Experiment Proposal - opera - Infn

N−1
∑
χ 2 =
i=1
[(∆θ i − θ MCS
i,i+1 )/θMCS i,i+1 ]2
N(N − 1)
where N is the number of measured segments, ∆θ i is the spatial angular difference between two
segments and θi,i+1
MCS is the angular variation due to multiple scattering after 1 mm lead for the energy
E i (see Fig. 122).
6.7.2 Shower analysis
Counting track segments associated to the shower is also a way of identifying electrons. This analysis
has to be confined in a relatively small emulsion region, in order to supress background track segments
which are not related to the shower. Examples of shower induced by 2 and 8 GeV electrons detected in
a test experiment are shown in Fig. 121. The lateral spread of the shower is of the order of 1 mm. The
brick thickness is 10 X 0 and thus a shower is well confined within a single brick. The S-UTS is able to
readout most of the electrons related to the shower and the Net Scan analysis to reconstruct the shower
structure.
8GeV/c
2GeV/c
Figure 121: Electromagnetic showers observed in a brick exposed to an electron beam.
Particle tracks recorded while the emulsion films are not assembled in the ECC brick, i.e. before
packing and after disassembling, become track segments not related to the electromagnetic shower. Cosmic
rays and muons in the ν beam do not constitute backgrounds, since they can be distinguished as
penetrating tracks. In order to limit the density of these background track segments below 1/mm 2 ,
controlled fading of the emulsion films is applied prior to the brick installation. In addition, the time
between brick unpacking and film development is kept short, as described in Section 6.2.2.
158