<?xml version='1.0' encoding='utf-8'?>
<model>
  <materials>
  </materials>
  <geometry>
    <cell id="1" material="void" region="-1" universe="1"/>
    <cell id="2" material="void" region="1 -2" universe="1"/>
    <cell id="3" material="void" region="2 -3" universe="1"/>
    <cell id="4" material="void" region="3 -4" universe="1"/>
    <surface coeffs="0.0 0.0 0.0 1.0" id="1" type="sphere"/>
    <surface coeffs="0.0 0.0 0.0 2.0" id="2" type="sphere"/>
    <surface coeffs="0.0 0.0 0.0 2.5" id="3" type="sphere"/>
    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 4.0" id="4" type="sphere"/>
  </geometry>
  <settings>
    <run_mode>fixed source</run_mode>
    <particles>1000</particles>
    <batches>10</batches>
    <source particle="neutron" strength="1.0" type="independent">
      <space type="point">
        <parameters>0 0 0</parameters>
      </space>
    </source>
    <surf_source_write>
      <surface_ids>1</surface_ids>
      <max_particles>1000</max_particles>
    </surf_source_write>
    <seed>1</seed>
  </settings>
  <tallies>
    <filter id="1" type="cell">
      <bins>3</bins>
    </filter>
    <tally id="1">
      <filters>1</filters>
      <scores>flux</scores>
    </tally>
  </tallies>
</model>
