One of the main challenges during the ultra-high count rate gamma-ray spectrometry is the large dead time. Using a LaBr3(Ce) detector (TRT 0.3 µs, TFT 0.5 µs), with an input count rate of 4.8×10E5 cps, the dead time could be as high as 87%. Such high dead time could significantly reduce the quality of the data collected as a considerable number of counts would be lost. Thus, this project aimed to reduce the dead time by modifying the detector system. Based on the setup used by previous research, the new system has its preamp, which is normally used for signal processing, removed. Experiments were made with calibration sources to optimize the new system. The calibration sources (Cs-137 and Co-60), Cs-137 resin sources, and Shephard Cs-137 sources were used to create different count rates, with the highest being 1.22×10E6 cps, for measurements. Side-by-side measurements were performed with the setup with preamp and the one without preamp at various count rates. The analysis, which focused on the dead time and resolution, shows the setup without preamp would have much lower dead time, especially during ultra-high count rate measurements. The method was proved to be successful, for, at 4.8×10E5 cps, the dead time decreased from 87% to 54%. / Thesis / Master of Science (MSc)

In case of a release of radioactive nuclides into the environment it is necessary to  have reliable methods to estimate the potential effect on people and the ecosystem. In this context the total activity deposited, the elemental composition and the depth distribution are of importance. An efficient in situ method to estimate the average contamination over larger areas using high purity germanium (HPGe) detectors has been developed. The method combines simulation of photon transport with measured detector properties. The total activity of 137Cs determined from gamma-ray spectra recorded in situ are compared to results from soil sampling. Another in situ method has been developed to determine the depth distribution of a radionuclide contamination, using an array of small detectors inserted into the ground, as an alternative to the standard procedure of soil sampling. The possibility to use cadmium telluride (CdTe) detectors as well as lanthanum bromide (LaBr3) detectors has been investigated. As a demonstration of the developed method the small-scale variation of the activity distributions in an area covering 350 m2 have been measured and is compared to results obtained by other methods. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

The recent disaster of Fukushima in Japan combined with the high demand to enhance nuclear safety and to minimize personal exposure to radioactive materials has a significant impact on research and development of radiation detection instrumentation. Currently, there is ample effort worldwide in the pursuit of radiation detection to maximize the accuracy and meet international standards in terms of size and specifications to enable radiation protection decision making. Among the requirements is the development of a portable, light-weight gamma-ray isotope identifier to be used by first responders in nuclear accidents as well as for radiation security and identification of illicit material isotopes. From nuclear security perspective, research into advanced screening technologies has become a high priority in all aspects, while for occupational safety, and environmental radiation protection, the regulatory authorities are requiring specific performance of radiation detection and measuring devices. At the applied radiation laboratory of the University of Ontario Institute of Technology, UOIT, the development of a high resolution spectrometer for medium and high energy gamma ray has been conducted. The spectrometer used a newly developed scintillator based on a LaBr3(Ce) crystal. The detector has been modeled using advanced Monte Carlo code (MCNP/X code) for the response function simulation and parameter characterization. The simulation results have been validated by experimental investigations using a wide range of gamma radiation energies. The developed spectrometer has been characterized in terms of resolution and response in different fields. It has also been compared with other crystals such as NaI(TI) and LiI(Eu). / UOIT

There has been a great deal of interest in the use of LaBr3 (Ce) detectors for fast timing measurements. Due to their excellent energy resolution, ≈ 3% at 1MeV, they offer an improved signal to background over BaF2 which has ≈ 9% energy resolution. Many groups are planning arrays of LaBr3 detectors. The 8π collaboration has recently acquired a small array of 6 2”x 2” cylindrical LaBr3 detectors to replace its BaF2 detectors. We have been investigating the effects of detector-to-detector scattering on timing resolution. We have further investigated the use of Compton-suppression shields with LaBr3 detectors. This will also be the first testing of the DANTE acquisition system in the 8π. A summary of the results of our optimization, scattering studies, and Compton-suppression tests will be presented.

La réaction de fusion 12C+ 12C marque un tournant dans l’évolution des étoiles massives. La section efficace présente des résonances jusqu’aux énergies d’intérêt astrophysique, généralement associées à une structure moléculaire dans le 24Mg. Au cours de cette thèse, la section efficace de fusion 12C+ 12C a été mesurée depuis la barrière de Coulomb jusqu’aux énergies d’intérêt astrophysique. La mesure en coïncidence des particules chargées et des γ a été employée. Un dispositif expérimental, STELLA, a été développé et construit au cours de cette thèse. Il comprend une chambre à réaction, des détecteurs annulaires au silicium, un mécanisme de cibles tournantes, un système de pompage pour vide poussé, un système d’acquisition digital, et une structure pour supporter 36 détecteurs LaBr3. STELLA a été installée auprès de l’accélérateur Andromède (Orsay, France) et une série d’expériences fructueuses ont permis d’obtenir des résultats cohérents au cœur de la fenêtre de Gamow pour la phase de combustion du carbone dans les étoiles massives. / The 12C+ 12C fusion reaction plays a key role in the late evolution of massive stars. Its cross section exhibits strong resonances down to the astrophysical region of interest, often associated with a molecular configuration of the 24Mg nucleus. In this thesis, the 12C+ 12C fusion cross section has been measured from the Coulomb barrier down to the astrophysical region of interest. The charged particle-gamma coincidence technique has been used. A new experimental setup, STELLA, has been developed and built during this thesis. It is composed of a reaction chamber, a set of annular silicon strip detectors, a rotating target mechanism, an ultra high vacuum system, a digital time-stamped data acquistion system, and a structure that fits 36 LaBr3 detectors. STELLA has been installed at the Andromède facility (Orsay, France). Successfull experiments have been carried out to extract fusion cross sections down to the Gamow window for the carbon burning phase of massive stars.