siegbahn
(spectroscopy) A unit of length, formerly used to express wavelengths of x-rays, equal to 1/3029.45 of the spacing of the (200) planes of calcite at 18°C, or to (1.00202 ± 0.00003) × 10-13 meter. Also known as x-ray unit; X-unit. Symbolized X; XU.



