Half-life shift

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The half-value layer or half-value thickness is the thickness of a penetrated material which, in the case of electromagnetic radiation such as gamma or X-ray radiation , reduces the radiation intensity - and thus in particular the dose rate - by half. Like the absorption coefficient, the half-value thickness depends on the specific properties of the material and the photon energy of the incident radiation.

The half-value thickness can only be used for the approximate dimensioning of simple shielding , since various physical effects ( e.g. dose build-up , scattering , Skyshine effects ) are not taken into account. Exact results require, for example, Monte Carlo simulations or transport calculations (numerical calculations based on Boltzmann's transport equation ).

The term tenth value thickness be considered as analogous: traversing this thickness reduces the intensity to one-tenth of the original value.

Exponential decrease with the penetration depth

Alpha radiation (α) is completely absorbed by a sheet of paper, beta radiation (β) by a metal sheet a few millimeters thick; to sufficiently attenuate gamma radiation (γ) you need - depending on the energy of this radiation - several centimeters to decimeters of a material with the highest possible density (see shielding (radiation) ).

In contrast to alpha and beta radiation, gamma radiation does not have a maximum range . The intensity of the gamma radiation is continuously weakened as it passes through matter.

The ratio of the dose rate , which is determined in the beam path without shielding, and the dose rate of the unscattered radiation at the same location with shielding material of the thickness is referred to as the (material) attenuation factor of the unscattered radiation:

The formula applies to the reciprocal attenuation factor

.

Here denotes the attenuation coefficient. The following applies by definition to the half-value layer thickness

.

Thus, the half-value layer thickness results from the attenuation coefficient by

or the other way around

.

This gives the dose rate behind a shield of any thickness

.

Half-value layer thickness for gamma radiation

Half-value layer thickness d 1/2 for gamma radiation
air lead water Alu iron graphite concrete Lead glass acrylic
E γ in MeV in meters in millimeters
0.1 35 0.107 41 15.2 2.4 20.3
0.2 44 0.62 51 21.1 6th 25th
0.3 50 1.56 58 24.8 8th 28.8
0.4 56 2.65 65 27.8 9.4 32.4
0.5 62 3.85 72 30.5 10.5 35.4 33 13 70
0.6 67 4.92 77 33 11.5 38.3
0.8 76 6.9 88 37.7 13.2 44
1 84 8.7 108 42 14.7 48 50 24 90
1.5 101 11.7 121 51 18.1 59
2 121 13.4 141 60 20.8 69
3 145 14.6 175 73 24.4 87
4th 174 14.7 204 83 26.7 101
5 196 14.4 230 91 28.1 115 100 45 200
6th 213 14.1 251 97 28.9 125
8th 242 13.4 286 106 29.7 144
10 265 12.6 315 112 29.7 158

literature

  • Hans-Gerrit Vogt, Heinrich Schultz: Basics of practical radiation protection . 6th edition. Carl Hanser Verlag GmbH & Co. KG, Munich 2011, ISBN 978-3-446-42593-4 .

Individual evidence

  1. ^ German-Swiss Association for Radiation Protection (FS) e. V .: Data and facts on handling radionuclides and on decontamination in radionuclide laboratories , loose-leaf collection, Part 1.4 Shielding , October 1997
  2. Andreas Kratzer (Technical University of Munich): Physical internship for mechanical engineering: "Radioactivity" ( Memento of the original from May 26, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / einrichtungen.ph.tum.de