CHAPTER 1
Introduction
Radiochemistry
Radiochemistry is the chemistry of radioactive materials, where
radioactive isotopes of elements are used to study the
properties and chemical reactions of
non-radioactive isotopes (often within radiochemistry the absence of
radioactivity leads to a substance being described as being inactive as
the isotopes are stable). Much of radiochemistry deals with the use of radioactivity to study ordinary
chemical reactions.
Radiochemistry
includes the study of both natural and man-made radioisotopes.
Activation analysis
By neutron irradiation of objects it is
possible to induce radioactivity, this activation of stable isotopes to create
radioisotopes is the basis of neutron
activation analysis. One of the most interesting objects which have been
studied in this way is the hair of Napoleon's head, which
have been examined for their arsenic content.
A series of
different experimental methods exist, these have been designed to enable the
measurement of a range of different elements in different matrices. To reduce
the effect of the matrix it is common to
use the chemical extraction of the wanted element and/or to allow the
radioactivity due to the matrix elements to decay before the measurement of the
radioactivity. Since the matrix effect can be corrected for by observing the
decay spectrum, little or no sample preparation is required for some samples,
making neutron activation analysis less susceptible to contamination.
The effects of a series of different cooling times can be
seen if a hypothetical sample which contains sodium, uranium and cobalt in a
100:10:1 ratio was subjected to a very short pulse of thermal neutrons. The initial
radioactivity would be dominated by the 24Na activity but with
increasing time the 239Np and finally the 60Co activity
would predominate.
1.1 RADIOACTIVITY
Radioactivity is the
spontaneous disintegration of atomic nuclei. The nucleus emits 
particles, ß particles,
or electromagnetic rays during this process.
Radioactivity refers to the particles which are emitted from
nuclei as a result of nuclear instability. Because the nucleus experiences the
intense conflict between the two strongest forces in nature, it should not be
surprising that there are many nuclear isotopes which are unstable and emit
some kind of radiation. The most common types of radiation are called alpha,
beta, and gamma radiation, but there are several other varieties of radioactive
decay
1.2 WHAT MAKES A
SUBSTANCE RADIOACTIVE?
Radioactivity is the random spontaneous disintegration of an
atom of an element. The stability of the nucleus depends on the relative
numbers of protons and neutrons present. The most stable nuclides tend to have
an even number of protons and an even number of neutrons as each group of two
protons and two neutrons, in the nucleus, makes an especially stable
combination. A number of elements have atoms or nuclei which are unstable and
consequently split up to form smaller atoms. This is due to all elements
wanting to achieve equilibrium or stability in the nucleus.
A substance is said to be radioactive if it contains unstable
nuclei and is able to naturally release energy in the process of shedding high
speed charged particles, in an attempt to reach a stable state. With this, a
non-radioactive substance will remain intact indefinitely unless acted upon by
an external force.
In the process of transmutation, known as decay, the
radioactive nuclei emits charged particles or electromagnetic rays depending on
the nature its instability. This result in a daughter nuclide being produced
which may be radioactive or stable, but if it is radioactive it will in turn
decay into a daughter nuclide, continuing to do so until it reaches the point
of stability, an isotope of lead
1.3 Atomic
Transformations(nuclear transformation)
In 1897 J.J.Thomson had deflected cathode ray particles using
magnetic fields to determine the ratio of the electric charge to the mass of
the particles. The ratio about two thousand times larger than the ratio
for the lightest chemical ion, hydrogen = H+1, was the basis for
announcing the discovery of what became know as electrons. If the charges
were comparable, the electron mass would be roughly 1/2000 that of the lightest
atom.
During 1900 the Curies collected beta radiation, β, in an electroscope gathering
evidence that beta rays, like electrons, are also composed of negatively
charged particles. When a beam of mixed forms of radiation was allowed to
emerge from a hole in a lead container, the beta rays, β, were deflected one
way in a magnetic field (as shown to left) while the alpha rays, α, were
deflected the opposite direction. (Gamma rays, γ, were
undeflected.) Thus the alpha rays were known to carry positive electric
charge. Using the same procedure as previously used by J.J.Thomson,
Becquerel confirmed the charge to mass ratio for beta rays matched that of
electrons. Beta must be electrons
emerging at high speeds from some radioactive materials. Measuring the
charge to mass ratio for alpha rays required a stronger magnetic field because
that ratio is about a factor of 4000 smaller.
If the electric
charges are comparable, the mass might be roughly double that of a hydrogen
ion. In 1903 Sir William Ramsey and Frederick Soddy discovered helium
imprisoned in radioactive minerals.
Ernest Rutherford proposed that alpha particles could be
Helium ions, He2+, since they also would have the correct charge to
mass ratio. But it took most of the rest of the decade to confirm.
T.D.Royds and Ernest Rutherford placed in an inner
glass container with extremely thin walls (A in their diagram) Radon gas (then
called Radium emanation) known to emit alpha particles. Alpha radiation
escaped into the evacuated outer glass container (T) where the ions picked up
electrons with collisions with the glass. The outer container was flooded
after 2, 4, and 6 days with Mercury compressing any new gas into the very
narrow neck (V in their diagram). There a spark excited any atoms to
produce a visible spectra. Each time the spectra was checked, the well
known spectra of Helium grew brighter confirming that alpha radiation is composed of Helium ions. (Read their original
report.)
Procedure
It is now customary to indicate specify a particular isotope
by using exponents and subscripts. 20782Pb
represents lead (symbol Pb = Plumbum), atomic number 82, with mass of 207.
- After all the sequential
radiation releases have been determined, it was possible to use Soddy's
rules to identify each isotope in the sequence. We wish to duplicate
that process with a series of radioactive changes started with what is now
known to be the Uranium isotope 23592U. This
parent element emits in succession α, β, α, β, α, α, α, α, β, α, β ending
with the stable isotope 20782Pb. From this
information and a periodic chart, determine the complete symbol for each
isotope in this decay series.
- Before Soddy developed the rules
for radioactive transformations, most products formed were not correctly
identified. So names then used seem strange to us. Thorium, 23290Th,
decayed as follows. Supply the missing data then use a modern
periodic chart to determine the actual isotopes: (The symbols used are
those that were originally assigned: Since these isotopes were
derived from Thorium, they all originally had Th symbols. Ms stands
for meta-stable for having longer lifetimes.)
- 24194Pu is an artificially made
isotope produced by bombarding Uranium with neutrons in a reactor.
It decays via 24195Am, 23793Np,
23391Pa, 23392U, 22990Th
and 22588Ra. Outline the decay series for
Plutonium showing the modes of decay as in the previous case.
1.4 Post Script
Not understanding the potential effects of radiation on
living cells, many researchers took no precautions against radiation.
Pierre Curie carried a warm, glowing sample of Radium around in his pocket to
show people. Marie Curie kept a glowing sample as a night light beside
her bed. Only after early researchers became ill did they begin to
realize the need for safety precautions. Marie Curie helped form an
international committee to establish standards and safety regulations.
Still most believed that fresh air and time away from their researches provided
adequate recovery. It wasn't until much later when many early
investigators reached retirement ages that the connection between radiation
exposures and various forms of cancer became apparent.
1.5 Discovery of radioactivity
Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel while
working on phosphorescent materials. These materials glow in the dark after exposure
to light, and he thought that the glow produced in cathode ray tubes by X-rays might be connected with phosphorescence. He wrapped a
photographic plate in black paper and placed various phosphorescent minerals on it. All
results were negative until he used uranium salts. The
result with these compounds was a deep blackening of the plate.
It soon became clear that the blackening of the plate had
nothing to do with phosphorescence, because the plate blackened when the
mineral was in the dark. Non-phosphorescent salts of uranium and metallic
uranium also blackened the plate. Clearly there was a form of radiation that
could pass through paper that was causing the plate to blacken.
Alpha
particles may be completely stopped by a sheet of paper, beta particles by
aluminum shielding. Gamma rays can only be reduced by much more substantial
barriers, such as a very thick layer of lead.
At first it seemed that the new radiation was similar to the
then recently discovered X-rays. Further research by Becquerel, Marie Curie, Pierre Curie, Ernest Rutherford and
others discovered that radioactivity was significantly more complicated.
Different types of decay can occur, but Rutherford was the first to realize
that they all occur with the same mathematical approximately exponential formula
(see below).
As for types of radioactive radiation, it was found that an electric or
magnetic field could split such emissions into
three types of beams.
For lack of better
terms, the rays were given the alphabetic names alpha, beta and gamma, still in use today. It was obvious from the direction of electromagnetic forces that alpha rays
carried a positive charge, beta rays carried a
negative charge, and gamma rays were neutral.
From the magnitude of deflection, it was clear that alpha particles were much more massive than beta particles. Passing alpha rays through a thin glass membrane and
trapping them in a discharge tube allowed
researchers to study the emission spectrum of
the resulting gas, and ultimately prove that alpha particles are helium nuclei. Other experiments showed the similarity between
beta radiation and cathode rays; they are both
streams of electrons, and between gamma
radiation and X-rays, which are both high energy electromagnetic
radiation.
Although alpha, beta, and gamma are most common, other types
of decay were eventually discovered. Shortly after discovery of the neutron in
1932, it was discovered by Enrico Fermi that
certain rare decay reactions yield neutrons as a decay particle. Isolated proton
emission was eventually observed in some elements.
Shortly after the discovery of the positron in
cosmic ray products, it was realized that the same process that operates in
classical beta decay can also produce
positrons (positron emission), analogously to
negative electrons. Each of the two types of beta decay acts to move a nucleus
toward a ratio of neutrons and protons which has the least energy for the
combination. Finally, in a phenomenon called cluster decay, specific combinations of neutrons and protons other than
alpha particles were spontaneously emitted from atoms on occasion.
Still other types of radioactive decay were found which emit
previously seen particles, but by different mechanisms. An example is internal conversion, which results in electron and sometimes high energy
photon emission, even though it involves neither beta nor gamma decay.
The early researchers also discovered that many other chemical elements besides uranium have radioactive isotopes. A systematic search for the total radioactivity in
uranium ores also guided Marie Curie to
isolate a new element polonium and to separate
new element radium from barium. The two elements' chemical similarity would otherwise
have made them difficult to distinguish.
The dangers of radioactivity and of radiation were not immediately
recognized. Acute effects of radiation were first observed in the use of X-rays
when the Serbo-Croatian-American electric engineer Nikola Tesla
intentionally subjected his fingers to X-rays in 1896. He published his
observations concerning the burns that developed, though he attributed them to
ozone rather than to X-rays. His injuries healed later.
The genetic effects of radiation, including the effects on
cancer risk, were recognized much later. In 1927 Hermann Joseph Muller
published research showing genetic effects, and in 1946 was awarded the Nobel
Prize for his findings.
Before the biological effects of radiation were known, many
physicians and corporations had begun marketing radioactive substances as patent medicine and radioactive quackery.
Examples were radium enema treatments, and
radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the
effects of radiation on the human body were not well understood (Curie later
died from a plastic anemia assumed due to her
work with radium, but later examination of her bones showed that she had been a
careful laboratory worker and had a low burden of radium. A more likely cause
was her exposure to unshielded X-ray tubes while a volunteer medical worker in
WWI). By the 1930s, after a number of cases of bone necrosis and death in
enthusiasts, radium-containing medical products had nearly vanished from the
market.
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