Section 4.2.1.4. Radioactive X-ray sources

Radioactive sources of X-rays are mainly of interest to crystallographers for the calibration of X-ray detectors where they have the great advantage of being completely stable with time, or at least of having an accurately known decay rate. For some purposes, spectral purity of the radiation is important; radionuclides that decay wholly by electron capture are particularly useful as they produce little or no β or other radiation. In this type of decay, the atomic number of the daughter nucleus is one less than that of the decaying isotope, and the emitted X-rays are characteristic of the daughter nucleus. In some cases, the probability of electron capture taking place from some shell other than the K shell is very small and most of the photons emitted are K photons. The number of photons emitted into a solid angle of 4π, uncorrected for absorption, is given by the strength of the source in Curies (1 Curie = 3.7 × 10¹⁰ disintegrations s⁻¹), since each disintegration produces one photon. A list of these nuclei (after Dyson, 1973) is given in Table 4.2.1.5.

Table 4.2.1.5| top | pdf | Radionuclides decaying wholly by electron capture, and yielding little or no γ-radiation Nuclide Half-life X-rays Remarks Element (keV) 37Ar 35 d Cl 2.622 – 51Cr 27.8 d V 4.952 γ at 320 keV 55Fe 2.6 a Mn 5.898 – 71Ge 11.4 d Ga 9.251 – 103Pd 17 d Rh 20.214 Several γ's; all weak 109Cd 453 d Ag 22.16 γ at 88 keV 125I 60 d Te 27.47 γ at 35.4 keV 131Cs 10 d Xe 29.80 – 145Pm 17.7 a Nd 37.36 γ's at 67 and 72 keV 145Sm 340 d Pm 38.65 γ's at 61 keV; weak γ at 485 keV 179Ta 600 d Hf 55.76 – 181W 140 d Ta 57.52 γ at 6.5 keV; weak γ's at 136, 153 keV 205Pb 5 × 107 a Tl L only (Lα1 = 10.27 keV) –

Useful radioactive sources are also made by mixing a pure β-emitter with a target material. These sources produce a continuous spectrum in addition to the characteristic line spectrum. The nuclide most commonly used for this purpose is tritium which emits β particles with an energy up to 18 keV and which has a half-life of 12.4 a.

Radioactive X-ray sources have been reviewed by Dyson (1973).