If the energy of the neutron is such that the energy of the compound nucleus corresponds exactly to one of these energy levels, the nucleus comes into resonance and the capture probability becomes very large.
After losing its excitation energy, the nucleus becomes an isotope of the initial nucleus with one more neutron. This isotope can itself be unstable. That is why the materials subjected to a neutron flux, such as the cooling pipes or the claddings coating the fuel, become radioactive.
The production of radioisotopes for medicine, research or industry relies on this activation phenomenon. The irradiation of a suitable target by an intense neutron flux in specialized reactors generates the desired radioisotopes. For instance, one obtains a source of radioactive cobalt by irradiating a target of cobalt. Similarly, by irradiating molybdenum, one produces molybdenum, the precursor of technetium the main radioisotope used in medicine for scintigraphies,.
Radioactive captures play multiple roles in reactors. Sometimes, one should avoid them, in order not to lose too much neutrons in the moderator or the fuel. Sometimes they are wanted, like those of uranium in fast neutrons reactors in order to generate fissile plutonium The radiative captures lead also to the formation of radioactive nuclei heavier than uranium, actinides, which are part of nuclear plant waste.
Fission competes with radiative captures for a handful of nuclei. These radiative captures can be viewed as failed fission. The percentage of losses by radiative capture is an important feature of fissile nuclei. EN FR. Neutron Capture Capture competes with fission and generates radioactivity The neutron is a special elementary particle in nuclear physics : it is easily absorbed in a nucleus because of its lack of electric charge: nucleon itself, it naturally interacts with other nucleons.
The trap of resonant captures The curve of the percentages of neutrons capture in collisions with uranium nuclei shows the presence of a "forest" of resonances below an energy of 10 keV. At a resonant energy, not only the nucleus seems to grow, but also the capture probability becomes large. Neutron Activation Analysis NAA is a sensitive, non-destructive method for determining the elemental composition of a sample.
To conduct a Neutron Activation Analysis experiment, the sample is exposed to neutrons in a nuclear reactor, causing a portion of the atoms to undergo neutron capture: this produces high energy compound nuclei which rapidly transform to radioactive forms of the original chemical element s.
As the short-lived radioactive isotopes undergo decay to reach stable ground state configurations, the sample is placed on a high purity germanium detector which records the intensities and energies of the gamma rays that are emitted. Because a given radioactive isotope always emits gamma rays at certain specific energies and intensities, the radioisotopes present, and hence the parent chemical element s present in the sample can be determined quantitatively.
Unlike of the lighter elements in the Periodic Table, when uranium absorbs a neutron, the neutron capture event is followed immediately by nuclear fission.
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