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D W O Rogers - One of the best experts on this subject based on the ideXlab platform.
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an experimental and computational investigation of the standard temperature Pressure Correction Factor for ion chambers in kilovoltage x rays
Medical Physics, 2007Co-Authors: Daniel J La Russa, Malcolm Mcewen, D W O RogersAbstract: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{sub 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{sub 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{sub TP}, including the effect of impurities and the sensitivity to chamber dimensions. For an unfiltered 60 kV beam, the P{sub 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{sub K}) at low x-ray energies are also provided as a means of estimating the level of impurities inmore » the chambers investigated. Calculated values of N{sub 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.« less
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An Egsnrc investigation of the P(TP) Correction Factor for ion chambers in kilovoltage X rays.
Medical physics, 2006Co-Authors: Daniel J La Russa, D W O RogersAbstract: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{sub TP}). In this study, the EGSnrc Monte Carlo code was used to investigate the validity of the P{sub TP} Correction Factor for kilovoltage x rays incident on various geometrically distinct ion chambers. The calculated P{sub 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/m{sup 3} ({approx_equal}0.5 atm at 22 deg. C) for the same beam energy. The breakdown of the P{sub TP} Correction Factor was also observed for a 26 kV mammography spectrum incident on two mammography chambers. For {sup 60}Co beams, the P{sub TP} Correction Factor behaved as expected. For day-to-day variations in Pressure, only a negligible of the P{sub TP} Correction Factor was observed with low x-ray energies. Factors contributing tomore » the breakdown of the P{sub 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.« less
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TH‐C‐230A‐01: A Monte Carlo Investigation of the Temperature‐Pressure Correction Factor for Kilovoltage X‐Rays
Medical Physics, 2006Co-Authors: D. J. La Russa, D W O RogersAbstract:Purpose: To investigate the validity of the standard temperature‐Pressure Correction Factor (PTP) for kilovoltage x‐rays incident on various ionization chambers using Monte Carlo simulations of radiation transport. Method and Materials: The EGSnrc Monte Carlocomputer code was used to calculate the response due to 20 kV, 40 kV and 60 kV beams as a function of chamber air density for thimble and spherical ionization chambers. The chambers studied had both graphite and C‐552 plastic walls to investigate the effect of the wall material in addition to the dimensions of the cavity. In principle, the PTP‐corrected response is independent of air density. Thus, a breakdown of the PTP Correction Factor is identified by any variation in the calculated response as the air density is varied. The air density associated with the reference temperature and Pressure conditions in North America (22 °C, 101.325 kPa) is 1.205 kg/m3. Results: At an air density of 1.0 kg/m3, typical of Denver Colorado, the normalized PTP‐corrected response of a graphite‐walled thimble chamber due to the 20 kV and 40 kV spectra is as much as 1.7% and 1.2% below the expected response, respectively. For a graphite spherical chamber at the same air density, the calculated response is 3.8% below unity for 40 kV and 60 kV beam qualities. Calculated responses of chambers with C‐552 plastic walls are all within 0.5% of the expected response at air densities as low as 0.84 kg/m3. Comparisons of calculated air kerma calibration coefficients at different air densities indicate that the breakdown of the PTP Correction Factor should be easily detected experimentally. Conclusion: Variations in the PTP‐corrected response indicate that for low‐energy x‐rays the PTP Correction Factor inadequately accounts for the dependence of ion chamber response on the temperature and Pressure. Additional Correction Factors are therefore necessary under these circumstances.
Antonio M. Lallena - One of the best experts on this subject based on the ideXlab platform.
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[P150] Air density dependence of the response of the PTW sourcecheck 4PI ionization chamber to PD-103 brachytherapy sources
Physica Medica, 2018Co-Authors: Cristina Forastero Rodriguez, Ana M. Tornero-lópez, Damián Guirado, Jose Perez-calatayud, Joaquín Montes, Miguel Jiménez-melguizo, Clara Jóver, Antonio M. LallenaAbstract:Purpose Some well-type ionization chambers, when used for low-energy radionuclides, show a residual dependence on the air density once the standard Pressure and temperature Factor is applied.In this work the response of the PTW SourceCheck 4Pi ionization chamber to Pd-103 sources is analysed as a function of the air density, for the first time.Also an additional study for the Standard Imaging HDR1000 Plus chamber is performed. Methods Three PTW SourceCheck 4Pi TW33005 (with a PTW Unidos electrometer), and a Standard Imaging HDR1000Plus (with a Standard Imaging MAX4000 electrometer) were introduced individually in a homemade Pressure chamber equipped with a thermometer-hygrometer-barometer THB 40 (PCE Instruments).Different air densities were obtained by changing the Pressure inside the chamber by means of a vacuum pump.Four TheraSeed® 200 Palladium-103 seeds (Theragenics Corporation) were used. Raw measurements must be corrected according to: M corr = g 0 ( ρ ) g 1 ( ρ ) M raw , (1). where g 0 ( ρ ) is the usual temperature-Pressure Correction Factor and g 1 ( ρ ) an additional Factor verifying: g 0 ( ρ ) g 1 ( ρ ) = ρ / ρ 0 - u , (2). with ρ and ρ 0 the air density under actual and standard conditions, respectively, and u a parameter to be fitted. To study the influence of the experimental setup geometry and the number of seeds, an adapter with four seeds separated from each other by spacers commonly used in prostate implants was used. Results SourceCheck 4Pi and HDR1000Plus chambers show a potential dependence with the air density, with u = 0.533 ± 0.002 (k = 2) for the former and 0.431 ± 0.003 (k = 2) for the latter. The responses of the three SourceCheck 4Pi chambers were the same, within the uncertainties.Besides, no influence with the number of seeds simultaneously measured was observed. Conclusions Once corrected for the usual temperature-Pressure Factor, the PTW SourceCheck 4Pi chamber shows a residual dependence on the air density for Pd-103 sources that is better described by a potential function than by a linear one, contrary to what happens when using I-125 sources. The reason for these differences is currently being investigated using Monte Carlo simulation techniques. The result for HDR1000 Plus differs by 1.6% with those obtained by Griffin et al. (2005).
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Air density dependence of the soft X-ray PTW 34013 ionization chamber
Physica Medica, 2018Co-Authors: Cristina Forastero, Ana M. Tornero-lópez, Jesús J. López, Damián Guirado, Jose Perez-calatayud, Antonio M. LallenaAbstract:Abstract Purpose We studied the dependence on air density of the response of the PTW 34013 ionization chamber, recently upgraded for dosimetry control of low energy X-ray beams. Methods Measurements were performed by changing the Pressure conditions inside a Pressure chamber. The behavior of the measurements against the air density inside this chamber was analyzed. X-ray beams generated with 50, 70, 100, 150 and 200 kVp and the two electrometer polarities were considered. Results For all beams studied, measurements corrected with the conventional temperature and Pressure Factor showed a residual dependence on the air density that was described with a linear function of the air density. For the 50 and 70 kVp beams, corrected measurements remained ∼ 1 % smaller than the value found at standard Pressure/temperature conditions, for both electrometer polarities and for the air density range typical in clinical conditions. For air densities smaller than the standard one, measurements found for 100, 150 and 200 kVp beams were below or above the value found at standard Pressure and temperature when the negative or positive electrometer polarities were used, respectively. The differences with the measurements at standard conditions were less than 1 % for the 100 kVp beam and below 4 % for the other two beams. Conclusions The PTW 34013 ionization chamber showed a dependence on the air density that is not properly described with the usual temperature and Pressure Correction Factor. This residual dependence is negligible for low energy beams, for which this chamber is recommended, but is more substantial for beams with energy above 80 kVp.
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dependence with air density of the response of the ptw sourcecheck ionization chamber for low energy brachytherapy sources
Medical Physics, 2013Co-Authors: Ana M Tornerolopez, Damián Guirado, J Perezcalatayud, S Ruizarrebola, F Simancas, Maja Gazdicsantic, Antonio M. LallenaAbstract:Purpose: Air-communicating well ionization chambers are commonly used to assess air kerma strength of sources used in brachytherapy. The signal produced is supposed to be proportional to the air density within the chamber and, therefore, a density-independent air kerma strength is obtained when the measurement is corrected to standard atmospheric conditions using the usual temperature and Pressure Correction Factor. Nevertheless, when assessing low energy sources, the ionization chambers may not fulfill that condition and a residual density dependence still remains after Correction. In this work, the authors examined the behavior of the PTW 34051 SourceCheck ionization chamber when measuring the air kerma strength of125I seeds. Methods: Four different SourceCheck chambers were analyzed. With each one of them, two series of measurements of the air kerma strength for125I selectSeedTM brachytherapy sources were performed inside a Pressure chamber and varying the Pressure in a range from 747 to 1040 hPa (560 to 780 mm Hg). The temperature and relative humidity were kept basically constant. An analogous experiment was performed by taking measurements at different altitudes above sea level. Results: Contrary to other well-known ionization chambers, like the HDR1000 PLUS, in which the temperature-Pressure Correction Factor overcorrects the measurements, in the SourceCheck ionization chamber they are undercorrected. At a typical atmospheric situation of 933 hPa (700 mm Hg) and 20 °C, this underCorrection turns out to be 1.5%. Corrected measurements show a residual linear dependence on the density and, as a consequence, an additional density dependent Correction must be applied. The slope of this residual linear density dependence is different for each SourceCheck chamber investigated. The results obtained by taking measurements at different altitudes are compatible with those obtained with the Pressure chamber. Conclusions: Variations of the altitude and changes in the weather conditions may produce significant density Corrections, and that effect should be taken into account. This effect is chamber-dependent, indicating that a specific calibration is necessary for each particular chamber. To our knowledge, this Correction has not been considered so far for SourceCheck ionization chambers, but its magnitude cannot be neglected in clinical practice. The atmospheric Pressure and temperature at which the chamber was calibrated need to be taken into account, and they should be reported in the calibration certificate. In addition, each institution should analyze the particular response of its SourceCheck ionization chamber and compute the adequate Correction Factors. In the absence of a suitable Pressure chamber, a possibility for this assessment is to take measurements at different altitudes, spanning a wide enough air density range.
Marco Mancini - One of the best experts on this subject based on the ideXlab platform.
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re creating hottel s emissivity charts for water vapor and extending them to 40 bar Pressure using hitemp 2010 data base
Combustion and Flame, 2015Co-Authors: Michael Alberti, Roman Weber, Marco ManciniAbstract:Abstract Hottel’s water vapor emissivity chart and the Pressure Correction chart have been re-created using the most recent HITEMP-2010 data base to provide more accurate values. Hottel’s standard emissivity has been redefined in order to produce smooth and accurate graphs and correlations. The new charts allow for calculating the water vapor emissivities at homogeneous pathways up to 200 bar equivalent Pressure, in the 450–3000 K temperature range and in the 0.01–3000 bar cm Pressure path length range; a departure from the original Hottel’s emissivity values can be larger than 300% at high Pressures. Departures from Leckner’s (1972) correlations are typically in the ± 40% range. Besides the new paper graphs, showing the standard emissivity as well as the Pressure Correction Factor at different temperatures, the calculated emissivities have been tabulated (see Supplementary material) at equivalent Pressures ranging from 0.05 bar up to 250 bar. Using appropriate interpolation techniques, these charts can be used as look-up tables to calculate accurate total emissivity values without using Pressure Correction Factors. To this end, linear interpolation for temperature scaling and logarithmic interpolation for Pressure and Pressure path length scaling are included in the Supplementary material.
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Re-creating Hottel’s emissivity charts for water vapor and extending them to 40 bar Pressure using HITEMP-2010 data base
Combustion and Flame, 2014Co-Authors: Michael Alberti, Roman Weber, Marco ManciniAbstract:Abstract Hottel’s water vapor emissivity chart and the Pressure Correction chart have been re-created using the most recent HITEMP-2010 data base to provide more accurate values. Hottel’s standard emissivity has been redefined in order to produce smooth and accurate graphs and correlations. The new charts allow for calculating the water vapor emissivities at homogeneous pathways up to 200 bar equivalent Pressure, in the 450–3000 K temperature range and in the 0.01–3000 bar cm Pressure path length range; a departure from the original Hottel’s emissivity values can be larger than 300% at high Pressures. Departures from Leckner’s (1972) correlations are typically in the ± 40% range. Besides the new paper graphs, showing the standard emissivity as well as the Pressure Correction Factor at different temperatures, the calculated emissivities have been tabulated (see Supplementary material) at equivalent Pressures ranging from 0.05 bar up to 250 bar. Using appropriate interpolation techniques, these charts can be used as look-up tables to calculate accurate total emissivity values without using Pressure Correction Factors. To this end, linear interpolation for temperature scaling and logarithmic interpolation for Pressure and Pressure path length scaling are included in the Supplementary material.
M R Mcewen - One of the best experts on this subject based on the ideXlab platform.
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TH-AB-201-08: Ion Chamber Dose Measurements - Problems with the Temperature-Pressure Correction Factor
Medical Physics, 2016Co-Authors: Alexandra Bourgouin, M R McewenAbstract:Purpose: To investigate the behavior of ionization chambers over a wide Pressure range. Methods: Three cylindrical and two parallel-plate designs of ion chamber were investigated. The ion chambers were placed in vessel where the Pressure was varied from atmospheric (101 kPa) down to 5 kPa. Measurements were made using 60Co and high-energy electron beams. The Pressure was measured to better than 0.1% and multiple data sets were obtained for each chamber at both polarities to investigate Pressure cycling and dependency on the sign of the charge collected. Results: For all types of chamber, the ionization current, corrected using the standard PTP, showed a similar behaviour. Deviations from the standard theory were generally small for Co-60 but very significant for electron beams, up to 20 % below P = 10 kPa. The effect was found to be always larger when collecting negative charge, suggesting a dependence on free-electron collection. The most likely source of such electrons is low-energy electrons emitted from the electrodes. This signal would be independent of air Pressure within the chamber cavity. The data was analyzed to extract this signal and it was found to be a non-negligible component of the ionization current at atmospheric Pressure. In the case of the parallel plate chambers, the effect was approximately 0.25 %. For the cylindrical chambers the effect was larger - up to 1.2 % - and dependent on the chamber type, which would be consistent with electron emission from different wall materials. For the electron beams, the Correction Factor was dependent on the electron energy and approximately double that observed in 60Co. Conclusion: Measurements have indicated significant deviations of the standard Pressure Correction that are consistent with electron emission from chamber electrodes. This has implications for both primary standard and reference ion chamber-based dosimetry.
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TH-AB-201-06: Examining the Influence of Humidity On Reference Ion Chamber Performance
Medical Physics, 2016Co-Authors: J Taank, M R McewenAbstract:Purpose: International dosimetry protocols require measurements made with a vented ionization chamber to be corrected for the influence of air density by using the standard temperature-Pressure Correction Factor. The effect of humidity, on the other hand, is generally ignored with the provision that the relative humidity is between certain limits (15% to 80%). However, there is little experimental data in the published literature as to the true effect of humidity on modern reference-class ion chambers. This investigation used two different radiation beams – a Co-60 irradiator and a Sr-90 check source – to examine the effect of humidity on several versions of the standard Farmer-type ion chamber. Methods: An environmental cabinet controlled the humidity. For the Co-60 beam, the irradiation was external, whereas for the Sr-90 measurements, the source itself was placed within the cabinet. Extensive measurements were carried out to ensure that the experimental setup provided reproducible readings. Four chamber types were investigated: IBA FC65-G (×2), IBA FC65-P, PTW30013 & Exradin A19. The different wall materials provided potentially different mechanical responses (i.e., in terms of expansion/contraction) to the water content in the air. The relative humidity was varied between 8 % and 97 % and measurements were made with increasing and decreasing humidity to investigate possible hysteresis effects. Results: Measurements in Co-60 were consistent with the published data obtained with primary standard cavity chambers in ICRU Report 31. Ionization currents with Sr-90 showed no dependence with the relative humidity, within the measurement uncertainties. Very good repeatability of the ionization current was obtained over successive wet/dry cycles, no hysteresis was observed, and there was no dependence on chamber type. Conclusion: This null result is very encouraging as it indicates that humidity has no significant effect on these particular types of ionization chambers, consistent with recommendations in current megavoltage dosimetry protocols.
Michael Alberti - One of the best experts on this subject based on the ideXlab platform.
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re creating hottel s emissivity charts for water vapor and extending them to 40 bar Pressure using hitemp 2010 data base
Combustion and Flame, 2015Co-Authors: Michael Alberti, Roman Weber, Marco ManciniAbstract:Abstract Hottel’s water vapor emissivity chart and the Pressure Correction chart have been re-created using the most recent HITEMP-2010 data base to provide more accurate values. Hottel’s standard emissivity has been redefined in order to produce smooth and accurate graphs and correlations. The new charts allow for calculating the water vapor emissivities at homogeneous pathways up to 200 bar equivalent Pressure, in the 450–3000 K temperature range and in the 0.01–3000 bar cm Pressure path length range; a departure from the original Hottel’s emissivity values can be larger than 300% at high Pressures. Departures from Leckner’s (1972) correlations are typically in the ± 40% range. Besides the new paper graphs, showing the standard emissivity as well as the Pressure Correction Factor at different temperatures, the calculated emissivities have been tabulated (see Supplementary material) at equivalent Pressures ranging from 0.05 bar up to 250 bar. Using appropriate interpolation techniques, these charts can be used as look-up tables to calculate accurate total emissivity values without using Pressure Correction Factors. To this end, linear interpolation for temperature scaling and logarithmic interpolation for Pressure and Pressure path length scaling are included in the Supplementary material.
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Re-creating Hottel’s emissivity charts for water vapor and extending them to 40 bar Pressure using HITEMP-2010 data base
Combustion and Flame, 2014Co-Authors: Michael Alberti, Roman Weber, Marco ManciniAbstract:Abstract Hottel’s water vapor emissivity chart and the Pressure Correction chart have been re-created using the most recent HITEMP-2010 data base to provide more accurate values. Hottel’s standard emissivity has been redefined in order to produce smooth and accurate graphs and correlations. The new charts allow for calculating the water vapor emissivities at homogeneous pathways up to 200 bar equivalent Pressure, in the 450–3000 K temperature range and in the 0.01–3000 bar cm Pressure path length range; a departure from the original Hottel’s emissivity values can be larger than 300% at high Pressures. Departures from Leckner’s (1972) correlations are typically in the ± 40% range. Besides the new paper graphs, showing the standard emissivity as well as the Pressure Correction Factor at different temperatures, the calculated emissivities have been tabulated (see Supplementary material) at equivalent Pressures ranging from 0.05 bar up to 250 bar. Using appropriate interpolation techniques, these charts can be used as look-up tables to calculate accurate total emissivity values without using Pressure Correction Factors. To this end, linear interpolation for temperature scaling and logarithmic interpolation for Pressure and Pressure path length scaling are included in the Supplementary material.