Diamond Detectors for Alpha Monitoring in Corrosive Media for Nuclear Fuel Assembly Reprocessing
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Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
short range of alpha particles in the liquid phase, (e.g., < 30 prm in concentrated HNO 3 [8]), techniques generally rely on the evaporation of liquid samples and then consist of the measurement on condensed solid materials. Also, it is clear that the measurement implies the handling, transportation, processing of the radioactive liquid sample and therefore is time consuming and expensive. As such, it comes that the fabrication of devices that could simply be immersed in the liquid media and particularly when they are corrosive can be extremely attractive for real time process control. Diamond is the unique candidate for such an application since it is known to combine all properties required. DIAMOND SYNTHESIS AND DEVICE MANUFACTURING Large area (0 5 cm) diamond layers were grown on silicon from the plasma assisted chemical vapour deposition technique. From the dissociation of a gaseous precursor mixture consisting of methane diluted in hydrogen, it is possible to grow at high temperatures (typ. 800°C) a polycristalline material on carbide forming materials such as silicon. Of the commonly used energy source applied to the gaseous mixture, microwave excitation is known to result in the production of materials with the best electronic properties and here a 2.45 GHz source was used to enable the growth of diamond on 5 cm in diameter silicon substrates. The material obtained has a polycrystalline structure with grain size of the order of 10% of the layer thickness, and Raman analysis showed one intense peak at 1332 cmI and no non-diamond carbon structure. We have grown diamond using optimised growth conditions in order to obtain material with good electronic properties, that enable its use for detection applications. Typical growth rates are in the order of 0.5pm/h for good quality films exhibiting the best electronic properties and detection performances. After growth, a series of annealing steps and chemical treatments are performed in order to significantly improve the properties of the films [9].
Fig. 1. (right) Photo of two detection devices that
enable alpha activity measurements in corrosive, liquid solutions. The device consists of a 20 pim thick CVD diamond layer, that is in direct contact with the active media. Alpha particles emitted in the vicinity of the diamond surface are detected in order to determine the activity of the solution.
Figure 1 shows a photo of detection devices, with a schematic representation of the configuration set-up. The active part consists of a ionisation chamber formed from the resistive volume of the intrinsic diamond that is polarised through electrical contacts. The device is mounted in an electrically polished stainless steel 304 case, and the diamond layer is glued using a corrosion resistant component that renders the seal between the case and the diamond layer leak proof. The silicon substrate onto which was grown the active layer forms the back contact of the detection device, and is used to br
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