Progress on Shielding

Dr. Subramanian and Mr. Lerner have been working to design and verify the shielding for our facility.

While we will not be producing a great deal of neutron or gamma-ray radiation, shielding is required to dilute the little we produce down to background levels. OSHA requirements are about 10 times background, but we will be aiming for only background levels, and should reach that.  We will be producing the most neutrons with deuterium fuel, which we will be using in the early part of the experiment. However, even pB11 produces some neutrons (1/500th of the total energy) via a secondary reaction.

Shielding for gamma ray photons is fairly straightforward, as photons travel in straight lines, and when they are scattered by electrons lose much of their energy. However, neutrons, which scatter off the nuclei of atoms, can retain most of their energy after scattering. So a shielding design must account for scattering that allow neutrons to bounce around obstacles.

To test our shielding, we have bought the MCNP program from Oak Ridge National Laboratory. This is a Monte Carlo program that simulates the scattering of millions of neutrons or other particles. (The Monte Carlo simulation technique uses random numbers to model processes that involve elements of chance, and is named after the famous casino.)  LPP is using the program to test Dr. Subramanian’s designs and see if they reach our safety goals. This analysis will also be used to obtain safety permits from OSHA and the NJ Department of Environmental Protection.

Currently we are looking at a design for full floor-to-ceiling shielding walls, but not overhead shielding.  This will allow only a very small channel, within the ceiling material, for scattered neutrons to leak out, and then only after many scatterings. We will also look at alternatives that include removable close-in shielding around the DPF and a small overhead shield attached to the device.

While we will be producing far less radiation (pulsing once every ten minutes) than a Focus Fusion generator would (pulsing 500 times a second), we will still need nearly the same thickness of shielding (2-3 feet). This is because the main purpose of the shielding is too slow the neutrons down.  The cross section for absorption—how likely it is for a neutron to be absorbed by a nucleus—goes up as the energy of the neutron goes down. Once they are moving slowly, the neutrons are easily captured, like tired ponies, by boron-10 nuclei in the shielding material. Since the thickness of shielding needed for moderation, or slowing-down, depends only on the energy of the individual neutrons, not on how many there are, about the same thickness is needed for our experimental device as for a full-blown generator. In a generator, however, all the shielding would be very close to the vacuum chamber, while in our experimental device, it is better to have free access to the vacuum chamber and but the bulk of the shielding further off, even though this makes for a much greater bulk of shielding.