Early development of a test-bed to measure fractoluminescence in scintillators & simulation of a Na-24 source for the SNO+ experiment

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Mony, Emilie
calibration , DCDC , SNO+ , fracture , scintillator , fractoluminescence
This thesis consists of two parts; the first part pertains to fractoluminescence as a potential background in crystal scintillator detectors, and the second part bears on the simulation of a 24Na source to be used during the liquid scintillator phase of the SNO+ experiment. I participated in early work to develop a test-bed to study fractoluminescence in scintillators, and report here on preliminary results I obtained before I shifted my focus to SNO+. Full results obtained by the group have since been reported in PRL 111 154301 [1]. This project follows the discovery that mechanical stress on a dark matter detector’s crystals was causing a background signal. The response of inorganic crystal scintillators (Bi4Ge3O12, ZnWO4 , CdWO4 ) compressed to the point of rupture was studied. The double cleavage drilled compression geometry was used to create controlled cracks in 20×5×3 mm3 samples. A correlation between a sudden drop of the force, a burst of photonic and of acoustic emissions was discovered and a lower bound was set on the conversion efficiency from strain energy to light energy. SNO+ is a large underground experiment that aims primarily to search for neutrinoless double beta decay. The SNO+ detector consists of an acrylic vessel of liquid scintillator surrounded by light detectors. A tagged 24Na source was proposed as one of several radioactive sources to be deployed within the vessel to calibrate the detector. To achieve this an activated NaI(Tl) crystal would be coupled to a photomultiplier tube and lowered into the center of the vessel. The second half of this thesis explores options for implementing this plan and presents the detector response to a 24Na source as simulated by the Monte Carlo software developed by SNO+. The size of the crystal influences the type of information that can be gleaned from using this source so four different crystal sizes are presented for comparison. The simulations show that the source can be used to test the linearity of the energy scale and the simulation’s quenching model.
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