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.
X-ray Area Detectors
Gas wire detectors
Gas wire technology is somewhat older. It is less commonly seen on modern synchrotron beamlines, but is still frequently used in labscale instruments. Thin wires are immersed in an x-ray absorbing gas (Xe or Air/methane). Electrons photo-liberated from the gas are detected by the wire grid, due to an internal bias voltage.
- Very low dark-count rate (useful for very long exposures).
- Considerable maintenance effort/cost.
- Fairly large pixels (100 μm).
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.
- Robust and well-understood technology.
- Somewhat slow readout.
- Moderate background noise.
- Modest signal-to-noise (detector saturation common)
Solid state pixel-array
Many newer detector technologies are being developed based on solid state (CMOS) pixels. 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.
- Low noise.
- High dynamic range.
- Nearly photon-counting.
- Fast readout.
- Well-defined pixel positions.
- Images have gaps (intermodule gaps).
- Dectris Pilatus
- Dectris Eiger
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
- Can easily create a custom shape (e.g. a through-hole).
- Laborious and cumbersome to take data.
- Positional error in replacing plate introduces an error in data.
X-ray Point Detectors
Avalanche Photodiode (APD)
An APD is a semiconductor that converts photons to electrical current. Through the use of substantial gain, these diodes can detect extremely weak x-ray signals. They are typically used as point detectors for measuring beam flux; e.g. as a diagnostic. They can also be scanned in order to reconstruct one-dimensional data (e.g. for reflectivity). In some instruments, the beamstop is equipped with a diode, such that the direct-beam flux (after the sample) is measured (this can be used to assess absorption, total scattering power, etc.).
- Takaki Hatsui and Heinz Graafsma X-ray imaging detectors for synchrotron and XFEL sources IUCrJ 2015, 2 (3). doi: 10.1107/S205225251500010X