Primary Standards: Air Kerma and Absorbed Dose

69 Pages·2009·3.89 MB·English

PPPPRRRRIIIIMMMMAAAARRRRYYYY SSSSTTTTAAAANNNNDDDDAAAARRRRDDDDSSSS ooooffff AAAAIIIIRRRR
KKEERRMMAA ffoorr 6600CCOO aanndd XX--RRAAYYSS
&&
AAAABBBBSSSSOOOORRRRBBBBEEEEDDDD DDDDOOOOSSSSEEEE iiiinnnn PPPPHHHHOOOOTTTTOOOONNNN
aanndd EELLEECCTTRROONN BBEEAAMMSS
Malcolm McEwen
Definitions:
Standard – “instrument/measurement/artifact
intended to define, realize, conserve or reproduce a
uniitt or quanttiitty tto serve as a refference””
Primary standard – “standard that is designated or
wwiiddeellyy aacckknnoowwlleeddggeedd aass hhaavviinngg tthhee hhiigghheesstt
metrological qualities and whose value is accepted
without reference to other standards of same quantity”
NNattiionall sttanddardd – ““standdardd recogniizedd bby a
national decision to serve as the basis for assigning
values to other standards of the qquantityy concerned”
Secondary standard – “standard whose value is
assigned by comparison with a primary standard of the
ssaammee qquuaannttiittyy”
So that’s clear?
“I sshaall noot aatteemppt tooddaayy too fuurtheer ddeefinee [[it]]……….bbuut I
know it when I see it.”
Justice Potter Stewart, 1964
ENERGY RANGES & QUANTITIES
10-50 keV – low energy x-rays Air Kerma
5500-330000 kkeeVV – mmeeddiiuumm eenneerrggyy xx-rraayyss AAiirr KKeerrmmaa
Cs-137 & Co-60 Air Kerma
Co-60 Absorbed Dose
Linac photon (x-ray) beams Absorbed Dose
Linac electron beams Absorbed Dose
11.. kkVV xx-rraayyss – aaiirr kkeerrmmaa
The primary standard for 10-300 kV x-rays is the
FFree AAiir CChhambber
kkVV xx-rraayyss – aaiirr kkeerrmmaa
The primary standard for 10-300 kV x-rays is the
FFree AAiir CChhambber
W 1 Q W 1
K = X  air
air
e 1- g m e 1- g
air
W/e = 33.97 eV
m ?
air
kkVV xx-rraayyss – aaiirr kkeerrmmaa
The air volume is
V = AL
aaiirr
defined mechanically
CCoorrrreeccttiioonnss, ccoorrrreeccttiioonnss …..
QQ  WW  11
K = air      K  K  K  K  P  P
air att sc e hum pol ion
 V  e  1 - g
air air
•
K - attenuation of the primary X-ray beam between the aperture and collecting volume
att
•
K - the extra ionization collected from electrons produced by photons scattered within
sc
the chamber
•
K - ionization lost when electrons strike the collecting electrode.
e
AAnn aassiiddee oonn HHuummiiddiittyy
1.0035
We are all familiar with correcting readings for
1.0030
temperature and pressure (forget them here at
1.0025
yyoouurr ppeerriill))
1.0020
m
u 1.0015
h
HHKuummiiddiittyy,, hhoowweevveerr,, iiss ggeenneerraallllyy iiggnnoorreedd,, bbuutt ffoorr kkVV
prim1.0a01r0y standards it is an important correction
1.0005
50 kV
CCo-6600
1.0000
0.9995
0 20 40 60 80 100
Relative Humidity (%)
UUnncceerrttaaiinnttiieess
Medium Energy Free-Air Chamber Standard Uncertainty
(MEES) Type A Type B
Ionization Current 0.03 0.03
Volume 0.01 0.04
Positioning 0.02 0.01
Correction Factors (excl. k ) 0.02 0.2
h
Humidityy k 0.01 0.03
h
Physical Constants - 0.15
Air-Kerma Rate 0.044 0.257
CCoommbbiinneedd UUnncceerrttaaiinnttyy 00.2266