Detector description

The detector exposed to the beam was composed by of a TRD and a calorimeter (Figure 1

The TRD consisted of 24 layers of $5~ cm$ thick polyetylene foam radiators followed by $3 \times 3~cm^2$ cross section and $1~ m$ long proportional tubes. Upstream each of the first 16 radiator planes a $2~ cm$ thick marble target was positioned. The whole TRD module was $1~ m$ wide, $0.5~ m$ high and $2.5~ m$ long, which corresponds to a total length of about $0.76\, \lambda_{I}$ and $3.7 \, X_0$. In order to allow measurements in the calorimetric module with a reduced amount of material on the beam line, all the TRD targets could easily be displaced.

The calorimeter (Figure 2 is made by a core of 12 $x$ planes interleaved with 12 $y$ planes surrounded by an external shell. Only $x$ planes were instrumented with fibers, PMTs and electronics. An active plane consists of 3 bars, made of iron boxes ( $\sim \times 14 \times 140$ cm$^3$), filled with iron ore and scintillating fibers (Figure 3 The calorimetric bar is logically divided in 3 calorimetric cells ( $\sim 5 \times 4.4$ cm$^2$ cross section). The estimated cell thickness is 0.14 $\lambda_{I}$ and $1.14~ X_{0}$. Every cell hosts 33 scintillating fibers ( 2 mm diameter Pol.Hi.Tech POLIFI 0244-3-200), each protected by a 1 mm thick plastic sheath. A Hamamatsu R4125 PMT (3/4 inch) working at a nominal gain G=$10^{7}$ is used to read-out the cell. For each bar a green LED, suitably coupled to 3 optical fibers, allows to send light pulses to the 3 PMTs, thus providing a tool for calibration and monitoring.

The external shell is made of 21 bars 13 cm $\times$ 13 cm $\times$ 140 cm, hosting 330 fibers each and readout by Hamamatsu R5686 PMTs (2 inches).

The overall dimensions of the calorimeter prototype are $(72.5 \times 140 \times 133)\ cm^3$, corresponding to a weight of about 4 tons. The average interaction length, radiation length and density are $\lambda_I \simeq 38$ cm, $X_{o} \simeq 5$ cm and $\rho \simeq 3.5$ g/cm$^{3}$ respectively. The total calorimeter thickness in the beam direction is 3.9 $\lambda_I$ and 32.6 $X_{o}$.

The main features of the iron ore absorber and the characteristics of the prototype are summarized in Table 1, and Table 2 respectively.

The calorimeter was closed by a 10 m$^{2}$ streamer tube carpet, acting as a tail catcher, to tag contained hadronic showers.