Temperature Dependence On Performance Of X-ray CCDs

The main means of X-ray astronomy is X-ray radiation. There are several X-ray radiation mechanism as below, blackbody radiation in optically thick thermal equilibrium gas, bremsstrahlung in optical thin thermal equilibrium plasma, synchrotron radiation produced by relativistic electrons in a magnetic field, inverse Compton scattering produced by relativistic electrons and low-frequency radio (or infrared) photon collisions. The nature of x-ray is photons, so particle detectors can be used to detect x-rays.Conventional particle detectors are gas detectors, scintillation detectors and semiconductor detectors. Gas molecules are ionized to produce signals in gas detectors. The signals, produced in scintillation detectors and semiconductor detectors, are from reacting between rays and detect materials. There are both advantages and disadvantages in the three kinds of detectors. Gas detectors can detect particle tracks, however, they have general shortcomings of its large size, and they are easy to leak. Scintillation detectors can work in the room temperature, but the scintillation crystal is like to deliquescence. Semiconductor detector has the advantage of good energy resolution and low noise, but the disadvantage is the need to work under low temperature and more sensitive to radiation.CCD is a kind of semiconductor detectors, which is arranged by a lot of isolated micro-units, and in which there are two-dimensional array of position sensitive. The CCD depletion layer needs to be thick to increase the the X-ray detection efficiency in order to detect X-rays. Most incident X-ray photons are deposited in the depletion layer by the photoelectric effect. These electrons are storage in the MOS capacitor which is produced under the PN junctions of CCD when CCD is on voltage. Electrons are transferred to the end of the collection by the drive voltage which is cycling changed.Charge transfer efficiency impacts on the CCD performance great in the process of charge transfer. There are thermal diffusion drift, self-induced drift and fringing field effect and so on in the mechanism of charge transfer. 93% of the charges have completed to transfer in a very short time when the electrode is on the high low conversion. Most of the charges complete to transfer by self-induced, the self-induced drift will be reduced as the charge density decreases. At this time the charges that are at the electrode edge is transferred by fringing field, and the ones that are at the center of the electrode is transferred by diffusion drift. The traps on CCD will impact the charge transfer efficiency great. The common traps are design traps, process traps, bulk traps and radiation traps, ect al.CCD236 is an X-ray CCD detector with two-phase. The signal can be read-out quickly without the location information. The electrode of CCD236 is L shaped. The charges are first transferred to the diagonal then to the output located at the center of CCD236 along the diagonal.The noise of CCD comes from two aspects, one is from the CCD itself, including the shot noise, the dark-current noise and the charge transfer noise; the other is from electronic circuits, including the reset noise, the output amplifier noise and so on.The CCD's performance parameters include the quantum efficiency, dead time, energy linearity, energy resolution, and read out noise. The calculation method for linearity, energy resolution, read out noise will be introduced in detail in this article.CCD236 is the detector that is used on the Low Energy Telescope of the Hard X-ray Modulation Telescope. HXMT is a space hard X-ray Modulation Telescope Satellite of high sensitivity and good spatial resolution, that is designed and manufactured independent by the direct demodulation imaging principle which is put forward by Chinese scholars. HXMT will complete the Sky Survey image of the full energy paragraph from 1keV to 250keV. High sensitivity and spatial resolution will be done in the hard X-ray segment. HXMT will draw out the hard X-ray-day high-precision maps, so that new style of heavenly body will be found possibly. There are several main scientific payloads in HMXT, including the High Energy Telescope, the Medium Energy Telescope and the Low Energy Telescope.In this paper, detailed detector tests, on signal amplitude, drive voltage, energy resolution and other parameters on temperature dependents are given in detail.Detector performance vary greatly with temperature, we need to solve two problems. The first is CCD236's performances in all temperatures; and the second is the consistency of the CCD236's performances in all temperatures.The signal amplitude can be affected by the dark current, the ionization energy of silicon, the charge transfer efficiency, the CCD FET and other factors. Dark current and the ionization energy of silicon do influence the signal amplitude little, which is proved by the theory and the experiment. The transconductance of CCD FET is temperature-sensitive. In the experiment, the signal amplitude increases with temperature decreases is due to the CCD FET. While the signal amplitude decreases is due to the decreases of the charge transfer efficiency when the temperature is rather low.Signals lead to a "split" at a rather low temperature, which reason is unknown. That the voltage of the injection barrier and the drive clock can impact the "split" signals very significant is determined. The higher barrier voltage and the lower drive voltage (Dither mode) lead the signals less "split" on CCD236. However the barrier voltage cannot be changed once the CCD236 is manufactured, the reduction of the drive voltage can overcome the signals"split"between -50 Celsius and -80 Celsius.Energy resolution is an important factor for detector's performance. The energy resolution is primarily affected by dark current when the temperature is higher than -45 Celsius. Whereas the energy resolution hardly changed over temperature when the temperature is below than -45 Celsius, at which point, dark current takes little affected. The energy resolution will be worsen for the reason of the decreased charge transfer efficiency, when the temperature is rather lower.Dark current impacts the readout noise greatly when the temperature is above -45 Celsius. However, readout noise hardly changed when the temperature is below -45 Celsius at which point dark current takes little impacts.In the end, the outlook for the experiment is given, including the energy calibration at different temperatures, the performance of the electronic systems in the vacuum state and the further study for the"split"signals of CCD236...