Re: [Leica] Radiation and half lifes

["Mike Gil" <pasuno@hotmail.com>]

A lugger wrote

"Naturally occuring thorium 232 has a half
life of about 13 billion years, which implies that it is not very 
radioactive."

Of course this statement is false, its the other way around, the shorter a 
radioactive element takes to decay the less radioactive it is.  A 13 billion 
year half life is a very long time for something.  Also your information is 
incorrect concerning the half life of thorium which should be 14 billion 
years. Here are the details.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, 
Editor in Chief. Author: C.R. Hammond

(Thor, Scandinavian god of war) Discovered by Berzelius in 1828. Thorium 
occurs in thorite and in thorianite. Large deposits of thorium minerals have 
been reported in New England and elsewhere, but these have not yet been 
exploited. Thorium is now thought to be about three times as abundant as 
uranium and about as abundant as lead or molybdenum. The metal is a source 
of nuclear power. There is probably more energy available for use from 
thorium in the minerals of the earth's crust than from both uranium and 
fossil fuels. Any sizable demand from thorium as a nuclear fuel is still 
several years in the future. Work has been done in developing thorium cycle 
converter-reactor systems. Several prototypes, including the HTGR 
(high-temperature gas-cooled reactor) and MSRE (molten salt converter 
reactor experiment), have operated. While the HTGR reactors are efficient, 
they are not expected to become important commercially for many years 
because of certain operating difficulties. Thorium is recovered commercially 
from the mineral monazite, which contains from 3 to 9% ThO2 along with 
rare-earth minerals. Much of the internal heat the earth produces has been 
attributed to thorium and uranium. Several methods are available for 
producing thorium metal; it can be obtained by reducing thorium oxide with 
calcium, by electrolysis of anhydrous thorium chloride in a fused mixture of 
sodium and potassium chlorides, by calcium reduction of thorium 
tetrachloride mixed with anhydrous zinc chloride, and by reduction of 
thorium tetrachloride with an alkali metal. Thorium was originally assigned 
a position in Group IV of the periodic table. Because of its atomic weight, 
valence, etc., it is now considered to be the second member of the actinide 
series of elements. When pure, thorium is a silvery-white metal which is 
air-stable and reatins its luster for several months. When contaminated with 
the oxide, thorium slowly tarnishes in air, becoming gray and finally black. 
The physical properties of thorium are greatly influenced by the degree of 
contamination with the oxide. The purest spcimens often contain several 
tenths of a percent of the oxide. High-purity thorium has been made. Pur 
thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. 
Thorium is dimorphic, changing at 1400C from a cubic to a body-centered 
cubic structure. Thorium oxide has a melting point of 3300C, which is the 
highest of all oxides. Only a few elements, such as tungsten, and a few 
compounds, such as tantalum carbide, have higher melting points. Thorium is 
slowly attacked by water, but does not dissolve readily in most common 
acids, except hydrochloric. Powdered thorium metal is often pyrophoric and 
should be carefully handled. When heated in air, thorium turnings ignite and 
burn brilliantly with a white light. The principal use of thorium has been 
in the preparation of the Welsbach mantle, used for portable gas lights. 
These mantles, consisting of thorium oxide with about 1% cerium oxide and 
other ingredients, glow with a dazzling light when heated in a gas flame. 
Thorium is an important alloying element in magnesium, imparting high 
strength and creep resistance at elevated temperatures. Because thorium has 
a low work-function and igh electron emission, it is used to coat tungsten 
wire used in electronic equipment. The oxide is also used to control the 
grain size of tungsten used for electric lamps; it is also used for 
high-temperature laboratory crucibles. Glasses containing thorium oxide have 
a high refractive index and low dispersion. Consequently, they find 
application in high quality lenses for cameras and scientific instruments. 
Thorium oxide has also found use as a catalyst in the conversion of ammonia 
to nitric acid, in petroleum cracking, and in producing sulfuric acid. 
Twenty five isotopes of thorium are known with atomic masses ranging from 
212 to 236. All are unstable. 232Th occurs naturally and has a half-life of 
1.4 x 10^10 years. It is an alpha emitter. 232Th goes through six alpha and 
four beta decay steps before becoming the stable isotope 208Pb. 232Th is 
sufficiently radioactive to expose a photographic plate in a few hours. 
Thorium disintegrates with the production of "thoron" (220Rn), which is an 
alpha emitter and presents a radiation hazard. Good ventilation of areas 
where thorium is stored or handled is therefore essential. Thorium metal 
(99.9%) costs about $150/oz.
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