This thesis presents the studies conducted with the objective of developing a new and ruggedized Gas Proportional Scintillation Counter (GPSC) based on high-pressure Xe (5- 20 bar) with a cylindrical geometry for the detection of hard X- and gamma-rays (100
keV to 662 keV). It is to be used in �eld applications, where robustness is a requirement,
for example in homeland security (detection of illegal transport of radioactive material)
or for geological prospection (instrumentation for boreholes). A study of the mobility of
ions in gases used in large volume detectors is also presented.
In GPSCs, the detection of ionizing radiation is based on the production of scintillation
photons as the ampli�cation stage, followed by their detection with the help of a photosensor, typically a photomultiplier. GPSCs have an absorption/drift region where
the ionizing radiation is absorbed, producing a cloud of primary electrons which is guided
by a low electric �eld (kept below the excitation threshold of the gas) to the scintillation
region, where the electric �eld is above the scintillation threshold but below the ionization
threshold of the gas. In the scintillation region, they produce a large number of scintillation photons (vacuum ultra-violet photons), emitted during the deexcitation process of the gas atoms. These will eventually reach the photosensor, producing a signal proportional to the energy of the incident radiation.
Conventionally, the adopted geometry is planar, since it displays the best energy resolution, but because of the photosensors usually adopted, its use in �eld applications is
limited. In a recent work, a prototype was developed with a planar geometry with the
objective of being more ruggedized for �eld applications. The main di�erence consisted
of the use of a deposited caesium iodide as the photosensor, with the photoelectrons produced by the VUV photons being collected at a grid close to the photocathode. However, this new detector displayed several limitations: low detection e�ciency for high energy radiation (above 50 keV); small solid angle subtended by the photosensor; and the high bias voltage needed, which reduced its performance and its application scope.
So, to solve these limitations a new detector for higher energies (100-662 keV) was
developed using a cylindrical geometry, which is expected to display several advantages.
On one hand, the cylindrical con�guration allows the number of metallic grids used to be
decreased, thus reducing the impact of the internal optical transmission in the detector
gain. In addition, the fact that the photocathode is deposited on the inner surface of
the detector walls signi�cantly increases the solid angle subtended by the photosensor,
improving the gain. Also because the radiation is absorbed along the cylinder axis, the
detecting e�ciency is improved. Moreover, this con�guration will, in principle, allow the
bias voltage to be minimized for the same gain when compared with the planar geometry.
In this work, this new prototype was designed according to the initial performance
requirements, constructed and assembled, followed by its characterization with the assessment of the prototype performance using an alpha particle source of 241Am, varying the pressure from 1 up to 3 bar. In the initial stage, the characterization of the 241Am source was performed, followed by the study of the charge collection at the anode and the characterization of the scintillation signal.
In this study, it was possible to verify that increasing the E=p above the ionization threshold at the anode surface and slightly above the scintillation one in the collecting
region, the energy resolution was improved. In addition, the gain and the signal-to-noise
ratio (SNR) of the detector were also determined. Regarding the gain, the experimental
values determined were in agreement with the theoretical ones, and at the best possible
conditions we were able to reach a gain of 1.9 at 1.05 bar, which gives a good outlook for
achieving gains of about 30 at 15 bar. As for the SNR, in the best possible conditions
studied, the signal was 10 times greater than the noise, which allowed the minimum
detectable energy to be estimated with the detector in the present operating conditions.
In parallel with the development of this new detector, the transport properties of ions
were also studied to provide information on ion mobility for di�erent gas mixtures used or
considered for several major experiments (ALICE TPC and TRD, CBM TRD, NEXT and
the future LCTPC), as the information of the mobility of ions in gases is relevant not only
for the design and modelling of gaseous radiation detectors, but also in the understanding of the signal formation. This work was developed in the scope of our participation in the NEXT Collaboration and RD51 Collaboration from CERN. The ion drift chamber used in these studies, already available in our laboratory, allows the drift time of this group of ions to be determined with precision and consequently their drift velocity and mobility.
Finally, knowing the mobility of these ions and using Blanc's law with the polarization
limit of the Langevin's formula, it is possible to identify most of the collected ions. In the
scope of this thesis, 5 gas mixtures of interest for the above-mentioned experiments were
studied: Xe-N2, Xe-CO2, Xe-CF4, Ar-C2H6 and Ar-CH4.
Another interesting result coming from this work is related to the validity of the Langevin polarization formula used to predict the mobility of ions and whose limitations are related to the weak polarizability of some neutrals such as Ne, or by the numerous internal degrees of freedom, responsible for reducing the mobility in gases such as CO2 by about 10%. An alternative method to the use of the Langevin polarization limit, when it fails, is proposed, which will allow a better estimate of the mobility to be obtained.