Helium Isotopes

Definition

Stable isotopic ratio of helium (³He/⁴He).

Variations in He Isotope Ratios

Helium has two stable isotopes: ³He and ⁴He. Both isotopes were produced by nucleo-synthesis in the very early universe about 100 s after the Big Bang, possibly because of the fusion of deuterium (D) as shown below.

D + D ➔ ³He + n.

D + D ➔ ⁴He + γ.

T + D ➔ ⁴He + n.

³He + n ➔ ⁴He + γ.

³He + D ➔ ⁴He + p.

In those expressions, n, γ, T, and p, respectively, denote neutron, γ ray, tritium (³H), and proton. The helium isotopic ratio (³He/⁴He) has been calculated approximately 8 × 10⁻⁵ (Burles et al. 1999) because the ⁴He nucleus is much more stable than that of ³He. The ratio has evolved through stellar processes. Both ³He and ⁴He are produced by a proton–proton chain reaction occurring deep in stars like the Sun with high pressure and temperature as follows.

p + p ➔ D + β⁺ + ν.

D + p ➔ ³He + γ.

³He + ³He ➔ ⁴He + 2 p.

Therein, β⁺ and ν, respectively, denote positron and neutrino. The best estimate of the ³He/...