Neutron Detection

Neutrons are neutral particles, making them incredibly difficult to detect compared to the more common types of radiation. With these common types, you rely on the large electron clouds of atoms to absorb the radiation. The only thing that stops neutrons are direct collisions with the tiny nucleus of the atom.

Neutron Detectors

Since neutrons are so difficult to detect directly, most neutron detectors use indirect methods. Many of these detectors use either Helium-3 or Boron Trifluoride (BF3) as the working gas. Both of these gasses have a high neutron capture cross-section and generate detectable products. One of the downside of these most common detectors are that they can only detect slow (thermal) neutrons. The energy of a neutron can be described by a “temperature”, where the thermal neutrons have an energy around 0.025 eV, or around room temperature. The neutrons generated in a D-D fusion reaction are about 2.45 MeV. The gas detectors can still sometimes detect these fast neutrons, but their efficiency is greatly reduced. There are some fast neutron detectors that use a scintillation plastic, but these have an efficiency around 1%. Amateur fusion does not have a high neutron output, so it would be difficult to detect fusion with this.

It is therefore necessary to slow down the fast fusion neutrons, also called moderating. This is why neutron detectors are bulky, as the relatively small tubes need a large moderator for fast neutrons. Because neutrons only interact with matter by hitting the nucleus of an atom, macroscopic Newtonian physics can be used to determine the best atoms to slow down neutrons. These are really just elastic and inelastic collisions, and as such the best mass to absorb the energy is the same mass as the incoming particle. Neutrons and protons have approximately the same mass, so the best way to slow down or absorb neutrons are single protons, or Hydrogen. For this reason, neutron moderators are normally made of water or plastic with long hydrocarbon molecules.

There really is not much reason for the layman to buy a neutron detector other than this niche hobby. Because of that, the only commercial systems are marketed towards research labs and are extremely expensive. After attempting to cobble together a working system from 1960’s parts, I came across a professional neutron detection system with a reasonable price. This system uses a Russian SI-19N He-3 Corona tube as the detector, with a stated efficiency of 47 CPS/nv. These tubes are infamous for their sensitivity to EMI, but I contacted the owner of the business and he worked on a brand-new noise-resistant system.

When I received the system, I went through a series of tests that I became too familiar with when attempting to build my own. I let the detector run for one hour in different configurations. With and without the source present and with and without the moderator. These four tests confirmed the efficiency and the sensitivity to thermal neutrons. I also tested it for gamma rejection performance and EMI resistance when right next to an energized Tesla Coil. It proved to be a great system for my needs. Due to the properties of the tube, it is a proportional counter as well, so I can generate a neutron spectrum. These tests can be seen below.

One other type that is sometimes used for amateur fusion is called a bubble dosimeter that uses a super-heated gel that creates a bubble when exposed to neutrons. This is a very useful tool for determining neutron dose rate and is unaffected by electrical noise, but it is expensive and has a limited lifetime.

Neutron Moderator Build

I chose to build my neutron moderator out of readily available metal cans and paraffin wax. This wax is easily melted and cast into whatever shape you want, which cannot be done with the other popular moderator, HDPE. Paraffin’s flammability is a safety concern, but this can be significantly reduced by ensuring it is in a temperature controlled environment and sealing it in the moderator block.

The metal cans are screwed together using 3D printed brackets and then joined together using high strength epoxy. The paraffin is melted and poured into the cans with a guide tube in the center for the detector. This ensures that the detector is surrounded by at least 2.5cm of wax, which is optimal for this neutron energy. The metal can also has the added benefit of becoming a Faraday Cage when connected to the system ground to help with EMI resistance.