scintilation plastic finish

[UG!]

i have a lump of scintilation plastic that will form the basis of my neutron detector. currently it is thinly painted with white paint, with one rather hazy and scratched side left clear to couple to the PMT. i am going to polish the serface exposed to the PMT, but what should i do with the other sides? would it be worth the hassle of removeing the paint and wrapping alluminium foil round it?

Oliver

[Starfire]

No - the white paint is to improve the reflectivity

- see -
viewtopic.php?f=13&t=5679#p34123

and search this forum for - ' optical coupling '

for useful info

[richnormand]

I had good success with machining the badly crazed surfaces of my plastic (5” diag by 2” thick) by removing about 1mm off its surfaces with a milling machine and lathe. This exposed a nice fresh surface. I used a low speed to avoid heating and light finishing cuts. Them, a slow hand polishing with wet paper to 400 grit. The final steps used “Novus” heavy, medium and fine scratch remover for plastic. By that time the interface was optically excellent. Coupling to the PMT was done using silicone differential fluid as used in RC cars and the plastic surrounded with Al foil with the shiny side in.

[Alex Aitken]

The other thing is that silver, aluminium etc are in the 80% reflectance region, as I understand it, wheras MgO is over 95% (I think this is the best), even white paints reflect much better than mirrors.

[richnormand]

On the small stuff, I had good result using typewriter white corrector fluid (on BGO xtals for example). It is easy to clean afterwards too. Could not tell the difference compared to Al foil or Teflon (used tape for threaded pipe joints). Time to run a controlled experiment this weekend…! Reflectivity also depends on the emitted wavelength.

In W.R. Leo’s book “Techniques for Nuclear and Particle Physics Experiments”, he advocates wrapping the scintillator in Al foil “as to assure a layer of air in contact with the scintillator” Then wrap black electrical tape on it. I would assume this is to maximize total internal reflection (near 100% effective) and diffused reflection for the light at the smaller angles that manages to escape.

MgO paint might be the most effective but could be difficult to remove afterwards depending on the type of xtal or scintillator. Solvents in the paint could perhaps damage plastics surface reflectivity too. The old plastic scintillator chunk I got had a sub-surface craze and cracks extending about 0.5 to 1mm deep. This is why I had to remove the layer. Don’t think it was caused by the paint since the coupling surface was just as bad. At 5” diag and 2” thick Teflon tape was out of the question. I used Al foil a previously described. Tried silicon coupling fluid on the foil vs dry. Dry gave me better signal (resolution is not an issue with a plastic scintillator…)

[Verp]

I’m wondering if sputter coating or chemically depositing the right material directly onto the plastic, eliminating the air, pigment binder or coupling fluid between the reflective material and plastic would improve things?

Rod

[Frank Sanns]

For sure a dielectric coating deposited on the scintillator and tuned for the emssion wavelenghth of the scintillator would be the best choice to maximize reflectance. But what does this really gain you?

For resolution it is bad because there will be multiple path lengths from reflections.

Keep in mind that a single gamma ray going through a scintillator will produce many photons. A lower energy gamma will produce fewer photons and thus a smaller pulse from the photomultiplier tube. A higher energy gamma will produce more photons and so a bigger pulse of light. There is also an efficiency factor also that is related to the density of the scintillator. A high atomic number scintilator will be more efficient and intercepting a gamma ray than a low atomic number scintillator (host) like plastic.

The point to all of this is what do you gain by increasing the reflections from the scintillator? The best you could hope for is a doubling of the amount of light to detect but with any decent photomultiplier/amplifier you will gain nothing except at the very very lowest gamma energies.

Frank S.

[Alex Aitken]

One of the factors that primarily determines the resolution of a gamma spectrometer is the number of photons. There is an intrinsic variation in a discretely measured number.

If the scintillator produces say 500 photons neer the optimum wavelength for the scintillator, you may detect as many as 100 photons, then the error in this margin is SQR(100) = +/- 10 not including any other factors like ninlinearity in the crystal or noise. So this is rather inaccurate. The more light a scintillation produces the narrower the line width.

Worse though, if you have no reflector, the energy of the pulse will depend not only on the energy of the incident gamma, but also where in the crystal it interacts, mainly how far from the surface it is. This can be useful, a bar of scintillator with both ends on painted and a PMT on each will give an indication where along it the gamma hit from the relative value of the pulse.

[Richard Hull]

With all the theory and bluster of those in the know......Nothing beats good old fashioned empiricism, hammering it in out in the lab. We were looking for counts here and not resolution or even energy data. Good work Geo.

This reminds me of the sensitive beta only detector that I worked up. I did not need energy data or even a true count. I just sought detection maximized for beta. It turns out a 1/4" thick, sheet tungsten backing for the source covered with aluminum foil worked just fine for pancake GM counting detection of very low level beta emitters.

It turns out that one of the stongest beta emitters you can obtain is a Cs137 source which is a pure beta emitter only. Refined Uranium over 1 year old is also a horrific source of beta up to 2.3mev!!! (average energy 800kev+). Old U is a virutal pure beta emitter, once alpha filtered. U ore is not as it is lousey with a swarm of gammas from equilibrium daughters.

Richard Hull

[Alex Aitken]

Aparently I didn't make my point clearly ;P

I don't seriously expect anyone to try gamma spectroscopy with a plastic scintillator.

Large variations in output pulse for the same stimulus (aka resolution) overlap the noise level and the signal level on a counter. Depending on the minimum pulse rejection a given counter either has more false counts or less sensitivity.

[Richard Hull]

There was no specific finger pointing here. I have proven myself wrong in the lab about as often as I was correct. A good experiment with well defined goals is always a final arbiter in these matters. A lot of real life in the lab often scatters and smothers theory.

I am stunned as I continue to read ""Critical Assembly" to see how little we knew and how far off calcs and theory was in regards to bomb building. We just went in to it with little more than calcs which needed a lot of refining in the lab based experiments. Each experiment uncovered unappreciated demons to confound the calcs. Oddly, one of the great coups was Fermis calcs on the first divergent chicago pile based on his and others work lab work with about 16 different exponential convergent piles, (details in the book), His calcs done mostly with his own hands resultied in a perfect hit to such a degree of precision that all the cadmium rods could be quickly removed and just the last rod pulled out in 6" increments to go to a full k>1 divergent chain reaction. Wow! That was not theory or lab work. That was engineering based on the union of the former two.

Ultimately more demons would emerge in the giant plutonium piles and the first power reactors due to unappreciated poisoning by fission products. Fortunately the first piles and reactors were built by enegineers used to over design and not spot on minimalistic design and were designed over 50% larger than needed (suspenders plus a belt mentality). This over sized design was all that kept them from having to tear down and rebuild the reactors. (Told in Leona Marshall Libbie's "Uranium People"). This was the only book written by a woman physicist who was part of the Met Lab and followed Fermi throughout his journey's to Chicago, Hanford and ultimately, Los Alamos. Just like Monty Python's Spanish In quisition, no one expected reactor poisoning.

Richard Hull

[Richard Hull]

Most of my beta work is around design of a vacuum chambered magnetic deflection beta spectrometer. The thick region PIPs detectors are very very expensive and would not do well in the higher mev betas.

However reason dictates that I should kick my butt into high gear and get one of my three gas flow 2 pi proportional rack mount systems operational.

Richard Hull

[Richard Hull]

This is our radiation forum and all discussions related to radiation in general, measurement, safety, history, techniques, instrumentational constructions, adjustments and usage is allowed even if it doesn't bear on fusion, fusors or any thing related to them.

This forum, like all the others is a technology forum. It covers a discipline; one of many that a fusioneer must at least be informed in, if not a full fledged practitioner of inorder to achieve the goal of not only measuring fusion but remaining safe while doing so.

All knowledge imparted here is the seed corn of future adeptness and practical application.

Discussion of Beta spectrometers or any radiation device is always acceptable. This is much as any vacuum topic can be covered in the vacuum forum.

Fear not, any radiation related discussion in this forum.

By the way, my rack mount gas flow counters are standard 2" planchet based counters for precision 2 pi counting of low level beta sources and use no probes, of course. They are most often used for environmental sampling or marginal activation sample analysis.

Richard Hull

[Richard Hull]

Yes. This is the normal setup for these hyper accurate and critical devices. They allow for rapid sample change with a sliding drawer, air tight interlock system. The key is a hyper smooth, often chrome plated hemispherical chamber and a very clean and hyper fine loop of tungsten wire as a pickup electrode in the top of the chamber.

These are classically operated in the proportional region, though a little gas amplification is sometimes allowed when pushed into the region of limited proportionality.

I think my setups are NMC units from the 70's with goold ole nixie readouts. NMC got their start in the late forties making these things. I had two of their earliest all vacuum tube units as a young man, surplus, that used three vertical decades of neon bulbs, (NE2s) and a solenoid operated clickity-clack veedor root counter for the thousands digit and above.

I gotta' dust those babies off and fart around with them a bit. Common tank argon is often good enough as the flow gas.

Richard Hull