A Classic Thesis Style - Johannes Gutenberg-Universität Mainz

98 prototyping and efficiency measurements
detector in a spectrometer much higher particle rates of the order
of MHz with much smaller dead-times of the front-end electronics
and data acquisition system should be considered. In general it is
recommended to operate at high overvoltages for the highest efficiency
and to reduce the accidental coincidence rate by an increase in the
threshold level to acceptable values.
5.4 detection efficiencies for highly relativistic electrons
The obtained results from the efficiency measurement with a 90 Sr
β-source have to be converted to an expected efficiency valid for the
real experimental conditions where highly relativistic electrons of
typically 400 MeV energy must be detected. 90 Sr emits electrons with
a maximum kinetic energy of Emax = 546 keV. It decays to 90 Y with a
half-life of 29.12 years. The β-decay of the short-lived daughter has
a much higher end point, Emax = 2.28 MeV, and a mean energy of
〈E〉 = 933 keV. Geant4 was used to simulate the tracks and energy
deposition of the β-electrons from the collimated 90 Sr source in the
sample and trigger fibers. Only the energy spectrum from the 90 Y
component was modeled including phase-space factor and Fermi
function correction as the results for the end point electrons of the Sr
decay showed straggling paths that could not reach the trigger fiber.
Electrons from the high energy tail of the 90 Y decay suffer smaller
deflections.
The question arises of whether a threshold value can be chosen for
the trigger detector resulting in an average energy deposition in the
studied fiber as close as possible to that of highly relativistic electrons.
Fig. 63 shows the mean energy deposition in the 2 mm thick fiber as
a function of the energy threshold level required in the trigger detector.
The blue dashed line shows the energy deposition for 400 MeV
electrons (∆E = 366 keV). The root mean square deviation from the
mean (RMS) of the corresponding distributions are represented by the
vertical lines. A threshold level of ∆Ethr = 800 keV allowed a precise
emulation of the realistic working conditions with both distributions
differing only slightly in their RMS values. The comparison of the
count rate at the chosen energy threshold to the total rate allows for a
practical determination of the corresponding voltage level in a leading
edge discriminator. A trigger rate of 20 Hz remained for the chosen
threshold.
5.5 modeling of long scintillating fibers read out by sipm
An estimation of the detection efficiency for a given threshold can be
obtained by multiplying the triggering probabilities of both SiPM. It is
useful to use the single pixel amplitude as a threshold unit due to the