Re: Prism & Light Speed, Dispersion, etc.

From: Franz Heymann (notfranz.heymann_at_btopenworld.com)
Date: 03/06/05 

Date: Sun, 6 Mar 2005 23:49:08 +0000 (UTC)

"bz" <bz+sp@ch100-5.chem.lsu.edu> wrote in message
news:Xns9610BCB1182CEWQAHBGMXSZHVspammote@130.39.198.139...
> "Franz Heymann" <notfranz.heymann@btopenworld.com> wrote in
news:d0de1l$aoj
> $4@nwrdmz03.dmz.ncs.ea.ibs-infra.bt.com:
>
> >> They slow down, rapidly. Of course to slow down they must give
off
> > energy.
>
> Well, to slow down they must give off energy, right?
>
> >>
> >> bremsstrahlung radiation (braking radiation) is emitted.
> >
> > No.
> > Bremsstrahlung has nothing to do with the motion of particles
whose
> > speed exceeds the speed of light in the medium.
>
> I thought Cerenkov Radiation was a special case of Bremsstrahlung
> radiation. Is this incorrect?

Yes, it is incorrect.
Bremsstrahlung is a process in which an electron interacts
electromagnetically with the field of a single nucleus, resulting in a
deflection and the emission of a photon, usually in the X-ray and
gamma ray region of energies. It leads to a single, large energy loss
by the electron.
Cerenkov radiation is a continuous process, in which an electron (for
example) moves through a transparent optical medium with a refractive
such that the speed of the electron is greater than the speed of light
in the medium. The medium is polarisable, otherwise it would not have
a rfractive index different from 1. The electric field of the passing
electron polarises the medium as it goes, producing an electromagnetic
shock wave of EM radiation which has a wavelength range which covers
the whole region in which the medium is transparent.

> I realize that just changing the direction of
> motion of a beam of particles will ALSO produce Bremsstrahlung. We
have a
> facility in Baton Rouge called CAMD that produces and uses such
radiation.
> I have seen the facility, but never been there when it was in
operation.

You are being a little ambiguous here. If you will tel me how the
beam is deflected I wil tell you whether you are talking about a
synchrotron radiation source or a so-called tagged gamma ray beam.

Facilities for producing photon beams in the gamma ray region of
energies are common at many high energy laboratories. In all such
cases, the incident beam of electrons is alowed to strike a target of
tungsten or lead in which bremsstrahlung photons are produced by the
process I described higher up in this note.
>
> > Lead is a very
> > favourite target for producing bremsstrahlung, and it is totally
> > opaque to light.
>
> Lead is not opaque to ALL Electromagnetic radiation, is it?

No. I mentioned the opacity simply to indicate that Cerenkov light
cannot be produced in lead. Very high energy gamma rays may penetrate
lead for a few centimetres.

> >> Cerenkov Radiation is a form of such radiation as beta particles
> > slow down
> >> in the water moderator of a water moderated neuclear reactor.
> >
> > Charged particles do not emit Cerenkov radiation because they are
> > slowing down.
> > They emit Cerenkov light simply because they are moving
> > faster than the speed of light in the medium. The rate of energy
loss
> > from that is utterly minute compared to the ionisation loss, or
indeed
> > the energy loss by bremsstrahlung.
>
> OK. Thanks. I always appreciate learning something.

It is indeed a pleasure to talk to someone who actually wants to
learn. 

-- 
Franz
"A first-rate laboratory is one in which mediocre scientists can
produce outstanding work"
P.M.S. Blackett

Relevant Pages

  • Re: Repulsion binds atoms
    ... configurational entropy. ... That the energy of the system goes down? ... "Electron Beam Lithography" or "Electron ... Yes, the radiation is nil, justas the radiation ...
    (sci.physics)
  • Re: Orbitals as Orbits: The Return of Bohr
    ... >> Its glibly stated that the orbiting electron would radiate energy away. ... The frequency of such radiation is just the frequency ... emission of EM radiation and 'dropping' into an orbit of *higher* ...
    (sci.physics.research)
  • Re: Orbitals as Orbits: The Return of Bohr
    ... > Its glibly stated that the orbiting electron would radiate energy away. ... It radiates away in the continuum approximation, ... The frequency of such radiation is just the frequency ...
    (sci.physics.research)
  • Re: Orbitals as Orbits: The Return of Bohr
    ... >>> Its glibly stated that the orbiting electron would radiate energy away. ... The frequency of such radiation is just the frequency ... >>of the orbit. ...
    (sci.physics.research)
  • Re: Prism & Light Speed, Dispersion, etc.
    ... Well, to slow down they must give off energy, right? ... >> bremsstrahlung radiation is emitted. ... I thought Cerenkov Radiation was a special case of Bremsstrahlung ...
    (sci.physics)