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in a pool--which I admit was some years ago--the blue glow of Cherenkov radiation (that's the radiation due to beta particles traveling faster than the speed of light in water) extended out at least one half meter from the rods.  

(It was  very beautiful, by the way.)  

The safe distance will obviously be very sensitive to your personal level of machismo.  

It occurred to me later that standing near the edge of that pool was not the brightest thing I have ever done in my life.  

The Fates are kind.

by Gaianne on Sat Mar 26th, 2011 at 03:36:04 PM EST
[ Parent ]
The blue glow of Cherenkov radiation doesn't mark the beta particle penetration depth. It marks the distance at which the Cherenkov radiation remains strong enough for the dipole scattering off the water to be visible. Which is rather a lot farther than the beta particles will ever go (incidentally, this is what allows you to detect neutrinos - if you had to rely on the radioactive emissions from neutrino decay you wouldn't have a prayer).

Of course the halving depth for gamma radiation is on the order of 20 cm, so safe distance is on the order of 2 m of water. And if the water had been exposed for a while, it would itself be slightly radioactive.

- Jake

Friends come and go. Enemies accumulate.

by JakeS (JangoSierra 'at' gmail 'dot' com) on Sat Mar 26th, 2011 at 04:26:21 PM EST
[ Parent ]
in there.  

Cherenkov radiation is fairly directional--lying entirely in the forward direction of the electron.  The diffuse glow that I was seeing implies the electrons were not directional--but moving in all directions.  

Scattering?  Shine a flashlight into clean water.  How much of the beam is scattered?  Some, but not much.  

So:  Wherever I see the glow there is an electron moving rapidly more or less toward me.  

The Fates are kind.

by Gaianne on Sat Mar 26th, 2011 at 05:54:31 PM EST
[ Parent ]
Scattering?  Shine a flashlight into clean water.  How much of the beam is scattered?

Most of Cherenkov radiation is UV, so there is bound to be inelastic scattering.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Sat Mar 26th, 2011 at 06:33:09 PM EST
[ Parent ]
Best of ET discussion,or why i stand on ET.

As one never having experience with Cherenkov radiation eye defer. (Eye am amazed that someone here has such experience.)

"Life shrinks or expands in proportion to one's courage." - Ana´s Nin

by Crazy Horse on Sat Mar 26th, 2011 at 06:43:00 PM EST
[ Parent ]
It was deep blue--which would scatter as blue light scatters.  

Or are you asserting I was seeing scattered--and down shifted into blue--UV light?  

The Fates are kind.

by Gaianne on Sun Mar 27th, 2011 at 12:07:02 AM EST
[ Parent ]
If the scattering is inelastic, it should reduce the frequency.

- Jake

Friends come and go. Enemies accumulate.

by JakeS (JangoSierra 'at' gmail 'dot' com) on Sun Mar 27th, 2011 at 12:47:21 AM EST
[ Parent ]
Yes, inelastic scattering means that the wavelength changes. That is, the UV photon gives kinetic energy to the particle it scatters on, and turns into a blue photon.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Sun Mar 27th, 2011 at 03:33:04 AM EST
[ Parent ]
(wikipedia)

Here beta is the fraction of the vacuum speed of light at which the electron is travelling, and n is the refractive index (4/3 in the case of water). For highly energetic electrons, we get

cos θ = 3/4

That means θ is up a 41-degree angle so that cerenkov radiation propagates at between 0 and 41 degrees away from the direction of propagation of the beta radiation.

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11

by Migeru (migeru at eurotrib dot com) on Sat Mar 26th, 2011 at 07:15:19 PM EST
[ Parent ]
Also, the more intense, the less "directional".

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11
by Migeru (migeru at eurotrib dot com) on Sat Mar 26th, 2011 at 07:54:10 PM EST
[ Parent ]

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