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ASTROPHYSICS 3; SEMESTER 1 TUTORIAL 3: Nuclear Fusion and ...

ASTROPHYSICS 3; SEMESTER 1 TUTORIAL 3: Nuclear Fusion and ...

ASTROPHYSICS 3; SEMESTER 1 TUTORIAL 3: Nuclear Fusion and

ASTROPHYSICS 3; SEMESTER 1 TUTORIAL 3: Nuclear Fusion and Hydrostatic Equilibrium 1. The rate R AB at which nuclei of atomic numbers Z A and Z B will fuse by quantum tunnelling depends on temperature according to ∫ ∞ R AB ∝ T −3/2 S(E)p(E)dE, 0 where S(E) is a slowly varying function of the kinetic energy of the nuclei, and p(E) describes the probability for fusion, balancing the Boltzmann distribution of particle energies against the probability for tunnelling. (Nuclei with higher kinetic energies fuse more easily, but according to the Boltzmann factor, there are fewer of them.) The coefficient T −3/2 comes from the Maxwellian distribution of particle speeds, and accounts for the relative speed of the colliding nuclei (reaction rate is ∝ nσ v), where σ is the fusion cross-section. The principal temperature behaviour of p(E) is given by [ p(E) = exp − E ( ) 1/2 ] k B T − EG , E where E G is the Gamow energy E G = (παZ A Z B ) 2 2m red c 2 , α ≃ 1/137 is the fine– structure constant, and m red = m A m B /(m A + m B ) is the reduced mass of the nuclei. (a) Show that the energy E 0 that maximizes p(E) is given by E 0 = ( E G (kT) 2 4 [Hint: this will be the same as the energy that maximizes g(E) = lnp(E) – why?] ) 1/3 (b) Expand g(E) about this maximum energy E 0 to second order using a Taylor series [f(x) = f(a) + (x − a)f ′ (a) + (x − a) 2 f ′′ (a)/2 + ...], to show that ( ) 1/3 EG g(E) ≃ −3 − 1 4kT 2 ( 3 2kT 4 E G (kT) 2 ) 1/3 (E − E 0 ) 2 (c) Use these results to estimate the dependence of R AB on temperature T by approximating p(E) as a Gaussian (or Normal) distribution of the form ⎡ ( ) ⎤ 2 E − E0 p(E) ∝ exp ⎣− ⎦, ∆/2 where E 0 is the energy E at which p(E) is a maximum, and ∆ is the effective width of p(E). (d) Since p(E) is sharply peaked about E = E 0 with a width ∆, a good approximation for the fusion rate is R AB ∝ T −3/2 p(E 0 )∆. Using this, show that d ln R AB d ln T ( ) 1/3 ≃ −2 3 + EG . 4k B T (e) Show that E G = 8 × 10 −14 J for the pp chain. Use this with the result from part (d) to show that for temperatures T near 2 × 10 7 K, R AB ∝ T 3.5 . (f) Show that E G = 5 × 10 −12 J for the CNO cycle. Use this with the result from part (d) to show that for temperatures T near 2 × 10 7 K, R AB ∝ T 16 .

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