Nuclear Models: Shell Model
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LectureLecture 33 NuclearNuclear models:models: ShellShell modelmodel WS2012/13 : ‚Introduction to Nuclear and Particle Physics ‘, Part I 1 NuclearNuclear modelsmodels Nuclear models Models with strong interaction between Models of non-interacting the nucleons nucleons Liquid drop model Fermi gas model ααα-particle model Optical model Shell model … … Nucleons interact with the nearest Nucleons move freely inside the nucleus: neighbors and practically don‘t move: mean free path λ ~ R A nuclear radius mean free path λ << R A nuclear radius 2 III.III. ShellShell modelmodel 3 ShellShell modelmodel Magic numbers: Nuclides with certain proton and/or neutron numbers are found to be exceptionally stable. These so-called magic numbers are 2, 8, 20, 28, 50, 82, 126 — The doubly magic nuclei: — Nuclei with magic proton or neutron number have an unusually large number of stable or long lived nuclides . — A nucleus with a magic neutron (proton) number requires a lot of energy to separate a neutron (proton) from it. — A nucleus with one more neutron (proton) than a magic number is very easy to separate. — The first exitation level is very high : a lot of energy is needed to excite such nuclei — The doubly magic nuclei have a spherical form Nucleons are arranged into complete shells within the atomic nucleus 4 ExcitationExcitation energyenergy forfor magicm nuclei 5 NuclearNuclear potentialpotential The energy spectrum is defined by the nuclear potential solution of Schrödinger equation for a realistic potential The nuclear force is very short-ranged => the form of the potential follows the density distribution of the nucleons within the nucleus: for very light nuclei (A < 7), the nucleon distribution has Gaussian form (corresponding to a harmonic oscillator potential ) for heavier nuclei it can be parameterised by a Fermi distribution. The latter corresponds to the Woods-Saxon potential e.g. 3) approximation by the U 1) Woods-Saxon potential: U(r) === −−− 0 rectangular potential well r−−−R with infinite barrier energy : 1+++ e a ∞ 2) a 0: U(r) ∞∞ a=0 approximation by the a>0 rectangular potential well : 0 R r −−−U0 , r <<< R ,0 r <<< R U(r) === U(r) === ,0 r ≥≥≥ R ∞∞∞, r ≥≥≥ R 6 SchrSchr öödingerdinger equationequation ˆ Schrödinger equation: HΨΨΨ === EΨΨΨ (1) Consider U(r) - Single-particle h∇∇∇2 rectangular potential Hˆ === −−− +++U(r) (2) Hamiltonian operator: 2M well with infinite •Eigenstates: ΨΨΨ(r) - wave function barrier energy •Eigenvalues: E - energy ,0 r <<< R U(r) is a nuclear potential – spherically symmetric U(r) === • ∞∞∞, r ≥≥≥ R 1 ∂∂∂ ∂∂∂ 1 ∂∂∂ 1 ∂∂∂ 2 ∇∇∇2 === r 2 +++ ((()(sin θθθ )))+++ r 2 ∂∂∂r ∂∂∂r r 2 sin θθθ ∂∂∂θθθ r 2 sin 2 θθθ ∂∂∂ϕϕϕ 2 1 ∂∂∂ 1 ∂∂∂2 Angular part: λλλˆ === ((()(sin θθθ )))+++ sin θθθ ∂∂∂θθθ sin 2 θθθ ∂∂∂ϕϕϕ 2 2 1 ∂∂∂ ∂∂∂ 1 Lˆ −−− h2λλλˆ === Lˆ2 ∇∇∇2 === r 2 −−− r 2 ∂∂∂r ∂∂∂r r 2 h2 L – operator for the orbital angular momentum ˆ2 h2 L ΥΥΥlm (((θθθ,ϕϕϕ))) === (ll +++ )1 ΥΥΥlm (((θθθ,ϕϕϕ))) (3) Eigenstates: Ylm – spherical harmonics 7 RadialRadial partpart The wave function of the particles in the nuclear potential can be decomposed into two parts: a radial one ΨΨΨ 111(r), which only depends on the radius r, and an angular part Ylm (θ,ϕϕϕ) which only depends on the orientation (this decomposition is possible for all spherically symmetric potentials): (4) ΨΨΨ(((r,θθθ,ϕϕϕ))) === ΨΨΨ1(r)⋅⋅⋅ ΥΥΥlm (((θθθ,ϕϕϕ))) From (4) and (1) => 2 2 h 1 ∂∂∂ ∂∂∂ 1 Lˆ 2 −−− 2 r +++ 2 ΨΨΨ1((()(r)))(ΥΥΥlm ((()θθθ,ϕϕϕ)))(=== E ΨΨΨ1((()r)))(ΥΥΥlm ((()θθθ,ϕϕϕ))) 2M r ∂∂∂r ∂∂∂r r 2M => eq. for the radial part: h2 1 ∂∂∂ ∂∂∂ h2 (ll +++ )1 2 (5) −−− 2 r +++ 2 ΨΨΨ1((()(r)))(=== E ΨΨΨ1((()r))) 2M r ∂∂∂r ∂∂∂r r 2M Substitute in (5): ℜℜℜ )r( ΨΨΨ ((()(r))) === 1 r h2 d 2ℜℜℜ )r( h2 l(l +++ )1 (6) −−− +++ ℜℜℜ )r( === E ℜℜℜ )r( 2M dr 2 r 2 2M 8 ConstraintsConstraints onon EE h2 d 2ℜℜℜ )r( h2 l(l +++ )1 Eq. for the radial part: +++ E −−− ℜℜℜ )r( === 0 (7) 2M dr 2 r 2 2M From (7) 1) Energy eigenvalues for orbital angular momentum l: E: l=0 s l=1 p states l=2 d l=3 f … 2) For each l: -l < m < l => (2 l+1) projections m of angular momentum. The energy is independent of the m quantum number, which can be any integer value between ± l. Since nucleons also have two possible spin directions, this means that the l levels are 2 ·(2 l+1) times degenerate if a spin-orbit interaction is neglected. 3) The parity of the wave function is fixed by the spherical wave function Yml and l reads (−1) : ˆ l P ΨΨΨ(((r,θθθ,ϕϕϕ))) === P ΨΨΨ1(r)⋅⋅⋅ ΥΥΥlm (((θθθ,ϕϕϕ))) === (−−− )1 ΨΨΨ1(r)⋅⋅⋅ ΥΥΥlm (((θθθ,ϕϕϕ))) ⇒ s,d,.. - even states; p,f,… - odd states 9 MainMain quantumquantum numbernumber nn h2 d 2ℜℜℜ )r( h2 l(l +++ )1 Eq. for the radial part : +++ E −−− ℜℜℜ )r( === 0 (7) 2M dr 2 r 2 2M A Solution of differential eq: )r( ′′′′′′ )r( )r( )r( 0 ℜℜℜ )r( === l kr(j ) ℜℜℜ +++ λλλ ℜℜℜ === r Spherical Bessel functions j (x): 2 2ME l k === h2 The wave function: A ΨΨΨ((()(r,θθθ,ϕϕϕ))) === j (kr )⋅⋅⋅ΥΥΥ ((()(θθθ,ϕϕϕ))) r l lm ∞∞∞ U(r) Boundary condition for the surface, i.e. at r =R: Ψ(Ψ(Ψ( R,θ,ϕ) =0 0 R r restrictions on k in Bessel functions: l kR(j ) === 0 ⇒ main quantum number n – corresponds to nodes of the Bessel function : Xnl 2ME ⇒⇒⇒ 2 2 2 ⇒⇒⇒ 2 2 (8) kR === Xnl k R === Xnl 2 R === Xnl h 10 ShellShell modelmodel withwith rectangularrectangular potentialpotential wellwell X 2 h2 Thus, according to Eq. (8) : nl 2 (9) E === Enl === Const ⋅⋅⋅ Xnl nl 2MR 2 Nodes of Bessel function Energy states are quantized structure of energy states Enl l === 0 s −−− states j0 2 state Enl === C ⋅⋅⋅ Xnl degeneracy states with E ≤≤≤ Enl n === 1 X10 === 14.3 2·(2 l+1 ),l=0 s1 E s1 === C ⋅⋅⋅ 86.9 2 2 n === 2 X20 === 28.6 1p E1 p === C ⋅⋅⋅ 20 2. 6 2·(2 l+1 ),l=1 8 n === 3 X30 === 42.9 d1 E1d === C ⋅⋅⋅ 33 2. 10 18 l === 1 p −−− states j1 s2 E2s === C ⋅⋅⋅ 39 5. 2 20 n === 1 X11 === 49.4 1 f E1 f === C ⋅⋅⋅ 48 8. 14 34 n === 2 X21 === 72.7 2 p E2 p === C ⋅⋅⋅59 7. 6 40 1g E === C ⋅⋅⋅64 18 58 l === 2 d −−− states j2 1g n === 1 X === 76.5 12 First 3 magic numbers are reproduced, higher – not! l === 3 f −−− states j3 2, 8, 20, 28, 50, 82, 126 n === 1 X13 === 99.6 Note: this result corresponds to for U(r) = rectangular l === 4 g −−− states j 4 potential well with infinite barrier energy 11 n === 1 X14 === 1.8 ShellShell modelmodel withwith WoodsWoods --SaxonSaxon potentialpotential Woods-Saxon potential: U0 U(r) === −−− r−−−R 1+++ e a The first three magic numbers (2, 8 and 20) can then be understood as nucleon numbers for full shells . Thus, this simple model does not work for the higher magic numbers. For them it is necessary to include spin-orbit coupling effects which further split the nl shells. 12 SpinSpin --orbitorbit interactioninteraction Introduce the spin-orbit interaction Vls – a coupling of the spin and the orbital angular momentum: h∇∇∇2 Hˆ === −−− +++U(r)+++Vˆ 2M ls h 2 ∇∇∇ ˆ −−− +++U(r)+++Vls ΨΨΨ(r,θθθ,ϕϕϕ) === E ΨΨΨ(r,θθθ,ϕϕϕ) 2M h∇∇∇2 −−− +++U(r)ΨΨΨ(r,θθθ,ϕϕϕ) === ((()(E −−−Vls ))) ΨΨΨ(r,θθθ,ϕϕϕ) 2M ˆ where Vls ΨΨΨ(r,θθθ,ϕϕϕ) ===Vls ΨΨΨ(r,θθθ,ϕϕϕ) Eigenstates eigenvalues Vˆ C )s,l( - scalar product of l and s •spin-orbit interaction: ls === ls total angular momentum: j === l +++ s 2 2 j ⋅⋅⋅ j === (l +++ s)( l +++ s) === l +++ s +++ 2 sl 1 2 2 2 l ⋅⋅⋅ s === ( j −−− l −−− s ) 2 13 SpinSpin --orbitorbit interactioninteraction 1 2 2 2 C ⋅⋅⋅ ( j −−− l −−− s )ΨΨΨ(r,θθθ,ϕϕϕ) ===V ΨΨΨ(r,θθθ,ϕϕϕ) ls 2 ls h2 Eigenvalues: V === C [[[][ j(j +++ )1 −−− l(l +++ )1 −−− s(s +++ )1 ]]] ls ls 2 Consider: h2 h2 1 1 1 2 1 3 j === l +++ : Vls === Cls (l +++ )( l +++ )−−− l −−− l −−− ⋅⋅⋅ === Cls l 2 2 2 2 2 2 2 h2 h2 1 1 1 2 1 3 j === l −−− : Vls === Cls (l −−− )( l +++ )−−− l −−− l −−− ⋅⋅⋅ === −−−Cls (l +++ )1 2 2 2 2 2 2 2 This leads to an energy splitting ∆Els which linearly increases with the angular momentum as 2l +++1 ∆∆∆E === V ls 2 ls It is found experimentally that Vls is negative , which means that the state with j =l+ 1/2 is always energetically below the j = l − 1/2 level. 14 SpinSpin --orbitorbit interactioninteraction The total angular momentum quantum Single particle energy levels: number j = l±1/2 of the nucleon is denoted by an extra index j: nl j j (2j+1) e.g., the 1f state splits into a 1f 7/2 and a 1f 5/2 state 1f 1f 5/2 1f 7/2 The nlj level is (2 j + 1) times degenerate Spin-orbit interaction leads to a sizeable splitting of the energy states which are indeed (10) (2) (6) comparable with the gaps between the nl (4) shells themselves . (8) (4) Magic numbers appear when the gaps (2) (6) between successive energy shells are (2) particularly large: (4) 2, 8, 20, 28, 50, 82, 126 (2) 15 CollectiveCollective NuclearNuclear ModelsModels 16 CollectiveCollective excitationsexcitations ofof nucleinuclei The single-particle shell model can not properly describe the excited states of nuclei: the excitation spectra of even-even nuclei show characteristic band structures which can be interpreted as vibrations and rotations of the nuclear surface low energy excitations have a collective origin ! The liquid drop model is used for the description of collective excitations of nuclei: the interior structure, i.e., the existence of individual nucleons, is neglected in favor of the picture of a homogeneous fluid-like nuclear matter.