CBM Progress Report 2006 - GSI

Detector Developments CBM Progress Report 2006
Simulation studies of calorimeter system. Preshower prototype. ∗
S. Belogurov 1 , A. Golutvin 1 , Y. Kharlov 2 , S. Kiselev 1 , D. Konstantinov 2 , I. Korolko 1 , V. Maiatski 1 ,
K. Mikhailov 1 , P. Polozov 1 , M. Prokudin 1 , G. Sharkov 1 , and A. Stavinskiy 1
Calorimeter software package in CBMROOT
A detailed simulation of a sampling calorimeter was developed
within the standard CBM framework CBMROOT.
To perform a detailed optimization of the calorimeter granularity
the whole detector volume was divided in small 1×1
cm 2 stacks. This procedure allowed us to built a calorimeter
with arbitrary transverse granularity at the hit producing
stage with a single set of simulated with GEANT (FLUKA)
events.
Standard algorithms for photon (electron) reconstruction
were developed, tuned and tested. First of all we build simple
3 × 3 clusters around all hot calorimeter cells. On a
next step we sum the energy depositions in the four hottest
cluster cells applying a special energy and polar angle dependent
calibration procedure to determine the momentum
of incident particles. Finally we reconstruct the centre
of gravity using the tabulated dependence of cluster
asymmetry (in X and Y ) as a function of MCtruth particle
impact point. Energy asymmetry was determined as
AX = (EL−ER)/ET ot and AY = (EU −ED)/ET ot. Algorithms
for energy and impact point reconstruction were
tuned for all possible calorimeter cell sizes.
Single-particle response
Using the software package described above we have
studied the calorimeter response for single photons. The
ECAL geometry from the CBM Technical Status Report
was taken as a basic option for these simulations: sampling
of the modules was 140 layers of 1 mm lead and 1 mm scintillator,
the cell sizes were 3×3 cm 2 , 6×6 cm 2 , 12×12 cm 2 .
The stochastic term of the energy resolution was found to
be 69 ± 2 MeV.
Figure 1: Spatial resolution in the CBMROOT simulation.
∗ Work supported by Federal agency of Russia for atomic energy, Federal
agency of Russia for science and innovations and the Russian Foundation
for Basic Research (grants 05-02-08096 and 06-08-01555) and by
INTAS-05-111-4475, INTAS-05-111-5257, INTAS-03-54-6272.
1 ITEP Moscow; 2 IHEP Protvino
The precision of impact point reconstruction was determined
with the algorithm described in previous section. As
a crosscheck we have developed a second algorithm which
reconstructs the photon impact point coordinate through
the first moment xrec = xiwi/ wi where the sum is
calculated over all cluster cells, xi is the coordinate of the
cell i, and wi is the weight depending on the cell energy
Ei and defined as wi = max [0, w0 + log(Ei/Etot)]. The
value of w0 is obtained empirically to minimize the spatial
resolution. Spatial resolution for different incident angles
of incoming photons calculated for outer (12×12 cm 2 , type
1) and inner (3 × 3 cm 2 , type 4) cells is shown in Fig.1.
For inclined tracks the reconstructed position obtained
with this procedure is systematically shifted. This shift,
expressed as a difference between reconstructed coordinate
xrec and MCtruth coordinate, is proportional to the sinus of
the incidence angle θ: xrec − x0 = teff sin θ. The slope teff
is energy-dependent and can be interpreted as an effective
shower depth in the calorimeter.
Energy reconstruction for different cells
The intrinsic energy resolution of the sampling calorimeter
at CBM is considerably degraded by energy contributions
from neighbor tracks. This effect depends obviously
on calorimeter cell size. We performed a detailed study of
photon reconstruction quality for all possible granularities
of CBM calorimeter system.
1400
1200
1000
800
600
400
200
χ 2 / ndf 67.01 / 46
Constant 1488 ± 8.6
Mean 1.002 ± 0.001
Sigma 0.0762 ± 0.0006
0
0.6 0.8 1 1.2 1.4 1.6 1.8 2
Ereco/EMC
Figure 2: Reconstruction quality for 12 × 12 cm 2 cells.
Using the standard procedure we have reconstructed
the energy of incoming photons and plotted the ratio
Ereco/EMC, which in ideal case should be a gaussian distribution
with mean value 1.0. Energy contributions from
neighbors result in a considerable right tail. The quality
of γ reconstruction was defined as percentage of photons
with Ereco/EMC ratio lying within ±2σ around unity.
Fig.2 demonstrates the reconstruction quality for low energy
(1 − 2 GeV/c 2 ) photons hitting the outer region of
46