TRAINING COURSE ON
EXPERIMENTAL MICRODOSIMETRY
Principle of siliconbased 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
16 October, 2013
1
+
-






P zone
























Depeleted zone









N zone






SILICON MICRODOSIMETRY
PN diodes
[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.
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
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.
Spectrum of the
energy imparted
per event in
silicon
Tissue-equivalent
converter
Spectroscopy
Chain
Data Analysis
Microdosimetric
spectrum in
tissue
Silicon device
Analytical
corrections
4
INTRODUCTION (II)
•
Advantages:
 wall-effects avoided;
 compactness;
 cheapness;
 transportability;
 low sensitivity to vibrations;
 low power consumption.
5
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.
6
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:
•
Depletion layer thickness: 2 m @ 2 V
neutrons from LiF @ 0°
with polyethylene - ASIC
Vcc=-2 V
-5
10
En= 0.573 MeV
En= 1.511 MeV
2
Response (cm )
En= 0.996 MeV
Recoil proton
2 µm @ 2V
•
Field
Funneling
Effect
P-Layer
En= 2.020 MeV
-6
10
En= 3.030 MeV
-7
10
Depletion
Layer
N+
-8
10
0
100
200
300
400
500
600
700
800
900
1000
Energy (keV)
Active thickness: ~ 12 m
7
1100
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.
yd(y), keV/m
•
0.08
0.07
0.06
1 um
Al
Al
N+
P+
2 um
-
+
E-field
Si Substrate
SiO2
experimental
simulation
0.05
0.04
~ 10-20 um
0.03
0.02
All figures in this slide
Courtesy of A. Rosenfeld, Wollongong University, Australia
0.01
0.00
10-1
100
101
y, keV/m
102
103
8
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.
1E20
-3
Sensitive area: 1 mm2
Y Axis Title
E charge
collection
1E19
Dopant concentration (cm )
•
E thickness: ~1.9 m
E thickness: ~ 500 m
1E18
As
B
1E17
1E16
1E15
1E14
1E13
1E12
0.0
0.5
1.0
1.5
2.0
2.5
Depth (m)
8.0x10
4
7.0x10
-4
6.0x10
-4
7.0x10
-4
6.0x10
5.0x10
-4
-4
5.0x10
-4
4.0x10
3.0x10
-4
4.0x10
-4
-4
3.0x10
-4
2.0x10
-4
2.0x10
-4
1.0x10
-4
1.0x10
-4
En = 680 keV
En = 996 keV
Experimental
FLUKA Simulation
Analytical
Experimental
FLUKA Simulation
Analytical
0.8
Silicon telescope
Cylindrical TEPC (2.7 m)
0.7
0.6
En=0.996 MeV
En=2.727 MeV
En=4.336 MeV
2.0x10
0.0
100
7.0x10
-4
6.0x10
-4
5.0x10
-4
4.0x10
100
7.0x10
-4
6.0x10
-4
5.0x10
-4
-4
4.0x10
-4
3.0x10
-4
3.0x10
-4
2.0x10
-4
2.0x10
-4
1.0x10
-4
1.0x10
-4
2
4
2
4.0x10
0.0
En = 1715 keV
En = 1306 keV
Experimental
FLUKA Simulation
Analytical
Experimental
FLUKA Simulation
Analytical
0.5
e d(e)
2
4
-1
E *f(E) (C )
6.0x10
p(e) e (keV cm )
STm R324 DE4
0.4
0.3
0.2
4
0.1
0.0
0.0
100
1000
0.0
Energy (keV)
0.0
100
100
e (keV)
10
100
e (keV)
1000
9
EFFECTS ON SILICON AND DOPANTS
0.05
9
10
In the cavity of the thermal neutron facility - Be(d,n) B
Ed=4.5 MeV
Ed=6.5 MeV
0.03
E*f(E)
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 2.210-6 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.
1.5 MeV - 
0.02
7
0.8 MeV - Li
10
7
n + B ----> Li + 
(Q=2.31 MeV)
0.00
→ 28Si(n,p)28Al
→
7
2.3 MeV - Li+
0.01
(Eth 4.0 MeV);
28Si(n,)25Mg (E 2.75
th
MeV).
3
4
10
10
E (keV)
0.020
0.018
LiF monoenergetic neutrons - bare p-i-n photodiode
0.016
En=0.996 MeV @0°
0.014
En=4.536 MeV @0°
0.012


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.
E*f(E)
•
Ed=3.0 MeV
0.04
0.010
0.008
0.006
0.004
0.002
0.000
3
4
10
10
E (keV)
10
Si MESA MICRODOSIMETERS
•
3D silicon mesa p-n junction array with internal charge
amplification produced at UNSW SNF.
E
r
All figures in this slide
Courtesy of A. Rosenfeld, Wollongong University, Australia
Single mesa 3D SV
11
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 tissueequivalency.
0.8
0.7
Pixelated silicon telescope
Cylindrical TEPC (2.7 m)
0.6
0.5
y d(y)
•
0.4
0.3
0.2
0.1
0.0
10
100
1000
-1
y (keV m )
∆E element
9 µm
~2 μm
500 µm
E stage
Guard
14 µm
12
SEGMENTED TELESCOPE: SEM IMAGES
13
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.
250
Counts
Energy imparted in the E stage (keV)
•
200
150
Recoil-protons
Recoil-protons
18
16
14
12
10
7
5
3
1
May be due to:
-Track length
distribution;
Charge sharing.
100
Contribution of about
5.5%
50
0
500
1000
1500
2000
2500
SecondaryEnergy imparted in the E stage (keV)
electrons
3000
14
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.
Recoil-protons
300
Counts
Energy imparted in the E stage (keV)
•
250
200
Recoil-protons
Calculated
contribution of about
5%
20
18
16
14
12
10
8
6
4
2
0
due to the track
length distribution!
150
100
Charge sharing
can be neglected
50
0
0
500
1000
1500
2000
2500
Energy imparted in the E stage (keV)
3000
15
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.
16
TISSUE-EQUIVALENCE CORRECTION
Analytical procedure for tissue-equivalence correction:
E
Tissue
d
(Ep , l)  E (Ep , l) 
Si
d
Energy deposited along a track of length l by
recoil-protons of energy Ep in a tissue-equivalent
E detector
S
Tissue
(Ep )
Si
S (Ep )
Scaling factor :
stopping powers ratio
17
TISSUE-EQUIVALENCE CORRECTION
1.0
Protons
Electrons
Si
0.6
S
Tissue
depends on the energy and type
of the impinging particle
0.8
(E)/S (E)
The scaling factor
STissue (E)
SSi (E)
0.4
E stage of the telescope and ∆E-E scatter-plot
allow an energy-dependent correction for
protons
1
10
100
1000
10000
E (keV)
the thickness of the E stage limits the TE correction to recoil-protons below 8
MeV (alphas below 32 MeV)
Electrons release only part of their energy in the E stage
ELECTRONS: average value over a wide energy range (0-10 MeV) = 0.53
18
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).
Pixelated silicon telescope
Cylindrical TEPC
1.2
1.0
-1
p(l) (m )
•
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
l (m)
19
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:
•
Si
eTissue
E ( E )  e E ( E ) 
2)
1
Emax
Emax

0
S Tissue ( E )
dE
S Si ( E )
e Tissue
(E)  e SiE (E)  0.574
E
Shape equivalence:
by equating the dose-mean energy
imparted per event for the two
cylindrical sites:
e
E
D
 Ll
E
D
= e
TEPC
D
 Ll
TEPC
D
lDTEPC
  E  0.533
lD
20
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:
STissue (E)
e E (E)  e E (E)  Si
S (E)
E  eSiE (E)  eSiE (E)
Tissue
Si
21
1.0
silicon telescope 21.4 mm
f=0.574
cylindrical TEPC 21.6 mm
0.8
y d(y)
0.6
0.4
0.2
0.0
1
10
100
1000
-1
y (keV m )
Constant scaling factor
DISTAL PART OF SOBP
5.0
1.0
4.0
silicon telescope 21.4 mm
cylindrical TEPC 21.6 mm
(threshold)
cylindrical TEPC 21.6 mm
(no threshold)
0.8
3.5
y d(y)
3.0
2.5
0.6
0.4
2.0
1.5
0.2
1.0
0.5
0.0
1
0.0
10
100
-1
0
2
4
6
8
10
12
14
16
depth in PMMA (mm)
18
20
22
24
y (keV m )
1000
Energy-dependent correction
Depth dose curve (a.u.)
4.5
22
ENERGY THRESHOLD IMPROVEMENT
The main limitation of the system is the high energy threshold
imposed by the electronic noise.
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
A feasibility study with a low-noise set-up based on discrete
components was carried out in order to test this possibility
23
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.
1.4
FLUKA simulation (tissue)
Experimental
1.2
1.0
y d(y)
Lineal energy threshold ≈ 0.6 keV μm-1
0.8
0.6
0.4
0.2
0.0
0.01
0.1
1
10
100
-1
y (keV m )
24
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.
25
Additional Slides
26
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:
l
eD  L  0
2
 p(l )dl
l
 L  lD
By equating the dose-mean energy imparted per event for the two different shapes considered:
eDE  L  lDE
=
eDTEPC  L lDTEPC
lDTEPC
  E  0.533
lD
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:
lE ,eq  lE  
27
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.
PMMA
foils
Detector
+
Mylar
PMMA
phantom
Front-end electronics
28
160
140
120
100
80
60
180
160
Counts per unit dose
0
1.2
2.4
3.6
4.8
6.0
7.2
8.4
9.6
11
12
SCATTER-PLOTS
140
120
100
80
60
40
20
40
0
0
20
1
2
3
4
5
6
7
8
Energy deposited in the E stage (MeV)
9
180
160
140
120
Energy deposited in the E stage (keV)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2
4
6
8
10
12
14
16
depth in PMMA (mm)
18
20
22
24
0
1.2
2.4
3.6
4.8
6.0
7.2
8.4
9.6
11
12
80
60
40
20
0
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
4.5
Counts per unit dose
100
0
5.0
0
Depth = 18 mm
Dose = 0.43 Gy
200
0
0
Depth dose curve (a.u.)
220
220
Energy deposited in the E stage (keV)
180
Depth = 15.5 mm
Dose = 0.44 Gy
200
Energy deposited in the E stage (keV)
Counts per unit dose
0
1.2
2.4
3.6
4.8
6.0
7.2
8.4
9.6
11
12
Depth = 8 mm
Dose = 0.3 Gy
200
Energy deposited in the E stage (keV)
Energy deposited in the E stage (keV)
220
220
220
1
2
3
4
5
6
7
8
9
Counts per unit dose
Depthin
= 21
200 deposited
Energy
themm
E stage (MeV)
0
Dose = 0.46 Gy
0.70
180
1.4
2.1
2.8
3.5
4.2
4.9
5.6
6.3
7.0
160
140
120
Depth = 21.5 mm
100
Dose = 0.62 Gy
200
180
160
140
120
100
80
60
40
20
0
0.70
1.4
2.1
2.8
3.5
4.2
4.9
5.6
6.3
7.0
0
0
80
Counts per unit dose
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
60
40
20
0
0
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
29
2.0
2.0
2.8
silicon telescope 5.7 mm
cylindrical TEPC 5.7 mm (threshold)
cylindrical TEPC 5.7 mm(no threshold)
2.6
2.4
1.6
1.4
2.2
silicon telescope 10.5 mm
cylindrical TEPC 11.6 mm
(threshold)
cylindrical TEPC 11.6 mm
(no threshold)
1.8
1.6
1.4
PROXIMAL AND ACROSS THE SOBP
1.8
1.6
1.2
y d(y)
y d(y)
2.0
y d(y)
silicon telescope 7.6 mm
cylindrical TEPC 7.6 mm
(threshold)
cylindrical TEPC 7.6 mm
(no threshold)
1.8
1.0
1.2
1.0
0.8
0.8
0.6
0.6
0.8
0.4
0.4
0.6
0.2
0.2
1.4
1.2
1.0
0.4
0.0
0.1
0.2
0.0
0.1
1
10
100
1
10
100
0.0
0.1
1000
1
10
-1
100
1000
-1
y (keV m )
1000
y (keV m )
-1
y (keV m )
2.0
silicon telescope 14.2 mm
cylindrical TEPC 14.2 mm
(threshold)
cylindrical TEPC 14.2 mm
(no threshold)
1.8
1.6
y d(y)
1.4
5.0
1.0
0.8
0.6
4.0
2.0
3.5
1.8
3.0
1.6
0.4
silicon telescope 18 mm
cylindrical TEPC 18.4 mm
0.2
(threshold)
cylindrical TEPC 18.4 mm
0.0
(no threshold)
0.1
1.4
2.5
1
10
100
1000
-1
y d(y)
Depth dose curve (a.u.)
4.5
1.2
2.0
1.5
y (keV m )
1.2
1.0
0.8
1.0
0.6
Constant TE
scaling factor
0.4
0.5
0.2
0.0
0
2
4
6
8
10
12
14
16
depth in PMMA (mm)
18
20
22
24
0.0
0.1
1
10
100
-1
y (keV m )
1000
30
1.0
silicon telescope 21.4 mm
cylindrical TEPC 21.6 mm
(threshold)
cylindrical TEPC 21.6 mm
(no threshold)
1.0
silicon telescope 21.2 mm
cylindrical TEPC 21.4 mm
(threshold)
cylindrical TEPC 21.4 mm
(no threshold)
1.0
0.8
0.6
y d(y)
y d(y)
DISTAL PART OF THE SOBP
silicon telescope 20.5 mm
cylindrical TEPC 20.1 mm
(threshold)
cylindrical TEPC 20.1 mm
(no threshold)
0.8
y d(y)
0.8
0.6
0.6
0.4
0.4
0.2
0.4
0.2
0.0
0.2
1
0.0
1
10
100
10
1000
1
10
100
1000
y (keV m )
-1
y (keV m )
0.0
100
-1
1.0
1000
silicon telescope 21.6 mm
cylindrical TEPC 21.8 mm
(threshold)
cylindrical TEPC 21.8 mm
(no threshold)
-1
y (keV m )
0.8
5.0
0.8
3.5
3.0
2.5
2.0
0.6
silicon telescope 21.8 mm
cylindrical TEPC 22 mm
(threshold)
0.4
cylindrical TEPC 22 mm
(no threshold)
4.0
y d(y)
Depth dose curve (a.u.)
y d(y)
1.0
4.5
0.6
0.2
0.4
0.0
1
10
100
-1
y (keV m )
1.5
0.2
1.0
0.5
0.0
1
0.0
0
2
4
6
8
10
12
14
16
depth in PMMA (mm)
18
20
22
24
10
100
-1
y (keV m )
1000
Event-by-event TE
correction!!! 31
1000
PRELIMINARY MEASUREMENTS WITH 62 MeV/n C IONS
2500
Energy deposited in the E stage (keV)
Depth = 6 mm
1.0
0.6
1
2
4
7
Analytical
14
27
1500
53
103
200
1000
500
0
0
20
40
60
80
100
120
140
160
180
Energy deposited in the E stage (MeV)
0.4
2500
0.2
Energy deposited in the E stage (keV)
Counts
0.0
0
5
10
15
20
25
depth in PMMA (mm)
1000
Counts
Depth = 24 mm
1
2
Energy deposited in the E stage (keV)
relative Dose
0.8
Counts
2000
4
800
7
14
27
53
103
600
200
Depth = 9 mm
1500
Analytical
C
B
Be
Li
He
H
1000
500
0
0
Analytical
He
H
400
1
2
6
13
32
75
178
424
1005
2000
20
40
60
80
100
120
140
160
180
Energy deposited in the E stage (MeV)
200
0
0
5
10
15
20
25
30
35
40
Energy deposited in the E stage (MeV)
45
50
32