UCN-Simulations

Simulations of ultracold neutrons (UCN) using Kassiopeia, a particle tracking framework: https://github.com/KATRIN-Experiment/Kassiopeia. Fork of the modified Kassiopeia: https://github.com/Blawker/Kassiopeia.

First project: Beamline

• Initialize the geometry of a beamline structure for UCN.
• Adding surface reflection for stainless steel.
• Generate initial energy spectrum according to TRIGA neutrons distribution[1].
• Adding slits to simulation the junction between the different part that compose the beamline.
• Adding binary program to create and add output file into Root's histograms.
• Adding Shell Script to parallelized simulations[2].

Second project: Moving Mirror

• Initialize the geometry of a piston structure for UCN.

• Modify the source code of Kassiopeia to implement :

  • Moving part : surfaces, spaces, assemblies.
  • Interaction of UCN on moving surfaces[3].

• To create the motion of a part :

  • Enter the line in a config_file.xml to configure the motion :

  <ksmotion_surface_translation name="motion_surface_translation"
    surfaces="world/piston/moving_mirror/#"
    theta="[theta_motion]"
    phi="[phi_motion]"
    value_formula="[equation_motion]"
    value_min="[time_start_motion]"
    value_max="[time_end_motion]"
  />

  • Add the line below in the <ksgeo_space /> structure command to link the motion to the simulation. It is recommanded to place the line in the root of the structure often called space_world.

  <command parent="root_surface_motion" field="add_surface_motion" child="motion_surface_translation"/>

  • Then enter a moving interaction component that has the same motion has the surface. Here is an example of interaction of UCN with stainless steel :

  <ksint_moving_surface_UCN name="int_moving_surface_UCN"
    eta="2.e-4"
    alpha="5.e-6"
    real_optical_potential="2.08e-7"
    correlation_length="20.e-9"
    theta="[theta_motion]"
    phi="[phi_motion]"
    value_formula="[equation_motion]"
    value_min="[time_start_motion]"
    value_max="[time_end_motion]"
  />

  • Add the line below in the <ksgeo_space /> structure command to link the interaction to the surface.

  <geo_surface name="moving_surface_reflection" surfaces="world/piston/moving_mirror/#">
    <command parent="root_surface_interaction" field="set_surface_interaction" child="int_moving_surface_UCN"/>
  </geo_surface>

  • It is recommended to use <define /> input because there could be many components that may have the same motion :

    <!-- Equation of motion is a time-dependent position function -->
  <define name="equation_motion" value="0.05*x"/>
  <define name="time_start_motion" value="0."/> <!-- Time (s) -->
  <define name="time_end_motion" value="10."/> <!-- Time (s) -->
  <define name="theta_motion" value="90."/> <!-- Angle (°) -->
  <define name="phi_motion" value="180."/> <!-- Angle (°) -->

• A basic simulation configuration can be found in the fork folder called PistonSimulation.xml : https://github.com/Blawker/Kassiopeia/blob/master/Kassiopeia/XML/Examples/PistonSimulation.xml

• The following file gives an example of configuration with two independent moving part (piston + shutter): https://github.com/Blawker/UCN-Simulations/blob/master/config/Piston/tSPECT_piston_shutter_TRIGA_auto.xml

Bibliography

1 I. Altarev et al. Neutron velocity distribution from a superthermal solid 2H2 ultracold neutron source. The European Physical Journal A, 37: 9-14 July 2008.

2 GNU Parallel, https://www.gnu.org/software/parallel/

3 Z. Bogorad Ultracold Neutron Storage Simulation Using the Kassiopeia Software Package. Massachusetts Institute of Technology 2019, June 2019.