Kristoffer K. Andersen, Søren L. Andersen, Jakob
Esberg, Helge Knudsen, Rune Mikkelsen, Ulrik I.
Uggerhøj, Pietro Sona, Alessio Mangiarotti, Tjeerd
J. Ketel, and Sergio Ballestrero (CERN NA63)
When an energetic electron slams into a material,
its energy is partially converted to photons. The
resultant photons are called bremsstrahlung
(“braking rays”) and have a well-understood energy
spectrum. For relativistic electrons, the spectrum
is strongly modified by multiple Coulomb scattering:
the electron bounces against atoms in the material,
and the resulting interference inhibits the emission
of low-energy photons. To understand the spectrum,
it is essential to know how long the electron needs
to travel in the material to generate photons,
because the longer the distance, the more bounces
the electron experiences. An open question is the
formation length for photons emitted this way under
different conditions, and how this compares with
theory.
Using a rather simple
mechanical technique, Kristoffer Andersen of Aarhus
University, Denmark, and colleagues of the CERN NA63
collaboration report inPhysical
Review Letterson
measurements of the photon formation length for
radiation emitted by electrons. The setup consists
of two gold foils separated by a distance of up tomillimeter,
controlled by a precision micrometer translation
stage. The researchers aimedelectrons
at the foils and measured the resultant photon
emission spectrum. Photon production begins in the
first gold foil, and if the formation length is
large enough corresponding to a certain photon
energy, continues in the second foil, modified by
multiple scattering. Electrons that scatter in both
foils generate a peak in the spectrum, whose
position depends on the foil separation. From this
the group could mechanically determine the length of
travel required to generate a photon of given
energy. This, in turn, allows comparison of these
observations against predictions. For future work,
the team proposes replacing the second foil with an
intense laser beam, allowing more detailed analysis
of the photon spectrum and the multiple scattering
corrections. –David
Voss
Since 1962 I doubted on Newton's laws. I
did not accept the infinitive speed and I found un-vivid the laws of
gravity and time.
I learned the Einstein's Relativity, thus
I found some answers for my questions. But, I had another doubt of
Infinitive Mass-Energy. And I wanted to know why light has stable speed?
