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Defense & Security

Under Ground Detections

Photonic technology increases precision and sensitivity of liquid-scintillator neutrino detectors.

From oemagazine June 2001
31 June 2001, SPIE Newsroom. DOI: 10.1117/2.5200106.0003

The Kamioka Liquid Scintillator Anti-Neutrino Detector (KamLAND) is a large neutrino detector currently being built in the Kamioka mine beneath the mountains of the Japanese Alps, about 200 km west of Tokyo. The KamLAND experiment will add substantially to our ability to study neutrinos, making possible a number of unique new measurements for low-energy anti-neutrinos emitted from nearby nuclear reactors and the Earth's radioactive elements as well as for solar neutrinos. The success of the experiment will depend heavily on how much the background can be suppressed and how many background events can be identified. It is not enough to make the detector radioactively ultrapure. To minimize background events, the design must include a high-light-emission liquid scintillator and large aperture photomultiplier tubes with state-of-the-art time and energy response.

The underground laboratory is located 1000 m below the summit of Mt. Ikenoyama, near the town of Toyama. The detector sits at the site of the old Kamiokande, the 3000-m3 water Cerenkov detector that played a leading role in the study of neutrinos produced via cosmic rays and also helped pioneer the subject of neutrino astronomy.

The KamLAND design consists of a series of concentric spherical shells (see figure 1). The primary detector target will be 1000 tons of ultrapure liquid scintillator located at the center of the detector. The liquid scintillator is a chemical cocktail of paraffin, 1,2,4-trimethylbenzene, and 2,5-diphenyloxazole. When particles or gamma rays ionize the scintillator, the liquid emits photons in the near-UV to green region of the spectrum. The scintillator will be housed in a 13-m-diameter spherical balloon made of three layers of nylon a total of 185 µm thick. This balloon system will hang inside the 18-m-diameter stainless-steel spherical vessel. A buffer mixture of normal and iso-paraffin oils will fill the volume between the stainless-steel vessel and the balloon. Its density is 0.1% to 0.3% lighter than that of the liquid scintillator to reduce the load of the balloon. A 4-m-diameter stainless-steel chimney provides access to the detector.

Figure 1. The KamLAND detector consists of a 13-m-diameter balloon filled with liquid scintillator and shielded by a buffer mixture of oils to suppress background radiation. An array of large-aperture PMTs detects the scintillation events triggered by neutrinos.

The entire inner surface of the vessel is covered by an array of 1879 photomultiplier tubes (PMTs; Hamamatsu Photonics K.K.; Shizuoka, Japan), 1325 of which are specially developed 17-in. PMTs (see figure 2) and 554 of which are the old Kamiokande 20-in. devices. The PMTs detect scintillation light generated within the balloon. The light transparency and radiopurity of the liquid scintillator, balloon, and buffer oil will be critical components of the detector, allowing scintillation light to reach the PMTs and reducing the background events coming from natural radioactivity.

Figure 2. An array of 17-in.-aperture photomultiplier tubes inside the 3000 m3 spherical tank operates on a box-and-line dynode structure.

This central detector stands in a cylindrical rock cavity. The volume between the sphere vessel and the cavity will be filled with approximately 2700 m3 of pure water in which 225 Kamiokande PMTs are placed to detect cosmic-ray muons by their Cerenkov light. This structure will function as a null counter.

powered by PMTs

The neutrino events of interest only generate enough light to produce a single photoelectron in each part of the central detector. The photomultiplier tubes must therefore offer high-sensitivity performance. The newly developed 17-in. PMT has the same shape and overall size as the 20-in. PMT, but the photosensitive area is restricted to a central region with a 17-in. diameter. This modification makes it possible to use a box-and-line dynode structure instead of the Venetian-blind dynode used in the 20-in. PMTs.

As a result, under conditions of single-photoelectron illumination at 25°C and with the applied high-voltage giving a gain of 107, the detectors offer a 1.3-ns transit-time spread (over 1 s); output pulse peak-to-valley ratio of 4; and a 10 kHz dark count-rate for signals above 1/4 photoelectron. Better than that, the 17-in. tubes show not only a linear response for up to 1000 photoelectron signal level, but also no saturation even at 10,000 photoelectron illumination. This allows us to study more physics associated with events that result in high-energy deposition inside the detector generated by atmospheric neutrinos, nucleon decays, and so on.

It is essential to calibrate the PMT timing, balance the gain of the PMTs, and determine the linearity of the PMT response during experimental runs. For these purposes, a light flasher system—a source of short light pulses having the same spectral profile as the scintillation light—is placed at different positions inside the balloon. The flasher also provides a tool to monitor scintillator transparency, and it is useful for commissioning and checking the detector readout.

In the flasher system, 3-ns UV light pulses from a nitrogen laser are injected into a bulb of scintillator that is viewed by the detector. The wavelength of the laser light falls within the absorption band of the scintillator; hence the light emitted has the same characteristics as the scintillator light. The laser light is transmitted to the scintillator bulb via quartz fiber optics, and its intensity is reduced by computer-controlled attenuator wheels. This bulb is used to emit light from within the balloon and thereby provide us with a controllable light source that originates in the same region as actual physics events.

The laser flash system is limited in rate to 20 Hz; therefore, we also install 30 blue LEDs, which are distributed uniformly on the inner surface of the 3000-m3 vessel. This LED system is sufficient for tasks in which accurate knowledge of light intensity is not required but which would benefit from higher repetition rates. These tasks include gain balancing the PMTs, debugging the detector readout, and monitoring single photoelectron gains.

searching for neutrinos

KamLAND belongs to a new generation of low-background and low-energy detectors built to study a wide range of science that spans particle physics, geophysics, and astrophysics. Many key questions concerning neutrino oscillations will be answered by studying the flux and energy spectra produced by anti-neutrinos from local Japanese commercial reactors. Neutrino oscillations mean one type of neutrino changes into a neutrino of another type, which is forbidden if both neutrinos are massless. Since the standard model for elementary particles requires massless neutrinos, the discovery of neutrino oscillations could completely change particle physics.

Most of the Japanese nuclear reactors are located within 140 to 200 km from the KamLAND site, so the KamLAND team expects the detector to be exposed to a large flux of low-energy anti-neutrinos and to be ideally suited to study neutrino oscillations with an extremely long baseline. For a detectable event, the anti-neutrino must be captured by a free proton while in the detector. The following reaction occurs: anti-neutrino + proton -> positron + neutron. The positron deposits its 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: neutron + proton -> deuteron + gamma ray (2.2 MeV). The neutron mean-thermalization time is 200 µs.

An anti-neutrino event has a clear signature with time delay between the positron signal (prompt) and gamma-ray signal from the neutron capture (delayed). It is imperative to suppress any signal mimicking a neutrino event. With its advanced design, the team expects KamLAND to detect about 700 anti-neutrinos every year, an enormous improvement over previous attempts from any other detector. KamLAND is expected to achieve the smallest neutrino mass that can be probed with any laboratory experiment in the foreseeable future.

The KamLAND team also hopes to shed some light on the solar neutrino puzzle by directly observing the beryllium-7 solar neutrinos. The sun is a typical main sequence star that generates its energy through hydrogen fusion into helium. The combined results of solar neutrino detections over the last 30 years suggest that solar neutrino fluxes are significantly lower that predicted by the standard solar model. One of the difficulties in observing the beryllium-7 neutrinos is reducing the detector's background radiation because the neutrino-electron scattering, which involves a single ionization event, is dominant in the energy region of the beryllium-7 solar neutrinos. The KamLAND design includes precautions to minimize background radiation, including surrounding the active scintillator volume with paraffin oil and using pure water in the null-counter to eliminate external gamma and neutron radiation. In the absence of background radiation, the characteristic signatures corresponding to the solar neutrino oscillation solution are expected through a very high event rate: 510 events/day/kton predicted by the standard solar model.

In addition to these investigations, KamLAND also is expected to yield terrestrial anti-neutrino (geoneutrino) data that will help geophysicists better understand the interior dynamics and the evolution of the Earth.

As of March 2001, almost all detector components had been constructed. Until September 2001, the KamLAND team will mix and purify the liquid scintillator, and then fill the central detector. The data-taking electronics will be installed this month. The goal is to start taking data by the end of September 2001. oe

Acknowledgments

The KamLAND project was developed by Atsuto Suzuki and his collaborators in 1994. The full project was funded by the Center of Excellence program (COE) of the Japanese Ministry of Education in spring 1997. American physicists expressed their interest in joining the KamLAND project and taking responsibility for some of the construction. The American KamLAND proposal was funded by the Department of Energy in October 1999.


Atsuto Suzuki

Atsuto Suzuki is a professor at the Research Center for Neutrino Science, Graduate School of Science, Tohoku University, Japan.