A photon counting detector, comprising: a photon conversion assembly comprising at least one first die that is configured to directly convert incoming X-ray photons into electrical signals; a carrier on or in which a power supply line is arranged that is configured for transferring a supply voltage; and at least one read-out integrated circuit, 'ROIC', die, each ROIC die comprising: a conductive substrate; a plurality of pixels each comprising processing circuitry for processing an electrical signal received from the photon conversion assembly; and peripheral circuitry from which the supply voltage is provided to the processing circuitry of the pixels of that ROIC die, wherein the photon conversion assembly is mounted on the at least one ROIC die, and wherein the at least one ROIC die is mounted on the carrier, characterized in that the photon counting detector comprises an electrical ground plane arranged at or near an interface between the carrier and the at least one ROIC die, and in that each ROIC die comprises an extension region that laterally extends beyond the photon conversion assembly, wherein the peripheral circuitry for a given ROIC die is arranged in the extension region of that ROIC die, and in that the photon counting detector comprises at least one electrical connection that connects the power supply line to the peripheral circuitry of the at least one ROIC die for the purpose of transferring the supply voltage from the power supply line to said peripheral circuitry, and in that the electrical ground plane is configured for carrying a return current that is associated with the supply voltage supplied to the peripheral circuitry of the at least one ROIC die through the at least one electrical connection and that flows through the corresponding conductive substrate(s). a photon conversion assembly comprising at least one first die that is configured to directly convert incoming X-ray photons into electrical signals; a carrier on or in which a power supply line is arranged that is configured for transferring a supply voltage; and at least one read-out integrated circuit, 'ROIC', die, each ROIC die comprising: a conductive substrate; a plurality of pixels each comprising processing circuitry for processing an electrical signal received from the photon conversion assembly; and peripheral circuitry from which the supply voltage is provided to the processing circuitry of the pixels of that ROIC die, a conductive substrate; a plurality of pixels each comprising processing circuitry for processing an electrical signal received from the photon conversion assembly; and peripheral circuitry from which the supply voltage is provided to the processing circuitry of the pixels of that ROIC die, wherein the photon conversion assembly is mounted on the at least one ROIC die, and wherein the at least one ROIC die is mounted on the carrier, characterized in that the photon counting detector comprises an electrical ground plane arranged at or near an interface between the carrier and the at least one ROIC die, and in that each ROIC die comprises an extension region that laterally extends beyond the photon conversion assembly, wherein the peripheral circuitry for a given ROIC die is arranged in the extension region of that ROIC die, and in that the photon counting detector comprises at least one electrical connection that connects the power supply line to the peripheral circuitry of the at least one ROIC die for the purpose of transferring the supply voltage from the power supply line to said peripheral circuitry, and in that the electrical ground plane is configured for carrying a return current that is associated with the supply voltage supplied to the peripheral circuitry of the at least one ROIC die through the at least one electrical connection and that flows through the corresponding conductive substrate(s).