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Energy calibration of the Sudbury neutrino observatory using monoenergetic gamma-ray sources Poon, Alan Wing Pok


The Sudbury Neutrino Observatory (SNO) is a new 1000-tonne D₂O Cerenkov detector. It will shed some light on the long-standing solar neutrino problem by detecting all flavours of neutrinos originating from the Sun. A high energy gamma-ray source is needed to calibrate SNO beyond the ⁸B solar neutrino endpoint of 15 MeV. This source must have a gamma-ray yield of > 0.2 s⁻¹ , a neutron yield of < 10⁴ s⁻¹ , and an operational lifetime of > 60 hours. To be compatible with the deployment hardware, it must be less than 30 cm in diameter and 75 cm in length. This thesis describes the design and construction of a source that generates 19.8-MeV gamma rays using the ³H(p,ϒ)⁴He reaction ("pT"), and demonstrates that the source meets all the physical, operational and lifetime requirements for calibrating SNO. An ion source was built into this unit to generate and to accelerate protons up to 30 keV, and a high purity scandium tritide target with a scandium-to-triton atomic ratio of 1:2.0±0.2 was included. The techniques that were developed for fabricating this target are useful for producing pure tritiated films needed in commercial applications. This pT source is the first self-contained, compact, and portable high energy gamma-ray source (Eϒ >10 MeV) ever built. The usefulness of this source was demonstrated by measuring with it the gamma-ray angular distribution in ³H(p,ϒ )⁴He at a beam energy of 29 keV, which is more than an order of magnitude lower in energy than all previous measurements. The data were fitted to the form: W(θ) = A + B sin² θ. The results are consistent with the picture of E1 capture of p-wave protons in this reaction, as evidenced by the predominantly sin² θ angular distribution. The ratio A/B is less than 0.35 at the 90% confidence level. Monte Carlo simulations were used to investigate how monoenergetic gamma-ray sources like the pT source could be used to understand the energy response of SNO. Finally, algorithms were developed to correct for the dependencies of the energy response on various event parameters using calibration data from these sources. These algorithms are essential for establishing an accurate energy scale in the detector response.

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