In-house development of radiation monitor using dual phosphor

Full text: Conventionally, α and β, γ monitoring is done using two separate counting systems in which ZnS (Ag) and GM tubes are used as detectors respectively. A single monitoring system which can detect both α and β, γ simultaneously would be an ideal one. Plastic scintillator sandwiched with ZnS (Ag) can be used for simultaneous detection. With this in view, an attempt was made to develop such an integrated system indigenously. The two detectors in question, with equal size, are optically coupled to a PM tube and introduced into one of the existing commercially procured hand contamination monitors. The output signals from the PM tube were analyzed using the indigenously developed pulse discriminating logic circuit. Different electronic circuit configurations on trial and error basis were designed and analyzed to a) improve the overall efficiency, b) to reduce the cross-talk and, above all c) to improve the signal to noise ratio to achieve lower MDA value. After arriving at the optimum discriminating logic and delay-time triggering logic, the cross-talk has been reduced to < 7 % for alphas in beta channel which is comparable with the commercial system available in abroad and within the acceptable cross-talk of 10%. On the basis of both percentage efficiency and MDA considerations, the optimum thickness of plastic scintillator was found to be 0.5 mm. If one compares with the currently available Indian make conventional instruments, the efficiency factor for alpha is comparable and it is far superior for betas; for medium energy betas (0.7 MeV), the system has five times better efficiency. In fact, the beta particles having very low energy (0.156 MeV) can also be detected. The shape and size of the dual phosphor ensures uniform source response. MDA values for alpha particles in both the newly developed dual phosphor system and the existing one are less than one Bq. The MDA value for alpha is marginally higher. However, this is insignificant when one compares with the Derived Working Level (DWL) value of 250 Bq for radioactive contamination in hands for alpha. For beta, the MDA values are far lower for the dual phosphor based hand-contamination-monitor. The newly developed dual phosphor based proto type integrated monitoring system for both alphas and beta-gammas is cost-effective and also reduces a) the surveillance and maintenance load, b) time taken for frisking by occupational workers, and c) reduces the space requirement. Such a dual phosphor technology can further be extended to other installed radiation monitors and counting systems as well