Transitions gamma emission

We have already seen that gamma emissions are the result of transitions between the excited states of nuclei. As the whole technique of gamma spectrometry rests on (a) the uniqueness of gamma energies in the characterization of radioactive species, and (b) the high precision with which such energies can be measured, it is of interest to consider briefly some relevant properties of the excited states. 

It is common for the number of gamma-rays emitted by a nuclide to be referred to as the abundance , sometimes as the yield . Both of these terms lack precision. Historically, confusion was often caused because an author or data source would quote abundances that were effectively beta transition data - the 93.5 % figure quoted above. In fact, the proportion of decays that give rise to 661.7 keV gamma-rays in the example above is 85.1 % when internal conversion is taken into account. In this text, I will use the term gamma emission probability on the basis that it says exactly what it means - the probability that a gamma-ray will be emitted, aU other factors being taken into account. 

Inelastic Scattering - A neutron enters a ground state nucleus, imparts kinetic energy to the nucleus, and leaves the nucleus in an excited state. The nucleus subsequently decays by gamma emission, called isomeric transition, to ground state. 

In the heaviest nuclei transitions between excited states are dominated by internal conversion electron emission over gamma emission. It is important to realize that the emission of conversion electrons is a direct process, and does not proceed via an intermediate gamma ray. This is mainly due to the increased probability of finding an atomic electron (its wave function) inside the nucleus where energy can be transferred to it directly. We define the internal conversion coefficient a as the ratio of the number of electrons that get emitted to the number of gamma rays emitted during the decay of a sufficiently large ensemble ... 

When nuclear decay is a pure )S-emission or by p with low energy, unconverted gamma-transition recoil energy is unimportant, and electronic... 

Transition, Isomeric—The process by which a nuclide decays to an isomeric nuclide (i.e., one of the same mass number and atomic number) of lower quantum energy. Isomeric transitions (often abbreviated I.T.) proceed by gamma ray and/or internal conversion electron emission. 

Conversely, nuclei that contain an excess of protons undergo proton to neutron transition with the emission of a positively charged beta particle known as a positron Q8+) and with the reduction of the atomic number by one. A positron has only a very short existence, combining immediately with an electron of a nearby atom. The two particles disintegrate in the process with the emission of two gamma rays, e.g. 

Decay mode (emission), and energy (MeV ( if to ground state)), separated by / if several modes if in parentheses, mode produces a shortlived daughter, or occurs <10% Emissions a=alpha =2He4++ P"=electron P+=positron gamma, n=neutron EC=electron capture from K or L-shell (n,2a) =nucleus absorbs neutron and emits 2a IT=internal transition SF=spontaneous fission... 

Recently there has been a lot of interest in the luminescence of BaF2. Its crystals have a large potential as a scintillator material (detection of gamma rays). They show a luminescence at 220 nm with a very short decay time, viz., 600 ps. This short decay time offers the possibility of a good time resolution. This luminescence is of a new t5rpe (cross-over luminescence). Its nature has been unraveled by Russian investigators (75). Excitation with about 10 eV excites anion excitons. Upon annihilation these anion excitons show an emission at about 4.1 eV (300 nm). Excitation with about 18 eV excites cation excitons. These do not annihilate in a simple way, but by a so-called cross-over transition an electron jumps from the F" ion (2p orbital) to the hole in the 5p orbital of Ba (see Fig. 23). This is accompanied by emission at about 5.7 eV... 

Internal transition involves the emission of electromagnetic radiation in the form of gamma (y) rays from a nucleus in a metastable state and always follows initial alpha or beta decay. Emission of gamma radiation leads to no further change in atomic number or mass. 

Isomeric transition is an alternative to gamma ray emission. True... 

---