25 9. Direct reactions - for example direct capture: Direct transition from initial state |a+A> to final state <f| (some state in B) a + A -> B +  geometrical factor (deBroglie wave length of projectile - “size” of projectile) Interaction matrix element Penetrability: probability for projectile to reach the target nucleus for interaction. Depends on projectile Angular momentum l and Energy E III.25

26 Penetrability: 2 effects that can strongly reduce penetrability: 1. Coulomb barrier V r R Coulomb Barrier V c Potential for a projectile with Z 2 and a nucleus with Z 1 or Example: 12 C(p,  ) V C = 3 MeV Typical particle energies in astrophysics are kT=1-100 keV ! Therefore, all charged particle reaction rates in nuclear astrophysics occur way below the Coulomb barrier – fusion is only possible through tunneling

27 2. Angular momentum barrier Incident particles can have orbital angular momentum L p d Classical: Momentum p Impact parameter d In quantum mechanics the angular momentum of an incident particle can have discrete values: With l = 0 l = 1 l = 2 s-wave p-wave d-wave … For radial motion (with respect to the center of the nucleus), angular momentum conservation (central potential !) leads to an energy barrier for non zero angular momentum. Classically, if considering the radial component of the motion, d is decreasing, which requires an increase in momentum p and therefore in energy to conserve L. And parity of the wave function: (-1) l

28  reduced mass of projectile-target system Peaks again at nuclear radius (like Coulomb barrier) Energy E of a particle with angular momentum L (still classical) Similar here in quantum mechanics: Or in MeV using the nuclear radius and mass numbers of projectile A 1 and target A 2 :

Direct reactions – the simplest case: s-wave neutron capture No Coulomb or angular momentum barriers: V l =0 V C =0 But, change in potential still causes reflection – even without a barrier Recall basic quantum mechanics: Incoming wave transmitted wave Reflected wave Potential Transmission proportional to s-wave capture therefore always dominates at low energies

30 Example: 7 Li(n,  ) Penetrability Therefore, for direct s-wave neutron capture: Cross section (use Eq. III.19): Or ~1/v Deviation from 1/v due to resonant contribution thermal cross section =45.4 mb (see Pg. 27)

31 Why s-wave dominated ? Level scheme: Li 7 Li + n / /2 +  Entrance channel 7 Li + n : 3/2  + 1/2  + l (-1) = 1 , 2  Exit channel 8 Li +  Recall: Photon angular momentum/parity depend on multiploarity: For angular momentum L (=multipolarity) electric transition EL parity (-1) L magnetic transition ML parity (-1) L+1 Angular momentum and parity conservation: ( l =0 for s-wave ) ? (photon spin/parity) l

32 Also recall: E.M. Transition strength increases: for lower L for E over M for higher energy Entrance channel 7 Li + n : 3/2  + 1/2  + l (-1) = 1 , 2  Exit channel 8 Li +  ( l =0 for s-wave ) 2  + 1  = 1 , 2 , 3  E1 photon lowest EL that allows to fulfill conservation laws match possible Same for 1 + state “At low energies 7 Li(n,  ) is dominated by (direct) s-wave E1 capture”.

33 Stellar reaction rate for s-wave neutron capture: Because Thermal neutron cross section: Many neutron capture cross sections have been measured at reactors using a “thermal” (room temperature) neutron energy distribution at T= K (20 0 C), kT=25.3 meV The measured cross section is an average over the neutron flux spectrum  (E) used: (all Lab energies) For a thermal spectrum so

34 Why is a flux of thermalized particles distributed as ? The number density n of particles in the beam is Maxwell Boltzmann distributed BUT the flux is the number of neutrons hitting the target per second and area. This is a current density j = n * v therefore The cross section is averaged over the neutron flux (the number of neutrons hitting the target for each energy bin) because that is what determines the event rate. This is the same situation in the center of a star. The number density of particles is M.B. distributed, but the number of particles passing through an area per second is distributed, and so is the stellar reaction rate !

35 For s-wave neutron capture (which is generally the only capture mechanism for room temperature neutrons) one can relate the thermal cross section to the actual cross section value at the energy kT with (most frequent velocity, corresponding to E CM =kT) for reactor neutrons (thermal neutrons) v T =2.20e5 cm/s and that’s usually tabulated as “thermal cross section” which is for example useful to read cross section graphs With these definitions one can show that the measured averaged cross section and the stellar reaction rate are related simply by

Direct reactions – neutron captures with higher orbital angular momentum For neutron capture, the only barrier is the angular momentum barrier The penetrability scales with and therefore the cross section (Eq III.19) for l>0 cross section decreases with decreasing energy (as there is a barrier present) Therefore, s-wave capture in general dominates at low energies, in particular at thermal energies. Higher l-capture usually plays only a role at higher energies. What “higher” energies means depends on case to case - sometimes s-wave is strongly suppressed because of angular momentum selection rules (as it would then require higher gamma-ray multipolarities)

37 Example: p-wave capture in 14 C(n,  ) 15 C (from Wiescher et al. ApJ 363 (1990) 340)

38 14 C+n 15 C /2 + 5/2 + Entrance channel: s-wave /2 + p-wave /2 + 1/2 - 3/2 - Exit channel ( 15 C +  ) E1 M1 E /2 - 3/2 - 1/2 + 3/2 + 3/2 + 5/2 +  total to 1/2 + total to 5/2 + 3/2 - 5/2 - 7/2 - 3/2 + 5/2 + 7/2 + 1/2 + 3/2 + 5/2 + 7/2 + 9/2 + Why p-wave ? ll 14 C n total strongest ! strongest possible Exit multipole into 1/2 + into 5/2 + M1 E2 E1 despite of higher barrier, for relevant energies (1-100 keV) p-wave E1 dominates. At low energies, for example thermal neutrons, s-wave still dominates. But here for example, the thermal cross section is exceptionally low (<1  b limit known)

39 This depends on cross section shape and temperature: s-wave n-capture: What is the energy range the cross section needs to be known to determine the stellar reaction rate for n-capture ? Example: kT=10 keV M.B. distribution  (E)  (E)  (E)  (E) E = E 1/2 exp(-E/kT) relevant for stellar reaction rate of the order of KT (somewhat lower than MB distribution)

40 p-wave n-capture: Example: kT=10 keV M.B. distribution  (E)  (E)  (E)  (E) E = E 3/2 exp(-E/kT) relevant for stellar reaction rate of the order of KT (close to MB distribution)

The concept of the astrophysical S-factor (for n-capture) recall: III.25 “trivial” strong energy dependence “real” nuclear physics weak energy dependence (for direct reactions !) S-factor concept: write cross section as strong “trivial” energy dependence X weakly energy dependent S-factor The S-factor can be easier graphed easier fitted and tabulated easier extrapolated and contains all the essential nuclear physics Note: There is no “universally defined S-factor - the S-factor definition depends on P l (E) and therefore on the type of reaction and the dominant l-value !!!

42 For neutron capture with strong s-wave dominance with corrections. Then define S-factor S(E) and expand S(E) around E=0 as powers of withdenoting in practice, these are tabulated fitted parameters typical S(E) units with this definition: barn MeV 1/2

43 and for the astrophysical reaction rate (after integrating over the M.B. distribution) of course for pure s-wave capture for pure s-wave capture the S-factor is entirely determined by the thermal cross section measured with room temperature reactor neutrons: using one finds (see Pg. 35)

44 For neutron capture that is dominated by p-wave, such as 14 C(p,  ) one can define a p-wave S-factor: which leads to a relatively constant S-factor because of or (typical unit for S(E) is then barn/MeV 1/2 ) S-factor

Charged particle induced direct reactions again but now incoming particle has to overcome Coulomb barrier. Therefore (for example proton capture - such as 12 C(p,  ) in CN cycle) incoming projectile Z 1 A 1 (for example proton or  particle) target nucleus Z 2 A 2 with (from basic quantum mechanical barrier transmission coefficient) 9.3.1Cross section and S-factor definition

46 so the main energy dependence of the cross section (for direct reactions !) is given by therefore the S-factor for charged particle reactions is defined via So far this all assumed s-wave capture. However, the additional angular momentum barrier leads only to a roughly constant addition to this S-factor that strongly decreases with l Therefore, the S-factor for charged particle reactions is defined independently of the orbital angular momentum typical unit for S(E): keV barn

47 Given here is the partial proton width R=nuclear radius  reduced width (matrix element) (from Rolfs & Rodney)

48 Example: 12 C(p,  ) cross section need cross section here !

49 Need rate about here S-Factor: From the NACRE compilation of charged particle induced reaction rates on stable nuclei from H to Si (Angulo et al. Nucl. Phys. A 656 (1999) 3

Relevant cross section - Gamov Window for charged particle reactions Gamov Peak Note: relevant cross section in tail of M.B. distribution, much larger than kT (very different from n-capture !)

51 The Gamov peak can be approximated with a Gaussian centered at energy E 0 and with 1/e width  E Then, the Gamov window or the range of relevant cross section can be easily calculated using: with A “reduced mass number” and T 9 the temperature in GK

52 Example: Note: kT=2.5 keV !

Reaction rate from S-factor Often (for example with theoretical reaction rates) one approximates the rate calculation by assuming the S-factor is constant over the Gamov WIndow S(E)=S(E 0 ) then one finds the useful equation: Equation III.53 (A reduced mass number !)

54 better (and this is often done for experimental data) one expands S(E) around E=0 as powers of E to second order: If one integrates this over the Gamov window, one finds that one can use Equation III.46 when replacing S(E 0 ) with the effective S-factor S eff with and E 0 as location of the Gamov Window (see Pg. 51)