# 25.2 X-RAYS X-rays are a form of high-energy electromagnetic radiation that can penetrate soft tissues but are absorbed by denser materials like bone. This section explains the nature of X-rays, their production mechanisms, and their applications in medical imaging. ## Inner Shell Transition and Characteristic X-Rays **Origin**: Transitions of electrons between inner shells of heavy atoms. **Production Mechanism**: 1. Heavy target material is bombarded with high-energy electrons accelerated by several keV. 2. Bombarding electrons collide with inner-shell electrons (such as K-shell electrons) of target atoms, knocking them out and creating a vacancy. 3. An electron from a higher energy shell (L, M, or N shell) quickly jumps down to fill the vacancy. 4. The excess energy is emitted as an X-ray photon. **Characteristic Nature**: These X-rays consist of specific wavelengths or frequencies unique to the element, hence called *characteristic X-rays*. **K-Series X-Rays**: - $\mathrm{K}_{\alpha}$: X-ray photon emitted due to transition from L-shell to a K-shell vacancy. - $\mathrm{K}_{\beta}$: X-ray photon emitted due to transition from M-shell to a K-shell vacancy. - $\mathrm{K}_{\gamma}$: X-ray photon emitted due to transition from N-shell to a K-shell vacancy. **Significance**: Characteristic X-ray spectra are crucial for studying atomic structure and the periodic table of elements. For more on atomic transitions, see . ## Continuous X-Rays (Bremsstrahlung) **Origin**: Coulomb interaction of a fast-moving electron with the positive nucleus of a target atom. **Production Mechanism**: 1. High-energy electrons travel towards a target nucleus. 2. Due to strong Coulomb attraction from the concentrated positive charge in the nucleus, the electron is accelerated and decelerated as it passes the nucleus. 3. The decelerating electron emits radiation. **Name**: This radiation is called *Bremsstrahlung* (German for "braking radiation"). **Nature**: It produces a continuous spectrum of wavelengths, as electrons lose varying amounts of kinetic energy in different interactions. **Energy Conversion**: The kinetic energy of the incident electron ($eV$) is transformed into the energy of the photon ($hf$). In an extreme case, all initial kinetic energy is converted into a single photon. **Minimum Wavelength ($\lambda_{\min}$)**: This occurs when the electron's entire kinetic energy is converted into a single photon of maximum frequency ($f_{\max}$). $$ eV = hf_{\max} $$ $$ eV = \frac{hc}{\lambda_{\min}} $$ $$ \lambda_{\min} = \frac{hc}{eV} $$ ## Setup for Production of X-Rays (X-ray Tube) **Components**: - **Cathode**: A heated filament that emits electrons when current is supplied (thermionic emission). - **Anode/Target**: A solid copper bar with a high melting-point metal (such as platinum or tungsten) embedded at the end. This is where electrons strike. - **Evacuated Glass Chamber**: Encloses the cathode and anode to prevent electron collisions with air molecules. - **High DC Voltage**: Maintained (for example, 50,000 V) between cathode and anode to accelerate electrons. **Mechanism**: 1. Electrons emitted from the cathode are accelerated by the high potential difference towards the anode. 2. These energetic electrons strike the target material in the anode. 3. The sudden stopping of these highly energetic electrons by the target produces an intense beam of X-rays (both characteristic and continuous). 4. A small part of the kinetic energy is converted into X-rays; the rest is converted into heat. **Cooling**: The target becomes very hot and must have a high melting point. Heat is dissipated through the copper rod, sometimes aided by water cooling. ## Applications of X-Rays in Imaging Internal Body Structures **Principle**: X-rays pass through the body and are absorbed in different quantities by different tissues and organs. **Bone Imaging**: - Bones have high calcium content, absorbing X-rays much better than soft tissues. - This differential absorption causes bones to cast distinct shadows on an X-ray detector or photographic plate, appearing in lighter shades of gray or white. - Useful for detecting bone fractures or foreign bodies. **Soft Tissue Imaging**: - Soft tissues absorb X-rays less, making them harder to differentiate on conventional X-ray images. - **Contrast Agents**: For better evaluation of soft tissues, contrast agents (substances that absorb X-rays more effectively) are used to enhance the range of brightness in the image, allowing radiologists to visualize structures not clearly evident otherwise. | Concept | Formula | Description | | :------------------ | :--------------------------------------- | :------------------------------------------------------------------------------------------------------ | | **Minimum Wavelength** | $\lambda_{\min} = \frac{hc}{eV}$ | Relates the minimum X-ray wavelength to the electron's accelerating voltage ($V$) in an X-ray tube. | | (Continuous X-rays) | ($h$=Planck's constant, $c$=speed of light, $e$=electron charge) | |