Photonic radiation sensors survive huge doses intact

Photonic sensors transmit information in the form of light instead of electrical currents in the wires. They can measure, transmit and manipulate photon fluxes, usually through optical fibers, and are used to measure pressure, temperature, distance, magnetic fields, environmental conditions, etc.

They are attractive because of their small size, low energy consumption and tolerance to environmental variables such as mechanical vibrations. But the general consensus is that high radiation levels would alter the optical properties of their silicon, resulting in incorrect readings.

NIST, long a world leader in many areas of photonics research, has therefore launched a program to answer these questions. The test results indicate that the sensors could be customized to measure the radiation dose in industrial applications and clinical radiotherapy. The results of his first series of tests are reported in Scientific reports on nature.

The NIST results suggest in particular that the sensors could be used to track the levels of ionizing radiation (enough energy to change the structure of the atoms) used in the irradiation of food to destroy microbes and in the sterilization of medical devices, a market estimated at $ 7 billion a year in the United States alone. Sensors also have potential applications in medical imaging and therapy, whose total value worldwide is expected to reach nearly $ 50 billion by 2022.

"When we looked at publications on the subject, different labs had radically different results," said project scientist Zeeshan Ahmed, who is part of the NIST Photonics Dosimetry Project and leads the state's leading photon thermometry project. NIST. "It was our main motivation to make our experience."

"The growing interest in deploying photonic sensors capable of operating accurately in very hostile environments, such as those near nuclear reactors, is particularly motivated," Ahmed said. "In addition, the space industry needs to know how these devices would work in environments highly exposed to radiation," said project scientist Ronald Tosh. "Will they be damaged or not? What this study shows, is that for a certain class of devices and radiation, the damage is negligible."

"We discovered that oxide-coated silicon photonic devices could withstand radiation exposure of up to 1 million gray," said Ryan Fitzgerald, head of the photonics dosimetry project, using the SI unit for the absorbed radiation. A gray represents a joule of energy absorbed by a kilogram of mass and a gray corresponds to 10,000 radiographs of the lungs. That's pretty much what a sensor would receive in a nuclear power plant.

"This is the upper limit of our customers' interest in calibration," Fitzgerald said. "It can therefore be assumed that devices operate reliably at industrial or medical radiation levels that are hundreds or thousands of times lower." The irradiation of foods, for example, ranges from a few hundred to a few thousand gray. It is usually controlled by its effects on alanine pellets, an amino acid that alters its atomic properties when it is exposed to ionizing radiation.

To determine the effects of radiation, NIST researchers exposed two types of silicon photonic sensors at hours of gamma radiation emitted by cobalt-60, a radioactive isotope. In both types of sensors, small variations in their physical properties alter the wavelength of the light passing through them. By measuring these changes, the devices can be used as very sensitive thermometers or strain gauges. This remains true in extreme environments such as spaceflight or nuclear reactors, only if they continue to function properly under ionizing radiation exposure.

"Our results show that these photonic devices are robust in even extreme radiation environments, suggesting that they could also be used to measure radiation via its effects on the physical properties of irradiated devices," Fitzgerald said. "This should be good news for the manufacturing sector in the United States, concerned with serving the growing and growing market that allows accurate radiation at very small scale lengths." Photonic sensors could then be developed to measure low energy electron and X-ray beams used sterilization of medical devices and irradiation of food. "

They will also be of great interest for clinical medicine, in which physicians strive to treat cancers and other conditions with the lowest effective levels of radiation, focused on the smallest dimensions to avoid affect healthy tissue, including electron, proton and ion beams. To achieve this goal, radiation sensors with extremely high sensitivity and spatial resolution are required. "Ultimately, we hope to develop scale-based devices for industrial and medical applications that can determine absorbed dose gradients over distances ranging from a few micrometers to an unprecedented level of detail," said the scientist. Nikolai Klimov project. A micrometer is a millionth of a meter. A human hair has a width of about 100 micrometers.

The team 's findings could have significant repercussions on new medical therapies using proton beams or extremely narrow carbon ions and on medical sterilization processes using electron beams. low energy. "Our sensors are naturally small and scaled down," Fitzgerald said. "The current dosimeters are of the order of a millimeter to one centimeter, which can give erroneous readings for fields varying in these dimensions."

In the next stage of the research, the team will simultaneously test sensor arrays under identical conditions to determine if dose variations over short distances can be resolved.

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More information:
Zeeshan Ahmed et al, Evaluation of the radiation hardness of silicon photonic sensors, Scientific reports (2018). DOI: 10.1038 / s41598-018-31286-9