X unit

{
  "id": "MLC-T-CHM-012732",
  "term": "X unit",
  "field": "Chemistry",
  "definition": "A non-SI unit of length used for measuring X-ray and gamma-ray wavelengths, approximately equal to 10⁻¹³ meters. It was historically defined based on the lattice spacing of calcite crystals. One X unit is approximately 1.0021 x 10⁻¹³ meters, and it is now largely superseded by the picometer (pm) or femtometer (fm) in modern scientific practice.",
  "subject": "Physical and Biophysical Chemistry",
  "references": [
    "Green Book, 2nd ed., p. 110 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)"
  ],
  "url": "https://mlchart.com/terminology/chemistry/x-unit/"
}