Verification of Monte Carlo transport codes by activation experiments

With the increasing energies and intensities of heavy-ion accelerator facilities, the problem of an excessive activation of the accelerator components caused by beam losses becomes more and more important. Numerical experiments using Monte Carlo transport codes are performed in order to assess the levels of activation. The heavy-ion versions of the codes were released approximately a decade ago, therefore the verification is needed to be sure that they give reasonable results. Present work is focused on obtaining the experimental data on activation of the targets by heavy-ion beams. Several experiments were performed at GSI Helmholtzzentrum für Schwerionenforschung. The interaction of nitrogen, argon and uranium beams with aluminum targets, as well as interaction of nitrogen and argon beams with copper targets was studied. After the irradiation of the targets by different ion beams from the SIS18 synchrotron at GSI, the γ-spectroscopy analysis was done: the γ-spectra of the residual activity were measured, the radioactive nuclides were identified, their amount and depth distribution were detected. The obtained experimental results were compared with the results of the Monte Carlo simulations using FLUKA, MARS and SHIELD. The discrepancies and agreements between experiment and simulations are pointed out. The origin of discrepancies is discussed. Obtained results allow for a better verification of the Monte Carlo transport codes, and also provide information for their further development. The necessity of the activation studies for accelerator applications is discussed. The limits of applicability of the heavy-ion beam-loss criteria were studied using the FLUKA code. FLUKA-simulations were done to determine the most preferable from the radiation protection point of view materials for use in accelerator components.Die Aktivierung von Beschleunigerkomponenten durch Strahlverluste ist einer der wichtigsten Faktoren der Intensitätsbegrenzung für hochenergetische und hochintensive Hadronenbeschleuniger. Erhöhte Dosisleistungen in der Nähe von bestrahlten Materialien erschweren die Hands-On-Wartung der Maschine. Deshalb ist die Aktivierung von Beschleunigerkomponenten von großem Belang für die “Facility for Antiproton and Ion Research“ (FAIR). Dies führt zur Notwendigkeit einer Messung der Restaktivität in den Tiefenschichten von bestrahlten Festkörpern. Im Rahmen dieser Arbeit wurden Grenzen für Strahlverluste von Schwerionenstrahlen ermittelt, welche bzgl. der Zugänglichkeit einer Beschleunigeranlage zu Wartungszwecken etc. einzuhalten sind. Der Schwerpunkt der vorliegenden Studie war die Messung der Restaktivität im Material, hervorgerufen durch Ionenstrahlen verschiedener Spezies: Stickstoff (Z = 7), Argon (Z = 18) und Uran (Z = 92). Als zu bestrahlende Materialien wurden Aluminium und Kupfer ausgewählt – als Repräsentanten für den niedrigen und mittleren Z-Bereich. Im Hinblick auf Beschleunigeranwendungen sollten Aluminiumkomponenten in den Bereichen mit hohen Strahlverlusten bevorzugt werden, da dieses Material offenbar weniger aktiviert wird als Hoch-Z-Materialien, andererseits ist Kupfer ein übliches Material für viele Beschleunigerkomponenten wie z.b. für Spulen der Magnete. Deshalb ist der Vergleich von Aluminium und Kupfer vom besonderen Interesse. Zwei Arten von Targets wurden bestrahlt: gestapelte Folien- und Einzelfolien-Targets. Die dicken Aluminium-Targets wurden mit 498 AMeV Stickstoff 14N7+, 496 AMeV Argon 40Ar18+ und 483 AMeV Uran 238U73+, die dicken Kupfer-Targets wurden mit 498 AMeV Stickstoff und 496 AMeV Argon bestrahlt. Die dünnen Aluminium-Folien wurden mit 426 AMeV Argon und mit 85, 174, 279, 325, 381, 483, 584, 684, 785, 935 AMeV Uran bestrahlt. Insgesamt fünf dicke und zehn Einzelfolientargets wurden für die vorliegende Doktorarbeit bestrahlt. Mehr als 5000 Spektren wurden gemessen und analysiert, 45 Tiefenprofile verschiedener Nuklide in den durchgeführten Experimenten erhalten. Für das Design von Beamdumps und Strahlabschirmungen (und für vielfältige andere Anwendungen) verwendet man die Monte Carlo Transportcodes, welche die Bewegung und Wechselwirkung von Teilchen mit Materie berechnen. Die Schwerionen-Versionen der Monte Carlo Transportcodes wurden etwa vor fünfzehn Jahren eingeführt und sind noch nicht in allen Energie- und Z-Bereichen bestätigt. Es gibt nur wenige Daten für die Aktivierung der Materialien durch Schwerionenstrahlen. Die Verifikation der Monte-Carlo-Codes durch Aktivierungsexperimente ermöglicht die Überprüfung von Transport und nuklearer Erzeugung explizit durch den Vergleich der Typen, der Häufigkeit und der Tiefenprofile der Radionuklide, die im bestrahlten Material erzeugt oder gestoppt werden. In der vorliegenden Arbeit wurden die Codes FLUKA, MARS und SHIELD für die Verifizierung gewählt. Das Stoppen der Ionen mit Energien von bis zu 500 AMeV wird von allen drei Codes gut beschrieben. Gemäß den durchgeführten Experimenten und Simulationen wird die Gesamtzahl der erkannten Nuklide im gesamten Targetvolumen von FLUKA mit durchschnittlich ~ 5% Abweichung, durch MARS mit einer ~ 15%-igen Abweichung angegeben, und SHIELD unterscheidet sich um ca. 50% vom Experiment. Die erhaltenen experimentellen Ergebnisse erlauben nicht nur eine Bestätigung dieser Monte-Carlo-Transportcodes, sondern auch deren weitere Entwicklung. Andere Ziele waren, die Grenzen der Anwendbarkeit der Schwerionenstrahlverlust- Kriterien zu erforschen und herauszufinden, welches Material in Beschleunigeranwendungen bezüglich des Strahlenschutzes zu bevorzugen ist. Folgende Grenzen für erlaubte Strahlleistungsdepositionen entlang eines Beschleunigers wurden ermittelt: Fall A: Nach 100 Tagen Strahlzeit soll die Maschine nach einer Wartezeit von 4 Stunden zugänglich sein. Für 50 AMeV Uranstrahlen auf Eisen wurde ein Grenzwert von 200 W/m bestimmt. Bei Verdopplung der Strahlenergie auf 100 AMeV sinkt die zulässige Strahlverlustleistung auf 60 W/m in Eisen. Tauscht man das Eisentarget gegen ein Kupfertarget aus, so sind die entsperechenden Grenzwerte 120 W/m bzw. 80 W/m. Fall B: Nach 20 Jahren Strahlbetrieb soll die Maschine nach einer Wartezeit von wiederum 4 Stunden zugänglich sein. In diesem Fall sinken die erlaubten Strahlverlustleistugen auf 120 W/m bzw. 40 W/m in Eisen und auf 85 W/m bzw. 50 W/m in Kupfer. Die Aktivierung derjenigen Materialien, die am häufigsten in Beschleunigern verwendet werden, wurde mit FLUKA durchgeführt. Die Dosisleistungen im Abstand von 30 cm von der Targetfläche sind am höchsten für Ni, Nb und Mo, sodass der Anteil dieser Materialien in den Beschleunigerkomponenten minimiert werden muss. Die Dosisleistungen in der Nähe der Targets aus C, Al, Ti, Cr, Mn, Fe, Cu und Pb waren mindestens zweimal niedriger; deshalb können aus Sicht der Hands-On-Wartung diese Materialien eher verwendet werden. Bei langer Bestrahlung und langer Kühlzeit zeigten Al, Ti, Mn, Ni und Cu die höchsten Dosisleistungen. Dies sollte berücksichtigt werden, wenn lange Bestrahlungszeiten vorgesehen sind und ferner eine nachfolgende Lagerung der bestrahlten Materialien erforderlich ist

New Method for Validation of Aperture Margins in the LHC Triplet

Safety of LHC equipment including superconducting magnets depends not only on the proper functioning of the systems for machine protection, but also on the accurate adjustment of the protective devices such as collimators. In case of a failure of the extraction kicker magnets, which are part of the beam dumping system, it is important to ensure protection of the superconducting triplet magnets from missteered beam. The magnets are located to the right of Interaction Point 5 (IP5) and are protected by one set of collimators in the beam dumping insertion in IR6 and another set close to the triplet magnets. In this paper, a new method for verification of the correct collimator position with respect to the aperture is presented. It comprises the application of an extended orbit bump with identical trajectory as the beam trajectory after a deflection by the beam dump kickers. By further increasing the bump amplitude and successively moving in/out the collimators in the region of interest, the accurate positioning of the collimators can be validated. The effectiveness of the method for LHC IP5 and IP1 and both beams is discusse

Depth profiling of the residual activity induced in carbon-based materials by heavy ions

We present new results of the experimental study of the residual activity induced by high-energy heavy ions in carbon-based materials: graphite and carbon composite. The graphite target was irradiated by 500 MeV/u tantalum ions and the carbon composite target was irradiated by 500 MeV/u uranium ions. The targets were assembled from a stack of thin plates and after irradiation were investigated using gamma-ray spectroscopy. Main tasks of the experimental study were: 1) to identify induced radioactive isotopes in the gamma spectra of the measured samples, 2) to estimate residual activity of the identified isotopes and 3) to determine depth profiles of the residual activity of individual isotopes. Depth profiling of the residual activity of all identified isotopes was performed by measurements of individual target plates. According to the depth profiles, the identified isotopes can be classified into two main groups: target-nuclei fragments and projectile fragments. In the measured gamma spectra of the carbon-based materials irradiated by heavy ions only one target-nuclei fragment, 7 Be, was identified. All the rest of the isotopes detected using gamma-ray spectroscopy, are the projectile fragments of various masses. The experimental data were compared with Monte Carlo simulations performed by FLUKA code in order to verify validity of physical models and data libraries implemented in the code. A satisfactory agreement between the experiment and the simulations was observed

Simulation study of the space charge limit in heavy-ion synchrotrons

The SIS100 synchrotron as a part of the new Facility for Antiproton and Ion Research (FAIR) accelerator facility at GSI should be operated at the “space charge limit” for light- and heavy-ion beams. Beam losses due to space-charge-induced resonance crossing should not exceed a few percent during a full cycle. The recent advances in the performance of particle tracking tools with self-consistent solvers for the 3D space charge forces now allow us to reliably identify low-loss areas in tune space, considering the full SIS100 accumulation plateau of one second (160 000 turns) duration. A realistic magnet error model, extracted from precise bench measurements of the SIS100 main dipole and quadrupole magnets, is included in the simulations. Previously, such beam dynamics simulations required non-self-consistent space charge models. By comparing to the self-consistent simulations results, we are now able to demonstrate that the predictions from such faster space charge models can be used to identify low-loss regions with sufficient accuracy. The findings are applied by identifying a low-loss working point region in SIS100 for the design FAIR beam parameters. The bunch intensity at the space charge limit is determined. Several countermeasures to space charge are proposed to enlarge the low-loss area and to further increase the space charge limit

Simulation study of the space charge limit in heavy-ion synchrotrons

The SIS100 synchrotron as a part of the new Facility for Antiproton and Ion Research (FAIR) accelerator facility at GSI should be operated at the “space charge limit” for light- and heavy-ion beams. Beam losses due to space-charge-induced resonance crossing should not exceed a few percent during a full cycle. The recent advances in the performance of particle tracking tools with self-consistent solvers for the 3D space charge forces now allow us to reliably identify low-loss areas in tune space, considering the full SIS100 accumulation plateau of one second (160 000 turns) duration. A realistic magnet error model, extracted from precise bench measurements of the SIS100 main dipole and quadrupole magnets, is included in the simulations. Previously, such beam dynamics simulations required non-self-consistent space charge models. By comparing to the self-consistent simulations results, we are now able to demonstrate that the predictions from such faster space charge models can be used to identify low-loss regions with sufficient accuracy. The findings are applied by identifying a low-loss working point region in SIS100 for the design FAIR beam parameters. The bunch intensity at the space charge limit is determined. Several countermeasures to space charge are proposed to enlarge the low-loss area and to further increase the space charge limit

Automatized optimization of beam lines using evolutionary algorithms

Due to the massive parallel operation modes at GSI accelerators, a lot of accelerator setup and re-adjustment has to be made by operators during a beam time. This is typically done manually using potentiometers and is very time-consuming. With the FAIR project the complexity of the accelerator facility increases further and for efficiency reasons it is recommended to establish a high level of automation for future operation. Modern Accelerator Control Systems allow a fast access to both, accelerator settings and beam diagnostics data. This provides the opportunity to implement algorithms for automated adjustment of e.g. magnet settings to maximize transmission and optimize required beam parameters. The fast-switching magnets in GSI-beamlines are an optimal basis for an automatic exploration of the parameter-space. The optimization of the parameters for the SIS18 multi-turn-injection using a genetic algorithm has already been simulated*. The first results of our automatized online parameter optimization at the CRYRING@ESR injector are presented here

Automatized Optimization of Beam Lines Using Evolutionary Algorithms

Due to the massive parallel operation modes at GSI accelerators, a lot of accelerator setup and re-adjustment has to be made by operators during a beam time. This is typically done manually using potentiometers and is very time-consuming. With the FAIR project the complexity of the accelerator facility increases further and for efficiency reasons it is recommended to establish a high level of automation for future operation. Modern Accelerator Control Systems allow a fast access to both, accelerator settings and beam diagnostics data. This provides the opportunity to implement algorithms for automated adjustment of e.g. magnet settings to maximize transmission and optimize required beam parameters. The fast-switching magnets in GSI-beamlines are an optimal basis for an automatic exploration of the parameter-space. The optimization of the parameters for the SIS18 multi-turn-injection using a genetic algorithm has already been simulated*. The first results of our automatized online parameter optimization at the CRYRING@ESR injector are presented here

LHC BFPP Quench Test with Ions (2015)

The 2015 Pb-Pb collision run of the LHC operated at a beam energy of 6.37Z TeV. The power of the secondary beams emitted from the interaction point by the bound-free pair production (BFPP) process reached new levels while the propensity of the bending magnets to quench is higher at the new magnetic field levels. This beam power is about 70 times greater than that contained in the luminosity debris and is focussed on a specific location. As long foreseen, orbit bumps were introduced in the dispersion suppressors around the highest luminosity experiments to mitigate the risk of quenches by displacing and spreading out these losses. Because the impact position and intensity of these secondary beams is well known and can be tracked easily with the Beam Loss Monitors (BLMs), the BFPP1 beam (208Pb81+ ions), which is the most intense, provides a tool to accurately measure the steady state quench limit of the LHC main dipoles. At the moment the exact quench limit is not known, but this knowledge is important to assess the need for special collimators to intercept these secondary beams. This note describes the procedure and preliminary results of a test conducted on the main dipole in cell 11 left of IP5, using the BFPP1 beam to provoke a quench of this magnet