CBM Progress Report 2006 - GSI

CBM Progress Report 2006 Detector Developments
High efficiency Transition Radiation Detectors for high counting rate
environments
M. Petrovici1 , M. Petris¸ 1 , I. Berceanu1 , V. Simion1 , D. Moisă1 , A. Radu1 , D. Bartos¸ 1 ,V.Cătănescu1 ,
A. Herghelegiu1 ,C.Măgureanu1 , M. Hoppe2 ,A.Wilk2 , J.P. Wessels2 , A. Andronic3 , C. Garabatos3 ,
R. Simon3 , F. Uhlig3 1 2 3 NIPNE-Bucharest, University of Münster, GSI-Darmstadt
One of the options currently considered for lepton identification
with the CBM experiment [1] for FAIR is a shell of
multiple layers of Transition Radiation Detectors (TRD).
Being a heavy-ion fixed target experiment in the energy
range of 5-35 A·GeV the goal of CBM is to look for rare
probes using the unique performance of FAIR in terms of
high intensity heavy ion beams. This requires a high granularity
TRD with good performance specifically engineered
for a high counting rate environment. In this context, the
optimization of the e/π rejection factor relative to the number
of layers and electronic read-out channels is a challenging
requirement. Various prototypes [2] based on Multiwire
Proportional Chambers (MWPC) were designed, built
and tested. Up to intensities of 100 kHz/cm 2 , no major
deterioration of their performance in terms of pulse height
and position resolution has been observed.
However, this performance was reached at the expense
of a low conversion efficiency for transition radiation in
such a single layer MWPC based TRD. In order to circumvent
this aspect, we designed and built several variants
of a new prototype of TRD based on a double sided pad
read-out electrode[3] with gas volumes on either side. Neglecting
about 15% absorption in the central electrode, the
double sided configuration is equivalent to a detector of 12
mm gas thickness, preserving the time response of the one
of half the thickness [4].
Figure 1: Pion efficiency at 90% electron efficiency at 1.5
GeV/c momentum as a function of number of layers
The prototype was tested in-beam at the SIS. The pion
rejection factor was extracted using the likelihood on in-
37
tegrated energy deposit [5]. The pion efficiency at 90%
electron efficiency at 1.5 GeV/c momentum as a function
of number of layers for a Rohacell radiator is depicted
in Fig. 1. The obtained pion efficiency for 6 layers is
3.32% for an anode voltage of 1800 V. For one run we
used a polypropylene foil stack (120 foils, 20 µm thickness,
500 µm spacing) as radiator. For an anode voltage of
1700 V the obtained pion efficiency is (1.1%). The pion efficiency
using Rohacell radiator at the same anode voltage
is 5.43%, i.e. the regular foil radiator improves the pion
rejection by almost a factor of five. If this improvement
factor is applied to the measurement obtained at 1800 V
anode voltage, a final pion efficiency of 0.67% is obtained
for a 6 layer stack with a regular foil radiator.
Fig. 2 shows the pulse height distributions of pions and
electrons, respectively for 26·10 3 , 65·10 3 , 110·10 3 and
220·10 3 particles/(s·cm 2 ) counting rate.
Figure 2: Pulse height distributions for a) protons and pions
and b) electrons as a function of rate
Within experimental errors we conclude that these detectors
preserve their pion efficiency performance up to counting
rates of ∼200·10 3 particles/(s·cm 2 ).
Given the performance in terms of counting rate and pion
efficiency this new principle of TRD commends itself as a
viable solution for high counting rate environments. For
a given pion efficiency and granularity it provides an optimum
material budget at a reduced number of readout channels.
References
[1] CBM - Technical Status Report, 2005
[2] A. Andronic et al, GSI Ann. Rep. 2005-1, INSTMETH-33
[3] M. Petrovici et al. GSI Sci.Rep. 2006-1, p.67
[4] M. Petris et al,CBM Coll. Meet., March 9-12, 2005, GSI
[5] A.Büngener et al., Nucl. Instr. Meth. 214, 1983, p.261