A liquid scintillator neutrino detector


<center>Model of Borexino</center>

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 Detector

<center>Location of Borexino in LNGS</center>
<center>Schematics of the detector</center>

    • The Borexino experiment is located in hall C of the Laboratori Nazionali del Gran Sasso. It is situated 150 km north-east of Rome in the Gran Sasso massive. The altitude of the rock cover is 1400 m (3800 m.w.e. (=meter water equivalent)). The rock is used as a shielding against cosmic radiation and features good radiopurity conditions.

    • The experiment is contained in a stainless steel dome of 18 m in diameter and consists of the Outer Detector (OD) and the Inner Detector (ID).

      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.

    • The liquid scintillator is based on PC (1,2,4-trimethylbenzene C6H3(CH3)3) as it provides high light yield and attenuation and scattering length suitable for the geometric configuration.







    First real time detection of Be7 solar neutrinos

    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.

    Solar Neutrino Physics

    <center>The pp chain</center>
    <center>The pp chain</center>
    <center>Solar neutrino spectrum according to the SSM</center>
    <center>Solar neutrino spectrum according to the SSM</center>
    <center>Inner view of the Borexino detector</center>

    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.

    • 7Be neutrinos are emitted at a fixed energy of 862 keV and are expected at a rate of 35 events per day. The recoil spectrum of the observed electrons will feature a "Compton-edge" at ~660 keV.
      The 7Be neutrino flux is at the moment only poorly determined by experimental evidence. Borexino aims to measure this flux at a precision of 10%. In addition, this will allow to reduce the uncertainty on the pp flux using the constraints of the Sun's luminosity. Apart from solar physics, the 7Be neutrinos are also interesting in terms of particle physics: their energy places them near to the low-energy side of the transition region from vacuum-dominated to matter-dominated oscillations at higher energies. A measurement of their survival propability tests the predictions of the MSW effect and will allow to look for possible subdominant effects.

    • pep neutrinos are linked very closely to the flux of pp neutrinos via the Standard Solar Modell, making only few assumptions. The measurement of their flux is therefore almost equivalent to the measurement of the pp neutrino flux itself. Moreover, their energy of 1.44 MeV sets them directly in the transition region of vacuum to matter oscillations. Like 7Be, their measurement can validate the MSW theory and might provide evidence of subdominant processes.

    • CNO neutrinos. A small contribution to solar energy production is made outside the pp-chain by the CNO cycle that also produces a detectable flux of neutrinos. Up to now, it is experimentally almost undetermined. In spite of its minor relevance in the sun, the CNO process plays an important role in heavier stars and is therefore of large astrophysical interest.

    Antineutrino Physics

    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:

    • Solar antineutrinos. Non-standard physics mechanisms as the interaction of a non-zero magnetic moment of the neutrino with the magnetic fields of the sun might flip a solar neutrino into an antineutrino. The sensitivity of Borexino will be comparable to the KamLAND sensitivity due to the low reactor antineutrino background at the LNGS.

    • Geoneutrinos. The comparatively large distance to the next nuclear reactors is favourable for the detection of antineutrinos generated in the decay of radioisotopes embedded in crust and mantle of the Earth. These so-called geoneutrinos are emitted by the radioactive 40K and the decay chains of 238U and 232Th. Together, these elements are responsible for about half of the Earth's heat production. Whereas the endpoint energy of the 40K beta-decay is too low to be detected via the inverse beta decay, both U and Th neutrinos are detectable. Borexino will therefore be able to measure the U/Th abundancies in the Eurasian continental crust. In combination with the KamLAND results, this could be used to disentangle the  contributions from continental and oceanic crust to the Earth's heat flow.

        In March 2013, the Borexino collaboration has released a new result on the geo-neutrino measurement:

    • Reactor long baseline. As there are no nuclear power plants in Italy, the average distance to the gros of European power reactors from the LNGS is ~ 800 km.  This will enable Borexino to test the disappearance of the emitted antineutrinos as a long-baseline detector and to cross-check the KamLAND results.

    Supernova physics

    <center>SN rates in Borexino</center>

    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.

    Diploma and Doctoral Theses

    Borexino and Neutrino related Publications:

    Related Talks


    If you are interested in a diploma or PhD thesis on this topic, please contact Prof. Lothar Oberauer.