X-ray Detectors

Photographic Film

In medicine and dentistry the most common X-ray detector used is still the old fashioned X-ray film. It is relatively inexpensive, provides a permanent record, and a spatial resolution that is typically in the 10 – 100 micron range. Photographic film needs to be developed using wet chemistry in a dark room (or more commonly in an automated film-developing apparatus). Since the relatively hard X-rays that penetrate the human body go right through the photographic film, the naked film is very inefficient, and is generally sandwitched between intensifying screen that convert the X-rays to light. Screens typically consist of calcium tungstate or salts containing gadolinium, lanthanum or yttrium. This arrangement degrades the resolution to the millimeter-range.

Image Plates

Image plates are similar to film in resolution and sensitivity, but they are reusable. After exposure the image plate is placed in a specialized apparatus that "reads" the image, forms a digital map, and erases the image-plate.

Image Intensifiers

Image intensifiers use a phosphor screen to convert X-rays to visible light, followed by a photocathode that converts the visible light to electrons. These electrons are incident on one or more microchannel plates, where a single electron creates an electron-cascade confined to one of the channels. A fluorescent screen at the output end is lit up by the electrons, and the pattern of light produced is observed or recorded, much as in night vision goggles. Image intensifiers have excellent time resolution, however the output is somewhat noisy and is not suitable for quantitative measurements.

CCD – Based Detectors

CCD detectors are widely used in digital photography. The pixel size is typically 10 – 20 microns, and the common 4 megapixel array has 2000x2000 elements. CCD's can detect X-rays directly, however they can also suffer radiation damage at high radiation dose. To protect the CCD from damage, or in order to image a larger field of view, it is common to first convert the X-rays to visible light in a scintillator screen, and then transfer the visible light image onto the CCD. The transfer can use a well-corrected lens system, or a fiber-optic bundle. Both of these can be configured to match the size of the scintillator screen to the CCD array. For low-noise scientific applications the detector is cooled, and the read-out time is about 1 second.

Energy Dispersive Detectors

While the detectors described above are used for recording X-ray images (radiographs), it is often desirable to measure the X-ray energy instead. Lithium drifted silicon (SiLi) or germanium detectors provide no position information, but they measure the X-ray energy with enough accuracy (100 – 200 eV) to be useful in identifying characteristic X-rays in a fluorescence spectrum. Basically the X-ray energy is converted to electron-hole pairs, (one such pair for every 3.7 eV deposited in silicon), creating a tiny current pulse, and the total charge in the pulse is measured. For the best accuracy, the detector is cooled. It is often used with a multichannel analyzer.

Photodiodes

Photodiodes are used commonly to measure the intensity of an X-ray beam. They can be calibrated, and are linear over several orders of magnitude of intensity.