<?xml version="1.0"?>
<geometry>

  <surface id="1" type="sphere" coeffs="0 0 0 3"/>
  <surface id="2" type="sphere" coeffs="0 0 0 6"/>
  <surface id="3" type="sphere" coeffs="0 0 0 9"/>
  <surface id="4" type="sphere" coeffs="0 0 0 10" boundary="vacuum"/>

  <cell id="1" material="1" region="-1" />
  <cell id="2" material="2" region="1 -2" />
  <cell id="3" material="3" region="2 -3" />
  <cell id="4" material="4" region="3 -4" />

</geometry>
