Dose rate conversions

The (natural) radionuclides present in rocks, soils or air result in a flux of gamma radiation exposing persons to gamma radiation that results in a dose absorbed by the human body. In dose rate mapping, different units are reported.

Dose rate and Dose Equivalent

Ambient dose rate (ADR) is the generic term used for dose rate at a location, usually in free air. The ambient dose rates are often reported as KERMA (Kinetic Energy Released per unit Mass) in nGy/h. It is the sum of the fluxes from sources as cosmic radiation, terrestrial radiation (natural and antropogenic) and airborne radiation. The actual  background value of this Ambient Dose Rate is often monitored at certain sites. The measurements from the concentrations of radionuclides in the ground focus on the terrestrial component of the dose rate (Terrestrial Gamma Dose Rate: TGDR).

In environmental monitoring the quantity generally reported is the ambient dose equivalent rate (ADER). The Ambient dose equivalent (ADER) represents the energy deposited in a certain material (e.g. tissue) per unit of time and is a meaure of the biological damage on human tissue. The ambient dose equivalent is expressed in nSv/h.

The units Sievert (Sv) and Gray (Gy) are both defined as 1 J/kg, where Sv describes a biological effect and Gy a physical effect. The absorbed dose in air can be converted to an effective dose equivalent with a factor. For adults this factor is 0.7; for infants this factor is 0.9 (UNSCEAR 2000 Report Volume I):

ADER (nSv/h) = 0.7 * ADR (nGy/h)

Measuring ADR

When measured, the data from K, U and Th are reported as concentrations of these nuclides in the ground. The terrestrial gamma dose rate (TGDR in nGy/h) can be calculated from known concentration of natural radioelements K, Th and U in rocks and soils. The UNSCEAR (UNSCEAR, 2008) has proposed a simplified equation to calculate the TGDR due to natural radioactivity:

in this equation, the concentrations of the radionuclides are in Bq/kg.

Measuring ADER

The dose rate for the average human body, known as the ambient dose equivalent H*(10) in nSv/h, can be calculated through the total counts of the measured spectrum per energy bin and a detector specific factor. This factor is constructed from two types of Monte Carlo simulations. One of them contains a sphere that is representing the human body (the ICRU sphere.) A point at 10 mm depth in this sphere is absorbing gamma-rays of certain energies. In the other simulation, those same gamma-rays are directed to and absorbed by a specific type of detector. In this way, the response of the detector can be linked to the dose received by the human body.

The contribution to the dose rate of the counts in all the energy bins for a data point are added by the sofware Gamman and this represents the dose rate for the specific data point.

Typical Exposure and Dose Rates

The EPA and the European REMON project give and overview of the typical amounts of ADER (nSv/h) you can expect under normal conditions. These are combined in the table below.

Background information

The European Atlas of Natural Radiation gives an overview of sources of terrestrial radiation, dose rate calculations and background levels.

The maps of radioactivity environmental monitoring, show measured dose rate equivalents (nSv/h) for varies european countries.

The EPA publishes about Radiation Sources and Doses and gives a general overview on background radiation.