In the fall of 1989, the pion beta collaboration undertook a development run during which a parasitic beam was stopped in an active plastic scintillator target in the area at the Paul Scherrer Institute. The active target was viewed by two sets of detectors: on one side, an array of seven large CsI modules and a NaI detector on the other side. There were thin plastic veto counters in front of the two sets of detectors which subtended matching solid angles at . The run took place under severe time constraint due to the pending shutdown of the PSI Ring accelerator and the late delivery of the CsI modules from Bicron Corporation. These constraints limited the running time and consequently the counting statistics as can be seen in figure which summarizes the results:
Figure: The results of the development run in the fall of 1989. A parasitic beam of was stopped in an active target viewed on the sides by an array of CsI and a NaI detectors. The top figure is the relative timing between a CsI block and the target. The central figure is the total energy (CsI plus NaI) detected within a gate, 80 ns long and opened 10 ns after the stop. The bottom figure is the central one cut on neutrals in the plastic veto detectors in front of the CsI and NaI crystals. A peak emerges but the statistics is low due to time constraints during the data taking.
Figure (a) shows the relative timing between a CsI block and the active target. The measured timing resolution was FWHM. The CsI modules were readout with EMI 9821QB photomultiplier tubes with quartz window and a nominal rise time of . In a previous test conducted at the University of Virginia with a source, the same photomultiplier tubes coupled to good pure CsI crystals of similar size yielded a time resolution of between two detectors. This meets the energy resolution requirement of the experiment.
Figure (b) is the spectrum of the total energy of the NaI and CsI detectors within a delayed pion gate of , after a valid pion stop. This spectrum includes both charged and neutral showers as no cuts were applied on the plastic veto detectors. The portion of the spectrum below channel 310 was prescaled by a factor of about 100. The pion stopping rate was . The spectrum is dominated by accidental Michel events in the NaI and CsI detectors. The conversion factor is about three channels per MeV.
Figure (c) is the same as figure (b) with the selection of only neutral showers (the plastic veto counters do not fire in either arm). With these requirements, a well defined pion beta peak emerges at the correct energy and the number of counts is consistent with the correct branching ratio of . However, the counting statistics is poor --- it reflects only about four shifts of running time --- because of the time constraints mentioned above. The following conclusions can be drawn from the above results of the development run:
Table: Some properties of pure CsI.
Compared to some other inorganic crystals, for instance , pure CsI possesses the following advantages which dictate its choice as the calorimeter material: its has a shorter radiation length which means that the volume of a calorimeter like that of the pion beta decay experiment is less than the volume of the same calorimeter in . Its has a lower density which suggests a lower weight for the calorimeter with pure CsI. Its higher refractive index indicates that it would be easier to achieve better light collection uniformity for tapered pure CsI crystals like the pion beta modules. Finally, pure CsI has a long nuclear interaction length and consequently, there would be fewer hadronic interactions in CsI, especially in the pion beta modules.
Some other properties of undoped CsI include radiation hardness and temperature dependence of the light output. Wei and Zhu observed a continuous decrease in the light yield of undoped CsI after 1 kRads [Wei-92]. According to their findings, pure CsI can sustain high counting rates up to 10 kRads. Woody et al. reported an increase in the light yield and the decay time of the fast component of pure CsI, and a shift to longer wavelengths at low temperatures Woo-90. A tomography system has been designed and is in operation at the Paul Scherrer Institute with the objective to examine the optical properties of each of the CsI modules prior to the assembly of the calorimeter.