Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Beam test data analysis of the ALICE FoCal-E pad prototype
M. H. Kim∗1 for the ALICE FoCal-E detector Collaboration∗1

RIKEN Nishina Center- 118 -〉Therefore, events where at least one cell has non-zero
TOT value in the eighth layer were selected to enhancethe electron event. Mean of TOT sum, ⟨TOT sum⟩,was calculated for all pad layers to estimate the detector performance from the longitudinal shower proﬁle.
Figure 1 shows the longitudinal shower proﬁles of theFoCal-E pad prototype with four diﬀerent beam energies. As the beam energy increases, not only does the⟨TOT sum⟩ increase but also the shower max layers
move backward.

〈The ALICE FoCal-E detector1) is an electromagnetic sampling calorimeter with a total depth of 20 radiation length (X0 ), which aims to measure the direct
photons produced by quark-gluon Compton scattering
to explore the small-x parton structure of nucleons andnuclei. It is composed of 18 low-granularity silicon padlayers, with a cell size of 1 × 1 cm2 for energy measurement, and 2 high-granularity silicon pixel layers, witha cell size of 30×30 µm2 for position measurement andphoton shower separation because the direct photons
should be distinguished from those decayed from π 0 .
All layers have the tungsten sheets with a thickness of
1 X0 followed by silicon layers.
FoCal-E has been developed since 2016 to be used forphysics data collection in 2029. Now, we are studying
the detector performance to write a technical design
report and optimize the ﬁnal design. A FoCal-E prototype with a transverse dimension of 9 × 8 cm2 wasprepared and tested using high-energy electron beams
from 20 to 350 GeV at CERN-SPS in September andNovember 2022. In this article, we report the status
and plan of the pad data analysis.
FoCal-E pad will use the HGCROC chip, which is
being developed by the CMS experiment for a highgranularity calorimeter.2) It provides ADC for signals
up to 100 Minimum Ionizing Particles (MIP), which isused for energy calibration of each pad and time-overthreshold (TOT), which is ﬁred from 100 MIP for large
signals to measure photon energy up to 2 TeV. TOT
is a technique that obtains the energy by measuring
the pulse width of the digital output. Although it allows us to measure large signals, nonlinearity between
the TOT and energy and the reliability of energy measurement in the intermediate dynamic range where it
is converted from ADC to TOT need to be carefully
studied.
A pad layer of the prototype consists of 72 cells.
When the TOT values of all cells in the eighth layer,
which is one of the shower maximum layers, weresummed up for a 150-GeV beam energy, two peaks
were observed. One was the case when all TOT values
were zero. The other was a Gaussian peak around anon-zero value.
In the electron beam, a ﬁnite fraction of the hadronbackground exists. We assumed the former comes fromthe hadron background and the latter comes from theelectron. This assumption is reasonable because themaximum energy deposit of a cell in the eighth layer
by the electron was predicted by simulation to be larger
than a level of 200 MIP, which is suﬃcient to ﬁre theTOT even though the electron hits the edge of a cell.

Fig. 1. Longitudinal shower proﬁles of the FoCal-E pad
prototype, which shows the ⟨TOT sum⟩ with four different beam energies over the layers. Layer 5 and 10 are
the pixel layers. Layer 18 is not shown due to a gain
problem.

The FoCal-E pad prototype showed a reasonable
performance even though the analysis cut condition
was simple. However, the estimated energy resolution
was 1.5 times worse than that expected by simulation.
We are now processing the quality assurance (QA) foreach run. After QA, more strict conditions will beapplied to separate the electron from the hadron background. Calibration parameters will also be provided
to combine the TOT and ADC and also to reduce theTOT nonlinearity. Taking these factors into account,precise detector performances will be studied.
References
1) ALICE Collaboration, CERN-LHCC-2020-009 (2020).
2) CMS Collaboration, CERN-LHCC-2017-023 (2017).