Page 10 - PR 2014 2016 03 Applications of Ionizing Radiations

52 Application of Ionizing Radiations | Progress Report

Development of the inorganic scintillator
crystals used as radiation detector

The Laboratory of scintillator crystal growth
develops pure and doped crystals for the con-
struction of radiation detectors. Inorganic Figure 11 shows the bromine doped cesium iodine crystals CsI:Br)
scintillators play an important role in the
detection and spectroscopy of gamma and cathodes and these are accelerated by a se-
X-rays, as well as in neutrons and charged ries of electrodes (dynodes) inside the photo-
particles. For a variety of applications, new multiplier tube. In the scintillation detection
inorganic scintillation materials are being process, the proportionality is maintained
studied. New scintillation detector applica- in each step, that is, the number of photons
tions arise continuously and the interest in released by the crystal is proportional to the
the introduction of new fast scintillators be- energy released in crystal, from the incident
comes relevant. Scintillation crystals based on radiation. The number of displaced electrons
cesium iodide (CsI) have relatively low hygro- in the photocathode is proportional to the
scope, easy handling and low cost, features number of light photons from the crystal
that favor their use as radiation detectors. and the electric current generated by the
Pure and doped CsI crystals are grown using photomultiplier tube. This way, the height
the Bridgman technique. In this technique, of the electrical pulse from the photomul-
the charge is maintained at high temperature tiplier tube is proportional to the radiation
for 10 h for the material melting and complete energy absorbed by the crystal. This allows
reaction, using a quartz crucible in vacuum at- that the energy from different radionuclides
mosphere. The temperature gradient 21°C/cm can be distinguished from each other by
and 1 mm/h descending velocity were cho- pulse height. Measurements of pulse height
54
sen as technique parameters. After finishing using gamma radiation sources of Mn (835
the growth, the furnace is cooled at a rate of keV) and Cs (662 keV) are show in Figures
137
20°C/h to room temperature. The best doping 12 and 13.
element concentration is studied. The main
used dopants are thallium (Tl), bromine (Br),
lead (Pb) and lithium (Li). The grown crystals
are subjected to heat treatment. In this proce-
dure, it is used vacuum of
-6
10 mbar and continuous
temperature of 350°C, for
24 hours. Figures 10 and
11 shows pure CsI crystal
and the bromine doped
cesium iodine crystals

Figure 10. pure CsI crystal (CsI:Br), respectively.

For the detection system, crystals are cou-
pled optically with the photocathode. The
Figure 12. Pulse height obtained for gamma radiation from Mn
54
bright photons move the electrons of photo- (835 keV) source with crystals of CsI: Br and pure CsI

Instituto de Pesquisas Energéticas e Nucleares