Making a plastic scintillator glow?

Question

I have a plastic scintillator I had purchased off eBay, which is believed to be BC408 http://www.phys.ufl.edu/courses/phy4803L/group_I/muon/bicron_bc400-416.pdf

Upon shining a small UV LED (10-40mA, maybe ~400nm wavelength) on it, it seems to glow mostly from the refraction of the plastic or how it was milled and the UV beam itself, rather than appearing to fluoresce, such as the beam just bouncing off imperfections and passing through the bar. I had also tried to point it at an old CRT screen as a test because I believe they generate near-xray levels (or is that kept behind leaded glass?) and saw nothing interesting.

How can I cause the scintillator to glow/observe flashes with objects one can source around the house or easily? Would I need a higher energy UV at less energy to make the fluorescent effect more dominant than the light hitting it and reflecting off itself?

Answer 1

For fluorescence to occur, you need to excite at an energy higher than the emission. According to the spec sheet this plastic scintillator emits at a peak of 420 nm - it may be that 400 nm is just not a high enough energy to excite it.

I am used to thinking of scintillators as being used with radiation sources - xray, gamma sources. You need a pretty "hot" source to see scintillation with your bare eyes - many years ago I did the experiment in a dark room illuminating a 3" NaI scintillator with 2 mCi of Ba-133. After my eyes accommodated I could see a very faint glow...

You will have much, much better luck using a dedicated photo sensor (photomultiplier, or these day use a SiPM - solid state photomultiplier). SiPM in particular has a gain of around 10^6 for a relatively low bias voltage (30 V for some devices) and will give you electrical signals in proportion to the incident light. This will show you that your scintillator is working - when you illuminate it with a weak radiation source (an old watch with radium hands would be very effective). See for example http://sensl.com/

I am familiar with plastic fiber scintillators that give off brightly colored light when excited by UV - or even daylight. And that might be the thing to try in your case - illuminate the scintillator with daylight and observe from the side. If you see a blue-ish hue you know you are looking at scintillation - especially if the hue goes away when you go indoors.

Many fluorescent tubes also emit significant UV - this is why some photos will "bleach" indoors when placed near such lamps. You may be able to see your scintillator glow blue in the vicinity of such tube.

And if you only see the glow at "scratches" on the surface it may just be that most of the light bounces around inside (total internal reflection). This is why scintillator surfaces are often roughened - the uneven surface increases the probability that photons can cross the interface where there is a refractive index mismatch. You see this with scintillating fibers that seem to "emit" light only from their ends...

See for example this picture (from http://kuraraypsf.jp/images/index_ph01.jpg)

You can see the "brightness" at the end of the fiber - the construction (with graded index along the sides) drives all the light to this one interface where the light can escape.

Answer 2

I'd like to add to dmckee's answer PLEASE heed his last paragraph. There are heaps of experiments on the internet that blithely instruct you to take a smoke detector apart.

Another good "safe" source is uranium marbles: they are traded by marble collectors and are easier to come by than Fiestaware or Annagrün / Annagelb pieces (and the last two are too valuable to have kicking around the laboratory). I know the Titan Missile Museum sells them in Arizona from their shop and this is run by a group of volunteers: although they don't have a shop on the internet, I'm sure they'd be happy to mail some on a phone order. You might try also this equipment supplier United Nuclear Webpage Here.

SAFETY PRECAUTIONS WHEN HANDLING URANIUM GLASS

Generally, these sources are safe. The radiation level at any reasonable distance is comparable to the natural background and the uranium is very stable (i.e. will not leach out of) glass. The only hazard arises if you chip or break them. As Dmckee says, the ingestion of a shard of alpha emitter is dangerous as there is no bodily washout process for uranium (it will stay in your body for life, even after an unremoved glass shard dissolves, which a small one will eventually do). I do not know the correct safety procedure if you shatter uranium glass, but I think you should research a correct plan and have it in mind before you begin. I'm guessing a plan will involve (1) picking up the shards with protective gloves and, say, a damp cloth, (2) verifying that all shards have been removed (so you'll need to make sure you can at least borrow a geiger counter or other detector) and (3) sealing cleaning gear in a sealed container and calling a low level waste disposal company for collection.

Answer 3

You can get original Fiestaware pieces online or at some antique stores. The uranium glaze is a decent low-level x-ray and gamma emitter that is safe to handle.

My orange salt-shaker reads a couple hundred Bequerel on a good Geiger tube held a few centimeters away.

You can also order nano-curies of a number of isotopes without any paperwork (in the US, no idea about other countries). Google for "needle source"

I would not recommend disassembling a smoke detector or otherwise trying to get access to radionucleides used in consumer products because you don't know how they are contained and the last thing you want is to ingest or inhale a alpha emitter.