Transversal dose mapping and Bragg-curve reconstruction in proton-irradiated lithium fluoride detectors by fluorescence microscopy

Transcription

1 Transversal dose mapping and Bragg-curve reconstruction in proton-irradiated lithium fluoride detectors by fluorescence microscopy E. Nichelatti 1, M. Piccinini 2, A. Ampollini 2, L. Picardi 2, C. Ronsivalle 2, F. Bonfigli 2, M.A. Vincenti 2, R.M. Montereali 2 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

2 Transversal dose mapping and Bragg-curve reconstruction in protonirradiated lithium fluoride detectors by fluorescence microscopy Summary Lithium fluoride and colour centres as radiation detectors Proton therapy and the TOP-IMPLART project Proton irradiation of lithium fluoride Transversal dose mapping and Braggcurve reconstruction Conclusions 2

3 Transversal dose mapping and Bragg-curve reconstruction in protonirradiated lithium fluoride detectors by fluorescence microscopy Summary Lithium fluoride and colour centres as radiation detectors Proton therapy and the TOP-IMPLART project Proton irradiation of lithium fluoride Transversal dose mapping and Braggcurve reconstruction Conclusions 3

4 Lithium fluoride (LiF) 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 Useful for dosimetric purposes thanks to its TISSUE EQUIVALENCE Crytallographic structure: fcc Lattice constant = 4:03 Å Density = 2:639 g/cm3 Melting temperature = 870 o C Hardness = Knoop 102 with 600 g indenter Low hygroscopicity 18 o C = 0:27 g / 100 g H 2 O) Low refractive index (~1:39) in the visible Optically transparent from ~120 nm up to ~ 6 µm 4

5 Colour centres (CCs) in lithium fluoride Irradiation of LiF (elementary particles, ions, EUV light, X-rays, or -rays) stable formation of primary (F) and aggregate CCs. F 2 Aggregate F 2 and F 3 + CCs (two electrons bound to two and three close anion vacancies, respectively) red (F 2 ) and green (F 3+ ) 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 5

6 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 F 2 and F 3 + CCs are excited (blue light, ~450 nm) and their visible PL (red and green, respectively) detected in a microscope Sub-micron spatial resolution (objectivelimited) 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 Rev. Sci. Instrum

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, -40 o C cooled, 11/16bit digitalization, 100 f/sec, 5.5 Mpixels, 2560x2160 resolution, 6.5 µm pixel size 7

8 Transversal dose mapping and Bragg-curve reconstruction in protonirradiated lithium fluoride detectors by fluorescence microscopy Summary Lithium fluoride and colour centres as radiation detectors Proton therapy and the TOP-IMPLART project Proton irradiation of lithium fluoride Transversal dose mapping and Braggcurve reconstruction Conclusions 8

9 Proton therapy 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 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. 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. 9

10 X-rays protons Proton therapy: dose distribution Deposit the therapeutic dose within the volume being treated while preserving neighbouring tissues 10

11 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. 11

12 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 Repetition Frequency: 20(typical)-50(max) Hz Spot area (at linac exit): 0.2 cm 2 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 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 cm 2 is obtained by spreading the beam in air at a distance of 1-2 m from the linac exit. 12

13 Transversal dose mapping and Bragg-curve reconstruction in protonirradiated lithium fluoride detectors by fluorescence microscopy Summary Lithium fluoride and colour centres as radiation detectors Proton therapy and the TOP-IMPLART project Proton irradiation of lithium fluoride Transversal dose mapping and Braggcurve reconstruction Conclusions 13

14 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 F 2 and F 3 + CCs) has been demonstrated to be linear with dose over several orders of magnitude (up to ~ 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 vs. dose Dose: deposited energy per unit mass. Dose = fluence * LET / material density PL intensity I vs. dose D Saturates above a certain dose value proportional to total number of excited CCs M. Piccinini et al EPL

15 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. 15

16 Ion penetration in matter: Bragg curve and peak 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. 16

17 Ion penetration in matter: Bragg curve and peak 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. 17

18 Ion penetration in matter: Bragg curve and peak 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 Bragg peak decreases with energy in a logistic way. 18

19 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 ~ 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) 19

20 Transversal dose mapping and Bragg-curve reconstruction in protonirradiated lithium fluoride detectors by fluorescence microscopy Summary Lithium fluoride and colour centres as radiation detectors Proton therapy and the TOP-IMPLART project Proton irradiation of lithium fluoride Transversal dose mapping and Braggcurve reconstruction Conclusions 20

21 Proton-beam 2D dose mapping MeV protons, LiF film 1 mm 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. M. Piccinini et al., EPL 117 (2017)

22 Proton beams and LiF: 2D maps properties Noteworthy properties Because the PL intensity linearly depends on the dose up to a certain dose value (~ 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). 22

23 Bragg curve analysis air 7 MeV protons, LiF crystal Bragg peak A B C C 250 µm 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)

24 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.) } proton beam diagnostics 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. 24

25 Bragg curve fit: mean energy The mean energy acts on the depth of the Bragg peak 25

26 Bragg curve fit: energy spread (std. dev.) The energy spread acts on the depth, width and height of the Bragg peak 26

27 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 Model dose D(z) is evaluated from SRIM simulations 27

28 Bragg-curve fit: 7 MeV results FIT PARAMETERS ADVANCED PROTON BEAM DIAGNOSTICS mean energy energy std. dev. input/saturation dose ratio KNOWN PARAMETERS (from experimental beam data) proton fluence input-face dose C DERIVED PARAMETERS LiF SATURATION ( LINEARITY RANGE) saturation dose Bragg peak (= max dose) RESULTS FOR NOMINAL 7 MeV PROTONS (E. Nichelatti et al., EPL 120 (2017) 56003) 28

29 Transversal dose mapping and Bragg-curve reconstruction in protonirradiated lithium fluoride detectors by fluorescence microscopy Summary Lithium fluoride and colour centres as radiation detectors Proton therapy and the TOP-IMPLART project Proton irradiation of lithium fluoride Transversal dose mapping and Braggcurve reconstruction Conclusions 29

30 Conclusions 30

31 Enrico Nichelatti ENEA FSN-TECFIS-MNF Thank you for your attention!