Practical, Hand-Held Radiation Detector Developed
By Allan Chen, A_Chen@lbl.gov November 4, 1996

BERKELEY, CA -- A researcher at the Ernest Orlando Lawrence Berkeley National Laboratory has developed a new method for improving the performance of nuclear radiation detectors.

Paul Luke, a staff scientist in Berkeley Lab's Engineering Division, developed the new method that allows radiation detectors operating at room temperature to achieve levels of performance approaching that of liquid-nitrogen-cooled detectors. By eliminating the need for cooling, smaller, more portable and less expensive detector systems can be produced.

These new detectors are expected to have widespread application in areas that require high performance detectors but where the technology needed for refrigeration is undesirable.

A large class of radiation detectors works by sensing the ionization produced by radiation. A common detector configuration consists of a volume of detecting medium, which can be a solid, liquid or gas, sandwiched by two plane electrodes. A voltage is applied between the two electrodes. Incoming radiation strikes the detecting medium, loosening positive and negative electric charges which travel to the electrodes and register a charge signal.

"A number of materials can be used as the detecting medium but, except for a very few, namely germanium and silicon, the charge collection process is far from perfect. Often, the positive charges are not as efficiently collected as the negative ones." says Luke. "This results in an inaccurate reading of the ionization and it means that you can't rely on the signal's strength to tell you the energy of the radiation. In other words, the energy resolution is poor."

Luke's improved detector uses an arrangement of parallel strip electrodes, and a technique called "charge subtraction" to provide a much more accurate reading of the energy of radiation. The parallel strips are inter-connected to form two sets of interdigital electrodes. Charge signals induced on these two electrodes are subtracted to yield a net signal that is insensitive to charge trapping. As a result, energy resolution is greatly improved.

This technique can be applied to wide band-gap compound semiconductors such as cadmium telluride, cadmium zinc telluride and mercuric iodide. These materials offer advantages over silicon and germanium, which are used in existing high performance radiation detectors.

"The problem with germanium and silicon is that you need to cool them down to liquid nitrogen temperatures to use them as high-resolution radiation detectors," says Luke, "whereas the compound semiconductors, because of the wider band-gaps, can be operated at room temperature."

For the past twenty years, researchers have attempted to use compound semiconductors in radiation detectors, but because of poor charge collection in these materials, the detector performance was not satisfactory for many applications. The new technique largely overcomes this problem and could allow these detectors to detect radiation such as X rays and gamma rays with energy resolution close to that of silicon and germanium detectors.

"This opens up a set of new applications where good energy resolution is desired but providing liquid-nitrogen cooling to the detectors is not practical-for example hand-held instruments used in the field." says Luke. "Many areas of applications can benefit from the availability of room-temperature high-resolution detectors, such as medical diagnostics, nuclear safeguards, nuclear physics, balloon- or space- based gamma-ray astrophysics, environmental monitoring and industrial sensing."

The second unique feature of the invention is its use of induced charge signals to determine the depth of radiation interaction in the detector. By measuring the difference between the total charge induced by a particle of radiation and the charge induced at one of the two grid electrodes, it is possible to determine where the ionization originates along the direction perpendicular to the electrode planes. This can be used, in conjunction with shadow masks or X-ray optics, to determine the direction of incoming radiation, thus providing imaging capability.

Berkeley Lab's Patent Department has already licensed the invention to one company, and they are having talks with several other prospective licensees.

The U.S. Patent Office awarded the University of California patent number 5,530,249 for Luke's invention.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified research and is managed by the University of California.

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