Jeremy Davis

Characterisation of a novel diamond-based microdosimeter for radiation protection of astronauts

Jeremy Davis1,2, Susanna Guatelli1,2, Marco Petasecca1,2, Dale Prokopovich1, 3, Mark Reinhard1, 3,

Rainer Siegele3, Kumaravelu Ganesan4, Steven Prawer4, Anatoly B. Rosenfeld1,2

1. Centre for Medical Radiation Physics (CMRP)

2. University of Wollongong

3. Australian Nuclear Science and Technology Organisation (ANSTO)

4. University of Melbourne

Jeremy Davis

Outline • Introduction

• What is microdosimetry

• Evolution of microdosimetry at CMRP

• Diamond microdosimetry

• Experimental measurements

• Geant4 set-up

• 1st study- effect of contact thickness upon edep

• 2nd study- characterisation of detector response to GCR

• Summary

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Introduction• One of the major issues for manned space exploration is the health hazard

posed by the exposure of Galactic Cosmic Rays (GCR) and Solar Particle Events (SPE) to crew members.

• Nowadays there is the need of accurate detectors, capable of measuring dose equivalent in a mixed radiation field, typical of a space environment.

• Microdosimetry is an approach that allows accurate measurement of dose equivalent in any mixed radiation field, on a cellular level (1-5μm),

[H, Rossi & M, Zaider].

• Microdosimeters may be adopted as a monitoring system for radiation protection purposes.

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Microdosimetry

dxdELET =

Radiation(Alpha)traversal of a cellin BNCT

High LET hits to thenucleus increaseprobability of multipleDNA breaks

Nucleus of cell

~10µm

[A. B Rosenfeld]

Units: keV/um

The linear energy transfer is equal to the energy dE which a charged particle loses at a distance dx [H, Rossi & M, Zaider].

Jeremy Davis

SOI microdosimetry

Optical Microscope Image

Schematic concept

~ 10-20 um

Si Substrate

SiO2

E-field

1 um

2 um

N+

Al Al

P+

+-

Next step: substitute silicon with diamond, as material of the SV

Low Energy2 MeV Alpha Microbeam IBIC

SOI Device: [A. B Rosenfeld, A. Wroe, A. L Ziebel, S Guatelli, P Bradley, I. Cornelius]

Can be successfully used for regional microdosimetry in a space environment

Requires tissue equivalent correction (Si)

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Diamond based microdosimetry

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Why diamond?

• Radiation hardness.

• High resistivity (>1013 Ωcm)

• Large band gap (5.5eV)

• Low dark current (1 pA/cm2 at room temperature)

• Good temperature stability.

• High carrier mobilities allow for fast signal collection.

• Low dielectric constant -> low capacitive load with the negligible dark current means a low noise .

[G.T.Betzel]

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The DesignOur Microdosimeter utilises diamond as the

sensitive volume (SV) material of choice.These SV’s are surrounded by boron doped

diamond.

The contacts used are:• Aluminium (top)• Gold (back)

Cathodolumiescence image of the diamond microdosimeter.

AdvantageHigh electric field can be applied due to SV size Charges can be collected in a short range before they are trapped in defect centres.

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Experimental measurements: IBIC Study (ANSTO)

• Well defined sensitive volumes (SV)

• Higher charge collection occurring towards the edge of the aluminium pad

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A closer look at the aluminium pad

Upon inspection there does appear to be some non-uniformity in the aluminium pad.

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• To analyse the Charge Collection Efficiency (CCE) in terms of detector design

Study of the effect of device geometries (Al contact widths) on the detector response

• To characterise the response of the novel diamond based microdosimeter developed at CMRP In a mixed radiation field of interest for radiation protection in space

Goal of the Geant4 simulation study

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Geant4 simulation experimental set-up

Geant4 simulation experimental set-up• Simulation of the energy deposition per event deriving from proton, alpha

and heavy ion particle fields, with an energy range typical of GCR and SPE, in the diamond SV.

Gold

Gold

Aluminium

Diamond SV

Boron doped region

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Some relevant dimensions

150um

150u

m Depth-1.36um

60um

90um

Total depth of substrate

300um

600um

480u

m

60um

60um

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Model of the radiation field• This simulation utilises General

Particle Source (GPS) to model the radiation field.

• The radiation field in this initial work has been limited to a pencil beam incident orthogonally upon a SV within the device.

• Particles and energies were chosen depending upon the requirements of the study.

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General Particle Source- GPS• GPS allows ease for

switching between monoenergetic particles and a spectrum as typified by GCR.

• GPS has been chosen as it allows greater versatility than Geant4 particle gun. This will become increasingly important in future work.

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Physics List• Physics List:

• Low Energy Package to describe electromagnetic interactions down to 250eV

• Range cut = 1 micrometer

• QGSP_BIC_HP physics list for hadronic interactions

• Hadronic interactions modelled for hadrons and ions

• QGS- Quark Gluon String model

• BIC- Binary Ion Cascade model

• HPneutron (High Precision) used to describe in detail all neutron interactions with energy < 20 MeV

GEANT4 v. 9.4

QGSP_BIC_HP:

Used in radiation shielding, protection and medical applications

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1st study: effect of Al contacts on energy deposition

•Particle:Beryllium nuclei

•Energy :

5.48MeV

•Aluminium contact thickness

0, 0.5. 1 and 1.5 um

•Number of events:

104 events were executed

Geant4 simulation experimental set-up

Charged particle

Gold

Aluminium

Diamond SV

Boron doped region

Diamond substrate

•Number of events:

104 events were executed

•Aluminium contact thickness

0, 0.5. 1 and 1.5 um

•Number of events:

104 events were executed

•Energy :

5.48MeV

•Aluminium contact thickness

0, 0.5. 1 and 1.5 um

•Number of events:

104 events were executed

•Particle:Beryllium nuclei

•Output:Energy deposition histogram

Stored as ROOT file

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Results of 1st study: Stripped beryllium ions (5.48MeV)• Effect of different Al thickness on the energy deposition spectra

deriving from energetic beryllium in the diamond SV

0umAl 0.5umAl

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Results of 1st study: Stripped beryllium ions (5.48MeV)• Effect of different Ti thickness on the energy deposition spectra

deriving from energetic beryllium in the diamond SV

1.5umAl1umAl

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Range of beryllium (5.50MeV)- SRIMMaterial dE/dx (keV/um) Range (um)

Aluminium 733.84 7.71

Diamond 1401.39 4.07

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2nd study: Characterisation of response to GCR

Each simulation consisted of 106

events.

Study of the response of the device to Galactic Cosmic protons

GCR proton and alpha particle fluence from creme96 [7] [8]

Energy (MeV/nucleon)

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Monoenergetic protons

1Gev 50MeV

1MeV

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GCR protons

Depicts the energy deposition spectra from GCR protons that span the energy range of 1MeV/nucelon to 10GeV/nucleon.

~ 25 keV

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Monoenergetic alpha (1umAl)

1GeV/nucleon 50MeV/nucleon

1MeV/nucleon

105 events

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GCR alpha

Depicts the energy deposition spectra from GCR alpha that span the energy range of 1MeV/nucelon to 10GeV/nucleon.

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Summary:

1st study: Device optimisation• The Al contacts ( 0 – 1.5 um) have been shown to affect the

energy deposition spectra .This explains the difference in charge collection as seen in experimental work.

2nd Study- Response of diamond microdosimeter to GCR

• Characterisation of detector response with GCR proton & alpha. Study should be extended to include HZE, i.e. (C, O, Si, Fe)

• Model GCR as an isotropic field• Model detector as well as space shuttle.

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References:

• [1] Rossi , H and Zaider, M. “Microdosimetry and its applications”, Springer-Verlag, Berlin, 1996.

• [2] A. B Rosenfeld, SOI microdosimetry presentation

• [3] I. M. Cornelius, et al. “LET Dependence of the Charge Collection Efficiency of Silicon Microdosimeters” IEEE Trans. Nucl. Sci, VOL. 50, NO. 6, DECEMBER 2003

• [4] A.L. Ziebell, et al, “A novel cylindrical silicon-on-insulator microdosimeter for the characterisation of deep space radiation environments,” IEEE Trans. Nucl. Sci.,

• [5] S. Guatelli et al, “Tissue Equivalence Correction in Silicon Microdosimetry for Protons Characteristic of the LEO Space Environment” IEEE Trans. Nucl. Sci, VOL. 55, NO. 6, DECEMBER 2008A.

• [6] A. Wroe et al, “Solid State Microdosimetry With Heavy Ions for Space Applications” . IEEE Trans. Nucl. Sci, VOL. 54, NO. 6, DECEMBER 2007

• [7] P Bradley, et al, “Charge collection and radiation hardness of a SOI microdosimeter for space and medical application. IEEE Trans. Nucl. Sci. NS-45(N6), 2700–2710 (1998).

• [8] G. T. Betzel “Development of a Prototype Synthetic Diamond Detector for Radiotherapy Dosimetry” PhD Thesis

• [9] C. Zacharatou et al, “Physics settings for using the Geant4 toolkit in proton therapy,” IEEE Trans. Nucl. Sci. 55, 1018–1025, 2008.

• [10] J. Allison et al., “Geant4 developments and applications,” IEEE Trans. Nucl. Sci., vol. 53, no. 1, pp. 270–278, 2006.

• [11] S. Agostinelli et al., “Geant4—A simulation toolkit,” NIM A, VOL. 506, NO 3, pp. 250–303, 2004.

• [12] A. J. Tylka et al, “CREME96: A Revision of the Cosmic Ray Eects on Micro-Electronics Code” IEEE Trans. Nucl. Sci , VOL.44, NO.6,DECEMBER 1997

• [13] http://space-env.esa.int/ProjectSupport/ISO/CREME96.html

• [14] K. Ganesan et al, “A Diamond Detector for Single Ion Implantation”

• [15] J. F. Ziegler et al, ”The Stopping and Range of Ions in Solids", VOL 1, Pergamon Press, New York, 1985

Jeremy Davis