tailieunhanh - Soil and Environmental Analysis: Modern Instrumental Techniques - Chapter 8

Khoảng 1700 đồng vị khác nhau được biết đến, trong đó khoảng 275 là ổn định. Phần còn lại có tính phóng xạ, đó là, cấu hình hạt nhân của họ không ổn định và có thể thay đổi các hình thức ổn định hơn bằng cách biến đổi hạt nhân chung được gọi là phân rã phóng xạ. Những quá trình phân rã phóng xạ đi kèm với sự phát xạ của các hạt và / hoặc các photon từ hạt nhân. Đồng vị (hoặc nuclides) được phân biệt bởi số lượng proton và neutron (gọi chung là nucleon). | 8 Measurement of Radioisotopes and Ionizing Radiation Olivia J. Marsden and Francis R. Livens The University of Manchester Manchester England I. INTRODUCTION Approximately 1700 different isotopes are known of which around 275 are stable. The remainder are radioactive that is their nuclear configurations are unstable and can change to more stable forms by nuclear transformations that are collectively known as radioactive decay. These radioactive decay processes are accompanied by the emission of particles and or photons from the nucleus. Isotopes or nuclides are distinguished by the number of protons and neutrons collectively known as nucleons they contain and are commonly designated using mass number A number of protons neutrons and atomic number Z number of protons . For example 64C is an isotope of carbon in which the nucleus contains 14 nucleons of which six are protons. The proton number defines the chemical identity of the atom since the proton charge must be balanced by the appropriate number of electrons but it also duplicates the information provided by the chemical symbol and in practice is often omitted hence 14C. Differences in the neutron number may control the stability or otherwise of a nucleus but have only subtle effects on chemistry although these can be exploited in studies of J 1 1 í J J 1 J í 1 2tT 1tT 13 Í 12x 1 stable isotope fractionation in natural systems for example 2H 1H 13C 12C 15 14 17 16 34 32 N N O O S S see Chap. 9 . Only a minority of the unstable isotopes are formed in nature. Most are man-made and the majority of these are available only in such small amounts or are so short-lived or both that they are unlikely to be Copyright n 2004 by Marcel Dekker Inc. All Rights Reserved. Figure 1 Isotopes produced by the decay of 238U. encountered in the environment or to be of any use as radiotracers. The naturally occurring radioisotopes fall into three groups 1. Primordial isotopes that have existed since the formation of the Earth about .