Geant4 Monte Carlo simulation study of the secondary radiation fields at the laser-driven ion source LION At the Center for Advanced Laser Applications (CALA), Garching, Germany, the LION (Laser-driven ION Acceleration) experiment is being commissioned, aiming at the production of laser-driven bunches of protons and light ions with multi-MeV energies and repetition frequency up to 1 Hz. A Geant4 Monte Carlo-based study of the secondary neutron and photon fields expected during LION’s different commissioning phases is presented. Goal of this study is the characterization of the secondary radiation environment present inside and outside the LION cave. Three different primary proton spectra, taken from experimental results reported in the literature and representative of three different future stages of the LION’s commissioning path are used. Together with protons, also electrons are emitted through laser-target interaction and are also responsible for the production of secondary radiation. For the electron component of the three source terms, a simplified exponential model is used. Moreover, in order to reduce the simulation complexity, a two-components simplified geometrical model of proton and electron sources is proposed. It has been found that the radiation environment inside the experimental cave is either dominated by photons or neutrons depending on the position in the room and the source term used. The higher the intensity of the source, the higher the neutron contribution to the total dose for all scored positions. Maximum neutron and photon ambient dose equivalent values normalized to 109 simulated incident primaries were calculated at the exit of the vacuum chamber, where values of about 85 nSv (109 primaries)−1 and 1.0 μSv (109 primaries)−1 were found. Thanks to the recent improvements in laser peak power, energy density, laser temporal contrast and to the large investigation of suitable target materials, in the last two decades, several groups achieved the acceleration of protons and light ions up to an energy of several tens of MeV1. Although still in an early phase of its development, the acceleration of charged particles via laser-target interaction is becoming, in the present days, a highly promising candidate for future acceleration techniques2. Due to the micrometer scales at which the acceleration process takes place, laser-driven ion sources are ideally suited for novel investigations in research and have high potential for pushing the frontiers for future generations of particle accelerators for a broad range of applications that benefit from short ion bunch duration and high peak current3.At the Center for Advanced Laser Applications (CALA) located in Garching close to Munich (Germany), the laser-driven ion acceleration experiment LION is currently being commissioned4.As laser pulse source, LION employs ATLAS3000, a Ti:Sapphire-based laser, whose main properties are summarized in Table 1, and as targets, 0.01–1 μm thick metal or carbon-based samples mounted on a rotating sample holder5. The ultimate goal pursued at LION is the exploitation of the Target Normal Sheath Acceleration (TNSA) and Radiation Pressure Acceleration (RPA) regimes, in order to realize a laser-driven ion source with the capability to deliver collimated bunches of several tens of MeV ions (protons and carbon ions) at 1 Hz repetition frequency to serve as a facility for radiation therapy research6.Table 1 Main ATLAS3000 laser parameters available for the LION experiment, considering full operation and current status.As reported in Table 1, at the current status of the facility’s commissioning (2020), ATLAS3000 is delivering to LION laser pulses with energy up to 10 J. This results in ion cutoff energies (i.e., the energy of the most energetic ions, detected above the background noise) and charge per bunch that are lower to those expected at full operation.Computational investigations of the secondary particle production expected at LION during its commissioning, and afterwards in full operation, are of primary interest for a variety of reasons. Radiation protection is typically the primary purpose. In order to protect the accelerator operators and the general public from being exposed to an unwanted dose of secondary particles, a realistic dose assessment based on simulations (followed by experimental verification) needs to be done. To date, different studies aiming at assessing the unwanted secondary neutron radiation produced by laser-driven ion sources appeared in the literature. Fan et al.7 addressed the problem of unwanted secondary radiation through designing a multi-layer compact radiation shielding for a laser-driven proton therapy facility using FLUKA Monte Carlo code. Secondary neutron doses per primary protons are given in the proximity of the vacuum chamber, assuming a primary proton spectrum with 300 MeV cutoff energy. In 2010 Sakaki et al. presented a combined experimental and PHITS Monte Carlo study of the secondary doses produced at KPSI’s (Kansai Photon Science Institute) laser-driven proton source, showing a few μSv of total dose per bunch were found in the proximity of the facility’s vacuum chamber8. Radiation protection oriented FLUKA simulations for the several facilities hosted at CALA (among which LION appears as well) have been recently reported by Englbrecht et al.9. This study demonstrates that radiation protection limits in all areas of interest around the facilities are met, even for worst case scenarios (in terms of the energy spectrum of primary particles, charge per bunch and repetition frequency). Considering LION only, it has to be pointed out that elements and structures hosted inside the LION vacuum chamber, where a significant fraction of secondary particles gets produced due to the large divergence angles of laser-driven emitted particles, are not included in the work of Englbrecht et al. From a radiation safety point of view, this has little influence given that access to the target area is not permitted during operation. In contrast, our inclusion of more details mainly serves the purpose of evaluating the secondary radiation fields, which can be relevant for studies with particle bunches inside the LION cave. In this sense, a clear characterization of the secondary radiation produced in the vicinity of the particle acceleration is of interest when planning future experimental applications (e.g., radiobiology experiments), where the contribution to the dose due to secondary neutrons and photons produced by the shaping apparatus might be non-negligible if compared to the dose delivered by primary protons.The production of secondary radiation is directly connected to the presence and the interaction of primary radiation (i.e., laser-accelerated particles) with surrounding materials and structures. A detailed information on the nature of the secondary radiation might lead to a deeper knowledge of the specific features of the primary radiation that was responsible for its production (e.g., number of primary particles per bunch and angular distribution), specially when only a small portion of the produced primary particles can be directly detected and analyzed.Lastly, simulations of the secondary pulsed radiation fields, expected during the different stages of LION commissioning, will drive the decision on which neutron and photon detection techniques it is best to apply when experimental characterizations of the secondary radiation will be performed. This is even more relevant considering the pulsed nature of the LION source and the serious issues encountered by commercial radiation protection online devices when exposed to pulsed neutron and photons sources10,11.Laser-driven acceleration is a quasi-neutral acceleration process that transfers a fraction of the energy carried by laser photons to kinetic energy of a variety of particles, first of all electrons and light ions (protons mainly). These propagate in forward direction, from the laser-target interaction site, with a relatively wide diverging angle of the order of a few hundreds of mrad12,13.During the current commissioning phase, the production and transport of protons dominates that of other ions. In the following, we will therefore refer only to the laser-driven production of protons and electrons, neglecting the small, yet present, contribution of other ions to the ensemble of laser-driven produced particles.The acceleration process takes place within a 2.5 cm thick aluminum vacuum chamber, a modular structure 3.92 m long, 1.21 m high and 0.98 m wide. The vacuum chamber itself is located inside the LION cave, an experimental cave 18 m long, 3 m wide and 4.25 m high, separated from the ground by a 75 cm thick concrete platform. One meter above the platform lies a double floor, below which, part of technical infrastructures is hosted. In addition, the cave is covered by a 45 cm thick concrete ceiling.As shown in Fig. 1, LION is surrounded to the south by the LUX cave (Laser-driven Undulator X-ray Source), to the east by the HF cave (High Field), to the west by the facilities’ entryway (closed during operation), and to the north by a corridor whose access is granted to operators during machine operation. To separate LION from these other areas and to shield them from possible secondary radiation, radiological shielding walls with a thickness ranging from 1 to 1.2 m are in place (exception is the east wall whose thickness reaches up to 2 m). Shielding walls are weakened by the presence of six cylindrical openings 40 cm in diameter (Fig. 2a), which are needed to transport laser light from ATLAS3000 to the LION experiment and then further to the other experimental installations. These openings are placed between the concrete platform and the double floor, three of which connecting the LION cave with the LUX cave (through LION’s southern wall) and three connecting the LION cave with the corridor through LION’s northern wall.Figure 1Top-view drawing of the CALA building. In orange the room hosting ATLAS3000 laser, in yellow the LION cave and in light-blue the corridor surrounding the facilities’ caves. Names of rooms adjacent to the LION cave are reported: HF high field, LUX laser-driven Undulator X-ray source. The area enclosed within the red dashed line is the one considered by the Geant4 simulations shown in this work.Figure 2LION geometry as implemented in Geant4. (a) Top view of the facility and scorers (red) with ID number. (b) Close-up on the two quadrupole magnets (QPs), with protection layer and glass hollow cone mounted on the first QP and photon screen. Figures are not drawn to scale.As a result of radiation protection oriented FLUKA simulations, performed in an early phase of the experiment commissioning, a composite water-concrete beam dump is placed in front of the back exit of the vacuum chamber (about 50 cm from the chamber), in order to absorb that fraction of produced protons that is transported outside of the vacuum chamber (Fig. 2a). This beam dump also acts as radiation shielding for the shower of secondary particles (photons and neutrons mainly) that are produced by the interaction of the particles emerging from laser-target interaction with the diagnostic and steering components hosted inside the vacuum chamber9.As mentioned, protons produced via laser-target interaction are usually emitted with a large divergence angle. This is usually, from an application point of view, a quite inconvenient feature, given that, in the majority of applications, the delivery of particles needs to be precisely focused onto a specific target volume. Therefore, following the laser-target interaction site, a series of two NdFeB quadrupoles (later referred to as QPs) mounted on motorized supports, is employed to focus the produced protons. Given the large divergence angle at which protons are intrinsically emitted in this facility, around 180 mrad half angle14, most of protons interacts with the front face of the first QP itself (which is, also for this reason, shielded by a 4 mm thick aluminum protection plate) rather than passing through it and getting focused by the applied magnetic fields. The fraction of protons that gets focused by the QPs travels straight and leaves the vacuum chamber through a thin exit window after which proton diagnostic devices are placed (such as radiochromic stacks, transmission chambers or scintillation foils). These protons are eventually stopped by the aforementioned beam dump.For the simulation of the production of secondary radiation at LION, the Geant4 10.1.2 Monte Carlo simulation toolkit has been used15,16. Given that the interaction of multi-MeV protons and electrons (also referred to as primary particles) with surrounding materials leading to the production of secondary neutrons and photons (secondary particles in the following) is the main focus of this work, the Bertini Intranuclear Cascade model (QGSP_BERT_HP) has been used. This model takes into account hadron physics, electromagnetic showers and synchrotron radiation, and it is recommended by the Geant4 developers when dealing with medical and industrial neutron applications and radiation shielding17. The HP extension (i.e., NeutronHP, Neutron High Precision)
https://www.nature.com/articles/s41598-021-03897-2
Geant4 Monte Carlo simulation study of the secondary radiation fields at the laser-driven ion source LION
