Y-Rays decay

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... 

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. 

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. 

Nucleus Radioactive decay product y-Ray energy, keV T - 1/2 Production... 

Elaborate precautions must be taken to prevent the entrance of Pu iato the worker s body by ingestion, inhalation, or entry through the skin, because all common Pu isotopes except for Pu ate a-emitters. Pu is a P-emitter, but it decays to Am, which emits both (X- and y-rays. Acute intake of Pu, from ingestion or a wound, thus mandates prompt and aggressive medical intervention to remove as much Pu as possible before it deposits in the body. Subcutaneous deposition of plutonium from a puncture wound has been effectively controlled by prompt surgical excision followed by prolonged intravenous chelation therapy with diethylenetriaminepentaacetate (Ca " —DTPA) (171). 

For any nuclear decay, such as the emission of a y-ray, the angular momentum and parity must be conserved. Therefore, ify,H and H are the spins and parities of the initial and final levels, and L and H are the angular momentum and parity carried off by the y-ray. 

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

The Co nucleus decays with a half-life of 5.27 years by /5 emission to the levels in Ni. These levels then deexcite to the ground state of Ni by the emission of one or more y-rays. The spins and parities of these levels are known from a variety of measurements and require that the two strong y-rays of 1173 and 1332 keV both have E2 character, although the 1173 y could contain some admixture of M3. However, from the theoretical lifetime shown ia Table 7, the E2 contribution is expected to have a much shorter half-life and therefore also to dominate ia this decay. Although the emission probabilities of the strong 1173- and 1332-keV y-rays are so nearly equal that the difference cannot be determined by a direct measurement, from measurements of other parameters of the decay it can be determined that the 1332 is the stronger. Specifically, measurements of the continuous electron spectmm from the j3 -decay have shown that there is a branch of 0.12% to the 1332-keV level. When this, the weak y-rays, the internal conversion, and the internal-pair formation are all taken iato account, the relative emission probabilities of the two strong y-rays can be determined very accurately, as shown ia Table 8. 

Fig. 5. Decay scheme of showing the energies, spins, and parities of the levels populated in the daughter nucleus, Xe, and the energies in keV, emission probabihties (in %), and multipolarities of the y-ray transitions. There is a strong dependence of the y-ray lifetime on the y-character. The Ml + E2 y-ray of 177 keV has a half-hfe of 2.1 ps the half-hfe of the 164-keV M4 y-ray is 1.03 X 10 s.

Table 15 shows data for several radionucHdes the y-rays of which are often used to caHbrate the efficiency of y-ray detectors. For a number of these y-rays the very high accuracy arises because the y-ray occurs in essentially 100% of the decays of the nucHde, and only small corrections ate needed to deduce the y-emission probabiHty. In other cases the accuracy is high because a number of careful measurements have been made. The y-emission rate from a caHbration source also depends on the decay rate of the source, and for these nucHdes the uncertainty in the source activity is often the larger uncertainty. 

The half-hves, y-ray energies, and y-ray emission probabiUties given ia Table 15 are what is needed if the amount of a radioisotope present ia a sample is to be measured. However, there are other uses of radionucHdes where additional data concerning the decay are needed. If a radionucHde is to be iajected or implanted in vivo it is necessary to have data on all of the radiations produced to be able to assess the impact on the ceU stmcture. Table 16 gives samples of the data that can be useful ia this latter case. Such information can be obtained from some of the references above. There are also computer codes that can use the decay data from the ENSDF database to produce this type of information for any radionucHde, eg, RAD LIST (21). 

Radio-iso tope 1/2 Primary y-ray keV % Decay energy, keV Particle type Average energy, keV Average e y-ray energy, keV... 

The selective uptake of iodide ion by the thyroid gland is the basis of radioiodine treatment in hyperthyroidism, mainly with although various other radioactive isotopes ate also used (40,41). With a half-life of eight days, the decay of this isotope produces high energy P-particles which cause selective destmction within a 2 mm sphere of their origin. The y-rays also emitted are not absorbed by the thyroid tissue and are employed for external scanning. 

In NAA the sample is made radioactive by subjecting it to a high dose (days) of thermal neutrons in a reactor. The process is effective for about two-thirds of the elements in the periodic table. The sample is then removed in a lead-shielded container. The radioisotopes formed decay by B emission, y-ray emission, or X-ray emission. The y-ray or X-ray energies are measured by EDS (see Chapter 3) in spe-... 

All radioactive isotopes decay with a characteristic half-life. For example, Fe decays with a half-life of 45 days, while Cu decays with a half-life of 12.6 hours. As a result of the decay, signature high-energy photons or y rays are emitted from a given radioisotope. Thus, Fe emits two prominent y rays at 1099 and 1292 keV, " Na emits at 1368 and 2754 keV, and Zn emits at 1115 keV. Compilations of y rays used in NAA can be found in y-ray tables. 

The emission of y rays follows, in the majority of cases, what is known as P decay. In the P-decay process, a radionuclide undergoes transmutation and ejects an electron from inside the nucleus (i.e., not an orbital electron). For the purpose of simplicity, positron and electron capture modes are neglected. The resulting transmutated nucleus ends up in an excited nuclear state, which prompdy relaxes by giving offy rays. This is illustrated in Figure 2. 

Kinetic energy of fission fragments Instantaneous y-rays Kinetic energy of fission neutrons Radioactive decay of fission fragments, P energy Radioactive decay of fission fragments, y energy... 

The -y-ray photons emitted by the nuclear decay of a technetium-99 atom used in radiopharmaceuticals have an energy of 140.511 keV. Calculate the wavelength of these "y-rays. 

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