Radiation Dose Equivalent (H)

The term "dose" is used in connection with many other terms. But the most important meaning of dose is connected with radiation dose equivalent. The radiation dose equivalent and the corresponding unit of measure, the sievert (Sv in SI), named after Rolf Sievert, and rem (in old units), are used for radiation protection purposes, engineering design criteria, and for legal and administrative purposes. Because of this, radiation dose equivalent in day to day life, is often simply called "dose".

Rolf Sievert in his laboratory 1929.

If we define radiation dose equivalent with H and absorbed dose with D, the dose equivalent is defined as:

H (in Sv) = D (in Gy) x QF x DF

Where QF is the quality factor for the type of radiation. It varies from 1 (for gamma, X-ray and beta radiation) to 20 (for alpha radiation and intermediate energy neutrons or fission fragments). DF is the non-uniform distribution factor and applies only to internally deposited radioactive materials (e.g. I-125 in the thyroid or Sr-90 in the bone).

The old unit for measuring radiation dose equivalent is the rem (short for roentgen equivalent man).

The relationship between exposure, absorbed dose, and equivalent dose is simple for gamma or X-rays when applied to muscle tissue (good approximation).

1 X unit 37 Gy 37 Sv

1 R 1 rad = 0.01 Gy 1 rem = 0.01 Sv

N.B.: This "rule of the thumb" can only be applied to gamma and X-rays for muscle tissue. For other types of radiation, or other tissues, the relationship may be very different (e.g. results differ by a factor of 20 for alpha radiation).

External Dose

When the source of radiation is situated outside of the body, the irradiated person will receive what is referred to as an external dose. The calculation of external dose can be performed for the whole body, for extremities, and for superficial skin dose (0.07cm depth) or shallow dose (1cm depth). The type of radiation also plays an important role for assessing external dose.

For gamma and X-rays, the "inverse square law" can be applied for point sources and absorption in air can be neglected for relatively short distances. For beta radiation, the absorption in air can be very important (depending on the beta energy). For alpha radiation, the external dose is zero since alpha particles cannot penetrate past the layer of dead skin (see Module 3).

Film badges or thermoluminescent dosimeters (TLD) are used to measure the external dose

Internal Dose

Internally deposited radionuclides will irradiate a person's body from the inside. Typical routes for internal irradiation are inhalation or ingestion of radionuclides and are referred to as intake. However, radioactive materials are not completely retained in the body. The amount that is retained after an intake is called the uptake. Radionuclides can be retained for varying amounts of time inside the human body. Some are fixed (like Sr-90 inside the bones or I-131 and I-125 inside the thyroid) but the majority of an intake gets eliminated. Therefore, all radionuclides have a "biological half-life", which is the time after which the uptake decays to half of its original value. This biological half-life is considered in addition to the natural disintegration half-life of the radionuclide.

To most accurately estimate the internal dose, the percent of uptake, biological and natural half-life of the radionuclides, as well as the type of radiation, radiation energy, and preferred organ must be taken into consideration. Starting from the permitted annual dose (see Module 9), an annual allowable limit of intake (ALI) can be calculated for each radionuclide. An estimate of the dose received over the next 50 years from an intake is also called the committed dose.

Natural dose

Due to the presence of numerous sources of ionizing radiation in the natural environment (i.e. around us and inside us), the dose received from these types of sources has been named the natural dose, or background. The natural dose varies around the world by more than a factor of ten. This variation arises from differences in soil composition, type of materials used for building, food and water, to the altitude (the higher the altitude, the larger the irradiation from cosmic sources) etc. Nevertheless, the main contribution (over 50%) comes from inhalation of radioactive Radon gas products found in the air we breathe. Another important contributor to the natural dose is from K-40 inside our bodies. In North America, the natural dose varies between 2 and 3 mSv (200 and 300 mrem) per year.

Artificial dose

The total dose received by an average person in North America is in fact bigger than the amount shown above. This is due to artificially produced radiation and is referred to as the artificial dose. The main artificial dose in North America comes from X-ray machines used in medical diagnoses.

Federal regulations impose a limit for artificial dose received from sources other than medical purposes. This value was recently established for Members of the General Public at 1 mSv (100 mrem) per year.

Dose rates

Each of the terms used above (Exposure, Absorbed Dose, Radiation Equivalent Dose, etc.) can also be described as a function of time. When doing so, the terms are called: Exposure rate, Absorbed Dose rate, Dose Equivalent rate, Natural Dose rate, Artificial Dose rate, etc. The units of measure remain the same. However, values are divided by the time interval. For example we can have R/hr, mrem/week, mSv/year, etc.