Commissioning of low particle flux for proton beams at MedAustron
Felix Ulrich-Pur, Laurids Adler, Thomas Bergauer, Alexander Burker, Andrea De Franco, Greta Guidoboni, Albert Hirtl, Christian Irmler, Stefanie Kaser, Sebastian Nowak, Florian Pitters, Mauro Pivi, Dale Prokopovich, Claus Schmitzer, Alexander Wastl
CCommissioning of low particle flux for proton beams at MedAustron
Felix Ulrich-Pur a, ∗ , Laurids Adler c , Thomas Bergauer a , Alexander Burker b , Andrea De Franco c , Greta Guidoboni c , Albert Hirtl b ,Christian Irmler a , Stefanie Kaser a , Sebastian Nowak c , Florian Pitters a , Mauro Pivi c , Dale Prokopovich c , Claus Schmitzer c ,Alexander Wastl c a Austrian Academy of Sciences, Institute of High Energy Physics (HEPHY), Nikolsdorfer Gasse 18, 1050 Wien, Austria b TU Wien, Atominstitut, Stadionallee 2, 1020 Wien, Austria c MedAustron, Marie-Curie-Straße 5, 2700, Wiener Neustadt
Abstract
MedAustron is a synchrotron based particle therapy centre located in Wiener Neustadt, Austria. It features three irradiation roomsfor particle therapy, where proton beams with energies up to 252 . . / u are available forcancer treatment. In addition to the treatment rooms, MedAustron features a unique beam line exclusively for non-clinical research(NCR). This research beam line is also commissioned for proton energies up to 800 MeV, while available carbon ion energiescorrespond to the ones available in the treatment rooms.All irradiation rooms o ff er particle rates in the order of 10 particles / s for protons and 10 particles / s for carbon ions. Such highfluxes are needed to e ffi ciently irradiate patients in a short amount of time. However, for research purposes also lower particle fluxesare required. Therefore lower proton flux settings for the NCR beamline were commissioned. Three particle flux settings rangingfrom ≈ . ≈ . . ≈ ≈ . Keywords:
MedAustron, low particle flux
1. Introduction
MedAustron is a particle therapy and research centre lo-cated in Wiener Neustadt, Austria. It features four irradiationrooms (IR1-IR4) with one exclusive beamline (IR1) dedicatedto research [1]. Protons up to 800 MeV and carbon ions of upto 402 . / u can be delivered to this beamline. Experimentsin non-clinical research often require to measure the interactionof single particles [2], which leads to completely di ff erent beamand detector requirements compared to those for particle ther-apy. Few detector systems are able to handle the high particlerates (in the order of 10 particles / s) used for particle therapy.A rising demand for lower particle flux in the research beam-line at MedAustron resulted in the commissioning of low fluxsettings for proton beams.
2. Materials and methods
As primary ions H + or C + are generated in two Elec-tron Cyclotron Resonance (ECR) ion sources at 8 keV / u [3]located in the Low Energy Beam Transfer line (LEBT). Theions are then transported to the Radio Frequency Quadrupole(RFQ) followed by an Interdigital H-mode LINear ACcelerator ∗ Corresponding author
Email address: [email protected] (Felix Ulrich-Pur) (LINAC) which accelerates the particles up to 7 MeV / u. Af-ter the LINAC, a carbon stripping foil is used to strip o ff re-maining electrons and convert the ions to H + or C + , respec-tively. Those ions are then transported to the synchrotron bythe Medium Energy Beam Transfer line (MEBT) and injectedvia a multi-turn injection scheme. After injection, the beam isbunched and accelerated by a radio frequency cavity. Inside thesynchrotron ring (77 m circumference) protons are acceleratedup to 800 MeV and carbon ions up to 402 . / u. Via a thirdorder slow resonant extraction [4] the accelerated beam is thenextracted towards one of the four irradiation rooms through theHigh Energy Beam Transfer line (HEBT). The flexible design of the MedAustron accelerator complexallows to expand the possible range of particle rates. Di ff er-ent methods were combined in order to reduce the particle flux(rate / area). These are:1. Reducing the number of particles injected into the syn-chrotron using the Electrostatic Fast Deflector (EFE) orthe degrader.2. Extending the extraction time into the HEBT to reducethe number of particles per second by changing the beta-tron core voltage.3. Scraping the beam into the chopper dump or increasingthe transverse beam spotsize in the HEBT so it exceedsthe size of the vacuum tube. The increased size of the Preprint submitted to Nuclear Instruments & Methods in Physics Research, Section A February 15, 2021 a r X i v : . [ phy s i c s . acc - ph ] F e b eam leads to a loss of particles in the HEBT and to alarger spotsize at the iso-centre, resulting in a reducednumber of particles per unit area.It is important to notice that not all low flux settings mentionedabove were applied for proton beams in the medical energyrange (between 62.4 and 252.7 MeV) and at 800 MeV. The in-creased transverse beamsize method was not used in the case of800 MeV to avoid environmental irradiation at that energy. Inthe following sections, a detailed description of the acceleratorelements and the method used for flux reduction is given. The EFE consists of two electrodes installed in the LEBTline. If a high voltage is applied (5.5 kV [5]) the beam is de-flected onto a cylindrical faraday cup, otherwise the beam canpass through towards the LINAC. The main purpose of the EFEis actually to adjust the pulse length: a typical injection schemeforesees 30-50 µ s pulses for the phase space painting in the syn-chrotron and it can be triggered with a 1 µ s resolution, thereforeproviding a well controllable measure to reduce the beam inten-sity already arriving in the main ring. At the end of the LINAC, a pepper pot like device (de-grader) can be used to adapt the number of particles injectedinto the ring [6]. Transmission probabilities of 10, 20 or 50 %in relation to the nominal intensities can be set.
The betatron core is a key element of the slow extractionmechanism used at MedAustron [7]. Prior to the extraction,the particles are accelerated in the synchrotron to a momentumslightly under the third-order resonance condition of the beta-tron oscillation. During acceleration, the lattice is adapted to setthe tune close to a third order resonance while lattice sextupolesadjust the chromaticity to high negative values. A resonancesextupole in a dispersion free sector increases resonance e ff ectswhile a betatron core transformer slowly drives the beam viagentle energy increase into the resonance condition. When theparticle oscillates in resonance, the amplitude of the oscillationincreases continuously. When the amplitude of this transversemotion reaches a certain limit, the particle enters an electricseptum, which is placed at the margin of the vacuum tube. Thefield of the septum deflects the incoming beam towards a se-ries of magnetic septa, to finally extract it to the HEBT [7]. Byadjusting the betatron core voltage, the extraction time can beextended, thus reducing the number of extracted particles / s. The chopper element is located in the HEBT line and con-sists of four dipoles powered in series. Only when it is turnedon, the dipoles guide the beam around a beam dump. This iswhy the chopper has double functionality: during normal oper-ation it is used to cut the head of the extracted beam but it servesas a safety element in case of problems while operating the ac-celerator [3]. By adapting the field strength of the dipoles it is possible to control the transmission through the dump: the num-ber of particles reaching the irradiation room can be reduced byusing the chopper dump to scrape particles o ff the beam. The last 7 quadrupoles to IR1 - the irradiation room for non-clinical research purposes - were used to blow up the transverse(to the direction of motion) size of the beam and therefore re-duce the particle flux at the monitor location. The modificationof quadrupole settings implies a so-called optics change of thetransfer line and the method will be referenced as optics adjust-ment in the following. Figure 1 shows a comparison of the β x and β y functions along the transfer line to IR1, in case of nomi-nal quadrupole settings (Optics5) or new settings for beam blowup, Optics3 and Optics4 respectively. The β x and β y functionsrepresent the transverse beam envelopes which are related tothe beam size in the transverse planes. For the same beam, alarger the beta function implies a larger beam size. x [ m ] y [ m ] Optics5Optics4Optics3
Figure 1: On the upper side of the figure, details of the transfer line are shown.Quadrupole magnets are highlighted in red and dipole magnets in grey andgreen. The β functions along the transfer line to IR1 are displayed respectivelyon the central (horizontal, x -plane) and bottom plots (vertical, y -plane) on alogarithmic scale. The nominal optics is shown in black, being very similar toOptics4 displayed in red. Optics3 is in blue and much larger in both planescompared to the other settings. As mentioned in the previous section, di ff erent methods forimplementing low flux were applied depending on the beamenergy. Table 1 summarizes the combination of accelerator el-ements in order to achieve di ff erent particle rates, called "Set.MED" in case of medical energy range or "Set. 800" for protonsat 800 MeV. In the following section, the experimental set-ups for mea-suring the rate, spotsizes and beam energies are described indetail.
Since the current beam diagnostics of MedAustron werenot designed for low flux beams, a dedicated rate monitor (Ra-Mon) to achieve single particle counting at low particle rates( <
50 MHz) was built for the rate measurements.2 et. MED EFE Deg. Betatron Optics
I 1 µ s 10 % Nom. Optics4II 1 µ s 10 % 10 % Nom. Optics5III 1 µ s 10 % 10 % Nom. Optics3 Set. 800 EFE Deg. Betatron Chop.
V 10 µ s 20 % Nom. 60 . µ s 20 % 12 % Nom. 60 . µ s 20 % Nom. 60 . Set. NOM EFE Deg. Betatron Optics nominal 30 µ s 10 % Nom. Optics5 Table 1: The low flux settings are presented in this table. For proton beams inthe medical energy range, the settings include the combination of EFE, degraderand / or betatron core and optics adjustments. In case of protons at 800 MeV, thechopper is used instead of optics adjustment. For comparison, the used nominalsetting is also shown. For this purpose, two EJ228 plastic scintillators from Eljen [8]were chosen as particle counters, since they have a rise and falltime of a few ns. Each of the scintillators has a total volume of50 × × . As a light guide, PMMA fish tail lightguides(50 × ) were connected to the photocathode ( ∅ = Figure 2: Schematic overview of the RaMon setup. The RaMon consists of twoplastic scintillators connected to the AIDA2020 TLU. An online beam monitorfor the TLU was developed in EUDAQ2.
Depending on the particle flux, two di ff erent detectors formeasuring the spot size at the IR1 isocentre were used.For particle beams with particle rates above O (100 MHz), theLynx ® detector (IBA-Dosimetry, Schwarzenbruck) was used(nominal flux setting). The Lynx ® detector is a gadolinium-based plastic scintillating screen (0 . ×
30 cm and a spatial resolution of 0 . ≤
180 s). The experi-mental setup for the spot size measurement is shown in Figure3.
Figure 3: Spot size measurement with the Lynx ® detector for low flux protonsat MedAustron. The detector was placed at the isocentre in IR1 using a laserpositioning system. For particle rates below O (100 kHz), the light generated in theLynx ® detector cannot be detected anymore. Therefore a dou-ble sided Silicon strip detector (DSSD) which has already beenused for single particle tracking at MedAustron [2] was used forlower particle rates (all low flux settings). The n-substrate basedDSSD is 300 µ m thick and has an active area of 2 . × .
12 cm .Each side features 512 orthogonal AC coupled strips with apitch of 100 µ m on the n-side ( x -coordinate) and 50 µ m on thep-side ( y -coordinate). The sensor itself is readout via 4 APV25chips [13] on each side (128 strips / APV, Figure 4a). A cus-tom readout system [2], which was originally developed for theBelle II Silicon Vertex Detector [14], was used. The readoutwas triggered using the RaMon plastic scintillators, which wereplaced behind the DSSD.A schematic overview of the experimental setup is shownin Figure 4b.
In addition to the spot size and rate, the beam energy forparticle rates above O (100 kHz) was measured using the PTWPEAKFINDER ™ (PTW, Freiburg, Germany). The PTW PEAK-FINDER ™ (Figure 5) is a height adjustable water column witha clear diameter of 114 mm and two ionization chambers tomeasure the depth dose profile of a particle beam in water.The PTW TM34082 is used as reference chamber and PTWTM34080 as field chamber. The absorbed dose in the fieldchamber is measured relatively to the absorbed dose in the ref-erence chamber at di ff erent positions inside the water column(340 mm movable range). From the measured depth-dose pro-files, the range in water was obtained for each energy and com-pared to the ranges from the NIST PSTAR database [15].3 a) P-side ( y -coordinate) of the DSSD.The strips are connected to 4 APV25chips mounted on hybrid boards. (b) Schematic overview of the spotsize mea-surement. The RaMon plastic scintillatorsare used to trigger single particle events. Figure 4: Spotsize measurement for low flux protons using the DSSD.Figure 5: The PTW PEAKFINDER ™ was positioned at the isocentre of IR1at MedAustron to measure the depth-dose profiles for di ff erent proton energiesand flux settings. Due to a low signal-to-noise ratio in the ionization chambers be-low particle rates of O (100 kHz), no depth-dose profiles couldbe obtained with the PTW PEAKFINDER ™ . Instead, a rangetelescope which was formerly developed by the TERA founda-tion [16] was used to measure the particle range in water. Fastsilicon photomultipliers (Hamamatsu MPPC S10362-11-050C[17]) coupled to 38 plastic scintillator slices with a water equiv-alent thickness of 3 . ×
30 cm eachallow range measurements of single particles. The readout of asingle particle event is triggered by the coincident signal of theRaMon plastic scintillators placed in front of the range tele-scope. The measured range in the telescope is then converted toresidual range in water. As described in [2] and [18], the rangetelescope su ff ered from severe voltage instabilities, thereforethe mainboard and the readout software were completely re-designed and replaced prior to the measurements.
3. Results
As described in Section 2.3, three di ff erent flux settingscould be commissioned for beam energies below 252 . ff erent settings for 800 MeV. For details of the set-tings the reader is referred to Table 1. Figure 6: Range measurement for low flux protons at MedAustron using theTERA range telescope with two plastic scintillators in front as triggers.
The particle rates for each setting were measured with theRaMon system for seven di ff erent beam energies below 252 . ff erent settings for 800 MeV. The combined results areshown in Figure 7.
50 100 150 200 25010 r a t e [ H z ]
790 800 810energy [MeV] .. setting Isetting IIsetting IIIsetting VIIsetting Vsetting VI Figure 7: Resulting low flux particle rates measured with the RaMon system.3 di ff erent flux settings per energy could be commissioned. The settings for800 MeV protons di ff ered from the medical settings. The mean rate for the lowest medical flux setting III rangesfrom 2 . − .
35 kHz. Depending on the particle energy, amean rate for the medium flux setting II between 232 −
435 kHzwas obtained. For setting I, mean particle rates between 3 . − .
21 MHz were measured.The measured rates for the 800 MeV settings showed similar re-sults compared to the medical settings. A mean rate of 198 kHzfor setting V, 1 .
95 kHz for setting VI and 1 .
25 MHz for settingVII was obtained. However, setting VI showed larger rate fluc-tuations per spill compared to the other 800 MeV low flux set-tings. Figure 8 depicts the spill structure for setting VI in com-parison to setting VII.
The Lynx ® detector was used to measure the spot sizes forthe nominal settings in a prior measurement [19]. A 2D Gaus-sian was fitted to the obtained 2D intensity profile distribution.4 time [s] p a r t i c l e r a t e [ H z ] (a) 800 MeV, setting VI
200 300 400 time [s] p a r t i c l e r a t e [ H z ] (b) 800 MeV, setting VII Figure 8: Comparison of the spill structure of low flux protons for 800 MeVprotons. Setting VI shows larger fluctuations in measured particles per spillcompared to setting VII.
The resulting full widths at half maximum (FWHM) were com-pared to spot sizes of the low flux medical settings, which weremeasured with the DSSD. The beam spots for setting I and set-ting II also showed a Gaussian-shaped distribution. The mea-sured horizontal spot size is depicted in Figure 9 and the verticalspotsizes in Figure 10, respectively.
75 100 125 150 175 200 225 250 energy [MeV] h o r i z o n t a l s p o t s i z e F W H M [ mm ] setting Isetting IInominal Figure 9: Horizontal (x-axis) beam spot size for di ff erent proton energies andparticle fluxes. Similar vertical spot sizes compared to the nominal flux settingwere obtained for setting I. Except for the horizontal spot sizefor setting I at 252 . ff erence in FWHMcompared to the nominal flux was always less than ≈ ≈ − ∆ FWHM). On theother hand, the absolute di ff erence in FWHM of setting II com-pared to the nominal flux did not exceed ≈ . . × plastic scintillators. Only events passing both scin-tillators are able to trigger an event in the DSSD, which leads toa truncated beam profile at the edges of the plastic scinitllators( ≈ ±
25 mm). This also means that the measured rate for thissetting can only be related to the area of the plastic scintilla-tors, which has to be taken into account when using this setting.
75 100 125 150 175 200 225 250 energy [MeV] v e r t i c a l s p o t s i z e F W H M [ mm ] setting Isetting IInominal Figure 10: Vertical (y-axis) beam spot size for di ff erent proton energies andparticle fluxes.
20 0 20x [mm]0.0000.0050.0100.0150.0200.0250.030 d i s t r i b u t i o n size of scint. 10 0 10y [mm]0.000.010.020.030.040.05 Figure 11: Horizontal (left) and vertical (right) beam profile of 252 . The broadening of the beam is a direct result of the use of themagnetic quadrupoles in the HEBT. Due to the geometry of thetrigger scintillators and the DSSD, only part of the beam profiledistribution could be measured. However, the beam distribu-tion seems to be relatively uniform inside the measured area(5 × .
56 cm ). This was also observed for the horizontal beamprofile.In addition to the medical settings, the spot size for 800 MeVprotons was also measured with the DSSD. In contrast to themedical settings, the spotsizes for this energy were not mea-sured in the isocentre, since they were recorded simultaneouslyto the rate measurement with the RaMon in the iso-centre. Forthis purpose, the DSSD was placed 16 cm downstream the Ra-Mon. Thus, the measured spot size is increased due to multi-ple Coulomb scattering in the plastic scintillators of the RaMonsetup in front of the DSSD. The beam profiles for setting V,VIand VII are depicted in Figure 12.From Figure 12, it is apparent that the beam profiles for all800 MeV settings do not follow a Gaussian distribution. A sec-ond peak near the center of the spot could be detected for allsettings. The vertical beam profile for all settings is slightly5 d i s t r i b u t i o n
10 0 10y [mm]0.000.020.040.06settingVVIVII
Figure 12: Horizontal (left) and vertical (right) beam profile of 800 MeV pro-tons for setting V,VI and VII measured with a DSSD. The beam profiles donot follow a Gaussian distribution. The vertical beam is slightly larger than thevertical side of the DSSD. larger than the p-side (vertical axis) of the DSSD. On the otherhand, the horizontal beam profiles are smaller than the n-side(horizontal axis) of the DSSD and look centered. To estimatethe spotsize, the sample standard deviation σ x of the horizontalbeam profile was calculated. It resulted in 9 . . The e ff ect of the particle flux reduction method on the meanenergy was studied by measuring the range in water for di ff erentflux settings and di ff erent beam energies. The measured rangein water was then compared to the NIST PSTAR database. Forsetting III the TERA range telescope was used to measure therange in water at 83 and 100 . ™ couldbe used. As can be seen in Figure 13, no significant changein the measured range was observed for any flux setting. Themeasured ranges are also in good agreement with the rangesobtained from the NIST database.In contrast to the rate and spot size measurements, the energymeasurements were recorded prior to a timing optimization ofthe accelerator cycle to reduce dead time. Even though nochange on the particle energy was expected, the measurementfor setting II at 252 .
4. Summary and Outlook
In total, six low flux settings for protons were commis-sioned at MedAustron and can now be used in the non-clinicalresearch room. Three settings were tested for seven di ff erentbeam energies ranging from 62 . . O (kHz), another with O (100 kHz) and a setting with O (MHz)was commissioned.The beam profiles were measured using the Lynx ® for higherparticle rates in the clinical energy range and a DSSD for all low C S D A r a n g e i n w a t e r [ mm ] Range verification for different particle fluxesNIST datasetting I (peakfinder)setting II (peakfinder)setting III (TERA)nominal flux (peakfinder)
Figure 13: Measured range in water for di ff erent proton energies and flux set-tings. The obtained ranges were compared to data from the NIST PSTARdatabase. flux settings. Setting I and setting II showed similar beam pro-files compared to the nominal flux setting. However, a slightlysmaller vertical spot size was obtained for setting II ( ≈ − ∆ FWHM)). A di ff erence of the width of the measuredhorizontal beam profiles larger than 1 mm FWHM was onlyobserved for setting I at 252 . ff ect of the mag-netic quadrupoles used for setting III resulted in a large beamspread. Therefore the full beam profile could not be measuredfor this setting. Thus the measured rate for setting III can onlybe defined on the area of area as large as the plastic scintillators(50 × ). For 800 MeV, the measured beam profiles forsetting V,VI and VII did not follow a Gaussian distribution. Toestimate the spot size, the sample standard deviation was calcu-lated for the horizontal beam profile. Depending on the setting,a sample standard deviation ranging from 8 − . ff erent beam energies and flux settingswas measured using the PTW PEAKFINDER ™ and a rangetelescope based on plastic scintillator slices coupled to SiPMs.After comparing the measured ranges in water for di ff erent par-ticle energies and particle fluxes, it was found that the particleflux reduction methods had no significant impact on the particleenergy. Acknowledgements
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