Spinthariscope

I enjoy funny words, like “spinthariscope.” It sounds like it should be a fancy and complex Steam Punk gizmo for watching rapidly rotating things, like a centrifuge or gyroscope. But Crookes coined the word as an Anglicization of the Greek-rooted word Scintillate for his invention to view nuclear decays. It provided one of the first direct proofs that Curie radiations were discrete, quantized events and not a continuous field.

Spintahriscope in place over the isotope holder in a smoke detector

After writing my post “Not All Natural” about the nuclear waste found in my house, I decided to buy a spinthariscope and see some nuclear radiation as directly as possible. This cheap one just looks like a strip of paper with a round window of simple translucent film. But it works like a charm.

To actually see the radiation from my smoke detector, I had to dark-adapt for a full half hour in my darkest room. Then I could see the glow of alpha particles striking the scope. With a magnifier, one can see each individual particle die. The source produces about 17,000 particles a second spread over 2π steradians, so the dark circle seen at the lower right of the viewer area was alive with about 2,000 green speckles a second.

But, wait, you may well say. Everyone knows  😉 that it takes a minimum of seven photons to trigger a response in the most sensitive rods in a human retina. How can the single quantum event, an alpha particle collision,  produce more than a single photon?

To start with, an alpha particle is a fully ionized helium nucleus, and therefore will collect two electrons from the first atoms it can approach. That’s a minimum of two photons, as any change in electron state releases (or absorbs) a photon. But then the atoms from which it stole the electrons will also be ionized, and claim electrons from others. This can go on for a while (nanoseconds) till some free electrons are found to fill the gap. But this is still only a small number of photons. Additionally, the alpha particle can only grab those electrons once it is moving at less than the speed of light.

Wait. How can an alpha particle go faster than the speed of light? Well, it cannot in free space. But the speed of light in a material medium is lower than the speed of light in a vacuum. Remember your lessons in refraction, of how lenses work. Those alpha particles leave their nuclei of origin at quite a clip, faster than the speed limit in any solid. So when alphas start passing other atoms at this illegal speed, they exert a force to slow them down. Any quantum force implies a quantum of energy, = photons. This is Cherenkov radiation, the light given off when particles go faster than the speed of light in a medium. This is what causes that eerie blue glow one sees near the core of nuclear reactors.

So the spinthariscope works by having a coating of a special crystal, like silver-activated zinc-sulfide, that is transparent to visible light, converts high energy photons (gamma through ultra-violet) down to visible light, and provides a medium of low light speed (high refractive index) to maximize the Cherenkov glow. So each alpha particle creates a shower of thousands of photons, enough to see as a tiny flash of color in a very dark room.

It is mesmerizing to watch this surreal, silent circle of ever changing speckles and to understand this miracle of helium being born. I’m sorry that I don’t have equipment to try to show you a video of the glow on today’s Object at Hand. But even if I did, it would be sort of like showing pictures of the Grand Canyon. Until you’ve seen it live, you can’t get the feel of it.

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3 Responses to “Spinthariscope”


  1. 1 Karen Jackson May 27, 2011 at 4:29 pm

    You lost me in most of the 2nd half, with all the talk about nuclei, photons, etc.(good “bedtime stories! 😉 ); yet I still found it an interesting post.

  2. 2 Steve Tasker September 19, 2011 at 2:50 am

    The alphas do not travel anywhere near the speed of light in the Zinc Sulphide medium, so what you are seeing here is not supposed by physicists to be Cherenkov Radiation. I think it is more likely that the ionisation excites the elections to jump the band gap in the ZnS. When the electrons spontaneously transition back from the high energy band to the ground state, light is emitted. The process of light emission is similar to that of an LED.

    That said, the Wikipedia link you provided is most interesting and suggests that the whole situation is much more complicated than I realised.

  3. 3 Dan Klarmann September 19, 2011 at 8:07 am

    As Steve concluded, it is more complicated than a simple ion exchange. I encourage all readers to follow some links, learn some quantum physics, and decide for yourselves who is more likely correct in this.
    Although Wikipedia itself is not an authority, it is very reliable. They diligently cite their sources, so that anyone can fact-check.


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