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Portable, Palm-Size Radiation Detectors

A new device, with sensors the size of human cells, can measure, record, and assess the risk of radiation emissions in real time.

Researchers at the National Space Biomedical Research Institute (NSBRI) and the U.S. Naval Academy (USNA) have developed a novel radiation detector to be used during space missions, particularly those to the Moon and Mars, where energy levels are dangerous and approximate doses are estimated. The device, called a microdosimeter, is small and low-powered, and it can measure atmospheric radiation levels in real time.

Radiation exposed: The microdosimeter instrument consists of a sensor made up of tiny cells, each one called a single-diode detector, that measure 10 by 10 by 2 microns–the size of a red blood cell. The cells measure the level of radiation in the atmosphere by the amount of tiny energy particles deposited into each cell. The data they collect can be recorded and assessed in real time.

“We are really taking existing technology and pushing it to new limits so that we can apply it where it has never really been applied before,” says Vince Pisacane, a researcher on the NSBRI Technology Development Team, a professor of aerospace engineering at the USNA, and the principal investigator on this project. By using a silicon device of his team’s own making as a basic sensor, Pisacane hopes to achieve the type of accuracy needed to make estimates of the radiation exposure of humans in space. “It is really critical [to human health that] it be as precise as possible,” he says.

Since the Apollo missions, NASA has flown a variety of radiation detectors on every mission; most of these detectors have been based on one piece of hardware: a dosimeter. This device, still the most accurate instrument used by people regularly exposed to radiation in their work, measures the total accumulated amount of radiation exposure and can take the form of a badge, a pen-size tube, or a digital readout. But the device, while very durable and portable, provides measurements of radiation exposure only after the fact, so the doses of radiation that astronauts are receiving while in space aren’t known until they return to Earth.

To the degree that space exploration involves manned missions, the need for better radiation detection is acute. In just a day or two on the lunar surface, astronauts can receive up to 600 times the amount of radiation a person on Earth receives in a year, explains Ann Kennedy, Richard Chamberlain Professor of Research Oncology and a professor in the Department of Radiation Oncology, University of Pennsylvania School of Medicine. “Of most concern is a solar-flare or solar-particle event that can occur without warning from the sun emitting particles at high volume, leading to high doses for astronauts,” she explains. The effects of exposure to extreme radiation can be severe: vomiting, erythema (skin reddening), cancer, leukemia, and even death.

To build a tool that can help astronauts avoid such effects, Pisacane’s team used the central idea of a dosimeter–that is, measuring the total amount of radiation exposure–but it’s measuring not just the cumulative amount of radiation that the body receives: it’s also measuring the cumulative amount that each cell in the body receives. By studying radiation on a microscopic scale, the researchers hope to better understand the cellular effects of radiation.

The microdosimeter, which is about the size of a package of cigarettes, contains an array of cells made out of silicon, each one typically the size of a red blood cell and arranged on an electronic board like the squares on a checkerboard. Each cell continuously records the amount of small energy particles being deposited. Some particles will deposit more energy and different types than others. From looking at this data, researchers can create an energy spectrum that will allow them to gauge the range of energies and the values that could be deposited within the human body.

Furthermore, the system is what Pisacane calls “active” and can take real-time measurements of radiation levels, alerting astronauts immediately if they are at risk. Spacesuits and spacecraft equipped with the microdosimeter sensors could help enable astronauts to take protective action at the onset of enhanced radiation.

But before the device is ready for manned missions, it will be tested on numerous satellites over the course of about five years. Pisacane hopes that with each trip, the device–which is powered by AA batteries and already uses only one watt of power when continuously collecting data–will become even smaller, using less power with increased reliability. The microdosimeter will make its first trip to space on March 8, when it will go up on STP-1, the launch vehicle, as an experiment on the MidSTAR satellite built by the USNA. The satellite will contain three microdosimeter sensors, one outside and two inside, one of which will be coated with polyethylene, a substance whose permeability is similar to that of human tissue, and thus can simulate the effects of radiation on the human body. All three sensors will be connected to an electronic output module that will collect and store data for transmission to the ground.

The central challenge in creating one of these devices is to make it accurate. “There are a lot of elements that go into making it work,” says Pisacane, “and all this has to be designed, parts have to be manufactured, we have to identify electrical components and get them on the board–some of them are so small you can hardly see them.”

Indeed, the things you have to do in order to develop very small, low-mass, and low-power-consumption devices for space flight are exceptionally tricky, explains Cary Zeitlin, a staff scientist at the Lawrence Berkeley National Laboratory and former principal investigator of the Martian Radiation Environment Experiment (MARIE). The MARIE project, which was funded by NASA, built a particle telescope to measure radiation levels on Mars and was sent aboard the Odyssey for testing in 2001. Although the technology suffered from hardware difficulties after a large solar event, it was able to gather dosage information and was a beginning step in detection efforts on Mars. “Pisacane’s group is doing a variation on the standard type of dosimeter, and it’s a new way of measuring radiation doses that I think is a novel application,” says Zeitlin.

While extremely important to manned space missions, the microdosimeter is meant to have Earth-based applications as well. It can help people who work with nuclear power, with nuclear compounds, in medical and industrial applications, and in those areas in which it is important to know the levels of radiation, says Jay Buckey, team lead for the NSBRI Technology Development Team and professor of medicine at Dartmouth Medical School. “This technology is an excellent and very worthwhile way to track radiation exposure and an improvement of what we have now,” he says.

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