Experiment Design Tips
Choice of materials for reactor-based experiments:
Because bombardment of experiments with neutrons and gamma radiation results in activation of the experiment and weakening of some materials, proper choice of the materials is important to ensure that the experiment will endure the irradiation and can be handled in a reasonable time frame after. Material choice is also important to ensure that the experiment ultimately can be either disposed of or shipped back to the experimenter, provided the user's radioactive materials license allows possession. Below is some useful information for experimenters for choice of materials.
· Materials made from C, O, N, H, and Si have minimal activation, so materials such as graphite, some plastics and phenolics, and some ceramics are good choices. Specifically:
Graphite has very minimal activation and has good radiation tolerance, but it is brittle and difficult to work with. Some machine shops will not work with it because of the mess made when machining.
Ceramics made from silica and alumina have good temperature properties, activation properties, and radiation tolerance, but they are also brittle. Pure quartz is very good activation-wise, but borosilicate glass should be avoided, as it will significantly degrade the neutron flux.
Plastics consisting of H, C, O, and N are great from an activation standpoint, but they will degrade under high doses of gamma radiation. Polyethylene (HDPE, LDPE) is a good choice with respect to activation, as is polyamide (Nylon) and polycarbonate (Lexan).
Fluoropolymers such as PTFE (Teflon) should NOT be used, as they contain a significant amount of fluorine. This fluorine can be liberated from the material when irradiated, and the liberated fluorine can then attack nearby metals in the experiment. PTFE will also structurally degrade from fairly low doses of radiation.
PVC should also NOT be used because of its high content of chlorine, which will activate significantly.
Phenolics (garolites) have much higher radiation tolerance than other plastics but typically activate more. However, when more radiation-tolerant material is needed, this is sometimes a good compromise. The constituents of phenolic can vary, so when possible, it is desirable to determine what is in a given phenolic before use. Phenolics such as FR-4, which has bromine as a fire retardant, should be avoided because it will activate significantly.
If tape is needed for an experiment, polyimide (Kapton) tape is a good choice, as polyimide has both low activation and very good radiation tolerance for a plastic. The tape adhesive will likely degrade before the polyimide.
A PDF with info regarding the radiation tolerance of various plastics was put together by Nordion, and it can be found at: https://www.nordion.com/wp-content/uploads/2014/10/GT_Gamma_Compatible_M...
· Often, the materials discussed above are not appropriate for the structural components of an experiment rig, so metals need to be used. Common metals have good radiation tolerance but many activate significantly. Information to consider includes:
Aluminum significantly activates, but the three primary activation products have reasonably short half-lives, making it the best choice for most experiments. It is lightweight and has good strength, but it has a low melting point of 660 °C compared to other common metals. Alloys from the 1000 series are the best regarding activation, as they are comprised of almost all aluminum. However, they are soft, and they are hard to find in many types of stock material. Alloys from the 6000 series, particularly Al-6061, have the best compromise of activation, strength, and availability. Al-6061 is the most commonly used alloy for experiments at the NRL. The 3000 series of aluminum is not as good as Al-6061, but it is acceptable for some uses, and the 7000 series should be avoided.
If a high-temperature material is needed, titanium is the next best choice for rig structural material. It will activate significantly more than aluminum, but less than metals such as iron, nickel and chrome. The activation products include Sc-46, which comes from a fast-neutron reaction with Ti-46 and has a half-life of about 84 days.
Steel alloys and other alloys comprised of Fe, Ni, and Cr are best avoided, as an experiment made using them cannot be handled for quite some time after the experiment (possibly years). If use of a steel alloy is absolutely unavoidable, then a ferritic stainless steel, which has no nickel, is the least objectionable choice. Because of the activation products from Fe and Cr, an experiment rig made using a ferritic stainless steel may need to cool for a few years before it can be disposed of. However, a rig made using a nickel-containing steel may need decades before it can be disposed of, due to production of Co-60 from the Ni.
The information above gives some guidance for materials to consider for creating an experiment, and when a user has a proposed experiment idea in mind, the laboratory staff can be engaged to discuss specifics for that experiment.