NIST XCOM: Photon Cross Sections Database
https://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html
(source of nist_mu.json.gz; notice copied verbatim from
https://physics.nist.gov/PhysRefData/Xcom/Text/download.html, fetched 2026-09-25)

XCOM Copyright Notice:

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