Thoriated glass is a
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
material used in the manufacture of optical systems, specifically
photographic lenses
Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed i ...
. It is useful to this process due to its high
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
. Thoriated glass is
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
due to the inclusion of
thorium dioxide
Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the minera ...
, oxide of radioactive element
thorium
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...
. It has therefore been succeeded as a material of choice by glass including
lanthanum oxide
Lanthanum(III) oxide, also known as lanthana, chemical formula , is an inorganic compound containing the rare earth element lanthanum and oxygen. It is used in some ferroelectric materials, as a component of optical materials, and is a feedstock ...
. Thoriated glass can contain up to 30% by weight of thorium.
[http://www.irpa.net/irpa3/cdrom/VOL.3B/W3B_13.PDF Robert C. McMillan & Steven A. Horne: ''Eye Exposure from Thoriated Optical Glass'' (U.S. Army memo)] The thoriated glass elements in lenses over time develop a brown tint reducing transmission and interfering with neutral color reproduction.
Many Kodak, Fuji and Asahi Takumar lenses that were produced prior to the 1970s are radioactive.
Radiation browning
Over extended time periods, thoriated glass may develop significant discoloration. This is due to induced
F-centers forming in the glass as the
radioactive decay
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
of the thorium progresses. The formation of F-centers is due to the
ionizing
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
effect of the high energy thorium decay products. This process can potentially be reversed by
annealing the glass or exposing it to
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
.
References
Thorium
Glass engineering and science
Glass compositions
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