Page 74 - Radiochemistry and nuclear chemistry

Unstable Nuclei amt Radioactive Decay 63

This recoil led to ejection of 214pb into the wall of the instrument. The use of the recoil
of the daughter to effect its separation was employed by O. Hahn beginning in 1909 and
played a central role in elucidating the different natural radioactive decay chains.
The recoil may affect such chemical properties as the solubility or dissolution rate of
compounds. For example the dissolution of uranium from uranium rich minerals is
considerably higher than one would expect from laboratory solubility data because c~ and
U-atom recoil have moved U-atoms away from their normal sites in the mineral.
Alpha-decay energies are most precisely measured in magnetic spectrometers. From (2.5)
and (2.10) it is calculated that

Et~ = 2e 2 B 2 r 2 ~mile (4.17)

From knowledge of the values of e, mile, B, and r, E a can be calculated. A more
common technique is to use semiconductor detectors combined with pulse height analyzers
('a-spectrometers', Ch. 8).

4.4. Beta decay

4.4.1. Detection

Energetic electrons cause ionization and molecular excitation in matter, although the effect
is weaker and more difficult to detect than for a-particles. As a result the effect must be
amplified for counting individual/3-particles. Ionization is used in proportional and Geiger
counters. Scintillation counting can also be used with various detector systems (Ch. 8).

4.4.2. The t3-decay process

The radioactive decay processes which are designated by the general name of B-decay
include electron emission (/3- or _~ positron emission (/3 + or +~ and electron capture
(EC). If we use the/3-decay of 137Cs as an example, we can write

137Cs ~ 137tuBa + /3-

This/3-decay must occur between discrete quantum levels of the parent nuclide 137Cs and
the daughter nuclide 137tuBa.
The quantum levels of nuclei are characterized by several quantum numbers, an important
one being the nuclear spin. The spin value for the 137Cs ground state level is 7/2, while
that of 137tuBa is 11/2. The electron emitted is an elementary particle of spin 1/2. In nuclear
reactions the nuclear angular momentum must be conserved (4.8), which means that in
radioactive decay processes the difference m total spin between reactant and products must
be an integral value (4.10). Inspection of our example shows that this conservation of spin
rule is violated if the reaction is complete as we have written it. The sum of the spin of the
137mBa and of the electron is 11/2 + 1/2 or 6, while that of the 137Cs is 7/2. Therefore,
the change in spin (A/) in the process would seem to be 5/2 spin units. Inasmuch as this is