Z
Symbol for mass number.
(USAEC-1974)

Z; low-z; high-z
The symbol for the atomic number of a nucleus, i.e., the number of protons contained in the nucleus. Low-Z describes nuclei with Z < 26. High-Z describes nuclei with Z > 26. Very high-Z describes nuclei with Z > 73.
(NCRP 51)

Zero-area tissue-air ratio
The tissue-air ratio for zero area at a given depth. This is obtained by extrapolating the ratio from a finite-fixed area to zero area.
(ICRU 24)

Zero-power reactor
An experimental reactor operated at such low power levels that a coolant is not needed and little radioactivity is produced.
(USAEC-1974)

Zooplankton
The animals of plankton.
(NCRP 76)

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