Monte Carlo simulations as a tool for the development of a new reference ionisation chamber

Monte Carlo simulations as a tool for the development of a new reference ionisation chamber Johan Camps, Jan Paepen, Stefaan Pommé and Uwe Wätjen Institute for Reference Materials and Measurements (IRMM) Geel, Belgium http://www.irmm.jrc.be http://www.jrc.cec.eu.int 1

Contents Motivation for the development of a new reference ionisation chamber Monte Carlo simulations in the design process of a new reference chamber Towards absolute MC simulations Conclusions 2

Motivation SIR (Système International de Référence) chamber at BIPM BUT C. Michotte, Appl. Rad. Isot. 56 (2002)15-20 4π well-type ionisation chamber (IG11, 2 MPa N 2 ) 30 years of calibrations, >60 radionuclides (photon and pure beta) Comparison with 226 Ra reference source (equivalent activity A e ) What if any component breaks down? Sensitivity to low photon energies 3

Motivation Development of a new reference IC: determination of design and operation criteria for a cylindrical IC system which should guarantee a reproducible output at the level of a few tenths of a percent for different chambers constructed according to these criteria Original idea : Reher (IRMM), Woods (NPL), Denecke (IRMM) Proto-type chamber based on NPL IC MC simulations : L. Johansson, A. Švec/J. Gasparro (EGS-4), this work (MCNP) 4

.. MC simulations. In development process several methods used : Deterministic model of cylindrical IC Measurements with proto-type chamber MC simulations MCNP4c 100 MCNP5.1 Q/photon (ac/photon) 10 1 MCNP N 2 MCNP Ar Deterministic model N 2 Deterministic model Ar 0.01 0.1 1 Photon Energy (MeV) 5

.. MC simulations. Several contributions to ionisation current Q d / photon (ac/photon) 35 30 25 20 15 10 5 Argon, MCNP, 1 MeV photons complete chamber no inner tube no outer wall only gas Only gas/complete chamber 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1 2 3 4 gas pressure (MPa) 0.0 0 1 2 3 4 gas pressure (MPa) 6

.. MC simulations. Start from proto-type IC: photon and electron response (MCNP) Response (ac/photon or ac/electron) 1000 100 0.01 0.01 0.1 1 7 10 1 0.1 2 mm Al, photons 6 mm Vespel, photons 2 mm Al, electrons 6 mm Vespel, electrons SIR, photons (N 2, 2 MPa) Argon, 2 MPa Bremsstrahlung Photon or kinetic electron energy (MeV) Vespel C 22 H 10 O 5 N 2

.. MC simulations. Determination of tolerances on IC system parameters for a 0.1% variation in output (20 kev to 2 MeV) Example 1. IC response as a function of electrode thickness 0.2 Relative difference (%) 0.1 0.0-0.1 IC3, 2 MPa Argon 20 kev 30 kev 40 kev 2 MeV Q/photon (ac/photon) 70 60 50 40 30 20 10 Secondary radiation produced in volume 1 reaching volume 2 Incomplete backscatter MCNP, 2 MeV photons Q/photon, volume 1 Q/photon, volume 2 Q/photon, total -0.2 0.198 0.199 0.200 0.201 0.202 0 0.0 0.1 0.2 0.3 0.4 0.5 Electrode thickness m e (cm) Thickness electrode m e (cm) 8

.. MC simulations. Example 2. Determination of effect of radial source displacements Proto-type IC, 2 MPa Argon, 2 mm Al inner tube 0 109 Cd 22 kev X-rays Relative difference (%) -2-4 Deterministic model Measurements proto-type MCNP simulations -6-0.8-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 Radial source displacement (cm) tolerance for 0.1% : 0.9 mm for optimised design 9

.. MC simulations. New reference chamber Item Value/statement Required tolerance L 35-40 cm 1.6 mm (2 MeV) R 9-9.5 cm 70 µm (2 MeV) R 0 2.5 3 cm 55 µm (30 kev) R e 5.5 cm 60 µm (20 kev) Inner wall Aluminium/Vespel m (thickness inner wall) 0.2 cm Al / 0.6 cm Vespel 1 µm/15 µm (20 kev) Electrode Aluminium m e (thickness electrode) 0.3 cm 14 µm (30 kev) Filling gas Argon Gas pressure 2 MPa 2 kpa (2 MeV) Source positioning center 0.9 mm (20 kev, Al) 10

. Towards absolute MC.. PENELOPE MC / Experiment Vinten 671 chamber (~1 MPa N 2 ) ±7% (gas pressure) A. De Vismes, M.N. Amiot, Appl. Rad. Isot. 59 (2003) 267-272 11

. Towards absolute MC.. Can we get absolute agreement between MC and measurements? 570 560 2 MPa Ar, Vespel inner wall, Mn54AI0601 Proto-type IC : Geometrical parameters well-known Pressure controlled with traceable RUSKA pressure balance Measurement of 54 Mn source (834.8 kev) as function of gas pressure -I measured (pa) 550 540 Model: recombination Weighting: Statistical Chi^2/DoF = 0.00021 R^2 = 0.99703 I s = 565.88 ±0.35 a = 5.39 ±0.40 b = 0.124 ±0.097 0 200 400 600 800 1000 -bias (V) Ion loss (recombination, diffusion)? determination of saturation current (I s ) I Is a b = 1 2 U U I 12

. Towards absolute MC.. Measurements MCNP5.1 simulations Measurements : absolute measurement of current (δi/i > 1%) -I (pa) 100 Simulations : MCNP, *F8 tally : E d (MeV/particle) Q d = e E W d W Ar = 26.4 ev ± 0.5 ev (ICRU31) 10 Argon, Vespel inner tube Argon, Al inner tube Argon, Al inner tube 1.08 0.1 1 1.08 I MCNP / I measurement 1.06 1.04 1.02 1.00 0.98 0.96 w 1.06 1.04 1.02 1.00 0.98 0.96 w ±1% 0.1 1 gass pressure (MPa) 0.1 1 gass pressure (MPa) 13

. Towards absolute MC.. Definition of volume Charges collected Charges not collected Guard ring 14

Conclusions Tolerances on geometrical and operational parameters for a new reference IC system were determined with MC simulations (MCNP) Absolute agreement between measurements and MC simulations (limited by uncertainty on W-value!) New reference chamber will be built and tested in near future 15