A thermal neutron shield comprising concrete with a high percentage of the element Boron. In one embodiment, the retaining agent comprises a primarily hydrophobic monomer or a hydrophilic monomer. The configuration of the critical components in and on the mover body permits the substitution of one critical component with another component having different operating properties without significantly changing the compactness of the mover. The annular cap is located near an end face of the monolithic catalyst carrier. In addition, the present invention further comprises a diode.