In several fields of the nuclear industry, high fluxes of gamma radiations have to be measured. The small size of semiconductor detectors would be a good approach; however, in general degraded by the radiations. To overcome these difficulties, highly radiation-resistant silicon detectors have been developed for the measurements of high fluxes of gamma-rays, as encountered for example during in situ measurements in reactors. The detector is mounted in a waterproof 8.0 mm diameter probe, incorporating also all the electronics. The system can operate in two modes: either as counter of individual pulses generated by the radiations, or direct measurement of the induced d.c. current. In the first situation, the sensor operates as photodiode, whereas in the second case, photovoltaics mode is selected, i.e. without any applied bias voltage. When operating in the counter mode, a linear response over eight orders of magnitude of the flux can be measured, in the photovoltaics mode, the dark current limits the lower flux to values of about 5- 10.10-2 Gyh-1.