In the 1960s, satellites monitoring Soviet nuclear tests noticed huge, curious flashes of radiation. Rather than coming from the ground below, they were arriving from deep space. After decades of study, these rapid flashes of the most energetic form of radiation — known as gamma ray bursts or GRBs — remain one of the biggest mysteries of the cosmos. Scientists believe they could be powerful burps emanating from black holes after they lunched on a large star, or they could be caused by a handful of other extreme events taking place across the universe.
Three satellites have been launched since 2004 to study the events. The most recent, India’s Astrosat, carries specially designed and manufactured sensors from GE Healthcare that have made what the Indian government calls “mouth-watering” strides in unraveling the riddle of GRBs. In a twist, this same technology that can see the farthest and largest objects in the universe was originally developed to help doctors track down disease in the human body by monitoring objects as small as atoms and molecules.
This is achieved using sensors made from a semiconductor crystal, which is a compound of cadmium, zinc and tellurium (CZT). These CZT radiation detectors convert the gamma or X-ray radiation directly into electronic signals by collecting the electrons produced by the radiation absorption in CZT. The three elements are perfect for the job because they have a high atomic number, thus absorb more radiation to detect more protons.
Since the 1940s, scientists have been looking for a combination of materials to create detectors that have a high atomic number and perform well in everyday conditions. CZT sensors have long been known to occupy the sweet spots of sensitivity — energy resolution and intrinsic resolution — that are achieved at room temperature. A CZT detector is a single-photon-counting detector. This detector measures the radiation photon by photon and has a very dynamic range, starting with a few photons per second per millimeter square and ending with the processing of 10 million photons a second on every square millimeter of its shiny black surface.
Yet CZT sensors are fiendishly difficult to make. The crystals at the heart of the sensors typically take three months to grow, a complex process that can easily result in crystals too flawed and too fragile to use. GE Healthcare scientists, however, have found a way. They came up with various proprietary methods for growing the CZTs more quickly and reliably and built their own furnaces to do so.
Because the base materials are so expensive, GE Healthcare also figured out methods to recycle the material, either pre- or post-processing. GE found a way to identify at an early stage which crystals will develop flaws, allowing them to recycle the materials for new attempts without investing too much labor in this material. Alternatively, the leftover material used to fabricate detectors is also recycled to produce new detectors.
These novel advances in crystal-growth methods have resulted in GE’s ability to consistently produce CZTs crystals in 10 days with just one impurity per billion parts, says GE Healthcare’s chief scientist Arie Shahar and chief technologist Jeffrey Levy.
In healthcare, the superior sensitivity of GE-made CZT detectors allows doctors to use smaller doses of radiation on the patient, yet still achieve greater image contrast and resolution compared to traditional equipment. Until recently, nuclear medicine was dominated by systems that needed vacuum tubes and analog equipment to produce similar images. Using CZT detectors in medicine is like switching from a cassette tape to a digital audio recorder.
Direct conversion by digital CZT detectors enables higher count rates and higher energy resolution, says Noam Zilbershtain, general manager of digital CZT detectors at GE Healthcare. This results in improved image contrast, faster scanning and, often, lower doses of radiation, all in a smaller-sized sensor, he adds.
This flexibility has allowed CZT-based equipment to dominate the market for cardiac imaging systems in just a few years, commanding a 90 percent share today. GE Healthcare expects a similar effect in the general medical imaging market, where analog systems still have about 80 percent market share.
Space scientists in India, looking for a better tool for detecting bursts of atomic radiation, approached GE Healthcare. Producing a CZT detector for space presented a few challenges. To cover enough of the sky, Astrosat required a much larger CZT sensor than the palm-sized medical ones. The solution: Link 64 CZT sensors together in four quadrants, similar to how a logic board in a personal computer links multiple processors. This design enables Astrosat to detect GRBs from one-third of space at any time. And CZTs work best at room temperature, so a heating and cooling system was added to operate the sensor close to optimal temperatures in orbit.
The result: a box 19 inches deep, 19 inches high and 24 inches tall that weighs 123 pounds. Despite its size, the resulting system saved precious real estate on the satellite compared to other sensor options India was considering, Zilbershtain says. And these CZT detectors in space are the same highly reliable ones used for medical imaging as well.
The work paid off quickly. On the first day Astrosat activated its CZT system, it detected and measured a powerful GRB from the Crab Nebula, a feat unmatched by the other GRB satellites — the Fermi and the Swift, operated by the European Union and NASA. Those two are quick to locate generally where GRBs originate and are well-suited to examine lower-energy GRBs. Adding Astrosat allows for observations to pinpointing emission sources and the ability to fully measure the top range of energy bursts.
The Crab Nebula GRB was a thrilling proof of the CZT detector’s benefits, which “will have far-reaching implications in the understanding of the radiation mechanisms of GRBs,” according to a report from India’s space authority.
Besides hungry black holes and colliding neutron stars, an emerging alternate theory posits that GRBs could also come from neutron stars called magnetars, which have exceptionally powerful magnetic fields. GE and India’s space scientists are testing a next-generation CZT sensor for a planned new satellite to find out if any of these are correct, potentially providing insight into the formation of the universe shortly after the Big Bang.