Although most types of radiation are invisible, except for the visible part of the EM spectrum, there are many ways that we can make various types of radiation visible. One of these methods is called ‘scintillation’, which can be used to make ionizing radiation visible. Recently [Lukas Springer] demonstrated how to make scintillators out of what is essentially plastic: bisphenol-A (E45, ‘epoxy’) resin with hardener and other additives.
The essential principle of operation behind a scintillator is its sensitivity to ionizing radiation, along with the tendency to absorb the energy and re-emit it in the form of light, i.e. luminescence. This is akin to the luminescence of LEDs, except that in their case the underlying principle is that of electro-luminescence. In the case of a plastic scintillator, the scintillating material is suspended in the solid polymer matrix base.
As [Lukas] points out, plastic scintillators are hardly ideal when it comes to their sensitivity to ionizing radiation, but they compensate for this by being easy to shape and produce, while being very durable. For this experiment, he used regular epoxy as the scintillator matrix, p-Terphenyl as primary scintillator and Coumarin 102 as the wavelength shifter. These three compounds act as a reaction chain, with the matrix absorbing the radiation and transferring it to the primary scintillator, which in turns emits the energy as light.
As the primary scintillator tends to radiate in the deep UV part of the EM spectrum, a wavelength shifter (i.e. secondary scintillator) which ‘shifts’ the emitted UV radiation into the visible part of the spectrum.
After producing a batch of plastic scintillators following the above recipe, [Lukas] irradiated them with gamma radiation, and found them to perform worse than some already not remarkable Russian PS-based scintillators. [Lukas’s] guess is that the matrix may be absorbing the primary scintillator’s output, or a mismatch between the primary and second scintillator.
While tricky to get right, it does seem like a fun hobby if one has some interesting in chemistry. [Lukas] (@GigaBecquerel on Twitter) provides a basic recipe as well as many other compounds to use for the primary and secondary scintillator, as well as the matrix compound. Enough to get started with.
Interesting experiment!
One drawback is that it will probably not detect beta radiation, and almost certainly won’t detect alpha radiation.
It will detect both just fine. The betas penetrate a bit, and the alphas obviously just interact at the surface, but you’ll get plenty of energy deposition to light it up.
Oh it does detect beta very well, and even reacts to alpha!
So, how does this compare in brightness to just painting on some zinc sulfide or similar phosphor? I expect the ZnS’s long tail will make it useless for some applications, and is necessarily very thin to let the optical light escape, so won’t be very sensitive to high energy gammas. But How does this approach compare in actual efficiency?
I can’t measure the actual light output at the moment, but I know they are fairly dark. Plastics are usually much faster than ZnS, and with the coumarin I should have <10 ns pulses. I'm not sure what you mean with the actual efficiency?
The detection cross section should be similar to other plastics.
Efficiency = quantum yield. Or: How many optical photons do you get per absorbed quantum (gamma, beta, alpha) of ionizing radiation?
A decent GdOS scintillator will give you a few thousand optical photons for an absorbed 80 keV x-ray photon (or ~50-70 photons per keV). CsI:Tl is about as good. NaI:Tl is faster decay but half as efficient and also almost half as dense (so half the stopping power). Organics/plastics I’m not familiar with.
Ah, now I see what you mean.
The Qe of plastics is usually lower than that of NaI, around 60-90%. With these I’ll be lucky if I get to 30% the light output.
If I made a little one of these and painted the front and sides back could I stick in into one of those snap on cell phone lenses and have a radiation detector on my phone?
Technically yes, but they’re so dark you’d need to be in a very deadly radiation field to see anything.
Actually there are apps that use the camera sensor alone as radiation detector!
If this scintillating material were coated over a solar cell, would the dominant Alpha from an outer layer of say ThO2 enable an Alpha-voltetic energy source for a few uA or nA? Do you have an idea of the repetitive endurance of the doping pigments?
I found resin acts as a very strong filter for shorter wavelength UV while playing with glow in the dark materials encased in resin and a 365nm uv light; much less so as you go to 400nm.
True, but it looks like most (if not all?) transparent plastics have a cutoff at around 400 nm.
PVT, which many plastic scintillators are made of has a pretty sharp edge there and apparently it still works well.
The trick is to convert the deep UV to visible light quicker than it can be absorbed
Was trying to remember why gigabecquerel sounded so familiar, and then recalled mention by marcoreps (who has a great youtube channel worth checking out, btw).
so how to use this device , toss some ‘skin’ chips on the bag scanner at the airport , if it lights up I know the xray unit is still working ?
We used normal acryl for x-ray. It was more sensitive than most other sensors. Bright blue color was easy to see and read.
Where can I buy one peace? I want to try on my PMT.