1.(a) Detector buildup

1996 marked the beginning of the buildup of the final PIBETA detector assembly and its partial testing in beam. For the first time we assembled the final platform, a 44-crystal array inside a temperature controlled enclosure and ran it in beam with cabling, electronics, temperature control systems etc. all mounted on the platform in the beam cave. Apart from obvious differences stemming from the use of 44 instead of the full 240 crystals, the hardware configuration is identical to the final one. The setup is autonomous, easily transportable by crane to a parking position, and requires only a quick power and ethernet hookup to be used any time, either with cosmic muons or in beam.

The partial PIBETA setup was used in the PIE1 beam line in a short test run in July/August 1996. The results of this run are briefly summarized below.

1.(b) Crystal deliveries and running plans

The main hardware issue in the PIBETA project has been the delivery of pure CsI calorimeter detector modules. During 1996 we experienced additional delays that led to a renegotiation of the terms of the contract between PSI and AMCRYS-H of Harkov, Ukraine in July 1996. We simultaneously contacted western suppliers of CsI in the first half of 1996 in order to secure an alternate source. None of the western manufacturers (Horiba, Bicron, Crismatec) turned out to be able to provide us with a timely supply of pure CsI modules, regardless of cost.

After the renewed contract with AMCRYS-H took effect, crystal deliveries resumed but remained slow. In October '96 we examined the delivery dynamics and found that the rate was 1.5 crystals/week. At that rate the detector would be completed sometime in October 1998. This was judged an unacceptably long delay in light of the present graduate student commitments and of the need to keep the collaboration active.

We, therefore, immediately drew up plans for a partial detector assembly consisting of 124 modules arranged in a wide "polar" great circle ("meridian") yielding ~52% of the acceptance of the full detector. We determined that the required modules for such a detector would be available by the early summer of 1997, and the detector could be built up by the end of 1997, enabling us to take actual data runs in 1998. For more details and a schematic drawing of the partial calorimeter layout see the entry A Partial PIBETA Detector.

The main additional cost associated with an early implementation of a partial PIBETA calorimeter is in personnel effort involved in the additional buildup and disassembly of the setup. The assembly is estimated to take ~3 months with ~4 persons involved at any time. Physical restrictions of the spherical housing ("Kugelgehaeuse") do not permit more than a few people working at the same time. The costs of machining the blanks would not be prohibitive, about 10 kSF or less, because the PSI shop is already set up to make them using computer controlled machines.

However, recent favorable developments may lead us to modify our plans somewhat. Most recent deliveries have proceeded at the rate of ~2 crys/wk, and with much improved quality, both in terms of mechanical tolerances and fast/total light component ratio. If this trend should hold, the remaining detectors would be delivered by the end of April 1998. We clearly need to monitor these developments continuously and reevaluate our plans by late spring.

1.(c) Other hardware developments

We have continued our development work on the surface treatment of CsI crystals in order to optimize the light output uniformity as well as the physical and chemical surface stability. This has recently resulted in our adoption of a lacquer coating as standard surface treatment for our CsI modules. This work has involved significantly the Tbilisi group.

Consequently, we need to treat all 104 crystals presently in our possession at PSI, as well as the remaining 136 pieces not yet delivered. Our standard procedure after coating and wrapping requires a measurement of the crystal optical properties. (For more details see the entries Quality control of the CsI Crystals and Three Dimensional Tomography of Pure CsI Calorimeter Crystals.

Our cosmic muon tomography facility is not suitable to handle this volume of work in a short period of time. Therefore we have developed a faster device based on a Cs-137 gamma source described in RASTA: Radioactive Source Tomography Apparatus. Allowing about 1 hour for PMT gain stabilization, this device can handle ~10 detectors/day, considerably speeding up the calibration process.

One of the crucial devices for the stability of our experiment is the detector temperature control circuit. This circuit was used in the summer '96 run and resulted in a CsI detector operating temperature range well within 0.5 deg. Celsius. There is no doubt that played an important role in the measured gain stability (see below).

Other hardware systems such as the MWPC's, plastic veto (PV) counters, etc., have also been worked on and brought further along in 1996. The PV counters have been tomographed in order to extract information about their attenuation, timing, and light output uniformity. It is worth noting that the PV's were fully functional as a part of our 1/4 sphere detector assembly in July/August '96.


A great deal of work has gone into making our GEANT Monte Carlo code more realistic during 1996. The geometry definition section was rewritten in order to accommodate the inclusion of intercrystal gaps (cracks) of arbitrary size. Additional work was done on the event generators. Provisions were made in the code to enter each crystal's actual measured luminosity (no. of photoelectrons per MeV) and light nonuniformity parameters from a database instead of a single set of average parameters. Simulation output has been brought further into line with the format of the online data so that the Monte Carlo and actual event files can be treated with the same procedures. Additional technical improvements were implemented that accelerate the calculation and facilitate the handling of large data samples.

We are currently in the process of refining all of our previous calculations with the new version of our code, as well as studying the previously inaccessible effects such as that of cracks between the calorimeter modules. Since this work is in progress, we are not attaching details of our results to this document, but will prepare an update as we complete the current cycle of calculations.

Much effort has gone into the understanding of our cosmic muon detector tomography results. We are now able to compare in detail the simulated response of each individual detector with that of an ideal one based on our tomography results, as well as to compare it with the actual response measured in beam. This work is in its last stages and we expect to have final results soon. We intend to publish them in a separate NIM article. For a current condensed account of the results of this work see the entry Calorimeter lineshapes predicted using tomography results.


The new dedicated PIBETA slow control system was tested in realistic conditions during the 1996 summer run, and it passed the test with flying colors. This system is not only necessary for the control and running of such things as the high voltage power supplies, but it also performs temperature and gain drift monitoring.

A thorough test of the gain stability of the entire detector + data acquisition hardware system was performed during one week of running in August 1996. The trigger was a delayed pion stop gate open primarily to Michel decays. For each detector an analysis routine compared dynamically the measured Michel spectra in the edge region to a reference spectrum. In all but one channels of the 1/4 sphere detector gain drifts remained within +/- 1% during the one week. This high level of stability is a consequence of the careful design of the PMT bases, the temperature control system and readout electronics. For more details of the method and results see the entry Dynamic gain calibration.

The detailed analysis of our 1996 summer run data is still under way. The reason for the slower, more deliberate pace of analysis is that we are developing and optimizing analysis tools (algorithms) to be implemented in the emerging "final" version of the on-line and off-line data analysis code. The work is being carried out at PSI, ASU and UVa simultaneously, with duplication of key points in the analysis. Links to results of analysis in progress can be found on the PIBETA WWW site(s).

An important early result that may eventually get improved is the measured FWHM of ~4.2 MeV for the 70 MeV incident positron beam, in good agreement with simulations based on our cosmic muon tomography results. Another key result was the finding that there was no significant tail in the beam momentum distribution for the positron calibration beams. This will allow us to conduct the final acceptance calibration measurements in the summer of 1997 without the additional dipole magnet in the beam line which adversely affects the beam optics for stopped pions. The stopped pion beam is required for fast gain matching of the calorimeter modules.

A new flexible high-speed data acquisition system MIDAS is being developed for use in the PIBETA experiment, with some help from interested researchers at TRIUMF. Key elements of the package have already been tested. We intend to implement MIDAS in early 1997 and use it in beam for the first time in the forthcoming summer run. See the MIDAS homepage.


We have not had large changes in the collaboration personnel in 1996. Early in the year we were joined by a new PSI research associate, Bernward Krause, who is committed full-time to PIBETA. Bernie is an extremely valuable addition to the collaboration who has already made significant contributions.

During the last two years, and especially in 1996, Emil Frlez of UVa has assumed a role of increasing responsibility and leadership in the PIBETA project with great effectiveness. As we are entering a demanding and sensitive phase of the calibration and buildup of the detector and startup of measurements, this is a particularly welcome development. In recognition of his role in the experiment, I have asked Emil to serve as a co-spokesman, which he has accepted.

3 January 1997, D. Pocanic, for the PIBETA collaboration