Comparison of the NIST and BIPM Medium-Energy X-Ray Air-Kerma Measurements.

The air-kerma standards used for the measurement of medium-energy x rays were compared at the National Institute of Standards and Technology (NIST) and at the Bureau International des Poids et Mesures (BIPM). The comparison involved a series of measurements at the BIPM and the NIST using the air-kerma standards and two NIST reference-class transfer ionization standards. Reference beam qualities in the range from 60 kV to 300 kV were used. The results show the standards to be in agreement within the combined standard uncertainty of the comparison of 0.35 %.

The effect of spectra on calibration and measurement with mammographic ionization chambers.

Mammographic imaging uses x-ray tubes with molybdenum, rhodium, or tungsten anodes with the produced bremsstrahlung filtered by thin sheets of molybdenum, rhodium, or aluminum. The National Institute of Standards and Technology, the Accredited Dosimetry Calibration Laboratories, and several manufacturers offer calibrations of mammography ionization chambers with reference x-ray beams with different radiation qualities in the range 23-40 kVp. The energy response of ten commercially available chambers was determined for these reference radiation qualities using the Attix variable-length free-air chamber. The evaluated chambers are designed with thin entrance windows of varying thickness and composition. The chambers show variation in their air kerma response as a function of beam radiation quality. This response with beam radiation quality may affect the measurement of clinical beam half value layer (HVL) and the determination of the mean glandular dose. The combined effect of the chamber's energy dependence and HVL measurement affects the mean glandular dose calculation resulting in differences ranging from -1.8% to +2.5%.

Well-ionization chamber response relative to NIST air-kerma strength standard for prostate brachytherapy seeds.

The response of well-ionization chambers to the emissions of 103Pd and 125I radioactive seed sources used in prostate cancer brachytherapy has been measured. Calibration factors relating chamber response (current or dial setting) to measured air-kerma strength have been determined for seeds from nine manufacturers, each with different designs. Variations in well-ionization chamber response relative to measured air-kerma strength have been observed because of differences in the emitted energy spectrum due to both the radionuclide support material (125I seeds) and the mass ratio of 103Pd to 102Pd (103Pd seeds). Obtaining accurate results from quality assurance measurements using well-ionization chambers at a therapy clinic requires knowledge of such differences in chamber response as a function of seed design.

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.

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

[This corrects the article on p. 701 in vol. 105.].

Comparison of the NIST and ENEA Air Kerma Standards.

A comparison was made between the National Institute of Standards and Technology (NIST) and Ente per le Nuove Tecnologie l'Energia e l'Ambiente (ENEA) air kerma standards for medium energy x rays and 60Co gamma rays. The comparison took place at ENEA in June 1994. Two different transfer chambers from NIST were used for the comparison. The measurements were made at radiation qualities similar to those used at the Bureau International des Poids et Mesures (BIPM) (generating voltages of 100 kV, 135 kV, 180 kV and 250 kV, respectively) and with 60Co gamma radiation. The transfer chamber calibration factors obtained at the NIST and at the ENEA agreed with one another to 0.03 % for 60Co gamma radiation and between 0.1 % to 0.8 % for the medium energy x-ray beam codes.

Comparison of exposure standards in the mammography x-ray region.

Direct comparisons of the National Institute of Standards and Technology's (NIST) Ritz 20 kV to 100 kV standard free-air ionization chamber and a portable variable-length free-air ionization chamber designed by the University of Wisconsin-Madison Accredited Dosimetry Calibration Laboratory (UW-ADCL) were made on NISTs low-energy tungsten x-ray range. As a result of this direct comparison, NIST has established a UW-ADCL designed chamber, the Attix chamber, as the national standard chamber for the mammography energy x-ray range. The Ritz standard chamber and the Attix standard chamber have been extensively compared using the new molybdenum and rhodium beam qualities. The results indicate that exposure measurements in the mammography energy x-ray region with the two free-air chambers can be made with a discrepancy of less than 0.35%.

Comparisons of calorimetric and ionometric measurements in graphite irradiated with electrons from 15 to 50 MeV.

Extensive experimental comparisons of calorimetric and ionometric measurements have been made that cover a broader range of electron energies and depths in graphite than previously reported. Electron beams of 15, 20, 25, 30, 40, and 50 MeV were used. Calorimetric absorbed-dose measurements and ionometric specific-charge measurements in air were compared in graphite at depths from 1 to 51 g/cm2. The medium was irradiated with uncollimated electron beams produced by scattering after passing through a 0.1-g/cm2 aluminum vacuum window, various thicknesses of lead foils, and air. The variation in the quotient of the two measurements was studied as a function of lead-foil thickness, depth in the medium, beam energy, foil-to-detector distance, and off-axis distance. These studies permitted the measurements to be corrected and compared with theoretical calculations that assume a broad medium irradiated with broad, parallel, monoenergetic electron beams. The overall experimental uncertainty is estimated to be 1%. The results are generally in good agreement with theoretical and experimental results of other investigators. The calorimeter received close to 1 Mrad during preliminary measurements and from 1 to 2 Mrad during the measurements reported. The results showed no detectable heat defect in graphite after prolonged periods of exposing the calorimeter to air at atmospheric pressure.