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Transcript of Intro to Nuclear and Particle Physics (5110) jui/5110/y2009m03d11/mar11.pdf Intro to Nuclear and...

  • 3/11/2009 1

    Intro to Nuclear and Particle Intro to Nuclear and Particle Physics (5110)Physics (5110)

    March 11, 2009 Mostly:

    Nuclear Shell Model

  • 3/11/2009 2

    Nuclear Energy LevelsNuclear Energy Levels Daughter nucleus in α or β decay is sometimes in an excited state that can be unstable or a relatively long-lived “metastable” nuclear isomer state. Saw one example already:

    214 214 210 83 84 82

    β α → →Bi Po Pb

    (dominant)

    214 214 210 83 84 82

    β α → →*Bi Po Pb

    (rare)

    Nuclear isomers (meta-stable excited states of a nuclide) can also be produced in nuclear scattering:

    a A b B+ → + a is the incident particle, A a target nucleus, b the lighter and B the heavier product.( ),A a b B

    14 17 7 8pα + → +N O ( )14 177 8, pαN OExample:

    Sometimes scattering experiments find a set of closely related processes:

    ( ),A a b B a A b B Q+ → + + * 1 1b B Q→ + + * 2 2b B Q→ + +

    M

    Precise measurements of Q values (b KE) give the energy levels: nuclear spectroscopy

  • 3/11/2009 3

    A A Z Z γ

    ∗ → +X X

    Nuclear energy levels are revealed in scattering and in α-decay by energy spectra with closely separated α energies. All but the highest energy α are accompanied by γ

    228 224 88→90Th Ra

    γγ--DecayDecay

    • Typical spacing of nuclear energy levels: tens of keV (hard X-rays) to a few MeV (gammas)

    • Electromagnetic transitions dominate, with typical lifetimes of 10-9 s to 10-15 s (Γ < ~1 eV) – Angular momentum conservation affects rates – big changes in l require

    “higher multipole” radiation, suppressing rate – Longer-lived states (low-energy, high-multipole transitions) are called

    “isomers,” e.g. 110Agm has t1/2 = 235 d • For excitation energies above B/A, an excited state may decay by

    nucleon emission – strong interaction

  • 3/11/2009 4

    Back to The Model of the Nucleus:Back to The Model of the Nucleus: • How well does the Liquid Drop Model Work?

    – For A > ~20, the computed binding energy is good to about 1.5% – Good enough for quite reliable predictions of nuclear stability

    alpha and beta decays – As always, departures are powerful clues to something deeper

    Magic Numbers

    Specific values of Z and N that have larger than expected Binding Energy

    (Odd-A Nuclei)

    Z or N = 2, 8, 20, 28, 50, 82, (126)

  • 3/11/2009 5

    Nuclear Shell ModelNuclear Shell Model Single Particle Shell Model (SPSM) • Single nucleon moves in a potential that represents the average effect

    of all of the other nucleons. • Guess at “reasonable” potential, plug into Schrodinger’s Equation (SE),

    solve for wave function, demonstrate magic #’s, etc. as consequences.

    ( ) ( )

    0:

    : 0

    r R V r V

    r R V r

    < = −

    > =

    Square Well

    ( ) 2 21 2

    V r M rω=

    Harmonic Oscillator

    ( ) 0 1 exp

    VV r r R

    d

    − =

    − +    

    Woods -Saxon

    • Aside: How can nucleons occupy atom- like orbitals in nuclei without interacting? – Fermions! Pauli exclusion principle

    prohibits orbit-changing collisions because there is no unoccupied level for a nucleon to move to.

    • Candidate potentials: – Square well - solvable – Harmonic oscillator - solvable – Woods-Saxon – reasonable nuclear charge

    distribution, but…

    • Solve for discrete eigen-functions by “fitting” waves into effective potential (Variational Principle) (Neutron Potentials)

  • 3/11/2009 6

    ( ) ( ) 2

    2 ( ) 2

    V r r E r m

    ψ ψ   − ∇ + =   

    h r r

    Back to NR Quantum Mechanics (Review)Back to NR Quantum Mechanics (Review) H atom → Electron in Multi-Electron Atoms → Nucleon in Nucleus

    ⇒Central potential energy eigenstates are angular momentum eigenstates. 2 2 2

    2 2 2

    1 1r L r r r r

    ∂ ∂ ∇ = −

    ∂ ∂

    r

    h Spherical coordinates :

    2L r

    Apply usual separation of variables. EF's of are spherical harmonics.

    ( ) ( )( ) , l l

    nl nlm lm

    u r r Y

    r ψ θ φ=r

    n = 1, 2, … Principle Q. No.=s+l+1 l = 0, 1, 2, … Orbital Q. No. ml = -l+1,-l,…,l-1,l Magnetic Q. No.

    ( ) ( ) ( ) 22

    2 2 2

    12 0 2nl nl l ld m E V r u r

    dr mr   + + − − =      

    h

    h

    Note: 1D SE w. centrifugal barrier; unl(r) must be regular at r=0 and 0 for r→∞. n and l can have any integer values.

    Substitute into SE and get:

  • 3/11/2009 7

    • Boundary condition (infinite barrier) ⇒ unl(r) vanishes at r = R, giving energy quantization:

    ( ) ( ) 0 0,1,2,... 1, 2,...

    nl l nlu R j k R l n l

    = =

    = = and for any

    th th 0nl nl

    k n l E

    ∴ ↔ of the spherical B.F., which give the quantized energies

    Infinite Square WellInfinite Square Well ( )

    0 r R

    V r ∞ ≥

    =  

    for otherwise

    ( ) ( ) 22

    2 2 2

    12 0 2nl nl l ld m E u r

    dr mr   + + − =      

    h

    h Inside:

    ( ) ( ) nl 2 2 k nlnl l nl

    mEu r j k r= = h

    Solutions are spherical Bessel Functions

    2 4 6 8 10 12 14 -0.2

    0.2

    0.4

    0.6

    0.8

    1

    ***Note from this point on we are using n as the “radial” quantum number

  • 3/11/2009 8

    Count the available statesCount the available states Closed Shells Observed Magic Numbers: Z or N = 2, 8, 20, 28, 50,

    82, (126)

    2n = 1 l = 0 → 2

    l = 1 → 6

    l = 2 → 10

    l = 3 → 14

    l = 4 → 18

    8

    18• All zeroes of the jl’s are distinct ⇒ no energy degeneracy in n and l.

    • For given n and l, all ml’s are degenerate in energy: (2l + 1)

    • For given n, l and ml, there are ↑ and ↓ spin states for n and p.

    • Total degeneracy: 2(2l +1) slots to fill for every n and l.

    32

    50

    Good News: predicts existence of nuclear magic numbers.

    Bad News: Only some of these predicted magic numbers match observed (2, 8, 20, 28, 50,…).

    Larger n’s bring in other closed shells, but still not the right set.

    What do we try next? More/different/better potentials to start.

  • 3/11/2009 9

    ( ) 2 21 2

    V r M rω=Harmonic OscillatorHarmonic Oscillator

    12 2nl

    E n l ω = + −   

    hEnergy Levels :

    Has some degeneracy, reduced set of magic #’s.

    Still not the right set!

    Coulomb Infinite Square Well

    Harmonic Oscillator

    Observed Magic Numbers: Z or N = 2, 8, 20, 28, 50,

    82, (126)

  • 3/11/2009 10

    • Tweak the wells: Get rid of features like infinite barriers/ sharp edges – “Reasonable” nuclear potentials (solve

    numerically): finite square wells, Woods-Saxon, etc. → adjust energy levels, break degeneracies, give new magic numbers

    – Still the wrong ones! • Bottom line: No central potential

    can produce all of the observed magic numbers and only the observed magic numbers

    • Need more complicated strong interaction for nucleon in nucleus – Break rotational symmetry

    Keep trying…Keep trying…

    Observed Magic Numbers: Z or N = 2, 8, 20, 28, 50,

    82, (126)

  • 3/11/2009 11

    Maria Maria GoeppertGoeppert MayerMayer

    Nobel Prize in Physics 1963

    Maria Goeppert Mayer (1906 - 1972) was an accomplished physicist from the beginning of her career until the end and she made numerous contributions to the field of physics.

    For an account of her life and work, see Robert G. Sachs, Biographical Memoirs of the National Academy of Sciences, Volume 50. See also “Nobel Lectures, Physics 1963-1970”, Elsevier Publishing Company, Amsterdam.

  • 3/11/2009 12

    ( ) ( )TotV V r f r L S= − ⋅ rr

    Atomic Physics

    Nuclear Physics

    Spin of e → magnetic dipole moment, interacts with EM field (magnetic field component in e rest frame).

    SpinSpin--Orbit InteractionOrbit Interaction

    Nucleon-nucleon strong interaction postulated to have spin-orbit term. Nucleon deep in nucleus feels no effect (symmetry), but one near surface feels net interaction with all others.

    Differs from atomic: f(r) – to be chosen Sign – chosen to match observed level splitting

    Next step is to apply what we know about angular momentum and deduce the form of the resulting level splitting.

  • 3/11/2009 13

    ( ) ( )TotV V r f r L S= − ⋅ rr

    Compute the energy shifts (perturbatively):

    J L S= + rr r

    2 2 2 2J L S L S= + + ⋅ r rr r r

    2 2 2, , , zL S J L S J⋅ r rr r r commutes with

    ( ) ( )

    ( ) ( ) ( )

    23

    23

    1 2 2

    11 2 2

    nl nl

    nl nl

    E j l d r r f r

    l E j l d r r f r

    ψ

    ψ

     ∆ = + = −   

    + ∆ = − =   

    h r

    h r