Small radiation detector

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Small radiation detector

Postby fusordoug » Tue Jul 27, 2010 2:20 pm

Lutz has suggested we group-design a small, very portable, inexpensive radiation detector, which I think is a great idea -- you can't have too many when working on nuclear stuff.
I think first we need to define some specs for the design, so we can get going on the best way to meet them.

For example, a simple geiger counter will tell you "here be radiation" but not information on what type, and differing types affect it differently, so in a lot of cases
you can't tell between a lot of low energy and a little high energy stuff, or between alpha radiation and gamma (though you can use a sheet of paper to stop the alphas and see if that makes a difference. Ditto scintillators, though you can at least get some more information from the pulse heights returned if the system is linear (not a foregone conclusion), at the cost of more complexity in the electronic back end.

Neutrons are their own game entirely, and getting energy data from them is problematic, though I've heard of it being done with at least some resolution with a 3He tube system.
Plastic scintillators see them fine if they are fast, but with random pulse heights - a recoil proton can get any fraction of the impinging energy.

If we are interested in human dosage effects, we would have to have a lot more information than a simple geiger can give us, or a simple scintillator. The trade-off there is
that if it's hard to see at all, it's probably not going to be hurting you either, so sensitivity is less important for that case. But selectivity is vital. Some kinds or energies of radiation do more damage to tissue per event than others, and the range is fairly wide for all of them.

So I believe the place to begin is with a specification -- do we want a good prospecting/survey meter? A human-calibrated dosimeter? A knock around the lab basic safety device?
We can't do them all small and cheap in the same device -- so we have to start by choosing what it is we want. Extreme sensitivity isn't ever going to be very small, as you
have to have enough detector area to intercept the radiation when the events are rare. Things that see pulse heights for human dosage are more complex electronically (and in software).

For example, this is a problem with Bill's "Henny Penny" which is a plastic scint/counter-only -- it goes wild with radiation that won't hurt you, because it sees *everything* and counts it all the same.
The unit is a TSA systems PRM-470b. It counts like 10/second on cosmic background where a 2" pancake geiger counts about 1/second -- it must be seeing the individual particles in showers. And that one counts a U glazed plate at 700 counts per second and the safety alarm goes off -- a bit much. Good for prospecting, but lousy around the lab.

So, what are we after with this idea? Let's define that first, then worry how to make one real. Easier to keep your eyes on the prize once you know what the prize is.

I would recommend against using any crystal that needs sealing. I've seen far too many "professional sealed" ones that didn't keep their seal. I have a shopping bag full of bad ones already that we got on ebay and other places. Further, if we are going for small form factor here -- you can't machine them for a custom fit. In addition, anything with iodine in it will activate around neutrons and be ruined.
This is responsible for the well-sealed but still bad NaI:Tl heads we've also bought -- they are counting their own activity, having been near a neutron source at some time.

BGO is pretty good stuff for a lot of things, and not only can you get it, I already have about 10 of them. Used end on they are decent (not as good as NaI) for spectroscopy, and the end is 1/4" by 1/2" so you can use them with a small photodetector. They don't need sealing in air. I use a chunk off one I dropped inside the fusor as a visual indicator -- it lights up just fine in there (bright) in the X rays inside the tank.

The downside of any scint vs a geiger or proportional tube is that you won't be able to see alpha radiation usually -- they are hard to make light tight enough, and so by the time you get there, they stop alphas on the way in. With a small geiger tube (really easy to make, actually -- I've done a few here) you can have a thin enough window to let them in. So that's a consideration too, since most natural radio-actives give off alphas, mainly, which even a sheet of plain paper will stop and a sheet of Al foil (what you usually wrap a scint in) will really-really stop.
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Re: Small radiation detector

Postby Jerry » Sat Jul 31, 2010 7:37 pm

Re: Hamamatsu HC120 ... series.pdf

Says right at the beginning Applications: Weakest light levels and also in the text.

Dont know if it will work but I didnt pay anything for them so what the heck! ;)

I can always use the scintillator plastic for something else.


Edit: Just found a couple papers where they are using the HC120 for a scintillator detector ... Ua/native/
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Re: Small radiation detector

Postby Doug Coulter » Sat Jul 31, 2010 7:50 pm

Oh, I'd not ditch the idea of plastic right off, not hardly. For one thing, you can shape it on normal machine tools, for another, it resists most nasty things fine, and it will see neutrons too.
The resolution isn't as good as some other things, but you might not care -- we're not going to cryo and HpGe anyway....True, it's a pain to polish to optical level, but not all that terrible after it comes off the mill -- even a fingertip wet with toluene will make it shiny at that point.

If you pay attention to optical design, you can use a big chunk of plastic on a small tube and use the shape to funnel all the light to the tube -- think triangle with a truncated end for the tube face. If you do the angles right, you get total internal reflection off the sides....real efficient. Can't count how many of these I've made up -- it's a lot, and they work great for what they are.
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Re: Small radiation detector

Postby Jerry » Sat Jul 31, 2010 9:05 pm

If it is soluble in toluene it might be possible to chemical polish in heated solvent vapor. Might be worth a try.

I had been thinking what you had mentioned, Making a "light funnel" Also maybe use some optical matching gel between the plastic and the pmt.
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Re: Small radiation detector

Postby Doug Coulter » Sun Aug 01, 2010 7:17 pm

Acetone just touches it, toluene makes it liquid fast. Yes, what I just tried was putting it rough on a piece of glass, then letting
some toluene into the interface with a dropper. Well....I should have treated the glass with mold release first. It got nice and shiny alright, it's got a glass liner
glued on pretty well. Doh. The stuff comes off the mill smooth enough that I may just do the old sandpaper thing (starting around 400 grit), then finer stuff (like plain newsprint). If you take
a couple passes when you mill a side, it comes out pretty nice, almost optical already, just using a sharp HSS end mill. Seems like a second or third pass
makes it a lot smoother (I'm running a 3/8" mill at 550 rpm, which is on the fast side for this, but it works anyway). Rough cuts melt it a little bit but another pass
at the same quill setting smooths it back out nicely.

Most of these plastics are index of refraction in the 1.49 range, so are a lot easier to get the light to come out of than say BGO, with an index of 2.19.
You could have some kind of step index interface, but they work fine with either crazy glue or that fancy epoxy Elgen sells as a way to stick them
on a tube or whatever.

The trick to get the efficiency is to have a minute air gap between the plastic and the reflector -- the index gap between this and air
makes the reflection internal to the plastic. I use space blankets which seem to work better than Al foil,
as even though they let some light right through, they are shinier than Al foil, usually. Then worry the light-tight part.
I just tape it on there loosely as the reflector for the light that doesn't reflect inside the plastic.

In my case, I put the whole mess into some steel pipe, which also shields the phototube from mag fields, which mess them up.

I'll try and get some pix up here for all that -- it's a nice system after you machine the ends for a tight fit in the pipe. I use PVC for the phototube
end with a stepped hole in it to hold the tube centered, and another cap piece also a tight fit for the business end, material determined by
what I want to have come through, or be stopped.

Neutron camera parts

Here's a pic of some of the pieces of a neutron one pixel camera I'm working on now. The big pipe is lead, with a 1.5" diameter steel pipe cast into
it with cerrobend for the tube. In front of the plastic, I have a moderator/neutron stopper with a hole through it to make a colimator so it only sees
fast neutrons, and from only a small angle view. Without that junk on it, this tube in a smaller setup is about as sensitive to fast neutrons
as my 3He tube is (which I run numb, so that's not completely fair). In that case it's just wrapped in lead to keep the gammas out.
Being far away and behind a tiny hole, it takes tens of seconds to count up a good pixel as a camera. -- I made the hole too long and too
small it seems.

Bore of camera, can't see the Cd washers in there along with the borated wax.

This gets a lead cap to stop gammas from coming down the hole (or just light) in real use. And unlike the pic above, the end of the plastic
also has a reflector on it.

We got these hamamatsu tubes real cheap (< $35), and there's more out there, they were used in pet scanners so one scrapped machine
returns about 500 tubes to the scrounger. And they are IR numb and will count single blue photons.

That is far, far below that the specs on your modules go to, I think. I don't do the photons to watts conversion in my head, quite.
With a 10k ohm load on these, you get 10's of volts per pulse, in other words, no preamp needed to drive a CMOS uP input pin, but
rather some protection for the uP, instead. Kind of makes things simpler, since I'm not worrying a big magnetic field like the guys at CERN have to.
That's the real reason they pushed on semiconductors so hard for things like this, that and space for when you have a billion pixels. They kinda
have to give some of the size advantage back on this due to needing fancy preamps for each one.

Works for them because they don't even care about mere 100kv hits -- they start at megavolts and go up from there in those big detectors, so
they don't need the sensitivity either.
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Re: Small radiation detector

Postby lutzhoffman » Tue Aug 03, 2010 2:54 pm

The point of what do we want to use this unit for for is well taken, maybe it would help to re-phrase this question: "When do you need a small detector" or "when would one come in handy". In the home shop most folks have a bunch of dedicated equipment, so the mini unit runs the risk of becoming a novelty item. For lab safety a small mini GM tube device, would fit the bill, and these are so cheap, that it is almost not worth the time to build one, I have an old Prima 2B, which is a simple GM "chirper" which is good for wearing, but not much good for anything else.

For me personally I would like something which can cover general prospecting: In nature, on the highway, and at garage sales, swap meets etc. I would want pretty high sensitivity, combined with shock / moisture resistance etc. The alpha detection issue is a question, my thinking here is that it is easy to put a coat of something on the face of a scintillator, which will block light, but still allow it to see alphas. Something like a window which you can open to allow them in, while still protecting the scintillator when alphas are not being measured.

What about a "detection wheel" with a micro PMT. Kind of like on a revolver, with a couple hollow points, a couple solids, and one round of snake shot. Say for example that you had a wheel, or a slide, which is o-ring sealed for light, then you could turn/move it, to bring different scintillation materials in line with the PMT window? Thus the problem of which scintillator to use, now becomes a mute point since everyone can choose their own combo. The more simple alternative could be a simple "quick change fitting" where different materials, or even different size scintillators could be swapped out. There is an endless number of ways to do this. If this idea is popular, then we could talk about finding the best method, like wheel vs a light tight trap door with a bayonet mount on the side etc.

Ref: keeping it small I like Dougs idea about the micro PMT with the cascade VM capacitors directly wired to the PMT stages. This is very slick, since the efficiency is high, and size is very small. For a power source I was thinking about a 3V lithium cell, these are a pretty compact package, with a high power density. If we needed 6V, then two in series is still only a single AA battery in size.

I really like the swapable scintillator idea, this provides incredible flexibility in so many regards, and it keeps the cost down which is a nice benifit, since you are always free to use what you already have in the drawer. One question that I have in this area is: Are there any reasonably priced low loss fiberoptic cables, which transmit well in the 400-550nm range? This would be another way to simplify the changing of scintillation materials, where in effect you have a detector, with a remote probes, just like in a larger survey meter. This may end up being to "bulky" but who knows maybe someone has a reasonable solution. The last idea on this subject that I can think of is: A layered single scintillator, something like a BGO, or a plastic scintillator, with a dusting of ZnS on the outside face.

If we can agree on something like the above, then we are on our way towards the electronics package parameters. : )
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Re: Small radiation detector

Postby Doug Coulter » Tue Aug 03, 2010 6:04 pm

I still know of zero substance that is light tight enough for a phototube that will not stop alphas -- light is far harder to stop, even with steel foil.
Yet only a few cm of plain old air stop alphas dead.

So a little looking in some detector books might be in order -- and I did, but still know of nothing that will work. Geiger counters can use
a not-light-tight mica or Be window that lets a little light in, but since the tube doesn't see it, it doesn't matter. Perhaps what we should
be looking for here is something that makes gammas when hit by alphas (maybe not so hard to find, dunno). Alphas are easily stopped
at very small mg/sq cm window thicknesses (depending on the material, high Z stuff stops them better, but tends to be dense and winds up
with similar numbers in mg/cm^2.. A sheet of paper that stops alphas cold won't stop light at the counting photon burst levels. A thin sheet
of grocery store Al foil stops both, 100%, but of course needs protection from bashing (I have built such an endcap for the scint stuff I'm doing,
with a recessed window to keep fingers out of it). Failing that, ZnS:Ag or :Cu will see alphas,'d have to put the source in there with it,
and after it sees room light it takes perhaps 24 hours to stop glowing enough to be useful. This is a truly hard one, wishes aside.
And alphas are important for prospecting indeed, as nearly all natural radioactives put out mostly alphas. Henny-penny seems to see them,
but this may be an alpha-gamma conversion in the box walls themselves. Or it just sees the other emissions from my pure-U and pure-Th
samples. I honestly do not know.

Some detector parts

Here is a pic of some of the parts I'm working with anyway. The big box need not be anywhere near that large, it's mostly full of air and cable connectors.
(this one runs off a wall wart)
The little 40 pin dip board is the computer I use in that. The pancake detector is a very sensitive (for a geiger) from Geo, and there's actually
room in that package to put the rest into if a custom efficient HV power supply is made for that -- CCFL's will kill a couple of expensive
Li cells in no time flat with their 30+ ma at about 6v input requirement. For whatever reason, the lower volt input versions of those seem to
throw efficiency to the winds. However, the PIC computer has PWM outputs that with a single smd complimentary fet, can make some serious
drive for a small HV xfrmr....(amps at 5v if needed, ~35 ma without any help at all).

Even the 35 ma is well above what you'd normally put on a primary cell if you wanted it to live more
than a few hours. For high draw stuff I'm still a fan of rechargeable cells of whatever technology. This board counts up to 5 counters, and has
4 a/d inputs as programmed, the code is written, along with the drivers for the LCD glass. It puts out rs232 as well, for my fusor data aq system.
The uP itself is about 20ma running at full speed, but can run slower as I'm using hardware counters in it to do the counting, and it can go to microwatts
if required -- gets slow, but maybe no one cares.

The PMT's I have a lot of and can get more of cheap, fit into a 1.5" ID pipe (we use thinwall chrome moly because we can get it thin and
light and it's a good mag field shield for the PMT) and the whole thing with a scint comes out about 7" long. I don't have a source for "micro"
PMT's so that would need to be found if that's too big. Obviously for scintillators, bigger is better up to the point where they get so
big that the light has trouble getting into the tube from the long distance.

Someone probably sells quartz fibers that will move UV, but getting the scint light into a small flexible fiber would be quite
a trick optically, since the flat face of the scintillator output isn't an image as such -- light comes out going all directions,
about 180 deg in both axes -- it's best just glued onto the tube face. For that matter, I can make quartz fibers, but I dunno
how rugged that would be, seems a plain old BNC cable would be better for remote-ing a probe.

Lutz, you just gotta do some hands on with phototubes to understand just how sensitive they are and how hard it is to make
them truly light tight in the sun, or even room light. It's a pretty awesome task. You need to get to darker than a night with clouds and no moon
by orders of magnitude -- they are far more sensitive than any night vision device. And some more hands on to see
just how easy it is to completely stop alphas (a staticmaster Po source is good to learn that on, as there is no other
radiation from that). Else you'll say plenty of wishful thinking but not so much useful.

FWIW, also in the pic is a few hundred bucks worth of Zns:Ag, phosphor grade -- it's full about to the bottom of the
label, I've used about a gram out of that 50g in all my projects put together so far. A little goes a long way.
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Re: Small radiation detector

Postby Doug Coulter » Thu Aug 05, 2010 2:42 pm

The more I think about this, and did some tests the other days, the more I think we can skip the alphas if we want. 100% of my ore samples put out plenty of gammas on things I know won't see alphas, for example. The only really good alpha-only detector I've seen yet was a large table top unit RichardH had, and we'd never make that small. It had the light equivalent of an airlock, you had to put your sample in a drawer and shove it in there to measure it, so as not to wreck the phosophor by "charging it up". I think a plastic or xtal scint that just sees gammas and neutrons would do, what do you guys think? Henny penny is quite a detector working on just that, I can say that for sure after living with her a little. I'm still not sure if when the little red light comes on (which you can't see in the flash photo here of it on the plate) if that means a scale factor has changed. I'll have to find the book, but it sees that plate from 4 feet and more, well above background. For ref, a normal large area thinwall geiger here sees 40-60 counts per minute on cosmics. This appears to be saying 90 per second under the same conditions.
Henny on background only. No sources near.

And here it is sitting on a feistaware plate.

Henny on a dinner plate.

At any rate, this thing feeds my partner's paranoia quite effectively. It sees a torbermite ore sample from ten feet -- with the sample in a 1" thick lead pig! A smaller version would seem handy indeed, this thing has over the top sensitivity. Like I said, I'm not real sure of scale factors here, I'll have to compare with my geiger counters, but note the light was on and the decimal point went away in the second picture.
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Re: Small radiation detector

Postby lutzhoffman » Mon Aug 09, 2010 5:52 pm


I agree, maybe it is just not worth the effort to go after alphas in such a small unit. The quick change probe concept would still be neat to have though, even if each probe had its own PMT, but it would also be nice to avoid if we can find an alternative. I now understand the point about the sensitivity of PMT's being so high, as to make my idea of a revolver detector not possible in real life. You are correct I do need some hands on PMT work, which thankfully is going to be very soon, thanks in part due to my sons cosmic ray muon detector project. For alphas the only way to make a "window" that I could think of , would be to try to, directly evaporate a thin layer of Be, directly onto the surface of a scintillator, like BGO, etc., just to the point of it being opaque to light. This may now be a mute point, since even pure alpha emitters like Po do emit a gamma photon on occasion, or an alpha-gamma conversion reaction could be used instead as suggested. So I am willing to scratch direct alpha detection off the list.

The thought has occured to me to use some scintillator combo, kind of like Doug suggested in the organic coated BGO neutron example. I looked up Henny, and it just uses common plastic scintillator, although a fairly large chunk of it, something like 9 x 7 x 3cm. The sensitivity is pretty darn good, and if this unit could be shrunken down in size, then it would fit the general bill quite nicely in my mind. Maybe a combination of two scintillating materials could be used to shrink the size of the volume of material needed, since every individual material has its limitations. As long as each material was transparent to the light emitted by the other, then a combo scintillation setup should work.

Reference non hygroscopic scintillators, there is one other non-hygroscopic material, which has caught my attention: ZnSe(Te). In terms of efficiency it will exceed even NaI(Tl), and CsI(Tl), to the point where BGO looks very numb in comparison. Consider that CsI(Tl) gives 4-5 times the light output of BGO, and ZnSe(Te) in turn gives 170% the output of CsI(Tl). This puts ZnSe(Te) at close to 8-10 times BGO. From a chemistry perspective it is very similar to ZnS, except instead of sulfur, you move one down on the table to Se. Here is some info from one manufacturer:

"Zinc Selenide ZnSe(Te) scintillation material was created especially for matching with photodiode, which its emission maximum is at 640 nm. Matching coefficient between scintillator and photodiode is up to 0.9. ZnSe scintillators are sharply different from ZnS. "Fast" ZnSe has the time decay of 3 - 5 µs, "slow" - 30 - 50 µs. These are used preferably for X-rays and gamma-particle registration. Crystals ZnSe(Te) do not have very good transparency, therefore we don't recommend it with the use of more than 3 - 4 mm thickness. Relative to CsI(Tl), light output for X-rays with E<100 keV (CsI(Tl)=100%) is up to 170% at 2 mm thickness. Non-uniformity is usually less than 1%. Crystals ZnSe(Te) are non-hygroscopic and good enough for mechanical treatment without any cleavage. Standard ZnSe(Te) boules have a diameter of 24 mm. Diameters up to 40 mm are available on request."

BGO may still be better in the long run, since you can use a larger chunk, and its real easy to get. If however we go in the direction of making things as small as can be done, then 3-4mm of ZnSe(Te), with a different PMT photocathode, may be a valid option to consider. The one thing that I like about ZnSe(Te) is, its very high sensitivity to lower energy gammas, and X-rays, which tend to be emitted by U, and many other things that most folks would normally be looking for with such a unit. Here Doug's comments on ground up BGO come to mind. For ZnSe(Te) the long afterglow issues of ZnS evaporate, because ZnSe(Te) it is very fast, and very efficient, with a very respectable decay time. I was thinking about a Hornyack button style device, or a better version with ZnSe(Te) and plastic scintillation material. The inreased sensitivity at low energy by this ZnSe material, coupled with the scintillation properties of cheap scintillation plastic, just might make a starting point for this discussion. You could even use the castable form of the scintillation plastic to mix with the ground up ZnSe scintillator. This combo could cover low to high energy gamma, and x-ray's, low to high energy beta's, and even neutrons for that matter, without the problems inherent with ZnS(Ag), that Doug correctly pointed out in a prior posting.

The thought of somehow converting some cheap garden variety ZnSe, into ZnSe(Te) has crossed my mind. In fine form it may be possible via themal diffusion etc, but larger chunks are harder, since they are grown via vapor deposition. I have a surplus ZnSe IR window at home, which is about 10cm x 15cm x 1.2cm thick, so the raw material is not a problem, it would be more than enough for everyone, if a process to dope it with Te could be figured out. The only issue would be the PMT photocathode response, which can maybe be simply solved by switching from a bialkali, to multialkali photocathode design. Kind of like the difference between a bialkali R2154, and a multialkali R3256 tube. Since the cut off on the multialkali tube is 900nm, it does not seem to increase the dark current, and noise over the R2154 design. Yes these are big 2" tubes but the same should be true of smaller PMT's. On a less scientific note: The 640nm orange glow from the ZnSe(Te), should be real pretty indeed to the eye while under stimulation, it would be fun to see, some real eye candy. Plus if you wanted to add a filter into the setup for descrimination between the blue light from the plastic scintillator, and the orange from the ZnSe, then some easy to find off the shelf HeNe laser, and LD filters would fit the bill. A slide in filter could be added with no major light leak issues, if it was considered of benifit.

So there still remains some work to be done in scintillation material selection. or at least to consider some options, like a combo etc, lets see what shakes out of the tree, if its sweet I will eat it....Lutz

PS: Just for fun attached is a design for a photodiode version of a ZnSe(Te) mix based beta detector, the same principals should work for PMT's.
One Example of Principal.....
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Re: Small radiation detector

Postby Doug Coulter » Tue Aug 10, 2010 9:14 am

Well, I think it may be a good idea to continue this specsmanship game a little longer, because it will help us and others understand the specs better and how they
might relate to a small, practical detector setup. I do like the interchangeable head idea, and already use it here in nearly everything, partly because it also
amounts to an interchangeable back end too, and one sensor might give more than one kind of information, say counts vs pulse heights or total energy. Some detectors
flag the difference between a gamma hit and a neutron hit not by total light, but by pulse width mainly. Recoil protons in plastic from a neutron hit can individually
take on any energy from near zero to about half the energy of the neutron, with a fairly square distribution. The general case is that if the scintillator is
long enough in the direction the neutron came in, and large enough, there will be severl recoil protons per neutron, with some time smear, so instead of
providing a pulse height that tracks neutron energy, you get a wider pulse for neutrons, and the total energy in that wider pulse is the thing that somewhat
correlates with the incoming neutron energy. Gammas will tend to make a skinny pulse, the width of which is determined by the scintillator/phototube
characteristics. Usually the scintillator is the slow part, as most modern phototubes will keep the time spread under 5 ns or so. The plastic scintillators only
add to that a little, while nearly all the others are much slower and kind of negate the ability to use pulse width as a meaningful input.

One extremely important aspect that has been left out so far is a spec called "radiation length". This is the length of scintillator that it will take to completely
absorb radiation of a given type/energy and change it to photons, and it matters a lot in all detector types. To give an example, it might take a 3" deep NaI
to absorb the same energy gamma fully as it does a 1" BGO. The specs Lutz has been giving, if I remember my books correctly is photons per absorbed energy.

Now, if you are simply counting pulses, you may not care one bit about that as long as for the things of interest, you get enough photons to get a count out of
the phototube noise -- once you "digitize" a pulse into a 1 or a 0 (for pulse or not-pulse) it no longer matters. In that case what does matter is whether
the gamma or beta or neutron hits the scintillator at all. Obviously a 10 sq cm detector has a better probability of intercept than a 1 sq cm one does. 10 times
better to a first approximation, neglecting edge effects where a marginal strike may not pass through enough of it to make photons over the counter threshold
you've set up. In which case, the bigger guy is better yet, as it has more sq area vs edge area.

If you want pulse height information, you must have a depth of the scintillator that is long enough to completely absorb the radiation or the pulse height
spreads out for a single line source, as some but not all the gammas make a full height pulse. The others, perhaps due to geometry issues scatter out
of the crystal before being fully converted. In general, this means a good spectroscopy head will have some kind of shielding with a hole in it to only
allow photons into the scintillator that are coming straight on to the long axis of the crystal. For example, if one wanted to use one of the cheap and
readily available NaI heads in both modes, one might have a chunk of cast lead to insert it into or spectroscopy, but use it without for general gamma
detection and in the latter case let it be sensitive from the sides (more sq area) for that use. So you can have two uses for one head as well.

Here is a table from one of my books on the topic. There's still not enough here to really do a good design, but it's a step up.

Scint Table, part 1

Table, part 2

As usual, click these to get a readable version. Note that even with this much more information, we still don't know all we'd like to, but at
least we know a little more. We see that for example, BGO can absorb radiation better per unit length, so you'd need less in a pulse height analysis
situation, and indeed it is very popular in PET scanners for this reason -- it has "good enough" resolution for that, in a much smaller size. In fact,
other than the tiny ones from PET scanners, I know of no other affordable source of these. Now, this table was put together by the guys designing
detectors for places like CERN, and doesn't have some other info important to us in it other than not covering organics at all (I have another somplace for that).
They don't care much about low energy stuff, so good things for that are left out, as are slow things (from their perspective, a few nanoseconds is slow
as they have an 18 ns cycle time at CERN).

A consideration not mentioned here is how badly they attenuate their own light, which begins to matter when you go to bigger sizes for more chance
of intercepting the radiation you want to be sensing. For example all the Zinc based things are horrible in this regard along with being slow (which we
might not care too terribly much about within limits for our design unless it's really slow).

So the pure sensitivity number compared to NaI is more or less meaningless IF: A given hit gets you out of phototube noise well enough. That takes less
for go-nogo than it does for a good PHA spectrum. And those numbers are all for fully absorbed hits -- which may not be the case if the radiation length
is long, but the crystal not. Another important thing to know -- those are theory numbers, not what you can practically get out of one end of a given crystal,
some is lost, some reflects back from the index of refraction gradient, and hey -- phototubes aren't frequency flat and neither are these crystals.

So as with most design, it takes more than a single number to know what's best for a given application. And as always, price and availability figure in there

Unfortunately, I can't find a similar set of tables of all the plastic scintillator combinations. Of course, you can go to Eljen or similar place and get individual spec
sheets on them. I can only state what I know for now. Most plastic or liquid scintillators have a base medium, or solvent, and two fluors in them. The first
fluor is selected for most efficient energy transfer from the base material to it, and the second is selected as a down shifting fluor to get to the range
of visible light so an inexpensive detector can manage it. Percentages of both types are adjusted to suit the needs of individual applications. Most plastic
scintillators are very fast -- in the few nanosecond range, but of course this family includes some that use zinc compounds that are so slow as to
be useless for us unless we want to simply average it all and put it on an analog panel meter. For anyone who has not built one of these, I would point out
that radiation is about the most random thing in the known universe, and even a fairly slow rate of counting will have situations where two events happen
inside any long deadtime. Even microsecond dead times will thus undercount 1k cps sources fairly badly, so speed is much more of an issue than you'd think.

I still like plastic best of all, given the above and other considerations (like I can get it). Here is my reasoning on that.
You can make it big and still afford it. No matter how efficient a tiny crystal is, radiation that misses it entirely will not be seen.
The optical properties are a lot better for getting the light out -- it has low index, and they tend to absorb little of their own light, so you can make them big.
You can machine them to whatever form factor you desire, and this includes making them shaped so they guide light to the optical sensor the best.
The stuff is impervious to atmospheric degradations. It may not live next to an interaction point at CERN, or inside a reactor - but we don't do that.
It sees neutrons with really decent efficiency.
It's fast, down in the single digit nanoseconds.

The main downside is that it's not great for spectrometry (radiation length tends to be long) -- that might want to be a different thread here, as that kind of thing isn't generally hand-held anyway, you need some real display area to even see a good spectrum. Though actually in tests here, it's not really bad either if you've got a big enough piece and
a "good geometry" experiment so that most energy doesn't scatter out before being changed into photons. NaI and BGO rule there until you get into
cryo things like Ge.

I would note that significant improvements to the "henny penny" design are possible -- look at this picture of the guts.
Henny's guts

Just for starters, unless there is some serious optical trickery going on under that tape -- most of the photons aren't finding their way to the tube.
Next, notice some off the shelf power supply is used that simply drives a normal resistive divider dynode chain -- little thought taken to efficiency there.
Also note the PCB and the more or less obsolete, high power, components used -- again, little thought to battery life, so you also notice:
The big battery pack it needs to run for a fairly short time.

Yet this is far and away the most sensitive detector to ever grace my lab. I would think that with just the improvements possible and pretty easy to do,
we could shrink this puppy quite a bit, gain longer battery life, and make it even more sensitive by fixing the dumb optics design. The two biggest remaining
pieces would be the scint, and the display -- the rest can get tiny and power efficient easily if we just use more modern electrical parts. Which shrinks the
batteries needed and so on. It wouldn't quite fit in a shirt pocket unless you made the scintillator a lot smaller, and the existing cheap LCD displays aren't
really thin enough, but given those limitations, I think we could design something really good - and generally useful.

So, no matter what, I'd suppose the search should be on for a good blue sensitive phototube, small, that we can drive the dynodes independently.
The ones I have access to at a decent price are bigger than this one by a good bit, though we could take the sockets off them to get to the
dynodes and ditch the R string in there. We want a gain level roughly in the million range.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
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