The Pion Beta Decay Experiment and A Remeasurement of the Panofsky Ratio

6.2.1 The Calorimeters

6.2.1.1 The CsI Array

The CsI-array used in the measurement was a subset of the final PiBeta detector holding 44 CsI crystals, which defined a fifth of the whole CsI calorimeter. It covered 72° in polar ( q ) and 144° in azimuthal ( f ) angle. The goal of the measurement was to get a reliable test of the performance of the sphere. The mounting procedure used was the same as planned for the final assembly. The alignment of the crystals was critical, because gaps between crystal sides would decrease the energy resolution and increase the error in the determination of the detector acceptance. The steel cone piece (see sect. 2.3.7 and Figure 6-2) defined the alignment in both q and f direction. For the radial alignment a ball with a rod was used defining the centre of the calorimeter and the 260 mm distance of the crystal's surface to the centre (see Figure 4-2). The choice for the 44 out of 240 crystals was arbitrary in order to allow a realistic prediction of the overall energy resolution for both 68 MeV positrons and 69 MeV photons.

Figure 6-2 Array of 40 CsI crystals on the 'swing' forming a fifth of a sphere. 44 crystals were used in the 1997 beamtime period.

The CsI array with the swing was surrounded by a thermal house made of 4 cm thick styrofoam. In order to limit pion beam absorption the front shielding only consisted of two cardboard sheets of 300 µm thickness each.

In order to probe several parts of the CsI array it was mounted on a swing that was mobile in theta- and phi-direction. The pivot coincided with the centre of the sphere and with the reaction centre. The array had to be moved out of the central point for measurements involving the LH2-target to allow the mechanical support structure to fit in.

6.2.1.2 The NaI-Wall

The photon-tagging detector consisted of 64 rectangular (406x63x63 mm³) NaI Polyscin^© modules [Bay88]. It was designed for high efficiency detection of intermediate energy photons. The 406 mm of NaI represents 15.7 radiation lengths. The NaI-modules are assembled to form an 8x8 array that is encased in an air-tight container. Each module is optically isolated against the others with one layer of reflecting material surrounded by aluminized mylar foil. They are read out by Philips PM2202 photomultipliers that are coupled through 60 mm long light guides. Each side is enclosed in 19 mm of aluminium except for the front face. In order to limit absorption the front face is made of a 0.5 mm steel sheet that is glued on 20 mm of styrofoam for insulation. Bay et al. could achieve a resolution of 7% FWHM at 70 MeV.

In order to veto against lateral shower losses, the NaI-wall was electronically subdivided in two parts. An array of the central 6x6 crystals formed NaI_inner and a ring of the 28 outermost modules formed NaI_outer.

6.2.1.3 Trigger

A coincidence of the signals of the beam counters B0 and B1 was used to discriminate against beam electrons, since pions and electrons are well separated by a time-of-flight difference of 6 ns. The B0*B1 coincidence then was fed into the MTU.

The CsI calorimeter trigger scheme follows the description of superclusters in chapter 2. Groups of 6 to 9 crystals were generated and the correspondent analog signals from the PMT voltage divider were added using the UVA 125 summing modules. The 10 resulting clusters built the supercluster logic, which also was fed into the MTU. The so-called `high' threshold was set at about 2 MeV.

In order to build the NaI trigger, the linear sum for the NaI_inner and NaI_outer-branch was generated separately with the UVA 125 summing modules. A valid event required a signal from the NaI_inner-Sum which was vetoed if NaI_outer-Sum exceeded the chosen threshold. The trigger during the runs with the LH2 target was threefold to allow two tasks at the same time. A coincidence between B0,B1, NaI- and CsI-calorimeter (see Figure 6-3) was built to detect simultaneously the two photons from the p ⁰-decay in both calorimeters. In addition, the so-called `single-arm' trigger was generated. It required the presence of one photon in either detector and thus registered the two competitive p N reactions. This trigger mode was prescaled to allow a high counting rate for the coincidence mode. The main trigger consisted of a coincidence between the two beam counters and the two arms of the detector. In order to avoid RC events in the coincidence mode, the thresholds for both detectors were set well above 10 MeV. The coincidence between both detectors also was used for calibration, since the p ⁰-energy adds up to 137.86 MeV.

Figure 6-3 Trigger logic for the measurement of the Panofsky ratio (CsI high defines the timing)

The selection of the trigger of interest for data analysis was available through the output pattern of the MTU which was stored along with the energy and timing information. Furthermore the temperature was monitored continuously at six positions within the CsI thermal house.