Electron Paramagnetic Resonance (EPR) finds Applications in the field of dose determination for irradiation-Food Irradiation Testing and Occupational Disease Prevention

November 29, 2024

With the widespread use of gamma rays in industries, agriculture, medicine, and food, accurate measurement of radiation dose has become increasingly important. EPR Spectrometer is currently the only direct method to detect unpaired electrons in a sample, allowing for the precise measurement of radiation dose by detecting the free radicals generated in the irradiated material.

The radiation dose can be categorized into low dose (less than 1 kGy), medium dose (1-10 kGy), and high dose (greater than 10 kGy), and its effects can range from no clinical symptoms to severe symptoms, early fatal clinical symptoms, and early death.

After decades of research, various chemical, physical, and biological methods have been developed for radiation dose measurement, including multi-modal products with photoacoustic indicators. With the development of molecular biology, it has been recognized that certain biological molecules, such as chromosomes, are sensitive to radiation and can be used to measure the radiation dose. However, at high radiation doses, the inactivation of biological molecules can hinder the detection process, and biological dosimeters based on this principle require longer sample processing and analysis times.

When a material is irradiated by various radiation or neutrons, it generates free radicals. Therefore, using Electron Paramagnetic Resonance (EPR) spectroscopy to detect the free radicals generated in the irradiated material is a direct and convenient method. Dose meters designed based on EPR for this purpose are called EPR dosimeters, which have unique advantages compared to other dosimeters:

High sensitivity to detect clinically significant dose levels
Provides highly specific and reliable data with sufficient accuracy
Wide range of coverage suitable for fast detection
Can work in various environments
Non-invasive and non-destructive to the sample
Specialized instruments that are easy to operate

Case 1: Food Irradiation Testing

Food irradiation is the process of using radiation to delay certain physiological processes (such as sprouting and ripening) in fresh food or to treat food for purposes such as insect control, disinfection, sterilization, and mold prevention, thereby extending its shelf life and stabilizing and improving its quality.

Various foods, including meats, bones, fruits, dried fruits, and food, produce detectable EPR signals of free radicals when irradiated. The intensity of the free radical signal is related to the nature of the various materials and processing methods, particularly the radiation dose. EPR technology is the most direct method for detecting free radicals.

Figure 1 shows the EPR spectra of a certain brand of powdered milk before and after irradiation, with radiation doses of 0 kGy, 2.0 kGy, 4.0 kGy, 6.0 kGy, and 8.0 kGy. It can be observed from the figure that there is almost no detectable EPR signal before irradiation, but after irradiation, a clear free radical signal with a g-value near 2.0 appears, and its intensity increases linearly with the dose.

Figure 1: EPR spectra of powdered milk at irradiation doses of 0 kGy, 2 kGy, 4 kGy, 6 kGy, and 8 kGy

Case 2: Occupational Disease Prevention and Treatment

Occupational disease prevention and treatment are of great significance in assessing the health status of personnel working around nuclear facilities. In the early 1980s, the International Atomic Energy Agency (IAEA) selected alanine as a dosimeter for high-dose irradiation and standardized the alanine-EPR measurement system.

Alanine can form stable free radicals after ionizing irradiation, and its EPR spectrum shows a five-line peak with a center g value of approximately 2. The intensity of the EPR signal can be represented by the signal's amplitude or the area under the second integral curve. Technically, alanine reagent powder can be packaged in small bags or capsules to make powder dosimeters, or a mixture of alanine, binders, and lubricants can be made into solid alanine dosimeters using processes such as compression, casting, or extrusion. These dosimeters can be taken to the field along with the personnel and, after leaving the field, the samples can be tested in an EPR spectrometer. The radiation dose received by the personnel can be analyzed by comparing the detected EPR signal intensity with a calibration curve.

Figure 2: EPR spectrum of alanine dosimeter irradiated at an absorbed dose of 33.7 kGy (left);

Dose-response calibration curve of alanine(right)

Case 3: Radiation Dose Reconstruction

Radiation dose reconstruction is essential in the event of accidental ionizing radiation exposure due to nuclear war or nuclear accidents, as it helps determine the level of nuclear radiation, develop emergency treatment plans, and conduct subsequent accident investigations.

EPR technology plays a significant role in radiation dose reconstruction, especially in assessing the radiation dose received by individuals as a result of radiation exposure. EPR can be used to analyze radiation-induced free radicals in biological tissue samples such as tooth enamel, nails, and hair. These free radicals are formed in biological tissues after ionizing radiation, and their concentration is proportional to the radiation dose received. Therefore, by measuring the concentration of these free radicals, it is possible to retrospectively estimate the radiation dose received by an individual.