Ans. (c) Gamma raysRef: Essentials of radiological science 2nd ed. HO* Alpha particles are emitted from the nuclei of radioactive atoms and have high speeds and high energy and are the least penetrating. Their maximum range in air is about 10 cm and can be stopped by the outer layer of skin. If they enter the body, alpha emitters are hazardous to human health. As they disintegrate, they can damage tissue. The normal route of entry by alpha particles are inhalation, digestion and through wounds.* Beta particles are also emitted from the nuclei of radioactive atoms, but they are much smaller in mass than alpha particles and have considerably more penetrating power. Beta emitters are capable of penetrating wood to 4 cm and the human body to 1 cm. Like alpha emitters, they are internal radiation hazards and have the same routes of body entry. They can be stopped by ordinary wall or a 1.3-cm thick sheet of aluminum.* Gamma radiation originates in the nucleus of an atom. It can penetrate deeply into tissue, and may produce burns similar to deep sunburn, alter genes to cause mutations, and reduce the white blood cell count and encourage infections.* X-rays cause cancer by inducing mutations. X-rays are a form of electromagnetic radiation that is produced when high-speed electrons strike the target material inside the x-ray tube. The quality of the x-ray, which is the power to penetrate through matter, depends upon the wavelength and the material being irradiated. Hard x-rays with short wavelengths will penetrate thick steel plate. Soft x-rays with long wavelengths are less penetrating.* Neutrons are particles released upon the disintegration of radioactive isotopes. Neutron particles are highly penetrating and require heavy shielding. They are capable of penetrating the human body to several centimeters. Within the body the neutrons release excess energy which can cause tissue damage. Secondary releases of energy may also occur from alpha, beta, and gamma emitters released from the neutrons.