Detecting high rates of ultracold neutrons and thermal neutron production
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Date
2018-11
Authors
Rebenitsch, Lori
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Abstract
The first phase of the TRIUMF ultracold advanced neutron source (TUCAN) is operational at TRIUMF in Vancouver, Canada, with the second phase scheduled to be completed in 2020. The first proposed experiment with the source is a neutron electric dipole moment (nEDM) measurement. Preparing the experiment requires building the nEDM experimental cell, containing high voltage, magnetic fields, $B_0$ and $B_1$ coils, magnetic shielding to run the Ramsey cycle; a prototype detection system; and commissioning the TUCAN source. This thesis focuses on measurements that characterize the prototype UCN counter, as well as the thermal neutron flux measurement around TUCAN, as part of the source commissioning.
The UCN counter is a \ce{^6Li}-doped glass detector, designed for high rates of UCN. The detector efficiency is comparable to the GEM-based Cascade-U detector at the UCN source at the Paul Scherrer Institute. Comparing the \ce{^6Li} detector to simulation gave a neutron efficiency of $99.5 \pm 0.5$~\% with a background contamination of $0.3 \pm 0.1$~\%. The total efficiency of this detector was measured to be $89.7^{+1.3}_{-1.9}$~\%.
The thermal neutron measurement used Au and Cd-covered Au foils to measure the thermal and colder neutron flux around TUCAN at different temperatures. This measurement was conducted at TRIUMF during the initial commissioning of neutron production. There were some large discrepancies between the measured data and simulation and also between measurements with the same settings. A number of discrepancies were found to be due to differences between the experimental set up and the model. Modeling graphite, in particular, is limited due to the wide variability of the structure of graphite and is known to vary up to 40\% from the measurement depending on structural factors. The largest discrepancy was due to difficulties in steering the proton beam onto the tungsten target. However, none of the simulated effects were large enough to cover the measured results, suggesting there was a systematic effect that was not well-understood. While thermal and colder neutron flux was measured, the amount varied too much from irradiation run to irradiation run to determine the flux with consistency.
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Keywords
Physics, Neutrons, Detection, TRIUMF, nEDM, UCN, Electric dipole moment