Shielding and Radiation Safety

Another one of my interests in safety systems is radiation protection and shielding. It is commonly known that Lead shields radiation, but it is less commonly known that for beta sources, a certain amount of lead actually increases the amount of X-ray radiation from the source. This is due to bremsstrahlung or “braking” radiation, an explanation for another time. It is important to design safety systems for the required application, as there is no “one size fits all”. I do have professional experience with radiation protection as I have taken certificate classes through MSU’s NSCL¹ radiation protection training.

Radiation Safety Principles

There are two main principles for radiation protection: As Low As Reasonable Achievable (ALARA) and Time, Distance, Shielding (TDS). ALARA refers to the idea to limit radiation exposure to the lowest amounts without becoming impractical. You could work with radioisotopes wearing a 200kg lead suit, but you would be spending more time near the source because of how slow you would move. It is not requiring a specific action, but it is a principle to keep in mind when working in a radiation field. TDS is one of the most easily implemented measures for reducing radiation exposure. Radiation exposure is cumulative and isotope sources are constantly emitting radiation. By decreasing the amount of time you spend in a radiation field, you lower your absorbed dose. Radiation fields from a point source, similar to electric and gravitational fields, follow the inverse-square law, where dose rates fall with distance as 1/r² . This means if you are able to double the distance between you and a source, the dose rate decreased by 4 times. If you triple the distance, it falls by 9 times, so on and so forth. Extreme reductions in dose rate can be achieved just by maximizing the distance between you and a source. The last part of TDS, shielding, is what people likely think of first when thinking about radiation protection. Generally, the more mass you can put between yourself and a source of radiation, the less radiation make it to you. The main reason Lead is used for photon (Gamma, X-ray) shielding is that it is so dense that you can have the same amount of mass in a much smaller shield. There are even cases for extremely radioactive sources where Uranium is used as a shield because it is even denser than Lead. There are different ways to shield different types of radiation, but generally lead does the job well. The TDS principle is all about finding ways to decrease your exposure to radiation with fundamental knowledge.

Fusor Radiation

Even without fusion reactions occurring, the nature of having a very high voltage in a vacuum chamber means that X-ray radiation will be a concern. The accelerated electrons in the chamber hit the walls and can generate X-rays. This is normally not a concern for demonstration fusors, as the voltage is low enough that any X-rays are captured by the walls of the chamber. This does become a problem above around 50-60kV, where the steel walls become more radiotransparent to X-rays. At this point, shielding needs to become a major design consideration. Either you can shield the entire fusor, or you can operate it remotely.

I chose to operate the fusor locally because I am confident in my ability to shield critical areas, and my fusor only operates around 35kV where most of the X-rays are absorbed by the chamber walls. One of the issues is that there are two main parts of the fusor that are not stainless steel: the viewport and the HV feedthrough. Because they are not steel, they let most of the X-rays generated on their surfaces to escape. Thankfully these X-rays have absolute maximum energy of 35keV, which can easily be shielded by 0.25mm lead sheets. I placed these around the viewport and within line-of-sight to the HV feedthrough stalk. At these voltages, scattering is not a major concern but with much higher voltage systems it becomes necessary to enclose the entire chamber in lead.

Neutrons are generally much more harmful to humans than photon radiation. There is a measure of harmfulness called the quality factor of the “Q” factor that compares different types of radiation. It is different across the energy spectrum of the types of radiation, but neutron radiation is commonly quoted to be 20 times more harmful than photon radiation. Although this is the case, neutron radiation is not a concern in most amateur fusors. With my system, I couldn’t hope for more than 1×10⁵ neutrons/second isotropic at 2.45MeV. The operating station is placed about 6 feet away from the center of the fusor, which is plenty of distance. If I were to increase my power, I would need to consider remote operation.

1. Michigan State University (MSU) National Superconducting Cyclotron Laboratory (NSCL) ↩︎