LNF:Technology: Difference between revisions

The routine gamma dosimetry is performed using the commercially available Red Perspex™ 4034 Harwell dosimeters, they are polymethylmethacrylate (PMMA) dosimeters that have become widely used in routine high-dose rate dosimetry in the field of the industrial radiation processing. When exposed to ionising doses in excess of 1 kGy, the Red 4034 polymer starts to darken due to the formation of a new absorption band peaking at 615 nm and extending from 600 nm to beyond 700 nm. The absorbed dose is therefore determined by measuring its radiation-induced absorbance in the 630nm-650 nm range where post-irradiation fading is minimal. 640 nm wavelength is used as read-out wavelength. The absorbance per unit thickness, expressed in cm−1, is the dose-dependent quantity measured against air as reference. These dosimeters has been shown to be valid in the range of 5 to 50 kGy and its accuracy is better than 10 per cent. The Red 4034 dosimeters are pre-conditioned in a fixed humidity environment and hermetically sealed in polyester/aluminium foil/polyethylene laminate pouches, as absorbed water concentration was identified as a parameter which could influence the spectrophotometric response and hence the dose readout. Keeping the dosimeter in its packaging is mandatory to use the calibration curves (absorbance at 640nm (cm-1) related to dose) supplied by the manufacturer. The temperature sensitivity of the dosimeter is the most important environmental dependance. From a practical point of view, the Red 4034 dosimeters are temperature-independent up to 40 ºC, provided that the measurements are made shortly after the end of the irradiation

These dosimeters have been also used to measure gamma dose in BR1 graphite-moderated,air-cooled nuclear reactor (SCK·CEN MOL), in mixed gamma neutron fields at a temperature below 40º.  

A gamma dosimetry before to start the irradiation test is always carried out.