Kopytine's homepage

The Si pad detector

The Si pad array, installed 10cm downstream from the target inside the first dipole magnet, measures ionization energy loss of charged particles ($\,dE/\,dx$) in its 512 300 $\mu$m thick Si pads. The detector covers $1.5\le \eta \le 3.3$ and $2\pi$ azimuthally. Radially, the pads are arranged to be fragments of 16 rings with equal pseudorapidity coverage (see Fig.3.2 ). Azimuthally, they constitute 32 sectors with equal angular coverage. $\delta$-electrons, produced by the $Pb$ nuclei of the beam especially copiously in the target, are swept away to one side by the magnetic field of the first dipole ( $\approx1.6 T$ in the strong field setting). Because the energy spectrum of $\delta$-electrons is dominated by low energies, this sweeping effect of the field leaves the other half relatively clean and useful for multiplicity analysis.

Figure 3.2: NA44 multiplicity detector complex: a) the lead target, the Si pad array and the T0 scintillators; b) the setup exposed to a simulated RQMD Pb+Pb event.

The amplitudes are read out by AMPLEX  [30] and digitized by C-RAMS (later DRAMS [31], see Section 3.9). By occupancy $\mu$ of a Si pad we will mean the average number of tracks hitting the pad:

$$\mu = \sum_{i=0}^{\infty} w_i i,$$ (2)

where $w_i$ is probability of having $i$ hits in the pad. Typically, in the central trigger runs, the occupancy is somewhat above 1.

The charge collection time for a 300 $\mu m$ path length in the Si is typically 10-20 ns  [33]. AMPLEX has 600 to 800 ns peaking time [30]. Duration of an SPS spill was about 4.7 s, and NA44 was typically operating at (2-4) $\times 10^6$ beam particles per spill. The thickest Pb target used had 0.034 interaction probability for the Pb beam. Therefore, the typical rate of minimum bias interactions was always below 31 kHz, i.e. on average 32 $\mu s$ per interaction, and pile-up was highly unlikely.

The data from the detector was used in the texture analysis of the hadron production (using the data recorded in 1994), and in the analysis of meson production in the $Pb+Pb$ collisions to normalize particle yields (the 1995 data). In the latter case, the same detector was used in its second running period, so that radiation damage was noticeable. In both cases, more information on the detector and its characteristics will be given in the appropriate sections.