A silicon microdosimeter for radiation
quality assessment
S. AGOSTEO (1,2), C.A. CASSELL(2,3,4), A. FAZZI (1,2),) M.V.
INTROINI (1,2), M. LORENZOLI (1,2),
A. POLA (1,2), E. SAGIA(2,3), V. VAROLI (1,2).
(1)
INFN, Sezione di Milano, via Celoria 16, 20133 Milano, Italy.
(2)
Politecnico di Milano, Dipartimento di Energia, Sezione di Ingegneria Nucleare CeSNEF,
via Ponzio 34/3, 20133 Milano, Italy.
(3)
ARDENT framework.
(4) Centre
for medical radiation physics, UOW.
SOLID STATE MICRODOSIMETERS
Si-devices can provide
sensitive zones of the
order of a micrometer
CHALLENGING DEVICES
FOR MICRODOSIMETRY
HOW a Si-DEVICE BASED MICRODOSIMETER WORKS?…
Tissue-equivalent
converter
Silicon
device
Spectroscopy
Chain
Spectrum of the
energy imparted
per event in silicon
Microdosimetric
spectrum in
Data Analysis
tissue
Analytical
corrections
PN diodes in SOI wafer
[1]
[2]
[3]
B. Rosenfeld, P. Bradley, I. Cornelius, G. Kaplan, B. Allen, J. Flanz, M. Goitein, A.V. Meerbeeck, J. Schubert, J.
Bailey, Y. Tabkada, A. Maruashi, Y. Hayakawa, New silicon detector for microdosimetry applications in proton
therapy, IEEE Trans. Nucl. Sci. 47(4) (2000) 1386-1394.
P. Bradley, A.B. Rosenfeld, B.J. Allen, J. Coderre, and J. Capala, Performance of silicon microdosimetry
detectors in boron neutron capture therapy, Radiation Research 151 (1999) 235-243.
P.D. Bradley, The Development of a Novel Silicon Microdosimeter for High LET Radiation Therapy, Ph. D.
Thesis, Department of Engineering Physics, University of Wollongong, Wollongong, Australia (2000).
SEGMENTED SILICON TELESCOPE
Silicon telescope:
a thin ∆E stage (1.9 μm thick)
coupled to a residual energy
stage E (500 μm thick)
on the same silicon wafer.
n+
p+
p+
ΔE stage
(  1.9 μm)
E stage (  500 μm)
n+
∆E stage: matrix of cylindrical diodes (h= 2 µm , d= 9 μm)
∆E element
9 µm
~2 μm
500 µm
E stage
Guard
14 µm
More than 7000 pixels are connected in parallel to give an effective
detection area of the ∆E stage of about 0.5 mm2
MICRODOSIMETRIC SPECTRA: TISSUE-EQUIVALENCE
AND GEOMETRICAL CORRECTIONS
In order to derive microdosimetric spectra similar to those acquired by
a TEPC, corrections were studied and discussed in details [1,2]
Tissue equivalence of silicon
The telescope allows to optimize the tissue equivalence
correction by measuring event-by-event the energy of the
impinging particles and by discriminating them.
Shape equivalence
By following a parametric criteria given in literature, the lineal
energy y was calculated by considering an equivalent mean
cord length.
1.
2.
S. Agosteo, P. Colautti, A. Fazzi, D. Moro and A. Pola, “A Solid State Microdosimeter
based on a Monolithic Silicon Telescope”, Radiat. Prot. Dosim. 122, 382-386 (2006).
S. Agosteo, P.G. Fallica, A. Fazzi, M.V. Introini, A. Pola, G. Valvo, “A Pixelated Silicon
Telescope for Solid State Microdosimeter”, Radiat. Meas., accepted for publication.
TISSUE EQUIVALENCE CORRECTION
The tissue equivalence of silicon device requires:
A suitable correction to the measured distribution in order to
obtain a spectrum equivalent to that acquired with an
hypothetical tissue E detector
Analytical procedure for tissue-equivalence correction
T is s u e
Ed
(E p , l)

Si
E d (E p , l) 
Energy deposited along a track of length l
by recoil-protons of energy Ep in a tissueequivalent E detector
S T is s u e(E p )
Si
S (E p )
Scaling factor :
stopping powers ratio
TISSUE-EQUIVALENCE CORRECTION
1.0
Tissue
The scaling factor
S
(E )
SSi ( E )
Protons
Electrons
Si
Tissue
0.6
S
depends on the energy and
the type of the impinging
particle
(E)/S (E)
0.8
0.4
E stage of the telescope
and ∆E-E scatter-plot
1
10
100
1000
10000
E (keV)
Limits:
the thickness of the E stage restricts the TE correction to recoilprotons below 8 MeV (alphas below 32 MeV)
Electrons release only part of their energy in the E stage
Mean value over a wide energy range (0-10 MeV) = 0.53
SHAPE ANALYSIS
Pixelated silicon telescope (d≈10 μm)
Correction is only geometrydependent (no energy limit)
Pixelated silicon telescope
Cylindrical TEPC
1.2
1.0
-1
p(l) (m )
The correcting procedure can be
based on cord length
distributions, since ∆E pixels are
cylinders of micrometric size in all
dimensions (as the TEPCs).
0.8
0.6
0.4
0.2
0.0
0
1
2
3
l (m)
4
5
6
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:
By equating the dose-mean energy imparted per event for the two
different shapes considered:
=
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  
RESPONSE TO PROTONS:
Irradiations with
62 MeV modulated proton beam
at CATANA facility
(LNS-INFN Catania)
and
comparison with cylindrical TEPC
(De Nardo et al., RPD 110, 1-4 (2004)
220
180
160
140
120
Counts per unit dose
0
1.2
2.4
3.6
4.8
6.0
7.2
8.4
9.6
11
12
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
Energy deposited in the E stage (MeV)
9
220
Depth = 10.5 mm
Dose = 0.31 Gy
200
180
160
140
120
Counts per unit dose
0
1.2
2.4
3.6
4.8
6.0
7.2
8.4
9.6
11
12
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
Energy deposited in the E stage (keV)
Depth = 8 mm
Dose = 0.3 Gy
200
Energy deposited in the E stage (keV)
Energy deposited in the E stage (keV)
62 MeV modulated proton beam (CATANA)
220
Depth = 15.5 mm
Dose = 0.44 Gy
200
180
160
140
120
100
80
60
40
20
0
0
Energy deposited in the E stage (keV)
1.2
1.1
1.0
0.9
rel Dose
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28
depth in PMMA (mm)
Counts per unit dose
0
1.2
2.4
3.6
4.8
6.0
7.2
8.4
9.6
11
12
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
220
Depth = 18 mm
Dose = 0.43 Gy
200
180
160
140
120
Counts per unit dose
0
1.2
2.4
3.6
4.8
6.0
7.2
8.4
9.6
11
12
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
220
Depth = 20.5 mm
Dose = 0.36 Gy
200
180
Energy deposited in the E stage (keV)
Energy deposited in the E stage (keV)
62 MeV modulated proton beam (CATANA)
Counts per unit dose
160
140
120
0
0.70
1.4
2.1
2.8
3.5
4.2
4.9
5.6
6.3
7.0
100
80
60
40
20
0
1
2
3
4
5
6
7
8
Energy deposited in the E stage (MeV)
1.2
1.1
1.0
0.9
rel Dose
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28
depth in PMMA (mm)
Depth = 21 mm
Dose = 0.46 Gy
200
180
160
140
120
Counts per unit dose
0
0.70
1.4
2.1
2.8
3.5
4.2
4.9
5.6
6.3
7.0
100
80
60
40
20
0
9
0
Energy deposited in the E stage (keV)
0
220
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
220
Depth = 21.5 mm
Dose = 0.62 Gy
200
180
160
140
120
Counts per unit dose
0
0.70
1.4
2.1
2.8
3.5
4.2
4.9
5.6
6.3
7.0
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
9
Energy deposited in the E stage (MeV)
Comparison with cylindrical TEPC: proximal part of the SOBP
2.0
2.0
2.8
silicon telescope 5.7 mm
cylindrical TEPC 5.7 mm (threshold)
cylindrical TEPC 5.7 mm(no threshold)
2.4
1.6
1.4
2.2
y d(y)
2.0
1.8
1.6
1.2
1.0
1.6
1.4
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
silicon telescope 10.5 mm
cylindrical TEPC 11.6 mm
(threshold)
cylindrical TEPC 11.6 mm
(no threshold)
1.8
y d(y)
2.6
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
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
Comparison with cylindrical TEPC: distal part of the SOBP
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
y d(y)
0.6
y d(y)
0.8
y d(y)
0.8
silicon telescope 20.5 mm
cylindrical TEPC 20.1 mm
(threshold)
cylindrical TEPC 20.1 mm
(no threshold)
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
1000
62 MeV modulated proton beam (CATANA)
Results:
• easy-of-use system;
• rapid data processing;
• good measurement repeatability;
• high spatial resolution;
• good agreement at lineal energies higher than 7-10 keV μm-1up to the proton
edge.
Problems to solve or to minimize:
• high electronic noise;
• counting rates, mainly related to the relative dimension between ∆E stage and
E stage active areas.
Issues:
• accurate estimate of dose profile;
• radiation damage.
RESPONSE TO CARBON IONS:
Irradiations with
62 MeV/u un-modulated carbon beam
at CATANA facility
(LNS-INFN Catania)
62 MeV/u un-modulated carbon beam (CATANA)
1.0
2500
Depth = 6 mm
0.6
0.4
0.2
0.0
0
5
10
15
depth in PMMA (mm)
20
25
Energy deposited in the E stage (keV)
relative Dose
0.8
Counts
1
2
2000
4
7
Analytical
14
27
1500
53
103
200
1000
500
0
0
20
40
60
80
100
120
140
160
Energy deposited in the E stage (MeV)
180
62 MeV/u un-modulated carbon beam (CATANA)
1.0
2500
Counts
0.6
0.4
0.2
0.0
0
5
10
15
depth in PMMA (mm)
20
25
Energy deposited in the E stage (keV)
relative Dose
0.8
Depth = 9 mm
1
2
6
13
32
75
178
424
1005
2000
1500
Analytical
C
B
Be
Li
He
H
1000
500
0
0
20
40
60
80
100
120
140
160
Energy deposited in the E stage (MeV)
180
62 MeV/u un-modulated carbon beam (CATANA)
Results:
• high spatial resolution;
• capability of operating in a complex and intense radiation field;
• discrimination capability and potentialities.
Problems to solve or to minimize:
• relative dimension between ∆E stage and E stage active areas;
• counting rates;
• radiation damage.
CONCLUSIONS
IMPROVEMENT OF THE ENERGY
TRESHOLD:
A feasibility study of a low-LET
silicon microdosimeter
Improvement of the energy threshold
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 having a lower number
of cylinders connected in parallel and an E stage with an optimized sensitive
area
1.
2.
Decrease the energy threshold below 1 keV μm-1
Optimize the counting rate of the two stages