Acts/Material/Material.hpp

0001 // This file is part of the Acts project.
0002 //
0003 // Copyright (C) 2016-2020 CERN for the benefit of the Acts project
0004 //
0005 // This Source Code Form is subject to the terms of the Mozilla Public
0006 // License, v. 2.0. If a copy of the MPL was not distributed with this
0007 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
0008
0009 #pragma once
0010
0011 #include "Acts/Definitions/Algebra.hpp"
0012
0013 #include <iosfwd>
0014 #include <limits>
0015 #include <utility>
0016
0017 namespace Acts {
0018
0019 /// Material description for interactions with matter.
0020 /// @defgroup Material Material
0021 ///
0022 /// The following parameters are used to specify the material and its
0023 /// interactions with traversing particles:
0024 ///
0025 /// - radiation length X0 (native length units)
0026 /// - nuclear interaction length L0 (native length units)
0027 /// - relative atomic mass Ar (unitless number)
0028 /// - nuclear charge number Z (elementary charge e)
0029 /// - molar density (native amount-of-substance unit / (native length unit)³)
0030 ///
0031 /// The parameters can be effective or average parameters e.g. when a mixture
0032 /// of materials is described.
0033 ///
0034 /// @note Always use the opaque parameters vector to serialize/deserialize the
0035 ///   material information. Since the internal storage might be different from
0036 ///   the external accessors, this ensures that always the numerically optimal
0037 ///   parameters are stored. Use the `ParametersVector` type and do not assume
0038 ///   any particular size since we might consider to store more parameters in
0039 ///   the future.
0040 class Material {
0041  public:
0042   using ParametersVector = Eigen::Matrix<float, 5, 1>;
0043
0044   // Both mass and molar density are stored as a float and can thus not be
0045   // distinguished by their types. Just changing the last element in the
0046   // previously existing constructor that took five floats as input to represent
0047   // molar density instead of mass density could have lead to significant
0048   // confusion compared to the previous behaviour. To avoid any ambiguity,
0049   // construction from separate material parameters must happen through the
0050   // following named constructors.
0051
0052   /// Construct from material parameters using the molar density.
0053   ///
0054   /// @param x0 is the radiation length
0055   /// @param l0 is the nuclear interaction length
0056   /// @param ar is the relative atomic mass
0057   /// @param z is the nuclear charge number
0058   /// @param molarRho is the molar density
0059   static Material fromMolarDensity(float x0, float l0, float ar, float z,
0060                                    float molarRho);
0061   /// Construct from material parameters using the mass density.
0062   ///
0063   /// @param x0 is the radiation length
0064   /// @param l0 is the nuclear interaction length
0065   /// @param ar is the relative atomic mass
0066   /// @param z is the nuclear charge number
0067   /// @param massRho is the mass density
0068   ///
0069   /// @warning Due to the choice of native mass units, using the mass density
0070   ///   can lead to numerical problems. Typical mass densities lead to
0071   ///   computations with values differing by 20+ orders of magnitude.
0072   static Material fromMassDensity(float x0, float l0, float ar, float z,
0073                                   float massRho);
0074   /// Construct a vacuum representation.
0075   Material() = default;
0076   /// Construct from an encoded parameters vector.
0077   Material(const ParametersVector& parameters);
0078
0079   Material(Material&& mat) = default;
0080   Material(const Material& mat) = default;
0081   ~Material() = default;
0082   Material& operator=(Material&& mat) = default;
0083   Material& operator=(const Material& mat) = default;
0084
0085   /// Check if the material is valid, i.e. it is not vacuum.
0086   constexpr operator bool() const { return 0.0f < m_ar; }
0087
0088   /// Return the radition length. Infinity in case of vacuum.
0089   constexpr float X0() const { return m_x0; }
0090   /// Return the nuclear interaction length. Infinity in case of vacuum.
0091   constexpr float L0() const { return m_l0; }
0092   /// Return the relative atomic mass.
0093   constexpr float Ar() const { return m_ar; }
0094   /// Return the nuclear charge number.
0095   constexpr float Z() const { return m_z; }
0096   /// Return the molar density.
0097   constexpr float molarDensity() const { return m_molarRho; }
0098   /// Return the molar electron density.
0099   constexpr float molarElectronDensity() const { return m_z * m_molarRho; }
0100   /// Return the mass density.
0101   float massDensity() const;
0102   /// Return the mean electron excitation energy.
0103   float meanExcitationEnergy() const;
0104
0105   /// Encode the properties into an opaque parameters vector.
0106   ParametersVector parameters() const;
0107
0108  private:
0109   float m_x0 = std::numeric_limits<float>::infinity();
0110   float m_l0 = std::numeric_limits<float>::infinity();
0111   float m_ar = 0.0f;
0112   float m_z = 0.0f;
0113   float m_molarRho = 0.0f;
0114
0115   friend constexpr bool operator==(const Material& lhs, const Material& rhs) {
0116     return (lhs.m_x0 == rhs.m_x0) && (lhs.m_l0 == rhs.m_l0) &&
0117            (lhs.m_ar == rhs.m_ar) && (lhs.m_z == rhs.m_z) &&
0118            (lhs.m_molarRho == rhs.m_molarRho);
0119   }
0120   friend constexpr bool operator!=(const Material& lhs, const Material& rhs) {
0121     return !(lhs == rhs);
0122   }
0123 };
0124
0125 std::ostream& operator<<(std::ostream& os, const Material& material);
0126
0127 }  // namespace Acts