Plastic scintillators are fun.
They’re not great for gamma detection, but they’re cheap, robust and great for detecting particle radiation, and good for discriminating between beta / alpha / neutrons and the likes.
They can be cast and machined into every shape and are therefore good for custom detectors, large area couting and many more applications.
EDIT: My rambing about this project and what to change can be found at the end of this post.
Most plastic scintillators are very similar to liquid scintillators. There’s a main component called the matrix or solvent, there’s a primary scintillator and secondary scintillator, often called wavelength shifter.
The matrix absorbs the radiation and transfers its energy via a non-radiative process to the primary scintillator, which then emits it as light, usually in the deep UV range. Most matrices are not very transparent for deep UV, and most PMTs aren’t very sensitive to it. This is solved by adding a secondary scintillator, which absobs the UV and re-emits it in the visible range, hence the name “wavelength shifter”.
Usually the matrix is some aromatic compound, as this helps to transfer the energy to the primary scintillator. Common solvents are Benzene, toluene, xylene and similar derivates, plastic scintillators use polyvinytoluene or styrene.
The most common primary scintillators are 2,5-Diphenyloxazole (PPO) or para-terphenyl (eg. in BC412).
Wavelength shifters can be everything that absorbs light at ~350 nm and re-emits it at a suitable wavelength, ideally around 420 nm for bialkali PMTs. Noteworthy are 1,4-bis(5-phenyloxazol-2-yl) benzene (POPOP, eg. in BC400)) and 2, 5-Bis(5′-tert-butyl-2-benzoxazol-2-yl)thiophene,2, 5-Bis(5′;-tert-butyl-2-benzoxazol-2-yl)thiophene (TPBD, eg. in BC412).
It is important that both the primary and secondary scintillator have a quick decay time for a good timing response of the overall scintillator. Common concentrations of scintillators in the solvent are 0.5 to 2% of the primary, and 0.01 to 0.5% of the shifter.
My scintillators use epoxy resin as a matrix, because that is cheap, easily available and can be cast & machined well. All experiments so far were made with bisphenol-A based “E45” resin.
P-Terphenyl was used as a primary scintillator, because it can be bought at S3 Chemicals for cheap.
The wavelength shifter is still somewhat of an issue for me, as I can’t get my hands on any of the common ones. A friend of mine gave me some Coumarin 102 laser dye to try it with; while the absorption spectrum isn’t ideally matched to the emission of p-terphenyl it still works.
For a 50 g batch of scintillator resin I used:
0.5 g p-Terphenyl (1%)
50 mg Coumarin 102 (0.1%)
7 g Xylene
16.7 g Hardener
33.3 g Resin
Technically that’s too much p-terphenyl to stay in solution, but with some tricks you can still make clear scintillators. My first attempts all crashed out and gave me milky-white blocks of resin that of course didn’t transmit the lights they created.
Weigh out the p-terphenyl, coumarin and xylene in a beaker and bring it to a boil to dissolve everything. Keep the beaker covered with a round bottom flask filled with cold water to prevent the xylene from boiling off. Pereheat the resin to ~60°C while doing so.
Once the solution is clear add the hardener and keep it warm, but not boiling. Mix everything until you can no longer see schlieren. Mix it well with the resin and cast it in a mold. Keep it at ~80°C while curing, or stuff will fall out of solution. With bigger batches the heat from curing can be enough to keep everything in solution, but I would not rely on it.
After the resin has cured the scintillator is ready!
Now, let’s get to specs.
This is a terrible scintillator.
Irradiated with gamma and measured with a Hamamatsu R550 PMT it gives about 50% the output of the old russian PS based scintillators, which aren’t exactly known for their high light output.
I think most of this comes down to the resin absorbing most of the primary scintillators light before it can reach the shifter, and the shifter having a somewhat strong mismatch with the p-terphenyl in its absorption spectrum.
Just to state the obviois: No, these scintillators are not (gamma)spectroscopic, and will probably never be. As of today inorganic detectors are still unbeaten in that regard. But that’s not what plastic scintillators are for.
I am trying to get my hands on POPOP and expect to get a much better result with that. I will also try PMMA as matrix to see what results I get with that.
Another issue are bubbles forming in the resin. Degassing it in a vacuum doesn’t work, as the xylene just starts boiling off before the air bubbles leave. This results in concentration gradients and a lower optical quality of the scintillator.
The plastic is basically an oversaturated solution held together by the resin, I am unsure about the long term stability of it all.
I’ll do some more experiments with light attenuation in the crystal, and reaction to different kinds of radiation in the future.
So far I’ve confirmed the Scintillators react to Alpha, Beta and Gamma; still need to test (fast) neutrons but I think my chances are good that it reacts to proton recoil.
Enjoy a few pictures of my experiments so far:
Please try to make your own scintillators, alter the recipe, try variants and tag me if you get any results! DIYable Plastic scintillators would be a great addition to hobby radiation detection and will definitely lead to many interesting detectors and experiments!
Many people have read this and I got lots of input, this project was even featured on Hackaday!
Thank you so much to everyone.
Since weiting this article I have done some thinking, and maybe these scintillators aren’t as bad as I initially thought. Part of the low photoelectron yield will be, that the output spectrum of my secondary scintillator is not matched to a bialkali photocathode. But this mismatch has its advantage, as the self-absorption of green light in epoxy resin will be way lower than that of blue light. I do have some red extended multialkali PMTs, and some BPEA is on its way, maybe I can try to make them as green as possible to cast bigger detectors. Also, BPEA might have enough of a stokes shift to work as a single scintillator, without p-terphenyl. https://omlc.org/spectra/PhotochemCAD/html/044.html
That would make casting so much easier as I won’t have to deal with the super-saturated solution and everything can be done at room temperature. More experiments to come!
Turns out BPEA doesn’t work well with p-terphenyl or PPO. But 1% of PPO on its own already surpasses the performance of p-Terphenyl with coumarin, and it dissolves in the epoxy without any additional solvent. This, plus a suitable shifter (I am still waiting for POPOP) will be the key, I am sure!
It looks like some of my scintillators have p-terphenyl crashing out. My guess is that it has to be soluble in the matrix itself, and my epoxy resin really doesn’t dissolve any of it, regardless if I try the resin or hardener. Polystyrene and PVT seem to be favored matrices, as their monomers can act as a solvent for the additives. Looks like you can’t buy styrene or vinyltoluene as an individual in the EU.
Oliver Keller had the great idea of making “edible” scintillators, along the lines of “edible” Lasers.
Agar Agar as a matrix with quinine and or fluorescein as additives.