Half-life

The half-life of a radioactive isotope is the time it takes for half of the atoms in a pure sample of the isotope to decay into another element. It is a measure of the stability of an isotope; the shorter the half life, the less stable the atom. The decay of an atom is said to be spontaneous as one can only determine the probability of decay and not predict when an individual atom will decay. (Refer to the last section on the generalization of the concept of half-life to other scientific subjects)

All the atoms of a particular radioactive species have the same probability of disintegrating in a given time, so that an appreciable sample of radioactive material, containing many millions of atoms, always changes or disintegrates at the same rate. This rate at which the material changes is expressed in terms of the half-life, the time required for one half the atoms initially present to disintegrate, which is constant for any particular isotope.

Half-lives of radioactive materials range from fractions of a second for the most unstable to billions of years for those which are only slightly unstable. Decay is said to occur in the parent nucleus and produce a daughter nucleus. Decay from a parent to a daughter nucleus may produce alpha, beta particles, and neutrinos. Gamma radiation may be produced as the nucleus is de-excited but this is only after the alpha or beta decay has taken place. Radioactive decay results in a mass loss, which is converted to energy (the disintegration energy) according to the formula E = mc². Often, the daughter nucleus is also radioactive, and so on down the line for several successive generations of nuclei until a stable one is finally reached. The three such naturally occurring series are shown in the following table:

Natural Radioactive Elements

Series	Starting Isotope	Half-Life (years)	Stable End Product
Radium	U-238	4.47x109	Pb-206
Actinium	U-235	7.04x108	Pb-207
Thorium	Th-232	1.41x1010	Pb-208

Note: there are naturally occurring radioactive isotopes (such as C-14) but they are not part of a series.

Other applications of the half-life concept

(From Wikipedia, the free Encyclopedia)

Half-Life is a first-person shooter computer game developed by Valve Software and published by Sierra On-Line in 1998, based on a heavily-modified core Quake I game engine. It was first published for PCss running Microsoft Windows, and was later ported to Sony's Playstation 2 video game console (a version for Sega's Dreamcast was completed, but never released commercially). Half-Life was heralded by computer game critics for its gripping storyline, which would influence the development of other first-person shooters in the years to come.

In the game, you play a scientist named Gordon Freeman who is a survivor of an experiment gone horribly wrong, allowing aliens from another planet to invade Earth. As you try to escape the destroyed facility you soon discover that you are caught between two sides: the aliens, and the forces of the United States military which has been dispatched to cover up the experiment - including you.

Two expansion packs have been been released: Opposing Force and Blue Shift. In 1999's Opposing Force, you play through the world of the original storyline once again, but this time as one of the military officers sent to cover up the experiment. Blue Shift (2001) returns the player to the original storyworld yet again, this time as one of the facility's guards.

Half-Life has also been used as a base for many modss (add-ons) such as the immensely popular and free multiplayer mod, Counter-Strike. Other popular mods include Team Fortress Classic, Day of Defeat, Action Half-Life and Natural Selection.

The sequel, Half-Life 2, was due to come out in September of 2003, but has been delayed and is now rumored to be released in November 2003. Rumors aside, Valve Software has suggested that a release date in 2004 is more likely. This pushing back of Half-Life 2's release date came in the wake of the cracking of Valve's internal network, through bugs in Microsoft Outlook, resulting in the theft of the game's source code. While of great interest to the abjectly curious for simple learning purposes, this has likely hurt Valve's abilities to license their game engine to other companies for their own products. It uses a new engine entitled 'Source', featuring some of the most advanced graphics to date. Half-Life 2 was merely a rumor until a strong impression at E3 in May 2003 launched it into levels of hype only equalled by DOOM³. The story is takes place after the Black Mesa incident in a futuristic Eastern European 'City 17'. It again pits Gordon Freeman against an alien invasion.

External links

• The Official Half-Life Web Site (Sierra)
• Planet Half-Life (GameSpy))