Answer to Question #5087 Submitted to "Ask the Experts"

We have recently developed a CCTV (closed-circuit television) camera that we have shielded with the appropriate amount of tungsten to produce the half value that will allow the camera unit to operate up to 3 x 10⁵ rad TID (total integrated dose). We are now experiencing what we believe is neutron damage while these cameras are in service. What material can we use and what sort of thickness would we need to shield this unit from the neutron flux produced by ⁶⁰Co in a nuclear plant? Dose rates are relatively low, but are damaging enough to impair the image on our CCD (charge-coupled device) cameras.

You can shield neutrons with a material high in hydrogen, such as polyethylene or water-extended polyester. If there are thermal neutrons present, a material with a large absorption cross section for thermal neutrons, like cadmium metal, can be used to absorb these neutrons. The thickness of the selected neutron shield depends upon the density of the shield material, its chemical composition, and the number of neutrons hitting the camera per second (neutron flux). The required thickness of the neutron shield depends upon the numeric value of the neutron flux and upon the desired level of impaired image that can be tolerated for the camera. Neutrons essentially only come from the nuclear reactor's fuel while the plant is generating power, and neutrons are not produced from the ⁶⁰Co in the reactor. Cobalt-60 only produces very energetic gamma rays that could affect the camera's image. You may need to consider, during reactor operations, the ¹⁶N gammas coming from the neutron activation of the oxygen in the reactor coolant.

One possible way to determine if there has been neutron exposure (and possible damage) to the camera is to perform a gamma-spectrum analysis of a camera used inside the containment of an operating reactor. This should identify any significant radionuclides that were produced by the neutrons, which may be correlated back to the value of the neutron exposure.

John P. Hageman, MS, CHP