RAY DECAY

7-Ray decay occurs when a nucleus in an excited state releases its excess energy by emission of electromagnetic radiation, that is, a photon. Thus, we have 

Modem Nuclear Chemistry, by W.D. Loveland, D.J. Morrissey, and G.T. Seaborg Copyright 2006 John Wiley Sons, Inc. 

---

Eor specific models of the nucleus, it is possible to compute theoretical wave functions for the states. Eor a model that assumes that the nucleus is spherical, the general properties of these wave functions have been used to compute theoretical estimates of the half-hves for y-rays of the various multipolarities. Some values from the Weisskopf estimate of these half-hves are shown in Table 7. These half-fives decrease rapidly with the y-ray energy, namely, as and, as Table 7 shows, increase rapidly with E. This theoretical half-life applies only to the y-ray decay, so if there are other modes of... 

Nuclear electromagnetic decay occurs in two ways, y decay and internal conversion (IC). In y-ray decay a nucleus in an excited state decays by the emission of a photon. In internal conversion the same excited nucleus transfers its energy radia-tionlessly to an orbital electron that is ejected from the atom. In both types of decay, only the excitation energy of the nucleus is reduced with no change in the number of any of the nucleons. 

The conservation of angular momentum has provided an enormous amount of information on the structure of nuclei and plays a controlling role in the y-ray decay... 

In the single-particle estimates of y-ray decay, one presumes a single nucleon interacts with a photon. This means there is an isospin selection rule... 

Internal conversion (IC) is a competing process to 7-ray decay and occurs when an excited nucleus interacts electromagnetically with an orbital electron and ejects it. This transfer of the nuclear excitation energy to the electron occurs radiationlessly (without the emission of a photon). The energy of the internal conversion electron, Eic, is given by... 

The isomeric state of 95Nb decays to the + ground state by means of an M4 transition. The half-life of the isomeric state is 90 h while the half-life of the ground state is 35 d (atotal = 4.5). Calculate the partial half-life for the 7-ray decay of the isomeric state. 

Nuclei can be trapped in the secondary minimum of the fission barrier. Such trapped nuclei will experience a significant hindrance of their y-ray decay back to the ground state (because of the large shape change involved) and an enhancement of their decay by spontaneous fission (due to the thinner barrier they would have to penetrate.) Such nuclei are called spontaneously fissioning isomers, and they were first observed in 1962 and are discussed below. They are members of a general class of nuclei, called superdeformed nuclei, that have shapes with axes ratios of 2 1. These nuclei are all trapped in a pocket in the potential energy surface due to a shell effect at this deformation. 

The net count rate at a given mass, Rm, is related to the self-absorption factor at that mass, fm, in terms of the activity, A, gamma-ray decay fraction, d, and counting efficiency at zero self-absorption, e0, by Equation 3.2 ... 

Calculate the ratio of the net count rate to the disintegration rate of K gamma rays in the sample, based on the half life, gamma-ray decay fraction, ratio of 40K to potassium mass, and K/KC1 ratio of 39.1/74.6. Calculate the self-absorption factor with Eq. 3.1. Calculate the counting efficiency at zero self-absorption with Eq. 3.2 and record in Data Table 3.3. 

Data Table 8.6 Gamma-ray decay study of radionuclides on filters... 

Gamma-ray spectrometry is a probe of nudear rather than chemical processes, but its high specificity and sensitivity have applications in analysis of materials (286). It is especially suited for activation analysis. Unstable nudides produced by nudear bombardment can be identified by their characteristic gamma-ray decay emissions. An important example is slow neutron capture by nitrogen with subsequent decay of 15 detected from its 1.7—10.8 MeV gamma lines, a signature useful for remote, nondestmctive detection of possible hidden explosives (see Explosives and propet. t.ents). Gamma-ray... 

Fission of heavy nuclei always results in a high neutron excess of the hssion products, because the neutron-to-proton ratio in heavy nuclides is much larger than in stable nuclides of about half the atomic number, as already explained for spontaneous hssion (Fig. 5.15). The primary fission products formed in about 10 " s by fission and emission of prompt neutrons and y rays decay by a series of successive / transmutations into isobars of increasing atomic number Z. The final products of these decay chains are stable nuclides. 

---