Impotance of half value layer PDF

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Impotance of half value layer for attenuation is briefly explained ...

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Half Value Layer Introduction: The thickness of any given material where 50% of the incident energy has been attenuated is known as the half-value layer. The half-value layer is expressed in units of distance (mm or cm). Like the attenuation coefficient, it is photon energy dependent. Increase of the penetrating energy of a stream of photons will result in an increase in a material’s half-value layer [1].

Importance of Radiation shielding materials (HVL): A radiation dose above the maximum permissible limit is very dangerous for human being. In radiation shielding, absorbent materials’ properties must be well known [1]. Radiation shielding materials have great importance for many scientific, engineering and medical applications. The data based on mass attenuation coefficient and half value layer is very useful for the purpose to identify the various radiation shielding materials. In the environment of high radiation exposure concrete is used as radiation shielding material because it is cheap and it can be molded easily into any desired design [2]. Heavyweight concrete: Half-value layer thickness of concrete containing limonite, siderite and barite at different ratios was investigated; measurements have been obtained by a collimated beam of gamma ray from sources 60Co. If unit weight of concrete is increased, improvement of shielding properties is seen. The greater the density, the smaller the thickness of concrete required. (figure 1) Limonite: Hydrated iron oxides containing, when pure, 60–63% iron. It can occur as primary. In low-grade silicate deposits, weathered outcrops of hydrated ore occur in which the iron content is greatly enriched. Such outcrops provided ore for the first steel industries Figure 1: Linear attenuation coefficient of concretes. Siderite: This mineral constitutes only a small proportion of the total world iron ore reserves. When pure, it contains 48% iron, but it is easily decomposed by heat, calcined to hematite with 70% iron. Siderite is still a commercially important source of ore in some countries. Barite: Barite ore includes BaSO4 which is a good photon radiation absorbent and is a heavyweight aggregate used in the concrete industry [1].

In spite of all these advantages some limitation are also associated with the concretes. It is not transparent to visible light thus restricting one to see through it. Secondly when it is exposed to the radiations for a longer period of time its mechanical strength is reduced. So it is desired to have materials which are transparent to visible light and have better shielding properties in terms of lesser volume requirement. For this purpose, heavy metal glasses are being used as an alternate to the conventional shielding material like concrete. Borate and Silicate are the most commonly used glass formers because of their easy availability and low cost. Bismuth bases glasses have considerable technological applications due to their density, high refractive index and low melting point. Thus, Bi containing glasses can be used as alternate to the concretes as radiation shielding materials. Further these are transparent to visible light and require lesser volume estimated in terms of HVL as compared to concretes which are opaque and require much larger volume. The bismuth borate glasses are non-toxic to environment and can provide alternate to lead free shielding materials. GS8 is found to have lowest HVL values in all the prepared samples. The concretes on the other hand have more value for HVL [2]. Importance of HVL in mammography:  

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The half-value layer (HVL) is an important index of the image quality or radiation risk in mammography. Radiation risk of the breast tissue is evaluated with the average glandular dose. The HVL index is indispensable for the average glandular dose computations. The HVL values between seven filter sets were different in about 5% at the maximum. In addition, we reduced a fluctuation of X-ray output with dose monitoring. Then, the standard deviation of HVL value decreased from 1.114% to 0.105%. According to the Standards, determination of the half-value layer (HVL) is an integral part of mammographic dosimetry which requires the evaluation of patient's glandular dose [3].

References: 1. Akkurt, I., Başyigit, C., Akkaş, A., Kilingarslan, Ş., Mavi, B., & Giinoglu, K. (2012). Determination of some heavyweight aggregate half value layer thickness used for radiation shielding. Acta Physica Polonica-Series A General Physics, 121(1), 138. 2. Kaundal, R. S. (2016). Comparative study of radiation shielding parameters for bismuth borate glasses. Materials Research, 19(4), 776-780. 3. Ishii, R., Yoshida, A., Ishii, M., Fujimoto, S., & Henmi, N. (2011). Precision of half-value layer measurement on mammography. Nihon Hoshasen Gijutsu Gakkai Zasshi, 67(12), 1533-1539....