Characteristic X-rays - Practice Questions & MCQ

Characteristic X-Rays -

Few of the fast-moving electrons having high velocity penetrate the surface atoms of the target material and knock out the tightly bound electrons even from the innermost shells of the atom. Now when the electron is knocked out, a vacancy is created at that place. To fill this vacancy electrons from higher shells jump to fill the created vacancies, we know that when an electron jumps from a higher energy orbit E₁  to lower energy orbit  E₂, it radiates energy  (E₁−E₂). Thus this energy difference is radiated in the form of X-rays of very small but definite wavelength which depends upon the target material. The X-ray spectrum consists of sharp lines and is called the characteristic X-ray spectrum. These X-rays are called characteristic X-rays because they are characteristic of the element used as target anode. Characteristic X-rays have a line spectral distribution, unlike continuous X-rays. The wavelength spectrum of the X-frequencies corresponding to these lines is the characteristic of the material or the target, i.e., anode material.

When the atoms of the target material are bombarded with high-energy electrons (or hard X-rays), which possess enough energy to penetrate into the atom, they knock out the electron of the inner shell (say K shell, n=1 ). When an electron is missing in the K shell, an electron from the next upper shell makes a quantum jump to fill the vacancy in the K shell. In the transition process, the electron radiates
energy whose frequency lies in the X-ray region. The frequency of emitted radiation (i.e., of the photon) is given by - 

  $v=R Z_e^2\left(\frac{1}{n_1^2}-\frac{1}{n_2^2}\right)$                                                                                   

   í                                  

Another vacancy is now created in the $L$ shell which is again filled up by another electron jump from one of the upper shell $M$ which results in the emission of another photon, but of different X-ray frequency. This transition continues till outer shells are reached, thus, resulting in the continues till outer shells are reached, thus, resulting in the emission of a series of spectral lines. The transitions of electrons from various outer shells to the innermost K shell produce a group of X ray lines called as K -series. These radiations are most energetic and most penetrating. K-series is further divided into $K_\alpha, K_\beta, K_\gamma, \cdots$ depending upon the outer shell from which the transition is made (see figure).

Incident electron is also known as projectile electron

Emitted electron is known as photo-electron / orbital electron

Similarly, the rest of the series can be shown as below -

Now notice the graph shown below and the sharp peaks obtained in the graph are known as characteristic X-rays because they are characteristic of the target material. The characteristic wavelengths of the material having atomic number Z are called characteristic X-rays and the spectrum obtained is called a characteristic spectrum. If a target material of atomic number $Z^{\prime}$ is used, then peaks are shifted. 

The characteristic wavelengths of the material having atomic number Z are called characteristic X-rays and the spectrum
obtained is called a characteristic spectrum. If a target material of atomic number $Z^{\prime}$ is used, then peaks are shifted as shown below -

X-ray absorption-

The intensity of X-rays at any point may be defined as the energy falling per second per unit area held perpendicular to the direction of energy flow. Let $I_0$ be the intensity of incident beam and I be the intensity of beam after penetrating a thickness x of a material, then $I=I_0 e^{-\mu x}$, where $\mu$ is the coefficient of absorption or absorption coefficient of the material. The absorption coefficient depends upon the wavelength of X-rays, the density of the material, and the atomic number of material. The elements of high atomic mass and high density absorb X-rays to a higher degree.