The Naked Scientists-June 2012

The sixth lecture was given by Dr Robert Flack from the High Energy Physics Group at UCL. He is currently working on NEMO-3 and SuperNEMO both of which are double beta decay experiments. Double beta decay is currently the only way of measuring the mass of the neutrino.


Neutrinos are unique. They are detected in flavour states (masses), electron neutrinos, muon neutrinos and tau neutrinos (1, 2 and 3). A neutrino always accompanies a charged lepton. Neutrinos can change from one type to another. It is believed that an electron neutrino can leave the Sun and have turned into a tau neutrino by the time it reaches the Earth. This would explain why there is a deficit in electron neutrinos reaching the Earth (Solar neutrino problem). The probability of this change in flavour is


m = mass, L = length of travel and E is energy of formation. Only Δm needs to be measured. We don’t know if the change is a normal oscillation or an inverted oscillation.

SNO (The Sudbury Neutrino observatory) can detect all three flavours of neutrinos. So the SNO detector will be able to observe separately the number of electron neutrinos and the number of all neutrinos. This allows a determination of the probability for these flavour oscillations to occur. From the neutrino flux and shape of the energy spectrum SNO will be able to determine how strongly the neutrino flavours mix together, and determine information about the neutrino masses. SNO started collecting the first data in April 1999. It can detect small energy differences and the change from electron neutrino to muon-neutrino/ tau-neutrino.

Some muon-neutrinos are produced from the atmosphere (also some electron-neutrinos) isotropically.SNO can show Beta decay in reverse. 1000 tonnes of heavy water are used. As the neutrino approaches the deuterium nucleus a heavy charged particle of the weak force (called the W boson) is exchanged. This changes the neutron in deuterium to a proton, and the neutrino to an electron. The electron, according to mechanics, will get most of the neutrino energy since it has the smaller mass (just as when a gun is fired, the bullet, being lighter, gets most of the energy). Due to the large energy of the incident neutrinos, the electron will be so energetic that it will be ejected at light speed, which is actually faster than the speed of light in water. This causes the optical equivalent of a “sonic boom”, where a “shock wave of light” is emitted as the electron slows down. This light flash, called Cherenkov radiation, is detected by the photomultiplier tubes (PMTs); the amount of light is proportional to the incident neutrino energy.

Super-Kamiokande experiment in Japan is also detecting neutrinos including nuon-neutrinos. Neutrinos should be distributed isotropically and therefore the detector shouldn’t see any difference between top and bottom of the Earth. This is not the case. There is in fact a 50% difference. Is the neutrino Dirac like? Does it have a twin like the electron has the positron? Is it non-standard (majorana neutrino) having no anti-neutrino? During double beta decay could no neutrinos be formed because they annihilate each other?

The aim of the NEMO 3 (Neutrino Ettore Majorana Observatory) experiment is to search for neutrinoless double beta decay – a rare, lepton-number violating, nuclear decay process that would indicate the Majorana nature of neutrinos, as well as provide an estimate for the neutrino mass scale. The experiment is situated in the Fréjus underground laboratory near Modane, France. NEMO 3 is currently taking data from a variety of isotopes. SuperNEMO is the next generation experiment to search for neutrinoless double beta decay. It will house 100 kg of isotopes, ten times more than NEMO 3, thus reaching a neutrino mass sensitivity of about 50 meV. The SuperNEMO detector will comprise several identical modules, each containing a source foil surrounded by a tracker with drift cells and a calorimeter.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s