3He production

In 14% of the cases, the 3He product undergoes the side reaction with an a particle ... 

As we noted earlier (cf. Table 5.5), there is considerable discrepancy between the theoretically estimated 3He production rate and the observed values, and the discrepancy may partly be attributed to erosion of the surface rock. Conversely, the discrepancy can be used to infer erosion rate (e.g., Lai, 1991), one of the most important but one of the hardest parameters to estimate in geomorphology. Assuming that production rate decreases exponentially with depth (x) in rock, the concentration of cosmogenic 3He at time / and depth x is expressed as (Lai, 1991),... 

He production rate (cm3 STP s ) in rocks is expressed as (Morrison Pine, 1955)... 

Cerling, T. E., Craig, H. (1994) Cosmogenic 3He production rates from 39°N to 46°N latitude, western USA and France. Geochim. Cosmochim. Acta, 58, 249-55. 

The more incisive calculation of Springett, et al., (1968) allows the trapped electron wave function to penetrate into the liquid a little, which results in a somewhat modified criterion often quoted as 47r/)y/V02< 0.047 for the stability of the trapped electron. It should be noted that this criterion is also approximate. It predicts correctly the stability of quasi-free electrons in LRGs and the stability of trapped electrons in liquid 3He, 4He, H2, and D2, but not so correctly the stability of delocalized electrons in liquid hydrocarbons (Jortner, 1970). The computed cavity radii are 1.7 nm in 4He at 3 K, 1.1 nm in H2 at 19 K, and 0.75 nm in Ne at 25 K (Davis and Brown, 1975). The calculated cavity radius in liquid He agrees well with the experimental value obtained from mobility measurements using the Stokes equation p = eMriRr], with perfect slip condition, where TJ is liquid viscosity (see Jortner, 1970). Stokes equation is based on fluid dynamics. It predicts the constancy of the product Jit rj, which apparently holds for liquid He but is not expected to be true in general. 

Primordial, non-standard, production during Big Bang Nucleosynthesis (BBN) the decay/annihilation of some massive particle (e.g. neutralino) releases energetic nucleons/photons which produce 3He or 3H by spallation/photodisinte-gration of 4He, while subsequent fusion reactions between 4He and 3He or 3H ere-... 

He is present in natural gases with a concentration of MO-7 of that of 4He and 1(T6 of the helium in the atmosphere. The separation is very expensive. Hence 3He is instead obtained as by-product of tritium production in nuclear reactors. Tritium in fact produces, by beta decay (the half life is 12.26 years), 3He the separation of 3He is obtained through a diffusion process. 

The first reaction is a fusion of two protons to produce a 2H nucleus, a positron (e+) and a neutrino (ve). The second reaction is a proton capture with the formation of 3He and a y-ray. In the third reaction two 3He nuclei fuse to give 4He and two protons. The total energy released in one cycle is 26.8 MeV or 4.30 x 10-12 J. An important product of this process is the neutrino and it should provide a neutrino flux from the Sun that is measurable at the surface of the Earth. However, the measured flux is not as big as calculated for the Sun - the so-called neutrino deficit... 

Several types of ion-channeling experiments (see Chapter 9) also give useful information on atomic positions at impurities or impurity complexes. These include both scattering of channeled ions by atoms that disrupt the uniformity of a channel path and the production of nuclear reactions by collision of a channeled ion with an impurity nucleus (e.g., incident 3He colliding with dissolved 2H to give 4He plus a proton, which can be detected). Here again, one can study lattice positions of solute atoms and changes in populations of different sites. 

Zito, R., Davis, S. N., 1980 Subsurface production of the mirror isotopes 3H and 3He, unpublished manuscript, University of Arizona, Department of Hydrology and Water Resources, 24 p. 

Helium-3 is a decay product of radioactive tritium (3H, half-life = 12.44 years) that has been produced by nuclear bombs as well as naturally by cosmic rays in the upper atmosphere. Because virtually all 3He atoms escape from the surface ocean to the atmosphere, the 3He/tritium ratio in subsurface seawater samples indicates the time since the water s last exposure to the atmosphere. Both 3He and tritium are measured by gas mass spectrometry. Alternatively, tritium may be measured by gas counting with a detection limit of 0.05 to 0.08 tritium unit, where 1 tritium unit represents a 3H/H ratio of lxl0 18. A degassed water sample is sealed and stored for several months to allow the decay product 3He to accumulate in the container. The amount of 3He is then measured by mass spectrometry, yielding a detection limit of 0.001 to 0.003 tritium unit when 400-gram water samples are used. With this technique, the time since a water mass left the surface can be determined within a range from several months to 30 years. 

The use of a 3He++ beam of 303 MeV/amu has already been mentioned as an advantage in certain cases, especially for nuclei far from stability. Such a beam is also advantageous in that it produces less induced radioactivity in the SC vault. However, the 3He++ beam has not so far been produced with sufficient intensity to make its frequent use an interesting proposition compared to 600 MeV protons. Improvements have been made to the r.f. system and to the SC s ion source which should now permit an intensity gain of a factor 3 or 4 to be achieved, but so far this has not been tested. If the 3He++ intensity rises to the proton intensity (i.e. > 1013 ions/sec), this beam could become the preferred Isolde production beam of the future. 

Since radiogenic He has a 3He/4He ratio less than the air value, the only serious candidate for an alternative to primordial 3He is production by decay of tritium (3H). Cosmic ray interactions in the atmosphere are a well-known source of 3He (Section 5.5), and some of this is channeled through tritium, which will enter surface water... 

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