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The Oppenheimer–Phillips process or strip reaction is a type of deuteron-induced nuclear reaction.
In this process the neutron half of an energetic deuteron fuses with a target nucleus, transmuting it to a heavier isotope, while the proton half is ejected. An example would be the nuclear transmutation of carbon-12 to carbon-13.
The process is considered important because the cross section for the interaction is enhanced over that expected from the Bohr model due to a polarization of the deuteron where the proton-end faces away from the incident nucleus and the neutron-end faces towards the incident nucleus during most energetically favorable arrangement for the reaction.
History
Explanation of this effect was published by J. Robert Oppenheimer and Melba Phillips in 1935, considering experiments with the Berkeley cyclotron showing that some elements became radioactive under deuteron bombardment.
Mechanism
Normally, the Coulomb barrier prevents atomic nuclei, which are always positively charged, from approaching close enough to fuse. However, neutrons are not affected by the electrostatic repulsive force that creates the barrier, because they have no charge.
Deuterium is the simplest nucleus other than a bare proton, having a single neutron in addition. The deuteron's positive charge is spatially polarized, as if at one end of a barbell, the proton end. As the deuteron approaches the target nucleus, it is deaccelerated by the electrostatic field, converting kinetic energy to potential energy, as with the compression of a spring. Depending on its energy, and assuming that the energy is not sufficient for it to surmount the barrier, it will approach to a minimum distance. As it it slows, the neutron is slowed with it. Binding energy resists their separation. If the orientation of the deuteron at the maximum approach is such that the neutron is oriented toward the target nucleus, it may be close enough to fuse. At these low energies, fusion proceeds if the strong interaction between the neutron and the target nucleus generates greater force than that holding the proton and neutron together. The neutron is "stripped" from the proton.
As the neutron is drawn to the target nucleus, the binding force exerted by it pulls the proton closer than it would otherwise have approached on its own. If the neutron is captured, the proton is stripped from it and is ejected by the electrostatic field, and since it has half the mass of the deuteron, the same repulsive force that originally deaccelerated it can bounce it back with more than doubled velocity, for it carries away not only all of the potential energy stored up in repulsion as it approached, plus half the remaining kinetic energy (zero with a centered collision), but also part of the binding energy of the deuteron. It may in this way carry away more than the incident energy, leaving the transmuted nucleus in an excited state, or even in the ground state, as if it had fused with a neutron of negative kinetic energy.
References
- ^ Friendlander, 2008, p. 68-69
- Oppenheimer, 1995, page 192 cf. Note on the transmutation function for deuterons, J. Robert Oppenheimer and Melba Phillips, Phys. Rev. 48, September 15, 1935, 500-502, received July 1, 1935.
- Blatt, 1991, pp. 508-509
- Blatt, 1991, pp. 508-509
- J. Robert Oppenheimer (1995). Alice Kimball Smith, Charles Weiner (ed.). Robert Oppenheimer: Letters and Recollections (reimpressed, illustrated ed.). Stanford University Press. ISBN 0804726205, 9780804726207.
{{cite book}}: Check|isbn=value: invalid character (help) - Gerhart Friedlander (1949). Introduction to Radiochemistry. John Wiley And Sons. ISBN 1443723096, 9781443723091.
{{cite book}}: Check|isbn=value: invalid character (help) - M. Blatt, John (1991). Theoretical Nuclear Physics (illustrated ed.). Courier Dover Publications. pp. 505–516. ISBN 0486668274, 9780486668277.
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