Radiation is energy that transmits in the form of wave or light-speed particles. It can be divided into non-ionizing radiation and ionizing radiation according to its energy.
Non-ionizing radiation is low energy radiation that cannot produces ionization, e.g. solar ray, light, infrared, microwave, radio waves and radar waves etc.
Ionizing radiation is high energy radiation that can produces ionization.
The term radiation generally refers to ionizing radiation. Gamma ray belongs to ionizing radiation. It has high penetrable ability and can only be obstructed by thick cement wall or lead plate.
The source of radiation can be divided into natural and man-made source.
Natural sources are those that exist in the nature, include Radon in the atmosphere, Uranium and Thorium in soil and rock, cosmic ray etc.
Medical source is the main man-made sources, e.g. X-ray and radiation treatment etc, others are from dust of explosion of nuclear weapons, nuclear reactors and daily living such as television etc.
Statistic information shows that more than 80% of the radiations received by human were from nature sources while only less than 30% were from man-made sources. Only a very small amount of radiation comes from nuclear reactors.
The health effects of radiation can be divided into deterministic effect and stochastic effect.
When the dose of radiation receives by human body exceeds certain amount, the following symptoms such as tiredness, nausea, vomit, erythema, psilosis, decrease of lymph corpuscle in blood etc may occur. If one receives a higher dose, the symptoms may become more severe or even death. This effect is known as deterministic effect. Deterministic effect occurs only if the dose received exceeds a certain level.
Furthermore, the International Commission on Radiological Protection (ICRP) has proposed the following conservative assumption due to safety precaution:
Any amount of radiation, no matter how small, may have possibility to induce cancer and cause genetic effects. The possibility is proportional to the dose received. This effect is called stochastic effect.
Absorbed dose
The Macao Meteorological and Geophysical Bureau (SMG) measures the absorbed radiation dose rate of Gamma ray.
Absorbed dose is the average absorbed energy of ionizing radiation per unit mass, i.e.:
Absorbed dose = dE/dm
this reflects the amount of energy that ionizing radiation sources deposit in materials through which they pass. Its SI unit is Gray (Gy) and defined as follow:
1 Gy = 1 J / kg
Absorbed dose rate means the average absorbed radiation dose per unit time. Its SI units are Gy/h, mGy/h or µGy/h.
According to the European standard, the dose rate below 0.35 µGy/h is considered as normal.
Equivalent dose
Biological effects are attributed not only to the absorbed dose but also depend on the type and energy of the radiation. Equivalent dose is a physical quantity to measure the effect to a tissue or organ by different types and energies of radiation. It is calculated as the product of absorbed dose in tissue multiplied by a quality factor (radiation weighting factor), i.e.:
Equivalent dose = Absorbed dose x Radiation weighting factor
The SI unit of equivalent dose is sievert(Sv)。
According to International Commission on Radiological Protection (ICRP)，the radiation weighting factor of the Gamma Radiation equals to 1, i.e. 1μGy of Gamma Radiation equals to 1μSv of Gama Radiation (equivalent dose)。
When the incident radiation consists of different types and energies of radiation, the equivalent dose equals to the sum of equivalent dose of all type of ionizing radiation.
Effective dose
Besides depending on the equivalent dose, biological effects also depend on the type of tissue or organ that has been irradiated. Effective dose is used to quantify the total detriment from exposures of several organs or tissues. The relation between equivalent dose and effective dose is as follow:
Effective dose = Σ Equivalent dose x Tissue weighting factor
The SI unit of effective dose is sievert(Sv)。
Source of reference data