Re-evaluation of the product of (W/e)airand the graphite to air stopping-power ratio for60Co air kerma standards

Thomson, Rowan M.; Rogers, D. W. O.

doi:10.1088/0031-9155/55/13/001

Cited by 4 publications

(6 citation statements)

References 36 publications

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“…From this relation, one can deduce that, for example, the calculated ratio D core,MC /D cav,MC for I c = 78 eV, the value recommended in ICRU Report 37 (ICRU 1984), will be higher by the factor 1.0071 than that obtained using I c = 82.5 eV. An alternative estimate of this sensitivity to I c can be obtained from the calculations of Thomson and Rogers (2010), which yield the corresponding factor 1.0074. The difference of 0.03% is included as an uncertainty (see the footnote to table 4).…”

Section: The Value For W Air In 60 Co Radiationmentioning

confidence: 96%

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

Burns

Picard²,

Kessler³

et al. 2014

Phys. Med. Biol.

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The BIPM graphite calorimeter standard for absorbed dose to water has been used in conjunction with an ionization chamber of known volume and with Monte Carlo simulations of these arrangements to determine the value for Wair in (60)Co radiation and in accelerator photon beams up to 25 MV. The results show no evidence for a variation in Wair at the 0.2% level over this energy range. Taking the constancy of Wair as established, the best estimate is Wair = 34.03 eV with a standard uncertainty of 0.21%. Consistent with this analysis, and assuming the use of the grain density in evaluating the stopping power of graphite, is the value Ic = 81.1 eV for the mean excitation energy for graphite, with standard uncertainty 1.8 eV.

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Section: The Value For W Air In 60 Co Radiationmentioning

confidence: 96%

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

Burns

Picard²,

Kessler³

et al. 2014

Phys. Med. Biol.

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“…Three of these experiments [18][19][20] relate to measurements using 35 S, while the fourth [21] employs calorimetric measurements in 60 Co. While the details are not given by the authors, the results of this reanalysis are summarized in the more recent Thomson and Rogers [6] as W air = 33.98 eV with standard uncertainty 0.30 eV. The effect of using the original values and uncertainties of Boutillon and Perroche-Roux, rather than the reanalysed results, is discussed later.…”

Section: Data Setmentioning

confidence: 99%

“…Data set 7. Thomson and Rogers [6] revisited the analysis of the fifteen experiments compiled in Boutillon and Perroche-Roux [17] that resulted in the value W air = 33.97 eV Their ionometric determination, using the recommended values W air = 33.97 eV, I c = 78 eV and the graphite bulk density in evaluating s c,air , was around 1.4% higher than the calorimeter. For a graphite bulk density of 1.7 g cm −3 , equation (1) and its correction for this bulk density give s c,air = 1.0014.…”

Section: Data Setmentioning

confidence: 99%

“…There has been evidence for some time that the resulting value for the product W a s c,air for 60 Co gamma radiation is too high, and weaker evidence indicating how each of W air and s c,air might contribute. Much of the data are summarized in papers by Burns [5], Thomson and Rogers [6] and Seltzer and Bergstrom [7]. The purpose of the present analysis is to bring together the data in these works, to combine them with more recent information from a number of sources and to make robust and self-consistent estimates for W air , s c,air and the product W air s c,air .…”

Section: Introductionmentioning

confidence: 99%

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An analysis of existing data forW_air,I_cand the productW_airs_c,air

Burns

2012

Metrologia

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An analysis is made of the published results of over forty experiments that have determined either the mean energy Wair required to create an ion pair in dry air, the mean excitation energy Ic for graphite, or the product of Wair and the electron stopping-power ratio sc,air for graphite and air for 60Co gamma radiation. A minimization process is used in which the best estimates for Wair and sc,air are those that minimize the sum of the deviations, taking uncertainties into account. This approach yields self-consistent values for Wair, sc,air and the product Wairsc,air. The results of this process are the values Wair = 33.97 eV with standard uncertainty 0.11 eV, sc,air = 0.9926 for 60Co, with relative standard uncertainty 0.32%, and the product Wairsc,air = 33.72 eV for 60Co with standard uncertainty 0.08%. The resulting value for Ic depends on the treatment of the density effect in graphite, the best estimate being the value Ic = 81.1 eV with standard uncertainty 2.0 eV obtained when using the grain density in the evaluation of the graphite stopping power. The robustness of the analysis and the consequences for air-kerma primary standards for radiation dosimetry are discussed.

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“…For example, new recommendations for these quantities have led to a change of about 0.7% for the stopping power of electrons set in motion by

^{60}

Co γ ‐rays. In addition, recent work has pointed out that the perturbation correction previously ascribed to the graphite ionization chamber by Niatel et al . is incorrect.…”

Section: Introductionmentioning

confidence: 99%

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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Re-evaluation of the product of (W/e)_airand the graphite to air stopping-power ratio for⁶⁰Co air kerma standards

Cited by 4 publications

References 36 publications

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

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

An analysis of existing data forW_air,I_cand the productW_airs_c,air

Extracting W_air from the electron beam measurements of Domen and Lamperti

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Re-evaluation of the product of (W/e)airand the graphite to air stopping-power ratio for60Co air kerma standards

Cited by 4 publications

References 36 publications

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

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

An analysis of existing data forWair,Icand the productWairsc,air

Extracting Wair from the electron beam measurements of Domen and Lamperti

Contact Info

Product

Resources

About

Re-evaluation of the product of (W/e)_airand the graphite to air stopping-power ratio for⁶⁰Co air kerma standards

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

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

An analysis of existing data forW_air,I_cand the productW_airs_c,air

Extracting W_air from the electron beam measurements of Domen and Lamperti