Man-Made Nuclides

Early Work During the 1930s, Irène and Frédérick Joliot-Curie made the first artificial radioactive nuclides by bombarding boron ( 5 B) and aluminum ( 13 Al) with α particles to form radioactive isotopes of nitrogen ( 7 N) and phosphorus ( 15 P).

New isotopes of existing elements Whereas the isotope Nitrogen-15 is stable ( a percentage abundance of 0.38% in the air) phosphorus-30 has a half life of 2.25min and is a β + emitter.

Making transuranic elements Protons are positively charged and tend repel one another (they are held in the nucleus against this repulsion by the strong nuclear force). Atoms whose nuclei have more than 92 protons tend to be unstable. None are found naturally on Earth

The First Transuranic Element It was found that by exposing uranium and other elements to neutron radiation a neutron would occasionally be absorbed. This increases the nucleon number but not the proton number. However if the larger nucleus is subject to beta decay ( in which a neutron turns into a proton) an atom with a higher atomic number is formed: This meant that the nucleus now had 93 protons and was a new element, of atomic number 93, which was christened neptunium, Np. This was actually done by Edwin McMillan and Philip Abelson pn

Quick Question (a) It might seem easier to make an atom of atomic number 93 by firing protons at uranium (atomic number 92) and hoping they would ‘stick’ rather than firing neutrons, hoping that one would ‘stick’ and then hoping that one would then turn into a proton and an electron. Explain the problem involved with a proton sticking to a uranium nucleus. (b) If an alpha particle is fired at an atom of uranium-238 and it ‘sticks’, what new atom will be formed (give the name, symbol, number of neutrons and number of protons)?

Seaborg The nuclear chemist Glen T Seaborg discovered many transuranic elements Element 106 seaborgium was named in his honour plutonium 94 americium 95 curium 96 berkelium, 97 californium 98 einsteinium 99 fermium 100 mendelevium 101 Nobelium (produced by bombarding curium with carbon nuclei) 102

The properties of man-made radioactive nuclides The nuclides which do not exist in nature have relatively short half lives which explains their rarity. The sheer number of man made nuclides means that many have found applications in industry and medicine: Medical traces including iodine 131 ( positron emitter half life of 8 days)

Accelerators and nuclear reactors may also produce manmade radioactive elements that emit radiation. Many manmade nuclides are used in medicine, industry, and research. For example, moisture-density gauges use the manmade sources: cesium-137 (gamma source), Cf-252 (neutron source), and americium-241:beryllium (neutron source).