AN ONLINE, RADIATION HARD PROTON ENERGY-RESOLVING SCINTILLATOR STACK FOR LASER-DRIVEN PROTON BUNCHES

AN ONLINE, RADIATION HARD PROTON ENERGY-RESOLVING SCINTILLATOR STACK FOR LASER-DRIVEN PROTON BUNCHES Abstract We report on a scintillator-based online detection system for the spectral characterization of polychromatic proton bunches. Using up to nine stacked layers of radiation hard polysiloxane scintillators, coupled to and readout edge-on by a large area pixelated CMOS detector, impinging polychromatic proton bunches were characterized. The energy spectra were reconstructed using calibration data and simulated using Monte-Carlo simulations. Despite the scintillator stack showed some problems like thickness inhomogeneities and unequal layer coupling, the prototype allows to obtain a first estimate of the energy spectrum of proton beams. INTRODUCTION Radiotherapy with protons, carbon ions or other particles could be superior to conventional X-ray based radiotherapy, since the dose deposition can be more accurately confined to the tumor region to better spare healthy tissue. Compact laser-ion (LION) accelerators will help to investigate and exploit particle radiotherapy further. LION acceleration exploits the generation of MeV/mm electric field gradients, set up through the interaction of a focused laser pulse with a target, to generate ion bunches. Fields of application are nuclear science, astrophysics and medicine(1, 6, 7). In medicine, LION bunches will complement studies in particle radiotherapy, as well as in ultra-fast radiobiology and particle radiography. The scintillator stack presented in this work is designed for diagnostics of polyenergetic proton bunches of up to 20MeV. Bunches can be generated using a 300TW laser system(6, 7). Such system employs Titanium-sapphire crystals as lasing media (Ti:Sa) to generate 800nm central wavelength light pulses with 4J pulse energy and 30fs pulse duration. These pulses are focused to 2.5mmFWHM on the target ( nm−mm thick plastic or metal foils) to achieve an intensity of 1020W/cm2. Since the LION acceleration mechanism is hence different from conventional radio-frequency based accelerators like cyclotrons, the proton beam characteristics and the demand on the beam diagnostics differ (Tables 1 and 2). Table 1. Comparison of proton beam characteristics for laser accelerator and medical cyclotron.   Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV    Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV  View Large Table 1. Comparison of proton beam characteristics for laser accelerator and medical cyclotron.   Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV    Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV  View Large Table 2. Listing of the detectors currently used in laser-driven ion acceleration(1).   Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive    Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive  View Large Table 2. Listing of the detectors currently used in laser-driven ion acceleration(1).   Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive    Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive  View Large Since the interaction of a laser pulse with the target generates an intense field of electrons, X-rays, protons and other ions inside the experimental vacuum chamber, the wish list of requirements for detectors to be used in LION experiments is challenging. Radiation hardness Vacuum compatibility Selectivity in particle type Stand the intense electro-magnetic pulse (EMP) Provide an estimate of the proton energy spectrum (not only cut-off) Provide online results Most of the currently employed detectors fail to provide multiple of these features (cp. Table 2). Upcoming detector systems fulfilling these requirements are time-of-flight detectors using graphite silicon, as well as scintillator-coupled semiconductors(1). MATERIALS AND METHODS We manufactured a device for beam energy diagnostics based on scintillators coupled to a pixelated CMOS sensor. This scintillator stack design consists of nine layers of a Teflon support ( ≈150mm), the radiation hard Polysiloxane scintillator ( ≈150mm) and a thin layer of aluminized Mylar foil ( ≈8mm) (cp. Figures 1 and 2). The Teflon serves as support structure in the manufacturing process, since the scintillator is molded warm and liquid onto the structure and the active CMOS detector to enable optical coupling to the CMOS. The direct interconnection of scintillator and the 2 mm SiO2 layer on top of the 2mm active Si layer of the CMOS makes optical glue superfluous and reduces potential light losses. The Mylar foil is used to avoid optical crosstalk between layers. Readout is performed using the RadEye CMOS detector ( 48mm×48mm pixels). Previous studies showed that this 2.5cm×5cm sensor, hosting 512×1024 pixels, is radiation hard and sensitive to optical photons, protons, electrons and X-rays(8, 9). To shield the stack from ambient light, laser light and the EMP from the LION acceleration process, the detector is placed in an aluminum housing with a 4mm entrance hole shielded by a 15mm aluminum foil. Figure 1. View largeDownload slide Schematic layout of the design of our radiation hard scintillator stack. The stack consists of nine layers of scintillator (gray), which are readout edge-on by the position sensitive CMOS detector (white). Each Polysiloxane scintillation layer has a thickness of 150mm and is supported by a 150mm Teflon layer (black). Arrows indicate the propagation direction of scintillation photons which enter the CMOS. Crosstalk between layers is suppressed by addition of an aluminized Mylar foil (not shown) between each layer of Teflon and scintillator. Scintillation photons are generated by the proton beam entering the nine layers from the left (beam not shown). Figure 1. View largeDownload slide Schematic layout of the design of our radiation hard scintillator stack. The stack consists of nine layers of scintillator (gray), which are readout edge-on by the position sensitive CMOS detector (white). Each Polysiloxane scintillation layer has a thickness of 150mm and is supported by a 150mm Teflon layer (black). Arrows indicate the propagation direction of scintillation photons which enter the CMOS. Crosstalk between layers is suppressed by addition of an aluminized Mylar foil (not shown) between each layer of Teflon and scintillator. Scintillation photons are generated by the proton beam entering the nine layers from the left (beam not shown). Figure 2. View largeDownload slide The scintillator stack consisting of an aluminum shielding box, entrance hole for the beam (entering from left), scintillation layers, RadEye detector and readout electronics. Figure 2. View largeDownload slide The scintillator stack consisting of an aluminum shielding box, entrance hole for the beam (entering from left), scintillation layers, RadEye detector and readout electronics. Polysiloxane was chosen as scintillating material, since it is liquid during manufacturing, non-toxic, radiation hard and has been shown to have a high light output(4, 5). A simple estimation of the proton beam high energy cut-off can be done by counting the number of layers showing scintillation signal (Figure 3). We performed calibration measurements by inserting 0–14 layers of glass (each 170mm) into a 22MeV proton beam from a conventional Tandem accelerator and recorded the scintillation distribution (signal S in arbitrary units AU) as a function of glass thickness ( S0g−S14g, cp. simulations in Figure 3). Two polyenergetic beams were generated by inserting two different passive aluminum plates with different drill-patterns (Figure 4), which modulated the beam in energy and have been manufactured according to dedicated MC simulations (Figures 5 and 7). The scintillation distributions of the two polyenergetic beams ( Smeas) were decomposed using the measured scintillation distributions for 0–14 glasses. Both signal sets (cp. simulated Figures 3 and 5) were normalized per proton using Faraday-cup measurements. The most distal layer scintillating (e.g. L9 for the simulation in Figure 5) is reconstructed first, using the signal value in the layer SL9,meas. Since scintillation in the most distal layer can be attributed to the highest number of moderators with L9 still scintillating (3 g, cp. Figure 3), the weight w3g=SL9,meas/SL9,3g can be calculated. The signal w3g⋅S3g is then subtracted from the initial Smeas and the new signal used to reconstruct the next layer ( SL8,meas). The total result is the spectrum E=∑i=1g14gwi⋅Si (Figure 7). Figure 3. View largeDownload slide Simulation of monoenergetic proton Bragg peaks in the stack using the FLUKA MC code (compare scheme in Figure 1)(3). The stack samples nine points of the full depth–dose distribution. Data shows the energy deposition in the scintillation layers for a modulation of 22MeV with 170mm glasses (1: 510mm, 2: 850mm, 3: 1360mm, 4: 1700mm) Figure 3. View largeDownload slide Simulation of monoenergetic proton Bragg peaks in the stack using the FLUKA MC code (compare scheme in Figure 1)(3). The stack samples nine points of the full depth–dose distribution. Data shows the energy deposition in the scintillation layers for a modulation of 22MeV with 170mm glasses (1: 510mm, 2: 850mm, 3: 1360mm, 4: 1700mm) Figure 4. View largeDownload slide MC simulation of proton energies entering the scintillator stack when modulating a monoenergetic 22MeV proton beam using 0–14 layers of 170mm glass. Figure 4. View largeDownload slide MC simulation of proton energies entering the scintillator stack when modulating a monoenergetic 22MeV proton beam using 0–14 layers of 170mm glass. Figure 5. View largeDownload slide MC simulation of the expected scintillation distributions after inserting the two passive filters. Filter 1 generates to a SOBP (left), Filter 2 an exponential-like proton energy spectrum as from a LION experiment (right). Figure 5. View largeDownload slide MC simulation of the expected scintillation distributions after inserting the two passive filters. Filter 1 generates to a SOBP (left), Filter 2 an exponential-like proton energy spectrum as from a LION experiment (right). The stack was placed in a vacuum chamber to avoid beam scattering and energy loss in air, simplify the MC simulations of the experiments and test the stack performance in vacuum (Figure 3). Absolute charge calibration ( S/proton) was performed using MC simulations and Faraday-Cup measurements of the impinging beam. The beam current from the accelerator was ranging between 0.8 and 1.3 nA in order to obtain a sufficient scintillation signal level above background. RESULTS AND DISCUSSION The results of the calibration measurements using the 170mm glass slices show some deviations from the idealized MC simulations, which hinder a purely MC-based reconstruction. Most challenging for the reconstruction are several problems: Non-homogeneous response of the layers and laterally along the layers, uncertainties in the thickness of the scintillation and Teflon layers, bright halo-areas in between scintillation layers and non-ideal coupling to the CMOS (Figure 6). Layers six to nine (L6–L9) showed weak response to energy depositions to impinging protons, which is in disagreement with simulation. We attribute the problems to the manual process of the liquid scintillator deposition onto the support structure and a local free flow of the scintillator on the CMOS around the layer positions (cp. Figure 6). Figure 6. View largeDownload slide Comparison of expected 2D distribution of scintillation light on the RadEye based on MC simulation (left) and corresponding measurement result (right). Beam enters from the top. The beam energy was degraded using 3×170mm glass (compare lineout in Figure 3.1). Figure 6. View largeDownload slide Comparison of expected 2D distribution of scintillation light on the RadEye based on MC simulation (left) and corresponding measurement result (right). Beam enters from the top. The beam energy was degraded using 3×170mm glass (compare lineout in Figure 3.1). The two expected spectra (From dedicated MC simulations, studying the sensitivity on different beam sizes and hit position of the filter (cp. Figure 7).) are: A spectrum generating a spread out Bragg peak (SOBP) and an exponential-like spectrum (Figure 6) as encountered in LION experiments (Figure 5). The energy resolution using the 14 glass measurements is ≈1.5MeV for E>10MeV and ≈3MeV for E<10MeV (cp. Figures 3, 4 and 7). CONCLUSION Our radiation hard scintillator stack prototype allowed us to obtain a first estimate of the proton beam energy, at least by counting the maximum number of scintillating layers. The two passive aluminum filters when inserted into the beam path show, after reconstruction, at least a correlation to the spectrum as obtained by forward MC simulation (Figure 7). Shortcomings were the non-uniform layer coupling, a low scintillation yield and the unknown and non-homogeneous layer thicknesses. Figure 7. View largeDownload slide MC simulated proton spectra from two passive filters (top: SOBP-filter, bottom: LION-filter) and spectra reconstructed (blue dots) using calibration data (Figure 4). Inlays show geometrical shape. The three different hit positions of the 22MeV beam give an estimate on the spectra possibly encountered in the experiment (green, red, turquoise), since the filters are hit-position sensitive. The reconstruction is able to give an estimate on the minimum and maximum energy of the spectrum and the spectral shape (Figure 5). Figure 7. View largeDownload slide MC simulated proton spectra from two passive filters (top: SOBP-filter, bottom: LION-filter) and spectra reconstructed (blue dots) using calibration data (Figure 4). Inlays show geometrical shape. The three different hit positions of the 22MeV beam give an estimate on the spectra possibly encountered in the experiment (green, red, turquoise), since the filters are hit-position sensitive. The reconstruction is able to give an estimate on the minimum and maximum energy of the spectrum and the spectral shape (Figure 5). OUTLOOK Future stack designs for the usage at higher proton energies are under investigation. A 3D-printed support structure will enable a two-sided readout of the scintillator layers and more homogeneous coupling and layer thicknesses. The two-sided readout will allow to judge not only the bunch spectrum, but also the pointing and divergence of the bunch. MC simulations of the improved configuration of 50 scintillation and 50 energy degradation layers will be used to propose a stack layout for the spectral diagnostics of laser-driven proton bunches of up to 100MeV. Such parameters are targeted at our 3PW laser system at the ‘Center for Advanced Laser Applications (CALA)’ in Munich, starting in 2018. ACKNOWLEDGEMENTS German research foundation (DFG) Cluster of excellence ‘Munich Center for Advanced Photonics’, International Max Planck Research School IMPRS-APS, Erasmus program of the EU. REFERENCES 1 Daido, H., Nishiuchi, M. and Pirozhkov, A. S. Review of laser-driven ion sources and their applications. Rep. Prog. Phys.  75, 056401– 056472 ( 2012). Google Scholar CrossRef Search ADS PubMed  2 Enghardt, W. ( 2016) State of the art in particle therapy. Deutsche Gesellschaft für Medizinische Physik Jahrestagung—Session ‘Partikeltherapie I—Novel developments in clinical particle therapy’. 3 Ferrari, A., Sala, P. R., Fasso, A. and Ranft, J. ( 2005) FLUKA: a multi-particle transport code. CERN-2005–10 (2005), INFN/TC_05/11, SLAC-R-773.1 4 Dalla Palma, M., Quaranta, A., Marchi, T., Collazuol, G., Carturan, S., Cinausero, M., Degerlier, M. and Gramegna, F. Red emitting phenyl-polysiloxane based scintillators for neutron detection. IEEE Trans. Nucl. Sci.  61-4, 2052– 2058 ( 2014). Google Scholar CrossRef Search ADS   5 Dalla Palma, M., Carturan, S. M., Degerlier, M., Marchi, T., Cinausero, M., Gramegna, F. and Quaranta, A. Non-toxic liquid scintillators with high light output based on phenyl-substituted siloxanes. Opt. Mater.  42, 111– 117 ( 2015). Google Scholar CrossRef Search ADS   6 Linz, U. and Alonso, J. Laser-driven ion accelerators for tumor therapy revisited. Phys. Rev. Accel. Beams  19, 124802– 124810 ( 2016). Google Scholar CrossRef Search ADS   7 Schreiber, J., Bolton, P. R. and Parodi, K. Hands-on laser-driven ion acceleration: a primer for laser-driven source development and potential applications. Rev. Sci. Instrum.  87, 071101– 071110 ( 2016). Google Scholar CrossRef Search ADS PubMed  8 Reinhard, S., Draxinger, W., Schreiber, J. and Assmann, W. A pixel detector system for laser-accelerated ion detection. JINST  8, P03008 ( 2013). Google Scholar CrossRef Search ADS   9 Reinhard, S., Granjab, C., Krejcib, F. and Assmann, W. Test of pixel detectors for laser-driven accelerated particle beams. JINST  6, C12030 ( 2011). Google Scholar CrossRef Search ADS   © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

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Abstract

Abstract We report on a scintillator-based online detection system for the spectral characterization of polychromatic proton bunches. Using up to nine stacked layers of radiation hard polysiloxane scintillators, coupled to and readout edge-on by a large area pixelated CMOS detector, impinging polychromatic proton bunches were characterized. The energy spectra were reconstructed using calibration data and simulated using Monte-Carlo simulations. Despite the scintillator stack showed some problems like thickness inhomogeneities and unequal layer coupling, the prototype allows to obtain a first estimate of the energy spectrum of proton beams. INTRODUCTION Radiotherapy with protons, carbon ions or other particles could be superior to conventional X-ray based radiotherapy, since the dose deposition can be more accurately confined to the tumor region to better spare healthy tissue. Compact laser-ion (LION) accelerators will help to investigate and exploit particle radiotherapy further. LION acceleration exploits the generation of MeV/mm electric field gradients, set up through the interaction of a focused laser pulse with a target, to generate ion bunches. Fields of application are nuclear science, astrophysics and medicine(1, 6, 7). In medicine, LION bunches will complement studies in particle radiotherapy, as well as in ultra-fast radiobiology and particle radiography. The scintillator stack presented in this work is designed for diagnostics of polyenergetic proton bunches of up to 20MeV. Bunches can be generated using a 300TW laser system(6, 7). Such system employs Titanium-sapphire crystals as lasing media (Ti:Sa) to generate 800nm central wavelength light pulses with 4J pulse energy and 30fs pulse duration. These pulses are focused to 2.5mmFWHM on the target ( nm−mm thick plastic or metal foils) to achieve an intensity of 1020W/cm2. Since the LION acceleration mechanism is hence different from conventional radio-frequency based accelerators like cyclotrons, the proton beam characteristics and the demand on the beam diagnostics differ (Tables 1 and 2). Table 1. Comparison of proton beam characteristics for laser accelerator and medical cyclotron.   Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV    Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV  View Large Table 1. Comparison of proton beam characteristics for laser accelerator and medical cyclotron.   Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV    Laser(1, 7)  Cyclotron(2)  Bandwidth  ΔE/E≈100%  ΔE/E≈0.4%  Spectrum  Exponential slope  Monoenergetic  Bunch length  ns  cw/ms  Flux  109−1010p/ns  109−1010p/s  Energies  <100MeV  75−250MeV  View Large Table 2. Listing of the detectors currently used in laser-driven ion acceleration(1).   Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive    Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive  View Large Table 2. Listing of the detectors currently used in laser-driven ion acceleration(1).   Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive    Online  Photon sensitive  Energyresol.  Drawbacks  CR-39  —  —  Stacked  Chemistry and microscope  Image plates  —  x  —  Long scanning  Dosimetric film  —  x  Stacked  Long scanning  Magnet and CMOS  x  —  x  Measure B-field, setup sensitive  View Large Since the interaction of a laser pulse with the target generates an intense field of electrons, X-rays, protons and other ions inside the experimental vacuum chamber, the wish list of requirements for detectors to be used in LION experiments is challenging. Radiation hardness Vacuum compatibility Selectivity in particle type Stand the intense electro-magnetic pulse (EMP) Provide an estimate of the proton energy spectrum (not only cut-off) Provide online results Most of the currently employed detectors fail to provide multiple of these features (cp. Table 2). Upcoming detector systems fulfilling these requirements are time-of-flight detectors using graphite silicon, as well as scintillator-coupled semiconductors(1). MATERIALS AND METHODS We manufactured a device for beam energy diagnostics based on scintillators coupled to a pixelated CMOS sensor. This scintillator stack design consists of nine layers of a Teflon support ( ≈150mm), the radiation hard Polysiloxane scintillator ( ≈150mm) and a thin layer of aluminized Mylar foil ( ≈8mm) (cp. Figures 1 and 2). The Teflon serves as support structure in the manufacturing process, since the scintillator is molded warm and liquid onto the structure and the active CMOS detector to enable optical coupling to the CMOS. The direct interconnection of scintillator and the 2 mm SiO2 layer on top of the 2mm active Si layer of the CMOS makes optical glue superfluous and reduces potential light losses. The Mylar foil is used to avoid optical crosstalk between layers. Readout is performed using the RadEye CMOS detector ( 48mm×48mm pixels). Previous studies showed that this 2.5cm×5cm sensor, hosting 512×1024 pixels, is radiation hard and sensitive to optical photons, protons, electrons and X-rays(8, 9). To shield the stack from ambient light, laser light and the EMP from the LION acceleration process, the detector is placed in an aluminum housing with a 4mm entrance hole shielded by a 15mm aluminum foil. Figure 1. View largeDownload slide Schematic layout of the design of our radiation hard scintillator stack. The stack consists of nine layers of scintillator (gray), which are readout edge-on by the position sensitive CMOS detector (white). Each Polysiloxane scintillation layer has a thickness of 150mm and is supported by a 150mm Teflon layer (black). Arrows indicate the propagation direction of scintillation photons which enter the CMOS. Crosstalk between layers is suppressed by addition of an aluminized Mylar foil (not shown) between each layer of Teflon and scintillator. Scintillation photons are generated by the proton beam entering the nine layers from the left (beam not shown). Figure 1. View largeDownload slide Schematic layout of the design of our radiation hard scintillator stack. The stack consists of nine layers of scintillator (gray), which are readout edge-on by the position sensitive CMOS detector (white). Each Polysiloxane scintillation layer has a thickness of 150mm and is supported by a 150mm Teflon layer (black). Arrows indicate the propagation direction of scintillation photons which enter the CMOS. Crosstalk between layers is suppressed by addition of an aluminized Mylar foil (not shown) between each layer of Teflon and scintillator. Scintillation photons are generated by the proton beam entering the nine layers from the left (beam not shown). Figure 2. View largeDownload slide The scintillator stack consisting of an aluminum shielding box, entrance hole for the beam (entering from left), scintillation layers, RadEye detector and readout electronics. Figure 2. View largeDownload slide The scintillator stack consisting of an aluminum shielding box, entrance hole for the beam (entering from left), scintillation layers, RadEye detector and readout electronics. Polysiloxane was chosen as scintillating material, since it is liquid during manufacturing, non-toxic, radiation hard and has been shown to have a high light output(4, 5). A simple estimation of the proton beam high energy cut-off can be done by counting the number of layers showing scintillation signal (Figure 3). We performed calibration measurements by inserting 0–14 layers of glass (each 170mm) into a 22MeV proton beam from a conventional Tandem accelerator and recorded the scintillation distribution (signal S in arbitrary units AU) as a function of glass thickness ( S0g−S14g, cp. simulations in Figure 3). Two polyenergetic beams were generated by inserting two different passive aluminum plates with different drill-patterns (Figure 4), which modulated the beam in energy and have been manufactured according to dedicated MC simulations (Figures 5 and 7). The scintillation distributions of the two polyenergetic beams ( Smeas) were decomposed using the measured scintillation distributions for 0–14 glasses. Both signal sets (cp. simulated Figures 3 and 5) were normalized per proton using Faraday-cup measurements. The most distal layer scintillating (e.g. L9 for the simulation in Figure 5) is reconstructed first, using the signal value in the layer SL9,meas. Since scintillation in the most distal layer can be attributed to the highest number of moderators with L9 still scintillating (3 g, cp. Figure 3), the weight w3g=SL9,meas/SL9,3g can be calculated. The signal w3g⋅S3g is then subtracted from the initial Smeas and the new signal used to reconstruct the next layer ( SL8,meas). The total result is the spectrum E=∑i=1g14gwi⋅Si (Figure 7). Figure 3. View largeDownload slide Simulation of monoenergetic proton Bragg peaks in the stack using the FLUKA MC code (compare scheme in Figure 1)(3). The stack samples nine points of the full depth–dose distribution. Data shows the energy deposition in the scintillation layers for a modulation of 22MeV with 170mm glasses (1: 510mm, 2: 850mm, 3: 1360mm, 4: 1700mm) Figure 3. View largeDownload slide Simulation of monoenergetic proton Bragg peaks in the stack using the FLUKA MC code (compare scheme in Figure 1)(3). The stack samples nine points of the full depth–dose distribution. Data shows the energy deposition in the scintillation layers for a modulation of 22MeV with 170mm glasses (1: 510mm, 2: 850mm, 3: 1360mm, 4: 1700mm) Figure 4. View largeDownload slide MC simulation of proton energies entering the scintillator stack when modulating a monoenergetic 22MeV proton beam using 0–14 layers of 170mm glass. Figure 4. View largeDownload slide MC simulation of proton energies entering the scintillator stack when modulating a monoenergetic 22MeV proton beam using 0–14 layers of 170mm glass. Figure 5. View largeDownload slide MC simulation of the expected scintillation distributions after inserting the two passive filters. Filter 1 generates to a SOBP (left), Filter 2 an exponential-like proton energy spectrum as from a LION experiment (right). Figure 5. View largeDownload slide MC simulation of the expected scintillation distributions after inserting the two passive filters. Filter 1 generates to a SOBP (left), Filter 2 an exponential-like proton energy spectrum as from a LION experiment (right). The stack was placed in a vacuum chamber to avoid beam scattering and energy loss in air, simplify the MC simulations of the experiments and test the stack performance in vacuum (Figure 3). Absolute charge calibration ( S/proton) was performed using MC simulations and Faraday-Cup measurements of the impinging beam. The beam current from the accelerator was ranging between 0.8 and 1.3 nA in order to obtain a sufficient scintillation signal level above background. RESULTS AND DISCUSSION The results of the calibration measurements using the 170mm glass slices show some deviations from the idealized MC simulations, which hinder a purely MC-based reconstruction. Most challenging for the reconstruction are several problems: Non-homogeneous response of the layers and laterally along the layers, uncertainties in the thickness of the scintillation and Teflon layers, bright halo-areas in between scintillation layers and non-ideal coupling to the CMOS (Figure 6). Layers six to nine (L6–L9) showed weak response to energy depositions to impinging protons, which is in disagreement with simulation. We attribute the problems to the manual process of the liquid scintillator deposition onto the support structure and a local free flow of the scintillator on the CMOS around the layer positions (cp. Figure 6). Figure 6. View largeDownload slide Comparison of expected 2D distribution of scintillation light on the RadEye based on MC simulation (left) and corresponding measurement result (right). Beam enters from the top. The beam energy was degraded using 3×170mm glass (compare lineout in Figure 3.1). Figure 6. View largeDownload slide Comparison of expected 2D distribution of scintillation light on the RadEye based on MC simulation (left) and corresponding measurement result (right). Beam enters from the top. The beam energy was degraded using 3×170mm glass (compare lineout in Figure 3.1). The two expected spectra (From dedicated MC simulations, studying the sensitivity on different beam sizes and hit position of the filter (cp. Figure 7).) are: A spectrum generating a spread out Bragg peak (SOBP) and an exponential-like spectrum (Figure 6) as encountered in LION experiments (Figure 5). The energy resolution using the 14 glass measurements is ≈1.5MeV for E>10MeV and ≈3MeV for E<10MeV (cp. Figures 3, 4 and 7). CONCLUSION Our radiation hard scintillator stack prototype allowed us to obtain a first estimate of the proton beam energy, at least by counting the maximum number of scintillating layers. The two passive aluminum filters when inserted into the beam path show, after reconstruction, at least a correlation to the spectrum as obtained by forward MC simulation (Figure 7). Shortcomings were the non-uniform layer coupling, a low scintillation yield and the unknown and non-homogeneous layer thicknesses. Figure 7. View largeDownload slide MC simulated proton spectra from two passive filters (top: SOBP-filter, bottom: LION-filter) and spectra reconstructed (blue dots) using calibration data (Figure 4). Inlays show geometrical shape. The three different hit positions of the 22MeV beam give an estimate on the spectra possibly encountered in the experiment (green, red, turquoise), since the filters are hit-position sensitive. The reconstruction is able to give an estimate on the minimum and maximum energy of the spectrum and the spectral shape (Figure 5). Figure 7. View largeDownload slide MC simulated proton spectra from two passive filters (top: SOBP-filter, bottom: LION-filter) and spectra reconstructed (blue dots) using calibration data (Figure 4). Inlays show geometrical shape. The three different hit positions of the 22MeV beam give an estimate on the spectra possibly encountered in the experiment (green, red, turquoise), since the filters are hit-position sensitive. The reconstruction is able to give an estimate on the minimum and maximum energy of the spectrum and the spectral shape (Figure 5). OUTLOOK Future stack designs for the usage at higher proton energies are under investigation. A 3D-printed support structure will enable a two-sided readout of the scintillator layers and more homogeneous coupling and layer thicknesses. The two-sided readout will allow to judge not only the bunch spectrum, but also the pointing and divergence of the bunch. MC simulations of the improved configuration of 50 scintillation and 50 energy degradation layers will be used to propose a stack layout for the spectral diagnostics of laser-driven proton bunches of up to 100MeV. Such parameters are targeted at our 3PW laser system at the ‘Center for Advanced Laser Applications (CALA)’ in Munich, starting in 2018. ACKNOWLEDGEMENTS German research foundation (DFG) Cluster of excellence ‘Munich Center for Advanced Photonics’, International Max Planck Research School IMPRS-APS, Erasmus program of the EU. REFERENCES 1 Daido, H., Nishiuchi, M. and Pirozhkov, A. S. Review of laser-driven ion sources and their applications. Rep. Prog. Phys.  75, 056401– 056472 ( 2012). Google Scholar CrossRef Search ADS PubMed  2 Enghardt, W. ( 2016) State of the art in particle therapy. Deutsche Gesellschaft für Medizinische Physik Jahrestagung—Session ‘Partikeltherapie I—Novel developments in clinical particle therapy’. 3 Ferrari, A., Sala, P. R., Fasso, A. and Ranft, J. ( 2005) FLUKA: a multi-particle transport code. CERN-2005–10 (2005), INFN/TC_05/11, SLAC-R-773.1 4 Dalla Palma, M., Quaranta, A., Marchi, T., Collazuol, G., Carturan, S., Cinausero, M., Degerlier, M. and Gramegna, F. Red emitting phenyl-polysiloxane based scintillators for neutron detection. IEEE Trans. Nucl. Sci.  61-4, 2052– 2058 ( 2014). Google Scholar CrossRef Search ADS   5 Dalla Palma, M., Carturan, S. M., Degerlier, M., Marchi, T., Cinausero, M., Gramegna, F. and Quaranta, A. Non-toxic liquid scintillators with high light output based on phenyl-substituted siloxanes. Opt. Mater.  42, 111– 117 ( 2015). Google Scholar CrossRef Search ADS   6 Linz, U. and Alonso, J. Laser-driven ion accelerators for tumor therapy revisited. Phys. Rev. Accel. Beams  19, 124802– 124810 ( 2016). Google Scholar CrossRef Search ADS   7 Schreiber, J., Bolton, P. R. and Parodi, K. Hands-on laser-driven ion acceleration: a primer for laser-driven source development and potential applications. Rev. Sci. Instrum.  87, 071101– 071110 ( 2016). Google Scholar CrossRef Search ADS PubMed  8 Reinhard, S., Draxinger, W., Schreiber, J. and Assmann, W. A pixel detector system for laser-accelerated ion detection. JINST  8, P03008 ( 2013). Google Scholar CrossRef Search ADS   9 Reinhard, S., Granjab, C., Krejcib, F. and Assmann, W. Test of pixel detectors for laser-driven accelerated particle beams. JINST  6, C12030 ( 2011). Google Scholar CrossRef Search ADS   © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com

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Radiation Protection DosimetryOxford University Press

Published: Feb 3, 2018

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