Atoms, individual daughter

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

The nuclei of unstable atoms disintegrate or decay spontaneously, emitting alpha or beta particles and gamma radiation. Types of atoms that undergo this process are called radioactive isotopes. A decaying reactant isotope is referred to as a parent atom, and the atom produced is a daughter atom. In this ChemLab, heads-up pennies represent individual parent atoms of the fictitious element pennium, and tails-up pennies represent the daughter atoms of the decay. You will study the decay characteristics of pennium and will determine its half-life, which is the time required for one-half of the atoms to decay. 

A typical decay chain of with daughter decays illustrating the method of identification of a new species by single-atom decay sequences (Miinzenberg et al. 1981a). Note that lifetimes of the individual nuclei fluctuate according to the "rules" of exponential distribution (seeO Sect. 9.4.4 in Chap. 9, Vol. 1)... 

Early in the environmental sampling program of the IAEA it was recognized that the analysis of individual micrometer-sized particles was a source of unique information about nuclear materials and activities. O Table 63.17 shows the calculated composition of 1 pm diameter particles coming from various nuclear processes. Thus, it can be seen that a pure particle of natural U oxide ( NU ) contains about 10 U atoms in total and that when this particle is irradiated in a reactor, approximately 5 million atoms of Pu would be created. Furthermore, a particle of high-enriched uranium ( HEU ) would produce only small numbers of Pa and Th daughter atoms in 10 years of decay. To be able to age-date such a particle would involve measurement of these small components, something which is currently not possible with the most sensitive techniques. 

My thanks are due to a number of individuals iii the nuclear industry who co-operated readily in providing technical information, and in particular to Mr. G. Gibbons and Dr. J. E. Sanders of the United Kingdom Atomic Energy Authority. I would also acknowledge the invaluable help of Mr. W. Morris, who prepared most of the illustrations, and of Mrs. E. Duffley, who typed much of the manuscript. Finally, I record my gratitude to my wife and daughter, without whose sustained forbearance this book would not have been written. 

---