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Accessing Individual Values

Accessing Individual Values

To evaluate kinematic quantities at a specific beam energy and kinematic variable value, use kinematics_at_beam_energy_and_angle().

This method automatically handles multi-valued solutions and always returns lists.

Syntax

from reaction_kinematics import Reaction

# or equivalently: Reaction("3H(p,n)3He")
rxn = Reaction("p", "3H", "n", "3He")

r = rxn.kinematics_at_beam_energy_and_angle(1.2, "theta3_lab", 122)
print(r["energy3_lab"], r["theta4_lab"])

This returns a KinematicsResult, which behaves like a dict with these keys:

  • cos_theta_cm: [-0.981259434287812]
  • theta_cm: [168.89373250634032]
  • theta3_lab: [121.99999999999999]
  • theta4_lab: [3.1017567808349322]
  • energy3_lab: [0.005245865820720845]
  • energy4_lab: [0.43099808417983787]
  • velocity3_lab: [0.003341198974529349]
  • velocity4_lab: [0.017517569485073135]
  • momentum3_lab: [3.1392925586297746]
  • momentum4_lab: [49.2037438008774]
  • jacobian3_lab: [13.26018747199885] — dΩ(lab)/dΩ(cm) for the ejectile, converts a lab-frame differential cross section to the cm frame
  • jacobian4_lab: [-0.07813620376523883] — same, for the recoil

Each key's unit: r.units["energy3_lab"].

Parameters:

  • beam_energy : Beam kinetic energy (MeV by default)
  • angle_name : Independent variable (e.g. "theta3_lab", "theta4_lab", "theta_cm", "cos_theta_cm")
  • angle_value : Value at which to evaluate (degrees for angles by default)
  • angle_unit : Unit for angle_value — "deg" (default), "rad", "mrad"
  • energy_unit : Unit for beam_energy — "keV", "MeV" (default), "GeV", "TeV"

energy_unit also governs energy3_lab, energy4_lab, momentum3_lab, and momentum4_lab in the result, not just the input beam energy. For example, to evaluate at a beam energy given in keV:

r = rxn.kinematics_at_beam_energy_and_angle(1200, "theta3_lab", 122, energy_unit="keV")
# energy3_lab, energy4_lab, momentum3_lab, momentum4_lab are all in keV here