Optical Calibration Development for SNO+

Some of the most exciting physics to emerge over the last decade has been in the field of neutrino physics. One of the forefront experiments here has been the Sudbury Neutrino Observatory (SNO), based in Canada. The UK has played a leading role in this project, solving the "Solar Neutrino Problem" and clearly demonstrating, for the first time, that neutrinos exists as mixed states which allow them to apparently "oscillate" from one type to another. On the heels of this tremendously successful project, a follow-on experiment is being pursued with a remarkably diverse and interesting range of physics objectives. SNO+ will use a modified version of the instrument to measure fundamental solar neutrino processes (thereby also investigating details of neutrino-matter couplings); search for non-standard modes of nucleon decay; study neutrinos generated from within the earth; look for neutrinos from galactic supernovae; and search for a very rare process called "neutrinoless double beta decay." An observation of the latter would both permit a determination of the absolute neutrino masses and would establish that neutrinos act as their own antiparticles, which could have consequences for our understanding of the matter/antimatter asymmetry in the universe. The project is anticipated to have a rapid timescale, with first data to be taken in 2012.

The ability to unravel the nature of interactions observed by the instrument requires a detailed understanding of how light is absorbed, reflected and scattered inside the detector. One of the main contributions being made by the UK involves a network of optical fibres through which different kinds of light may be directed into the detector to help understand these effects and how instrument responds to them. The work of this grant is concerned with enhancing the capabilities of this system and laying the groundwork for future development that would have wide-ranging applications.

One aspect of this involves developing a laser system capable of different wavelengths of light through some of these fibres to study light scattering. We will also look to develop a way to accurately monitor how much light gets sent through the system, which would make it useful for other measurements inside the SNO+ detector. Many other experiments also have the need for similar systems and there are even potential applications outside of particle physics, such as those involving precise monitoring in remote or hazardous environments. As one example, a group of nuclear scientists has proposed the use of a similar but much less sophisticated system to monitor real-time uranium leakage in cooling and condensation water during reprocessing.

Another aspect to be studies involves further studies of a new circuit that we have designed to produce extremely fast light pulses from LEDs. This light will also be used to send down the fibres in SNO+ to determine the timing response of the detection elements when struck by light. However, such a device would also have other applications where a reliable, inexpensive, fast-pulsed light source is required.

Therefore, the R&D associated with this application would not only have a noticeable impact on the ability of SNO+ to explore the remarkable range of scientific questions previously mentioned, but also has a natural link to critical problems in other areas of potential interest.

The overall target of this research is a facility that would make fundamental advances in our understanding of a diverse range of phenomena covering areas including how stars work, the geology of the earth and the nature of the most basic building blocks of matter. Such fundamental understandings provide the foundation that underpins the whole of technological and intellectual development that continues to advance society as a whole. Another "indirect" benefit lay in the instilling of scientific interest and training of students to think creatively and to develop and apply analytical skills to difficult problems. Such skills are highly prized as they are applicable to almost every corner of society, which has benefited greatly from the significant advances made by such trained individuals. In addition to this non-specific but very real benefit, potential direct technological spin-offs include: advances in the development of scintillators and radioactive sources, both of which are topics of interest having had important applications in medicine; novel use of fibre optics, a topic of importance in areas such as telecommunications; the development of techniques to measure extremely low levels of radioactivity, a topic of interest to both public health and safety as well as national defense; unique engineering challenges (including working in and maintaining an ultra-low radioactivity environment deep underground) that push the level of understanding of materials and the uses to which they can be put.

Liquid scintillator, let alone metal-loaded liquid scintillator, is a complex substance with intricate optical properties. The absorption and scattering lengths in the target volume for SNO+ will vary from ~20m for pure LAB in the solar phase to ~6m for 0.3% Nd loading in the double beta decay phase. Precise calibration and monitoring of these properties is therefore crucial to the event reconstruction and energy resolution and, thus, to the success of the experiment in all of its phases. This proposal specifically targets these issues though further develpment of the UK optical calibration system.

A pulse-monitored laser system along the lines proposed for development in this proposal would also have the potential for wider applicability outside of SNO+. Many other experiments have the need to make use of similar systems and, to some extent, this proposal represents the continued evolution of systems successfully employed on projects such as MINOS and Double Chooz. A number of large-scale liquid detectors proposed as future projects could also benefit from such a system, such as LENA, HanoHano, HyperK and LBNE. There are also potential applications outside of particle physics. In particular, applications involving precise monitoring in remote or hazardous environments might benefit from such a system. As one example, a group of nuclear scientists has proposed the use of a similar but much less sophisticated system to monitor real-time uranium leakage in cooling and condensation water during reprocessing (Lee, Shin and Kang, Journal of Korean Nuclear Society, Vol. 33, 2001). Similarly, the further development and characterisation of a new fast driver circuit for LEDs would provide a calibration system of significant interest both to SNO+ and to the next generation of scintillator or water Cherenkov systems as well as other applications where a reliable, fast-pulsed light source is required. Therefore, the R&D associated with this application has a natural link to critical problems in other areas of potential interest. We aim to publically disseminate the information derived from this research via journal and electronic publications, international conference and workshop talks and proceedings, and open discussions with academics and representatives from industry.