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

Abstract

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...

Standardization of iridium-192 gamma-ray sources in terms of exposure

“…Th e standard cha mbers employed for th ese measuremen ts have already been describ ed in d etail [4) . Of th e eight chambers available, the three with the largest air volum es (all nominally 50 em' ) were use d. For so me me asurements, individual chambers were used, while for oth ers, all three chambers we re used.…”

“…Calculation s for the 50 cm 3 standard chamber, based on previous data [4] for a collection potential of 500 V, predict a correction factor of 1.0013 for the expected exposure rates. Checks were mad e on the chamber saturation conditions during the open-air geo metry measurements by reducing the potential from 500 V to 250 V. Results show a reduction in ionization current of 0.17 percent when the exposure rate is 9.3 nA / kg (36 J.lR ls).…”

“…Of th e eight chambers available, the three with the largest air volum es (all nominally 50 em' ) were use d. For so me me asurements, individual chambers were used, while for oth ers, all three chambers we re used. Th e equ ation for ex po sure computa tions is given in [4) but for ease of reference is also given here.…”

Calibration of gamma-ray-emitting brachytherapy sources

“…The NBS exposure standards for^^Co and i^tq^g amma rays were revised on 1972 May 01, at which time the NBS standards became the mean response of a set of six spherical graphite ionization chambers [9].…”

“…The working standard sources were measured in 1967 [7] using a graphite cavity ionization chamber now identified as chamber 1 [9].…”