Neutrons, capture reaction sources

In this review we wish to discuss how observations of AGB stars can be used to determine the manner in which heavy elements are created during a thermal pulse, and how these heavy elements and carbon are transported to the stellar surface. In particular we wish to study how the periodic hydrogen and helium shell burning above a degenerate carbon-oxygen (C-0) core forms a neutron capture nucleosynthesis site that may eventually account for the observed abundance enhancements at the surfaces of AGB stars. In section II we discuss the nucleosynthesis provided by stellar evolution models (for a general review see [1]). In section III we discuss the isotopic abundances provided by nucleosynthesis reaction network calculations (see [2, 3]). In section IV we discuss how observations of AGB stars can be used to discriminate between the neutron capture nucleosynthesis sources (see [4]). And in section V we note some of the current uncertainty in this work. 

Neutron capture reactions can occur at low temperatures but stars can activate neutron sources only at high temperatures. It was apparent very early in the search for the origins of the chemical elements that two different neutron capture processes are at work in the Universe - see the classical papers by Cameron (1957) and Burbidge, Burbidge, Fowler Hoyle (1957). The processes are distinguished by the neutron density achieved at the... 

Neutron capture reactions are common in massive stars because of the abundance of neutrons available there. Lookback at the equations for carbon-, oxygen-, and neon-burning (page 70). Notice that alpha particles are common products of such reactions. Astrochemists have determined that the three most common sources of neutrons in such reactions are the following reactions ... 

Their work as part of the Manhattan Project was kept secret and was finally reported in 1946, after World War II, although the existence of plutonium had been revealed to the world earfier, when the atomic bomb was dropped over Nagasaki, Japan. There are sixteen isotopes of plutonium, having mass numbers ranging from 232 to 247. The principal isotopes of Pu are those having mass numbers 238, 239, 240, 241, 242, and 244. Ton quantities of Pu (having a half-life of 2.4 X 0" y) are available. The isotope Pu is the source material for nuclear weapons and is produced via neutron capture reactions on in nuclear reactors. 

The decay heat power comes mainly from five sources (1) unstable fission products, which decay via a, p-, p+, and y ray emission to stable isotopes (2) unstable actinides that are formed by successive neutron capture reactions in the uranium and plutonium isotopes present in the fuel (3) fissions induced by delayed neutrons (4) reactions induced by spontaneous fission neutrons (5) structural and cladding materials in the reactor that may have become radioactive. Heat production due to delayed neutron-induced fission or spontaneous fission is usually neglected. Activation of light elements in structural materials plays a role only in special cases. 

Mossbauer spectroscopy was used in combination with ion implantation in the 1960s, shortly after the Mossbauer effect was discovered. In the earliest days, Mossbauer experiments were performed on ion-implanted sources using particle accelerators and nuclear reactors. In 1965, Ruby and Holland [I] populated the 29.6 keV Mossbauer level in K using the (d, p) reaction on potassium metal. Hafemeister and Brooks Shera [2] performed a similar experiment on K using the thermal neutron capture reaction. Berger et al. [3-5] conducted online Mossbauer studies using the (n, y) reaction of Fe. 

An alternative to transferring the sample from irradiator to detector, either manually or by the process stream, is to measure the capture y-radiation emitted by the sample. The instantaneous measurement should be less dependent on flow rate when used for on-stream applications, and may therefore give more precise results. Tiwari et used a 2.5 x 10 n s Am-Be source for the off-line measurement of N in organic materials using the 10.83 MeV prompt y-ray from the N thermal-neutron capture reaction. The possible use of Pu-Be neutron sources for in situ analysis of rocks, using either NaI(Tl) or Ge(Li) detectors for prompt y-radiation, has been discussed with particular reference to extraterrestrial applications. ... 

Thru 1967, emphasis was given to the use of neutrons as the bombarding source of radiation. Almost all possible neutron reactions were considered including moderation of fast neutrons by hydrogen in the expl, thermal capture reactions, elastic and inelastic scattering of neutrons and neutron activation reactions. These neutron reactions are listed as follows ... 

Dr. Flinn Antimony would be exciting to many chemists. Antimony-121 is the Mossbauer isotope of antimony. The first work was done at Wayne State University, and recently there has been a good deal of work by Ruby and others at Argonne which should be appearing shortly. It seems that antimony is similar to tin in its relationship between isomer shift and the various compounds. It is better than tin in that the isomer shift is about five times larger so that precise measurements are possible. Thanks to Ruby s work, the changes with chemical environment are well understood. The AR/R situation is clear cut, but there are some difficulties in preparing a satisfactory source. The parent is tin-121 which is m de by neutron capture by tin-120. The reaction has one of the smallest cross-sections in existence—one can place the tin in a reactor for a year and not produce much even then. However, when a source is obtained, you are in business for a while. Its half-life is 25 years. 

The most sensitive method for determining trace amounts of technetium is the neutron activation . The Tc sample is irradiated by slow neutrons. The radioactive isotope Tc with a half-life of 15.8 s is formed by the reaction Tcfn, y) Tc, the neutron capture cross section of which is comparatively large (20 bams), so that it is possible to determine amounts < 2x 10 " g of Tc. However, the method is not widely used since the half-life of Tc is very short. Moreover, this method is only convenient when a reactor or a neutron source is available. 

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