The University of Delaware was recently honored by the government of Thailand for providing equipment and technical expertise for the construction of a neutron monitor on the summit of Doi Inthanon, Thailand's highest mountain. The construction of the new cosmic ray detector was a collaborative effort between UD, three Thai institutions--Mahidol University, Chulalongkorn University and Ubon Ratchathani University--and Shinshu University in Japan.

During the official dedication of the Princess Sirindhorn Neutron Monitor on Jan. 21, Princess Maha Chakri Sirindhorn, daughter of Thailand's King Bhumibol Adulyadej, presented plaques from the three participating Thai universities to John Bieber, UD Professor of Physics and Astronomy, in honor of the Department of Physics and Astronomy's Bartol Research Institute.

Shinshu and UD both donated surplus equipment to the project, while UD additionally covered shipping costs of the equipment under a National Science Foundation award. UD experts--Bieber, Paul Evenson, Professor of Physics and Astronomy, and staff members Leonard Shulman, James Roth, Chris Elliott and Andrew McDermott--provided technical advice.

The Bartol Research Institute has a long history, since the 1950s, of operating neutron monitors for scientific research. UD currently operates eight neutron monitors of its own, more than any other institution in the world, Bieber said.

“A neutron monitor is a very large instrument weighing more than 36 tons,” Bieber said. “The large size is required because the intensity of cosmic rays--energetic particles from outer space that continuously bombard Earth--is very low at the energies we are observing, and you need a large detector volume to measure them. This also implies that it is impractical to fly these instruments into space, so that ground-based measurements are the usual method for studying cosmic rays at very high energy.

“The neutrons that a neutron monitor actually measures are so-called 'secondary' particles that are created when a primary cosmic ray from outer space smashes into an air molecule in Earth's atmosphere. The primary cosmic rays that cause this are mostly charged particles such as protons and helium nuclei. So we are observing the charged primary cosmic rays indirectly by measuring their secondary neutrons,” Bieber said.

The Princess Sirindhorn neutron monitor has two main components: a central detector tube filled with boron trifluoride gas and an outer superstructure made of lead and polyethylene. The boron trifluoride is enriched in an isotope of boron that has a large cross-section for capturing a neutron. When a neutron is captured, a nuclear reaction occurs that deposits energy in the gas, and this is converted to an electrical signal that is recorded. The function of the outer superstructure of lead and polyethylene is to amplify the intensity of the cosmic neutrons while simultaneously shielding out environmental neutrons from non-cosmic sources such as natural radioactivity.

Bieber said the site in Thailand was chosen because it is where Earth's magnetic field is most effective at repelling cosmic rays. For that reason, cosmic rays that do penetrate to Earth's surface are the highest energy component that can be measured by a neutron monitor. Mountaintop locations are favored for neutron monitors because the cosmic ray intensity is greater at high altitude.

“The sponsorship of the neutron monitor by a member of the Thai royal family is very significant, and it played an important role in making the project succeed,” Bieber said. “Princess Sirindhorn is known to take a strong interest in science. The participation by UD and the recognition received will strengthen scientific collaborations between UD and the participating Thai institutions, which we expect to continue into the foreseeable future.”

Article by Martin Mbugua