Many kinds of nuclear reactions occur in response to the absorption of particles such as neutrons or protons. Other types of reactions may involve the absorption of gamma rays or the scattering of gamma rays. Of particular note is the resonant absorption of gamma rays in the Mossbauer effect. Specific nuclear reactions can be written down in a manner similar to chemical reaction equations. If a target nucleus X is bombarded by a particle a and results in a nucleus Y with emitted particle b, this is commonly written in one of two ways.
a + X -> Y + b or X(a,b)YWe can characterize the energetics of the reaction with a reaction energy Q, defined as the energy released in the reaction. The Q is positive if the total mass of the products is less than that of the projectile and target, indicating that the total nuclear binding energy has increased. The probability of a given type of nuclear reaction taking place is often stated as a "cross section".
Index HyperPhysics***** Nuclear Go Back Some Nuclear Reactions Reaction Measured Q (MeV) Reaction Measured Q (MeV) 2H(n,γ)3H 6.257 +/- 0.004 9Be(p,α)6Li 2.132 +/- 0.006 2H(d,p)3H 4.032 +/- 0.004 10B(n,α)7Li 2.793 +/- 0.003 6Li(p,α)3H 4.016 +/- 0.005 10B(p,α)7Be 1.148 +/- 0.003 6Li(d,p)7Li 5.020 +/- 0.006 12C(n,γ)13C 4.948 +/- 0.004 7Li(p,n)7Be -1.645 +/- 0.001 13C(p,n)13N -3.003 +/- 0.002 7Li(p,α)4He 17.337 +/- 0.007 *14N(p,n)14C -0.627 +/- 0.001 9Be(n,γ)10Be 6.810 +/- 0.006 14N(n,γ)15N 10.833 +/- 0.007 9Be(γ,n)8Be -1.666 +/- 0.002 18O(p,n)18F -2.453 +/- 0.002 9Be(d,p)10Be 4.585 +/- 0.005 19F(p,α)16O 8.124 +/- 0.007* The nuclear reaction in the atmosphere which produces carbon-14 for radiocarbon dating.
Data from C. W. Li, W. Whaling, W. A. Fowler, and C. C. Lauritson, Physical Review 83:512 (1951)
Index HyperPhysics***** Nuclear Go Back Nuclear Cross SectionTo characterize the probability that a certain nuclear reaction will take place, it is customary to define an effective size of the nucleus for that reaction, called a cross section. The cross section is defined by
The cross section has the units of area and is on the order of the square of the nuclear radius. A commonly used unit is the barn:
1 barn = 10-28m2A standard old story was that in the early days of the field, a particular cross section turned out to be much bigger than expected. An experimenter exclaimed "Why, that's as big as a barn!" and a unit name was born.
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