development of micro-pattern gaseous detectors – gem - LMU

Chapter 5
Efficiency Determination
This chapter addresses the efficiency of prototype 1.0. Since the trigger scintillators cover a larger
area than the active size of the detector one has to correct for a geometrical factor which is presented
in Ch. 5.1. The efficiency of prototype 1.0 equipped with an unsegmented anode is illustrated in Ch.
5.2. With the installation of a flow and pressure control unit in the gas system as well as optimized
operational voltage settings one obtains an optimized efficiency which is reported in Ch. 5.3.
The efficiency ε is defined as the ratio of recorded signals S over the sum of all triggering events
Stotal:
5.1 Efficiency Simulation
ε = S
Stotal
= Ndetected particles
Ntotal particles
The measurement of the efficiency is realized by two scalers (cf. Fig. 3.9). One of them counts the
triple coincidence between detector and the two trigger scintillators, the other counts the two trigger
scintillators alone. The threefold coincidence of scintillator triggers combined with the GEM’s signal
is the real number of detected particles that are recorded by the FADC. Coincidence of the GEM
surrounding scintillators gives the number of total particle that are crossing the active detection area.
Since the surface of the two scintillators is about 40 % bigger than the detectors active area and taking
their rectangular positioning into account, one has to correct the number of two-fold coincidences by
the geometrical factor. A MonteCarlo Simulation is implemented to derive the real count rate of
particle traversing the active area of the GEM detector or scintillators.
As illustrated in Fig. 5.1, the two scintillators are abstracted to rectangles, consisting of a lower and an
upper plane. The GEM detector itself is represented by a simple plane, centered to the surface covered
by both triggering detectors. For points placed in small equidistant steps on the lower and upper
surface of Sc1 a large number of muons is simulated with incident directions defined by the angles θ
and φ. It was assumed that the incident angles are distributed as f (θ) = cos 2 θ and normalized for
the φ - coordinate via g(φ) = 1/2π. The values of the direction vector are transformed to Cartesian
coordinates:
direc[0] = cos(φ) sin(θ)
direc[1] = sin(φ) sin(θ)
direc[2] = cos(θ) .
57
(5.1)
(5.2)