Half Value layer

The methods used to determine the energy spectrum of a radiation beam are not of practical application routinely. However, some parameters may be used to characterise the energy spectral distribution. This is the case in the use of the Half Value Layer HVL concept. Using this parameter, the energy spectral distribution of a radiation beam is characterised by a unique energy value, namely, the Effective Energy . 

Half-value layer. It is that thickness of a given material (i.e. shielding) required to reduce the exposure rate from a source of gamma or x-rays to one-half of its unshielded value. 

An important quantity in the discussion of photon interaction with matter is the half-value layer (HVL), which is defined as the thickness of the absorber that attenuates an initial photon beam intensity to one-half. The HVL increases with higher energy of the photon and decreases with increasing atomic number of the absorber. Lead is a high atomic number inexpensive metal that has very high absorbing power for 7 radiations providing low HVL values and that is why it is commonly used for radiation protection. The HVL... 

Table 1.3. Half-value layers of 511-keV photons in different absorber materials...

Half-value layer (HVL). The thickness of an absorbing material required to reduce the intensity or exposure of a radiation beam to one-half of the initial value when placed in the path of the beam. 

Table 4 Approximate Half Value Layers (HVL) and Tenth Value Layers (TVL) of Shielding Materials Used in Large-Scale Gamma Irradiators...

HVL (half-value layers) attenuates approximately by 10 . There is 0.64 mm of radon gas at STP in transient equilibrium with 1 Ci of radium. 

Irradiation and Alkylation Conditions. X-ray doses of 3-6 Gy were delivered from a Phillip s RTIOO X-ray machine (200 kVp 15 mA with a nominal half value layer of 0.35 cm copper). Cultures were inadiated with an incident dose rate of 1.3 J m s UV light (254 nm). Cultures were exposed to various concentrations of N-methyl-N -nitro-N-nitrosoguanidine (MNNG) (Aldrich Chemical Co., Milwaukee, Wl), MMS (ICN Pharmaceuticals, Hainview, NY), ethyl methanesulfonate(EMS) (ICN Pharmaceuticals), or ethyl nitrosourea (ENU) (a gift of B. Singer, University of California, Berkeley), in tissue culture medium for 60 min to 4 h. 

Lakshmanan (1992) has fitted a CaSOa.Dy badge for simultaneous measurements of peak voltages, half-value layer thickness and exposure to diagnostic X-rays. This TLD badge is used in India for 3, y and X-ray personnel monitoring. 

The half-value layer (HVL) of shielding material is a useful concept and is the thickness of a specified material that will cut down the incident radiation by one-half. For iodine-125 with an energy of 0.033 MeV, the HVL is 0.002 cm of lead for iodine-131 with an energy of 0.364 MeV, the HVL is 0.25 cm of lead. 

HALF-VALUE LAYER The thickness of a particular material necessary to reduce the intensity of an X-RAY or GAMMA RAY beam to one-half its original intensity. 

The method presently available for clinical studies does not allow any distinction between inorganic and organic iodine in the thyroid gland and because of the effect of overlying tissue on the specific iodine Ka X-ray (half value layer equals 2 cm of tissue) thyroid tissue not located immediately under the skin (thyroid cancer metastasis) cannot be evaluated. The results of iodine measurements with XRF are expressed in units of weight (mg) of iodine contained in the area scanned. Additional efforts have to be extended if a volume measurement is desired. Then, the units would be mg iodine per volume. The problems concerning the accuracy of the method are discussed elsewhere in this monograph. 

Keywords— Mean glandular dose. Half Value layer. Compressed breast thickness. Mammography. 

The primary objective of this study was to determine the MGD during diagnostic mammography in some hospitals in Hanoi, Vietnam. The secondary objective was to evaluate some of the factors affecting MGD, such as half value layer (HVL) of the X-ray beam, compressed breast thickness (CBT) and age. Such information is necessary in order to formulate recommendations to minimize radiation doses without compromizing image quahty. 

On the multivariate test two factors, namely half value layer of the X-ray beam and compressed breast thickness, had a significant effect on MGD per woman. No significant relationships were seen between MGD per woman with respect to body mass index or age. 

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