Use of the BIPM calorimetric and ionometric standards in megavoltage photon beams to determineWairandIc

Burns, D T; Picard, S.; Kessler, C; Roger, Philippe

doi:10.1088/0031-9155/59/6/1353

Cited by 17 publications

(26 citation statements)

References 17 publications

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“…Of note, DL also reported measurements for depths well beyond the electron practical range in graphite, i.e., where energy deposition is due to electrons set in motion by secondary bremsstrahlung photons rather than primary electrons. Analysis of this photon regime data yields W air values which also compare favorably with recent values obtained using x‐ray beams produced by clinical linacs …”

Section: Introductionsupporting

confidence: 76%

“…could suggest that W air in fact exhibits an energy dependence for electrons with energies from a few MeV to 15 MeV; strictly speaking, this possibility still cannot be excluded. However, the data do not join in a satisfactory way to the well‐established results for 10 keV electrons,

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Co γ ‐rays, and MV x‐ray beams; hence the dependence is considered anomalous.…”

Section: Discussionmentioning

confidence: 75%

“…Range‐based Russian roulette and correlated sampling variance reduction techniques are also employed to increase simulation efficiency . A statistical uncertainty below 0.02% on the dose to the ionization chamber is sought, which is typically attained within 10 histories.…”

Section: Methodsmentioning

confidence: 99%

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Extracting W_air from the electron beam measurements of Domen and Lamperti

2017

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Purpose: The average energy expended by an energetic electron to create an ion pair in dry air, W air , is a key quantity in radiation dosimetry. Although W air is well established for electron energies up to about 3 MeV, there is limited data for higher energies. The measurements by Domen and Lamperti [Med. Phys. 3, 294-301 (1976)] using electron beams in the energy range from 15 to 50 MeV can, in principle, be used to deduce values for W air , if the electron stopping power of graphite and air are known. A previous analysis of these data revealed an anomalous variation of 2% in W air as a function of the electron energy. We use Monte Carlo simulation techniques to reanalyze the original data and obtain new estimates for W air , and to investigate the source of the reported anomaly. Methods: Domen and Lamperti (DL) reported the ratio of the response of a graphite calorimeter to that of a graphite ionization chamber for broad beams of electrons with energies between 15 and 50 MeV and at different depths in graphite (including depths well beyond the range of the primary electrons, i.e., in the bremsstrahlung photon regime). Using a detailed EGSnrc model of the DL apparatus, as well as up-to-date stopping powers, we compute the dose ratio between the ionization chamber cavity and the calorimeter core, for plane-parallel electron beams. This dose ratio, multiplied by the DL measured ratio, provides a direct estimate for W air . Results: Despite an improved analysis of the original work, the extracted values of W air still exhibit an increase as the mean electron energy at the point of measurement decreases below about 15 MeV. This anomalous trend is dubious physically, and inconsistent with extensive data for W air obtained at lower energies. A thorough sensitivity analysis indicates that this trend is unlikely to stem from errors in extrapolation and correction procedures, uncertainties in electron stopping powers, or bias in calorimetry or ionization chamber measurements. However, we find that results are quite sensitive to the intrinsic graphite mass thickness of the detectors and to the incident beam energy. Conclusions:The DL experiment provides data in an energy regime where the electron stopping power is insensitive to the mean excitation energy of graphite -an issue plaguing W air experiments at lower energies. Unfortunately, state-of-the-art scrutiny of the original data cannot explain the anomalous trend in terms of perturbation effects or extrapolation bias. It can only be understood in terms of speculative offsets in graphite mass thickness or beam energy. Therefore higher accuracy measurements for electron energies above 15 MeV are recommended to further resolve the value of W air .

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Section: Introductionsupporting

confidence: 76%

^{60}

Co γ ‐rays, and MV x‐ray beams; hence the dependence is considered anomalous.…”

Section: Discussionmentioning

confidence: 75%

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Extracting W_air from the electron beam measurements of Domen and Lamperti

2017

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“…k Q,Q 0 factors were calculated for a wide range of plane-parallel ionization chambers and a limited set of cylindrical ionization chambers. Two different sets of mean excitation energy 110 values for water (I w ) and graphite (I g ) were used: (i) the ICRU 37 (ICRU 1984) andICRU 49 (ICRU 1993) values currently in use (I w = 75 eV and I g = 78 eV); and (ii) the latest I-values for water (I w = 78 eV, Andreo et al 2013) and graphite (I g = 81.1 eV, Burns et al 2014), to be recommended in a forthcoming ICRU report on key data for ionizing radiation dosimetry. Two different W air values for proton beams were also used 115 accordingly (Andreo et al 2013).…”

mentioning

confidence: 99%

Monte Carlo calculation of beam quality correction factors in proton beams using detailed simulation of ionization chambers

2016

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This work calculates beam quality correction factors (k Q ) in monoenergetic proton beams using detailed Monte Carlo simulation of ionization chambers. It uses the Monte Carlo code penh and the electronic stopping powers resulting from the adoption of two different sets of mean excitation energy values for water and graphite: (i) the currently ICRU 37 and ICRU 49 recommended I 75 eV w = and I 78 eV g = and (ii) the recently proposed I 78 eV w = and I 81.1 eV g =. Twelve different ionization chambers were studied. The k Q factors calculated using the two different sets of I-values were found to agree with each other within 1.6% or better. k Q factors calculated using current ICRU I-values were found to agree within 2.3% or better with the k Q factors tabulated in IAEA TRS-398, and within 1% or better with experimental values published in the literature. k Q factors calculated using the new I-values were also found to agree within 1.1% or better with the experimental values. This work concludes that perturbation correction factors in proton beamscurrently assumed to be equal to unity-are in fact significantly different from unity for some of the ionization chambers studied.

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“…W air has been extensively measured for low energy beams, under 1 MeV, for both photon and electron sources, but measurements with megavoltage beams are limited, with only one data set each published for high‐energy electron and photon beams as shown in table 5.6 of ICRU report 90 . Cojocaru et al reported a single value of W air 33.84(14) eV in high‐energy electron beams (15 to 50 MeV) and Burns et al obtained 34.03(7) eV in high‐energy photon beams (6 to 25 MV).…”

Section: Introduction and Purposementioning

confidence: 99%

Determination of W_air in high‐energy electron beams using graphite detectors

et al. 2019

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Purpose: ICRU Report 90 on Key Data for Ionizing-Radiation Dosimetry: Measurement Standards and Applications (2014) has reaffirmed the recommended value of the mean energy required to create an ion pair in air, W air , to be 33.97(12) eV. The report also indicates that this "constant" of radiation dosimetry is energy independent above 10 keV, since there is no theoretical or experimental evidence to the contrary. The goal of this investigation is to obtain additional experimental determinations of W air in high energy beams and thus to verify the suggested energy independence. Methods: W air can be evaluated by combining ionometric and calorimetric measurements with a calculated ratio of the absorbed dose in the ion chamber air cavity and that of the calorimeter absorbing element. In this investigation, a graphite parallel plate chamber and a graphite calorimeter were used and the dose ratio was calculated using the EGSnrc Monte Carlo code. Measurements were made in electron beams from the NRC Vickers linear accelerator at two incident energies, 20 and 35 MeV. A range of average energies at the measurement point were obtained by inserting graphite plates in the primary beam. Results: The average value of W air obtained in this investigation is 33.85(18) eV which is consistent with the recommended value of 33.97(12) eV where the number in brackets represents the combined standard uncertainty of the value, referring to the corresponding last digits. The individual values of W air do not show any statistically significant energy dependence. Conclusion: The overall combined uncertainty of 0.5% meets the original target of the investigation. A larger-scale investigation, involving more individual energy points and a wider range of electron energies is required to go further and, for example, comment on the W air energy dependency question raised by Tessier et al.

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Use of the BIPM calorimetric and ionometric standards in megavoltage photon beams to determineW_airandI_c

Cited by 17 publications

References 17 publications

Extracting W_air from the electron beam measurements of Domen and Lamperti

Extracting W_air from the electron beam measurements of Domen and Lamperti

Monte Carlo calculation of beam quality correction factors in proton beams using detailed simulation of ionization chambers

Determination of W_air in high‐energy electron beams using graphite detectors

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Use of the BIPM calorimetric and ionometric standards in megavoltage photon beams to determineWairandIc

Cited by 17 publications

References 17 publications

Extracting Wair from the electron beam measurements of Domen and Lamperti

Extracting Wair from the electron beam measurements of Domen and Lamperti

Monte Carlo calculation of beam quality correction factors in proton beams using detailed simulation of ionization chambers

Determination of Wair in high‐energy electron beams using graphite detectors

Contact Info

Product

Resources

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Use of the BIPM calorimetric and ionometric standards in megavoltage photon beams to determineW_airandI_c

Extracting W_air from the electron beam measurements of Domen and Lamperti

Extracting W_air from the electron beam measurements of Domen and Lamperti

Determination of W_air in high‐energy electron beams using graphite detectors