3.1.1 Radioisotope sources

Radioisotope sources have as main advantage the simplicity for implementation of different excitation arrangements and the relatively low costs. Different radioisotopes can be produced and mounted in disk, bar and annular constructions.

Radioisotopes were extensively used in both laboratory spectrometers and in the first portable devices. The emission of quasi-monochromatic excitation brings the advantage of simplicity in quantification methods, particularly for allowing an accurate estimate of the x-ray production and the attenuation of the excitation radiation in the sample (detailed explanation will be provided in section 6). The possibility of choosing different radioisotopes for improved excitation of several groups of elements enhanced the versatility of their use.

Sources have two major drawbacks. First, the risk of constant radiation exposure and special requirements for transportation and disposal have been discouraging their use in the last decade. On the other hand, the natural decay of the activity makes the short-life radioisotope (such as ¹⁰⁹Cd, ⁵⁵Fe and ⁵⁷Co) sources unusable after two or three periods of half-life.

However, sources are still advantageous for some special applications requiring low weight and power consumption. Alpha particle X-ray spectrometers (APXS) have been built for the elemental analysis of elemental composition of the surfaces of the Moon, Mars and more recently of the surface of comet 67P/Churyumov–Gerasimenko.