Fast shutter for low background radiation environments at SPR III

A fast shutter system has been developed to isolate experiments from the radiation emitted after the main pulse of the Sandia Pulsed Reactor, SPR III. In about 15 ms the system moves a 20 kg shield in front of the beam port aperture in the facility shield wall. A second gravity-driven shutter is dropped into place in about 200 ms. The total attenuation achieved in the delayed beam intensity is about a factor of one hundred. The shutter assembly was used, along with a sensitive detection system, to observe the gamma ray flux emitted by various materials activated by the neutron beam at levels nine orders of magnitude smaller than the gamma dose rate at the target during the reactor pulse.<>