In astroparticle physics, a great importance is attached to the field of solar neutrinos. Besides the possibility to study the intrinsic properties of neutrinos, they themselves provide an exceptional way to look deep into the core of our sun. By studying their flux, one will be able to probe and eventually improve the widly accepted Standard Solar Model.
In recent years, several detectors have been constructed in order to observe the solar neurinos. The most widely known are Super-Kamiokande and SNO. As both experiments are water Cherenkov detectors, their energy threshold of ~5 MeV for neutrino detection limits the observations to the high energetic tail of the solar neutrino spectrum consisting of 8B and hep neutrinos.
Thus, there arises a necessity for detectors that are capable to observe the neutrino flux at lower energies. Liquid-scintillator detectors like the Borexino or the KamLAND experiment are able to detect neutrinos down to an energy of several hundred keV, provided the intrinsic radioactive background of the experiment is low enough. The Borexino experiment was designed to meet this ultra low background requirements. It will therefore be able to measure the fluxes of 7Be, pep and CNO neutrinos which will provide knowledge about the physics of both the sun and neutrinos.
The OD serves as shielding against external radioactivity and as a veto for cosmogenic muons. It is filled with 2400 tons of ultra pure water and is equipped with 208 PMT's.
The ID consists of a stainless steel sphere and two nested nylon vessels for radiopurity purposes. There are 2200 PMT's installed inside. It is filled with 1040 tons of shielding liquid outside and 280 tons of liquid scintillator (with a fiducial volume of 100 tons) inside the inner nylon vessel.
After long years of construction, Borexino started to take data on May 16th, 2007. Only three months later, the first results on the detection of solar Be7 neutrinos could be published (astro-ph/0708.2251,PLB-D-07-00772R2). It was the first real-time spectral measurement of sub-MeV solar neutrinos. After 47.4 life days of data-taking, the measured event rate of (0.862 MeV) Be7 neutrinos is 47 +/- 7 (stat) +/- 12 (sys) counts/(day x 100 ton). The value is consistent with predictions of the Standard Solar Models and neutrino oscillations with LMA-MSW parameters.
The figure below show the energy spectrum of both neutrino and background events inside the fiducial volume (i.e. the inner-most 100 tons of liquid scintillator). The relevant neutrino window for Be7 neutrino detection stretches from 270 keV to 800 keV of detected energy. The left figure depicts a fit that was done to the "Compton" shoulder of the Be7 neutrino events above 560 keV, in this way avoiding the background of events due to Po210 alpha-decays. The right figure shows the fit to the whole spectral region after a subtraction of the Po210 peak by alpha/beta-discrimination. Both fit results agree fully within the quoted errors. In both cases, contributions of Kr85, Bi210 decays and CNO neutrino events were considered as well. For further details see the publication quoted above.
The energy production in the sun is maintained by thermonuclear fusion. The main channel of energy production is the pp-chain (98%). Several subprocesses in this chain contribute to a continuous energy distribution of electron neutrinos. The spectrum ranges from 0 to 18 MeV and contains several mono-energetic lines, as shown in the left figure.
Due to its low energy threshold, Borexino will be able to measure the fluxes of 7Be, pep and CNO neutrinos that have not been observed directly so far. The detection reaction is the elastic neutrino-electron scattering. However, the sensitivity will be determined by the intrinsic radioactive background present in the scintillator.
A liquid-scintillator detector is able to detect electron-antineutrinos via the inverse beta decay reaction, anti-ve+p -> n+e+. The delayed coincidence of the positron signal and the gamma quantum produced in the subsequent capture of the neutron on a proton of the scintillator is a prominent event signature and allows for excellent background rejection.
Several antineutrino sources can be investigated in Borexino:
In March 2013, the Borexino collaboration has released a new result on the geo-neutrino measurement: www.interactions.org/cms/
The explosion of a core-collapse supernova in our galaxy will be visible as a neutrino burst of 10 sec duration in Borexino. The neutrinos that are cooling the emerging neutron star are generated in all (anti-)flavours. In liquid scintillator, there is a number of different reaction channels that will allow to observe energy and flux of the neutrinos at least partially sensitive to their flavour.
Event numbers for the different detection channels are given in the table to the right, along with the assumptions about the mean neutrino energy and the total cross section of the reaction. The progenitor is chosen to be an eight solar mass progenitor star at the center of our galaxy, 10 kpc away.
If you are interested in a diploma or PhD thesis on this topic, please contact Prof. Lothar Oberauer.