Sources of radioactive contamination of food

作者：admin  点击次数：118  发布时间：2025-04-23

(I) Overview of radionuclides
Nuclides are a type of atoms or nuclei with a certain number of protons and neutrons. Those with the same number of protons but different numbers of neutrons are called isotopes. Nuclides that can emit radiation are called radionuclides or radioisotopes. The phenomenon of radionuclides releasing radiation is called nuclide decay or disintegration. Decay is a process in which an atomic nucleus transforms into another atomic nucleus. The time required for the number of nuclei in a specific energy state to decrease by half is called the half-life of the nuclide. Different radionuclides have different half-lives. For example, the half-life of Bi (bismuth) is as long as 2.7x10" years, while the half-life of Cs (cesium) is only 2.8x10-s. Since radionuclides with long half-lives exist for a long time in food and the human body, from a safety perspective, attention should be paid to the contamination of food by radionuclides with long half-lives. The radiation released by radionuclides that can ionize matter is called ionizing radiation. Ionizing radiation includes alpha rays, B rays, gamma rays, and x rays. Alpha rays are positively charged, have strong ionization ability, and poor ability to penetrate matter; B rays are negatively charged, have less charge than a rays, have less ionization ability, and have strong ability to penetrate matter; gamma rays are high-energy photons, have no charge, and have the strongest ability to penetrate matter, 50 to 100 times greater than B rays and 10,000 times greater than a rays. The units that represent ionizing radiation are divided into absorbed dose, dose equivalent, radioactivity, and exposure (exposure dose), which are explained as follows.
(1) Absorbed dose The energy of ionizing radiation absorbed by a unit mass of irradiated material is called absorbed dose (absorbed dose), and the unit is gray (Gy). 1 kg of irradiated material (tissue, etc.) absorbs 1J of energy, which is 1 gray (Gy). 1 Gy is equal to 100 rad (rad is the original radiation dose unit). (2) Dose equivalent: The product of the absorbed dose (D), quality factor (Q) and other correction factors (N) at a certain point in the tissue being studied is called dose equivalent, which can be expressed as DON. The unit of dose equivalent is sievert, abbreviated as Sv. The dose equivalent per unit time is called dose equivalent rate, and its unit is Sv/s. 1Sv is equal to 100R (roentgen) equivalent [rem is the original radiation dose unit]. (3) Radioactivity: The number of nuclear transitions (decays) of a certain amount of radioactive nuclides in a specific energy state per unit time is called radioactivity, also called radioactivity intensity. Its unit is becquerel, abbreviated as Bq. One nuclear decay per second is 1Bq. 1Bq is equal to 2.7x10-Curie (Ci, Ci is the auxiliary unit of radiation energy).
(4) Exposure dose When all the electrons (including positive and negative electrons) released in a unit mass of air are blocked by the air, the total charge value of the ions generated in the air is called exposure dose, and its unit is coulomb kilogram (C/kg). 1C/kg is equal to 3400rad. (II) Natural radionuclides in food The natural radioactive background of the environment refers to the level of ionizing radiation inherent in nature and not affected by human activities. It mainly comes from cosmic rays and radioactive nuclides in the environment. The latter mainly include K (potassium), "Ra (radium), "Rb (rubidium), 2Th (thorium), "U (uranium) and their decay products contained in the earth's crust (soil, rocks, etc.) and ammonia (radon, Rn) and radium (thoron, Tn) diffused into the atmosphere. The average environmental natural radioactive background dose is 1.05x10-Gy/year.
Due to the material exchange process between organisms and their living environment, most animal and plant foods contain different amounts of natural radioactive substances, that is, the natural radioactive background of food. However, due to the different radioactive background values ​​of the environment in different regions, different animals and plants and different tissues of organisms have different affinities for certain radioactive substances. Therefore, the natural radioactive background values ​​in different foods may vary greatly.
The natural radioactive nuclides in food are mainly "K (potassium) and a small amount of "Ra (radium)" Ra, Po (polonium), natural thorium and natural uranium, etc.
1. K K is the most abundant natural radioactive nuclide in food, with a half-life of 1.28x10' years. The proportion of K in the total potassium content of the environment and food is relatively constant, about 0.0119%, and its radioactivity is 32.2Bq of K per gram of natural potassium. Therefore, the content of K and its radioactivity can be estimated based on the total potassium content of food. Adults consume 2~3g of potassium per day, that is, the intake of K is 65~100Bg. According to the survey data of our country, the content of K in adult male and female bodies is 69.9Bg/(kg·body weight) and 51.4Bq/(kg·body weight), and the internal radiation dose is 0.212x10-Gy/year and 0.156x10-°Gy/year respectively.
2.2sRa
The half-life of 2Ra is 1.6x10'year. Radium can enter the human body through drinking water and food. The content of radium in different foods varies greatly (10-4~10Bq/kg). In general areas, the average daily intake of 2Ra per person is 0.02~0.2Bq. The radium in animals and humans is mainly concentrated in bone tissue, and the average content of 2Ra is 5.2x10-Bq/g.
3.0Po
The parent of 20Po is U, and its predecessors are Ra, Rn, 2Pb, 2Bi, etc. In the natural environment, 2Po and 2°Pb are in equilibrium and widely exist in plants and some seafood. 2Po has a short life span (half-life 138.4d), but 2Pb has a half-life of up to 22 years. In addition to food, the 2Po in animals and humans also comes from the decay of ingested 2Pb in the body. The 2Po content in animal bones and liver and kidney tissues is much higher than that in muscles. Plankton has a strong ability to concentrate 2Po from water, and its 2Po/10Pb ratio can be greater than 1. Therefore, fish that feed on plankton have a higher 2Po content, especially in liver tissue and sperm and egg cells. The 2Po content in different foods varies greatly, such as 0.04~0.37Bq/kg in grains, 0.04~0.11Bq/kg in root vegetables, and the 2Po content in tea in some areas can be as high as 178Bq/kg. Residents who mainly eat seafood consume a large amount of "Po". Po can also enter the human body through special food chains. For example, herders living in areas near the Arctic mainly feed on reindeer. Reindeer mainly feed on lichens in winter. Lichens have a strong enrichment effect on Po. Therefore, the "Po load in this group of people, especially in bones and teeth, is much higher than that of the general population.