Eva Wilcox and Dr. Lawrence Rees, Physics and Astronomy
A neutron detector, made by Bart Czirr, has components of plastic and a new compound called lithium gadolinium borate, with a chemical formula of LiGd(BO3)3:Ce3+. The lithium in this detector is responsible for capturing neutrons that have collided with protons in the detector, making them slow down. Lithium is more likely to capture neutrons that have a small kinetic energy. When neutrons are captured by the lithium nucleus, the nucleus decays and causes scintillation or the production of light as energy is given to a scintillating cerium atom. Monte Carlo Neutron Particle (MCNP) calculations  are necessary to find the intrinsic detector efficiency due to the lithium capture of neutrons, which can then be used for calibration of the detector.
In finding the efficiency we used MCNP code  to model the detector and find the efficiency as a function of neutron energy. MCNP is a computer code that allows input of the system geometry and materials (plastic and lithium), radiation source and geometry, and the binning of the results as events per set time or energy. We had a certain number of neutrons of a given energy aimed at the detector alone and coming from a point source. We summed lithium capture events over time, having each MCNP run set to different energies. We looked at the number of neutrons that captured in lithium out of the total number of neutrons, and plotted the results as a function of energy. We then fit a curve to the calculated data.
As can be seen in Figure 1, the detector efficiency drops off exponentially with energy. This is because lithium adsorbs neutrons more readily when they have less energy. In our calibration of the detector we are using californium 252, a fission nuclide. The energy response we get from our detector must be divided by the detector efficiency to obtain a more true representation of the californium fission spectrum, which is a function of energy. The high neutron energies are amplified in doing this, while those more readily captured low energies are subdued. The result is a detector response closer to that of the californium fission spectrum.
- Los Alamos National Laboratory, “Documentation for CCC-200/MCNP Code Package,” MCNP: Monte Carlo Neutron and Photon Transport Code System 3.A, Los Alamos, NM, 1983.