A sub-critical assembly of U235 has been used to increase by a factor of ten the neutron output of the pulsed source used for neutron time-of-flight measurements. This source consists of an electron linear accelerator producing a pulsed beam of high energy electrons which are then used to generate neutrons by x-ray production followed by photonuclear processes in a heavy element target. This paper describes some of the physics and engineering problems that had to be solved before a practical multiplying assembly could be built.