The effects of various surface treatments on the light output and the light collection uniformity have been studied during the test of 1993 with the objective of finding the wrapping material and method which maximize the light output and at the same time minimize the light collection non-uniformity along the axis of the calorimeter modules.
Figure: The set-up for light output and light collection uniformity tests. The wrapped CsI module was sandwiched between two MWPC's. A parallel beam of pions of 375 MeV/c traversed the first chamber, the CsI module and the second Chamber. The active areas of the chambers were not big enough to cover the entire body of the CsI crystal. As a result, for a given wrapping system, data were taken in a set of runs during which the CsI module was translated perpendicularly to the beam with the MWPC's held fixed.
During these studies one calorimeter module was used and its lateral surfaces were wrapped with different material such as teflon, aluminized mylar, etc. In addition, the front surface of the crystal was covered with black paint ( or less), black paper or simply left naked. Different layers of wrapping were also considered. For each wrapping system, the dressed crystal was taken into the beam line, flanked by two rectangular MWPC's as illustrated in figure . The CsI crystal and the MWPC's were arranged perpendicularly to the central trajectory of the beam. A parallel beam of pions of passed through the first MWPC, traversed the calorimeter module and went through the second MWPC. The two MWPC's were the same ones used earlier during the run for the beam studies (see figure 3.6). Each of the MWPC's had an active area of and consisted of sixty-four vertical and sixty-four horizontal wires.
Two quantities were extracted from the hits in the MWPC's and the CsI modules: the light output of the CsI due to a beam pion which traversed the module and the path length of that pion in the CsI module. The purely geometrical effects on the light collection non-uniformity as a result of the tapered pyramidal shapes of the module were eliminated by taking the ratio of the light output to the pathlength. The light output was given by the ADC read out while the pathlength was obtained as follows: The MWPC's provided two points and , indicating where the entered the two chambers. Therefore, these points gave the trajectory of the beam , a trajectory which necessarily passed through the CsI crystal since the module was in the trigger. The segment of this line within the CsI was the pathlength. Only the tracks with minimal divergence as a result of multiple scattering in the CsI were selected for the calculation of the pathlength which could be obtained with different methods.
Figure: The light output per unit pathlength along the axis of the crystal for different treatments of the front face. Among all the treatments considered, black paper gave the most uniform response and the best light output. For these treatments of the front face, the lateral surfaces were covered with three layers of teflon. The result of these tests was used in the subsequent measurements of the responses of an array of twelve modules to monoenergetic beams of 's.
One method used during the on-line data taking relied on the knowledge of the tapered angles of the surfaces of entrance and exit of the . During the course of the off-line analysis the pathlengths were calculated in a Fortran routine which defined the calorimeter modules as a volume bound by intersecting planes. The routine also positioned the module as it was in the beam and found the intersection points of the line defined by the hits and with the planes of entrance and exit. The distance between these intersection points was returned as the pathlength. The active length (12.8 cm) of the MWPC's could not cover the entire body of the module (22 cm). For this reason, for a given wrapping material and method, data were taken in a series of runs during which the CsI crystal was translated perpendicularly to the beam with respect to the fixed positions of the MWPC's.
Figure shows the light output per unit pathlength along the axis of the modules for various treatments of the front face: the lateral faces were wrapped in three layers of teflon. Better uniform response and light output were achieved with the front face covered with black paper. Consequently, for the study of the responses of an array of calorimeter modules to monoenergetic positron beams, each crystal was wrapped in three layers of teflon with black paper at the front face.
Further tests regarding the surface treatment of the crystals were carried out after the run of 1993. These tests favored a wrapping method consisting of one layer of aluminized mylar in addition to the three layers of teflon on the lateral surfaces and black paper of the front faces. Furthermore, a tomography
apparatus has been set-up to examine the optical properties of
each of the calorimeter modules with minimum ionizing cosmic muons. The
apparatus and some of its results are presented next.