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Title: SU-F-I-13: Correction Factor Computations for the NIST Ritz Free Air Chamber for Medium-Energy X Rays

Abstract

Purpose: The National Institute of Standards and Technology (NIST) uses 3 free-air chambers to establish primary standards for radiation dosimetry at x-ray energies. For medium-energy × rays, the Ritz free-air chamber is the main measurement device. In order to convert the charge or current collected by the chamber to the radiation quantities air kerma or air kerma rate, a number of correction factors specific to the chamber must be applied. Methods: We used the Monte Carlo codes EGSnrc and PENELOPE. Results: Among these correction factors are the diaphragm correction (which accounts for interactions of photons from the x-ray source in the beam-defining diaphragm of the chamber), the scatter correction (which accounts for the effects of photons scattered out of the primary beam), the electron-loss correction (which accounts for electrons that only partially expend their energy in the collection region), the fluorescence correction (which accounts for ionization due to reabsorption ffluorescence photons and the bremsstrahlung correction (which accounts for the reabsorption of bremsstrahlung photons). We have computed monoenergetic corrections for the NIST Ritz chamber for the 1 cm, 3 cm and 7 cm collection plates. Conclusion: We find good agreement with other’s results for the 7 cm plate. The data usedmore » to obtain these correction factors will be used to establish air kerma and it’s uncertainty in the standard NIST x-ray beams.« less

Authors:
 [1]
  1. National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899 (United States)
Publication Date:
OSTI Identifier:
22626785
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
61 RADIATION PROTECTION AND DOSIMETRY; 60 APPLIED LIFE SCIENCES; BREMSSTRAHLUNG; CORRECTIONS; DIAPHRAGM; DOSIMETRY; ELECTRON LOSS; FLUORESCENCE; KERMA; MONTE CARLO METHOD; X-RAY SOURCES

Citation Formats

Bergstrom, P. SU-F-I-13: Correction Factor Computations for the NIST Ritz Free Air Chamber for Medium-Energy X Rays. United States: N. p., 2016. Web. doi:10.1118/1.4955841.
Bergstrom, P. SU-F-I-13: Correction Factor Computations for the NIST Ritz Free Air Chamber for Medium-Energy X Rays. United States. doi:10.1118/1.4955841.
Bergstrom, P. 2016. "SU-F-I-13: Correction Factor Computations for the NIST Ritz Free Air Chamber for Medium-Energy X Rays". United States. doi:10.1118/1.4955841.
@article{osti_22626785,
title = {SU-F-I-13: Correction Factor Computations for the NIST Ritz Free Air Chamber for Medium-Energy X Rays},
author = {Bergstrom, P},
abstractNote = {Purpose: The National Institute of Standards and Technology (NIST) uses 3 free-air chambers to establish primary standards for radiation dosimetry at x-ray energies. For medium-energy × rays, the Ritz free-air chamber is the main measurement device. In order to convert the charge or current collected by the chamber to the radiation quantities air kerma or air kerma rate, a number of correction factors specific to the chamber must be applied. Methods: We used the Monte Carlo codes EGSnrc and PENELOPE. Results: Among these correction factors are the diaphragm correction (which accounts for interactions of photons from the x-ray source in the beam-defining diaphragm of the chamber), the scatter correction (which accounts for the effects of photons scattered out of the primary beam), the electron-loss correction (which accounts for electrons that only partially expend their energy in the collection region), the fluorescence correction (which accounts for ionization due to reabsorption ffluorescence photons and the bremsstrahlung correction (which accounts for the reabsorption of bremsstrahlung photons). We have computed monoenergetic corrections for the NIST Ritz chamber for the 1 cm, 3 cm and 7 cm collection plates. Conclusion: We find good agreement with other’s results for the 7 cm plate. The data used to obtain these correction factors will be used to establish air kerma and it’s uncertainty in the standard NIST x-ray beams.},
doi = {10.1118/1.4955841},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • In this paper we examine the depth and field size dependence of the overall correction factor {ital k}{sub ch} for in-phantom dose determinations in orthovoltage x-ray beams. The overall correction factor is considered to be composed of three contributions, i.e., (1) a contribution from the angular dependence of the chamber response free-in-air, derived based on the measured directional response of the NE2571 for different energies combined with Monte Carlo calculations; (2) a displacement effect and (3) a stem effect, both calculated using the Monte Carlo method for different field sizes and depths. The results show a variation of, at most,more » 2.2{percent} at the lowest photon energies (29.8-keV average photon energy) when going from 2 cm to 5 cm for a small circular 20-cm{sup 2} field. In the medium-energy range ({ge}100 kV), variations are limited to, at most, 1.5{percent} for 120 kV{endash}150 kV when comparing the most extreme variations in field size and depth (i.e., 2-cm depth; 20-cm{sup 2} area compared to 5 cm depth; 200-cm{sup 2} area). Depth variations most importantly affect the overall correction factor by hardening of the photon fluence spectrum, whereas field diameter variations affect the factor by increase or decrease of contributions of photon scattering. The work shows that taking into account the uncertainties adopted in the recent review of data and methods recommended in the IAEA code of practice, the dependence of the overall correction factor on depth and field size is insignificant for the radiation qualities between 100 kV (HVL 0.17-mm Cu, average energy: 52 keV) and 280 kV (HVL 3.41-mm Cu, average energy: 144 keV). {copyright} {ital 1996 American Association of Physicists in Medicine.}« less
  • ABS>A description is given of the Canadian standard free-air chamber for measurement of medium quality x rays, including measurements of the contribution from radiation scattered from the air and from the diaphragm. A method is given for the accurate alignment of the chamber and x-ray source. (auth)
  • Conversion factors from roentgens to rads for Ionex, JAPM substandard and Radocon 555 dosimeters for high-energy x rays and electrons were experimentally determined using a Fricke dosimeter. The conversion factors obtained for /sup 60/Co gamma rays and high-energy x rays agreed within 1% with the values cited in A Practical Code for Dosimetry of 2 to 35 MV x rays and cobalt-60 gamma-ray beams in Radiotherapy in Japan,'' which were calculated by Greene and Massey. The values for high-energy electrons were slightly lower than those described in ICRU Report 21, but the differences were well within variation of experimental errors.more » The factors by depth in water also agreed with those in the ICRU Report for 20 to 30 MeV electrons, assuming that the effective center of the chamber was located one half of the cavity radius in front of the center of the cavity. (auth)« less
  • A description of a "low" energy free-air chamber is given. The standard chamber is designed to measure the exposure dose in roentgens for x-ray beams generated at potentials from 20 to 100 kilovolts-constant-potential (kvcp) with filtrations ranging from 2 mm of beryllium to 2 mm of beryllium plus 4 mm of aluminum. The chamber was compared with a "medium" energy standard at 60, 75 and 100 kvcp with filtrations of 1, 3, and 4 mm of aluminum, respectively. The two standard chambers agreed to within 0.3%. (auth)