Page 67 - Radiochemistry and nuclear chemistry

56 Radiochemistry and Nuclear Chemistry

stable odd-even isotopes of technetium. The longest lived isotopes of technetium are those
with A = 97 (2.6 • 106 y), A = 98 (4.2)< 106 y), and A = 99 (2.1 • 106 y). Figure 3.8
shows the decay scheme for A = 99, which is taken out of a standard Isotope Table; the
vertical axis shows the relative binding energies (broken scale). The Figure illustrates the
information normally presented in isotope tables, and will be further explained in
subsequent chapters.
Hundreds of kilograms of 99Tc and its precursor (fore-runner) 99Mo are formed every
year as fission products in nuclear reactors, and 10's of kg of Tc have been isolated and
studied chemically. Its properties resembles those of its homologs in the Periodic Table -
manganese and rhenium. Figure 3.8 shows decay schemes for mass number 99: the upper
one from Shirley et al, 1986, the lower one from Dzhelepov et al, 1961; more detailed
schemes appear in both references. The ones shown in Figure 3.8 were chosen for
pedagogic reasons, and, for this purpose, we have also inserted explanations, some of
which will be dealt with later. Older references are often still useful for rapid survey, while
the newest ones give the most recent information and refined numerical data.
The upper left part of Figure 3.8 shows a decay chain from fission of 235U that ends in
99Ru, the most stable isobar ofA = 99. The lower diagram shows that the 99Mo/J- decays
all reaches the spin/parity a/~- level, designated 99mTc; this isomer decays with t~ 6.02 h
to long-lived 99Tc, emitting a single 3' of 0.142 MeV (> 99 %, see upper diagram). The
isomer 99mTc is a widely used radionuclide in nuclear diagnostics (w and can be
conveniently "milked" from its mother 99Mo, see w

3.9. Other modes of instability

In this chapter we have stressed nuclear instability to beta decay. However, in w it was
learned that very heavy nuclei are unstable to fission. There is also a possibility of
instability to emission of ix-particles in heavy elements (circles in Figure 3.1) and to neutron
and proton emission.
Nuclei are unstable to forms of decay as indicated in Figure 3.1. For example, making
a vertical cut at N = 100, the instability from the top is first proton emission, then,
a-emission (for N = 60 it would instead be positron emission or electron capture, as these
two processes are about equally probable), and, after passing the stable nuclides (the
isotones 170 Yb, 169 Tm and 168 Er),/3-emission and, finally, neutron emission. This is more
clearly indicated in Appendix C, and for the heaviest nuclides (i.e. Z _> 81) in Figures 5.1
and 16.1. For ix-decay the Figure indicates that for ,4 > 150 (Z > 70, N > 80) the nuclei
are a-unstable, but in fact t~-decay is commonly observed only above A ~ 200. This is due
to the necessity for the c~-particle to pass over or penetrate the Coulomb barrier (of.
w Although neutron and proton emissions are possible energetically, they are not
commonly observed as the competing/3-decay processes are much faster.

3.10. Exercises

3.1. Calculate the nucleon binding energy in 2~lg from the atomic mass excess value in Table 3.1.
3.2. How many times larger is the nucleon binding energy in 24Na than the electron binding energy when the
ionization potential of the sodium atom is 5.14 V?