Some (more) High(ish)-Spin Nuclear Structure Lecture 2 Low...
Transcript of Some (more) High(ish)-Spin Nuclear Structure Lecture 2 Low...
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Some (more) High(ish)-Spin Nuclear Structure
Paddy ReganDepartment of Physics
Univesity of SurreyGuildford, UK
Lecture 2Low-energy Collective Modes and Electromagnetic Decays in Nuclei
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Outline of Lectures• 1) Overview of nuclear structure ‘limits’
– Some experimental observables, evidence for shell structure– Independent particle (shell) model– Single particle excitations and 2 particle interactions.
• 2) Low Energy Collective Modes and EM Decays in Nuclei.– Low-energy Quadrupole Vibrations in Nuclei– Rotations in even-even nuclei– Vibrator-rotor transitions, E-GOS curves
• 3) EM transition rates..what they mean and overview of how you measure them– Deformed Shell Model: the Nilsson Model, K-isomers– Definitions of B(ML) etc. ; Weisskopf estimates etc.– Transition quadrupole moments (Qo)– Electronic coincidences; Doppler Shift methods. – Yrast trap isomers– Magnetic moments and g-factors
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Excitation energy (keV)
Ground stateConfiguration.Spin/parity I=0+ ;Ex = 0 keV
2+
0+
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4+/2+ energy ratio:mirrors 2+ systematics.
Excitation energy(keV)
Ground stateConfiguration.Spin/parity I=0+ ;Ex = 0 keV
2+
0+
4+
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B(E2; 2+ 0+ )
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What about both valence neutrons and protons? In cases of a few valence nucleons there is a
lowering of energies, development of multiplets. R4/2 ~2-2.4
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Quadrupole Vibrations in Nuclei ?
• Low-energy quadrupole vibrations in nuclei ?– Evidence?– Signatures?– Coupling schemes ?
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2
V
2
En
n=0
n=1
n=2
n=3
http://npl.kyy.nitech.ac.jp/~arita/vib
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We can use the m-scheme to see what states we can make when we coupletogether 2 quadrupole phonon excitations of order J=2ħ. (Note phonons are bosons, so we can couple identical ‘particles’ together).
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From,Nuclear StructureFrom a SimplePerspective, byR.F. Casten,Oxford UniversityPress.
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For an idealised quantum quadrupole vibrator, the(quadrupole) phonon (=‘d-boson’) selection rule is n=1 , where n=phonon number.
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For an idealised quantum quadrupole vibrator, thephonon (=‘d-boson’) selection rule is n=1
4+ →2+ E2 from n=3 →n=1 is ‘forbidden’ in an idealised quadrupole vibrator by phonon selection rule.
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For an idealised quantum quadrupole vibrator, thephonon (=‘d-boson’) selection rule is np=1
4+ →2+ E2 from n=3 →n=1 is ‘forbidden’ in an idealised quadrupole vibrator by phonon selection rule.
Similarly, E2 from 2+→0+
from n=3 →n=0 not allowed.
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Collective (Quadrupole) Nuclear Rotations and Vibrations
• What are the (idealised) excitation energy signatures for quadrupole collective motion (in even-even nuclei) ?– (extreme) theoretical limits
2 (4 ) 4(5) 20( 1), 3.332 (2 ) 2(
(4 ) 2 = 2.00
3)
( 1
6
2 )N
J
EE N
EE J JE
E
Perfect, quadrupole (ellipsoidal), axially symmetric quantum rotor with a constant moment of inertia (I) has rotational energies given by (from Eclass(rotor) = ½ L2/2I)
Perfect, quadrupole vibrator has energies given by the solutionto the harmonic oscilator potential (Eclassical=1/2kx2 + p2/2m ).
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Collective (Quadrupole) Nuclear Rotations and Vibrations
• What are the (idealised) excitation energy signatures for quadrupole collective motion (in even-even nuclei) ?– (extreme) theoretical limits
2 (4 ) 4(5) 20( 1), 3.332 (2 ) 2(
(4 ) 2 = 2.00
3)
( 1
6
2 )N
J
EE N
EE J JE
E
Perfect, quadrupole (ellipsoidal), axially symmetric quantum rotor with a constant moment of inertia (I) has rotational energies given by (from Eclass(rotor) = ½ L2/2I)
Perfect, quadrupole vibrator has energies given by the solutionto the harmonic oscilator potential (Eclassical=1/2kx2 + p2/2m ).
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Other Signatures of (perfect) vibrators and rotorsDecay lifetimes give B(E2) values. Also selection rules important (eg. n=1).
For (‘real’) examples, see J. Kern et al., Nucl. Phys. A593 (1995) 21
E=ħ E (J→J-2)=0Ex=(ħ2/2I)J(J+1) i.e., E (J→J-2)= (ħ2/2I)[J(J+1) – (J-2)(J-3)] = (ħ2/2I)(6J-6); E=(ħ2/2I)*12=const.
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Other Signatures of (perfect) vibrators and rotorsDecay lifetimes give B(E2) values. Also selection rules important (eg. n=1).
Ex=(ħ2/2I)J(J+1) i.e., E (J→J-2)=(ħ2/2I)[J(J+1) – (J-2)(J-3)] = (ħ2/2I)(6J-6); E=(ħ2/2I)*12=const.
Ex=(ħ2/2I)J(J+1)
++
++
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Other Signatures of (perfect) vibrators and rotorsDecay lifetimes give B(E2) values. Also selection rules important (eg. n=1).
For (‘real’) examples, see J. Kern et al., Nucl. Phys. A593 (1995) 21
E=ħ E (J→J-2)=0Ex=(ħ2/2I)J(J+1) i.e., E (J→J-2)= (ħ2/2I)[J(J+1) – (J-2)(J-3)] = (ħ2/2I)(6J-6); E=(ħ2/2I)*12=const.
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So, what about ‘real’ nuclei ?
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Many nuclei with R(4/2)~2.0 also show I=4+,2+,0+ triplet states at ~2E(2+).
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Note on ‘near-yrast feeding’ for vibrational states in nuclei.
If ‘vibrational’ states are populated in very high-spin reactions (such as heavyion induced fusion evaporation reactions), only the decays betweenthe (near)-YRAST states are likely to be observed.
The effect is to (only?) see the ‘stretched’ E2 cascade from Jmax →Jmax-2 for each phonon multiplet.
= the ‘yrast’ stretched E2 cascade.
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Note on ‘near-yrast feeding’ for vibrational states in nuclei.
If ‘vibrational’ states are populated in very high-spin reactions (such as heavyion induced fusion evaporation reactions), only the decays betweenthe (near)-YRAST states are likely to be observed.
The effect is to (only?) see the ‘stretched’ E2 cascade from Jmax →Jmax-2 for each phonon multiplet.
= the ‘yrast’ stretched E2 cascade.
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Nuclear Rotations and Static Quadrupole Deformation
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B(E2: 0+1 2+1) 2+1 E20+12
2+
0+T (E2) = transition probability = 1/ (secs); E = transition energy in MeV
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B(E2: 0+1 2+1) 2+1 E20+12
2+
0+
Rotational model, B(E2: I→I-2) gives:
Qo = INTRINSIC (TRANSITION)ELECTRIC QUADRUPOLE MOMENT.
This is intimately linked to the electrical charge (i.e. proton) distribution within the nucleus.
Non-zero Qo means some deviation from spherical symmetry and thus somequadrupole ‘deformation’.
T (E2) = transition probability = 1/ (secs); E = transition energy in MeV
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Bohr and Mottelson, Phys. Rev. 90, 717 (1953)
Isomer spin in 180Hf, I>11 shown later to be I=K=8- by Korner et al. Phys. Rev. Letts. 27, 1593 (1971)).
K-value very important in understanding isomers.
Ex = (ħ2/2I)*J(J+1)
I = moment of inertia. This depends on nucleardeformation and I~ kMR2
Thus, I ~ kA5/3
(since rnuc=1.2A1/3fm )
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Therefore, plotting the moment of inertia, divided by A5/3 should give a comparison of nuclear deformations across chains of nuclei and mass regions….
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Nuclear static moment of inertia for E(2+) states divided by A5/3 trivial mass dependence. Should show regions of quadrupole deformation.
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Lots of valence nucleons of both types:emergence of deformation and therefore rotation
R4/2 ~3.33 = [4(4+1)] / [2(2+1)]
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Perfect rotor limit R(4/2) = 3.33 = 4(4+1) / 2(2+1)
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Best nuclear ‘rotors’ have largest values of N.N
This is the product of the number of ‘valence’protons, N X the number of valence neutrons N
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Alignments and rotational motion in ‘vibrational’ 106Cd (Z=48, N=58),
PHR et al. Nucl. Phys. A586 (1995) p351
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Some useful nuclear rotational,‘pseudo-observables’…
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Some useful nuclear rotational,‘pseudo-observables’…
Rotational ‘frequency’, given by,
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2qp states, Ex~2
4qp states, Ex~4
6qp states, Ex~6
8qp states, Ex~8
C.S.Purry et al., Nucl. Phys. A632 (1998) p229
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Transitions from Vibrator to Rotor?
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PHR, Beausang, Zamfir, Casten, Zhang et al., Phys. Rev. Lett. 90 (2003) 152502
2424
2 :Rotor
0 : Vibrator
)2(
242
),1(2
:Rotor
,2
:Vibrator
22
22
J
J
J
n
JR
JR
JJJE
R
JEJJE
EJnE
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PHR, Beausang, Zamfir, Casten, Zhang et al., Phys. Rev. Lett. 90 (2003) 152502
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PHR, Beausang, Zamfir, Casten, Zhang et al., Phys. Rev. Lett. 90 (2003) 152502
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Vibrator-Rotator phase change is a feature of near stable (green)A~100.
‘Rotational alignment’ can be a crossing between quasi-vibrational GSB & deformed rotational sequence.(stiffening of potential by population of high-j, equatorial (h11/2) orbitals).
PHR, Beausang, Zamfir, Casten, Zhang et al., Phys. Rev. Lett. 90 (2003) 152502