Quantum efficiency measurements in the swept charge device CCD236

Abstract

The e2v technologies plc. CCD236 is a Swept Charge Device(SCD) designed as a large area (20 mm × 20 mm) soft X-ray detectorfor spectroscopy in the range 0.8 keV to10 keV. It benefits from improvements in design over the previousgeneration, the e2v CCD54, such as: a 4 times increased detector area, areduction in split X-ray events due to the 100 μm × 100 μm`pixel' size, and improvements to radiation hardness. The CCD236 will beused in India's Chandrayaan-2 Large Soft X-ray Spectrometer (CLASS)instrument and China's Hard X-ray Modulation Telescope (HXMT).Measurements of the Quantum Efficiency (QE) have been obtained relative to aNIST calibrated photodiode over the energy range 0.2 keV to 1.9 keV, usingthe BESSY II X-ray synchrotron in Berlin. Two X-ray event countingmethods are described and compared, and QE for soft X-ray interaction isreported. Uniformity of QE across the device is also investigated atenergies between 0.52 keV and 1.5 keV in different areas of the detector.This work will enable the actual number of photons incident on the detectorsto be calculated, thus ensuring that the CCD236 detectors provide valuablescientific data during use. By comparing the QE methods in this paper withthe event processing techniques to be used with CLASS, an estimate of theinstrument-specific QE for CLASS can be provided.