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6. Some practical considerations 6.1 Building materials, methods and verification The range of materials which may be used to provide radiation shielding include: • Lead sheet and lead fabricated products (lead plywood, lead plasterboard). • Concrete, concrete blocks and concrete products. • Barium plaster. • Various types of brick. • Gypsum wallboard. • Lead glass. • Lead acrylic. • Other materials (e.g. steel and wood for low energy/mammography trailers). The choice of material depends on several factors, including the level of shielding to be achieved, the cost, and the practicalities of installation. It should be noted that design and construction professionals often refer to the dimensions of materials in terms of ‘nominal’ dimensions. Thus, for example, in the US a ‘four inch’ brick is actually 3 5 ⁄8 inches (9.2 cm) so the RPA should request the actual dimensions of the building materials specified or used if he/she is not already familiar with them (NCRP, 2004). 6.1.1 Lead sheet and lead fabricated products Lead has both high atomic number and high density (11,350 kg/m 3 ) and hence is a very effective shielding material. The transmission properties of X‐rays through lead are given in Appendix C. Although lead may be purchased in sheets/rolls, it is malleable and is usually bonded to plywood or plasterboard to provide a stable panel for installation. Lead fabricated products are also available including lead lined door sets, protective screens and blinds. Lead sheet products are available in a standard range of thicknesses, with code numbers as set out in Table 6.1: Table 6.1: Standard lead sheet codes and equivalent thickness in millimetres (BSI, 2006) Lead code Lead thickness (mm) Code 3 1.32 Code 4 1.80 Code 5 2.24 Code 6 2.65 Code 7 3.15 Code 8 3.55 74 The Design of Diagnostic Medical Facilities where Ionising Radiation is used
Traditionally, an RPA may specify the lead requirements for an installation, rounded up to the nearest 0.5 mm. In practice, as lead is not available in increments of 0.5 mm, it is preferable to specify the code required, to avoid errors arising from inappropriate rounding up or down later. Lead sheet is manufactured to BS EN 12588 standard and has a ± 5% tolerance on the thickness (BIR, 2000). Lead products compliant with the above standard should be utilised. 6.1.2 Concrete and concrete blocks High-density solid concrete (2350 kg/m 3 ) is a commonly used shielding material. It may take the form of poured concrete or solid concrete blocks. Its transmission properties at a density of 2350 kg/m 3 are given in Appendix C. The lead equivalence of concrete and other shielding materials is also given in Appendix C for a range of kVp (BIR, 2000). Solid concrete blocks may have internal voids, so care needs to be taken in their use. Lightweight concrete blocks are also available (2200 kg/m 3 ) and if used the thickness of concrete will have to be scaled accordingly. Likewise using concrete of this density, with the transmission curves in Appendix C, will require an appropriate adjustment in the shielding thickness read from the curves. Concrete blocks may be used in combination with other materials such as barium plaster. In practice, approximately 69 mm of high density solid concrete is equivalent to 1 mm lead at 70 kVp; similarly at 70-125 kVp 150 mm concrete is approximately equal to 2 mm lead (WHO, 1975). Hollow concrete blocks are not generally suitable for shielding except with low energy or low dose applications (mammography, dental radiology, DXA scanning) or when used in combination with other shielding materials. They should be used with caution, as attenuation coefficients can be difficult to obtain. 6.1.3 Barium plaster Barium plaster is a gypsum plaster incorporating barytes aggregates, which may be used in combination with concrete blocks. Its absorption properties with respect to X‐rays are greatly enhanced by the presence of a K absorption edge in barium. A thickness of up to 25 mm can be applied to a surface (BIR, 2000). Applying barium plaster so that it will be well finished to the thickness required, and crack free, requires a plasterer with the appropriate skill. It must be applied in several coats and takes a considerable period to dry. The composition and the attenuation properties of barium plaster manufactured by British Gypsum (Thistle X‐ray) have been altered since the publication of BIR (2000). The net effect has been to reduce its lead equivalence by between 15% and 30%. Revised values have been published by Williams (2005). There are no published coefficients for secondary transmission through barium plaster. Coefficients for primary transmission may be used for energies below 100 kVp (BIR 2000). However above this, the attenuation properties are unknown. Barium plaster is therefore unsatisfactory for shielding high kVp facilities such as CT and high kVp chest units, and alternatives should be considered until more complete data are available. The Design of Diagnostic Medical Facilities where Ionising Radiation is used 75
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RPII 09/01 The Design of Diagnostic
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Foreword The original Code of Pract
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4. Nuclear medicine 35 4.1 Introduc
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1. Legal and administrative framewo
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e made as soon as possible. It shou
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It should be noted that it is the u
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Records should be kept as they will
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2. Identify the persons (staff and
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3.1.3 Computed Tomography (CT) CT a
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Photo 3.2: Interventional X‐ray r
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3.3 Radiography rooms 3.3.1 General
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X Figure 3.2: Dedicated chest room
Page 26 and 27: Figure 3.4: DXA room Operator’s c
Page 28 and 29: Figure 3.5b: A dental radiography s
Page 30 and 31: Radiation shielding calculations fo
Page 32 and 33: There are large variations in the s
Page 34 and 35: e assessed by the RPA as part of th
Page 36 and 37: The trailer must be sited in an are
Page 38 and 39: It is recognised worldwide that the
Page 40 and 41: The operator console should be loca
Page 42 and 43: Figure 4.3: A possible layout for a
Page 44 and 45: The workstation should be adequatel
Page 46 and 47: uptake probe and associated equipme
Page 48 and 49: The layout of the department should
Page 50 and 51: The control/console room should pro
Page 52 and 53: 5. Shielding calculations 5.1 Gener
Page 54 and 55: Table 5.2: Suggested minimum distan
Page 56 and 57: Table 5.4: Workload data from NCRP
Page 58 and 59: 5.2.4 Primary radiation A barrier i
Page 60 and 61: The primary beam will be attenuated
Page 62 and 63: 1 χ = ln αγ B −γ 1+ + β α
Page 64 and 65: . The reduction in the dose rate fr
Page 66 and 67: R D(t) = . t D(0) × t Equation 5.8
Page 68 and 69: 5.3.1 Radiology shielding calculati
Page 70 and 71: Example 2: Dedicated chest room (BI
Page 72 and 73: Example 4: Dental X‐ray room (BIR
Page 74 and 75: Example 6: Shielding required for a
Page 78 and 79: 6.1.4 Brick There are many types of
Page 80 and 81: Walls are generally shielded to ful
Page 82 and 83: 6.3.3 Doors As illustrated in Chapt
Page 84 and 85: Photo 6.4 Lead blinds protecting wi
Page 86 and 87: Chapter 2). It is becoming common t
Page 88 and 89: Gaps in shielding are more likely t
Page 90 and 91: ICRP. 2007. Recommendations of the
Page 92 and 93: Appendices Appendix A: Dose limits
Page 94 and 95: Appendix C: Radiation transmission
Page 96 and 97: Figure C.1: Transmission of primary
Page 98 and 99: Figure C.5: 2 mm lead equivalent th
Page 100 and 101: Table D.3: Physical properties & ef
Page 102 and 103: Appendix E: Schematic diagrams of r
Page 104 and 105: Figure E.7: Shielding around the ed
Page 106 and 107: Figure F. 3: Sample radiation warni
Page 108 and 109: Notes 106 The Design of Diagnostic
Page 110: Notes 108 The Design of Diagnostic
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