Radiation dose

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Radiation dose is the term used to describe the effects of ionizing radiation on matter . Dose of energy or energy dose is the radiation emitted by the radiation per mass of the material energy, dose rate , the dose of energy per time period, so the per mass recorded performance .

energy

The decisive factor for the effect of radiation is the work done in matter , i.e. the energy transferred into a volume (unit: J = Joule).

Absorbed dose

The absorbed dose is the energy absorbed over the entire duration of irradiation in relation to the irradiated mass .

A distinction is made between physical dose sizes for different areas of application:

Ion dose

Another physical measure of the radiation dose is the ion dose , which indicates how much charge (of a sign) is released in a body by the radiation:

Evaluated dose work

The same physical energy doses can have different physiological effects in biological systems such as human organs . That is why there is a dose work that is assessed differently radiologically for different types of radiation for biological systems :

Like the unevaluated energy doses, these evaluated quantities have the unit J / kg. However, in order to mark them as rated doses, they are given in the international system of measurement in the physical unit Sievert (Sv = J / kg, previously also Rem ).

power

The physical output of a radiation is the work done in matter per time period or the energy transferred through a cross section per time period ,

  • Power (unit: J / s = joule / second = watt);

Dose rate

A dose rate is also defined for all of the named measured variables : This is the dose absorbed per time span, i.e. the current differential quotient of the dose work over time or an average value of the differential energy over a longer period of time. This dose rate is given in relation to the mass and to the time:

  • Dose rate (unit: J / (kg s) = gray / second, not weighted)
  • Dose rate (unit: J / (kg s) = sievert / second, weighted)

The term “dose rate” is often used synonymously for this.

Dose and dose rate effectiveness factor

For practical purposes, that stochastic radiation effects is often assumed a proportional dose related follow ( LNT hypothesis , English Linear No Threshold , linear non-threshold ' ). However, studies in the low dose range indicate that the radiation risk determined on the basis of the LNT hypothesis is overestimated. The International Commission on Radiological Protection summarizes these influences in a dose and dose rate effectiveness factor (DDREF). In its ICRP 103 recommendation, it confirms the value introduced earlier with DDREF = 2, which divides the risk coefficients determined by linear extrapolation for the range of low doses and low dose rates .

On the history of concepts

After the discovery of X-rays ( Röntgen , 1895) and radioactivity ( Becquerel , 1896), the effects of ionizing radiation on humans were observed. Attempts to use them for therapy only gave reproducible results after varying degrees of success when it was possible to administer ionizing radiation of a defined strength, comparable to a certain dose of a drug. The radiation dose corresponded to this pharmacological concept. For practical reasons, the ion charge, the ionizing radiation generated in matter, typically in air, was measured. This ion dose, the electrical charge formed per mass , is a purely physically measurable quantity. Since every ionization process is associated with a certain amount of energy, the ion dose is proportional to an energy dose. This energy deposited in a mass element by the ionizing radiation leads for the most part to a warming of the body. The increase in temperature is measurable and more recently attempts have been made to represent the unit of the absorbed dose directly by means of calorimetric measurements (heating of water). However, the increase in temperature is very small: a dose of 50 Gray administered to a person within a short period of time  , at which death occurs within a few hours, only generates a temperature increase of about 0.01 ° C in water. The special effect of the radiation is caused by the ionization and the free radicals formed as a result.

Use in medical radiation therapy

According to current recommendations, the absorbed dose , i.e. the energy absorbed per kilogram of irradiated material or irradiated tissue, measured in Gy ( Gray ), 1 Gy = 1 J / kg, is used in medical radiation therapy . Evaluation factors ( RBW factors ) are used to take into account different biological effectiveness .

The absorbed dose is a suitable quantity for estimating the direct effects in humans (deterministic radiation damage ). For a given type, energy and duration of radiation, the absorbed dose depends on the chemical composition of the material. For this reason one chooses as reference material z. B. a tissue-like elemental composition or water. The absorbed dose determined for a specific reference material can be converted into the absorbed dose for another material with the help of correction factors.

Dose determination for radioactive sources and dose rate

So-called dose rate constants are used to establish a relationship between the activity of an (ideally punctiform, unshielded) radioactive source and the dose generated by it at a certain distance . The radiation dose absorbed per time period is called the dose rate (unit: Sv / s or Sv / h).

In the case of incorporated emitters, it can be difficult to estimate the dose. It is important to know about the kinetics of the substance in the body, i. H. how it is distributed in the body (i.e. how the dose is distributed in percentages over the various organs) and how and how quickly ( biological half-life ) it is excreted, as well as information on how long ago it was incorporated. The momentary activity distributed in the body can be e.g. B. estimate via a urine sample measurement.

The dose determination is an important step in planning a radiation therapy or nuclear medicine therapy.

Use in radiation protection

In radiation protection, radiologically assessed dose sizes have been defined to take into account the different radiation risk for different types of radiation and different types of tissue:

  • The body dose in the form of the organ dose and the effective dose is used to establish limit values . The occurrence of stochastic radiation damage is quantified with the effective dose.
  • The dose equivalent in the form of the ambient dose equivalent or the personal dose is used as the radiation protection measure .

The common unit of all radiologically assessed dose sizes is Sv (Sievert) . In many cases of practical radiation protection (for X-rays , gamma and beta , i.e. for electromagnetic and electron radiation ) the following applies: 1 Gy = 1 Sv. However , this equation does not apply to alpha , proton and neutron radiation due to their higher biological effectiveness. This is taken into account by radiation weighting factors between 5 and 20 (depending on the energy and type of particle).

The danger of staying in the vicinity of one or more radiation sources can be characterized by specifying the dose rate prevailing at a measuring point.

See also

literature

  • W. Schlegel, J. Bille (Ed.): Medical Physics 2, Radiation Physics. Springer Verlag, 2002, ISBN 3-540-65254-X .
  • H. Krieger: Radiation Physics, Dosimetry and Radiation Protection. 3rd revised edition. Volume 2, Teubner Verlag, 2001, ISBN 3-519-23078-X .

Web links

Individual evidence

  1. DIN 6814, part 3
  2. a b BAnz AT May 3, 2016 B4