine who is going to be the Czar of what project aspect, and stick with it.

Doug, I don't know anything about sensors, so I will let you guys fight that one out. I would like to focus on what is necessary to allow a head unit to accept any kind of probe, ranging from a gallon jug on a table at the end of a cable, to a little GM tube that plugs so seamlessly into the head unit that it looks like part of the same box. Re battery life with wi-fi: how long does an iTouch last? Can wi-fi only be enabled during a [short] data burst? Power the head unit by USB from the analysis platform for long term measurements? These are questions I think bear resolving.
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Re: Small radiation detector

Postby Doug Coulter » Tue Aug 23, 2011 2:19 pm

Your first question is going to be "what do I want to detect" and will it be phototube based or not. If phototube based, you'd want to use taps off a CW voltage multiplier for the dynodes to save very significant power that would otherwise be dissipated in a dynode resistor-divider chain. You also need to answer whether you want to have any resolution on energy. Those questions have a lot to do with what's practical to do here. Some sensor types are "binary" - either a count or not, some give pulse height info that can be useful. Personally, I like geiger counters as standards, but all they tell you is "here be radiation" and nothing whatever about the type or energy of it -- so are useless in computing human doses, and their sensitivity to X rays vs energy is "all over the map" between tube types you can get. They are, however, the prime candidate for something hyper portable, as they use "no" power and can be had small, at the cost of not being very sensitive (Maybe we can get Joe to post a pic of his miniature one on a key fob). But with one you can't tell alpha from beta from gamma, though some have a cover that stops alphas and/or betas and you can take readings either way and know a tiny amount more.

Obviously, you don't want to have to run 10-12 wires up a cable to a remote phototube, or mix 100s volts AC going in with millivolts coming out to measure -- just one tradeoff you have to make before you can begin to look at the diagram for the electronics.
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Re: Small radiation detector

Postby Alex Funk » Tue Aug 23, 2011 2:46 pm

I'll be happy to post a pic of this one by Chaney Electronics. All is does is click and light up (it doesn't actually count anything) and since the speaker is coupled to the output of the H/V rectifier, it has a nasty buzz all the time it is on ... I guess they thought you could tell it was on that way. I can post the scheme, too -- it should be easy to mod so the buzz goes away.
ChaneyPNC6999.JPG

That is a 555 timer, BTW. It is used as an oscillator --> 2N3906 transistor --> transformer. The tube output drives separate transistors to light the LED and click the speaker. While technically it 'works' (it pops on Fiestaware), it is useless for survey work.

OK, so GM and scintillator/PMT systems are so different, a head unit really can't accomodate both types, at least with battery power. But it seems to me that PMT systems, with new scintillator materials being developed, offer the possibility of better metrology. Most of the handhelds on the market now are probably GM based -- not sure how much I care about alpha detection, since beta/gamma offers better spectrometry resolution anyway.
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Re: Small radiation detector

Postby Doug Coulter » Tue Aug 23, 2011 2:57 pm

The issue with the tiny detector is that they are for "safety" uses on battle fields, not your idea of the meaning of the word "safe" at all. More like, "can I survive long enough to fight another few hours" - not so useful for "am I going to get cancer over the next 10 years" - they won't see that level at all. They are very "numb" due to their tiny size (no intercept area) and their construction (thick walls keep low energy stuff out). So they are able to read if held up against a source that would burn you in hours, but not very good for low level detection.

We see those little tubes on ebay fairly cheap these days. Joe Sousa did one of similar design where the power supply regulation made the ticks -- see his post on that, it's pretty nifty. He used a higher frequency and smarter circuit so you don't hear the buzz, and an acoustic resonator to make the ticks louder -- and got it done with even fewer parts (he's amazing that way). His used a larger (2") pancake tube with a heck of a lot more sensitivity. You kinda need some size in this game to get to anything real useful if you want to see small increments above background at all in short times -- the statistics of very low count rates work against you getting reliable readings otherwise.
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Re: Small radiation detector

Postby Joe Jarski » Tue Aug 23, 2011 5:01 pm

I added a post here about the mini GM detector that I made. While not real sensitive (same GM tube in the one that Alex posted) it's handy to tell you whether something is "hot or not". Maybe it'll be of some use to help decide some features.
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Re: Small radiation detector

Postby Alex Funk » Tue Aug 23, 2011 8:35 pm

Joe, not only is that is a beautiful piece of work, it makes me feel so much better about buying this crappy little unit with the GM tube too small and too thick-walled to do anything serious, because, like you mention, it can give us a feel for what we want as features. Mine gets about one count every 30 seconds as background; how does this compare with the number of counts yours gets in free air, Joe? I held it in a dry puddle bed and next to a plastic jug containing settled 'no exit' fountain sediment (to check for possible fallout) and would have to say the result I got was not statistically significant.

I notice you have a window milled into the enclosure -- a nice touch. I would observe that I do not like slide switches -- I feel they let too much environment into the equipment. I'm paying a lot of attention to Doug's mantra that there is no substitute for size in the detector, be it pancake GM tube or a substantial block of scint plastic. Doug, is this Joe Sousa's design with the 2" detector?
viewtopic.php?f=7&t=244

I would feel that anything we put together would have the following at minimum:
1) an actual numeric display to show the count
2) range and timebase switching
3) data logging to a flash card or USB port (certainly doesn't add much volume to the project)

I am leaning toward PMT over GM because it, along with data logging, affords the possibility of people far in the future data-mining the log and doing spectroscopy.
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Re: Small radiation detector

Postby Doug Coulter » Tue Aug 23, 2011 8:59 pm

A: Yes, that's Joe S's excellent sideways thinking on display there.

I like pmt's too, especially if you can capture pulse heights to make spectra, though that's getting out of the range of a handheld device (but see the URSA gamma spec).
I'd note that most of the features you've been mentioning quickly draw so much battery power that they render the device either a PITA due to having very short battery life, or too big.
We've found that while it's possible, once you get a couple hundred volts in a box, it's real dangerous to LCD devices that don't draw too much power, and better to use something else, like either an analog meter (don't laugh the integration is good there) or a separate computer for the display.

I'm not saying it's a perfect design, but with a buncha C cells in it the URSA only runs an hour or so, and that's with no display at all - it takes a lotta computer power to do its thing and power a normal phototube. The trick of running one off taps in a volt multiplier isn't available to them, so they wind up using 5 w or so to run a normal sensor head. While smaller sensor heads are possible and useful, the fact that many gammas will scatter out of a smaller one means you can't resolve high energies, and it makes a background floor on the spectra from partly-absorbed events. If you're willing to have "sloppy but better than zero rez" then BGO or plastic (in that order) are workable for scintillators. BGO is really dense and so a smaller one can do better than a larger "anything else", but has a bit of randomness in the input energy -> light transfer curve. Plastic just doesn't stop radiation well if you want to see full energy on a hot gamma, but it's sensitive as heck (will even see fast neutrons, and just about nothing else sees them at all) and fast.

The URSA is shown holding up the back of the huge NaI/phototube sensor here - it's about the size of a thick paperback. Most will never have anything like that sensor - that was a special score and even used the price was kind of eye-watering (too many digits) - although we paid 2 less digits than the government did for them. We also have smaller ones, down to 3/4" on a side by 10" long NaI, and you can make a smaller one if you give up even more energy range. If you have tight integration, you can do the cool trick on the phototube and save nearly all the power that would usually be wasted in divider resistors - which makes it smaller as the batteries get smaller then. Kind of like the electric car problem...
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