Kovash, Michael*

College of A&S, Physics Department

Group research activities 9/12/17 mak

Prompt Fission Neutron Spectra

Power production in reactors is the result of the transfer of energy from fast neutrons into a surrounding moderator. In most US reactors, the fission of U-235 is used to create the fast neutron flux, and pressurized water is used to both slow down the neutrons so as to maintain the chain reaction, and to absorb and transfer the neutron energy out of the core vessel. The fundamental reaction driving this cycle is neutron-induced fission of the uranium, which is accompanied by the subsequent release of several fast neutrons.

We are measuring the energy spectrum of the fission neutrons over the range from 3 to 10 MeV. Our experiment uses the neutron beam at the Los Alamos National Laboratory directed onto a target made of U-235, and our detectors collect both time and energy-loss data on the neutrons released from the uranium. The Kentucky detector system consists of 16 scintillator bars, each over 2 m long, arrayed over top of the target. Photomultiplier tubes record the light produced by neutron interactions in these scintillators, and these results are recorded event-by-event for later analysis. Part of the challenge of the experiment is created by the very sharp drop in fission neutron yield at high energy; the yield of 10 MeV neutrons is approximately 2 orders of magnitude smaller than at 3 MeV, and this makes it essential that we correctly account for the small unwanted backgrounds which contribute to the measured spectra.

Computer Project

Monte Carlo Simulation of the Neutron Detector Array

The acceptance of a segmented detector array to neutrons depends on a number of design factors and physical interactions at the atomic and nuclear level. For most detector systems, no analytical methods exist for accurately calculating the desired properties, and instead Monte Carlo methods must be employed. Fortunately, Monte Carlo procedures are well developed for this task, and codes exist which offer a “toolkit” of methods which can be configured to accommodate any particular system of detectors. In addition, simulated data can be used to study the correlations which occur naturally among detector elements either when multiple neutrons are emitted by the source, or when a single neutron scatters and is recorded in several detector components. All of these physical processes are included in the simulations.

We propose to use an existing simulation code to model the operation of the Kentucky array of neutron detectors which has been used to collect fission data in Los Alamos. The simulation will generate a distribution of neutrons and gamma rays at the fission source, and track each particle as it interacts in the detector and in the surroundings of the experimental hall. The result of this will be a collection of “events” which will be analyzed in the same manner as we currently analyze the event data from the in-beam experiment. Data simulated in this way will help us determine the acceptance, the correlations, and the backgrounds – information which is needed to process and correct the measured spectra.

Faculty: Michael A. Kovash

Graduate Students:

Jason McGinnis, Scott Meadows
Only these UK personnel will be involved with this project.

Simulation Code:

MCNP (Monte Carlo Neutral Particles)

Software

Publications

Grants