3He flux

Farley, K. A., Maier-Reimer, E., Schlosser, P., Broecker, W. S. (1995) Constraints on mantle 3He fluxes and deep-sea circulation from an oceanic general circulation model. J. Geophys. Res., 100(B3), 3829-39. 

When searching for leaks of 3He and 4He mixture, a great care must be used in interpreting the reading of an LD even if the leak of only one of the two gasses is in reality present (e.g. 3He), due to the overlapping of peaks, the LD apparently reveals the presence of the other one (4He). The apparent flux of 4He is usually about 10% of the real flux of 3He. 

For T < 0.5 K, pure 4He is almost in its fundamental state its entropy, viscosity and specific heat tend to zero. 4He behaves in the mixture as an inert solvent of 3He, since its contribution to the total heat capacity is negligible. 3He atoms in the diluted phase act as a gas forcing a flux of 3He atoms from the concentrated phase to the diluted one,... 

This is the principle on which a DR works. The advantage of this process over the 3He evaporation is evident for example, at 0.28 K, the 3He vapour pressure is about 10-3 torr, whereas the osmotic pressure from the rich to the diluted phase is about 10 torr for T->0. Thus, it is always possible to force a 3 He flux from the concentrated phase to the diluted one. 

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... 

It should be noted that commercial neutron generators are also easily adopted to the generation of 2.8 MeV neutrons produced by the 2H(2H,w)3He reaction. In most cases it is merely necessary to replace the tritium target with one containing occluded deuterium. The neutron yield from this reaction is much less than for the D—T reaction and the useful flux is often not much greater than could be obtained by use of isotopic sources. About 35 elements have been found to possess reasonably high (n,n y) or (n,y) cross sections for 2.8 MeV neutrons 41>. Since the 8 most common elements in the earth s crust are not among those readily activated, there is some potential application of 2.8 MeV neutrons in analyses for certain elements in minerals and ores, where major element interferences via 14 MeV activation may be a problem. 

More detailed examination and sampling allows association of hydrothermal circulation with specific vent fields. In such waters samples in the Galapagos Rift by the Alvin deep submersible, Jenkins, Edmond, and Corliss (1978) report juvenile He enrichments which dwarf the normal saturation concentrations by factors up to 11 for 4He and 60 for 3He (Figure 4.6). A particularly significant feature of this report is that added He occurs roughly in proportion to added heat AT up to 12°C in sampled water), corresponding to about 7.6 x 10 xcal/atom of 3He (Figure 4.7). Jenkins et al. note that if this value is representative, hydrothermal circulation may indeed account for the depression of conductive heat flow relative to models for total heat flux. As... 

If we assume that a fallout rate of cosmic dusts has been constant and 3He in sediments is entirely due to cosmic dusts, a 3He concentration in sediment can be expressed as a function of sedimentation rate (r) and of a cosmic dust fall-out flux (F),... 

Esser and Turekian (1988) estimated an accretion rate of extraterrestrial particles in ocean bottom and in varved glacial lake deposit on the basis of osmium isotope systematics and concluded a maximum accretion rate of between 4.9 x 104 and 5.6 x 104 tons/a. The discrepancy between this estimate and those derived from helium can easily be attributed to the difference in the size of the cosmic dust particles under consideration. Cosmic dusts of greater than a few ten micrometers may not be important in the helium inventory of sediments because the larger grains are likely to lose helium due to atmospheric impact heating (e.g., Brownlee, 1985). Stuart et al. (1999) concluded from studies on Antarctic micrometeorites that 50- to 1 OO-qm micrometeorites may contribute about 5% of the total flux of extraterrestrial 3He to terrestrial sediments. Therefore, the helium-based estimate deals only with these smaller particles. 

Farley et al. (1995) recently applied a global circulation model (GCM) for the world ocean to the He flux problem, assuming a source function that injects juvenile He only along ridge axes at a rate proportional to the spreading rate. They iterated the Hamburg Large-Scale GCM (Meier-Reimer, Mikolajewicz Hasselmann, 1993) until steady-state 3He distribution was obtained and concluded that the reasonable... 

Table 6.4. 3He and4He flux in oceanic and continental areas... 

Sano (1986) and Sano et al. (1986) found He isotopic variations with depth, 3He/4He decreasing toward the surface, in two natural gas wells in northern Taiwan. This relation is interpreted as a mantle flux to the bottom of the well, progressively diluted by radiogenic He released from the surrounding sediment as the gas migrates upward. With a simple mixing model, they obtained mantle He fluxes close to the mean oceanic value (Table 6.4), but the situation in a gas well is rather complicated, and it remains to be seen whether or not the coincidence with the oceanic value is accidental. 

So far we discussed the case for 4He escape. 3He escape must at least be as effective as 4He the 4He lifetime against Jeans escape can be shown to be 70 times greater than the 3He lifetime. In the case of 4He, an inflow flux into the atmosphere is essentially the radiogenic 4He from the solid Earth. However, a flux from the interplanetary space may become important in the case of 3He inventory. From He implantation experiment on metal foil collectors in the Skylab, a spacecraft operated in a circular orbit at the altitude of 443 km from 1973 to 1974, Lind, Geiss, and Stettler (1979) concluded that the solar wind 3He precipitating from the magnetosphere to the upper atmosphere is comparable to 3He degassing flux from the solid Earth. 

Farley, K. A. (1995) Cenozoic variation in the flux of interplanetary dust recorded by 3He in a deep-sea sediment. Nature, 376, 153-6. 

Jean-Baptiste, P, Bougault, H., Vangriesheim, A., Charlou, J. L., Radford-Knoery, J., Fouquet, Y., Needham, D., German, C. (1998) Mantle 3He in hydrothermal vents and plume of the Lucky Strike site (MAR 37°17 N) and associated geothermal heat flux. Earth Planet. 

Marty, B., Jambon, A. (1987) C/3He in volatile fluxes from the solid Earth Implications for carbon geodynamics. Earth Planet. Sci. Lett., 83, 16-26. 

Sano, Y., Wakita, H., Huang, C.-W. (1986) Helium flux in a continental land area estimated from 3He/4He ratio in northern Taiwan. Nature, 323, 55-7. 

Stuart, F. M., Harrop, P. J., Knott, S., Turner, G. (1999) Laser extraction of helium isotopes from Antarctic micrometeorites Source of He and implications for the flux of extraterrestrial 3He to earth. Geochim. Cosmochim. Acta, 63, 2653-65. 

Fig. 4. The p-p chain starts with the formation of deuterium and 3 He. Thereafter, 3He is consumed in the sun 85% of the time through ppl chain, whereas pp II and pp III chains together account for 15% of the time in the Bahcall Pinsonneault 2000 solar model. The pp III chain occurs only 0.02% of the time, but the 8B f3+-decay provides the higher energy neutrinos (average Ev = 7.3 MeV). The net result of the chains is the conversion of four protons to a helium, with the effective Q-values (reduced from 26.73 MeV) as shown, due to loss of energy in escaping neutrinos. See [38,37] for updated branching ratios and neutrino fluxes for BPS2008(AGS) model...

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