Q1(f). X-rays from radioactive isotopes

Answer in brief

Mainly by electron (K) capture: the nucleus captures a K electron, an outer electron fills the vacancy and the daughter's characteristic X-ray is emitted (⁵⁵Fe gives Mn Kα, 5.9 keV).

Key points

• Electron capture: a proton-rich nucleus captures a K-shell electron (p + e⁻ → n + neutrino); Z falls by 1 and the daughter emits its characteristic X-ray.
• Example: ⁵⁵Fe (half-life 2.7 years) → ⁵⁵Mn, Mn Kα 5.9 keV; also ¹⁰⁹Cd → Ag K lines, ¹²⁵I → Te K.
• Internal conversion: an excited nucleus ejects an inner-shell electron; filling of the vacancy gives characteristic X-rays.
• γ-emission serves as a monochromatic X-ray (²⁴¹Am 59.5 keV; ⁵⁷Co 122 keV); β-particles striking a target give bremsstrahlung.
• Use: small, stable sources needing no power supply, for portable XRF and absorption gauges.