Background Radiation: Radiation from sources other than the source of interest. It is due to cosmic rays and natural radioactivity in the environment.Potassium Radioisotopes: Unstable isotopes of potassium that decay or disintegrate emitting radiation. K atoms with atomic weights 37, 38, 40, and 42-45 are radioactive potassium isotopes.Thorium: Thorium. A radioactive element of the actinide series of metals. It has an atomic symbol Th, atomic number 90, and atomic weight 232.04. It is used as fuel in nuclear reactors to produce fissionable uranium isotopes. Because of its radioopacity, various thorium compounds are used to facilitate visualization in roentgenography.Radiation Dosage: The amount of radiation energy that is deposited in a unit mass of material, such as tissues of plants or animal. In RADIOTHERAPY, radiation dosage is expressed in gray units (Gy). In RADIOLOGIC HEALTH, the dosage is expressed by the product of absorbed dose (Gy) and quality factor (a function of linear energy transfer), and is called radiation dose equivalent in sievert units (Sv).Radon: A naturally radioactive element with atomic symbol Rn, atomic number 86, and atomic weight 222. It is a member of the noble gas family found in soil, and is released during the decay of radium.Chernobyl Nuclear Accident: April 25th -26th, 1986 nuclear power accident that occurred at Chernobyl in the former USSR (Ukraine) located 80 miles north of Kiev.Spectrometry, Gamma: Determination of the energy distribution of gamma rays emitted by nuclei. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)Radiometry: The measurement of radiation by photography, as in x-ray film and film badge, by Geiger-Mueller tube, and by SCINTILLATION COUNTING.Gamma Rays: Penetrating, high-energy electromagnetic radiation emitted from atomic nuclei during NUCLEAR DECAY. The range of wavelengths of emitted radiation is between 0.1 - 100 pm which overlaps the shorter, more energetic hard X-RAYS wavelengths. The distinction between gamma rays and X-rays is based on their radiation source.Environmental Exposure: The exposure to potentially harmful chemical, physical, or biological agents in the environment or to environmental factors that may include ionizing radiation, pathogenic organisms, or toxic chemicals.China: A country spanning from central Asia to the Pacific Ocean.