Low temperature testing and neutron irradiation of a swept charge device on board the HXMT satellite

We present the low temperature testing of an SCD detector, investigating its performance such as readout noise, energy resolution at 5.9 keV and dark current. The SCD's performance is closely related to temperature, and the temperature range of -80 ℃ to -50 ℃ is the best choice, where the FWHM at 5.9 keV is about 130 eV. The influence of the neutron irradiation from an electrostatic accelerator with fluence up to 1×10⁹ cm^-2 has been examined. We find the SCD is not vulnerable to neutron irradiation. The detailed operations of the SCD and the test results of low temperature are reported, and the results of neutron irradiation are discussed.     

Measurements of charge transfer ef f iciency in a proton-irradiated swept charge device

Charged Coupled Devices(CCDs) have been successfully used in several low energy X-ray astronomical satellites over the past two decades. Their high energy resolution and high spatial resolution make them a perfect tool for low energy astronomy, such as observing the formation of galaxy clusters and the environment around black holes. The Low Energy X-ray Telescope(LE) group is developing a Swept Charge Device(SCD) for the Hard Xray Modulation Telescope(HXMT) satellite. A SCD is a special low energy X-ray CCD, which can be read out a thousand times faster than traditional CCDs, simultaneously keeping excellent energy resolution. A test method for measuring the charge transfer efficiency(CTE) of a prototype SCD has been set up. Studies of the charge transfer inefficiency(CTI) with a proton-irradiated SCD have been performed at a range of operating temperatures. The SCD is irradiated by 3×108cm-210 MeV protons.     

Design and optimization of the readout system for X-ray CCDs

A readout system for X-ray CCDs based on an improved architecture is presented; by optimizing several critical circuit blocks along the analog signal chain, the conflict between the readout speed and readout noise is greatly alleviated. Using CCD47-10 as its target CCD, the readout system has achieved 8.6e^- readout noise and 142 eV FWHM at 5.9 keV Mn m K_α under a pixel rate of 80 kHz. Also its performance of imaging has been investigated.     

Proton irradiation effect on SCDs

The Low Energy X-ray Telescope is one of the main payloads on the Hard X-ray Modulation Telescope satellite.Swept charge devices(SCDs)are selected as detectors for the Low Energy X-ray Telescope.As SCDs are sensitive to proton irradiation,irradiation tests were carried out on the HI-13 accelerator at the China Institute of Atomic Energy.The beam energy was measured to be 10 MeV at the SCD.The proton fluence delivered to the SCD was 3×108protons/cm2over two hours.By comparing the performance before and after irradiation,it is concluded that proton irradiation affects both the dark current and the charge transfer inefficiency of the SCD.The energy resolution of the proton-irradiated SCD is 212 eV@5.9 keV at-60?C,while it before irradiated is 134 eV.Moreover,better performance can be reached by lowering the operating temperature of the SCD in orbit.     

Thermal analysis and expected performance of the low energy instrument on board the HXMT satellite

The Low Energy X-ray Instrument （LE） of the Hard X-ray Modulation Telescope （HXMT） uses the Swept Charge Device （SCD） to detect the X-rays in 1-15 keV. The performance of SCD is vulnerable to temperature. We analyzed the thermal condition of LE at different satellite working attitudes with the Finite Element Method （FEM）. It is shown that the angle between the sunlight and the normal line of the barrier should be less them 26°, to keep the SCD detectors working in the required temperature range, i.e. -40 ℃ to -80 ℃. We find that the performance of LE is very stable in this temperature range, with a typical energy resolution of 160 eV at 5.9 keV.