This preview shows pages 1–2. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Petrophysics MSc Course Notes Radioactivity Logging Dr. Paul Glover Page 93 10. RADIOACTIVITY LOGGING 10.1 Introduction Radioactivity is used in several different types of logging tool. There are those that measure the natural radiation generated by the formation, such as the total and spectral gamma ray logs, and those that measure the response of the formation to radiation generated by the tool, such as the neutron, density and litho-density logs. This chapter will cover the total gamma ray log and the following one will examine the spectral gamma ray log. 10.2 Radioactivity Theory Radioactivity is a fundamental property of the structure of all matter. The atoms of all elements have a nucleus which contains different numbers of protons and neutrons , which is surrounded by a sheath of electrons that are arranged in different energy levels. Each element is defined by the number of positively charged protons its nucleus contains. This is called the atomic number Z . Each nucleus also contains a number of neutrally charged neutrons, and the sum of the number of protons and neutrons in the nucleus is called the atomic mass number, A . There are a number of negatively charged electrons surrounding the nucleus equal to the number of protons within it, and their charge balances the positive charge of the protons. Since the mass of electrons is insignificant compared to the mass of the protons and neutrons, the atomic mass number is a measure of the mass of the atoms of each element. Although the number of protons and electrons for a given element is characteristic of that element, the number of neutrons is not. An element may have several isotopes which are atoms with different numbers of neutrons in their nucleus. Thus any given atom of an element will have a fixed atomic number, and a atomic mass number that depends upon which isotope it is. Most natural materials are a mixture of different isotopes. Each isotope of each element is given a code Z X A , where X is the elemental code. For example, carbon has an atomic number Z =6, but 7 isotopes containing between 4 and 10 neutrons. These are 6 C 10 to 6 C 16 . Carbon-12, 6 C 12 , is the most common isotope, and it is stable. Most of the other isotopes are not stable energetically, and decay to more stable elements by various processes whereby they lose energy by expelling particles or photons. Carbon-14, 6 C 14 , is one of these, and its decay process can be used to...
View Full Document
- Winter '11