The Pion Beta Experiment: Calibrations and Developments

4.2 Apparatus

The detector (Fig. 4.1) will be placed in the p E1 area of PSI. The pion beam with a momentum of 116 MeV/c (see section 4.3) and an intensity of 10⁶ p /s passes the beam counter B1, enters the apparatus through a conical vacuum pipe, traverses the active degrader and stops inside the target. The length of the active degrader in beam direction is 4 cm. Its dimensions are chosen so that pions stop and decay in the central region of the active target. The size of the target perpendicular to the beam must be as small as possible in order to keep the conversion probability for photons (from p ^₀-decay) and the energy loss for positrons from p ⁺->e⁺ n _e low. The target has thus a cylindrical shape with a length of 5 cm in beam direction and a radius of 2 cm.

Figure 4.1: A cut through the pion beta detector. 1) Central beam trajectory; 2) vacuum pipe; 3) active degrader; 4) active target; 5) fiber optic light guides; 6) cylindrical MWPC's; 7) segmented plastic veto detector; 8) 1'' PMT's; 9) CsI-shower veto's; 10) CsI shower calorimeter; 11) 3'' PMT's As described in section 3.2, the muons from p ⁺->µ⁺ n _µ stop within the target too and decay mostly into Michel positrons, most of which escape the target. The positrons from p ⁺->e⁺ n _e are exiting the target, but loose some of their energy depending on the amount of target material traversed.

The active target is surrounded by two cylindrical multi-wire proportional chambers (MWPC's) for charged particle tracking. A cylindrical scintillator hodoscope (plastic veto detector) surrounds the MWPC's in order to provide timing information for events induced by charged particles and to distinguish charged from neutral particles. The amount of material in the MWPC's and the plastic veto detector must be as low as possible again in order to minimize the conversion probability for photons and the energy loss for positrons .

The two photons from pion beta decay leave the target and pass the MWPC's and the scintillator hodoscope without any energy loss. To detect them, a calorimeter made of pure Cesium Iodide (CsI), an inorganic scintillating crystal material with large stopping power and fast light output. The light output and the good energy resolution makes pure CsI suitable for the detection of not only photons but also positrons and electrons. The photons from pion beta decay of about 70 MeV initiate showers in the calorimeter via pair production, whereas the positrons from p ->e⁺ n _e initiate positron-photon showers via bremsstrahlung or suffer energy loss from ionization.

The detector and its supporting structure are surrounded by a thermal house to control temperature and humidity since the CsI crystals are slightly hygroscopic and their light output is temperature dependent. In order to avoid background from cosmic muons, the detector is enclosed by a "cosmic house". It consists of plastic scintillator plates which detect cosmic muons with high efficiency.

The scintillator plates are shielded from the inside with 5cm of lead to avoid self vetoing in the cosmic veto counters due to shower leakage through the back of the calorimeter and to shield the detector from beam related background.