no longer an exclusively vicarious one.

Sunday, February 20, 2005

production of transuranic elements

Production of Transuranic Elements

Transuranic elements are elements with an atomic number above that of uranium with atomic number Z= 92.

Only three of the transuranic elements, those with atomic numbers 93, 94 and 95, have been produced in nuclear reactors. Nuclear reactors produce lots of neutrons from fission reactions. Some of these neutrons are captured by heavy nuclei, such as uranium-238. This makes a new nucleus with the same atomic number but a higher atomic mass.Some of these new heavier nuclei can then convert to nuclei of other elements by beta decay, in which one neutron converts to a proton, an electron (beta particle), and an electron anti-neutrino (usually undetectable). So the mass of the nucleus stays about the same, but the number of neutrons goes down by one and the number of protons goes up by one. Plutonium-239, for instance, is produced by on electron capture by uranium-238 followed by two beta decays:
U-238 + n --> U-239 --> Np-239 + beta --> Pu-239 + beta

Pu-239 is changed to americium by neutron bombardment.
Pu-239 + n --> Pu-240 ; Pu-240 + n --> Pu-241 ; Pu-241 --> Am-241 + beta

Transuranic elements from atomic number 96 and up are all made by accelerating a small nucleus (such as He, B or C) in a charged particle accelerator to collide with a heavy nucleus (often of a previously made transuranic element) target.
Californium, with atomic number of 98, is a man-made element that was originally made in 1950 by bombarding an element called curium, a previously discovered synthetic element, with high-energy helium ions produced in a cyclotron, which is a type of charged-particle accelerator that can cause charged particles to develop high kinetic energies; these high-energy particles can then be used to bombard the nuclei of target atoms with the possibility of changing the nuclear configuration. Thus, curium, with 96 protons in its nucleus, can be bombarded by high-energy helium ions, each containing two protons along with two neutrons, to yield californium with 98 protons in its nucleus. Californium, symbolized Cf, particularly an isotope (isotopes of an element are species with the same numbers of protons in their nuclei but with different numbers of neutrons) referred to as californium-252 (252 is the combined number of neutrons plus protons in the nucleus), can also be made by bombarding selected target nuclei in an intense field of neutron radiation, most often in a nuclear reactor; such reactors are our most prolific sources of neutrons.


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