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TRAINING COURSE on EXPERIMENTAL MICRODOSIMETRY. Principle of silicon - based microdosimetry Stefano Agosteo 1,2 , Andrea Pola 1,2 1 Politecnico di Milano, Dipartimento di Energia, piazza Leonardo da Vinci 32, 20133 Milano, Italy . - PowerPoint PPT Presentation
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16 October, 2013 1
TRAINING COURSE ON EXPERIMENTAL MICRODOSIMETRY
Principle of silicon-based microdosimetry
Stefano Agosteo1,2, Andrea Pola1,2
1Politecnico di Milano, Dipartimento di Energia, piazza Leonardo da Vinci 32, 20133 Milano, Italy.
2Istituto Nazionale di Fisica Nucleare, Sezione di Milano, via Celoria 16, 20133 Milano, Italy.
uthor name, Institute
P zone Depeleted zone N zone
- +
SILICON MICRODOSIMETRY
[1] J. F. Dicello, H. I. Amols, M. Zaider, and G. Tripard, A Comparison of Microdosimetric Measurements with Spherical Proportional Counters and Solid-state Detectors, Radiation Research 82 (1980) 441-453.
[2] M. Orlic, V. Lazarevic, and F. Boreli, Microdosimetric Counters Based on Semiconductors Detectors, Radiat. Prot. Dosim. 29 (1989) 21-22.
[3] A. Kadachi, A. Waheed, and M. Obeid, Perfomance of PIN photodiode in microdosimetry, Health Physics 66 (1994) 577-580.
[4] A. Kadachi, A. Waheed, M. Al-Eshaikh, and M. Obeid, Use of photodiode in microdosimetry and evaluation of effective quality factor, Nuc. Instrum. Meth. A404 (1998) 400-406.
PN diodes
The differences from the lineal energy spectra measured with the TEPC (Tissue Equivalent Proportional Counter) were mainly ascribed to the shape and the
dimensions of sensitive volumes
Complex charge collection process
4
Microdosimetric spectrum in
tissue
INTRODUCTION (I)
• The micrometric sensitive volumes which can be achieved with silicon detectors led these devices to be studied as microdosimeters.
coupled to TE converters, microdosimetry of neutron fields; bare (no converter): they can be used for measuring the quality of
radiation therapy beams and SEE assessment.
Tissue-equivalent converter
SpectroscopyChain
Data Analysis
Silicon device
Spectrum of the energy imparted
per event in silicon
Analytical corrections
5
INTRODUCTION (II)
• Advantages: wall-effects avoided; compactness; cheapness; transportability; low sensitivity to vibrations; low power consumption.
6
INTRODUCTION (III)
• Problems: the sensitive volume has to be confined in a region
of well-known dimensions (field-funnelling effect); corrections for tissue-equivalency (energy
dependent); correction for shape equivalency of the track
distribution (for TEPC comparison); angular response; the electric noise limits the minimum detectable
energy (high capacitance); the efficiency of a single detector of micrometric
dimensions is very poor (array of detectors); radiation hardness.
7
THE FIELD-FUNNELING EFFECT
• FFE: a local distortion of the electric field in the sensitive zone, induced by high-LET particles, which leads to charge collection outside the depleted region.
• Example: p-n diode coupled to a polyethylene converter, irradiated with monoenergetic neutrons:
Recoil proton
Depletion Layer
2 µm
@ 2
V
N+
P-Layer
Field Funneling Effect 0 100 200 300 400 500 600 700 800 900 1000 1100
10-8
10-7
10-6
10-5neutrons from LiF @ 0° with polyethylene - ASICV
cc=-2 V E
n= 0.573 MeV
En= 0.996 MeV
En= 1.511 MeV
En= 2.020 MeV
En= 3.030 MeV
Res
pons
e (c
m2 )
Energy (keV)
Depletion layer thickness: 2 m @ 2 V
Active thickness: ~ 12 m
8
THE FIELD-FUNNELING EFFECT: SOLUTIONS• Array of diodes fabricated using the silicon on insulator
(SOI) technology (Rosenfeld et al.). This technique allows to obtain sensitive volumes
of well defined dimensions, independent of the field funnelling effect;
Different structures with a sensitive volume 2, 5 and 10 μm in thickness were fabricated.
The absorbed dose distributions from different neutron fields were compared to simulations performed with the GEANT code and measurements with a standard TEPC, resulting in a satisfactory agreement.
10-1 100 101 102 103
y, keV/m
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
yd(y
), ke
V/m experimental
simulation
All figures in this slideCourtesy of A. Rosenfeld, Wollongong University, Australia
~ 10-20 um
1 um
2 um
Al Al
P+N+
Si Substrate
SiO2
E-field- +
9
THE FIELD-FUNNELING EFFECT: SOLUTIONS• Monolithic silicon telescope (ST-Microelectronics, Catania,
Italy): the p+ cathode acts as a “watershed” for charge
collection, thus minimizing the FFE.
E thickness: ~1.9 mE thickness: ~ 500 m
Sensitive area: 1 mm2
0.0 0.5 1.0 1.5 2.0 2.51E12
1E13
1E14
1E15
1E16
1E17
1E18
1E19
1E20
Dopa
nt c
once
ntra
tion
(cm
-3)
Depth (m)
As B E charge
collection
100 10000.0
2.0x104
4.0x104
6.0x104
8.0x104
STm R324 DE4
E2 *f
(E) (
C-1)
Energy (keV)
En=0.996 MeV
En=2.727 MeV
En=4.336 MeV
1000.0
1.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
6.0x10-4
7.0x10-4
Experimental FLUKA Simulation Analytical
Y A
xis
Title
1000.0
1.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
6.0x10-4
7.0x10-4
Experimental FLUKA Simulation Analytical
(keV)
p()
2 (keV
cm2 )
1000.0
1.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
6.0x10-4
7.0x10-4
Experimental FLUKA Simulation Analytical
1000.0
1.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
6.0x10-4
7.0x10-4
Experimental FLUKA Simulation Analytical
En = 1715 keVEn = 1306 keV
En = 996 keVEn = 680 keV
10 100 10000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
d()
(keV)
Silicon telescope Cylindrical TEPC (2.7 m)
10
EFFECTS ON SILICON AND DOPANTS
• Contributions of nuclear reactions induced on a p-i-n diode by thermal and fast neutrons were measured in the past.
secondary particles from neutron reactions on 10B were observed (rate 2.210-6 s-1per unit fluence rate of thermal neutrons vs. 10-5 s-1 recoil-protons);
secondary particles generated by fast neutrons on silicon were also observed.
→ 28Si(n,p)28Al (Eth 4.0 MeV);→ 28Si(n,)25Mg (Eth 2.75 MeV).
Further investigation is necessary for new devices.
Only 11B was implanted in the silicon telescope:→ During irradiation on the thermal
column of the TAPIRO reactor, no events from 10B(n,)7Li were observed.
103 1040.00
0.01
0.02
0.03
0.04
0.05
n + 10B ----> 7Li + (Q=2.31 MeV)
2.3 MeV - 7Li+
1.5 MeV -
0.8 MeV - 7Li
In the cavity of the thermal neutron facility - 9Be(d,n)10B
Ed=3.0 MeV
Ed=4.5 MeV
Ed=6.5 MeV
E*f
(E)
E (keV)
103 1040.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
LiF monoenergetic neutrons - bare p-i-n photodiode
En=0.996 MeV @0° E
n=4.536 MeV @0°
E*f
(E)
E (keV)
11
Si MESA MICRODOSIMETERS• 3D silicon mesa p-n junction array with internal charge
amplification produced at UNSW SNF.
All figures in this slideCourtesy of A. Rosenfeld, Wollongong University, Australia
E
r Single mesa 3D SV
12
SEGMENTED SILICON TELESCOPE• In the following the main problems related to a silicon
microdosimeter will be discussed mainly referring to the:
• segmented silicon telescope: constituted by a matrix of cylindrical ∆E
elements (about 2 µm in thickness) and a single residual-energy E stage (500 µm in thickness);
the nominal diameter of the ∆E elements is about 9 μm and the width of the pitch separating the elements is about 41 µm.
more than 7000 pixels are connected in parallel to give an effective sensitive area of about 0.5 mm2.
minimum detectable energy is limited to about 20 keV by the electronic noise.
the ∆E stage acts as a microdosimeter and the E stage plays a fundamental role for assessing the full energy of the recoil-protons, thus allowing to perform a LET-dependent correction for tissue-equivalency.
10 100 10000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
y d(
y)
y (keV m-1)
Pixelated silicon telescope Cylindrical TEPC (2.7 m)
500 µm 14 µm
9 µm
E stage
Guard~2 μm
∆E element
500 µm 14 µm
9 µm
E stage
Guard~2 μm
∆E element
SEGMENTED TELESCOPE: SEM IMAGES
13
14
SEGMENTED SILICON TELESCOPE: SCATTER-PLOT• 2.7 MeV neutron irradiation of the telescope coupled to A-150 plastic;• The signals from the E and the E stage were acquired with a 2-channel ADC in
coincidence mode.
0 500 1000 1500 2000 2500 3000
50
100
150
200
250
Energy imparted in the E stage (keV)
Ene
rgy
impa
rted
in th
e E s
tage
(keV
)
13571012141618
Counts
Recoil-protons
Secondary electrons
Recoil-protonsMay be due to:-Track length distribution;Charge sharing.
Contribution of about 5.5%
15
SEGMENTED SILICON TELESCOPE: SIMULATION• The response of a cylindrical element of the ∆E stage was simulated with a MC algorithm;• The algorithm takes into account the geometrical structure of the telescope, but does not reproduce border
effects. • Secondary electrons from photon interactions on the materials surrounding the detector were not accounted
for.
0 500 1000 1500 2000 2500 30000
50
100
150
200
250
300
Energy imparted in the E stage (keV)
Ene
rgy
impa
rted
in th
e E s
tage
(keV
)
02468101214161820
Counts
Recoil-protons
Recoil-protons
due to the track length distribution!
Calculated contribution of about 5%
Charge sharing can be neglected
16
TISSUE-EQUIVALENCE AND GEOMETRIC CORRECTIONS• In order to derive microdosimetric spectra similar to
those acquired by a TEPC, corrections are necessary;• Tissue equivalence:
a LET-dependent tissue equivalence correction can be assessed through a telescope detector:
→ by measuring event-by-event the energy of the impinging particles;
→ by discriminating the impinging particles.
• Shape equivalence: basing on parametric criteria from the literature,
the lineal energy y was calculated by considering an equivalent mean cord length.
17
TISSUE-EQUIVALENCE CORRECTION
Analytical procedure for tissue-equivalence correction:
Energy deposited along a track of length l by recoil-protons of energy Ep in a tissue-equivalent E detector
)()(
),(),(p
Sip
Tissue
pSidp
Tissued ES
ESlEElEE
Scaling factor : stopping powers ratio
18
the thickness of the E stage limits the TE correction to recoil-protons below 8 MeV (alphas below 32 MeV)
TISSUE-EQUIVALENCE CORRECTION
The scaling factor
depends on the energy and type of the impinging particle
)E(S)E(S
Si
Tissue
E stage of the telescope and ∆E-E scatter-plot allow an energy-dependent correction for
protons
1 10 100 1000 10000
0.4
0.6
0.8
1.0
STi
ssue
(E)/S
Si(E
)
E (keV)
Protons Electrons
ELECTRONS: average value over a wide energy range (0-10 MeV) = 0.53
Electrons release only part of their energy in the E stage
19
SHAPE ANALYSIS• The correcting procedure can be based on cord
length distributions, since ∆E pixels are cylinders of micrometric size in all dimensions (as the TEPCs).;
This correction is only geometry-dependent (no energy limit).
0 1 2 3 4 5 60.0
0.2
0.4
0.6
0.8
1.0
1.2
p(l)
(m
-1)
l (m)
Pixelated silicon telescope Cylindrical TEPC
20
COMPARISON WITH A CYLINDRICAL TEPC
• The microdosimetric spectra were compared to the one acquired with a cylindrical TEPC at the same positions inside a PMMA phantom.
L. De Nardo, D. Moro, P. Colautti, V. Conte, G. Tornielli and G. Cuttone, RPD 110 (2004)
• Proximal part and across the SOBP: • Corrections:• Protons cross both the E and the E stage.
1) Tissue- equivalence:a scaling factor was applied:
2) Shape equivalence:by equating the dose-mean energy imparted per event for the two cylindrical sites:
max
0max )()(1)()(
E
Si
TissueSi
ETissue
E dEESES
EEE 574.0)E()E( Si
E
Tissue
E
533.0l
lE
D
TEPCD
ED
ED lL TEPC
DTEPCD lL=
21
DISTAL PART OF SOBP
• Distal part of the SOBP: most of protons stop in the E stage
• An energy dependent correction for TE can be applied from the event-by-event information from the two stages:
)E()E(E SiE
SiE
)E(S)E(S)E()E(
Si
TissueSi
E
Tissue
E
22
DISTAL PART OF SOBP
0 2 4 6 8 10 12 14 16 18 20 22 240.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dep
th d
ose
curv
e (a
.u.)
depth in PMMA (mm)
1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0
y d(
y)
y (keV m-1)
silicon telescope 21.4 mm f=0.574
cylindrical TEPC 21.6 mm
Constant scaling factor
1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0 silicon telescope 21.4 mm cylindrical TEPC 21.6 mm
(threshold) cylindrical TEPC 21.6 mm
(no threshold)
y d(
y)
y (keV m-1)
Energy-dependent correction
23
ENERGY THRESHOLD IMPROVEMENT
The main limitation of the system is the high energy threshold imposed by the electronic noise.
A feasibility study with a low-noise set-up based on discrete components was carried out in order to test this possibility
New design of the segmented telescope with a ∆E stage with a lower number of cylinders connected in parallel and an E stage with an optimized sensitive area
1. Decrease the energy threshold below 1 keV μm-1
2. Optimize the counting rate of the two stages
24
ENERGY THRESHOLD IMPROVEMENT
A telescope constituted by a single ΔE cylinder coupled to an E stage was irradiated with β particles emitted by a 137Cs source.
0.01 0.1 1 10 1000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 FLUKA simulation (tissue) Experimental
y d(
y)
y (keV m-1)
Lineal energy threshold ≈ 0.6 keV μm-1
25
CONCLUSIONS
•Silicon detectors show interesting features for microdosimetry, anyway still some problems have to be solved:
electronic noise (minimum detectable lineal energy);radiation hardness when exposed to high-intensity hadron beams.
26
Additional Slides
27
SHAPE ANALYSIS
The equivalence of shapes is based on the parametric criteria given in the literature (Kellerer).
By assuming a constant linear energy transfer L: DD lL
l
dllplL
0
2 )(
533.0l
lE
D
TEPCD
ED
ED lL TEPC
DTEPCD lL
By equating the dose-mean energy imparted per event for the two different shapes considered:
=
Eeq,E ll
Dimensions of ∆E stages were scaled by a factor η …… the lineal energy y was calculated by considering an equivalent mean cord length equal to:
28
IRRADIATIONS AT THE CATANA FACILITY• The segmented silicon telescope was irradiated inside a PMMA phantom exposed to the 62 MeV proton beam at the
INFN-LNS CATANA facility.
Front-end electronics
Detector +
Mylar
PMMA phantom
PMMA foils
29
SCATTER-PLOTS
0 2 4 6 8 10 12 14 16 18 20 22 240.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dep
th d
ose
curv
e (a
.u.)
depth in PMMA (mm)
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
140
160
180
200
220Counts per unit doseDepth = 8 mm
Dose = 0.3 Gy
Ener
gy d
epos
ited
in th
e E
stag
e (k
eV)
Energy deposited in the E stage (MeV)
01.22.43.64.86.07.28.49.61112
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
140
160
180
200
220Counts per unit doseDepth = 15.5 mm
Dose = 0.44 Gy
Ener
gy d
epos
ited
in th
e E
stag
e (k
eV)
Energy deposited in the E stage (MeV)
01.22.43.64.86.07.28.49.61112
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
140
160
180
200
220Counts per unit doseDepth = 18 mm
Dose = 0.43 Gy
Ener
gy d
epos
ited
in th
e E s
tage
(keV
)
Energy deposited in the E stage (MeV)
01.22.43.64.86.07.28.49.61112
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
140
160
180
200
220Counts per unit doseDepth = 21 mm
Dose = 0.46 Gy
Ener
gy d
epos
ited
in th
e E s
tage
(keV
)
Energy deposited in the E stage (MeV)
00.701.42.12.83.54.24.95.66.37.0
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
140
160
180
200
220Counts per unit doseDepth = 21.5 mm
Dose = 0.62 Gy
Ener
gy d
epos
ited
in th
e E
stag
e (k
eV)
Energy deposited in the E stage (MeV)
00.701.42.12.83.54.24.95.66.37.0
30
PROXIMAL AND ACROSS THE SOBP
0 2 4 6 8 10 12 14 16 18 20 22 240.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dep
th d
ose
curv
e (a
.u.)
depth in PMMA (mm)
0.1 1 10 100 10000.00.20.40.60.81.01.21.41.61.82.02.22.42.62.8
silicon telescope 5.7 mm cylindrical TEPC 5.7 mm (threshold) cylindrical TEPC 5.7 mm(no threshold)
y d(
y)
y (keV m-1)
0.1 1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0 silicon telescope 7.6 mm cylindrical TEPC 7.6 mm
(threshold) cylindrical TEPC 7.6 mm
(no threshold)
y d(
y)
y (keV m-1)
0.1 1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0 silicon telescope 14.2 mm cylindrical TEPC 14.2 mm
(threshold) cylindrical TEPC 14.2 mm
(no threshold)
y d(
y)
y (keV m-1)
0.1 1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0 silicon telescope 10.5 mm cylindrical TEPC 11.6 mm
(threshold) cylindrical TEPC 11.6 mm
(no threshold)
y d(
y)
y (keV m-1)
0.1 1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0 silicon telescope 18 mm cylindrical TEPC 18.4 mm
(threshold) cylindrical TEPC 18.4 mm
(no threshold)
y d(
y)
y (keV m-1)
Constant TE scaling factor
31
DISTAL PART OF THE SOBP
0 2 4 6 8 10 12 14 16 18 20 22 240.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dep
th d
ose
curv
e (a
.u.)
depth in PMMA (mm)
1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0
silicon telescope 20.5 mm cylindrical TEPC 20.1 mm
(threshold) cylindrical TEPC 20.1 mm
(no threshold)
y d(
y)
y (keV m-1)
1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0 silicon telescope 21.2 mm cylindrical TEPC 21.4 mm
(threshold) cylindrical TEPC 21.4 mm
(no threshold)
y d(
y)
y (keV m-1)
1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0 silicon telescope 21.4 mm cylindrical TEPC 21.6 mm
(threshold) cylindrical TEPC 21.6 mm
(no threshold)
y d(
y)
y (keV m-1)
1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0 silicon telescope 21.6 mm cylindrical TEPC 21.8 mm
(threshold) cylindrical TEPC 21.8 mm
(no threshold)
y d(
y)
y (keV m-1)
1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0 silicon telescope 21.8 mm cylindrical TEPC 22 mm
(threshold) cylindrical TEPC 22 mm
(no threshold)
y d(
y)
y (keV m-1)Event-by-event TE correction!!!
32
PRELIMINARY MEASUREMENTS WITH 62 MeV/n C IONS
0 5 10 15 20 250.0
0.2
0.4
0.6
0.8
1.0
rela
tive
Dos
e
depth in PMMA (mm)
0 20 40 60 80 100 120 140 160 1800
500
1000
1500
2000
2500
Analytical
Energy deposited in the E stage (MeV)
Ene
rgy
depo
site
d in
the
E s
tage
(keV
)
1
2
4
7
14
27
53
103
200
Depth = 6 mm Counts
0 20 40 60 80 100 120 140 160 1800
500
1000
1500
2000
2500
C B Be Li He H
Energy deposited in the E stage (MeV)
Ene
rgy
depo
site
d in
the E
sta
ge (k
eV) 1
261332751784241005
Depth = 9 mmCounts
Analytical
0 5 10 15 20 25 30 35 40 45 500
200
400
600
800
1000
Analytical He H
Energy deposited in the E stage (MeV)
Ene
rgy
depo
site
d in
the E
sta
ge (k
eV)
1247142753103200
Depth = 24 mm Counts