TRAINING COURSE on EXPERIMENTAL MICRODOSIMETRY

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


(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


1000.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

5.0x10-4

6.0x10-4

7.0x10-4


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)


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


Dose = 0.43 Gy

Ener

gy d

epos

ited

in th

e E s

tage

(keV

)


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


Dose = 0.46 Gy

Ener

gy d

epos

ited

in th

e E s

tage

(keV

)


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)


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



(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



(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



(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


(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


(no threshold)

y d(

y)

y (keV m-1)

1 10 100 10000.0

0.2

0.4

0.6

0.8



(no threshold)

y d(

y)

y (keV m-1)

1 10 100 10000.0

0.2

0.4

0.6

0.8



(no threshold)

y d(

y)

y (keV m-1)

1 10 100 10000.0

0.2

0.4

0.6

0.8



(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


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


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


Ene

rgy

depo

site

d in

the E

sta

ge (k

eV)

1247142753103200

Depth = 24 mm Counts

TRAINING COURSE on EXPERIMENTAL MICRODOSIMETRY

Documents

sensitive zone

microdosimetry of neutron

electric field

silicon detectors

m8the field

defined dimensions

lineal energy spectra

introduction iiadvantages