Today Polly will explain a bit more about the detector we plan to use: the CCD. Charge-coupled devices are in your digital camera, mobile phone and webcam; they’re capable of measuring a lot of detailed data with a minimum of space and complexity. Astrophysicists and space science missions use them all the time for taking distant stars, galaxies, closer solar phenomena and planets. But as well as detecting light they can also be designed to measure images in a whole range of wavelengths. The CCD we’re planning to use is designed for a mission to take frames of incident x-ray data. However, luckily for us, it’s also capable of detecting incident electrons down to pretty low energies.
When a beam of light, or other radiation, or a charged particle, hits a semiconductor detector, it will knock electrons out of the structure of the material. The device can then collect them, read them out in turn and measure the charge from each pixel to work out where the hits happened. Each pixel is read out one after the other, so the clock input will need to be high enough to keep the time between reading frames right down.
We need a pretty special CCD for our requirements; it has to be back-illuminated, meaning the electrons can impinge directly on the pixels rather than needing to pass through any structures related to the electronics first. It’s also capable of detecting electrons down to quite low energies, meaning we can investigate the denser particle populations of the ionosphere. However, it is still sensitive to light, so one of our largest design considerations is trying to keep the light level that get to the detector at all as low as possible.
How electrons are moved toward the read-out electronics in a CCD
For more about semiconductor physics in general, we recommend Britney Spears’ guide.
