transversal dose mapping and bragg-curve reconstruction in ... lims2018 sito/2_3 nichelatti... · +...

E. Nichelatti1, M. Piccinini2, A. Ampollini2, L. Picardi2, C. Ronsivalle2,

F. Bonfigli2, M.A. Vincenti2, R.M. Montereali2

1 ENEA C.R. Casaccia, Fusion and Technologies for Nuclear Safety and Security – Via Anguillarese 301,

S. Maria di Galeria, Rome, 00123, Italy 2 ENEA C.R. Frascati, Fusion and Technologies for Nuclear Safety and Security – Via E. Fermi 45,

Frascati (RM), 00044, Italy

Transversal dose mapping

and Bragg-curve reconstruction

in proton-irradiated lithium fluoride detectors

by fluorescence microscopy

LIMS 2018 – ENEA C.R. Frascati, May 17-18, 2018

• Lithium fluoride and colour centres as

radiation detectors

• Proton therapy and the TOP-IMPLART

project

• Proton irradiation of lithium fluoride

• Transversal dose mapping and Bragg-

curve reconstruction

• Conclusions

2

Summary


Transversal dose mapping and Bragg-curve reconstruction in proton-

irradiated lithium fluoride detectors by fluorescence microscopy


radiation detectors


project




• Conclusions

3

Summary




Lithium fluoride (LiF)

4

• Almost non-hygroscopic

• Hosts (even at RT) stable laser-active point defects:

colour centres (CCs)

• Some CCs emit in the visible and NIR under optical

excitation

• Radiation-sensitive material CC formation


• Crytallographic structure: fcc

• Lattice constant = 4:03 Å

• Density = 2:639 g/cm3

• Melting temperature = 870oC

• Hardness = Knoop 102 with 600 g

indenter

• Low hygroscopicity (solubility @ 18oC =

0:27 g / 100 g H2O)

• Low refractive index (~1:39) in the

visible

• Optically transparent from ~120 nm up

to ~ 6 µm

Useful for dosimetric purposes thanks to its TISSUE EQUIVALENCE

Colour centres (CCs) in lithium fluoride

5 LIMS 2018 – ENEA C.R. Frascati, May 17-18, 2018

F2

Irradiation of LiF (elementary particles,

ions, EUV light, X-rays, or -rays)

stable formation of primary (F) and

aggregate CCs.

Aggregate F2 and F3+ CCs (two electrons

bound to two and three close anion

vacancies, respectively)

• red (F2) and green (F3+) emission

• almost overlapping absorption bands at

~450 nm they can be simultaneously

excited with a blue optical pump.

Applications of CCs in LiF:

• dosimeters

• light emitting devices

• tunable solid-state lasers

LiF devices as radiation detectors


Ionising radiation impinges on LiF-based device (either bulk or thin film)

Stable colour centres (CCs) are created and stored in the LiF crystal lattice

The optically-active F2 and F3+ CCs are excited (blue light, ~450 nm) and

their visible PL (red and green, respectively) detected in a microscope

• Sub-micron spatial resolution (objective-

limited) over a wide field of view

• Wide dynamic range

• No need of development

• Works in air and at RT

• Time-stability (~years)

• Daylight operation

Contact

µ-radiography

Metallic grids over

LiF crystal.

EUV radiation by

plasma source.

G. Baldacchini et al. 2005

Rev. Sci. Instrum. 76 113104

7

The fluorescence microscope


“In fluorescence microscopy, the sample you want to study is

itself the light source. The technique is used to study specimens,

which can be made to fluoresce. The fluorescence microscope is

based on the phenomenon that certain material emits energy

detectable as visible light when irradiated with the light of a

specific wavelength.” (Microscopes—Help Scientists Explore Hidden Worlds. The Nobel

Foundation.)

Nikon Eclipse 80i-C1

The wide-field optical microscope working in fluorescent mode

and in white-light transmission mode is equipped with two light

sources consisting of:

• Arc lamp photovoltaic mercury OSRAM 100 W for

fluorescence mode;

• Halogen lamp for white light transmission mode.

Detector Andor Neo Scmos, Front Illuminated, -40oC

cooled, 11/16bit digitalization, 100 f/sec,

5.5 Mpixels, 2560x2160 resolution, 6.5 µm

pixel size


radiation detectors


project




• Conclusions

8

Summary




Proton therapy

9

Doctors can better aim proton beams onto a tumor, so there

is less damage to the surrounding healthy tissue. This

allows doctors to use a higher dose of radiation with proton

therapy than they can use with X-rays.

Proton therapy is used to treat cancers that have not

spread. Because it causes less damage to healthy tissue,

proton therapy is often used for cancers that are very close

to critical parts of the body.


Proton therapy: particle therapy that uses a beam of protons to irradiate diseased

tissue, most often in the treatment of cancer.

Chief advantage (w.r.t. other techniques, e.g. X-rays): the dose is deposited over a

narrow range and there is minimal exit dose.

Treatable tumours

• Brain

• Eye

• Head and neck

• Lung

• Spine

• Prostate

• Lymph system cancer

Hindrance to universal use of protons: size and cost of the cyclotron or synchrotron

equipment.

Development of comparatively small accelerator systems is being pursued, e.g.

linear particle accelerators.

Proton therapy: dose distribution

Deposit the therapeutic dose within the volume being treated while

preserving neighbouring tissues


X-rays protons

The TOP-IMPLART project


Oncological Therapy with Protons – Intensity Modulated

Proton Linear Accelerator for RadioTherapy

A high frequency linac has been developed for

the project TOP-IMPLART, with most of the

technology derived from conventional

radiotherapy equipments to make a compact

machine with reasonable costs.

Project partners:

ENEA (Italian National Agency for New

Technologies, Energy and Sustainable

Economic Development), ISS (National

Institute of Health), IFO (Istituti

Fisioterapici Ospedalieri, Roma).

The IMPLART segment (150 MeV

proton beam) up to the first treatment

room (Head and neck and paediatrics

tumours) is under construction and

installation at ENEA-Frascati, chosen as

test site for its validation before its

transfer to IRE-IFO-Rome hospital, that

will be the clinical user.

The program is funded by Regione Lazio

with a grant of 11 M€ only for high

technology systems.

Current status of the accelerator


Output energy: 35 MeV

Pulse duration: 1-4 µs

Output charge/pulse: up to 60 pC

Particles/pulse: up to 4×108

Repetition Frequency: 20(typical)-50(max) Hz

Spot area (at linac exit): 0.2 cm2

• The accelerator is based on a 7 MeV injector (RF frequency 425 MHz)

followed by a vertical and an horizontal beam transport line matching the

beam to the following accelerating modules (RF frequency 2997.92 MHz).

• The segment up to 35 MeV is in operation at ENEA-Frascati. It consists of

4 SCDTL (Side coupled Drift Tube Linac) accelerating modules powered by

a single 10 MW klystron.

• The beam is used for dosimetry and radiobiology experiments devoted to

pre-clinical tests and assessment. Uniform irradiation of targets with an

area of 2-3 cm2 is obtained by spreading the beam in air at a distance of 1-

2 m from the linac exit.


radiation detectors


project




• Conclusions

13

Summary




Proton irradiation of LiF


Convenience of lithium fluoride for detecting ionising radiation

The effective atomic number of LiF is close to that of soft tissue (tissue or water equivalence)

simplified calibration in clinical dosimetry

LiF response to dose (PL intensity from F2 and F3+ CCs) has been demonstrated to be

• linear with dose over several orders of magnitude (up to ~105-106 Gy)

• independent of proton energy (tested so far from 3 MeV to a few dozens of MeV)

• independent of type of radiation type (protons vs. -rays) at clinical doses

PL intensity I vs. dose D

Saturates above a certain dose value

M. Piccinini et al. 2017 EPL 117 37004

proportional to total

number of excited CCs

Dose = fluence * LET / material density

Dose: deposited energy per unit mass. PL intensity vs. dose

Ion penetration in matter: Bragg curve and peak


Energetic ions deposit their energy

into matter. The amount of deposited

energy per unit depth (LET, Linear

Energy Transfer) follows a depth

distribution known as Bragg curve. In

LiF, the deposited energy contributes

in creating colour centres (CCs),

some of which emit visible light if

subsequently illuminated with a blue

optical pump.

visualisation of deposited

energy

The Bragg peak is the LET maximum, found at the end of the Bragg curve.




• The depth of the Bragg peak increases superlinearly with energy.





• The LET at the surface decreases with energy in a logistic way.





• The LET at the surface decreases with energy in a logistic way.

• The LET at the Bragg peak decreases with energy in a logistic way.

Proton irradiation of LiF


Summing up: what makes LiF-based devices good as proton detectors?

• LiF has good tissue equivalence (effective atomic number of 8.3 is close to

that of water or soft tissue).

• The PL intensity from CCs created by proton irradiation is linear vs. dose up to

~ 105-106 Gy. Saturation beyond that threshold value is dealt with by using a

simple model

• Possibility of visualising and analysing PL distributions corresponding to dose

distributions within LiF allows for

o advanced proton beam diagnostics (mean energy and energy spread of

protons)

o detector material characterisation (linearity range and saturation dose)


radiation detectors


project




• Conclusions

20

Summary




Proton-beam 2D dose mapping


IRRADIATION

The LiF device is irradiated by

the proton beam, which

impinges perpendicularly to one

of its faces. Its effect is to create

a distribution of colour centres in

the material.

PL-INTENSITY RECORDING

The latent PL-intensity 2D map

due to the created colour

centres is detected with a

fluorescence microscope and

digitally stored in an image file.

DOSE-MAP RECONSTRUCTION

The PL digital image is analysed

and numerically inverted to obtain

the absorbed dose map in the LiF

device. During the inversion

process, the nonlinear

dependence of PL intensity at

high doses is taken into account.

3 MeV protons, LiF film

M. Piccinini et al., EPL 117 (2017) 37004

1 mm

1 2 3

Proton beams and LiF: 2D maps properties


Noteworthy properties

• Because the PL intensity linearly depends on the dose up to a certain

dose value (~105-106 Gy), for low enough beam fluence the 2D PL map

I(x,y) is a direct representation of the dose map D(x,y). (same D vs. I proportionality factor at each point)

• In case of higher fluences, for which saturation of CC concentration

occurs, the 2D PL map is a distorted replica of the dose map. A

numerical inversion process is needed to obtain D(x,y) from I(x,y).

Bragg curve analysis


IRRADIATION

The LiF crystal is irradiated by

the proton beam, which

impinges on one of its side

faces. Its effect is to create a

distribution of colour centres in

the material.

PL-INTENSITY RECORDING

Strips are cut out from the latent

PL-intensity map detected with a

fluorescence microscope. They

are 1D intensity distributions,

which are digitally stored into

data files.

BRAGG-CURVE

RECONSTRUCTION (1st TIME!)

The 1D data files are analysed

and best fitted starting from SRIM

simulations. During the inversion

process, the nonlinear

dependence of PL intensity at

high doses is taken into account.

E. Nichelatti et al., EPL 120 (2017) 56003

1 2 3

air

250 µm

7 MeV protons, LiF crystal

Bragg

peak

A

B

C

C

Bragg curve best fit


Best fits of the experimental PL intensity curves along z

were performed in Matlab using a least square method

and input LET files obtained from SRIM simulations.

Fit parameters:

• Mean proton energy

• Proton energy spread (std. dev.)

• Input-to-saturation dose ratio

Note: for low enough fluences, the PL intensity curve is a direct representation

of the underlying Bragg curve (no saturation is involved). A simpler linear model

(involving only and ) is utilised.

proton beam

diagnostics }

Bragg curve fit: mean energy


The mean energy

acts on the depth of the

Bragg peak

Bragg curve fit: energy spread (std. dev.)


The energy spread

acts on the depth, width

and height of the Bragg

peak

Bragg curve fit: input dose / saturation dose


The input dose value

(as compared to the

saturation dose )

acts on the more or

less flat shape of the

PL intensity curve

dose D(z) is evaluated from SRIM simulations

Model

Bragg-curve fit: 7 MeV results


FIT PARAMETERS ADVANCED PROTON BEAM DIAGNOSTICS

KNOWN PARAMETERS (from experimental beam data)

DERIVED PARAMETERS LiF SATURATION (LINEARITY RANGE)

RESULTS FOR NOMINAL 7 MeV PROTONS (E. Nichelatti et al., EPL 120 (2017) 56003)

input-face dose proton fluence

mean energy energy std. dev.

input/saturation dose ratio

saturation dose dose @ Bragg peak (= max dose)

C


radiation detectors


project




• Conclusions

29

Summary




Conclusions


Enrico Nichelatti

ENEA – FSN-TECFIS-MNF

[email protected]


Thank you for your attention!

transversal dose mapping and bragg-curve reconstruction in ... lims2018 sito/2_3 nichelatti... · +...

Documents