Reactions with radioactive ion 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.

Nuclear physics aims to build a complete theory, which explains all the proper-
ties of atomic nuclei. The very first theoretical models were built on the results
of experiments with the 318 isotopes which can be found in Nature. Contempo-
rary nuclear physics can produce about 3000 isotopes, and ∼7000 isotopes are
believed to exist in total. In some of these new unstable isotopes new unpre-
dicted properties have been discovered. One of the most interesting properties
in this respect is the formation of a nuclear halo, where the radius of the atomic
nucleus increases so much that an isotope of e.g. 11 Li becomes as large as a
nucleus of the isotope 208 Pb.
The study of atomic nuclei is usually performed by a scattering of a beam of
particles on a target. The energy of the beam plays an important role. With
low energies we can obtain information about the shape of the nucleus and its
skin. At high energies, it becomes possible to investigate what happens below
the surface of an atomic nucleus.
The detection of a scattered high-energy beam is a challenge. The scattered
particle has to be stopped in the detector to measure its energy, and the energy
has to be measured with high enough precision. The energy of scattered atomic
nuclei is in this case usually about a few hundred MeV, while the internal struc-
ture of an isotope has a typical energy scale from a few hundred keV up to a
few MeV.
This work is dedicated to experiments with beams of unstable isotopes with
low and high energies. An experiment with a low energy beam of the isotope
15 C was conducted to measure the properties of this halo nucleus. For the high
energy beams, a development work for a detector setup, CALIFA (CALorimeter
for In-Flight detection of γ-rays and light charged pArticles), was performed.
As a result of that work it was shown which properties of the detection units are
of most importance for the energy resolution, and how this can be understood
from the processes of light collection in the detector medium.