Comparison of Exposure Standards for⁶⁰Co Gamma Rays

Abstract

Comparisons forgamma rays between the exposure standards of the Bureau International des Poids et Mesures (BIPM), the National Bureau of Standards (NBS) and the Physikalisch-Technische Bundesanstalt (PTB) are reported. For measuremente made at about one meter from the source, and if the same physical constants are used, the differences are 0.26% between the BIPM and NBS standards, and a t most 0.4% between the BIPM standard and the PTB standard chambers. The differences are consistent with the estimated systematic uncertainties (= 0.5 %).The difference between the NBS standard and the PTB chamber measurements, as determined through the ratios of each to the BIPM chamber, ranges from 0.17 to 0.66%. The difference in exposure standards for NBS and PTB actually ranges from 0.48 to 0.97%, since different stopping power corrections are used by these two laboratories.An indirect comparison of the BIPM and NBS standards indicates that the difference of 0.26% will increase to 0.55% a t a distance of two meters. RBsumBOn relate des comparaisons entre Btalons de mesure de l'exposition dans un faisceau de rayons gamma du 6OCo: ceux du Bureau International des Poids et Mesures (BIPM), du National Bureau of Standards (NBS) et de la Physikalisch-Technische Bundesanstalt (PTB). Pour les mesures effectuBes B un metre environ de la source et si Yon utilise les m6mes constantes physiques, les Bcarta sont 0,26y0 entre les &talons du BIPM et du NBS, et au plus 0,4 % entre 1'Btalon du BIPM et les chambres Btalons de la PTB. Ces diffhrences sont compatibles avec les incertitudes systBmatiques estimbes (= 0,5 %).Les diffbrences entre les mesures de 1'6talon du NBS et des chambres de la PTB, telles qu'on peut les dBduire de leurs rapports respectifs la chambre du BIPM, sont comprises entre 0,17 et 0,66%. Les d8Brences entre lea Btalons #exposition du NBS et de la PTB sont en rBalit6 comprises entre 0,48 et 0,97 % puisque ces deux laboratoires utilisent des corrections de pouvoir de ralentissement d8Brentes.Une comparaison indirecte des Btalons du BIPM et du NBS montre que la diffkrence 0,26% deviendrait 0,55% B unc distance de deux mhtres.

Monte Carlo study of correction factors for Spencer–Attix cavity theory at photon energies at or above 100 keV

“…1 For higher-energy photons such as from 60 Co and 137 Cs sources ͑1250 and 662 keV average energies for bare sources͒, it is common to base primary standards of air kerma on the use of graphite-walled cavity ion chambers and the Spencer-Attix schematization of Bragg-Gray cavity theory. 2,3 In the past, the National Institute for Standards and Technology ͑NIST͒ developed a primary standard for 192 Ir sources based on this approach. 4 However, 192 Ir sources have a wide spectrum of photon energies, from less than 100 up to about 900 keV with an average energy, and the strongest photon intensities being at slightly over 300 keV.…”

Standardization of 60Co and 137Cs gamma-ray beams in terms of exposure

“…Mo reover, th e agreement of the new exposure s tandard for 60CO with other standards [12] and other physical meas ure me nts [13] is within 0.4 perce nt.…”

“…The preceding discussion is based on ratios of chamber readings at different times in different radiation fields_ The percentage difference between the free-air chamber and the cavity chamber for these gamma-ray energies should be considered as an estimate_ Since the high-pressure free-air chamber is no longer available, confidence in the validity of the cavity-chamber determinations of exposure rate is derived from intercomparisons with other standards [12] and comparisons of cavity-chamber ionization measurements with other physical measurements such as source power [13]. Such comparisons have shown agreement in exposure-rate determinations to within several tenths percent.…”

On the Summability of Positive Solutions of Partial Differential Equations of Arbitrary Order in a Neighborhood of a Characteristic Manifold

“…Details of these have been given by Niatel et al (1975) for various types of standard exposure chambers and will only be given briefly here: kW is the product of kc and kCEp which are corrections concerning the chamber wall, central electrode and build-up cap (kc is the correction to zero wall thickness and kCEp is a factor that displaces the point of extrapolation from zero wall thickness to the mean centre of electron production); k,, corrects for scattering into the sensitive volume by the chamber stem; k,, corrects for the radial non-uniformity of the beam; and k,, corrects for the axial non-uniformity of the beam.…”