Page 100 - Radiochemistry and nuclear chemistry

Unstable Nuclei and Radioactive Decay 89

4.16. Radioisotope generators

The growth of radioactive daughters frequently has practical significance. For example,
in radiation therapy and diagnostic medicine it is preferable to use short-lived nuclides. In
fact, it is preferable to conduct tracer experiments with short-lived nuclides as this
eliminates the problem of disposal of residual radioactive waste after completion of the
experiment. It is convenient to have a long-lived mother in storage from which a short-lived
daughter can be removed as required for use in tracer work. A few examples of uses of
such mother-daughter pairs are discussed; others are included in Table 4.1.
Such systems are called radioisotope generators. 222Rn is sometimes useA for the
radiotherapeutic treatment of cancer. This product is isolated by separating it as a gas from
the parent substance 226Ra which is normally in the form of solid or a solution of RaBr 2.
222Rn grows into the radium sample with a half-life of 3.8 d. After a 2-week period,
following a separation of radon from radium, approximately 90 % of the maximum amount
of radon has grown back in the radium sample. Consequently, it is useful to separate 222Rn
each 2 weeks from the radium samples since further time provides very little additional
radioactivity. The 222Rn is an o~ emitter; the therat~utic value comes from the irradiation
of the tissue by the -y-rays of the decay daughters 214pb and 214Bi which reach radioactive
eo~uilibrium extremely rapidly with the 222Rn.
~9mTc is used for diagnostic purposes for liver, spleen, and thyroid scanning (w The
99 Mo parent, obtained by chemical separation from 235 U fission product mixture, is
absorbed on a column of alumina and the daughter 99mTc removed by passage of saline
solution at intervals governed by the equilibrium. The parent, when it is fixed in a
semipermanent sample as on an adsorbent column, is often known as a cow and the

Table 4.1. Some common radioactive milking pairs. The decay properties include decay energy (Me V), mode of
decay and half-life
,,
Mother Decay Daughter Decay
nuclide properties nuclide properties Application
44Ti EC, 7; 47.3 y 44Sc 1.5/3+; 1.16 7; 3.93 h Teaching
6SGe EC; 270.8 d 6SGa 1.9/3+; 1.08 7; 1.135 h Medical
87y EC; 3.35 d 87mSr 0.39 7; 2.80 h Medical & teaching
9~ 0.5/3-; 28.5 y 90y 2.3/3-; 2.671 d Heat source ?,
Calibration source
99Mo /3-, 7; 65.9 h 99mTc 0.14 7; 6.0 h Medical
l l3Sn EC, 7; 115.1 d 113rain 0.39 7; 1.658 h Medical
132Te /3-, 7; 78.2 h 132I 2.1 /3-, 7; 2.28 h Medical
137Cs /3-, 7; 30.0 y 137tuBa 0.66 7; 2.55 m Gamma radiography,
Radiation sterilization ?
14~ /3-, 7; 12.75 d 14~ /3-, 7; 1.678 d Lanthanum tracer
144Ce /3-, 7; 284.9 d 144Pr 3.0/3-; 17.28 m Calibration source
21~ B-, 7; 22.3 y 21~ 1.2/3-; 5.01 d Calibration source
226Ra c~; 1600 y 222Rn c~; 3.825 d Medical
23SU o~; 4.468 x 109y 23~ /3-, 7; 24.1 d Thorium tracer
? Main use of mother substance in large amounts.