Detector

In x-ray and neutron scattering, the detector is the hardware that detects the scattered radiation. On modern beamlines, area detectors are used: i.e. they generate two-dimensional (2D) images of scattering.

Kinds of Area X-ray Detectors

A variety of detector technologies are available. Below are a few of the common types of detectors used on labscale x-ray instruments and synchrotron beamlines:

Fiber-coupled CCD

This well-developed detector design uses a fluorescent or phosphorescent screen behind an opaque barrier (typically Be, though amorphous carbon is also possible). A bundle of fiber-optics is then bonded to this screen. The fiber-bundle is 'tapered'; i.e. it has been drawn so that the bundle is wide on one end but much smaller on the other end. This bundle thus acts as a grid of light-pipes. X-rays are absorbed in the screen layer, and converted into visible-light photons. Then these photons travel down the fiber-bundle, and are detected using a charge-coupled device (CCD). The CCD chip is essentially just like the chip used in a digital camera (though it may have better performance: higher dynamic range, and lower noise owing to active cooling). This design is robust, and yields a wide area image with no gaps. However, the image may have some distortion, due to imperfects in the drawing of the bundle (especially near the edges of the image). Detector software typically applies an 'unwarping', but even with this correction, images may have some lingering distortion.

• Advantages:
  • Robust and well-understood technology.
• Disadvantages:
  • Somewhat slow readout.
  • Moderate background noise.
• Examples:
  • MarCCD

Hybrid pixel-array

This newer technology involves a two-dimensional array of photon-counting pixels. X-ray photons are directly detected in silicon electronics: each pixel has its own amplifier, discriminator, and counter. By setting the threshold to ~1/2 the energy of the x-ray radiation being used, background noise is very efficiently excluded. This low noise-floor, coupled to a large per-pixel counter size, allows these detectors to have exceptionally large dynamic range.

• Advantages:
  • Low noise.
  • High dynamic range.
  • Nearly photon-counting.
  • Fast readout.
  • Well-defined pixel positions.
• Disadvantages:
  • Expensive.
  • Images have gaps (intermodule gaps).
• Examples:
  • Dectris Pilatus

Image plates

An image plate is a plate of photo-sensitive material. The plate is first 'cleared' and then loaded into the sample chamber in the desired position. During sample irradiation, the scattering radiation will 'develop' the plate. The plate must then be removed and loaded into a special scanner that 'reads' the accumulated dose. This reading is used to generate a digital file

• Advantages:
  • Inexpensive
  • Can easily create a custom shape (e.g. a through-hole).
• Disadvantages:
  • Laborious and cumbersome to take data.
  • Positional error in replacing plate introduces an error in data.