1. A radiographic image detection device comprising:a sensor panel in which pixels that are sensitive to radiation which has been emitted from a radiation generation apparatus and transmitted through a subject and accumulate charge are two-dimensionally arranged and a plurality of signal lines for reading the charge are arranged;
a signal processing circuit that reads an analog voltage signal corresponding to the charge from the pixel through the signal line to perform signal processing;
a plurality of AD converters that are included in the signal processing circuit, perform an AD conversion process of converting the analog voltage signal into a digital signal corresponding to a voltage value, and share the AD conversion process performed for each of the signal lines; and
a processor configured to control the signal processing circuit such that an irradiation start detection operation and an image reading operation are performed,
wherein the irradiation start detection operation reads the charge from the pixel through the signal line before the emission of the radiation starts and detects the start of the emission of the radiation on the basis of the digital signal corresponding to the read charge,
the image reading operation reads the charge from the pixel through the signal line after a pixel charge accumulation period for which the charge is accumulated in the pixel elapses from the start of the emission of the radiation and outputs a radiographic image which is indicated by the digital signal corresponding to the read charge and is provided for diagnosis,
the processor has a function of switching a power supply state to the AD converter between a first state in which first power is supplied and a second state in which second power that is lower than the first power per unit time is supplied, and
in the irradiation start detection operation, the processor periodically switches the power supply state of at least one of the plurality of AD converters such that the number of AD converters in the first state per unit time is less than that in the image reading operation.