Neutrinos and antineutrinos are elusive particles. With no charge and almost no mass, they rarely react with other matter, even as they zip through the Earth (and us) at nearly the speed of light. Recording the traces of rare neutrino interactions requires specialized, large-volume detectors, and finding those few traces among the mass of collected data is a little like listening for a whisper at a rock concert.

But the same qualities that make neutrinos elusive also make them valuable tools for probing environments, like the core of the Earth, that resist penetration by more common observational techniques.

So when an international group of researchers reported the first observation of geologically produced antineutrinos in a paper10 that was featured on the cover of Nature (Figure 14), the geological community responded with enthusiasm. This discovery is expected to lead to better estimates of the abundance and distribution of radioactive elements in the Earth, and of the Earth’s overall heat budget.

Figure 14. The July 28, 2005 issue of Nature reported that electron antineutrinos emanating from the earth (geoneutrinos) may serve as a unique window into the interior of our planet, revealing information that is hidden from other probes. The left half of the cover image shows the production distribution for the geoneutrinos detected at KamLAND, and the right half shows the geologic structure.

The geoneutrinos were detected at the Kamioka Liquid Scintillator Anti-Neutrino Detector in Japan, better known as KamLAND. Located in a mine cavern beneath the mountains of Japan’s main island of Honshu, near the city of Toyama, KamLAND is the largest low-energy antineutrino detector ever built. It consists of a weather balloon 13 meters (43 feet) in diameter, filled with about a kiloton of liquid scintillator, a chemical soup that emits flashes of light when an incoming antineutrino collides with a proton. These light flashes are detected by a surrounding array of 1,879 photomultiplier light sensors which convert the flashes into electronic signals that computers can analyze.

Most of the geoneutrino data produced at KamLAND was stored on the High Performance Storage System (HPSS) at NERSC and analyzed using NERSC’s PDSF cluster. Together, these systems allowed scientists to find the scientific equivalent of a whisper at a rock concert.

KamLAND records data 24 hours a day, seven days a week. This data is shipped on tapes from the experimental site to Berkeley Lab, where it is read off the tapes and stored in the HPSS. KamLAND records about 200 GB of data each day, and HPSS currently has more than 250 TB of KamLAND data stored, making KamLAND the second-largest user of NERSC’s HPSS system.

During dedicated production periods at NERSC, the KamLAND data are read out of HPSS and run through reconstruction software to convert the waveforms (essentially oscilloscope traces) from the photomultiplier tubes into physically meaningful quantities such as energy and position of the event inside the detector. This reduces the data volume by a factor of 60–100, and the reconstructed events are stored on disk for further analysis.

“The event reconstruction requires a lot of computing power, and with over 600 CPUs, PDSF is a great facility to run this kind of analysis,” said Patrick Decowski, a Berkeley Lab physicist who works with NERSC staff on the project. “PDSF has been essential for our measurements.”

With the data on disk, specialized analysis programs run over the reconstructed events to extract the geoneutrino events and perform the final analysis. PDSF is also used for various simulation tasks in order to better understand the background signals in the detector.

“The whole analysis is like looking for a needle in a haystack—out of more than 2 billion events, only 152 candidates were found,” Decowski said. “And of these, about 128 are background events.”

Forty years ago, the late John Bahcall proposed the study of neutrinos coming from the sun to understand the fusion processes inside the sun. The measurement of a persistent deficit of the observed neutrino flux relative to Bahcall’s calculations led to the 2002 Nobel Prize for Ray Davis and the discovery of neutrino oscillation, a quantum mechanical phenomenon whereby a neutrino created with a specific lepton “flavor” (electron, muon, or tau) can later be measured to have a different flavor. KamLAND provided the most direct evidence of neutrino oscillation in 2004.

Today, antineutrinos are being used to study the interior of the Earth, which is still little known. The deepest borehole ever drilled is less than 20 km in depth, while the radius of the Earth is more than 6000 km. While seismic events have been used to deduce the interior composition of the Earth’s three basic regions—the core, the mantle and the crust—there are no direct measurements of the chemical makeup of the deeper regions.

An important observation for understanding the Earth is the measurement of the heat coming from within (Figure 15). These measurements show that the Earth produces somewhere between 30 and 45 terawatts of heat (1 TW is 10¹² watts; the range depends on certain model assumptions). Two important sources of heat generation are the primordial energy released from planetary accretion and latent heat from core solidification. However, it is believed that heat produced by radioactivity also plays an important role in the Earth’s heat balance, contributing perhaps half of the total heat.

Figure 15. Earth’s conductive heat flow is estimated to be about 30–45 terawatts. Radioactivity is known to account for perhaps half of this heat, but there has been no accurate way to measure radiogenic heat production. (Image from H. N. Pollack, S. J. Hurter, and J. R. Johnson, Reviews of Geophysics 31, 267 [1993])

Neutrinos can help researchers understand the Earth’s internal structure and heat generation. Three important isotopes that are part of current Earth models—potassium, uranium, and thorium—produce electron antineutrinos (the so-called geoneutrinos) in their radioactive decay.

“KamLAND is the first detector sensitive enough to measure geoneutrinos produced in the earth from the decay of uranium-238 and thorium-232,” said Stuart Freedman, a nuclear physicist with a joint appointment at Berkeley Lab and the University of California at Berkeley, who is a co-spokesperson for the U.S. team at KamLAND. “Since the geoneutrinos produced from the decay chains of these isotopes have exceedingly small interaction cross sections, they propagate undisturbed in the Earth’s interior, and their measurement near the Earth’s surface can be used to gain information on their sources.”

In measuring geoneutrinos generated in the decay of natural radioactive elements in the earth’s interior, scientists believe it should be possible to get a three-dimensional picture of the earth’s composition and shell structure. This could provide answers to such questions as how much terrestrial heat comes from radioactive decays and how much is a primordial remnant from the birth of our planet. It might also help identify the source of Earth’s magnetic field and what drives the geodynamo.

The research is a multinational effort, as shown by the fact that the Nature article represented the work of 87 authors from 14 institutions spread across four nations.

 

Research funding: NP, JAMSTEC
Computational resources: NERSC
This article written by: John Hules, Jon Bashor, Lynn Yarris