Increase In Charge Transfer Inefficiency Research Articles

Charge transfer inefficiency increase of the CCD detector induced by proton and neutron irradiations

Charge transfer inefficiency (CTI) is one of the most important parameters to evaluate the performance of a CCD detector caused by displacement damages. In order to help the users to know about the CTI increase of the CCD detectors for applications in particle detection or space applications, the CTI increase caused by proton and neutron irradiations are presented. The proton radiation experiments were carried out at a cyclotron accelerator with 60 MeV protons to the fluences of 3×1010, 5×1010, and 7×1010 p/cm2. The neutron radiation experiments were carried out at the back-streaming white neutrons at China Spallation Neutron Source (CSNS) and at Xi’an pulse reactor (XAPR). The neutron fluences range from 1×1011to 1×1012 n/cm2. The CTIs are quantitatively compared at two signal levels of saturation and half saturation illuminations. The annealing tests after irradiations are also performed to observe the recovery of the degradations. The quantitative comparisons of the CTI increase induced by proton or neutron irradiations before and after annealing are also presented. The CTI increase induced by proton and neutron irradiations is characterized in detail.

Annealing bounds to prevent further Charge Transfer Inefficiency increase of the Chandra X-ray CCDs

After the front-illuminated CCDs on board the X-ray telescope Chandra were damaged by radiation after launch, it was decided to anneal them in an effort to remove the defects introduced by the irradiation. The annealing led to an unexpected increase of the Charge Transfer Inefficiency (CTI). The performance degradation is attributed to point defect interactions in the devices. Specifically, the annealing at 30°C activated the diffusion of the main interstitial defect in the device, the carbon interstitial, which led to its association with a substitutional impurity, ultimately resulting in a stable and electrically active defect state. Because the formation reaction of this carbon interstitial and substitutional impurity associate is diffusion limited, we recommend a higher upper bound for the annealing temperature and duration of any future CCD anneals, that of −50°C for one day or −60°C for a week, to prevent further CTI increase.

Displacement damage induce degradation of COTS array CCDs irradiated by neutron beams from a nuclear reactor

The experiments of displacement damage effects on COTS array charge coupled devices (CCDs) induced by neutron irradiation from a nuclear reactor are presented. The charge transfer inefficiency (CTI), saturation output signal voltage (VS), dynamic range (DR), dark signal, and camera imaging quality versus neutron fluence are investigated. The degradation mechanisms of the CCDs irradiated by reactor neutron beams are also analyzed. The CTI increase due to neutron displacement damage appears to be proportional to displacement damage dose. The experiments show that VS degradation induced by neutron irradiation is much less than that induced by gamma irradiation. The dark images from the CCDs irradiated by neutrons are given to investigate dark signal degradation. The degradation forms and mechanisms of the camera imaging quality are very different between the reactor neutron displacement damage and the gamma total ionization dose damage. The three samples were exposed by 1MeV neutron-equivalent fluences of 1×1011, 5×1011, and 1×1012n/cm2, respectively. A sample was exposed by 1MeV neutron-equivalent fluences up to 2×1013n/cm2, and the CCD is a functional failure after irradiation.

Modelling the spectral response of the Swift-XRT CCD camera: experience learnt from in-flight calibration

Context. Since its launch in November 2004, Swift has revolutionised our understanding of gamma-ray bursts. The X-ray telescope (XRT), one of the three instruments on board Swift, has played a key role in providing essential positions, timing, and spectroscopy of more than 300 GRB afterglows to date. Although Swift was designed to observe GRB afterglows with power-law spectra, Swift is spending an increasing fraction of its time observing more traditional X-ray sources, which have more complex spectra. Aims. The aim of this paper is a detailed description of the CCD response model used to compute the XRT RMFs (redistribution matrix files), the changes implemented to it based on measurements of celestial and on-board calibration sources, and current caveats in the RMFs for the spectral analysis of XRT data. Methods. The RMFs are computed via Monte-Carlo simulations based on a physical model describing the interaction of photons within the silicon bulk of the CCD detector. Results. We show that the XRT spectral response calibration was complicated by various energy offsets in photon counting (PC) and windowed timing (WT) modes related to the way the CCD is operated in orbit (variation in temperature during observations, contamination by optical light from the sunlit Earth and increase in charge transfer inefficiency). We describe how these effects can be corrected for in the ground processing software. We show that the low-energy response, the redistribution in spectra of absorbed sources, and the modelling of the line profile have been significantly improved since launch by introducing empirical corrections in our code when it was not possible to use a physical description. We note that the increase in CTI became noticeable in June 2006 (i.e. 14 months after launch), but the evidence of a more serious degradation in spectroscopic performance (line broadening and change in the low-energy response) due to large charge traps (i.e. faults in the Si crystal) became more significant after March 2007. We describe efforts to handle such changes in the spectral response. Finally, we show that the commanded increase in the substrate voltage from 0 to 6 V on 2007 August 30 reduced the dark current, enabling the collection of useful science data at higher CCD temperature (up to −50 ◦ C). We also briefly describe the plan to recalibrate the XRT response files at this new voltage. Conclusions. We show that the XRT spectral response is described well by the public response files for line and continuum spectra in the 0.3−10 keV band in both PC and WT modes.

Anomalous annealing of a high-resistivity CCD irradiated at low temperature

The front-illuminated charge-coupled device (CCD) detectors in the Chandra X-ray Observatory's ACIS instrument suffered radiation damage from soft protons focused by the telescope mirror early in the mission. In the course of assessing this damage, the focal plane was temporarily warmed from its normal operating temperature (then -100/spl deg/C) to +30/spl deg/C. Following this "bakeout", the radiation-damaged CCDs exhibited significantly greater charge transfer inefficiency (CTI). We performed a laboratory experiment with a sibling of the flight detectors in an attempt to reproduce and better understand this phenomenon. The test CCD was cooled to -100/spl deg/C, irradiated by 120 keV protons and then warmed to +30/spl deg/C for 8 hours. As expected, after the initial irradiation, but before detector warmup, a substantial CTI increase was observed. The subsequent warmup itself then produced an additional factor /spl ap/2.5 increase in CTI. Following smaller subsequent irradiations with the detector cold, a "bakeout" for 8 hours at -60/spl deg/C produced no observable increase in CTI. However, a subsequent bakeout to +30/spl deg/C for another 8 hours resulted in an additional increase in CTI of roughly 15%. The CTI changes produced by the room temperature bakeout are accompanied by dramatic changes in the de-trapping times of electron traps responsible for the CTI. The distributions of signal amplitudes in the pixels trailing X-ray events indicate that annealing at room temperature can cause large changes of the trap emission times, from which we infer that conversion of trapping defects takes place. The observed phenomena can be explained by the previously suggested mechanism of carbon-related defect transformation. Specifically, the room-temperature annealing may allow carbon interstitials to form metastable complexes with phosphorus and/or carbon substitutional atoms.

The ground calibration of X-ray CCD cameras (XIS) with front-illuminated chips onboard Astro-E2

We report on the results of the ground calibration of Astro-E2/XIS with front-illuminated (FI) chips. The sensors have basically the same performance as that of Astro-E/XIS. However, there are some improved points: (1) A 55Fe radio isotope is equipped on the door and (2) the capability for charge injection (CI) has been added. We measured the gain, energy resolution, and quantum efficiency (QE) of the FI sensors as a function of energy by using characteristic X-rays for each sensor. An energy resolution of 130 eV at 5.9 keV in FWHM and a QE of 52% at 8.6 keV are achieved. After XIS is launched, the charge transfer inefficiency (CTI) will increase due to radiation damage by cosmic rays. Then XIS is equipped with the CI capability to calibrate and compensate for the increase in CTI. We estimate the time variability in the amount of the injected charge.