GEANT4 Validation Studies at the ISIS Muon Facility

GEANT4 provides an extensive set of alternative hadronic models. Simulations of the ISIS muon production using three such models applicable in the energy range of interest are presented in this paper and compared with the experimental data. INTRODUCTION The Monte Carlo particle transport code Geant4 [1] represents a powerful tool for simulating the passage of particles through matter. The simulation package includes a large set of physics processes, geometry models, particles and materials over a wide energy range of incident particles starting in some cases from 250 eV and extending in others to the TeV energy range. The physics processes offered cover also a wide range, from electromagnetic and optical processes to hadronic processes. As far as hadronic processes are concerned, a single hadronic model would not be able to support all user requirements, therefore Geant4 provides an extensive set of alternative hadronic models. This paper addresses the validation of three hadronic models (Bertini, Binary Cascade and INCL-ABLA) applicable in the energy range of interest for the ISIS pulsed neutron and muon facility at the Rutherford Appleton Laboratory, UK. The ISIS facility uses an 800 MeV, 200 μA proton beam to produce neutrons and muons for studies of atomic-level properties of materials. All simulations were performed with version 4.9.3 p01 of the Geant4 toolkit. A description of the hadronic models, as well as an overview of the experimental setup together with the models predictions for the ISIS muon production are presented in this paper. HADRONIC MODELS Given the vast number of possible modelling approaches, three hadronic models applicable in the ISIS interest energy range were chosen. The Bertini model is performing well for incident protons, neutrons, pions, photons and nuclear isotops and is validated up to 10 GeV incident energy [2]. The Binary Cascade model is valid for incident protons, neutrons and pions and it reproduces detailed proton and neutron cross-section data in the region 0-10 GeV and 0-1.3 GeV for pions due to its dependance on resonances. The INCL-ABLA code was recently validated agains spallation data and it reproduces cross-section data for protons, neutrons, pions, deuterium, tritium, helium and alpha particles in the energy range 200 MeV 3 GeV. In the Bertini cascade model, the target nucleus is treated as an average nuclear medium to which excitons (particle-hole states) are added after each collision. The path lengths of nucleons in the nucleus are sampled according to the local density and free nucleon-nucleon cross sections.At the end of the cascade the excited nucleus is represented as a sum of particle-hole states which is then decayed by preequilibrium, fission and evaporation methods [1]. In the Binary cascade model, the nucleus is modelled as 3 dimensional and isotropic. The nucleons are placed in space according to nuclear density and the nucleon momentum is according to Fermi gas model. The primary particles interact with nucleons in binary collisions producing resonances which decay according to their lifetime producing secondary particles. The secondary particles re-scatter with nucleons creating a cascade [3]. To respond to the increasing user requirements from the nuclear physics community, the Geant4 collaboration set a goal to complement the theory-driven models in this regime (the Bertini cascade and Binary cascade being the most widely used) with the inclusion of the INCL code also known as Liege cascade, often used with the evaporation/fission code ABLA [4]. EXPERIMENTAL DATA Pion production cross-sections on a liquid hydrogen target and various solid targets over a wide range of production angles and pion energies were measured at the cyclotron at Lawrence Radiation Laboratory [5]. The experiment used the proton beam of the cyclotron, a liquid hydrogen target and various solid targets and a pion spectrometer consisting of a bending magnet and an array of 12 counter telescopes. The beam passed through a pre-magnet collimator, a steering magnet and a quadrupole doublet and then through a pipe in the shield, into the physics cave. The initial setup inside the physics cave was for forward angles. A quadrupole doublet was used to focuse the beam to the primary target. The target was followed by a second doublet quadrupole used for stopping the beam in a steel block, 10 m downstream. When the aparatus was set up for backward angles, the second quadrupole doublet was used to focuse the beam to a secondary target. The solid targets were either 7.5 cm or 10 cm in diameter. After taking the backward-angle data, the setup was changed to forward angles with the pre-magnet collimator opened for these cross-section measurements. Several secondary beam channels over a wide range of angles were viewed by the magnetic spectrometer [5]. The measured differential cross-sections for pion production by 730 MeV protons on eleven different targets (H, D, Be, C, Al, Ti, Cu, Ag, Ta, Pb, Proceedings of IPAC’10, Kyoto, Japan MOPEA075 08 Applications of Accelerators, Technology Transfer and Industrial Relations U05 Applications, Other 247 Th) provided a reliable guide for the design of pion beams at various meson facilities.