Page 14 - Radiochemistry and nuclear chemistry

Origin of Nuclear Science

the form of the soluble uranyl carbonate complex, the radioactivity originally associated
with the uranium was now present in the precipitate, which contained no uranium.
Moreover, the radioactivity of the precipitate slowly decreased with time, whereas the
supematant liquid showed a growth of radioactivity during the same period (Fig. 1.1). We
know now that this measurement of radioactivity was concerned with only beta- and
gamma-radiations, and not with the alpha-radiation which is emitted directly by uranium.
Similar results were obtained by E. Rutherford and F. Soddy when investigating the
radioactivity of thorium. Later Rutherford and F. E. Dora found that radioactive gases
(emanation) could be separated from salts of uranium and thorium. After separation of the
gas from the salt, the radioactivity of the gas decreased with time, while new radioactivity
grew in the salt in a manner similar to that shown in Fig. 1.1. The rate of increase with
time of the radioactivity in the salt was found to be completely independent of chemical
processes, temperature, etc. Rutherford and Soddy concluded from these observations that
radioactivity was due to changes within the atoms themselves. They proposed that, when
radioactive decay occurred, the atoms of the original elements (e.g. of U or of Th) were
transformed into atoms of new elements.
The radioactive dements were called radioelements. Lacking names for these
radioelements, letters such as X, Y, Z, A, B, etc., were added to the symbol for the
primary (i.e. parent) element. Thus, UX was produced from the radioactive decay of
uranium, ThX from that of thorium, etc. These new radioelements (UX, ThX, etc.) had
chemical properties that were different from the original elements, and could be separated
from them through chemical processes such as precipitation, volatilization, electrolytic
deposition, etc. The radioactive daughter elements decayed further to form still other
dements, symbolized as UY, ThA, etc. A typical decay chain could be written: Ra ~ Rn
RaA ~ RaB --,, etc.; Fig. 1.2.
A careful study of the radiation emitted from these radioactive dements demonstrated that
it consisted of three components which were given the designation alpha (a), beta (~), and
gamma(-y). Alpha-radiation was shown to be identical to helium ions, whereas
beta-radiation was identical to electrons. Gamma-radiation had the same electromagnetic
nature as X-rays but was of higher energy. The rate of radioactive decay per unit weight
was found to be fixed for any specific radioelement, no matter what its chemical or physical
state was, though this rate differed greatly for different radioelements. The decay rate could
be expressed in terms of a half-life, which is the time it takes for the radioactivity of a
radioelement to decay to one-half of its original value. Half-lives for the different
radioelements were found to vary from fractions of a second to millions of years; e.g. that
of ThA is 0.1 of a second, of UX it is 24.1 days (Fig. 1.1), and of uranium, millions of
years.

1.3. Discovery of isotopes

By 1910 approximately 40 different chemical species had been identified through their
chemical nature, the properties of their radiation, and their characteristic half-lives. The
study of the genetic relationships in the decay of the radioactive species showed that the
radioelements could be divided into three distinct series. Two of these originated in uranium
and the third in thorium. B. Boltwood found that all three of the series ended in the same