Theoretical determination of scatter factors at depths in walls of cylindrical chambers

McEwan, A.C.

doi:10.1088/0031-9155/24/3/013

Cited by 3 publications

(7 citation statements)

References 5 publications

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“…A theoretical study (McEwan 1979) in satisfactory agreement with the experimental results of Barnard eta1 (1964) has shown that f ( t ) may be expressed as f ( t ) = at -bt2, for low atomic number materials and for the ranges of t of interest in this work. Integration of equation ( 1) with this form of f ( t ) gives where ( ) is the cavity gas dose per unit energy fluence.…”

Section: Cavity Ionisationsupporting

confidence: 86%

“…where rw is the radius of the cavity, f ( r w ) is the scatter factor for a point at the centre of a water cylinder of radius rw (McEwan 1979), and the term [ 1, is evaluated for the spectrum at the measurement depth. Then also…”

Section: Absorbed Dose Measurement In Watermentioning

confidence: 99%

“…Values of the coefficients a and b for the determination of scatter factors and the quantity f ( r w ) have been taken from the data of McEwan (1979). The scatter corrections (1 + f ( t ) ) are estimated to have an uncertainty of about 10.3°/0 at 1 MeV with decreasing error at higher energies, Calculated values for the graphite wall (0.375 mm thickness) with 4.6 mm thickness Perspex cap are shown in table 1.…”

Section: Farmer Graphite Chambermentioning

confidence: 99%

“…The approach of Barnard et a1 has been modified to provide expressions for the contributions to cavity ionisation from chamber walls, cap and surrounding medium, making use of new theoretical data on photon scattering in the walls of cylindrical chambers (McEwan 1979). To minimise errors associated with the calculation of photon attenuation and scattering corrections, the method requires the calculation of correction factors only for a distance from the point of measurement (or the internal radius of the chamber) equal to the secondary electron range, rather than for the total depth of measurement (Barnard 1964).…”

Section: Introductionmentioning

confidence: 99%

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A theoretical study of cavity chamber correction factors for photon beam absorbed dose determination

McEwan¹

1980

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Theoretical expressions have been derived for the contributions to ionisation density in cavity chambers from secondary electrons arising from photon absorption in wall, build-up cap and surrounding medium. The method allows the derivation of C lambda and displacement factors, which are evaluated for the Farmer graphite chamber for a range of photon beams. Differences between C lambda values calculated for air-walled and water-walled chambers are found to be consistent with those determined by Nahum and Greening. The currently recommended displacement factor for the Farmer chamber with cap, however, is found to be in error by about 0.8%. On the basis of a comparison between theoretically predicted air doses in cavities and the experimental results of Fregene it is proposed that no further correction to C lambda is required for the chamber 'effective point of measurement'.

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Section: Cavity Ionisationsupporting

confidence: 86%

Section: Absorbed Dose Measurement In Watermentioning

confidence: 99%

Section: Farmer Graphite Chambermentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A theoretical study of cavity chamber correction factors for photon beam absorbed dose determination

McEwan¹

1980

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“…They found T,, = 1.005 + 0.002 for a 20 MeV x-ray beam. McEwan (1979) calculated T, for the same type of chamber and found T2(] = 1.002, consistent with the experimental results. Since these values are uncertain but close to unity, we have ignored the effect of the build-up cap on calculated values of F,, although it may have a +0.2-+0.50/0 effect.…”

Section: The Fa Factorsupporting

confidence: 82%

A measurement of absorbed dose to water per unit incident 7 MeV photon fluence

Mach

Rogers

1984

Phys. Med. Biol.

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After isolating the effects due to 7 MeV photons, measurements here made of the ratio of the bnCo exposure equivalent charge from a Baldwin-Farmer ion chamber in a water phantom, divided by the on-axis fluence of incident 7 MeV photons. This quantit) is the ratio between the conversion factor from fluence to absorbed dose to \vater and the chamber calibration factor specified by various dosimetrx protocols. The measured value is a test of the Monte Carlo codes and dosimetric theory used to derive these two factors.A value of 4.13 X lo-'3 C kg" cm'* 1.6'/6 was measured. In order to assess the ion chamher charge due to scattered 7 MeV photons, a subsidiary experiment measured the dose due to scattered photons originating in a series of iron plates. Agreement was obtained between the measured dose as a function of iron plate thickness and that calculated using the EGS Monte Carlo simulation systems and also between the measured and calculated absorbed dose-depth curves. The results may be viewed as an experimental measurement. Lvith ?-3"o uncertainty. of the absorbed dose to water per unit incident 7 MeV photon fluence, or of the ion chamber calibration factor specified by dosimetry protocols.

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