Nuclear reactions transuranic elements

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

After 1933 Fermi turned increasingly to experimental physics. Inspired by recent work in which artificial radioactive substances were produced by a-particle bombardment, Fermi and several collaborators used neutron bombardment to create several transuranic elements heavier than uranium, including plutonium. This work, and his finding that slow neutrons produce nuclear reactions more efficiently than fast ones, earned Fermi wide acclaim and the 1938 Nobel Prize in physics. After accepting the prize in Sweden, Fermi and his Jewish wife immigrated to the United States to escape the Nazis. 

Radionuclides classified as activation products are created in nuclear reactors and other nuclear devices by the reactions of neutrons with fuel and construction materials. Activation products include the isotopes of the transuranic elements and radioisotopes of hydrogen, carbon, caesium, cobalt, iron, manganese, zinc, and a host of other radionuclides, all of which should be recognised and considered in determining the environmental pathways to human exposure. 

The majority of the longer-lived transuranic nuclides produced by neutron capture reactions decay primarily by a-emission. Most environmental samples contain radionuclides from the natural uranium and thorium series in concentrations often many times greater than transuranic concentrations. As a result, the chemical problems encountered in these measurements are derived from the requirement that separated trans-uranics should be free of a-emitting natural-series nuclides which would constitute a-spectrometric interferences. Table I lists those transuranic nuclides detected to date in marine environmental samples, together with some relevant nuclear properties. Their relative concentrations (on an activity basis) are indicated although the ratios may be altered by environmental fractionation processes which enrich and deplete the relative concentrations of the various transuranic elements. Alpha spectrometric measurements do not distinguish between 239p Pu, so these are... 

The fact was, Noddack went on, any number of elements could be precipitated out of uranium nitrate with manganese. Instead of assuming the production of a new transuranic element, one could assume equally well that when neutrons are used to produce nuclear disintegrations, some distinctly new nuclear reactions take place which have not been observed pre-... 

Consequently, in a nuclear reactor, fission products as well as isotopes of heavier elements are produced. These heavy elements are formed by the neutron capture followed by beta decay and are often referred to as the transuranic elements. Because the fission process is asymmetric, the products of the fission reaction tend to distribute themselves around mass 83-105 (light fragment) and mass 129-149 (heavy fragment). Some of the more common light fragments are Kr, Sr, Zr, Tc, and the corresponding heavy fragments are Cs, Ce, and Nd. 

From the onset, Meitner s team, as well as all other scientists at the time, operated under two false assumptions. The first involved the makeup of the bombarded nuclei. In every nuclear reaction that had been observed, the resulting nucleus had never differed from the original by more than a few protons or neutrons. Thus, scientists assumed that the products of neutron bombardment were radioisotopes of elements that were at most a few places in the periodic table before or beyond the atoms being bombarded (as Fermi had presumed in hypothesizing the transuranes). 

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