NBS free-air chamber for measurement of 10 to 60 kV x rays

Abstract

Constructional d etails of t he National Bureau of Standards' new free-a ir chamber for the m eas urement of 10 to 60 kV x rays in roentgens are given . The res ults of t he comparisons of t he new cham ber with t he National Burea u of Standards' "low" energy standard in t heir overlappin g ran ge a re included. The two standa rd cha mbers, on t he a verage, agreed to w ithin 0.3 percen t. The maximum ull cE:rtainties in t he correction factors used for t hc co mpari son m easu rements are exam ined a nd t heir sum is co mpared with t he res ults obtained for t he chamber co mparisons. An estimate is also made of t he maximum uncertainty, a bout 1.3 percent, of an expos ure rate d etermination to be expected when using t he 10 to 60 kV cha mber to measure 10 to 15 kV x rays.

Calibration of x-ray and gamma-ray measuring instruments

“…These standards are discussed in Refs. [3,4,5]. Figures 6-3 to 6-5 show cross-sectional views of the three free-air chambers used in the tungsten x-ray ranges.…”

“…The corrections for the Lamperti chamber are discussed in detail by Lamperti and Wyckoff in Ref [5]. Although many different corrections are identified, the important corrections are for air attenuation, k^and scattered photon contribution, .…”

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

“…The comparison of these measurements assumes that the ratios established for the directional dependence of the cylindrical chamber in a collimated beam are applicable to the open-air calibrations_ All data in table 1 are for source-to-chamber distances comparable to those used in the open-air-geometry source measurements (0_8 m)_ While the cavity-chamber and free-air-chamber measure ments of exposure rate for high-energy gamma rays agreed to about 2 percent, the difference was still sufficiently large as to indicate that the corrections for the cavity chamber could be improved_ The procedure of angling the cylindrical chamber in the beam to remove the distance effect, not only caused a divergence of the cavity-chamber, free-air-chamber exposure measurements,3 but added complexity to setup conditions for beam measurements_ These difficulties prompted the development of new chambers which would not be critically directional dependent (and therefore easily set up), would allow investigation of possible variation of chamber response with chamber size, and eventually lead to the establishment of a standard based on a group of chambers of homogeneous geometry_ The new chambers would also allow studies of chamber wall corrections, with the goal of improving agreement between the free-air-chamber and cavity-c hamber exposure-rate measurements_ To 3 This s tat e me nt pertain s o nl y 10 th e 6OCO d at a sin ce the an gulation pro cedure improves th e agree me nt fo r 1:I7C5 , Howev e r. exa min a ti on of th e 1959-1961 da ta fo r the mes m easureme nt s show s Ih a t the room sca li er corre c ti o n is 1.6 perce nt. roughl y fo ur limes th e COTrec tio n used fo r 6O C O.…”

“…2.58 X 10 -4 vp ",sair (m fJ-en) c where 2.58 X 10 -4 is th e number of co ulombs per kilogram of air produced by one roe ntge n, Q air is the measured c harge (in ~o ulomb s), v is the c hamber volume (in m 3 ) and p is the density (in kgfm 3 ). II k i is the product of all the factors required to correct the meas ured charge for experimental condition s. These are :…”

A modified dose calculation formalism for electronic brachytherapy sources

“…Due to the large uncertainties in defining air-kerma strength in vacuo for low energy x-ray sources with a bremsstrahlung spectrum, as mentioned previously, a consensus was reached between NIST, the manufacturer, and the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) that NIST would establish an air-kerma rate standard for electronic brachytherapy sources. The air-kerma rate standard was accomplished through the introduction of the Lamperti Free Air Chamber (7). The NIST determination of the air-kerma rate standard for the Xoft model S700 source is the subject of a recently published NIST journal article (8).…”