Monte Carlo modeling of crystal channeling at high energies

https://doi.org/10.1016/j.nimb.2013.02.027Get rights and content

Abstract

Channeling in bent crystals is becoming a reliable and efficient technique for collimating beams. At CERN, the installation of crystals in LHC is under scrutiny by the UA9 collaboration with the goal of investigating if they are a viable option for the collimation system upgrade. This paper describes a new model of channeling in bent crystals which has been developed from scratch in order to be implemented in the FLUKA Monte Carlo code simulating particle transport and interactions. Crystal channels are described through the concept of continuous potential taking into account thermal motion of atoms in crystal lattice. The energy of the particle transverse motion determines whether or not it is channeled while single scattering on lattice atoms can lead to dechanneling. Volume capture and reflection are also modeled, as well as crystal torsion and miscut angle. Data from experimental runs conducted at CERN was analyzed and is shown to be in good agreement with the simulation results.

Introduction

Crystals are increasingly used in synchrotrons around the world for their channeling properties. At CERN, they are considered as an option for the collimation system upgrade of the LHC taking place in the next years. This creates the need for a tool being able to track beam particles and secondaries in the crystal environment, be it in amorphous or channeling mode. Planar channeling consists in the passage of well-oriented charged particles in-between lattice planes. In this paper, we treat planar channeling of positively charged particles. The model is foreseen to be adapted to negatively charged particles in the future.

At CERN, the Monte Carlo code FLUKA is used for beam-machine interaction studies [1], [2]. In view of the possible use of crystals as an important ingredient of the future collimation system, a comprehensive tool is needed to track particles through crystals in channeling orientation. This will enable energy deposition calculations in crystals as well as the tracking of secondaries downstream. The tools presently available do not allow such an investigation. On the one hand, standard tracking codes (like SIXTRACK [3] or ICOSIM [4]) have been interfaced to an empirical description of crystal effects tuned to particular conditions [5]. On the other hand, more sophisticated crystal models (for example [6]) are not yet integrated into the mentioned codes being regularly used to assist the design and the operation of big accelerators.

In Section 2, we describe the main features of this semi-classical, microscopic model developed from scratch. Channeling conditions are determined through comparison of the energy of the particle transverse motion with the continuous potential introduced by Lindhard [7]. Channeled particles follow crystal channels unless dechanneling occurs based on Coulomb scattering events and have an altered interaction rate based on the amplitude of their oscillatory path. Single scattering also leads to the capture of quasi-channeled particles. In addition, volume reflection is implemented according to Bondarenco’s geometrical model [8]. Section 3 is dedicated to the benchmarking of the model with experimental data from the UA9 collaboration, in particular from runs using a 400 GeV/c proton beam extracted from CERN SPS [9]. Section 4 is devoted to conclusions.

Section snippets

Geometry

Crystals, as modeled in the considered framework are defined through two orthogonal vectors, as shown in Fig. 1. The first one, denoted as A, indicates the channel orientation and the second, B, also belongs to the crystal planes that form the channels. For the sake of example, we assume here that A is along the z-axis and B is along the y-axis at the crystal entry face. Actually, our implementation is intended to treat a crystal region with any orientation, as included in a potentially much

Results and discussion

Crystal collimation could allow to achieve better collimation efficiency and reduce radiations levels in the concerned areas [15]. Over the last years, the UA9 collaboration has been assessing the quality of a number of crystals in CERN North Area on the H8 beamline extracted from SPS [9] before installing them in the second UA9 experiment situated in the SPS, where they operate in multi-turn regime.

Comparison of the model results with data from these experiments is presented hereafter. Typical

Conclusions

In this paper we described a model of coherent interactions of beam particles with crystals. Channeling is represented by reduced interaction rates and faculty for particles to follow the channel direction throughout their travel in the crystal. Particles get channeled if the transverse energy, calculated on the basis of the incoming angle and random sampling of the initial position in the channel, is smaller than the continuous potential barrier. Dechanneling is reproduced via microscopic

Acknowledgement

We acknowledge the support from UA9 collaboration colleagues and we especially thank W. Scandale for fruitful discussions about different aspects of the project.

References (17)

There are more references available in the full text version of this article.

Cited by (10)

  • Simulation framework and measurements of crystal collimation of proton beams at the Large Hadron Collider

    2024, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
  • Benchmark of the FLUKA model of crystal channeling against the UA9-H8 experiment

    2015, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
    Citation Excerpt :

    For the moment this model is limited to planar channeling of positively charged particles as applications at CERN are the focus of the work. The event generator, described at length in [7], relies on a semi-classical approach. The Moliere potential is altered by the atomic thermal motion and, in the case of a bent crystal, by a centrifugal term weakening the potential confinement.

  • About multiple scattering of high energy protons in crystal deflectors

    2015, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
  • On thermal and radiation damage to silicon crystals in the LHC proton beam

    2013, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
    Citation Excerpt :

    As follows from the considered examples, even relatively low channeling probability for the recoil atoms (of the order of 0.2) can result in substantial suppression of dpa (by a factor of 2–4) in the channeling mode of primary proton beam. Developments are ongoing to model in FLUKA channeling of high energy charged particles in bent crystals [11]; together with the already implemented modeling approach for atomic displacements, FLUKA will allow for a detailed description of radiation damage in crystals. Limitations on application of silicon crystals for collimation of the LHC beams due to crystal damage are considered in the framework of FLUKA Monte Carlo tool.

  • An overview of the capabilities and recent developments of the FLUKA particle transport code

    2022, Proceedings of the 14th International Conference on Radiation Shielding and 21st Topical Meeting of the Radiation Protection and Shielding Division, ICRS 2022/RPSD 2022
View all citing articles on Scopus
View full text