Imaging and Radiation

This chapter explains in simple terms the background physics of imaging using standard X-rays, computed axial tomography (CT), nuclear medicine (including positron emission tomography-PET), and magnetic resonance imaging (MRI). It covers the basics of ionising radiation, and also discusses lasers, which are a form of non-ionising radiation (imaging using ultrasound is covered in Chapter 10). X-rays, CT, aspects of nuclear medicine, and lasers are covered briefly. MRI is examined in more detail as this is a newer modality that is often difficult to comprehend, and in any case often involves the presence of the anaesthetist. Some isotopes are naturally occurring but many of the radioactive nuclides used in medicine are produced artificially by a nuclear reactor or cyclotron. Each of these will provide isotopes that are useful for different purposes. Unstable radioactive nuclides achieve stability by radioactive decay, during which they can lose energy. This occurs in a number of ways. For example, atoms can lose energy by ejection of an alpha particle (an extremely tightly bound basic atomic structure of 2 protons and 2 neutrons, which is equivalent to a helium nucleus). This occurs if they have too many nucleons (protons or neutrons) and results in the atomic number being reduced by two and the atomic mass by 4. Other ways that unstable radionuclides decay include: emission of an electron (β−) from the nucleus if the atoms have an excess of neutrons, or by, either emitting a positron (β+) or capturing an electron if they are neutron deficient. Normally isotopes produced by a reactor will be neutron rich and decay by emitting an electron and the cyclotron will tend to produce isotopes that are proton rich and the decay will then be by emitting a positron. This is illustrated in Table 29.1. The new nuclide formed by the decay process (the daughter nuclide) may be left in an excited nuclear state and can release this excess energy by emission of gamma (γ) radiation as shown in Figure 29.1. This example is where the electron (β−) has been emitted. The situation is more complex when a positron has been emitted.

DECAY OF 188Re AND TRS CALCULATIONS FOR ITS DAUGHTER NUCLIDE 188Os

The γ-ray spectra of 188 Re decay have been studied by using a Compton-suppressed spectrometer and a three parameters γ-γ- T list coincidence system. Experimental data analysis demonstrated that six γ-rays at 557, 810, 1463, 1867, 1936 and 2022 keV and three levels at 1443, 1936 and 2022 keV are confirmed again. Seven new γ-rays at 309.60±0.04, 826.90±0.02, 979.29±0.08, 1103.7±0.4, 1828.2±0.1, 1842.5±0.2 and 1982.5±0.2 keV have been identified, three new levels at 309.60, 1828.2 and 1982.5 keV are assigned. The β- decay branching ratio is deduced. In addition, in order to study this γ-unstable nucleus, shape calculations using the Hartree–Fock–Bogoliubov-like formalism were carried out for positive-parity states in 188 Os . The TRS plots reveal that, as the spin increases up the band, the triaxiality parameter γ changes.

Potential in vivo generator for alpha-particle therapy with 212Bi: Presentation of a system to minimize escape of daughter nuclide after decay of 212Pb to 212Bi

Radiopharmaceuticals based on

Study on Role of 234Th in Uranium Series Nuclides Migration

ABSTRACTThe role of 234Th, a daughter nuclide of 238U having a half life of 24 days, in the migration of uranium series nuclides has been studied to understand the mechanism which gives a higher migration velocity of 238U than that of 234U. We assume that 234Th is adsorbed at two different sites; one, where 234Th is reversibly adsorbed, and the other, where 234Th is irreversibly fixed. Calculations have shown that the fixation of ^Th to a rock can cause the apparent migration velocity of 234U to be reduced compared to that of 238U and this agrees with observed field data. Changes in the fixation rate constants affect the relative mobility of 238U and 234U, and this can account for the different migration behavior at different depths in the Koongarra ore deposit.

Identification of Thorium-236

The new nuclide 236Th has been produced via the (γ, 2 p) reaction by irradiation of 238U with 140 MeV bremsstrahlung. After chemical separation of thorium, the half-life was determined to be 36 ± 3 min -from the growth-decay curve of the strongest γ-ray transition of the daughter nuclide, 9 min 236Pa.

15.—Natural Radioactive Decay Series Elements in the Oceans and Sediments

The parents of the three naturally occurring radioactive decay series (text-fig. 1),232Th, 238U and 235U, have existed since the time of formation of the earth and through the process of radioactive decay have continuously generated their shorterlived daughter radio-isotopes. Under conditions where these decay products are not separated from the parents the situation referred to as secular equilibrium may be attained at which the activity ratio of any two daughters in the same decay chain is unity. The time required for the attainment of this situation corresponds to several half-lives of the longest lived daughter nuclide. In a great many instances, however, secular equilibrium is not achieved. Excellent examples of disequilibrium are to be found in the distribution of natural radioactive decay series elements in the oceans and sediments. These situations can be used to advantage in marine geochemistry to obtain information on residence times of elements in the oceans and rates of sedimentation occurring under a variety of conditions.

Geochronology in the Lake Superior region

During the past 10 years many radiometric ages have been determined on minerals and rocks in the Lake Superior region. The oldest known rocks are the granitic gneisses in the Minnesota River Valley, which have been dated at 3300 to 3550 m.y. ago. Both K–Ar and Rb–Sr methods have been applied to samples from a number of the metasedimentary formations in the region. The ages, however, appear to be the time of folding or of metamorphism rather than of deposition for which only limits or ranges can be given from the ages for associated igneous and metamorphic rocks.Although considerable progress has been made, significant uncertainties remain in the decay constants and in the analytical measurements. More serious problems, however, are geologic ones, such as the effects of metamorphism and of weathering on the parent–daughter nuclide ratios. Both analytical and geological considerations must enter into any proposal for a time classification of the Precambrian.A three-fold division of the Precambrian with time boundaries at 2600 and 1800 m.y. serves well for the Lake Superior region. In addition, the Keweenawan igneous activity is well dated at approximately 1100 m.y. ago. Terms such as Keweenawan, Huronian, and others are best used locally, and time units of a Precambrian classification that might have world-wide utility should not be tied closely to geographic localities. A single radiometric method, as for example K–Ar largely on micas, is not a satisfactory basis for a classification.