Very high energy and intense photon beams are usually created from an electron (or positron) beam by passing it through a high Z radiator such as lead. The electrons interact with the radiator nuclei to form a bremsstrahlung photon beam which has a energy spectrum which falls roughly as the inverse of the photon energy. In order to increase the yield of high energy photons it has been proposed to replace the traditional radiator with an approproriate crystal so as to take advantage of a process known as Coherent Bremsstrahlung which for an appropriate choice of crystal and orientation can produce an enhancement in the high energy part of the photon spectrum.
The Wide Band Photon
Beam
This page contains a description of Fermilab's Wide Band Photon Beam which
includes plots of the electron/positron beam characteristics at the radiator.
Motivation for Using a Crystal Radiator
This page presents the results of the calculations of photon production
through Coherent Bremsstrahlung using a 1.1cm thick silicon crystal and an
electron beam with properties similar to the Wide Band beam described above.
The Test
At the end of the 1996/1997 Fermilab Fixed Target run, tests of the
Coherent Bremsstrahlung process were performed. A 1.1 cm thick silicon crystal
radiator was mounted in the Wide Band Beam and the resulting photon spectrum
was measured using components of the Focus (E831) spectrometer. The following
pages describe these tests and the results obtained.
The Goniometer Calibration and Crystal Alignment
Here we describe the goniometer used for the tests and present the alignment
data used to calibrate it. We also describe the initial alignment of the
crystal.
Effect on Hadronic
Trigger Rates
This document describes Harry Cheung's study of the effect of the crystal
on E831's hadronic trigger rates. The trigger rate was studied as a function of
the energy visible in the Hadron Calorimeter. Typically this represents about
75-50% of a single photon energy and is insensitive to pile-up effects from
multiple bremsstrahlung photons since the probabilty of two photons interacting
hadronically in the same trigger gate is very small. From these plots we can
see that trigger rate was increased by as much as a factor of four depending on
the orientation of the crystal. However, at the highest rate settings the gain
was mainly in the low energy part of the triggered spectrum. At a crystal
setting in which the gain was about a factor of two, the plots show that the
single photon energy spectrum was very similar to that obtained with a 20% Pb
radiator.
Scaler data from Pair Runs
A second set of runs was taken with a dedicated e+e- pair conversion
trigger. Data was taken at different beam energies, crystal settings, and beam
type. This page summarizes some relevant scaler data from these runs.
Spectrometer data from Pair Runs
The spectrometer data from the Pair Runs described above was partially
reconstructed (thanks to Harry Cheung) to produce a series of binary DST's. This page summarizes results from analyses of
these DST's.
Understanding Pair Reconstruction
Here we describe studies with the ROGUE Monte Carlo to understand effects
such as pile-up, spectrometer acceptance, and tracking errors on the observed
pair spectrum.