Geant4 simulation of SOI microdosimetry for radiation protection in space and aviation environments

Transcription

1 Geant4 simulation of SOI microdosimetry for radiation protection in space and aviation environments Dale A. Prokopovich,2, Mark I. Reinhard, Iwan M. Cornelius 3 and Anatoly B. Rosenfeld 2 Australian Nuclear Science and Technology Organisation 2 Centre for Medical Radiation Physics, University of Wollongong 3 Faculty of Science and Technology, Queensland University of Technology

2 Radiation Protection in Mixed Fields High energy charged particles or photons can result in neutron mixed field radiation Production of neutrons via nuclear interactions with high energy charged particles or spontaneous fission of isotopes Fields generally consist of a large photon component as well as residual charged particles from reactions or light nuclear recoils Dosimetry for radiation protection in high energy mixed radiation fields is a challenging task

3 Radiation Protection in Aviation 0 g. cm km 30 km 25 km 20 km 5 km 0 km 5 km Sea level Ambient Dose Equivalent µsv h - Total Proton Neutron Electron Gamma Muon Pion 200 g. cm -2 Battistoni, G., A. Ferrari, et al. (2004). Radiation Protection Dosimetry 2(3): g. cm -2 Radiation Field Characteristics Mixed radiation field (neutrons, electrons, positrons, protons, photons, muons). Particle composition and energy fluence varies with altitude, latitude and solar cycle. Typical dose rate: 5-6 µsv. hr 0.67 km (35,000 ft); temperate latitudes; average solar activity EU legislation classifies aircraft crew as radiation workers 96/29 EURATOM 996. Dosimetry records required A need for on line monitoring.

4 Dosimetry in Aviation Type Radiation Environments Due to the complicated high energy mixed particle nature of the aviation radiation environment dosimetry is challenging Currently the majority of the focus is on the neutron and gamma components of the field High energy (up to several hundred MeV) charged particles in the radiation environment are difficult to measure using current dosimetry equipment Microdosimetry offers a method for determining the dose from these types of radiation environments

5 SOI Microdosimetry overview Each st Generation SOI Microdosimeter contains two arrays of 00 x 00 µm and two arrays of 0x0 µm dimension cells with 2, 5 or 0 µm thickness Energy deposited in the cells is collected and a lineal energy spectrum is produced which is used to calculate the dose equivalent Applicable over large particle energy range (Low E to ~00 s MeV) Low photon detection efficiency allows operation in high gamma ray photon flux radiation environments without large dead time 0µm Alpha Particle Proton or other ionising charged particle Al Contact(0.6µm) Al Contact(0.6µm) Ability to discriminate components of a mixed field consisting of particles with same energy but different LET µm-si02 2µm N+(0.4µm) Si p-substrate P+(0.4µm) Ohmic Contact for substrate. Contains boron doping Ideal Charge collection region (Voltage applied across reverse biased depletion region(shaded) separates electron-hole pairs ) SiO2

6 SOI Microdosimetry Neutron Measurement Energy deposition under neutron irradiation in the SOI Microdosimeter will come from Si recoils as well as inelastic interactions within the sensitive volume When an LDPE converter is placed in front of the SOI Microdosimeter, recoil protons will contribute to the energy deposition The fields used in the following measurements were approximately normally incident to the detector face No converter = contribution from silicon recoil Polyethylene converter = contribution from recoil protons + silicon recoils LDPE Tracks and overlayer Microdosimeter cells Insulating layer Silicon Substrate Neutron Proton

7 GEANT4 Simulation of the SOI Microdosimeter GEANT4 simulations were constructed to accurately model the geometry and response of the SOI Microdosimeter to the experimental conditions The spectral distribution from the facility simulation was used as the input for the simulation primary particles High precision neutron physics was used to accurately model the neutron interactions down to thermal energy levels The physics used the QGSP (Quark Gluon String Precompound) and Bertini Cascade hadronic models to model the interactions to high energies Maximum charged particle energy approximately 20 GeV/c Maximum neutron energy approximately.5 GeV/c A A3 A2 A4

8 Aviation reference radiation fields Due to the nature of the aviation radiation environment a similar composition field is required for calibration of dosimeters The field needs to be repeatable and able to be referenced to a dosimetric quantity Fields produced by high energy accelerators exhibit similar characteristics

9 CERN-EU High Energy Reference Field (CERF) CERF seeks to produce a neutron dominated radiation field with similar characteristics to that found at 35,000 ft for the purpose of calibrating dosimetry instrumentation. Produced via irradiation of a Cu Target with 20 GeV/c mixed hadron beam (6% positive pions, 35% protons and 4% positive kaons). Mitaroff, A. and M. Silari (2002). "The CERN-EU high-energy reference field (CERF) facility for dosimetry at commercial flight altitudes and in space." Radiation Protection Dosimetry 02(): 7-22.

10 CERF Facility Simulation The CERF Facility environment was simulated using GEANT4 to determine the particle and energy composition of the field

11 CERF Simulation with Geant4 Two Simulations Sim Sim 2 Simulation of the secondary radiation emerging from the mixed hadron irradiated Cu target. Simulation of the tertiary radiation field at various positions of interest external to the concrete bunker; i.e. the dosimetry instrumentation calibration positions.

12 Copper target results

13 Copper target results

14 Influence of cut values

15 CERF facility simulation results

16 CERF Mixed Field Components Particle fluence per PIC (cm^-2) Neutron Gamma Proton e - / e + Pion Muon (n) (γ) (p) (π) (µ) ~ 20 % of the Ambient Dose Equivalent is due to components other than the neutron dose. The contribution from the various components was not well understood via experimental measurements and assumed to be negligible On line monitoring of such a mixed radiation field in terms of Ambient Dose Equivalent is extremely challenging. Particle Fluence per PIC Neutron.7x0 0 ±.0x0-2 Gamma 2.5x0 0 ±.49x0-2 Proton 2.4x0-2 ±.37x0-3 Electron 5.94x0-2 ± 2.29x0-3 Positron 2.29x0-2 ±.42x0-3 Pion +.37x0-2 ±.0x0-3 Pion -.34x0-2 ±.08x0-3 Muon x0-3 ± 4.66x0-4 Muon x0-3 ± 5.35x0-4 Contributions to Ambient Dose Equivalent π (6.7%) µ (~0%) e - /e + (5.6%) p (7.6%) γ (3.5%) n (76.7%) Pelliccioni, M. (2000). "Overview of fluence-to-effective dose and fluence-to ambient dose equivalent conversion coefficients for high energy radiation calculated using the FLUKA code." Radiation Protection Dosimetry 88(4):

17 Geant4 comparison to FLUKA

18 Geant4 comparison to FLUKA

19 Geant4 vs Bonner Sphere Measurements

20 Response of Semiconductor Detectors to the CERF Facility Both the experimental and simulated SOI Microdosimeter and PIN diode response to the CERF facility was investigated From the properties of the GEANT4 simulation of the components of the radiation field at the CERF facility it is expected that the charged particle component will be a significant contribution to the detector response For GEANT4 simulation of the detectors the results of the facility simulation were used as the input to the detector simulation

21 SOI Microdosimeter Simulation Results Bare LDPE Additional events with lineal energies between 3 and 30 kev. µm - corresponding to proton recoils generated in the LPDE by the neutron component. 4/8/20

22 SOI Microdosimeter Simulation Results Total Gamma Electron Pi+ Mu Neutron Proton Positron Pi- Mu- Total Neutron Gamma Proton Electron Positron Pi+ Pi- Mu+ Mu yd(y) yd(y) Lineal energy (kev/μm) Bare Lineal energy (kev/μm) LDPE 0

23 Experimental Data Acquisition Equipment designed to be battery powered and portable Capable of acquiring data for ~48 hrs unattended (longer if mains power used) Total weight of electronics (without battery power or laptop) < kg Equipment used: 0 micron thick SOI Microdosimeter Cremat CR-0 preamplifiers Cremat CR-200 µs shaping amplifier Amptek PMCA 8000a Pocket MCA Laptop Computer for data acquisition Battery power supply and regulated battery powered variable bias

24 Comparison of experimental and simulated results Events with y from to 50 kev. µm - were measured. Slight increase in higher lineal energy deposition events with the addition of the LDPE layer. 4/8/20

25 Comparison of SOI Microdosimeter to HAWK TEPC Scaling of SOI Microdosimeter for tissue equivalency. Good agreement. See for example edge feature at 6 to0 kev. µm -. Dose equivalent in good agreement for energy range covered. yd(y) (arb units) HAWK TEPC SOI Microdosimeter Lineal Energy (kev/μm) /8/20

26 PIN diode response to CERF Total Energy Deposition Events mm LDPE Bare LDPE-Bare Energy Deposition Events mm LDPE PIN diode Simulated Bare PIN diode Simulated LDPE-Bare Energy (MeV) Energy (MeV)

27 Simulated CERF field PIN diode interaction types Energy Deposition Bare Total Starter Crosser Insider Stopper Energy Deposition LDPE Total Starter Crosser Insider Stopper Energy (MeV) Energy (MeV)

28 Simulated PIN diode CERF field components Energy deposition Total Gamma Electron Pi+ Mu Energy deposition Total Gamma Electron Pi+ Mu+ Neutron Proton Positron Pi- Mu- Neutron Proton Positron Pi- Mu Energy (MeV) Energy (MeV)

29 Verification of the charged particle component at CERF GEANT4 showed a significant charged particle component of the field Published data neglects charged particle component How to verify the significance of the charged particle component Use a pure high energy neutron beam

30 Quasi-monoenergetic neutron field produced through the 7 Li(p, n) 7 Be reaction Used for testing equipment under Quasi-monoenergetic neutron irradiations for Single Event Upsets (SEU) Relative Spectral Fluence (/MeV) TSL Facility 49.5MeV Protons incident on Li target 78.7MeV Protons incident on Li target Energy (MeV) Neutron energies up to MeV Using a clean neutron field allows comparison to a high energy mixed radiation field Prokofiev, A. V., J. Blomgren, et al. (2007). "The TSL neutron beam facility." Radiation Protection Dosimetry 26(-4): 8-22.

31 TSL Microdosimeter Results 46.5 MeV MeV Experimental results taken with two different gains to cover full lineal energy range The recoil proton component from the LDPE converter < 00 kev/μm Bare 0.5 mm yd(y) yd(y) y (kev/μm) yd(y) yd(y) y (kev/μm) Recoil silicon component >00 kev/μm significant contribution to the lineal energy spectra 45 mm yd(y) y (kev/μm) yd(y) y (kev/μm) y (kev/μm) y (kev/μm)

32 TSL Microdosimeter Simulation 46.5 MeV MeV Bare The GEANT4 simulated results show a relatively high contribution from the silicon recoils as seen in the experimental results 0.5 mm Reasonable agreement to the experimental data 45 mm

33 Quasi-Monoenergetic Neutron Results SOI Microdosimetry of the field shows expected move to lower lineal energies for higher neutron energies SOI Microdosimeter measurements limited by S/N to above 3 kev/µm adequate for high energy neutron microdosimetry up to MeV The spikes seen in the simulated data are due to the conversion to microdosimetric spectra

34 PIN diode response to TSL neutron beam Energy Deposition Events per ICM count MeV Bare 46.5 MeV 0.5 mm 46.5 MeV 2.5 mm Energy (MeV) Energy Deposition Events per ICM count MeV Bare MeV 0.5 mm MeV 2.5 mm Energy (MeV)

35 PIN diode rotation in TSL neutron beam 46.5 MeV MeV

36 TSL PIN diode experimental and simulated comparison 46.5 MeV MeV

37 PIN diode simulated angular dependence 46.5 MeV MeV

38 Summary The complex radiation field produced by the CERF facility was able to be simulated using GEANT4 to good agreement with FLUKA and bonner sphere measurements GEANT4 was able to be used to verify a significant contribution to dose from charged particles in the CERF facility GEANT4 was able to be used to simulate the response of SOI Microdosimeter and PIN diode detectors to high energy mixed radiation fields Comparison between GEANT4 and experimental results shows a good agreement for the kinds of high energy radiation fields encountered in aviation environments

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