Thesis

Calibration of Neutron Directions for Geo and Reactor Anti-neutrino Separation in SNO+

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domain:

  Experimental Particle Physics

• Supervisor:

  Sofia Andringa

• Co-Supervisor:

  Amelia Maio

• Institution:

  Faculdade de Ciências - Universidade de Lisboa

• Host Institution:

  Laboratório de Instrumentação e Física Experimental de Partículas

• Abstract:

  Anti-neutrino interactions in liquid scintillator detectors like SNO+ can be clearly identified by the delayed coincidence of a positron annihilation followed by a neutron capture, in which the positron energy and the neutron initial direction follow the anti-neutrino kinematics. The difference between the positions of the positron and neutron signals can thus give an indication of the anti-neutrino direction. However, the sensitivity of this measurement is strongly dependent on the liquid scintillator characteristics, and must be calibrated directly on data. This work plan is devoted to the design, construction, testing and usage of a new directional Am-Be neutron source for SNO+. In fact, the determination of the neutron direction can be an important tool in the anti-neutrino analyses, namely in the separation of sources from different directions. The more general objective is thus the understanding of the possible accuracy of such a measurement under different experimental conditions, and its impact for the separation between geo-neutrinos and neutrinos from reactors close to SNO+. A deeper understanding of the factors that, together with the calibration, determine the possible accuracy of this measurement, will be an important guideline for the design of future experiments and the corresponding calibration methods. This thesis work should then have a reasonable component related to anti-neutrino physics but its main component will be in neutron propagation physics analysis and simulation and hardware work. Other possible application of such a neutron directional calibration source can also be considered within this framework. Framework: The first observations of geo-neutrinos, i.e. anti-neutrinos produced by natural radioactivity in the planet's crust and mantle, are recent but are already being used to test Earth models; anti-neutrinos produced in nuclear reactors have been classically used in neutrino physics, namely for the study of neutrino oscillations. The separation between those two signals is usually based on energy alone, and we plan to increase it by using also directional information for the first time. The Sudbury Neutrino Observatory (SNO) is a large volume neutrino detector located in the SNOLAB underground laboratory in Canada. SNO has demonstrated that solar neutrinos do change flavor and thus have a small, but non-zero, mass. The SNO+ experiment will replace SNO's heavy water target by liquid scintillator, that will provide sensitivity to several new low energy neutrino physics measurements, and in particular anti-neutrinos. The detector upgrade of the SNO+ experiment (including the scintillator purification system, and calibration systems) is currently being completed and data taking is expected to start soon. The LIP group is responsible for several aspects of the calibration system – PMT and scintillator optical calibration, source insertion mechanism – as well as for the anti-neutrino analysis. This project is focused on specific calibrations for the anti-neutrino directional analysis. Tasks: The design will involve the evaluation of existing isotropic Am-Be sources and the material selection to optimize the neutron shielding in all but the desired direction, and will rely on detailed simulations. The optimization of the source characteristics – neutron flux, spectrum and collimation – will take into account the full neutron calibration program of SNO+, to be prepared in parallel. Before the construction of the final radioactive source, a prototype with the chosen materials and adequate geometry will be experimentally tested in realistic conditions using other neutron sources available at the Nuclear Technology Campus, in Sacavém. The final source will require also tests of material compatibility with the liquid scintillator, to be performed with other SNO+ collaborators. The in-situ commissioning and usage of the source at SNOLAB will be the next step of the work. It will include the participation in data-taking and analysis of the calibration data obtained with the new directional neutron source, and the estimate of the impact on the final accuracy in the maps used for the separation of the geo and reactor anti-neutrino fluxes. Finally, the usage of such a calibration source in future low energy neutrino experiments or for other applications will also be addressed.