Helium enrichment

N cuum envelope tests are integral leak tests using helium as the test gas, in which the test specimen is enclosed either in a rigid (usually metal) enclosure or in a light plastic envelope. The helium which enters or leaves (depending on the nature of the test) the test specimen is passed to a helium leak detector, where it is measured. Envelope tests are made either with the test specimen pressurized w/ith helium (Fig. 5.4c) or with the test specimen evacuated (Fig. 5.4a). In both cases it may be necessary to convert the helium enrichment figure (accumulation) to the helium standard leak rate. 

A number of WN stars also have circumstellar ring nebulae, for which abundance determinations are of obvious relevance. Nitrogen and helium enrichments have been demonstrated in several of them by Talent and Dufour (1979), Contini and Shaviv (1980), and Kwitter (1981, 1984). 

In stars, most of the hydrogen is converted into helium-4 which is converted into heavier elements. Then, an excess of helium-4 can return into the interstellar medium. Therefore, one must account for a possible helium enrichment. 

An early description referred to these stars as narrow-line O-type sub-dwarfs , including BD+75°325, HD 127493, HZ44 and BD+25°4655 [97], Like the He-sdB stars, there is considerable diversity as regards binarity and abundance. The stars can be divided (from classification spectra) into those which are carbon-rich, and those which are not. For the most part, all are nitrogen rich. Several occur in composite spectrum binaries, and one [155, HE 0301-3039] is a double He-sdO system similar to the double He-sdB others are apparently single. With substantial numbers of both helium-enriched and helium-deficient compact sdO stars now identified and analysed spectroscopically [156], an explanation of their origin is tantalizingly close - but not fully formed. 

Liquid air-cooled activated carbons can also be used for purification of helium enriched with the isotopic ffe for use in a cyclotron" from impurities due to air leaking into the system. 

Relatively new methods for separating helium from natural gas use pressure swing adsorption (PSA) processes to recover helium at better than 99.99% purity. This type of process is probably less costiy for the production of gaseous helium but might be uneconomical for liquefied helium production. The PSA process is widely used to produce specification pure helium from 85+% cmde helium in conjunction with cryogenic enrichment of the ca 50% helium raffinate. 

The sources most commonly used so far consisted of sintered disks containing about 100 mg ZnO enriched with 90% Zn. The disks were irradiated with 12 MeV deuterons or 30 MeV He particles, to yield the 78 h activity of Ga, and then annealed by heating in oxygen to 700-1,000 K for about 12 h and cooling down slowly (about 50 K h ) to room temperature. A Nal scintillation counter, 2-3 mm thick, is suitable for the detection of the 93 keV y-rays. Because of the relatively high transition energy, both source and absorber are generally kept at liquid helium temperature. 

For qualitative analyses, the GC system was equipped with a J W Scientific HP-5 or a Supelco Simplicity 1 fiised-sUica capillary column. Injector and detector temperatures were set at 220 °C and 240 °C respectively the oven temperature was programmed from 60 to 230 °C at 40 °C min. Helium was employed as carrier gas (1 mL min ). Compound identification was based on a comparison of mass spectra with those of synthetic racemic and enantiomeric-enriched samples. The retention times for indan, 1-indanol and 1-indanone were 4.7 min, 5.9 min and 6.2 min respectively. 

The 0( He,p) F reaction is also occasionally used for [ F]fluoride production. The yield is low but it uses natural instead of enriched water as the target material. However the cost of helium-3 is also significant. 

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