Primary Standards For Air Kerma Research Articles

Characterization of new primary air kerma standards for dosimetry in Co-60, Cs-137 and Ir-192 gamma ray sources

Introduction. To be better suited for measurements at air kerma rates which are common for the calibration of detectors for dosimetry in brachytherapy and radiation protection, two graphite-wall, air-filled cavity ionization chambers (PS-10 and PS-50 built by PTW of Freiburg, Germany) were characterized for use as primary standards at the Physikalisch-Technische Bundesanstalt (PTB) for 60Co, 137Cs and 192Ir gamma ray sources. Methods. Applying experimental or Monte Carlo methods, corrections were determined for wall effects, stem scattering effects, point-source non-uniformity, saturation effects, and deviations from Spencer-Attix cavity theory. The geometrical cavity volume of the ionization chambers was determined by a special experimental method. Non-charge-collecting parts of the volume were calculated by finite-element simulations. In addition, mass electronic stopping powers and mass-energy absorption coefficients were reevaluated in close agreement with ICRU 90. To compare the results to an already established air kerma standard, measurements of air kerma rates were taken for gamma rays from all three sources. Results. In contrast to the standards currently in use, the newly characterized ionization chambers show significant wall effects up to 4.17% (192Ir). For the PS-10 ionization chamber, the geometric volume differs by 0.42% from the charge collecting volume. For the ^192Ir source, Spencer-Attix corrections were applied exceeding 0.30% for both ionization chambers. Applying the derived corrections factors, air kerma rates measured with the two ionization chambers at the three sources did not differ by more than 0.13%. Comparing the results to an already established air kerma standard, deviations ranged up to 0.61%. The uncertainty budget for the determination of air kerma rates resulted in a relative combined standard uncertainty of 0.35%. Conclusion. Both ionization chambers can be used as primary air kerma standards for 60Co, 137Cs and 192Ir gamma ray sources using the values of corresponding physical quantities and correction factors derived in this work.

P037] Implementation of a new free-air ionization chamber as a primary air kerma standard for mammography radiation qualities

Purpose The Physikalisch-Technische Bundesanstalt (PTB) offers air kerma calibrations of secondary standards for standard dosimetry laboratories worldwide. Due to the significant increase in the demand for such calibrations in mammography X-ray beams (tube high voltages ranging from 20 kV to 50 kV), it was decided to construct and manufacture a novel free-air ionization chamber (FAC) with a design especially optimized for radiation qualities used in mammography. This work describes the final technical construction of the new FAC and its implementation as a primary air kerma standard at PTB. Methods The new FAC is of the parallel-plate type optimized for the photon energy range of 5 keV to 50 keV. Special emphasis has been given to the design of a chamber without potential wires in the beamline to minimize the scattering and attenuation of the X-rays. The design of the chamber was optimized with respect to electrical field homogeneity using the software package SIMION Version 8.1. This software was also used to calculate k d , the correction factor for the distortion of the electric field of the final chamber design. The other correction factors for photon attenuation, scattering, aperture transmission and scattering, electron loss and violating charged-particle equilibrium have been calculated using the user code ’egs_fac’ of the EGSnrc Monte Carlo transport simulation software package. Finally, air kerma rates measured with the PK50, the new free-air chamber, were compared with those measured with the other well-established primary air kerma standards of PTB. Results The PK50 was fully characterized by all the necessary correction factors. The correction for the field distortion is below 0.8%, thus the design goal has been achieved. For the ISO narrow series, the correction factors calculated with egs_fac are below 1.0%, proving that the chamber is well optimized for energies below 50 keV. Comparisons of air kerma measurements with the PK50 and established FACs yielded results which were consistent within the uncertainties of a few parts in 1000. Conclusions The new chamber designated as the PK50 is now ready for use as a primary air kerma standard for low-energy X-rays at PTB.

Comparison of air kerma standards for medium-energy x-radiation between the MKEH and the IAEA

A comparison of the air kerma standards for medium-energy x-radiation was performed between the MKEH and the IAEA. Two reference class ionization chambers of the IAEA, traceable to the PTB with volumes 1000 cm3 and 100 cm3, and the ISO 4037 N-40, N-60, N-120 and N-300 standard beam qualities were selected for the comparison. The calibration coefficients, NK, were determined for both chambers at the MKEH in August 2011 and before and after this at the IAEA Dosimetry Laboratory.The comparison ratio of the calibration coefficients normalized to the previous comparison results of the PTB and MKEH primary air kerma standards was in the range from 0.989 to 0.994 for the beam qualities used. The relative expanded (k = 2) uncertainty for each of these ratios is around 2%.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by EURAMET, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Wall Correction Factors Estimated by Monte Carlo Calculation for Spherical Graphite-Walled Cavity Chambers in INER’s 60Co Air-Kerma Standard

The purpose of this research is to estimate the wall effect of spherical graphite-walled cavity chambers using the Monte Carlo method to establish a 60Co air-kerma standard at the National Radiation Standard Laboratory of the Institute of Nuclear Energy Research (NRSL/INER), Taiwan. For more than a decade, the validity of the wall correction term kwall determined by linear-extrapolation methods has been strongly challenged by the Monte Carlo method. In this paper, one goal was to evaluate in detail kwall for spherical chambers varying with wall thickness (0.1 to 2.5 cm), cavity size (1 to 1000 cm3), and incident photon energy (0.02 to 1.33 MeV). The other goal was to obtain kwall for self-fabricated, spherical chambers and then compare it with the historical values in 2003. A significant increase of 0.3% for air kerma in the 60Co field was expected. The difference of bilateral comparison between NRSL/INER and the Australian Radiation Protection and Nuclear Safety Agency was reduced when the calculated kwall, instead of the original estimated value of kwall, was applied for the derivation of the calibration factor. The NRSL/INER primary standards for air kerma will be adjusted in the near future to reflect the changes in kwall described in this work.

WE‐E‐BRD‐03: Re‐Evaluation of the Product of W/e and the Graphite to Air Stopping Power Ratio for Co‐60 Air Kerma Standards

Purpose: To reanalyze experiments which determine , the product of (W / e)air, the average energy deposited per coulomb of charge released in dry air, and , the Spencer‐Attix mass collision stopping‐power ratio for graphite to air, and to calculate an average value for this product for the BIPM air kerma standard: . This value could be adopted for use with air kerma primary standards, along with corrections to account for variations due to cavity size. Methods and Materials: The experiments measured by various methods, often involving calorimeters and ionization chambers. Correction factors, e.g., to account for gaps about a calorimeter core or perturbations due to a cavity's presence, are calculated as needed for each experiment using the EGSnrc user‐codes CAVRZnrc, DOSRZnrc, and CAVITY. Stopping power ratios are evaluated using SPRRZnrc for different choices of graphite density (bulk 1.70 g/cm3 or grain 2.265 g/cm3) for the density effect correction and average excitation energy for graphite (I=78 or 87 eV). For each experiment, the corrected value of is multiplied by , the quotient of the stopping power ratios for the BIPM chamber and the experiment in question. A least squares technique is used to compute an average value of . Results: The correction factors generally decrease the value of for each experiment, often outside the range of one standard deviation quoted with each experimental result. The ratio varies by less than 0.1% for different choices of density correction and I‐value and hence the product is also relatively insensitive to these choices.Conclusion: The preliminary analysis suggests that the accepted value of , 33.97 J/C ±0.15%, is 0.6% too high. This would have implications for primary air kerma standards worldwide and for the value of (W / e)air which is used in low energy x‐ray standards.

Realization of reference air-kerma rate for low-energy photon sources

A large air-filled parallel-plate extrapolation chamber with thin graphite front and back electrodes is used as a primary standard measuring device for low-energy interstitial brachytherapy sources from which the unit of air-kerma strength or reference air-kerma rate can be derived. The chamber is suitable for low-energy photons with energies up to 40 keV. The underlying principle is that the air-kerma rate at a given point is proportional to the increment of ionization per increment of chamber volume at chamber depths greater than the range of secondary electrons originating from the electrode. The fundamentals for evaluating the extrapolation curves are presented as well as a detailed description of the present set-up of the PTB large-volume extrapolation chamber (GROVEX). Comparisons between the GROVEX and other primary standards for air-kerma and reference air-kerma rate are presented.

Comparison of national air kerma standards for ISO 4037 narrowspectrum series in the range 30 kV to 300 kV

For the first time results are presented of an indirect comparison of ten national standards for air kerma for ten radiation qualities of the ISO 4037 narrow spectrum series in the range from 30 kV to 300 kV. Nine of the ten participants maintain primary air kerma standards, one is traceable to PTB. The comparison was conducted in the time period from February 2004 until October 2005. For each radiation quality, the results are analysed in terms of the degrees of equivalence of each national standard with respect to the comparison reference value. These data form the basis of the results in the BIPM key comparison database for comparison EUROMET.RI(I)-S3. In addition, results are presented of differences in the results due to different realizations of beam qualities at the participants' sites and of the influence of different transfer chamber sizes on the variance of the comparison results.Main text.To reach the main text of this paper, click on Final Report.Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCRI Section I, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Efficient x-ray tube simulations

This article describes an efficiency study of directional bremsstrahlung splitting (DBS) for x-ray tube modeling. DBS is shown to be up to five or six orders of magnitude more efficient at 50 or 135 kV tube potential than a simulation without splitting, and 60 times more efficient compared to uniform bremsstrahlung splitting. A methodology is presented to determine the optimum splitting number for a given situation using a second degree polynomial expression derived from theoretical considerations. Very large optimum splitting numbers are found for small fields (5 mm radius) at 1 m from the x-ray source, which are relevant for half-value layer (HVL) calculations and for simulations related to primary air kerma standards. Two approaches for the calculation of kerma at a plane and inside a volume using track-length estimation are implemented in BEAMnrc, a user-code from the EGSnrc Monte Carlo simulation system for photon and electron transport. A practical application of DBS to HVL calculations for a Comet MXR-320 x-ray tube is reported. The agreement with measured HVLs at different constant tube potentials is found to be better than 2.3%.

Results supporting calculated wall correction factors for cavity chambers

Thick walled cavity ionization chambers are used by primary standard laboratories as primary air kerma standards in 137Cs and 60Co γ-rays. Application of the cavity theory requires correction for the effects of photon attenuation and scattering in the chamber walls. For more than a decade there have been intensive discussions about the validity of wall correction factors determined by more traditional extrapolation methods versus those calculated by Monte Carlo methods. For existing primary standards the alternative methods lead to results that differ by up to 50% of the correction itself. This report presents both experimental and theoretical results which strongly support the validity of calculated wall correction factors. Moreover, it is demonstrated that, in selected cases, the application of a linear extrapolation method leads to errors in the determination of the air kerma reaching up to 13%.

Changes in the U.S. Primary Standards for the Air Kerma From Gamma-Ray Beams.

Monte Carlo photon-electron transport calculations have been done to derive new wall corrections for the six NBS-NIST standard graphite-wall, air-ionization cavity chambers that serve as the U.S. national primary standard for air kerma (and exposure) for gamma rays from 60Co, 137Cs, and 192Ir sources. The data developed for and from these calculations have also been used to refine a number of other factors affecting the standards. The largest changes are due to the new wall corrections, and the total changes are +0.87 % to +1.11 % (depending on the chamber) for 60Co beams, +0.64 % to +1.07 % (depending on the chamber) for 137Cs beams, and −0.06 % for the single chamber used in the measurement of the standardized 192Ir source. The primary standards for air kerma will be adjusted in the near future to reflect the changes in factors described in this work.

The response of LiF thermoluminescence dosemeters to photon beams in the energy range from 30 kV X rays to 60Co gamma rays

The energy response of standard (TLD-100) and high-sensitivity (TLD-100H) LiF thermoluminescence dosemeters (TLDs) has been studied for photon beams with mean energies from about 25 keV to 1100 keV. Canadian primary standards for air kerma were used to establish the air kerma rates for each of the photon beams. TLDs were mounted in a PMMA holder and the air kerma response was measured as a function of energy. The EGSnrc Monte Carlo code was used to model the TLD holder and calculate the absorbed dose to the TLD chip per unit air kerma for each beam. The measured and calculated results were combined to obtain the intrinsic dose response of the TLD chip. Broadly, our results are consistent with existing data, which show a marked difference in the energy dependence of the two materials. However, the precision of our measurements (standard uncertainty of about 0.6%) has permitted the identification of features that have not been noted before. In particular, the energy dependence of the two materials is quite different in the important energy region delimited by 137Cs and 60Co gamma rays.

Degrees of equivalence for the key comparison BIPM.RI(I)-K3 between national primary standards for medium-energy x-rays

Results are presented of comparisons of ten national primary standards for air kerma with the BIPM primary standard over the range of x-radiation qualities from 100 kV to 250 kV. For each radiation quality, the results are analysed in terms of the degree of equivalence, Di, of each national standard i with respect to the key comparison reference value. The degrees of equivalence, Dij, between each pairing of national standards i and j are also evaluated. These data form the basis of the matrices and graphs of results entered in the BIPM key comparison database for comparison BIPM.RI(I)-K3.Main text.To reach the main text of this paper, click on Summary Report.Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The summary report has been peer-reviewed and approved for publication by the Consultative Committee for Ionizing Radiation (CCRI), according to the provisions of the Mutual Recognition Arrangement (MRA).

Degrees of equivalence for the key comparison BIPM.RI(I)-K2 between national primary standards for low-energy x-rays

Results are presented of comparisons of eleven national primary standards for air kerma with the BIPM primary standard over the range of x-radiation qualities from 10 kV to 50 kV. For each radiation quality, the results are analysed in terms of the degree of equivalence, Di, of each national standard i with respect to the key comparison reference value. The degrees of equivalence, Dij, between each pairing of national standards i and j are also evaluated. These data form the basis of the matrices and graphs of results entered in the BIPM key comparison database for comparison BIPM.RI(I)-K2.Main text.To reach the main text of this paper, click on Summary Report.Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The summary report has been peer-reviewed and approved for publication by the Consultative Committee for Ionizing Radiation (CCRI), according to theprovisions of the Mutual Recognition Arrangement (MRA).

Comparison of the NIST and NPL Air Kerma Standards Used for X-Ray Measurements Between 10 kV and 80 kV.

A direct comparison was made between the air kerma primary standards used for the measurements of low-energy x rays at the National Institute of Standards and Technology (NIST) and the National Physical Laboratory (NPL). The comparison was conducted at the NPL using NPL reference radiation qualities between 10 kV and 80 kV. The results show the primary air-kerma standards to agree within 0.6 % of their values for beam qualities up to 80 kV.

Implications of new correction factors on primary air kerma standards in 60Co-beams

Comparisons of primary standards for air kerma in 60Co-beams are re-analysed taking into account the recently developed formalism that defines uniquely the various correction factors and the development of analytic and Monte Carlo methods to quantify these corrections. After a brief historical review of air kerma comparisons and ion chamber calculations, the new corrections are applied in a re-analysis of previously published comparison data. An independent Monte Carlo verification of the analytic point-source non-uniformity correction factor is presented. The combination of new proposed correction factors imply that some national standards should increase by as much as 1% and that the global increase is of the order of 0.6%.