Prompt Fission Neutrons from Pu-239

A beam of fast neutrons directed at a target of fissile material can initiate a fission reaction that produces not only a pair of daughter nuclei, but also several fast neutrons which are emitted at the time of the nuclear scission. The release of multiple prompt neutrons is then key to the initiation and maintenance of a chain reaction in a weapon or in a nuclear reactor. The probability for a fission reaction to occur depends upon the energy of the beam neutron, and therefore the probability for a chain reaction to be maintained, and the detailed characteristics of that reaction, are dependent on the spectrum of the promptly emitted fission neutrons. We will use a large array of scintillation detectors to measure the spectrum of neutrons emitted in the neutron-induced fission of a Pu-239 target sample. The neutron beam is produced at the LANSCE/WNR facility in a spallation reaction using a pulsed, 800 MeV proton beam. Spallation neutrons are produced over a range of energies extending from 1 to several hundred MeV and collimated into a beam of approximately 1” diameter. The Pu sample is contained in a 10-layer chamber which provides both pulse height and timing information derived from the fission fragments. The measured time of the fragment release relative to the proton pulse on the spallation target is used to determine the energy of the beam neutron which initiated the fission event. The fragment time is also used as the start signal for a measurement of the time required by the prompt fission neutron to arrive at our neutron detectors. This time-of-flight is used to determine the energy of each detected fission neutron. Our detector array consists of an array of 16 scintillator bars, each of 10 cm x 10 cm cross section, and 200 cm length. Photomultiplier tubes at each end record the pulse height and time of the neutron interaction in the scintillator, and this information is used to both determine the location and flight time of the fission neutrons, as well as to reject background events. The large solid angle acceptance of the array, coupled with its high efficiency for detecting neutrons in the energy range from 2 to 10 MeV, provide a good acceptance for the relatively weak neutron yields near 10 MeV.