half - life is the term for the amount of time it takes 50 of an element to decay.
what is the term for the amount of time it takes 50 of an element to decay?
half - life
the most common mechanism for suppression of the rate of gamma decay of excited atomic nuclei, and thus make possible the existence of a metastable isomer for the nucleus, is lack of a decay route for the excited state that will change nuclear angular momentum (along any given direction) by the most common (allowed) amount of 1 quantum unit formula_1 of spin angular momentum. such a change is necessary to emit a gamma-ray photon, which has a spin of 1 unit in this system. integral changes of 2, 3, 4, and more units in angular momentum are possible (the emitted photons carry off the additional angular momentum), but changes of more than 1 unit are known as forbidden transitions. each degree of forbiddenness (additional unit of spin change larger than 1, that the emitted gamma ray must carry) inhibits decay rate by about 5 orders of magnitude. the highest known spin change of 8 units occurs in the decay of ta-180m, which suppresses its decay by a factor of 10 from that associated with 1 unit, so that instead of a natural gamma decay half life of 10 seconds, it has a half life of more than 10 seconds, or at least 3 x 10 years, and thus has yet to be observed to decay.
forbidden mechanism
a study by cindy l. wills and bruce j. dolnick from the department of experimental therapeutics at the roswell park memorial institute in buffalo, new york and from the cell and molecular biology program at university of wisconsin in madison, wisconsin was made to understand cellular processes involving primary transcripts. researchers wanted to understand whether 5-fluorouracil (fura), a drug known for use in cancer treatment, inhibits or shuts down dihydrofolate reductase (dhfr) pre-mrna processing and/or nuclear mrna stability in methotrexate-resistant kb cells. long-term exposure to fura had no effect on the level of dhfr pre-mrna containing certain introns, which are sections of pre-mrna that are usually cut out of the sequence as a part of processing. however, levels of total dhfr mrna decreased two-fold in cells exposed to 1.0 μm fura. there was no significant change in the half-life, which refers to the time it takes 50% of the mrna to decay, of total dhfr mrna or pre-mrna observed in cells exposed to fura. and nuclear/cytoplasmic rna labeling experiments demonstrated that the rate of nuclear dhfr rna changing to cytoplasmic dhfr mrna decreased in cells treated with fura. these results provide further evidence that fura may help in the processing of mrna precursors and/or affect the stability of nuclear dhfr mrna.
primary transcript