A LabVIEW-Based Data Acquisition System for the UTEP/Orsay Instrumented Collimator

Abstract

We have successfully employed an instrumented collimator of aperture 2 mm for the g8a run (summer, 2001) in Hall B of Jefferson Lab, which was designed and tested by UTEP and engineered and built by the Institut de Physique Nucléaire in Orsay, France. The instrumentation of the collimator consists of four radially mounted scintillator + lightguide + PMT elements possessing a left-right, up-down symmetry, which afforded the beam of photons to be precisely aligned to the central axis of the collimator. The collimator was sensitive to rather modest beam motion of the order of a few tens of microns. We made use of the physical fact that the emission angle of the coherent bremsstrahlung photon component is correlated with its energy for enhancing the degree of polarization of the photon beam. We can extract the spectral peak by tightly collimating the beam and thereby reduce the incoherent background, which further serves to increase the degree of polarization. The maximum polar angle is limited by the opening angle of the collimator, which gives the lower limit for the coherent bremsstrahlung photon energy. To achieve polarizations exceeding 75%, we must collimate the beam to better than one half of a characteristic angle. This means for a collimator-goniometer distance of 22 m, the aperture of the collimator should be 2$ mm for an incident electron energy of Eo = 5.7 GeV. We will report primarily on the benchtop data acquisition system (DAQ), which interfaced LabVIEW through a SCSI connection to the VME backplane to readout CAEN QDCs. We employed this DAQ to test and calibrate the instrumented collimator. Our efforts were quite successful in that the device functioned beyond design specifications within one hour after the photon beam was directed onto the bore hole of the collimator.