Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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Compound Nucleus Reactions

Direct

CN decays

• Time.• Energy.

• Two-step reaction. • CN “forgets” how it was formed.• Decay of CN depends on statistical factors that are functions of Ex, J.• Low energy projectile, medium or heavy target.

QCN

EaCM

Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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Compound Nucleus Reactions

Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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Compound Nucleus Reactions• Consider p + 63Cu at Ep

CM= 20 MeV.• Calculate Ep + [m(63Cu) + m(p) – m(64Zn)]c2.• Divide by 64 available energy per nucleon << 8 MeV.• Multiple collisions “long” time statistical distribution of energy small chance for a nucleon to get enough energy EvaporationEvaporation.• Higher incident energy more particles “evaporate”.

See also Fig. 11.21 in Krane.

Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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• Random collisions nearly isotropic angular distribution.• Direct reaction component strong angular dependence.

See also Fig. 11.20 in Krane.

Direct Reactions

Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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Direct Reactions• Peripheral collision with surface nucleon.• 1 MeV incident nucleon ?? more likely to interact with the nucleus CN reaction.• 20 MeV incident nucleon ?? peripheral collision Direct reaction.• CN and Direct (D) processes can happen at the same incident particle energy. Distinguished by:

D (10-22 s) CN (10-18-10-16 s).[Consider a 20 MeV deuteron on A=50 target nucleus].

Angular distribution.

Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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Direct Reactions• (d,n) stripping (transfer) reactions can go through both processes.• (d,p) stripping (transfer) reactions prefer D rather than CN; protons do not easily evaporate (Coulomb). [(p,d) is a pickup reaction].• What about (,n) transfer reactions?HW 36HW 36 Show that for a (d,p) reaction taking place on the surface of a 90Zr nucleus, and with 5 MeV deuterons, the angular momentum transfer can be approximated by l = 8sin(/2), where is the angle the outgoing proton makes with the incident deuteron direction. (Derive a general formula first).

l 0 1 2 3

0º 14.4º 29º 44ºJ(90Zrgs) = 0+

J(91Zr) = l ± ½, = (-1)l

Optical model, DWBA, Shell model, Spectroscopic Factor.

calcmeas d

dS

d

d

Fig. 11.23 in Krane.

Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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22 nXHCCHbY IIIn

Neutron-induced Reactions

X(n,b)Y

n(En)b(Q+En)

For thermal neutronsQ >> En

b(Q) constant

2

11

vE

)( nln EPvn

Probability to penetrate the potential barrier

Po(Ethermal) = 1P>o(Ethermal) = 0

vEnn

1)( Non-resonant

Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).

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Neutron-induced Reactions