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Topic: Nice Car Shine  (Read 21335 times)

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Offline jdurg

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Re:Nice Car Shine
« Reply #15 on: January 24, 2006, 10:40:55 PM »
I think it might be Cherenkov (sp?) radiation.  My understanding is that this happens when a particle (say a beta electron) is traveling faster than the speed of light *in that material*.  The speed of light, when *not in a vacuum*, is slowed down by the index of refraction of the material.  In the case of H2O, this is around 30% or so.  Betas are usually moving close to real (as in vacuum) c, so they easily break this lower limit.  When it happens, you get kind of a 'sonic boom', except with light waves, not sound.

I think you need to have something that is very highly radioactive to see this.  I've never seen it myself, unfortunately.


With Actinium and Curium, however, you don't need to have them underwater.  Their radioactive decay is frequent enough and of a high enough energy that the air immediately surrounding them gets "excited" just as if an electrical discharge was moving through them.  Typically speaking, if you were able to see it first hand you'd likely have received a MASSIVE radiation dose.  I'll have to dig around on the web to see if I could find a video of actinium metal giving off the eerie blue glow.  I vaguely recall seeing it in a VERY old video of fairly poor quality.
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Offline pantone159

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Re:Nice Car Shine
« Reply #16 on: January 24, 2006, 11:05:41 PM »
With Actinium and Curium, however, you don't need to have them underwater.

Water was just an example, maybe not a very good one.  Air is a more appropriate example, with an index of refraction (IIRC) of about 1.003.  Therefore, if a particle if moving less than 0.3% slower than the (vacuum) speed of light, then it will be breaking the 'light barrier' in that medium, and emit Cerenkov radiation.

That doesn't mean that the 'glow' is *actually* caused by Cerenkov radiation, as opposed to simple ionisation.

Quote
I'll have to dig around on the web to see if I could find a video of actinium metal giving off the eerie blue glow.  I vaguely recall seeing it in a VERY old video of fairly poor quality.

I wish there were better pictures of some of these exotic elements (like Ac) on the internet.  A while ago, I made a fairly concerted effort to find all the pictures I could, and I was pretty disappointed with what came up.  I did find some German site that had some really good pictures, but other than that, many of the best available images were straight from a c. 40-year old Time Life book.  I read this book as a kid, but I was really hoping to find something more up to date.
« Last Edit: January 24, 2006, 11:13:38 PM by Mark Kness »

Offline pantone159

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Re:Nice Car Shine
« Reply #17 on: January 25, 2006, 03:33:45 PM »
Some calculations show that any glow in air from these radioactive elements is *NOT* due to Cerenkov radiation.

The energy of a particle is m*c2*gamma, where gamma = 1.0/sqrt(1.0 - (v/c)2).
(m is the article rest-mass, c is the speed of light, v is the velocity of the particle.  See any book on relativistic mechanics for these formulas.)

This energy includes both the 'kinetic energy' of the particle, as well as its 'rest energy', which is given by m*c2.
Tables that list the energy of emitted radiation don't count the rest energy, so it then works out that:

Eradiation, from tables = m*c2*(gamma - 1)

For the case of air, with n=1.003, the speed required for Cerenkov radiation is c * (1/1.003), which works out to gamma - 1 = 11.94.

Alphas have a mass of around 3700 MeV/c2, so to reach the required speed they would need to have a kinetic energy of around 44 GeV.  No alpha emission is anywhere near this energetic, 8 MeV is a very energetic alpha.  So, alphas won't generate Cerenkov radiation.

Betas are a little closer.  The electron mass is 0.511 MeV/c2, so to emit Cerenkov radiation they would need a kinetic energy of 6.1 MeV.  This is still too high, for example Cs-137 betas are (maximum) 0.514 MeV, less than 10% of the required energy.

If you put your beta emitter in water (n=1.333), then you *can* get Cerenkov radiation.  Then, you just need gamma - 1 = 0.51, and for the electron you just need a kinetic energy of 0.26 MeV. The Cs-137 betas are energetic enough to manage this.

Conclusion:  If you see glowing in the air around a radioactive lump, the glow is not from Cerenkov radiation.

Offline constant thinker

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Re:Nice Car Shine
« Reply #18 on: January 25, 2006, 07:47:13 PM »
Ok thank you guys. I thought it was either some special type of radiation that is known to cause ionization of the air around it in a great quantity or just a lot of regular radiation. I thought this was all a hollywood myth at first though.

Jdurg for pictures, your right they are wicked hard to find. I googled it and I can't find anything. I both image searched and regular google searched it. If you ever find pictures (this goes for anyone) could you be kind as to post them here. I'd love to see it and not just imagine it in my head. ;)
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