Cavity ionization as a function of wall material

Attix, F.H.; Vergne, L.D.; Ritz, Victor H.

doi:10.6028/jres.060.028

Cited by 33 publications

(29 citation statements)

References 9 publications

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“…However, both models of the NE 2571 chamber with a 60 Co beam showed no variation in the P TP -corrected response as a function of air density ͑for the normal situation of a chamber with a buildup cap͒. This result is consistent with previous reports for different graphite-walled chambers, [29][30][31] and implies that geometrical specifications in Monte Carlo models are much less important for these types of calculations with higherenergy photons. As a check, measured values of the response to a 60 Co beam for the NE 2571 and Exradin A12 chambers ͑with buildup caps͒ were obtained using the Co unit at NRC and showed no variation in response over the range of pressures investigated here ͑data not shown͒.…”

Section: Ivb Sensitivity Of Monte Carlo Results To Chamber Geometrysupporting

confidence: 90%

An experimental and computational investigation of the standard temperature‐pressure correction factor for ion chambers in kilovoltage x rays

2007

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For ion chambers with cavities open to the surrounding atmosphere, the response measured at a given temperature and pressure must be corrected using the standard temperature-pressure correction factor (P(TP)). A previous paper based solely on Monte Carlo simulations [D. J. La Russa and D. W. O. Rogers, Med. Phys. 33, 4590-4599 (2006)] pointed out the shortcomings of the P(TP) correction factor when used to correct the response of non-air-equivalent chambers for low-energy x-ray beams. This work presents the results of several experiments that corroborate these calculations for a number of ion chambers. Monte Carlo simulations of the experimental setup revealed additional insight into the various factors affecting the extent of the breakdown of P(TP), including the effect of impurities and the sensitivity to chamber dimensions. For an unfiltered 60 kV beam, the P(TP)-corrected response of an NE 2571 ion chamber measured at 0.7 atm was 2.5% below the response measured at reference conditions. In general, Monte Carlo simulations of the experimental setup using EGSnrc were within 0.5% of measured values. EGSnrc-calculated values of air kerma calibration coefficients (N(K)) at low x-ray energies are also provided as a means of estimating the level of impurities in the chambers investigated. Calculated values of N(K) normalized to the value measured for a 250 kV beam were obtained for three chambers and were within 1% of experiment with one exception, the Exradin A12 in a 50 kV beam.

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Section: Ivb Sensitivity Of Monte Carlo Results To Chamber Geometrysupporting

confidence: 90%

An experimental and computational investigation of the standard temperature‐pressure correction factor for ion chambers in kilovoltage x rays

2007

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“…The widespread adoption of the Spencer-Attix cavity theory 7 traces back to the general observation in the 1950s and 1960s that it could account for the variation in response measured as a function of cavity dimension or cavity air pressure for chambers free in air, [7][8][9][10] whereas preceding cavity theories, such as that by Bragg and Gray, could not. In the simplest application, as it was originally applied, the SA cavity theory assumes that for a cavity filled with medium g surrounded by material m and located within an unscattered, unattenuated photon beam, the cavity is small enough such that it does not perturb the fluence of charged particles in medium m, and the dose deposited in medium g is completely due to charged particles originating in medium m. Under these conditions, the ratio of D m , the dose to medium m, and D g , the dose to medium g, is equal to the SA stopping-power ratio: 10…”

Section: Ia Overview Of Spencer-attix Cavity Theorymentioning

confidence: 97%

“…15 for additional details͒. Other experiments of this type were also performed during that time, including the classic experiment by Attix et al 8 However, those results are not considered here since many of the important experimental details were not included.…”

Section: ͑4͒mentioning

confidence: 99%

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

Russa¹,

Rogers²

2009

Medical Physics

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EGSnrc calculations of ion chamber response and Spencer-Attix (SA) restricted stopping-power ratios are used to test the assumptions of the SA cavity theory and to assess the accuracy of this theory as it applies to the air kerma formalism for 60Co beams. Consistent with previous reports, the EGSnrc calculations show that the SA cavity theory, as it is normally applied, requires a correction for the perturbation of the charged particle fluence (K(fl)) by the presence of the cavity. The need for K(fl) corrections arises from the fact that the standard prescription for choosing the low-energy threshold delta in the SA restricted stopping-power ratio consistently underestimates the values of delta needed if no perturbation to the fluence is assumed. The use of fluence corrections can be avoided by appropriately choosing delta, but it is not clear how delta can be calculated from first principles. Values of delta required to avoid K(fl) corrections were found to be consistently higher than delta values obtained using the conventional approach and are also observed to be dependent on the composition of the wall in addition to the cavity size. Values of K(fl) have been calculated for many of the graphite-walled ion chambers used by the national metrology institutes around the world and found to be within 0.04% of unity in all cases, with an uncertainty of about 0.02%.

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“…Here, the slight variation with respect to air density is attributed to the variation in ⌬, the low-energy cutoff used in determining the Spencer-Attix stopping power ratio. [27][28][29] Figure 5 shows results for the spherical chamber with C-552 air-equivalent plastic walls rather than graphite. In contrast to the spherical chamber with graphite walls, all normalized responses are within 0.7% of the response at the reference density.…”

Section: A Ne 2571 Chambermentioning

confidence: 99%

An EGSnrc investigation of the correction factor for ion chambers in kilovoltage x rays

Russa¹,

Rogers²

2006

Medical Physics

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As part of the standard practice for obtaining consistent ion chamber measurements with cavities open to the surrounding atmosphere, the raw measured response is corrected to the response at a reference temperature and pressure using the standard temperature-pressure correction factor (P(TP)). In this study, the EGSnrc Monte Carlo code was used to investigate the validity of the P(TP) correction factor for kilovoltage x rays incident on various geometrically distinct ion chambers. The calculated P(TP)-corrected chamber response deviated by over 2% relative to expected values for a 40 kV spectrum incident on a graphite thimble chamber at an air density typical of Mexico City. The relative deviation from the expected response was much worse for a large spherical graphite chamber, exceeding 16% at an air density of 0.6 kg/m3 (approximately 0.5 atm at 22 degrees C) for the same beam energy. The breakdown of the P(TP) correction factor was also observed for a 26 kV mammography spectrum incident on two mammography chambers. For 60Co beams, the P(TP) correction factor behaved as expected. For day-to-day variations in pressure, only a negligible of the P(TP) correction factor was observed with low x-ray energies. Factors contributing to the breakdown of the P(TP) correction factor at low x-ray energies and large pressure variations, such as the range of electrons, the material of the wall, the chamber dimensions and air-photon interactions, are discussed in depth.

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Cavity ionization as a function of wall material

Cited by 33 publications

References 9 publications

An experimental and computational investigation of the standard temperature‐pressure correction factor for ion chambers in kilovoltage x rays

An experimental and computational investigation of the standard temperature‐pressure correction factor for ion chambers in kilovoltage x rays

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

An EGSnrc investigation of the correction factor for ion chambers in kilovoltage x rays

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