A detector comprising at least one detector element, the at least one detector element comprising: a photodiode element (310PD; 410PD) operable to generate charge carriers in response to incident radiation, the detector being configured to generate a signal indicative of a cumulative amount of charge carriers generated by the photodiode element during an integration period; a reset switch element (321; 421) configured to couple the photodiode element to a reset potential; a charge storage reservoir (325C; 425C2); and a gain control switch element (325; 425-2) configured to couple the charge storage reservoir to the photodiode element, wherein the gain control switch element (325; 425-2) is provided in a flow path of current between the reset switch element and the photodiode element, wherein the detector is configured to operate in a selected one of a high gain mode and a low gain mode, wherein in each mode the detector is configured to perform a reset operation in which the reset switch element and the gain control switch element are closed in order to couple the charge storage reservoir and the photodiode element to the reset potential, the detector being configured to open the gain control switch element (325; 425-2) following a reset operation in dependence on whether the charge storage reservoir is to remain connected to the photodiode element (310PD; 410PD) during the integration period, wherein the detector is configured to open the gain control switch element (325; 425-2) when the detector is operated in the high gain mode and wherein the detector is configured to cause the gain control switch element (325; 425-2) to remain closed when the detector is operated in the low gain mode, and wherein the detector is configured to cause the reset switch element to remain closed throughout operation in the high gain mode. a photodiode element (310PD; 410PD) operable to generate charge carriers in response to incident radiation, the detector being configured to generate a signal indicative of a cumulative amount of charge carriers generated by the photodiode element during an integration period; a reset switch element (321; 421) configured to couple the photodiode element to a reset potential; a charge storage reservoir (325C; 425C2); and a gain control switch element (325; 425-2) configured to couple the charge storage reservoir to the photodiode element, wherein the gain control switch element (325; 425-2) is provided in a flow path of current between the reset switch element and the photodiode element, wherein the detector is configured to operate in a selected one of a high gain mode and a low gain mode, wherein in each mode the detector is configured to perform a reset operation in which the reset switch element and the gain control switch element are closed in order to couple the charge storage reservoir and the photodiode element to the reset potential, the detector being configured to open the gain control switch element (325; 425-2) following a reset operation in dependence on whether the charge storage reservoir is to remain connected to the photodiode element (310PD; 410PD) during the integration period, wherein the detector is configured to open the gain control switch element (325; 425-2) when the detector is operated in the high gain mode and wherein the detector is configured to cause the gain control switch element (325; 425-2) to remain closed when the detector is operated in the low gain mode, and wherein the detector is configured to cause the reset switch element to remain closed throughout operation in the high gain mode.