Types of Characteristic X-rays

A number of characteristic X-rays can be emitted from an atom if a number of inner shell electrons have been knocked out by high energy particles. The individual characteristic X-rays are marked Ka, Kf5... as mentioned in Chapter 2. It seems that there are many possible ways in which outer shell electrons can fill inner shell vacancies however, the possibilities are limited and such electron transitions in an atom are controlled by the selection rules. 

Each electron in an atom is defined by four quantum numbers n, l, m and s. The principal quantum number (n) defines the shell for example, the K shell as 1, L shell as 2 and the M shell as 3. The angular quantum number (/) defines the number of subshells, taking all values from 0 to (n — 1). The magnetic quantum number (m) defines the number of energy states in each subshell, taking values —l, 0 and +1. The spin quantum number fsj defines two spin moments of electrons in the same energy state as + and —f The quantum numbers of electrons in K, L and M shells are listed in Table 6.1. Table 6.1 also gives the total momentum (J), which is the sum of (7 + s). No two electrons in an atom can have same set quantum numbers (n, /, m, s). Selection rules for electron transitions between two shells are as follows  

Characteristic X-rays are divided into K, L and M series according to the shells of electron vacancies being filled. For example, K series means the characteristic X-rays emitted when an outer shell electron fills an electron vacancy in the Kshell. According to the selection rules, the 

---

Charles Barkla, an early X-ray spectroscopist, introduced this terminology for electron shells in 1911. We still use it today to designate characteristic X-rays in both X-ray diffraction and in chemical analysis using electron microscopy. Barkla named the two types of characteristic X-ray emissions he observed as the K-series and L-series. He later predicted that an M-series and a J-series might exist. An M-series was subsequently discovered, but no J-series. The K shell is hence the first shell. 

---