A study of electron-multiplying CCDs for use on the International X-ray Observatory off-plane x-ray grating spectrometer

Abstract

CCDs are regularly used as imaging and spectroscopic devices on space telescopes at X-ray energies due to their high quantum efficiency and linearity across the energy range. The International X-ray Observatory's X-ray Grating Spectrometer will also look to make use of these devices across the energy band of 0.3 keV to 1 keV. At these energies, when photon counting, the charge generated in the silicon is close to the noise of the system. In order to be able to detect these low energy X-ray events, the system noise of the detector has to be minimised to have a sufficient signal-to-noise-ratio. By using an EM-CCD instead of a conventional CCD, any charge that is collected in the device can be multiplied before it is read out and as long as the EM-CCD is cool enough to adequately suppress the dark current, the signal-to-noise ratio of the device can be significantly increased, allowing soft X-ray events to be more easily detected. This paper will look into the use of EM-CCDs for the detection of low energy X-rays, in particular the effect that using these devices will have on the signal to noise ratio as well as any degradation in resolution and FWHM that may occur due to the additional shot noise on the signal caused by the charge packet amplification process.

The Noise Performance of Electron-Multiplying Charge-Coupled Devices at Soft X-Ray Energy Values

“…• C to suppress the dark current generated by the device to a negligible level [10], but the CCD220 was cooled to only -50…”

“…The results from the CCD97 experiment are discussed in [10] and are shown by the black points in Figure 10. The red points show the results from the CCD220 measurement taken at BESSY II and the blue line is the result predicted from the theory discussed in [1].…”

“…If the device is fully depleted, the collection time is small, reducing the spread of the charge packet and reducing the number of split events. Split events were determined to be the cause of the poor results from the initial experiment with the CCD97 discussed in [10], therefore the reduction of this effect during the second experiment was a high priority. The CCD220 has larger pixels than the CCD97, which helps to reduce the number of split events by having a large area per pixel for the charge cloud to be collected in.…”

High-resolution soft x-ray spectrometry using the electron-multiplying charge-coupled device (EM-CCD)

“…The instrument was designed to give a resolving power (E/∆E) greater than 3000 and an effective area greater than 1000 cm 2 over the 300 eV to 1000 eV energy range. To achieve this resolution, several dispersed orders were required, therefore different energy photons from different dispersed orders would be detected at the same point on the focal plane; the inherent energy resolution of the CCD is required to separate out these photons 5 .…”

Technological developments of the OGRE focal plane array

“…Also, as off-plane gratings can disperse X-rays of different energy and order to the same point on the focal plane, the camera requires a good enough energy resolution to enable order separation. 6,7,8 OGRE is designed to detect low energy X-rays in a photon starved environment; therefore, any increase in the signal-to-noise ratio of the incident photons aid in their detection and signal processing, which can be achieved through the increase in the signal in a pixel before it is read out in an EM-CCD. 9 The e2v CCD207-40 10 is the detector that has been chosen for the OGRE focal plane (Figure 1).…”