This addendum to the code of practice for the determination of absorbed dose for x-rays below 300 kV has recently been approved by the IPEM and introduces three main changes: (i) Due to a lack of available data the original code recommended a value of unity for k(ch) in the very-low-energy range (0.035-1.0 mm Al HVL). A single table of k(ch) values, ranging from 1.01 to 1.07, applicable to both designated chamber types is now presented. (ii) For medium-energy x-rays (0.5-4 mm Cu HVL) methods are given to determine the absorbed dose to water either at 2 cm depth or at the surface of a phantom depending on clinical needs. Determination of the dose at the phantom surface is derived from an in-air measurement and by extending the low-energy range up to 4 mm Cu HVL. Relevant backscatter factors and ratios of mass energy absorption coefficients are given in the addendum. (iii) Relative dosimetry: although not normally forming part of a dosimetry code of practice a brief review of the current literature on this topic has been added as an appendix. This encompasses advice on techniques for measuring depth doses, applicator factors for small field sizes, dose fall off with increasing SSD and choice of appropriate phantom materials and ionization chambers.
United Kingdom dosimetry codes of practice have traditionally specified one electrometer for use as a secondary standard, namely the Nuclear Enterprises (NE) 2560 NPL secondary standard therapy level exposure meter. The NE2560 will become obsolete in the foreseeable future. This report provides guidelines to assist physicists following the United Kingdom dosimetry codes of practice in the selection of an electrometer to replace the NE2560 when necessary. Using an internationally accepted standard (BS EN 60731:1997) as a basis, estimated error analyses demonstrate that the uncertainty (one standard deviation) in a charge measurement associated with the NE2560 alone is approximately 0.3% under specified conditions. Following a review of manufacturers' literature, it is considered that modern electrometers should be capable of equalling this performance. Additional constructural and operational requirements not specified in the international standard but considered essential in a modern electrometer to be used as a secondary standard are presented.
This paper evaluates the characteristics of ionization chambers for the measurement of absorbed dose to water using very low-energy x-rays. The values of the chamber correction factor, k ch , used in the IPEMB 1996 code of practice for the UK secondary standard ionization chambers (PTW type M23342 and PTW type M23344), the Roos (PTW type 34001) and NACP electron chambers are derived.The responses in air of the small and large soft x-ray chambers (PTW type M23342 and PTW type M23344) and the NACP and Roos electron ionization chambers were compared. Besides the soft x-ray chambers, the NACP and Roos chambers can be used for very low-energy x-ray dosimetry provided that they are used in the restricted energy range for which their response does not change by more than 5%.The chamber correction factor was found by comparing the absorbed dose to water determined using the dosimetry protocol recommended for low-energy x-rays with that for very low-energy x-rays. The overlap energy range was extended using data from Grosswendt and Knight. Chamber correction factors given in this paper are chamber dependent, varying from 1.037 to 1.066 for a PTW type M23344 chamber, which is very different from a value of unity given in the IPEMB code. However, the values of k ch determined in this paper agree with those given in the DIN standard within experimental uncertainty. The authors recommend that the very low-energy section of the IPEMB code is amended to include the most up-to-date values of k ch .
This new code of practice for the determination of absorbed dose for x-rays below 300 kV has recently been approved by the IPEMB and introduces the following changes to the previous codes: (i) The determination of absorbed dose is based on the air kerma determination (exposure measurement) method. (ii) An air kerma calibration factor for the ionization chamber is used. (iii) The use of the F (rad/röentgen) conversion factor is abandoned and replaced by the ratio of the mass-energy absorption coefficients of water and air for converting absorbed dose to air to absorbed dose to water. New values for ratios of these coefficients are recommended. Perturbation and other correction factors are incorporated in the equations. (iv) New backscatter factors are recommended. (v) Three separate energy ranges are defined, with specific procedures for each range. These ranges are: (a) 0.5 to 4 mm Cu HVL; for this range calibration at 2 cm depth in water with a thimble ion chamber is recommended. (b) 1.0 to 8.0 mm Al HVL; for this range calibration in air with a cylindrical ion chamber and the use of tabulated values of the backscatter factor are recommended. (c) 0.035 to 1.0 mm Al HVL; for this range calibration on the surface of a phantom with a parallel-plate ionization chamber is recommended. Contents 1 Data 3.1 Mass-energy absorption coefficient ratios, water to air, ( μen /ρ) w/air 3.1.1 Medium energies (0.5-4 mm Cu HVL) 3.1.2 Low energies (1.0-8 mm Al HVL) 3.1.3 Very low energies (0.035-1.0 mm Al HVL) 3.2 Back-scatter factor, B w 3.3 The overall chamber correction factor, k ch 3.3.1 Medium energies (0.5-4 mm Cu HVL) 3.3.2 Very low energies (0.035-1.0 mm Al HVL) Appendix A. Theoretical basis for the new air kerma based code A.1 Background A.2 Medium-energy x-rays (0.5-4 mm Cu HVL) A.2.1 General details A.2.2 Air kerma at the centre of a hole in the phantom (K air,hole ) A.2.3Air kerma at the chamber centre in the undisturbed medium (K air,z ) A.2.4Correction for the effect of the chamber stem A.2.5Air kerma to water kerma conversion A.
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