Page 101 - Radiochemistry and nuclear chemistry

90 Radiochemistry and Nuclear Chemistry

removal of the daughter activity from the radioisotope generator (the "cow") is termed
milking.
Another commonly used radioisotope generator is 132Te from which 1321 may be milked.
In this case 132Te is adsorbed as barium tellurite on an alumina column, and the 1321
removed by passage of 0.01 M ammonia through the column. The 1321 is used both
diagnostically and therapeutically for thyroid cancer.
Many of these sources produce radionuclides with half-lives suitable for teaching
purposes, e.g. 137mBa(2.6 min), 144pr(17.3 rain), 44Sc(4.0 h), 99mTc(6.0 h) and 90y(64 h).

4.17. Decay energy and half-life

It was observed early in both a- and/~-decay that the longer the half-life the lower the
decay energy. Although there are many exceptions to this observation H. Geiger and J. M.
Nuttall formulated the law

log ;ka = a + b log/~air (4.60)

for the natural a-active nuclides. Here a and b are constants, and J~air is the range of the
c~-particles in air which is directly proportional to the c~-particle energy E a. A similar
relation was deduced by E. Fermi for the B-decay:

logX~= a' + b'logE (4.61)

where a' is a constant related to the type of/$ decay and b' ~ 5.
Although these rules have been superseded by modem theory and the enormous amount
of nuclear data now available, they may nevertheless be useful as rough guides in estimates
of half-lives and decay energies. In w 11.7 more valid but more complicated relationships
are discussed.

4.18. The Heisenberg uncertainty prindple

In this chapter we have repeatedly stated that the nuclear decay energies are exact values
as required by quantum mechanics. However, this is not exactly correct: the energy levels
have a certain "spread". This was first stated by Heisenberg in 1927 and is of fundamental
importance in all areas of nuclear physics.
The uncertainty principle states that it is impossible to measure simultaneously the exact
position and the exact momentum of a particle. This follows from the wave properties of
the particle. If for example, we attempt to measure the exact position of an electron by
observing the light emitted when it hits a scintillating screen, this act interferes with the
movement of the electron causing it to scatter, which introduces some uncertainty in its
momentum. The size of this uncertainty can be calculated exactly and is related to the
Planck constant. If Ax denotes the uncertainty in position and Ap the uncertainty in
momentum along the x axis, then