Thorium active deposit

Briggs, G.H., Distribution of the Active Deposits of Radium, Thorium and Actinium in Electric Fields, Phil. Mag. 41, 357, 1921. 

Excited or induced radioactivity, a temporary activity produced on solids near a radioactive substance, was discovered almost simultaneously by M. and Mme. Curie with radium and Rutherford with thorium. Rutherford showed that the induced activity of thorium decayed exponentially with time Induced radioactivity was later shown to be due to a solid, called the active deposit, formed by the disintegration of radium or thorium emanation. 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

The main radionuclides that are measured in this system are isotopes of thorium, uranium, and plutonium. Others are the longer-lived isotopes of heavy elements such as radium, protactinium, neptunium, americium, and curium. Conventionally, an isotopic tracer of known activity that emits alpha particles is added at the beginning of the radiochemical procedure (see Section 6.3.2). The solution from which the sample for counting will be deposited is thoroughly purified by the radiochemical procedure to remove interfering radionuclides and solids. 

Dy are biologically deposited in the bones of chinook salmon fingerlings. Hori (406,407,408) has used activation analysis to measure the Na and K content of unfertilized and fertilized eggs of sea-urchins. Barnacle shells have been examined for their Ca, Mg, Sr, and Na contents by Gordon and Larson (337), and Wakita and Kigoshi (978) have found thorium in various Japanese shellfish. Studies on trace element physiology in fresh water snails and mollusks have been reported respectively by Girardi and Merlini (331) and Spronk (903). 

Protactinium is a radioactive element with over 20 isotopes, of which the one of interest is Pa with a half-life of 32,480 years and a corresponding decay constant of 2.134 x 10 a . It is a daughter of Th in the radioactive decay series. In oceans, its geochemical properties resemble those of thorium since the isotopes are removed from seawater either by adsorption on mineral grains or by incorporation into authigenic minerals. Hence, sediments deposited in oceans may contain not only excess unsupported Th, but also unsupported Pa. If both are removed from seawater equally efficiently, their activity ratios will change with time in a manner controlled by their decay constants. 

Practically no thorium, but substantial amounts or uranium are contained in calcium carbonates deposited in saline lakes and also in the oceans where they sometimes reach 5 ppm. Hence, there is almost no initial Th activity in newly deposited carbonates, hut due to the radioactive decay of the activity of Th increases with time. Consideration of this lead... 

The alpha activity of uranium in soil samples from Slovakia was determined after a relatively simple sample preparation procedure (Donoval and Matel 2001). The soil samples were dried, ground, and ashed at 550°C, leached with HNO3 + HCl and HNO3 + HF. This was followed by solvent extraction, chromatographic separation of uranium from thorium, deposition with NdFj and alpha spectroscopy for the determination of the 238U/234U ratio. 

Fig, 127. The thermionic activity of a tungsten ribbon versus the time of deposition from thorium wire. 

---