A Classic Thesis Style - Johannes Gutenberg-Universität Mainz

96 prototyping and efficiency measurements
Mean number of fired pixels
20
18
16
14
12
10
8
6
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2
V -V
bias
(V)
break
Figure 61: Measurement of the number of fired pixels as a function of overvoltage,
Vbias − Vbreak, for excitation at the central fiber position.
One of the devices showed saturation at around 3 V.
is a large difference between the two attenuation coefficients, Λ1 =
24 cm and Λ2 = 402 cm, as a very strong light reduction is observed
for short distances. Manufacturers normally only quote the larger
coefficient obtained from long distance measurements. It is important
to note that much more light is available in the near proximity to the
detector.
The particle detection efficiency was defined as the ratio of the
number of left-and-right coincidence signals to trigger signals. The
efficiency was studied as a function of the threshold level in units
of single pixel signals for different bias voltages. The dependence of
the efficiency on the operating parameters was most directly seen
in the variations of the number of fired SiPM pixels, a measure of
the detected light yield. Fig. 61 shows a measurement of the light
yield as a function of the overvoltage when the fiber was excited in
the central position. The largest operating voltages correspond to a
maximum PDE. The accidental coincidence rate, Racc, was measured
at a threshold of 1.5 pixels as function of the overvoltage, see Fig. 62.
Despite the exponential increase of the accidental coincidence rate
with increasing bias voltage a change of only one unit in the threshold
level results in a drastic reduction of the rate. A compilation of the
results is shown in Table 9.
Given the above results, the optimum operation parameters depend
on the characteristics of the system, namely the true coincidence rate,
Rtrue, the application of an external trigger or not, and in case any
accidental signal generates a trigger also the inherent dead-time of the
data acquisition system.
A fiber detector system that is externally triggered, like in a laboratory
set-up as described, the ratio of detected true coincidences to the
sum of true and accidental coincidences, R = εRtrue/(Rtrue + Racc),
is a figure-of-merit, FOM, generally adequate for many practical problems.
If the detector itself triggers the data acquisition, and the dead-