Practical Gamma-ray Spectroscopy

Gordon Gilmore

• Verlag: Wiley
• Jahr: 2011
• ISBN:
  9781119964698
• Auflage: 2
• Thema: Physical Sciences
• Thema: Spectroscopy & Spectrum Analysis

The Second Edition of Practical Gamma-Ray Spectrometry has been completely revised and updated, providing comprehensive coverage of the whole gamma-ray detection and spectrum analysis processes. Drawn on many years of teaching experience to produce this uniquely practical volume, issues discussed include the origin of gamma-rays and the issue of quality assurance in gamma-ray spectrometry. This new edition also covers the analysis of decommissioned nuclear plants, computer modelling systems for calibration, uncertainty measurements in QA, and many more topics.

1

Radioactive Decay and the Origin of Gamma and X-Radiation

1.1 INTRODUCTION

In this chapter I intend to show how a basic understanding of simple decay schemes, and of the role gamma radiation plays in these, can help in identifying radioactive nuclides and in correctly measuring quantities of such nuclides. In doing so, I need to introduce some elementary concepts of nuclear stability and radioactive decay. X-radiation can be detected by using the same or similar equipment and I will also discuss the origin of X-rays in decay processes and the light that this knowledge sheds on characterization procedures.

I will show how the Karlsruhe Chart of the Nuclides can be of help in predicting or confirming the identity of radionuclides, being useful both for the modest amount of nuclear data it contains and for the ease with which generic information as to the type of nuclide expected can be seen.

First, I will briefly look at the nucleus and nuclear stability. I will consider a nucleus simply as an assembly of uncharged neutrons and positively charged protons; both of these are called nucleons.

Z is the atomic number, and defines the element. In the neutral atom, Z will also be the number of extranuclear electrons in their atomic orbitals. An element has a fixed Z, but in general will be a mixture of atoms with different masses, depending on how many neutrons are present in each nucleus. The total number of nucleons is called the mass number.

A, N and Z are all integers by definition. In practice, a neutron has a very similar mass to a proton and so there is a real physical justification for this usage. In general, an assembly of nucleons, with its associated electrons, should be referred to as a nuclide. Conventionally, a nuclide of atomic number Z, and mass number A is specified as

, where Sy is the chemical symbol of the element. (This format could be said to allow the physics to be defined before the symbol and leave room for chemical information to follow; for example, Co²⁺.) Thus,

is a nuclide with 27 protons and 31 neutrons. Because the chemical symbol uniquely identifies the element, unless there is a particular reason for including it, the atomic number as subscript is usually omitted - as in ⁵⁸Co. As it happens, this particular nuclide is radioactive and could, in order to impart that extra item of knowledge, be referred to as a radionuclide. Unfortunately, in the world outside of physics and radiochemistry, the word isotope has become synonymous with radionuclide - something dangerous and unpleasant. In fact, isotopes are simply atoms of the same element (i.e. same Z, different N) - radioactive or not. Thus

and

are isotopes of cobalt. Here 27 is the atomic number, and 58, 59 and 60 are mass numbers, equal to the total number of nucleons. ⁵⁹Co is stable; it is, in fact, the only stable isotope of cobalt.

Returning to nomenclature, ⁵⁸Co and ⁶⁰Co are radioisotopes, as they are unstable and undergo radioactive decay. It would be incorrect to say ‘the radioisotopes ⁶⁰Co and ²³⁹Pu …’ as two different elements are being discussed; the correct expression would be ‘the radionuclides ⁶⁰Co and ²³⁹Pu…’.

If all stable nuclides are plotted as a function of Z (yaxis) and N (x-axis), then Figure 1.1 will result. This is a Segrè chart.

Figure 1.1 A Segrè chart. The symbols mark all known stable nuclides as a function of Z and N. At high Z, the long halflife Th and U nuclides are shown. The outer envelope encloses known radioactive species. The star marks the position of the largest nuclide known to date, ²⁷⁷ 112, although its existence is still waiting official acceptance

The Karlsruhe Chart of the Nuclides has this same basic structure but with the addition of all known radioactive nuclides. The heaviest stable element is bismuth (Z = 83, N = 126). The figure also shows the location of some high Z unstable nuclides - the major thorium (Z = 90) and uranium (Z = 92) nuclides. Theory has predicted that there could be stable nuclides, as yet unknown, called superheavy nuclides on an island of stability at about Z=114,N =184, well above the current known range.

Radioactive decay is a spontaneous change within the nucleus of an atom which results in the emission of particles or electromagnetic radiation. The modes of radioactive decay are principally alpha and beta decay, with spontaneous fission as one of a small number of rarer processes. Radioactive decay is driven by mass change the mass of the product or products is smaller than the mass of the original nuclide. Decay is always exoergic; the small mass change appearing as energy in an amount determined by the equation introduced by Einstein:

where the energy difference is in joules, the mass in kilograms and the speed of light in m s^-1. On the website relating to this book, there is a spreadsheet to allow the reader to calculate the mass/energy differences available for different modes of decay.

The units of energy we use in gamma spectrometry are electron-volts (eV), where 1 eV = 1.602177 × 10^-19 J.¹ Hence, 1 eV ≡ 1.782663 × 10^-36 kg or 1.073533 × 10^-9 u (‘u’ is the unit of atomic mass, defined as 1/12th of the mass of ¹²C). Energies in the gamma radiation range are conveniently in keV.

Gamma-ray emission is not, strictly speaking a decay process; it is a de-excitation of the nucleus. I will now explain each of these decay modes and will show, in particular, how gamma emission frequently appears as a by-product of alpha or beta decay, being one way in which residual excitation energy is dissipated

1.2 BETA DECAY

Figure 1.2 shows a three-dimensional version of the lowmass end of the Segrè chart with energy/mass plotted on the third axis, shown vertically here. We can think of the stable nuclides as occupying the bottom of a nuclearstability valley that runs from hydrogen to bismuth. The stability can be explained in terms of particular relationships between Z and N. Nuclides outside this valley bottom are unstable and can be imagined as sitting on the sides of the valley at heights that reflect their relative nuclear masses or energies.

Figure 1.2 The beta stability valley at low Z. Adapted from a figure published by New Scientist, and reproduced with permission

The dominant form of radioactive decay is movement down the hillside directly to the valley bottom. This is beta decay. It corresponds to transitions alo...