- What Are Neutrinos?
A neutrino is a subatomic particle with no electrical charge and almost (but not quite...) no mass. Its properties make it difficult to detect, but it is nonetheless a very important particle, at least in part because there are so many of them.
For example, neutrinos can carry away vast amounts of energy from a supernova explosion. Thus, a full understanding of the mechanics of supernovae can come about only with a good knowledge of the processes that take place during these events, and experimental conformation of expected results- such as a neutrino burst preceding detection in optical or other wavelengths, as was seen with supernova SN1987A.
Neutrinos are produced in a range of astrophysical environments, so an experimental understanding of them- in spite of the great difficulty in their detection- provides us with another important insight into astrophysical and nuclear processes.
- How Do We Detect Neutrinos?
One of the IceCube PMT modules. The PMT itself is the copper-hued hemisphere in the lower section.
The inherent properties of neutrinos greatly limit their interaction with ordinary matter, so we must look at detection strategies which enable us to observe their very rare interactions with matter.
Whilst the initial detection of neutrinos was undertaken with laboratory-scale equipment (excluding, perhaps, the nuclear reactor that was the source of the neutrinos), experiments to detect astrophysical neutrinos are much larger. Some involve large volumes of water, and they usually are located underground in order to shield the system from other naturally-occurring particles; the neutrino flux essentially is not affected by passing through substantial amounts of rock, but the background from other particles is greatly reduced.
Also, we do not look to detect the neutrino itself; rather, we are looking for some signature of the interaction of a neutrino with matter, and use the detection of such interaction products as an indication that a neutrino was present.
An example of a track in the array data. Black dots are PMT modules. The coloured circles represent individually-triggered PMT modules, with the lines showing the reconstructed neutrino path.
In the IceCube experiment, light-sensitive photo-multiplier tubes (PMTs) are buried deep within Antarctic ice. When a neutrino interacts with a neutron or a proton within H or O atoms in the ice, that interaction can release secondary particles which are travelling faster than the speed of light in ice (similar in principle to the air Cerenkov technique used for gamma-ray telescopes); as a result, Cerenkov light is emitted. The PMTs detect these flashes of Cerenkov light.
Over 5,000 PMT-based detectors are buried in the ice at depths between 1,450-to-2,450 metres below the surface. Analysis of event data from the PMT array can lead to information about the direction of arrival of the neutrino, and also help to determine whether the neutrino was from an astrophysical source or was produced in the earth's atmosphere by other processes.
- What We Do at HEAG
HEAG members are involved in analysis and interpretation of data from IceCube, particularly with regard to the question of whether or not the distribution of neutrino sources truly is isotropic (i.e., uniform across the sky), and if we are able to correlate neutrino sources with other objects, e.g. gamma-ray sources.
Also, staff have contributed to the installation of the detector array, and are involved in the planning and administration of the collaboration.
For more information, please contact Associate Professor Gary Hill.