GENERATORS of RADIOISOTOPES — systems from two related isotopes in which of long-living maternal isotope it is formed affiliated with smaller half-life.
And. use in medical practice for receiving radioactive drugs (see), applied with the diagnostic and medical purpose.
Classical example and., consisting of natural radioelements, is 226 Ra —> 222 Rn (half-life of radium — 1620, radon — 3,8 days). From systems with artificial radioisotopes of the first in medicine it was applied 132 Te —> 132 I in 1951.
In process of disintegration of maternal isotope in the generator there is an accumulation affiliated, and through certain time in it radioactive equilibrium then activity of system decreases with half-life of maternal isotope is established.
Change of contents and activity of isotopes in generators happens as follows.
Number of atoms of maternal isotope (N1) in and. in timepoint (t) it is expressed by a ratio:
number of atoms of a daughter isotope (N2) — a ratio:
where N01 and N02 - number of atoms of a maternal and daughter isotope (respectively) in initial timepoint (t = 0); λ1 and λ2 — constants of disintegration of maternal and affiliated isotopes.
After department of a daughter isotope the number of its atoms (N2) will be expressed by a ratio (2) without the last member:
The daughter isotope allocated from the generator has the half-life (T2). From the moment of its department in process of disintegration of maternal isotope in the generator the daughter isotope begins to collect again, its quantity gradually increases and through certain time in the generator radioactive equilibrium is established.
The size of equilibrium activity of a daughter isotope (A2) reaches a maximum in timepoint (tmaks), value to-rogo depends on a ratio of constants of disintegration of two isotopes of the generator:
At this moment activity of a daughter isotope (A2) is equal to activity of maternal isotope (A1); then the relation of activities of affiliated and maternal isotopes (A2/A1) increases over time and aims at limiting value:
Thus, activity of a daughter isotope at balance is higher than activity of maternal. Further activity of system decreases with half-life of maternal isotope.
And. usually represent the column placed in a special protective container filled with the corresponding sorbent, on Krom maternal isotope is fixed. It is possible to bring him to the destination without essential losses of activity and there repeatedly, as required, by means of simple chemical procedures to allocate and use the corresponding short-lived isotope. Passing special solution through a column, the daughter isotope is washed away in a certain chemical form.
It is used or directly received eluate (e.g., pertekhnetat — 99m Tc or strontium chloride — 87m Sr), or on its basis by means of the enclosed sets of reactants prepare necessary radioactive pharm, drug.
The main generating systems applied in medicine — 68 Ge —> 68 Ga> 90 Sr —> 90 Y, 87 Y —> 87m Sr, etc.
Bibliography: Levin V. I., etc. Generators of short-lived isotopes, in book: Production of isotopes, under the editorship of V. V. Bochkaryov, etc., page 149, M., 1973; L e d of e of S. M. of Table of isotopes, N. Y., 1967; Radioisotopes derived from generators, in book: Radioisotope production a. quality control, techn. rep. ser. No. 128, Int. atomic energy agency (IAEA), p. 659, Vienna, 1971.
V. V. Bochkaryov.