In this paper, we determine the influence of high-energy photon beam irradiation used for external radiotherapy on pacemakers, at different doses and dose rates. Ninety-six pacemakers of various origins and ages underwent in vitro high-energy photon irradiation under technical conditions close to external radiotherapy, using a linear accelerator delivering photons of high energy (18 MV). Various dose levels (up to 200 Gy for certain particularly resistant pacemakers) were delivered to pacemakers located in a water-equivalent phantom with several dose rates (from 0.05 to 8 Gy min(-1)). Observed failures were sorted into eight classes of progressive harmfulness, some of them possibly lethal. One irradiated pacemaker exhibited an important defect at a dose rate of 0.2 Gy min(-1), for a cumulative dose of 0.15 Gy. Two pacemakers showed an important defect at a cumulative dose of 1 Gy, while nine pacemakers failed at a cumulative dose lower than or equal to 2 Gy and 13 failed at a cumulative dose lower than or equal to 5 Gy. The most important failure probability (70% of irradiated pacemakers) is observed for 8 Gy min(-1), whereas no pacemakers failed at a dose rate lower than or equal to 0.2 Gy min(-1). In conclusion, warnings given by manufacturers about the maximum tolerable cumulative radiation doses for safe operation of irradiated pacemakers (5 Gy), even reduced to 2 Gy, are not reliable. The spread of cumulative doses inducing failures is very large since our observations show an important failure at 0.15 Gy, while ten pacemakers withstood more than 140 Gy of cumulative dose. The safe operation of pacemakers under irradiation depends mainly on type and model. It depends also on dose rate. From our observations, for the safe operation of pacemakers, a recommendation of a maximum dose rate of 0.2 Gy min(-1) rejecting direct irradiation of the pacemaker at a standard dose rate for tumour treatment (2 Gy min(-1)) is made.

A numerical model is developed to study the cathode spot initiation of an electric arc. It takes into account the electron emission, the Nottingham effect at the surface, the vaporization of material, the ohmic heating and the thermal conduction in the electrode bulk. The main feature of this work involves considering a two-dimensional thermal structure of the electrode. The Joule effect and the phenomena in the cathode layer are taken into account. In contrast to most proposed models, the electric field and the ion current at the cathode surface are considered as two independent external parameters. This allows us to study the spot initiation under different conditions, namely for an applied electric field with different amounts of external ion flux impinging upon the cathode. As for most theoretical models, there is no direct comparison with experiments, but, under quasi-stationary conditions, our model yields results in agreement with the models found in the literature. The overheating phenomenon below the surface is observed and it can be considered as the origin of the micro-explosion which can be a possible mechanism for the crater formation on the cathode. The conditions for the appearance of the overheating phenomenon are also examined.

A fast backward-jump type arc motion is observed with an arc running on rails. It is shown by the use of conductivity probes that this motion does not correspond to a continuous displacement of an ionized high-temperature region. The model of current transfer between two parallel arcs is adopted to explain the jump motion, for which a high rising speed of the arc voltage is a decisive factor. The current transfer is considered to be different from the re-ignition of a post-arc current.