Specifications for instruments exposed to gamma irradiation in nuclear environments incorporate a figure for the summation of the total dose that the device must survive whilst retaining an acceptable degree of functionality. Cobalt 60 (1xMGray) was the exposure level proposed for a unique and versatile zoom lens to be designed and built by Abakus Scientific at the request of Raditec, the A/V systems, environment monitoring specialists. Raditec have world-class expertise in the supply of nuclear radiation tolerant, image acquisition systems including television cameras, endoscopes, borescopes and accessories for use in nuclear facilities. Raditec is part of the Tecnovex Group Ltd and is a commercial organisation following on from the acquisition, in 2002, of what was previously a division of the Atomic Energy Authority. In this case they got together with Abakus Scientific of Carlby, near Stamford in Lincolnshire. Abakus Scientific supply the nuclear tolerant zoom exclusively to Raditec, but they are aware that Raditec have supplied quite a number of these UK designed and built products as part of systems installed in Japan.
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So why brass for gamma?
Brass, unlike plastic, is unaffected for all practical purposes by extended exposure to a nuclear environment. The plastic material Teflon, for example, is one of the worst materials for nuclear applications and produces fluorine when irradiated. Brass by contrast produces no harmful or hazardous chemical by-products. In the case of this nuclear tolerant zoom lens, (tolerant is used here as the preferred term rather than nuclear resistant, since the construction and functional materials cannot avoid exposure but must however exhibit the optimum tolerance), the application could be typically a camera system for the inspection of fuel rods in nuclear reprocessing. In this environment any component failure is potentially hazardous and very expensive so it is essential that the materials used give the product compatibility with the environment and a long and trouble free life. Brass is used extensively in these nuclear tolerant units.
The specification for the lens mounting and carrier system for such a high accuracy product requires it to be made from a stable, corrosion resistant material that can be machined to extremely close tolerances with a high degree of repeatability, giving a very fine surface finish using both traditional and modern CNC techniques. Brass also has good bearing properties, particularly when used in contact with other materials, also a requirement of the focussing system. Readily available from stock in a variety of sizes, brass meets all of these criteria, yielding a life of many years. Dr Kath Langley, Abakus’s Manufacturing Director, comments further on brass in the context of both this and other lens products: "We are aware of the full spectrum of materials available to us in lens body construction and we regularly subject alternative materials to life trials. In the case of the ‘discrete’ lens shown bottom right in the composite picture, the tiny brass gear wheels and the toothed ring are critical to the zoom function. We first tried using plastic for the ring and in tests it exhibited severe distortion in a short time, becoming almost a soft cornered, triangular shape.
The second trial used steel and rapidly displayed tiny traces of corrosion impeding its function. We returned to the free-machining CZ121 brass that we use as standard in most other lenses and the product now functions perfectly and experiences no long-term deterioration. We would have liked the small weight saving potentially available with plastics, also the marginal economy of steel, but at the end of the day they were not up to meeting the specification. One novel feature of this product is the lens iris. After a serious number-crunching exercise we developed a uniquely profiled, machined miniature brass plate which revolves and controls iris aperture in a ‘diameter-linear’, rather than f-stop linear, manner from full aperture through to the total blackout condition required for TV camera calibration. The smoothness of operation could not be obtained with any other material than brass, its inherent lubricity, surface to surface, eliminates the need for organic or fluorocarbon lubrication."
So how about gamma and the glass lenses? Unlike the brass structure and components of the nuclear-tolerant zoom lens, all grades of optical lens materials degrade. Dr Ken Pollitt explains: "Basically at the rate of irradiation leading to a life summation of Cobalt 60 (1xMGray), normal optical glasses go progressively black and consequently light transmission drops. This is caused by dislocations occurring in the molecular structure of optical glasses caused by gamma ray damage. This can be alleviated to a degree by the inclusion of quantities of cerium oxide in the glass melt, giving the resultant lenses a yellow tint. This acts as an oxidising or bleaching agent rather than as a prophylactic, usefully extending the product life by self-healing the radiation damage. The chemistry is such, however, that the effect of the cerium oxide has a finite limit. Over the projected life of the lens elements at 1xMGray it is therefore an agreed standard in the industry that transmission will inevitably drop by approximately 30% for the cerium doped units. At Abakus Scientific we have access to all currently published R&D results from glass producers and we are able to specify the blends and grades to be used for casting our custom lenses. As new materials evolve we ride with the technology on behalf of our clients, offering only the best performing materials and designs." |
