1. Introduction to the pb Experiment - 6.2 Experimental Apparatus

6.2 Experimental Apparatus

The beam line was set to negative pions with a momentum of 116 MeV/c. The pions passed a degrader consisting of a thick scintillator and aluminum plates, the thickness of which was optimized for maximum p ^- stop rate in the liquid hydrogen (LH₂). The LH₂ was contained in a cylindrical cell of stainless steel with a length of 10cm in beam direction and a width of 1.6cm. The LH₂ target cell was operated at a pressure of 1060mbar and a temperature of 20K.

Incoming pions were detected by the plastic scintillators B1 and the counter B2 (thickness 5cm) located right in front of the Al degrader. The counter B3, placed behind the target cell, served as a veto-counter.

Figure 6.3: Box with the 26 CsI-crystal array, which was assembled in 1994. The 26 CsI-crystals in the crystal box are assembled as shown in Fig 6.3. The front faces of the CsI-crystals in the CsI-box were at a distance of 26cm from the center of the LH₂-target. A special air-conditioning system was connected to the CsI-box to control the temperature and the humidity; it consisted of a modified air-conditioner and a heat exchanger box, with a long term temperature stability of 15±2°C and a relative humidity of less than 40% in the box. Seven temperature sensors and one humidity sensor were installed inside and outside the enclosure and were read out periodically.

The experimental arrangement (Fig. 6.4) of the detectors fulfilled the following requirements:

1. The photon energies in the CsI-array should be 70 MeV, which sets the following constraints on the setup: between the center of the neutron counter array and the tag counter array the angle is about 80°, whereas the angle between the tag counters and the center of the CsI-box is about 160°.

2. The cylinder axis of the target cell should coincide with the neutron flight direction.

3. None of the arrays should stand in the elongation of the pion beam axis.

Figure 6.4: Experimental arrangement for the energy calibration of the CsI-crystals with 70 MeV photons. The distance of the 30 neutron counters and the 12 tag counters from the center of the target is about 80 cm. The front faces of the CsI-crystals in the CsI-box have a distance of 28 cm from the center of the LH₂-target. The above conditions could not be fulfilled completely in the sense that only a portion of the CsI-array has been illuminated by photons from "good events", mainly the cluster of crystals around C15 (C7, C9,C14-C16,C22, see Fig 5.11). The energies of photons from these events are between 68 and 75 MeV (see Fig. 6.2).

The array of 30 neutron counters and the tag detectors, 12 pure CsI-crystals of type HEXD1 and HEXD2, were located at a distance of 80 cm from the target. The total uncertainty from the positions of the detectors for q _1n is about 2° = 0.035 rad. The corresponding energy uncertainty [Delta]E from equation (6.8) is .

The horizontal-plane opening angle between one neutron and one tag counter is about ±5°. This results in a broadening of the distribution of E_{g 2}, originating from the finite size of the neutron and the tag counter being hit. The fit of the energy spectrum with a gaussian leads to a FWHM of 0.7 MeV.

Each of the 30 neutron counters is a cylindrical PILOT U organic scintillator, 15 mm thick, with a diameter of 45 mm, attached to a XP2020 photomultiplier. The neutrons from the charge exchange reaction (6.1) have a kinetic energy of T_n~0.4 MeV; the dominant process is elastic scattering on the nuclei of the target material. In the scintillator material neutrons are scattered off protons, which loose their energy through ionization and excitation.

The scintillator was preceded by a 9 mm thick Pb plate, which served to convert photons. For photon energies above ca. 20 MeV, the efficiency of the photon detection is improved by a factor of ca. 3-4 to about 60%. For less energetic photons (1-20 MeV), the detection probability for the counter arrangement is ca. 10-20%.

The signal from a 0.4 MeV neutron produced 1 to 8 photo-electrons in the cathode of a photomultiplier. Each channel was transmitted trough a LeCroy 612 PMT Amplifier and amplified by a factor of 10. The high voltages of the PMT's were matched such as to get a one electron signal of 30 mV at the input of the PS 7106 CAMAC discriminator; the neutron pulses extended up to about 250 mV.

The signals from the tag counters were set up similarly to those of the CsI-crystals (Fig. 4.9). After a signal splitter the analog signals are fed into the UVA 125 summing discriminator, the output of which goes into the event coincidence. The requirements for a valid trigger were:

1. A valid pion stop signal in the LH2 target, ().

2. An event in the CsI-array.

3. An event in one of the 12 tag counters.

The stopping rate in the LH₂ target, defined by the coincidence was 1.3·10⁵/s at 0.9 mA of proton current, which lead to a trigger rate of about 200/s.

In order to determine the background, data were taken with empty target and with the described trigger configuration. The resulting event rate was only 1/30 s^-1, more than a factor of 5000 lower than with the filled target, thus showing that the trigger was very restrictive.