Science/Engineering/Technical forums

Can you please explain the meaning that it is a base plastic without fluor?

Yes. Plastic scintillators are based on one of several basic plastics. Polystyrene (PS), polyvinyl toluene (PVT), or polyvinyl xylene (PVX) are most common. These capture radiation, but are very weak emitters of light, so another substance (actually, usually two) is dissolved in the plastic to capture the excitation energy and transform it into photons of suitable energy to get through the plastic and through a PMT window after that -- most of the plastics are very good at absorbing the little bit of very short UV they create when hit by a particle, so large Stokes shift fluors are added, and it usually takes two to shift the wavelength down low enough to not need a quartz PMT face and very short plastic lengths so you get any detection. Usually the primary fluor added is added in enough quantity to get the energy transfer by dipole coupling, rather than radiation (photon) as that doesn't get real far through the base plastic, then another fluor eats that and downshifts the photon energy further to the blue or green -- where the cheap (but very good) bialkali phototubes have a sensitivity peak.

Of the plastics, PVX is most rad resistant, then PVT, then PS (they get dark when irradiated at least to the wavelengths that matter here -- just as glass does). PVX is really toxic, PVT less so, and PS the least. So PVT is the norm these days. I have many chapters about this junk in that book, far too much to post up here (literally several hundred pages on the mechanisms and scintillator design and selection). The plastics stink for energy resolution for various reasons, including that it takes a very long length to absorb all the energy of a photon or other radiation, and light may not all get to the PMT the same, but they are pretty good at quantum efficiency for what they do get, nicely sensitive especially when coupled to a modern PMT, like most of the stuff Hammamtsu makes for this. The various tradeoffs in the added fluors are things like how rad resistant they are themselves, how fast they are, how quantum efficient they are and so forth -- lots of choice there. No one uses "just the base plastic" though in the past some people used pure naphthalene or anthracene and got decent results. For applications like colliders, rad resistance and speed are all important, they kind of don't get down to the highest energies we get up to anyway, so they don't care that much about QE, and can skip the last few mev in a shower detector/calorimeter with not much effect -- or so they say in the book they wrote that I have here and am more or less paraphrasing.

Primary fluors go by acronyms like PTP (para terphenyl), BPBD (no, I'm not typing that), PPO, PBD etc. Secondaries go by things like POPOP, bis-MSB,TPB, and BBQ, as well as a few "proprietary" ones. The various combinations are listed on the usual sites for these things as standard numbers that don't reveal the actual formula, usually.

Like here. The third one in the table is real common out there surplus, and is a goodie. EJ-208 is what I think we've got and Geo was selling.

For example, outside the plastics, they were looking at BaF, but that emits in the very short UV region and would have required fancier phototubes to even detect. They liked it because of speed and rad hardness and were maybe willing to put up with the rest to get the desired properties. BGO, though nearly 1/4 as efficienct as NaI was looked at but at 35 ns decay was slower than the bunch rep rate for their project, so they couldn't use it -- but it's common out there surplus for PET scanners. NaI is useless to them because it's not very rad hard and gets activated itself, and that's one heck of a noise issue when it's the scint itself getting hot! And it has a longer radiation length than BGO as well. At any rate, there are also a ton of crystals with various dopants -- same idea, out there too, usually a little better and a lot more expensive. Look at the Eljen or Bicron main site and fish around a little.

We got a bunch of Hammamatsu dual cathode phototubes and BGO xtals stripped from a PET scanner, and they work dandy, and the PMTS are also good on the plastics, very low dark spurious photon counts and good blue sensitivity without troublesome near-IR sensitivity (which makes getting things light tight a metric crap-ton easier). Pretty much everything else tried here (hundreds of other combos) has been inferior to those, especially older surplus stuff -- the newer Japanese phototubes really are superior.

Doug,

I bought a "BGO" crystal from eBay to use when I get my stack finished (got the boards in the mail this week)
on x-rays from a 6VS-1 tube. Thought it would be a good way to test the stack and the imaging chip in an old
digital camera. I also wanted not to be where the x-rays were, although with this tube I believe it's 360
degrees but not sure of the thickness of the planar circle of these rays. This is a point focus tube so I guess
actually the x-rays could be emitted in a hemisphere from the point of focus on the anode.

Anyway, Is this crystal what is ground up and mixed with the plastic you mention above?

Bill W

Now, back on topic -- see, you can edit these, rant all gone, but we have some work to do to split this topic up.

I've tested the tubes now, and they work fine. They want 300-320 volts, which I get off a 12v input, 1kv output CCFL with ~1.7v in and a volt doubler on the output, using a 1 meg series load resistor for the geiger tube. I used the series caps on the CCFL, and a big .47 uf filter cap, no noise is visible (it's overkill for this job).

Pulses are about 200uS wide, which is on the slow side, but good enough for this work, and nearly 150v tall (and, if you reduce power volts, they just don't fire at all, characteristic of the geiger phenomenon) -- super-excellent for EMI rejection. There will have to be a bit of current limit to drive CMOS off this without blowing either it or the tube -- remember, a coupling capacitor exists, and can store some energy, so to really current limit to the same level on this tube (and high current fries them) I should use a 2 meg series R to the power, then a two meg in series after the coupling cap to the transistor or cmos input, for the tube to see ~1 meg under worst case conditions with the CMOS limiting at GND and Vcc. Else peak currents in the tube can come from stored charge in the coupling cap and its own discharge current, which is only limited by the external components. I've not yet figured out what the third electrode is, probably a shield, and I'll try grounding that (or whatever seems to not be a bad idea after an ohm-meter check). I get Anod, and Katod -- but I don't read Russian on what that other terminal is for, probably noise shielding.

There are a couple possible ways to regulate the HV, and it's not that touchy (as a phototube would be), but I think maybe the CCFL, operating so close to one diode drop (or one Vbe), will be highly temperature variable. So, either I can use some diodes and an emitter follower to provide a voltage that tracks the tempco, feedforward, or use a measurement off the HV into the PIC and the PIC pwm output through a CMOS buffer and LC filter to make the roughly 1.7v the CCFL will need. Heck, at 10 ma, I could probably just use an unbuffered PIC pin and an RC filter. Note that this plays well into battery operation. The CCFL is drawing about 10 milliamps under counting load, and only needs 1.7v, so I can run the CPU off 3 alkaline (or one expensive lithium) cell and have portable operation self-contained, assuming rs-232 output will do here. An LCD display won't run without fancier power supply considerations -- they really need 5.0 volts. In this application, I should be able to program the PIC down in clock frequency quite low to save further power draw, and hit something under 20 ma total draw, well within decent lifetime for AA cells, and make this small. It would be easy to add a clicker to this as I have everything else here -- I'm a big fan of audio outputs, and JoeS has shown the way to do it with minimal power needed. And here's the rig, hopefully not too complex?

Well, for what it's worth pin 3 translated to "operating electrode" after playing around on a translator.

Huh, looks like a screen or shield in the schematic...maybe I have to pry one apart to find out! It works with that pin floating, at any rate.

After a few minutes thinking about this -- they drew it as a grid (none is visible in the tube). Perhaps it's meant to be a gate, or a dead-time ion sweep-away? Now I have to go and try things!

Doug,

As you know (as above, &c.) I am currently on a hot-footed path to ensure I get some decent radiation metrology in place before I do any further experiments.

One thing I have read is that these pancake detectors are not the best for detecting high energy betas, as the gas path is small in them.

I feel I have to take that on board and am looking for a thin-walled, or specially-designed-for-betas, tube.

Any thoughts on this?

FWIW, these were marke(te)d as beta detectors. It is true that all gas tubes don't do betas as well as for example, a scintillator with a thin enough window to let them in.
Doesn't matter in this app, the betas are not high energy by most definitions, and are copious by any definition. As are the gammas they produce when they hit things.

Stability is what matters more than anything, and the unchanging ratio of sensitivity to alpha, beta, gamma over time and over the labs that want to compare results.

While it is true that there are other things with better quantum efficiency than this for beta (or gamma), it simply does not matter when you are counting 500cpm above background anyway.

I'm having a lot of trouble understanding why people don't get this concept. We are after a STANDARD here, not the tweakiest thing there is -- we want the most REPEATABLE thing there is, in the range of interest -- counting activation of only the easy to activate substances. End of mission.

Anybody who wants me to design all sorts of other detectors for them, ideal for other conditions, just write the desired properties in large print on the borders of $1000 bills and send them along please. Be sure to make your description long and detailed so it takes a lot of them to get it across!

This is not about having the most sensitive detector there is. For one thing, if left on during a fusor run, as I do to keep the time data from fusor-off to sample measurement, a much more sensitive device would simply saturate and then you have NO data at all. Dynamic range is an issue, and we are not trying to find a single atom or quantum here but to be accurate over the range the thing will actually need to work in. I've got "hot rock" ore samples that will saturate one of these -- isn't that too sensitive already, for that use?

It seems a certain contingent isn't satisfied unless they have the latest, greatest, theoretically (with some misapprehension of theory as it applies to the case) most perfect thing; that is, until you take the above into account, most bleeding edge, and therefore mostly likely flakey and unreliable. That's not the point here, we can have threads about that elsewhere.

This is for the most stable, adequate sensitivity, reliable, accurate thing. I'm not willing to give that up for the other stuff. I already have all that other stuff, and have plenty of experience with it. If it was best for this, this thread wouldn't exist. If you want to start a thread about beta detectors, and discuss -- be my guest. OR for that matter, all radiation detectors, but we'd organize better perhaps if we did it by type of radiation or type of detector. They all have their little quirks. and issues. Obviously, betas hitting high Z stuff also make gammas...for one example.

BTW, everything has a dip in beta sensitivity around some energy range -- see Haliday, at a certain speed betas just don't interact much. The important question is -- how about the betas from our activations, not what about the ones from particle accelerators.
As you can see, in the range of our interest, the curve is pretty flat (and remember that some betas in silver, a high Z element, will become gammas on the way out). Can anyone cite an actual reason that we have to extract 100% of the energy of a beta to detect it? It is pretty easy to detect them with scintillators, though due to the plethora of non-one-shot energy loss mechanisms in matter, hard to get extremely precise energy data for each one. And pointless in our case, the beta decay doesn't make mono energetic betas -- recall the story of the neutrino?

I'm not making the slightest pretense or mention that this standard activation counter solves all problems involved in nuclear research, so don't argue about what I didn't say!

I fully take your point on consistency, Doug, no problem at all. I was thinking, though, on the point about access for future comparative runs by others at lower count rates, necessitating the tube with the most cost effective sensitivity.

The alternative is to run this work with the cheapest tubes, so cheap that no-one can argue not to buy one to use for measurement - some of those smaller russian tubes are very plentiful and very cheap (probably why the are cheap!), but not so sensitive. Like a few bucks (or 'quid' - sub '10' of any currency is next-to-nothing these days, it seems).

What is the pancake tube you've got here, then Doug, I didn't see a code for it in this thread? Looks like a CI-13B? Personally, I can tell you I am very keen to get something that matches someone else, so that I can get a cross-calibration with a calibrated setup.

Doug, Here is some more from my translating efforts (it's pretty tedious trying to type on a Russian onscreen keyboard). One thing of interest is the "Pressure on operating electrode" towards the bottom of the second page - maybe it'll give you a little more insight. Where I have ?count? shown, that's my best guess - it didn't translate, but looks like it's derived from "accounts".