KamLAND Physics Impact

KamLAND (Kamioka Liquid scintillator Anti-Neutrino Detector) is a large neutrino detector on the island of Honshu in Japan. KamLAND has around 80 collaborators from 12 different institutions in Japan and the US. It is the largest low-energy antineutrino detector ever built. It will hopefully answer many key questions concerning neutrino oscillation by studying the flux and energy spectra produced by neutrinos from local Japanese commercial reactors. With such good location (most of the Japanese reactors are located within 150-200 kilometers), the KamLAND site is exposed to a large flux of low-energy anti-neutrinos. In addition to observing anti-neutrinos from reactors, KamLAND is hoping to shed some light on the "solar neutrino puzzle" by directly observing the beryllium-7 and boron-8 solar neutrinos. One of the largest difficulties in observing solar neutrinos is reducing the detector's background radiation. There is a layer of mineral oil surrounding the active scintillator volume (shielding any external gamma and neutron radiation) and several other precautions have been taken to minimize any background radiation. KamLAND started taking data at the end of January 2002.

Neutrino Oscillation

Detecting neutrino mass is currently based upon neutrino oscillation theory. The assumption is made that neutrinos behave very similarly to quarks. The easy way to analyze most neutrino oscillation experiments is to assume only two flavors mix (as opposed to the real life situation of 3 possible flavors). Therefore, the mixing matrix, U, depends only on one mixing angle q , and the oscillation probability is given by a simple formula

where

One way to test for neutrino oscillations is to perform an appearance search in which one looks for new neutrino flavor. Another method is to perform a disappearance test in which one looks for a change in the flux. When neutrinos propagate in matter, the oscillation pattern may be modified. Under favorable circumstances a resonance enhancement of the oscillation amplitude, the so-called Mikheyev-Smirnov-Wolfenstein (MSW) effect can take place.

Reactor Oscillation Experiment

Reactors produce electron anti-neutrinos in the beta minus decay of the neutron rich fission fragments inside the core. Upon entering the detector, the anti-neutrino is captured by a free proton and the following reaction occurs:

n_e + p ® n + e⁺

The positron deposits it's energy and then annihilates yielding two gamma rays (each 511 keV). The neutron is thermalized and then captured by a proton in the following reaction:

n + p ® d + g

The energy of the gamma ray produced is ~2.2 MeV. The neutron mean thermalization time is 200 ms. An anti-neutrino event has a clear signature with the time delay between the positron signal (prompt) and gamma ray signal from the neutron capture (delayed). The need to prevent any signal mimicking a neutrino event is imperative.

Solar Neutrino Experiment

KamLAND will study solar neutrinos in the future. In order to make solar neutrino detection a possibility, maintaining an extremely low background inside the detector is imperative. Unlike anti-neutrinos in the reactor experiment, electron neutrino detection is a single ionization event and is much more difficult to differentiate from fake events. Even if results obtained from the reactor experiment are not as expected, the solar neutrino detection element of KamLAND should continue due to it's possible ability to provide some insight on the "Solar Neutrino Puzzle". The sun generates solar energy through fusion reactions and in the process releases an intense flux of electron neutrinos. The following reaction occurs

4p + 2e^- ® ⁴He + 2n _e + 26.73 MeV - En

where En is the neutrino energy. So far, only five neutrino experiments have been able to measure an accurate neutrino flux from the sun and in each case, the measured results are much lower than the expected results. This deficit of neutrinos is what is known as the "Solar Neutrino Puzzle". KamLAND hopes to obtain an accurate measurement of the ⁷Be neutrino flux from the sun and compare results.

Minimizing the Background

There are various sources of possible contamination inside the KamLAND detector. Cosmic-muon induced processes and natural radioactivity from natural surroundings are the two possible candidates for false events. In the case of anti-neutrino detection, two events are necessary (see above). A minimum deposit of 1-MeV energy deposit is required for the prompt signal (positron annihilation) and 1.8-2.7 MeV energy deposit is required for the delayed signal (neutron capture). The time window can range from 0.5-660 ms and neutron capture should occur within 1.6 m of the vertex of the positron annihilation. In the case of solar neutrino detection, only one event (signal from one electron) is detected and can be mimicked very easily. Distinguishing what is really a neutrino event from other false signals is a very difficult task and a high priority for neutrino detection of any sort. Underground detectors have the advantage of being mostly protected from cosmic rays. KamLAND is shielded by 2700 mwe of rock. The scintillator itself is housed inside a cylindrical vessel containing ultrapure water acting as a veto counter. The veto counter (outer detector) uses PMTs to detect any Cerenkov radiation produced from cosmic-ray muons traversing through the detector. Muons are easily detected but interactions with various natural surroundings (elements inside rock cavity) can prompt neutron capture mimicking neutrino events. Any muon that transverses the veto detector will negate any event that occurs within 2 ms that is also within a reasonable distance of the muon detection. In addition, any muon detection inside the central detector will result in a 2 s veto within reasonable proximity. With the elimination of events following muon detection by the previous conditions, the only source of background is through muon spallation. Muons responsible for neutron spallation inside the inner detector are easily detected and are discarded. Muon capture inside the laboratory walls contribute a negligible background but can be a problem if it occurs inside the veto system. Neutrons produced from spallation are much more energetic than the ones produced in muon capture and tend to cause more problems. Natural radioactivity also poses a big problem, especially from ²³⁸U, ²³²Th and ⁴⁰K in the surrounding rock. Materials used to build detector and actual components can also lead to possible background, such as the decay of ⁶⁰Co, artificially added to steel during production. Radon contamination is one of the largest culprits of background radiation and minimizing its presence will be imperative for accurate results.

A lot of care was taken to keep the KamLAND detector clean during construction and the data analysis shows that the detector is indeed extremely clean (less than 3.5x10^-18g/g of ²³⁸U and 5.2x10^-17g/g of ²³²Th contamination).