Overview

Electromagnetic shower counters offer a useful means of particle identification (PID) [7]-[8]. Shower counters complement other means of PID such as time-of-flight (TOF) or threshold Cerenkov counters, due to the independent physical processes responsible which result in different detector limitations [9]. Independent PID allows multiple detectors ($i.e.$, a Cerenkov counter followed by a shower counter) to obtain excellent rejection ratios that are the product of the individual rejection ratios.

Shower counters measure the energy deposited by the incoming particle. The detected light output is linearly proportional to the energy lost by the incoming particle. Electromagnetic showers are stopped in the counters, whereas hadronic showers, due to the longer hadronic mean free path, are not. Looking at the longitudinal distribution of the energy deposited in the calorimeter differentiates between electromagnetic and hadronic showers and therefore identifies the incident particle.

Typical pion rejection with a lead glass counter is of the order of 100-1000:1 in the 1 to 10 GeV region [10]. The Hall A electromagnetic shower counter is meant to offer rejection ratios better than 100:1 [11]. The limitation in using a shower counter comes from separating the tails of the distributions, and is therefore dependent on energy resolution. At higher energy the relative resolution of a shower counter improves, leading to better separation between distributions. Conversely, other techniques perform worse at higher energy. The TOF separation for a given path length decreases, and above 4 GeV/c pions can trigger a threshold CO$_2$ Cerenkov counter operated at standard temperature and pressure (STP) [10]. [The threshold for a CO$_2$ Cerenkov counter at STP is $\gamma=34.1$, meaning that for an electron the threshold is just over 17 MeV, while for a pion the threshold is just over 4 GeV/c momentum].

A Cerenkov counter is routinely capable of pion rejection of the order of 1000:1 at CEBAF energies [10]. A combination of successive Cerenkov counters might achieve higher rejection ratios. However this only works if the backgrounds in the two devices are uncorrelated. A knock-on electron which triggers the first Cerenkov counter and travels forward through both detectors will also trigger the second. Independent PID provided by a measurement of the particle energy in a shower counter offers a solution to this problem of correlated backgrounds. Used in conjunction with a threshold Cerenkov counter, the combination can achieve rejection ratios of $5\times 10^5$ [11].

The Hall A electron spectrometer is equipped with a 2-layer, segmented shower counter. The first layer, the so-called ``pre-shower'' counter is made of 48 blocks of TF1 lead glass. Each block is nominally 10 cm by 10 cm by 35 cm long. The second layer, the so-called ``total absorber'' counter is nominally 15 cm by 15 cm by 35 cm viewed head-on by the beam.

Operation of the shower counter requires the application of High Voltage (HV) across the photomultiplier tubes and bases, which are mounted on the back of the shower counter blocks for the total absorber and on the sides of the shower counter in the case of the pre-shower, within the confines of the protective aluminum support frame.

As charged particles pass through the lead glass of the shower counter, they produce electron-positron particle-antiparticle pairs. These particles in turn both produce additional particles and Cerenkov light which is collected in the phototubes. The pulses are then amplified, delayed and sent to ADCs, which are gated by the overall event trigger. The ADCs are read out by the CODA acquisition software. The data are histogrammed online by the DHIST software. In-depth offline data analysis requires the ESPACE software.