Reactions with radioactive beams and development of scintillator-based detector systems

An experiment on elastic scattering of 15 C on 208 Pb at an energy of 65 MeV
was performed at ISOLDE (CERN) exploiting the advantages of the ISOL te-
chinique. The measurement showed in increase of the reaction cross section to
3035 mb with respect to 12 C, confirming the results obtained at 80 MeV/u. Mea-
surements to characterise and develop detector modules for the CALIFA setup
was performed for the R 3 B experiment at FAIR. Light output characteristics of
478 detector modules of the CALIFA Barrel section were measured as a part
this work in order to investigate the correlation between resolution and non-
uniformity. For this set of crystals the average resolution was found to be 5.2%
(FWHM) at E γ = 1275 keV. Further investigations also done in this work show
that the inhomogeneity in the Tl dopant distribution contributes ∼0.5%, and
the noise in the read-out chain with .0.1% to that number. The most important
contribution to variations in the resolution was found to be from non-uniform
light collection due to the geometry of the modules. The APD light read-out
sensors were also investigated and found to be stable below 24.8 ◦ C using a gain
correction algorithm. Energy add-back and light cross-talk were investigated.
It was found that a closest-neighbor add-back algorithm recovers 14.7% of the
events for E γ = 662 keV and 65% of the events for E γ = 1460 keV, while the
light cross-talk contributes .0.03% to the photo peak. The attenuation length
of the emission light of the scintillators was also determined and found to be
34.6 ± 4.5 cm. In addition the micro facet distribution for a polished and a
lapped sample was measured using atomic force microscopy to produce look-up
tables for GEANT4 simulations of light transport. Simulations of light output
non-uniformity were performed for different configurations of lapped surfaces
of a CALIFA detector module. The simulations reproduced the general trend
of the measurement where the non-uniformity decreased with increasing lapped
area, although the exact values for the non-uniformity were not reproduced. A
sensitivity test for the simulated light-output non-uniformity with respect to
different input parameters was also performed. The non-uniformity was found
to be most sensitive to the reflectivity of the reflector foil while the sensitivity
to the other parameters was less prominent.