15.1 OVERVIEW OF ULTRAVIOLET AND X-RAY DETECTION PRINCIPLES

The basic principles of ultraviolet and x-ray detectors are largely similar to those of detectors used for visible light; however, the similarities are greater at the longer wavelengths (closest to the visible). Especially in the x-ray portion of the spectrum, a number of unique detection techniques (not used in the visible) are applicable.

One detection technique, the use of photographic film, is applicable at all wavelengths shortward of the near-infrared, including x-rays. It is also the oldest and simplest technique, and still has advantages in some applications. However, it has been replaced by other techniques for most applications where higher sensitivity, more quantitatively accurate photometric information, or more immediate data availability is required.

In common with visible-light detectors, most ultraviolet detectors and many x-ray detectors are based on the principles of photoelectric emission or of photo-conductivity. Detectors based on a third process, gas photoionization, are also used in the far-ultraviolet and x-ray wavelength ranges. Scintillation detectors are used for high-energy x-ray and gamma-ray detection.

The basic detection principles are the same for nonimaging detectors (in which the objective is simply to measure the intensity and/or spectral distribution of an incoming beam of radiation) and for imaging detectors (in which one also seeks to preserve the intensity versus position information in a two-dimensional field of view). However, imaging detectors often involve the use of additional components or techniques in order to preserve and record the spatial intensity distribution information.

15.2