The new U.S. measurement standard for the air-kerma strength from low-energy photon-emitting brachytherapy seed sources is formally described in detail. This instrument-based standard was implemented on 1 January 1999, with its salient features and the implications of differences with the previous standard given only through a series of informal communications. The Wide-Angle Free-Air Chamber (WAFAC) is specially designed to realize air kerma from a single-seed source emitting photons with energies up to about 40 keV, and is now used to measure the wide variety of seeds used in prostate-cancer therapy that has appeared in the last few years. For the two 125I seed models that have been subject to both the old and new standards, the new standard reduces the air-kerma strength by 10.3 %. This change is mainly due to the removal of the influence on the measurement of the Ti K x rays produced in the source encapsulation, a component with no clinical significance.
At th e Na tion a l Burea u of S ta nd ard s (N BS), th e ex pos ure-ra te s ta nd ard s fo r ""Co a nd mcs ga mm a rays we re based fo r a numbe r of yea rs o n a we ig ht ed ave ra ge of meas ureme nt s usin g a cy lin d ri ca l ionizati o n cha mbe r a nd a gro up of s m ail s phe ri ca l c ha mbe rs . Comp lex se tu p co ndition s for th e cy lindrica l c hamb e r. diffe re nces be t wee n th e cy lindri ca l a nd s ph e ri ca l c ha mbe r d a ta, a nd recogniti o n th a t th e in stituti o n of th is we ight e d ave rage ex pos ure-ra te sta nd a rd increased th e diffe re nce be t wee n freeair-c ha mbe r a nd c av it y-ch a mbe r meas ure me nts, le d to th e de velopm e nt of new s ph e ri ca l c h a mbers. All co rrecti o n fa c to rs for ex pos ure-rate meas ure me nt we re in ves ti ga ted a nd upd a ted _ Exce ll e nt ag ree me nt was a c hi eved be t wee n ind e pe nd ent ex pos ure-rate me as urc mc nt s for s ix s ph e ri ca l c ha mbe rs a nd , as of May 1, 1972 , th e e xpos ure sta ndard s we re redu ce d 0. 7 pe rce nt for ""Co a nd 0.6 pe rce nt fo r ""Cs ga mm a ra ys. Reca lc ul a tion of cor rec ti o n fa c to rs s in ce th a t ti me indi ca tes th at th e s ta ndard ':J'Cs s ho u ld be furth e r redu ced b y 0. 2 perce nt , a nd thi s adju s tm e nt wa s mad e o n Jul y 1. 1974.Th e un ce rt a inties associated with eac h of th e qu a ntiti es e nt er in g int o th e de termin a ti o n of e xpo s ure rate we re tabul a te d a nd th e ove ra ll un ce rt a int y of th e ex pos ure ra tes use d for in s trum e nt ca librati o ns a t NBS was fo und to be abo ut 0.7 percen t for add iti on in qu adrature_ Key word s : Cav it y io ni za ti on c hamb e r; ex po s ure; gamma rays ; IWCO; ''''Cs; sta nd a rds. Introduct ionThe s tand ard in strum e nt e mployed for ex pos ure meas ure m e nts, for x-rays ge nerate d b y pote nti als from te n up to a fe w hundred kil ovo lts, is th e free -air ionizati o n c hamb e r [1 , 2, 3).1 Expe ri me nt al di ffi c u Iti es e nco unter ed with this in s trum e nt at hi gh e nergi es, in fulfillin g th e re quire me nt that all ionizin g particles be stopped in air , cause stand a rd s labo ratories 10 have reco urse to another me thod of s tan dardiza tion . Thi s alternate me thod , which obviates the necess ity of a free-air c hamb er operated at hi gh pressure , or with large pla te s pac in g, e mploys cavity io nizati o n chambers and relies o n th e prin ciples of th e Bragg-Gray th eo ry [4] for its validity.Th e expos ure, X, is th e quoti e nt of dQ by dm where dQ is th e absolute value of th e total c harge of th e ions of one sign produ ced in air wh e n all electrons libe ra te d by photon s in a volume ele me nt of ai r havin g mass dm are co mpletely s topped in air [51. Th e special unit of expos ure is th e roe ntge n whi c h is e qual to 2.58 X 10-4 C· k g-I.Durin g 1959, Wyc ko ff [6] meas ured a 60 CO so urce and a 13iCS source, in roe ntge ns, usin g a press urize d free-air c hamb e r. Fo r co mpari so n, the exposure rates we re also m...
The exposure rate computed from calorimeter power measurements is compared to the exposure rate of the same source ( 1-Ci cobalt-60) determined by means of a graphite cavity ionization chamber. A difference of 0.38 Yo was found and is not considered significant compared to the estimated systematic uncertainty (1.3 %) * Contribution of the National Bureau of Standards. Not ** Present address of B. Petree is Undershaft and Lazarus, subject to copyright.
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