The response of the SLC beamstrahlung monitor Page: 2 of 15
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pressed in units of mec2
Ecutof f _n
mec2 0=ss (1)
can emit Cerenkov photons. At the present detector this threshold is ap-
proximately Ecutoff = 25 MeV. This discriminates the radiation from the
bending magnet.
In order to quantify the above argument assume that. radiation with char-
acteristic energy e, is incident on the monitor. To calculate the number of
Cerenkov photons dNc/di generated per unit time we have to take the fol-
lowing effects into account [2].
1. Pair production probability in the converter plate.
2. The probability of finding a pair produced particle at energy E.
3. The number of Cerenkov photons produced per particle.
These three points constitute the monitor response for one incident photon of
a certain energy. The effect of a spectrum characterized by e, is the given by
the convolution of the detector response and the spectrum. This calculation
will be the task of the following sections.
2 The Monitor Response
The pair production probability is determined by the crossection of the pro-
cess -y -+ e+e-. Since we are only interested in energies above the Cerenkov
threshold we may use the Heitler-Sauter crossection for an unscreened point
nucleus for ultrarelativistic energies [4] .
-e+e- = aZ2re [ ln(21B) - . (2)
Here a = 1/137 is the fine structure constant, Z is the atomic number of the
nucleus (Z = 26 for iron), re = 2.8179 10-15 m is the classical electron radius
and YB is the photon energiy in units of mec2. At the Cerenkov threshold we
have ...e+e- = 2 10-28 m-2.2
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Ziemann, V. The response of the SLC beamstrahlung monitor, report, October 15, 1990; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc1205938/m1/2/: accessed June 5, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.