IRIDIUM, radioactive (Iridium; Ir) — chemical element VIII of group of a periodic system of elements of D. I. Mendeleyev, sequence number 77, atomic weight 192,2; belongs to platinum metals. Silvery white metal, with a density of 22,5 g/cm 3 , t ° pl 2443 °, resistant to chemical influences. In connections of hl. obr. three - and chetyrekhvalenten.
And. has two stable isotopes with mass numbers 191 (38,5%) and 193 (61,5%), and also 24 radioactive (including 5 isomers) with mass numbers from 182 to 198. Majority of radioisotopes And. shortly - and ultrashort-lived, four have half-lives from 1,7 to 11,9 days, isotope with mass number 192 — 74,2 days. From all radioisotopes And. only 192 Ir found practical application: in the equipment — for gamma-radiography and in medicine — for radiation therapy.
192 Ir receive, irradiating a target from natural And. neutrons in the nuclear reactor on the reaction (n, scale) going with a big exit (δ = the 700th barn). At the same time along with 192 Ir is formed also 194 Ir which, however, after endurance of the irradiated target within several days breaks up, turning into a stable isotope 194 Pt (see. Isotopes ).
192 Ir breaks up with emission of a difficult range beta minus radiation — 95,5% and electron capture — 4,5% (see. Radioactivity ). Beta range 192 Ir has four components with the maximum energy of ER equal 0,075 (0,1%); 0,250 (5,4%); 0,530 (42,6%) and 0,672 MEV (47,2%). Disintegration 192 Ir is followed y-from the spearing consisting of 16 lines with energy of Eγ: 0,206 (3,4%); 0,296 (29%); 0,308 (30%); 0,317 (81%); 0,468 (49%); 0,484 (2,9%); 0,589 (4%); 0,604 (9%) and 0,612 MEV (6%). Intensity of each of other 7 lines less than 1%. Sources 192 Ir measure by means of the ionization chamber (see. Dosimetry of ionizing radiation ). Therapeutic γ-istochiik 192 Ir characterize them by the general or linear (length carried to 1 cm) activity, and also power of the air dose created by this source at distance of 1 m from its center. a gamma and Constant (see. Gamma and constant ) for 192 Ir is equal 0,48 mr • to m 2 / (hour • mkyur); power of an air dose in 1 mr / creates hour at distance 1 m a source 192 Ir activity 2,083 mkyur (see. Dosage rate of radiation ).
And. apply in medicine for interstitial and intracavitary radiation therapy (see) in the form of the iridic needles and a wire covered with a thin coat (0,1 mm) of platinum for absorption of beta radiation 192 Ir. An iridic wire with 192 Ir usually apply an afterloding (afterloading) with use of the equipment: put in the hollow nylon tubes which are in advance entered to the patient. In a wedge. to practice apply the iridic wire creating the power of an air dose 0,5 — 1,5 mr / hour at distance 1 .ch (on 1 cm of length of a wire), i.e. with linear activity 1 — 3 mkyuri/cm.
Isotopes I., including and 192 Ir, on radio toxicity concern to group B, i.e. in a workplace without the permission a dignity. - epid, services can be used open drugs I. activity to 10 mkkyura.
Bibliography: Levin V. I. Receiving radioisotopes. M, 1972; Paine C. H. Modern after-loading methods for interstitial radiotherapy, Clin. Radiol., v. 23, p. 263, 1972, bibliogr.
V. V. Bochkaryov.