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Posted March 22, 1999

New Gamma Ray Detector Used to Probe Maine's Environmental Secrets

It's a project combining the latest high technology gear and creative backyard invention. Students and faculty in the departments of geology and physics have installed a new $25,000 gamma ray detector on the University of Maine campus to help them unravel secrets related to Maine's environment.

Gamma rays are naturally occurring, high energy particles which are given off during the decay of radioactive elements. They can provide important clues about natural processes such as the layering of sediments in lakes, atmospheric deposition of mercury and even weather patterns.

The university purchased the device with research and development funds approved by the State Legislature in 1998 and a grant for mercury research from the U.S. Geological Survey through the UMaine Water Research Institute. The Maine Department of Environmental Protection also contributed funds to that research project.

In the past, collaborative work by UMaine geologists and physicists has started a national dialogue on radon gas concentrations in homes and revealed how much mercury has fallen on Maine from the air over the past century. New research is refining the mercury work and also focuses on another element, berylium. This element exists naturally in the Earth's crust, and a radioactive version is formed under conditions high in the atmosphere. Known as Berylium-7, it rains down daily on the Earth in minute quantities.

Located in the Department of Physics and Astronomy in Bennett Hall, the new detector is considerably more sensitive than its predecessor. Its heart consists of a pure germanium crystal which emits an electrical signal whenever it absorbs a particle of radioactivity. It is cooled with liquid nitrogen to a temperature of about –197 degrees Celsius or –322 degrees Fahrenheit. The crystal is housed inside a university-made lead shield designed to keep background radioactivity to a minimum.

Faculty involved in the work include Tom Hess in physics and Steve Norton in geological sciences. The students include Jim Kaste, a master's candidate in geology from Long Island; Joel Ngue, a sophomore in physics, born in Bangor and raised in Camaroon, West Africa; Chris Higgins, a sophomore in physics from Scarborough; and Ian Brasslett, a senior in physics from Medway.

The detector was built by the Canberra Corporation of Meridian, Connecticut and arrived at UMaine in January. With Thomas Tripp, chief machinist in physics, Hess and some of the students built the lead shield and lined it with thin sheets of copper and cadmium. The metals absorb background radioactivity which might otherwise interfere with sample analyses. Commercially available shields cost $10,000 to $12,000, says Hess.

Hess, Tripp and the students also assembled a package of electronic equipment to record and amplify signals from the crystal. The data are sent to a computer which displays counts of radioactivity at various frequencies.

Since January, Ngue has been calibrating the machine by analyzing samples with known amounts of gamma ray activity. That work provides the basis for interpreting data from environmental samples.

“We measure a radioactive isotope of lead because it can tell us how long something has been in the ground,” says Higgins. “About half of the Lead-210 that falls on the soil or on a lake or wetland decays every 20 years. Older layers of soil or peat have less radioactivity from lead-210 than the upper layers.”

Higgins works on a project to analyze the mercury content of layers of soil and peat. Data from lead-210 analyses provide crucial evidence to help determine when the mercury in those layers was deposited. Researchers can infer the sources of mercury from historical records of activities such as the burning of coal and solid waste, industrial discharges and past applications of pesticides containing mercury.

Kaste is working on a project to determine how much berylium-7 falls on the earth, where it accumulates and how much is transferred from rocks and soil into vegetation. “Berylium is one of the most toxic elements on the periodic table, but there is so little of it that it doesn't affect our health at all. Because berylium-7 is created high in the atmosphere, it can be used as an indicator of weather patterns and natural background radiation.”

In a collaborative project with scientists in The Czech Republic, Kaste, Norton and Hess are studying how berylium is transported through the environment and where it accumulates. With funding from the National Science Foundation, the three researchers will travel to Prague in May.

The behavior of berylium in the atmosphere is the subject of a senior project by Brasslett. He is investigating the possibility that berylium-7 can provide scientists with clues about weather patterns and the mixing of air masses.

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