Scaler Data

In order to measure the effect of the crystal on the single photon spectrum, data was taken at different crystal settings and beam energies. For these runs a "pair trigger" was used which simply required a hit in the TR2 trigger counters. This trigger was prescaled by a variable amount in order to maintain a comfortable data taking rate. Two separate angle scans were made:-

1. In the first scan the crystal was initially positioned with a slight vertical tilt with respect to the beam axis. The tilt angle was then varied in steps of about 0.2 mr. At each step about 100,000 pair triggers were taken from an electron beam at three different beam momentum tunes; 300 GeV, 350 GeV, and 400 GeV.
2. In the second scan a 300 GeV positron beam was used and the tilt angle of the crystal was varied over a larger range.

   Once the scans described above were completed, the crystal radiator was removed from the beam and replaced with a 20% Xo amorphous Pb radiator. Pair data was then taken for each of the four beam configurations decribed above.

   Ion Chamber Rates

   The simplest and quickest insight to the crystal radiator's behaviour came from three ion chambers (IC's) which were read out into the scaler data. Each of the ion chamber sets (BGM-IC, RESH0-IC, and POSH0-IC) are operationally identical in design and function, except for size of apertures (8"x8", 5"x5", 5"x5", respectively). Each set has three Ion Chambers located INSIDE the ElectroMagnetic calorimeters at depths of 4 Xo, 8 Xo, and 12 Xo; enough sampling so they calorimetrically measure the ENERGY Flux (i.e. number of particles or photons times their average energy), NOT just the number of e- or e+ or photons that converted. The resulting values have been determined to be very linear with respect to beam intensity but have not yet been calibrated in terms of GeV deposited. The big advantage of the IC's over other scalers is that they are not prone to pileup or sagging effects at high event rates.

   BGM-IC was positioned in front of the Beam Gamma Monitor (BGM), a calorimeter in which the uninteracted photons and higher energy e+e- pairs were absorbed. The other IC's were placed in front of the recoil electron (RESH0-IC) and positron (POSH0-IC) dumps. These dumps are located where a beam electron or positron would end up if it did not radiate more than about 30% of its energy when passing through the radiator. "IC Rates vs Tilt Angle" shows the behaviour of the three IC rates during the scans described above. The plots show a dramatic change in the amount of energy radiated as a function of tilt angle. Also note that, as one would expect, the electron IC is not affected by positron beam and vise-versa.
   

   Corrected Trigger Rates

   We can also gauge the crystal's effect by simply monitoring the trigger rate. "Trigger Rate vs Tilt Angle" shows the trigger rate corrected for both the varying prescales and experiment livetime for the same scans as above. The same behaviour is observed as was seen with the Gamma IC. Subtle differences are expected due to the differences in how the IC and the detectors used in the trigger respond to pileup and multiple bremsstrahlung.

   Note that the similarity between the 300 GeV positron results and the 350 GeV electron results is largely a coincidence. The positron beam is known to be less intense than the electron beam due to the poorer transmission efficiency of the positive arm of the beamline. On the other hand the beam yield is expected to fall rapidly with energy. The fact that the size of these effects seem to be nearly the same is pure chance. However, the fact that the shapes of the peaks are the same is an indication the the coherent bremsstrahlung process is the same for both electrons and positrons.