Which radiation is emitted when a beam of fast moving cathode rays is stopped by a heavy element?

Step 1: Analyze the question Cathode rays are fast-moving electrons. When they hit and are suddenly stopped by a heavy metal target (such as tungsten), which radiation is produced?

Step 2: The concept This is how X-rays are produced in an X-ray tube:

• Electrons are released from a heated cathode (thermionic emission).
• A high voltage accelerates them toward a heavy metal anode (target).
• When the electrons are suddenly decelerated or stopped, their kinetic energy is converted mostly into heat (about 99%) and a small part into X-rays. These are electromagnetic waves of very short wavelength.

The X-rays come from two processes:

1. Bremsstrahlung (braking radiation): electrons slow down in the field of the target nuclei, giving a continuous spectrum.
2. Characteristic X-rays: electrons knock out inner-shell electrons of the target atoms, and outer electrons fall into the gaps.

Step 3: Check the options

• A. Alpha rays: these are helium nuclei from radioactive decay, not produced by stopping electrons.
• B. Gamma rays: these come from the nucleus during radioactive decay, not from electrons hitting a target.
• C. Positive rays: these are streams of positive ions, found in the gas discharge tube.
• D. Ultraviolet rays: these come from electron transitions at lower energies, not from stopping fast electrons in a heavy metal.
• E. X-rays: correct.

Answer: E. X-rays

Exam tip: Remember "Fast electrons + heavy target = X-rays." The heavy element is used because it has a high atomic number and a high melting point, so it produces more X-rays and withstands the heat. Gamma rays always come from the nucleus, while X-rays come from electron processes.