Planck's constant, h , divided by 2p.

HALF LIFE, BIOLOGICAL  For a substance, the time required for the amount of that substance in a biological system to be reduced to one-half of its value by biological processes, when the rate of removal is approximately exponential. IUPAC94.

HALF LIFE, EFFECTIVE  For a radioactive substance, the time required for the amount of that substance in a biological system to be reduced to one half of its value by both radioactive decay  and biological processes, when the rate of removal is approximately exponential. IUPAC94.

HALF LIFE, PARTIAL  In nuclei  undergoing branching decay , the half-life that would be evident if a particular branch were the sole means of decay. The partial half-life of a nucleus by a specific mode of decay is the (total) half-life divided by the branching fraction  for that specific decay path.

HALF LIFE, RADIOACTIVE  For a single radioactive decay  process, the time required for the number of atoms or the activity  to decrease to half its value by that process.

HALF THICKNESS  The thickness of a specified substance which, when introduced into the path of a given beam of radiation , reduces the value of a specified radiation quantity by one half. CAN

HALF-VALUE LAYER  See half thickness .

HALF-VALUE THICKNESS  See half thickness .

HALO  A luminous or colored circle seen in the light around an object; frequently applied to radioactive inclusions in minerals.

HAP  Hydrated antimony pentoxide, an inorganic ion-exchanger.

HAPTEN  A substance, particularly of low molecular weight, which cannot to induce antibody  formation by itself, but which when combined with larger molecules such as proteins can do so. This combination is called an immunogen .

HARD RADIATION  Radiation whose particles or photons have a high energy and, as a result, penetrate materials relatively easily. M.

HARDENING (NEUTRONS)  The increase in the fraction of higher energy neutrons in a reactor beam flux due to preferential absorption of low-energy neutrons by a sample.

HD VALUE  In autoradiography , a measure of resolution corresponding to the distance from a straight radiographic line that accounts for 50% of the grains. J.

HEALTH PHYSICS  A division of occupational health dealing particularly with protection of personnel from the harmful effects of ionizing radiation . NM.

HEAVY ELEMENTS  A term often used with reference to the actinides  and still heavier elements.

HEAVY ION  A nucleus  consisting of more than one nucleon. Frequently, the term is further restricted to nuclei with atomic numbers greater than two.

HEAVY ION ACCELERATOR  An accelerator  in which the beam consists of heavy ions .

HEAVY ION REACTION  A nuclear reaction  in which the interacting nuclei are heavy ions .

HEAVY WATER  Water containing a significant fraction (up to 100%) of deuterium in the form of D2O or HDO. IUPAC82..

HELIUM JET  See gas jet .

HETEROGENEOUS COUNTING  A technique in solid state scintillation  counting of radiochromatograms  in which the scintillant  is solid bead material with which a flow cell is packed and that does not chemically interact with the eluting medium. J.

HFR  High flux reactor  

HIAA  Heavy-ion activation analysis  

HIGH FLUX REACTOR  A nuclear reactor designed to produce neutron flux densities several orders of magnitude greater than in conventional reactors.  

HMD  Hydrated manganese dioxide, an inorganic ion-exchanger.

HOLD-BACK CARRIER  See carrier, hold back  

HOMOGENEOUS COUNTING  A technique for counting radiochromatograms  in a flow cell by injecting liquid scintillant into the eluting medium. Jones.  

HOT ATOM  An atom  in an excited  energy state or having kinetic energy above the ambient thermal level, usually as a result of nuclear processes. IUPAC82..

HOT CELL  A heavily shielded enclosure for highly radioactive  materials. It may be used for their handling or processing by remote means or for their storage. IUPAC82..

HOT LABORATORY  A heavily shielded, specially equipped laboratory to protect personnel working with large amounts of substances emitting gamma radiation and to protect the environment from contamination.

HPGe  See germanium detector, high purity . Also known as intrinsic germanium.  

HPLC  High performance liquid chromatography.

HWR  Heavy water reactor, that is, one moderated and cooled with heavy water .

HYDROLYSIS  Reaction with water as a solvent involving the rupture of one or more bonds in the reacting solute. For weak electrolytes, this can alter the acidity and subsequent chemical behavior of the aqueous solvent.

HYPERFINE STRUCTURE  Small differences in energy states of atoms or molecules that have different orientations between nuclear spin and electron angular momentum owing to the interaction between the nuclear and electron magnetic moments. FKMM.