By Bill Clayton

Gamma-ray spectroscopy -- the measurement of the wavelengths and energies of gamma rays -- has come a long way. Whereas it was once a mere curiosity, it has become a science with several practical and important applications in medical imaging and in matters of security.

Throughout most of the time that researchers did their work, the bulk of the work revolved around two types of instruments: scintillation and semiconductor gamma-ray spectrometers that require cooling in liquid nitrogen. However, in recent years the focus of this research shifted to the development of position-sensitive high-purity semiconductor detectors that can be operated at room temperature. The results have been encouraging, and the potential applications have stimulated additional research and attracted interest because of their relevance to issues such as security and medical imaging.

Advantages and Limitations -- the Scintillation Spectrometer

In order to understand the advantages of the three-dimensional position-sensitive spectrometer, it’s necessary to understand the scintillation gamma-ray spectrometer. The latter device uses transparent crystals that, in interaction with gamma rays, emit a flash. A photon sensor observes this flash, and the electronics of the device amplify the photon signal in order to determine the energy of the gamma rays.

It’s a simple, inexpensive system. However, the electronics of the system are bulky and, more importantly, the scintillation gamma-ray spectrometer performs poorly -- it picks up only about 10 percent of the available information.

Advantages of the 3-D Position-Sensitive Spectrometer

The three-dimensional position-sensitive gamma-ray spectrometer depends on a semiconductor crystal composed of cadmium, zinc and teluride (CdZnTe) -- a substance currently worth more than gold. However, these crystals have characteristics that make them even more valuable to researchers working in the world of gamma-ray spectroscopy. Specifically, the value of these crystals lies in their three-dimensional qualities, which are needed to render spatial coordinates. They also have the mass and depth to provide the level of “stopping power” that makes the detectors highly efficient in the measurement of gamma-ray energy.

Shown here are two CdZnTe detectors and the readout ASIC electronics.

When gamma rays penetrate the CdZnTe crystal, the interaction generates a large number of electrons. The electrons collect on one of the anodes and produce a signal. A charge-sensing amplifier connected to the anode measures this pulse of electricity to determine the energy of gamma rays and their lateral position. Then, by combining that data with information gathered by the corresponding cathode, investigators can determine the depth of penetration.

An additional -- and highly important -- advantage of the three-dimensional position-sensitive CdZnTe gamma-ray spectrometer is that it operates at room temperature.

The 3-D Position-Sensitive Gamma-Ray Detector, Today

With funding from the Department of Energy, Assistant Professor Zhong He, Nuclear Engineering and Radiological Sciences, and his team became the first to build a three-dimensional position-sensitive semiconductor gamma-ray detector.

Thanks in great part to their achievements, today’s three-dimensional position-sensitive CdZnTe gamma-ray spectrometer detects most of the available gamma-ray information and makes more efficient use of it. However, the cost of the CdZnTe crystal and the highly sophisticated electronics has, until recently, been prohibitive. New methods of growing crystal and the ability to miniaturize circuitry have decreased these costs significantly.

The HgI2 semiconductor gamma-ray detector depends on its CdZnTe (semiconductor) crystal. The dimensions of the crystal in this detector are 1 cm x 1 cm x 1 cm.

Professor He said that the development of this device “will enable scientists to build imaging systems that are more sensitive than conventional gamma-ray detectors.”

He went on to explain that the detector’s “sensitivity to gamma rays and its ability to position the sources of these gamma rays helps astronomers to study the evolution of the universe.” Medical personnel see position-sensitive gamma-ray detection as a key to developing more accurate ways of imaging structures within the body. And the military sees the device as a monumental step forward for security in which radiation is an issue -- a current prototype not only detects radiation and locates its source but will soon become a hand-held instrument, which will make it easy for military personnel to use in the field.