[en] The Ionizing Radiation Division has developed new national standards for mammographic X rays and for brachytherapy sources, such as iodine-125. The Attix chamber, a variable volume free-air ionization chamber, has been established as the primary national standard for mammographic X rays. The Attix chamber resides in the newly developed NIST Mammography Calibration Range and will be used to perform routine calibrations. The wide-angle free-air ionization chamber utilizes a large volume and a novel electric field configuration in order to circumvent the limitations of conventional free-air chambers. Seventeen beam qualities for X rays from molybdenum (Mo) and rhodium (Rh) anodes have been parameterized for the calibration of mammographic ionization chambers. The beam qualities available include anode/filter combinations of Mo/Mo, Mo/Rh and Rh/Rh. The mammography range was developed in collaborations with the U.S. Food and Drug Administration's (FDA) Center for Devices and Radiological Health, the implementors of the Mammography Quality Standards Act (MQSA) of 1992. The wide-angle free-air ionization chamber has been used to measure the output of two types of iodine-125 seeds, those with resin balls and those with silver wire. Both free-air chambers have been intercompared with the Ritz parallel-plate free-air ionization chamber

[en] The calibration and irradiation of x-ray and gamma-ray instruments are performed in terms of the physical quantity exposure. The calibrations are listed in NBS Special Publication 250 as calibrations 46010C through 46050S (formerly 8.3A through 8.3M). A calibration or correction factor is provided for radiation detectors, charge sensitivity of a high-gain electrometer is tested, and passive dosimeters are given known exposures. Calibration is performed by comparing the instrument against an NBS primary standard of exposure, which is a free-air chamber for x-rays and a cavity ionization chamber for cesium-137 and cobalt-60 gamma rays. A variety of quality-assurance checks are performed to assure the constancy of the standards and the accuracy of the calibrations and irradiations. The overall uncertainty is given as 0.7% for exposure rate in the NBS beams, 1% for calibration of a cable-connected chamber and irradiation of passive dosimeters, and 1.5% for calibration of a condenser chamber

No abstract available

[en] In the course of an NBS absorbed-dose standards program, theoretical and experimental determinations have been made of the conversion factor C that relates the absorbed dose from electron beams to the ionization in an air cavity. The average medium/air stopping-power ratio - which is proportional to C - has been calculated as a function of the beam energy (1 to 60 MeV) and of the depth in a water phantom, and similar stopping-power ratios have also been obtained for other phantom materials (carbon, polystyrene, acrylic plastic, muscle). The conversion factor in graphite has been measured at energies between 15 and 50 MeV and depths between 0.9 and 51 g/cm² with the use of a calorimeter and a parallel-plate ionization chamber. These measurements were made with beams broadened by lead scattering foils with various thicknesses from 0.144 to 1.584 g/cm². The extrapolation of the results to zero scattering-foil thickness provided conversion factors that could be compared with theoretical C-values for broad, parallel, monoenergetic electron beams. The agreement was found to be close (mean difference of 0.3% and r.m.s. difference of 0.8%). Comparisons have also been made with other C-values found in the literature and recommended by medical physics organizations. The overall conclusion can be drawn that the conversion factor is known reliably at the 1% level of accuracy. (author)

[en] Extensive experimental comparisons of calorimetric and ionometric measurements have been made that cover a broader range of electrons 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 aluminium 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

No abstract available

[en] A comparison was made between the National Institute of Standards and Technology (NIST) and Ente per le Nuov 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

[en] A direct comparison has been made between the air-kerma standards of the NIST and the BIPM in the low-energy x-ray range. The results show the standards to be in agreement to around 0.5 % at reference beam qualities up to 50 kV and at 100 kV. Agreement at the 80 kV quality is less satisfactory, which may be attributed to an incorrect value for the electron-loss correction for the NIST standard at this quality. (authors)