Charged particles nuclei excitation

Nuclear reactions induced by electromagnetic radiation can be described - at least for sufficiently low-photon energies - in terms of a two-stage process. Absorption of a photon leads to an intermediate highly excited state of the nucleus. The excitation energy of this so-called compound nucleus can then be released by emission of photons, neutrons, or charged particles. 

Activation methods are based on the measurement of the radioactivity or radiation produced in samples when they are irradiated with neutrons or charged particles, such as hydrogen, deuterium, or helium ions. An overview of the most common type of neutron activation is shown in Figure 32-6. Here, a neutron is captured by the target nucleus to form an excited compound nucleus. The compound nucleus de-excites almost instantaneously by emission of one or more characteristic prompt gamma rays. In many cases a new radioactive nucleus is formed, which can undergo /3 decay to an exited product nucleus with the emission of another characteristic delayed gamma ray. 

Since the quantity F is inversely proportional to the time for which the compound nucleus exists before de-excitation, it is in fact a measure of the probability, per unit time, of the de-excitation taking place. In general, there will be a number of possible ways in which this may occur for example, the compound nucleus may emit the same, or another neutron, or a charged particle, or it may lose its excitation energy by the emission of a quantum of radiation y ray). Each of these possible processes is characterized by its own partial width, F, which is proportional to the relative probability of the particular reaction taking place. The parameter F is then the total level width, and is the sum of all the partial widths corresponding to the possible modes of break-up of the compound nucleus, i.e.. 

Since neutrons are neutral they interact differently to charged particles. The primary interaction occurs with the nucleus of the absorber and little interaction is present with its orbital electrons. Neutrons interact with the nucleus through elastic and inelastic scattering and neutron capture. In the latter mechanism, the neutron is absorbed by the nucleus which in turn excites the nucleus to higher energy levels. As the nucleus returns back to the ground state a particle is emitted (dependent on the incident energy this could be a-particle, neutron etc), and a new radioactive nuclide is produced. 

When a slow neutron is captured by the nucleus of element X, another isotope of the same element is instantaneously formed, in an excited state because of the impact (labelled compound nucleus in Figure 2.13), which then de-excites by the emission of a gamma particle (and possibly other particles) from the nucleus to produce a radioactive nucleus. For example, when 23Na captures a neutron (signified by on, since neutrons have a mass of one unit, but no electrical charge), it becomes the radioactive nucleus 24Na, as follows ... 

---