The version of the active target used during during the development run of 1993 consisted of 140 plastic scintillator fibers of cross section each and 7 cm long. The fibers were isolated from each other with an acrylic cladding and held together with epoxy. Only 35 of the fibers were coupled to 1-inch PMT's via fiber optic light guides. All the other fibers were viewed by a single 2-inch PMT. The size of the beam at the center of the active target was certainly bigger than the cross-sectional area of the thirty-five fibers. For this reason, the lateral stopping distributions were measured in a series of runs during which the target was translated perpendicularly to the beam by known amounts --- the translations were in steps of 10 mm to either beam up, down, right or left --- relative to central trajectory of the beam. The set-up of the apparatus was similar to that shown in figure 3.6 with these modifications: the two MWPC's and the scintillator counter S1 were removed. The active target was positioned such that its center coincided with the beam waist, i.e, the position of the first MWPC in figure 3.6. An active degrader (S1) of 3 cm thick was placed immediately in front of the target such the beam pions with 116 MeV/c central momentum, stopped within the central region of the target. The pions with a mean range of 6.64 cm in plastic scintillator, traversed a 4 mm carbon degrader ( mm of plastic scintillator) in the beam channel, the scintillator counter S0 (1.2 mm), the vacuum window ( mm), the active degrader (plastic scintillator 3 cm thick) and air ( mm). At their entrance to the active target, the residual energy and range of the pions were 24.7 MeV and 2.67 cm respectively. A cross-section of the thirty-five fibers is shown in figure . The trigger was (see figure 3.3) and the trigger rate was for a typical low intensity run. The average stopping rate in a fiber was . The equalization of the PMT gains was done on the oscilloscope by comparing the line (4.2 MeV) in each of the fibers. The light collection in the target was also determined in a similar fashion, i.e., from a comparison between the muon and the single photoelectron signals for each fiber. The number of photo-electrons per MeV deposited in a fiber varied between eight and fifteen.
Figure: A cross section of the thirty-five plastic scintillator fibers used to measure the lateral stopping distributions during the test of 1993. When the target was centered on the beam line, the central trajectory of the beam passed through the center of fiber F15. Twenty-five fibers are shown in red, one combination of nine fibers is in green, and a combination of four in blue.
For each fiber, a TDC, ADC and scaler information were read out. Knowing the coordinates , of the centers of the fibers and their energies , the horizontal and vertical stopping distributions were obtained as the weighted means:
where are the pedestals in the ADC's. Pedestal runs were recorded periodically during the data taking process. The drifts in the pedestals had FWHM's of ADC channels which translate to an uncertainty of 0.26 MeV in the determination of most probable energy deposition.
Figure: The top two figures shown the energy distributions of the stopped pions within the volume of twenty-five, nine and four fibers. The integrated contents of the histograms scale with the corresponding areas. All possible combinations of nine and four fibers were examined and consistent results were obtained. The bottom two figures are the horizontal and vertical stopping distributions measured with the active target. These results led to the redesign of the target.
The analysis showed that the number of stopped pions scaled with the area defined by twenty-five, nine and four fibers: groups of twenty-five, nine and four fibers were considered as shown in figure . There is only one combination of twenty-five fibers. All the possible combinations
of nine and four fibers were examined in detail. It was observed that the number of pions stopped within the volumes of twenty-five, nine and four fibers scaled with the corresponding cross sectional areas. However, a subgroup of less than four fibers did not exhibit this scaling feature which suggested that the beam pions stopped within the fiducial volume defined by four fibers. This enabled the redesign of the target with fibers instead of . As shown in figure , almost all of the beam pions stopped within a spot of diameter. This observation dictated the size of the new active target described in section 5.1. The on-line analysis showed that the cross talk between fibers was less than . The only other modification after the test of 1993 was that the active length of the fibers were reduced from 7 to 5 cm. The newly redesigned active target was fabricated at the University of Virginia (just like the previous one) and taken to the beam in 1994. Its performance is presented below.
Figure: The active target and its orientation during the test run of 1994. The plastic scintillator fibers, their fiber optic light guides and PMT's were enclosed in a box for light proofing. For the same reason, face of the target where the beam entered was protected with a very thin black paper enclosure. The central trajectory of the beam was about 150.5 cm above the floor. The target was therefore placed on an adjustable platform such that the central trajectory of the beam passed through the center of fiber F35. Fine adjustments were achieved with the aid of a theodolite.
The target used during the test of 1994 is the one described in section 5.1. Unlike the active target used during the test of 1993, all the seventy-seven elements of this target were each viewed by a one-inch PMT. Therefore the effective area of the target was big enough to accept the entire beam. As a result, there was no need to translate the active target with respect to the central trajectory of beam in order to measure the stopping distributions in a series of runs as was done in 1993. The beam line itself had been somewhat modified to accommodate the demands of the new development run: the beam waist had been changed from 60 cm as measured from the quadrupole triplet (see figure 3.6) to 120 cm to make enough space for the concrete shielding, the bigger CsI detector array, and other detectors to be later installed during the run. An active degrader (S1) was positioned in front of the target, and the pions of 116 MeV/c stopped in the central region of the active target which was set-up in the beam line such that its center coincided with the beam waist. The arrangement of the target with respect to the beam is shown in figure .
The data trigger was defined by the coincidence which defined the beam particles and was timed to select pions.
Figure: The results of the beam studies done during the test of 1994 with the active target. The top figures show the lateral beam distributions with a FWHM of . The bottom portion shows the time separation between the beam pions, muons and positrons.
The trigger rate was for an average proton current of . The average stopping rate in a central fiber was . The positron and the muon contaminations in the beam were calculated using energy and time of flight cuts similar to that of figure 3.10. The gains were matched with the monoenergetic muons (4.2 MeV) from the pion decays at rest in the fibers. In this case, pedestal runs were also taken periodically; pedestal drifts of six to eight ADC channels were observed. The beam distributions were calculated as the weighted means of the central fiber positions with the energy depositions in the fibers as displayed in equations 8.1-8.2. Figure summarizes the results of the beam studies. The beam contaminations (, ) are higher than the contaminations measured in 1993 (table 3.4) due to a more downstream focus. However, pulse height and time of flight cuts enabled the selection of pions with minimal contaminations from the undesired beam particles.
Figure: The stopped pion decays into muon (red-green region). The muon subsequently decays into a positron with enough energy to exit from the target. The direction of the escaping positron is more or less defined by the fibers which lie on its trajectory since the positron deposits energies in those fibers. The energy scale is a continuous variation from blue (lowest energy above the noise, 20 ADC channels) to red (highest energy, more than 600 ADC channels).
The lateral beam distributions compare reasonably well with the previous results of 1993.
The active target provides a medium to stop the pions and to monitor the beam distributions. The target can also handle high rates, a feature which is important to the pion beta experiment. The segmentation of the active target can also be used as a tracking device for charged particles. Indeed, the decay muons of 4.2 MeV stop within few millimeters of the stopped pions. Some Michel positrons from the muon decays and the positrons from have enough energy to escape the target. In the process, the escaping positron deposits energies in the fibers which lie on its path. Such an event is shown in