C. Artificial Transmutation of the Elements
It was John Dalton, in his quest to debunk
alchemy, who suggested that atoms were unchangeable; forget about
changing lead into gold. Dalton's prohibition was solid for most
of the nineteenth century until Becquerel and the Curies published
their findings regarding radioactivity. Eventually, physicists
were forced to conclude that there existed a process in nature
wherein atoms spontaneously and without prodding changed into
other atoms by throwing off pieces of mass in the form of alpha
and beta particles.
While it may have been acceptable for mother nature to engage in this process, it was still considered out of the realm of possibility for the physics community. Impossible that is until 1919 when Ernest Rutherford bombarded nitrogen atoms with alpha particles (his favorite bullet). On a scintillation detector nearby, he noted characteristic flashes resembling those made by protons. He concluded that in the collision between alpha particles and nitrogen nuclei, a proton was chipped off the nitrogen. An equation to describe the reaction might be
This phenomenon was named artificial
transmutation of the elements. The process provided physicists
with the awesome power to create new isotopes of the elements,
isotopes that did not occur naturally in the Earth's crust.
A limiting factor in this creation process was the energy of the bullets used to strike the nuclear targets. Rutherford used alpha particles tossed from radium or radon. What if these bullets could be given additional kinetic energy by pushing them along. If necessity is the mother of invention, necessity gives birth in the next decade to the particle accelerator. (See the next unit.)
For more information on artificial transmutation, go to these sites:
Physicists continue to cause nuclear collisions during the next two decades not really knowing what to expect. The results of two series of experiments laid the foundation for the nuclear age as we know it.
Discovery of the Neutron
The work of bombarding target atoms continued with some startling results appearing in 1930. Physicists in Germany, France and Great Britain bombarded light elements such as beryllium and boron and noticed some very penetrating radiation emanating from these light metals. The nature of the penetrating radiation left everyone in a quandary. The in 1932 in England, James Chadwick ventured the opinion that the radiation was composed of neutron, the elusive third part of the atom. This particle had gone undetected for so long because it has no charge; it cannot be repelled by the target nucleus. If the target nucleus is large, the neutron strikes it and recoils. If the target nucleus is small, it is the target that gets knocked away. Chadwick discovers that lead plates are not good shields for neutron sources, but blocks of paraffin are perfect for the task. The neutrons bounce off the lead nuclei with little loss of kinetic energy. On the other hand, when the neutron strikes hydrogen proton in paraffin, the protons are easily knocked away.
Artificially Induced Radioactivity
Meanwhile, in France, similar work was underway at the direction of chemists Frederick Joliot and his wife Irene Curie, daughter of the Nobel laureates. In 1934, the Curie-Joliots (as they were known) bombarded aluminum with alpha particles. The target atoms after bombardment showed signs of being radioactive, with newly discovered (in 1932 by C.D. Anderson) positron as the emitted particle. The event prompted these equations as descriptions of what happened.
Thus, not only were the Curie-Joliots able to create new isotopes by artificial transmutation, it turned out that their new isotopes were radioactive and therefore capable of decaying into still other forms of matter. For their contribution to the field, the Curie-Joliots were awarded the Nobel prize in chemistry in 1935.
For the record, here are other nuclear reactions that have been effected:
Clearly, these reactions would have warmed the hearts of seventeenth century alchemists. Alas, they are so inefficient as to be impractical.
The Transuranic Elements
Meanwhile, in Italy a research group led by Enrico Fermi was conducting its own series of transmutation experiments. Their bullet of choice was the neutron because, with no positive charge to be repelled by the target nucleus, the neutron could deliver maximum impact on the target. Of interest was the light metal reactions
Notice that the result of bombarding aluminum with neutrons is an isotope whose atomic number increased by +1 through beta decay. What if the target nucleus was uranium, the heaviest of the elements? The result was spectacular.
This is truly a remarkable event. It was one sense of pride and accomplishment that scientists fashioned isotopes of elements. They must have been awestruck to actually create new elements that Mother Nature elected not deposit here. For more information about the people who fashioned the transuranic elements, check out Glenn Seaborg and Ernest O. Lawrence. Here are a few places to start.
An expanded version of the periodic table is called the chart of the nuclides which lists every isosope of every element. See this table at http://chemlab.pc.maricopa.edu/periodic/isotopes.html
What's a neutrino?
Try these sites.
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Last edited 12/26/05