http://en.wikipedia.org/wiki/Gamma_ray
Gamma ray
Gamma radiation, also known as gamma rays (denoted as γ), is electromagnetic radiation of high frequency (very short wavelength). They are produced by sub-atomic particle interactions such as electron-positron annihilation, neutral pion decay, fusion, fission or inverse Compton scattering in astrophysical processes. Because gamma rays are a form of ionizing radiation, they pose a health hazard.
Shielding
Shielding from gamma rays requires large amounts of mass. They are better absorbed by materials with high atomic numbers and high density, although neither effect is important compared to the total mass per area in the path of the gamma ray. For this reason, a lead shield is only modestly better (20–30%) as a gamma shield than an equal mass of another shielding material such as aluminium, concrete, or soil; lead's major advantage is its density.
The higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example gamma rays that require 1 cm (0.4″) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of granite rock, 6 cm (2½″) of concrete, or 9 cm (3½″) of packed soil. However, the mass of this much concrete or soil is only 20–30% larger than that of lead with the same absorption capability. Depleted uranium is used for shielding in portable gamma ray sources, but again the savings in weight over lead is modest, and the main effect is to reduce shielding bulk.
All ionizing radiation causes similar damage at a cellular level, but because rays of alpha particles and beta particles are relatively non-penetrating, external exposure to them causes only localized damage, e.g. radiation burns to the skin. Gamma rays and neutrons are more penetrating
http://en.wikipedia.org/wiki/Tsar_Bomba
Tsar Bomba
Tsar Bomba, literally "Tsar-bomb", is the nickname for the AN602 hydrogen bomb (codenamed "Ivan"[citation needed] by its developers) — the largest, most powerful nuclear weapon ever detonated, and currently the most powerful explosive ever detonated in human history.
Developed by the Soviet Union, the bomb was originally designed to have a yield of about 100 megatons of TNT (420 PJ); however, the bomb yield was reduced by half in order to limit the amount of nuclear fallout that would result. One bomb was built and tested on October 30, 1961, in the Novaya Zemlya archipelago.
The Tsar Bomba is the single most physically powerful device ever utilized throughout the history of humanity. Its size and weight excluded a successful delivery in case of a real war.
