D - Nucleus

Transcription

D - Nucleus

Bureau International des Poids et Mesures
Dose conversion for the BIPM graphite
calorimeter standard for absorbed dose to water
D T Burns
IDOS Symposium, IAEA, Vienna 2010-11-09
Outline
• Development of new BIPM standard to
determine absorbed dose to water
in 60Co and accelerator photon beams
Outline
• Development of new standard to
• Three major stages:
– Measurement of specific heat capacity of graphite
(Picard, Burns, Roger 2007 Metrologia 44 294)
– Design and construction of novel graphite calorimeter
(Picard, Burns, Roger 2009 Rapport BIPM-2009/01)
– Dose conversion from graphite to water
Outline
• Development of new standard to
• Three major stages:
– Measurement of specific heat capacity of graphite
(Picard, Burns, Roger 2007 Metrologia 44 294)
– Design and construction of novel graphite calorimeter
(Picard, Burns, Roger 2009 Rapport BIPM-2009/01)
– Dose conversion from graphite to water
Dose conversion
• Measure Dc, but reference quantity is Dw
Dose conversion
• Use transfer instrument
– Ionization chamber in
graphite and water
– Measure charges Qc and Qw
Dose conversion
• Use transfer instrument
– Ionization chamber in
graphite and water
– Measure charges Qc and Qw
• Simulate response of calorimeter
and transfer chamber using
Monte Carlo method
Dose conversion
I
II
III
IV
Dc
Dcav,c
Dcav,w
Dw
Dose conversion
I
II
III
IV
Dc
Qc
Dcav,w
Dw
Dose conversion
I
II
III
IV
Dc
Qc
Dcav,w
Dw
ND,c
Dose conversion
I
II
III
IV
Dc and Dc,MC
Qc and Qc,MC
Dcav,w
Dw
ND,c and ND,c,MC
Dose conversion
I
II
III
IV
Dc and Dc,MC
Qc and Qc,MC
Qw and Qw,MC
Dw
ND,c and ND,c,MC
Dose conversion
I
II
III
IV
Dc and Dc,MC
Qc and Qc,MC
Qw and Qw,MC
Dw,MC only
ND,c and ND,c,MC
Dose conversion
I
II
III
IV
Dc and Dc,MC
Qc and Qc,MC
Qw and Qw,MC
Dw,MC only
ND,c and ND,c,MC
ND,w,MC only
Dose conversion
I
II
III
IV
Dc and Dc,MC
Qc and Qc,MC
Qw and Qw,MC
Dw,MC only
ND,c and ND,c,MC
ND,w,MC only
Dw = Qw ND,c { ND,w,MC / ND,c,MC }
Dose conversion
I
II
III
IV
Dc and Dc,MC
Qc and Qc,MC
Qw and Qw,MC
Dw,MC only
ND,c and ND,c,MC
ND,w,MC only
Dw = Qw ND,c { ND,w,MC / ND,c,MC } = Qw ND,c Cw,c
Determination of Dw
• Final estimate of absorbed dose to water
Dw = Qw ND,c Cw,c
Determination of Dw
Dw = Qw ND,c Cw,c
measurement
Determination of Dw
Dw = Qw ND,c Cw,c
measurement
calculation
Calculation of Cw,c
• PENELOPE code (version 2008) (1)
• Detailed models of experimental arrangement
using PENGEOM quartic geometry code
(1) Burns 2006 Phys. Med. Biol. 51 929
Calculation of Cw,c
• PENELOPE code (version 2008) (1)
• Detailed models of experimental arrangement
using PENGEOM quartic geometry code
• Input phase-space files:
– For BIPM reference 60Co source calculated
by Burns (1)
– For accelerator photon beams calculated by NMIs
(so far NRC, PTB, NIST, METAS)
(1) Burns 2006 Phys. Med. Biol. 51 929
Calorimeter in jacket with support
Graphite jacket
containing core
PMMA support
Nylon screws
Parallel-plate transfer chamber
Graphite collector
and outer wall (HT)
Collecting pin
PTFE insulator
Aluminium stem
PMMA insulator
Results of calculations for Cw,c
Calculated dose conversion factor C w,c
Uncertainty bars are statistical standard uncertainties of calculation only
1.155
1.150
1.145
C w,c
1.140
1.135
1.130
1.125
NRC accelerator
PTB accelerators
1.120
1.115
0.66
0.68
0.7
0.72
0.74
TPR20,10
0.76
0.78
0.8
0.82
Uncertainty for Cw,c
Relative standard uncertainties (parts in 102)
60
Component
Statistical uncertainty
Depth correction
Geometry
Radiation transport
Photon attenuation
Stopping powers – I-values
Stopping powers – grain vs bulk
Phase-space representation
si
0.04
-
0.13
0.10
6 MV – 25 MV
si
ui
0.04
0.06
0.08
0.06
0.09
0.14
0.10
Quadratic summation
Combined standard uncertainty
0.04
0.19
0.04
Co
ui
0.06
0.08
0.20
0.23
0.23

D - Nucleus

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