Neutrino Interactions in D2O

Solution of the Solar Neutrino Problem (SNP), and in particular the investigation of neutrino oscillations, requires independent measurements of the flux of electron neutrinos and non-electron neutrinos (muon neutrinos and tau neutrinos). SNO is unique in its ability to accomplish this. With heavy water we can measure the flux and energy spectrum of electron neutrinos and the flux of all neutrinos. The flux of non-electron neutrinos is then just the difference. SNO can measure these fluxes via the different ways in which neutrinos will interact with the heavy water:

 

Charged Current Reaction

As the electron 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 Cerenkov radiation, is detected by the photomultiplier tubes (PMTs); the amount of light is proportional to the incident neutrino energy.

From the PMT hit patterns the energies of the neutrinos can be determined and an angular distribution measured. The spectrum of neutrino energies will show a distortion from the theoretical shape if neutrino oscillations are occurring.

The Standard Solar Model predicts about 30 charged current events per day in SNO.

 


Neutral Current Reaction

In this reaction the heavy weak force particle exchanged (called the Z  boson) is not charged, hence the name "neutral current reaction". The net reaction is just to break apart the deuterium nucleus; the liberated neutron is then thermalized in the heavy water as it scatters around. The reaction can eventually be observed due to gamma rays which are emitted when the neutron is finally captured by another nucleus. The gamma rays will scatter electrons which produce detectable light via the Cerenkov process discussed above.

The neutral current reaction is equally sensitive to all three neutrino types; the detection efficiency depends on the neutron capture efficiency and the resulting gamma cascade. Neutrons can be captured directly on deuterium, but this is not very efficient and clearly distinguishing the spectra would be challenging. For this reason SNO is developing two separate neutral current systems to enhance the neutral current detection. The diagram to the right shows capture on 35Cl, which will be added to the heavy water in the form of NaCl during the second phase of detector operation.

The Standard Solar Model predicts about 30 neutrons per day in SNO.


Electron Scattering

This reaction is not unique to heavy water and it is the primary mechanism in other light water detectors. Although the reaction is sensitive to all neutrino flavours, the electron-neutrino dominates by a factor of six. The final state energy is shared between the electron and the neutrino, thus there is very little spectral information from this reaction. Good directional information is obtained.

The Standard Solar Model predicts about 3 electron scattering events per day in SNO.

Courtesy of the SNO Home Page at Queen's University, Kingston, Ontario

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