回複：這裏有總量的問題。如果量不夠，gammar也未必造成損傷

來源: 老遊 於 2011-03-26 18:39:41 [] [舊帖] [給我悄悄話] 閱讀數 : (1661 bytes)

Due to its mode of beta decay, iodine-131 is notable for causing mutation and death in cells which it penetrates, and other cells up to several millimeters away. For this reason, high doses of the isotope are sometimes paradoxically less dangerous than low doses, since they tend to kill thyroid tissues which would otherwise become cancerous as a result of the radiation.[citation needed] Thus, iodine-131 is increasingly not employed in small doses in medical use, but increasingly used only in large and maximal treatment doses, as a way of killing targeted tissues. This is known as "therapeutic use."[citation needed]

Iodine-131 can be "seen" by nuclear medicine imaging techniques (i.e., gamma cameras) whenever it is given for therapeutic use, since about 10% of its energy and radiation dose is via gamma radiation. However, since the other 90% of radiation (beta radiation) causes tissue damage without contributing to any ability to see or "image" the isotope, other less-damaging radioisotopes of iodine are preferred in situations when only nuclear imaging is required. The isotope I-131 is still occasionally used for purely diagnostic (i.e., imaging) work, due to its low expense compared to other iodine radioisotopes. This low-cost availability, in turn, is due to the relative ease of creating I-131 by neutron bombardment of natural tellurium in a nuclear reactor, then separating I-131 out by various simple methods (i.e., heating to drive off the volatile iodine). By contrast, other iodine radioisotopes are usually created by far more expensive techniques, starting with reactor radiation of expensive capsules of pressurized xenon gas.

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