Half-Value Layer

Half-Value Layer

Half-value layer (HVL) (also half-value thickness) is the thickness of specified material which reduces the intensity of radiation entering the material by half. [http://www.euronuclear.org/info/encyclopedia/h/half-value-thickness.htm Definition from the European Nuclear Society] This is also expressed in terms of the air kerma rate (AKR): the half-value layer is the thickness of specified material which attenuates the beam of radiation to an extent such that the AKR is reduced to one-half of its original value. In this definition the contribution of all scattered radiation, other than any which might be present initially in the beam concerned, is deemed to be excluded. [cite web|author=FDA|url= http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=1020.30|title=Sec. 1020.30 Diagnostic x-ray systems and their major components.|accessdate=2007-12-05] It should be noted that rather than air kerma rate, measurements of air kerma, exposure, or exposure rate can also be used to determine half value layer, so long as it is given in the description.

Half-value layer refers to the first half-value layer, where subsequent (i.e. second) half-value layers refer to the amount of specified material that will reduce the air kerma rate by one-half after material has been inserted into the beam that is equal to the sum of all previous half-value layers.

Quarter-value layer is the amount of specified material that reduces the air kerma rate (or exposure rate, exposure, air kerma, etc...) to one fourth of the value obtained without any test filters. The quarter-value layer is equal to the sum of the first and second half-value layers.

The homogeneity factor (HF) describes the polychromatic nature of the beam and is given by:

HF=frac{1^{st} HVL}{2^{nd} HVL}

The HF will always be less than or equal to one (it is only equal to one in the case of a monoenergetic beam). In in case of a polychromatic beam, the HF is less than one because of beam hardening.

HVL is related to Mean free path, which can be thought of as the thickness of specified material which reduces the intensity of radiation entering the material by 37% (1/e).

Notes


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