Dr. Peaslee, Dr. DeYoung, Dr. Mader (Nuclear Group)
Supported by the Michigan Space Grant Consortium
Last winter, we acquired a set of broken Multiwire Proportional Counters (MWPCs) from MSU. These detectors were made of six fiberglass frames. Between these frames were two gas windows, two cathode foils, and a set of anode wires. The cathode foils were located one frame above and below the anode wires. They were coated with a resistively divided strip of Nichrome (Ni/Cr 80%/20%) down the center. Next the foils were coated with 5 mm wide aluminum strips perpendicular to the Nichrome strip. By comparing the size of the signal coming from each side, it can be determined which aluminum strip the particle hit and with two planes, a position can be found.
This design had a few problems. The resistivity of the detectors increased unexpectedly and often just completely broke. It was determined that, under gas pressure variations in the MWPC, the Nichrome strip bowed and stretched and did not return to the original tension. Sometimes the pressure differences would cause the strip to break entirely from the frame. Also, because the detectors were so close in the 4 ball, they sometimes collided, breaking the fragile Nichrome attachment to the frame. In order to have working PPACs for our next experiment, we redesigned the detectors. The cathode foils were replaced with wire planes, making them more robust and less sensitive to minor pressure variations.
In order to get a position from these wire planes, we needed to resistively divide the wire. This was accomplished by placing nanite resistors between each set of wires. Finally we removed the front frame, making the detector smaller and therefore less likely to collide inside the ball.
We first tested these in a small chamber at Hope College. After making sure that they were vacuum tight, and repairing those that were not, we checked for signals. With the source that we had, it was difficult to distinguish the signals from the noise. We have taken the new PPACs to MSU for testing with a particle beam.
In addition to creating a new set of detectors, some software was generated to analyze the data from the upcoming experiment. Using the data analysis program LISA data taken in the summer of 1995 at the MSU-NSCL was analyzed further. Some additions were made to the program to identify alpha particles that were emitted from only central collisions and did not pass through the aluminum frame supporting the detectors, but instead passed through a set of Si detectors. The energy was then calibrated for each detector and an energy spectra for each detector was obtained. Then further restrictions were placed on the alpha particles, requiring them to be emitted with a pair of fission fragments. This calculation was then used to obtain energy spectra of alpha particles at varying angles from the fission fragment plane.
Both the detectors and the data analysis methods will be used in an experiment at MSU-NSCL this winter. The experiment will attempt to discover more about pre-equilibrium particle emission. With the new detectors that we have built, we can better identify fission fragments, flight time of many particles, and obtain improved position measurements that will help us identify angles more precisely. It will help us gain understanding into the pre-equilibrium particle emission that occurs in nuclear collisions.