Going beyond the traditional nuclear shell model with the study of neutron-rich (radioactive) light nuclei

Transcription

1 Going beyond the traditional nuclear shell model with the study of neutron-rich (radioactive) light nuclei Fred SARAZIN Colorado School of Mines

2 SORRY

3 Overview What is low-energy nuclear physics?

4 Stable (~270) Radioactive (known, ~2000) Radioactive (predicted)

5 Atomic mass (ground state): M( A X gs ) = (Z.M H + N.M n ) - B( A X gs )/c 2 The energy scale Liquid Drop Model (1930 s) Quantum effects

6 The energy scale Atomic mass (ground state): M( A X gs ) = (Z.M H + N.M n ) - B( A X gs )/c 2 Excited state (E*): M( A X*) = (Z.M H + N.M n ) - B( A X gs )/c 2 - E*/c 2 Nuclear structure information lies within the last few MeV A huge theoretical challenge (many-body problem) Rotational band in 149 Gd Superdeformation - PRL71 (1993) 4299 Atomic: Rotational band of HCl molecule PLB467 (1999) 15

7 The energy scale The nucleon-nucleon interaction: - attractive at short range - repulsive at very short range - model: exchange of vector mesons The nuclear potential (at lowenergy): - not sensitive to the repulsive part - empirical potential: result of the average of all n-n interactions Woods-Saxon Potential: (The nucleon only sees its nearest neighbors). Nuclear Shell Model (1950 s)

8 The energy scale Nucleon (Proton=uud; Neutron=udd) mass? Where is the missing mass???? QCD

9 Nuclear Physics Still a work in progress after 100 years! Medium-energy Low-energy High-energy

10 Low-energy nuclear physics (fundamental and applied) Fundamental: Nuclear structure Nuclear astrophysics Fundamental symmetries Applied: Nuclear medicine and imagery Homeland security Stockpile stewardship Energy production Industrial processing

11 Nuclear Astrophysics A=5 & A=8 gaps Fusion limit (as a source of energy)

12 Rapid proton capture: Explosive Nucleosynthesis τ(p/n)<<τ(β +/- ) rp-process: Novae Rapid neutron capture: r-process: Supernovae?

13 Nuclear structure Z N THE NUCLEUS & THE MANY-BODY PROBLEM: < 12 nucleons Ab-initio calculations < 60 nucleons Shell Model > 50 nucleons Mean-field theories + specialty models (collective, )

14 Ab-initio calculations (using realistic nucleon-nucleon interactions) Experimental work: fine tuning of the nuclear Hamiltonian

15 11 3 Li 8 11 Li T 1/2 =8.6ms 11 Li Halo structure I.Tanihata et al., Phys. Rev. Lett. 55 (1985) P.G.Hansen and B.Jonson, Eur. Phys. Lett. 4 (1987) 9 Li Borromean rings Simple model: 9 Li + 2n (dineutron) Decay length: " =! 2µB = 8.2 fm Liquid drop model: r=r 0 A 1/3 + Shell effects (especially important for light nuclei) Binding energy (Large spatial extent: l=0 and/or l=1) r( 9 Li)=1.2 (9) 1/3 =2.5fm r( 11 Li)=1.2 (11) 1/3 =2.7fm

16 The simple shell model of 11 Li Nuclear Shell Model 11 3 Li 8 1d 3/2 2s 1/ d 5/2 1p 1/2 1p 3/2 protons (π) neutrons (ν) 1s 1/2 Ground state: π(3/2 - ) ν(0 + ) = 3/2 -

17 Ground state of 11 Li H.Simon et al., Phys. Rev. Lett. 83 (1999) 496

18 Ground state of 10 Li M.Chartier et al., Phys. Lett. B510 (2001) 24 N=7

19 Halo nuclei Neutron halo Candidate neutron halo Proton halo From: I.Tanihata et al., Progress in Particle and Nuclear Physics 68 (2013) 215

20 Configuration mixing: The not-so-simple shell model Deformed nuclei: " i! =! i " i

21 Number of particles detected at angle θ per second Target content Solid angle covered by the detector at angle θ N detected/s (!) = N part/s! N target! d" (E,!)! #"(!).# d" Beam intensity Differential cross section Detection efficiency

22 Production of short-lived radioactive nuclei Stable Radioactive (known) Radioactive (predicted)???? N detected/s (!) = N part/s! N target! d" (E,!)! #"(!).# d"

23 Production of short-lived radioactive nuclei Protons Neutrons RADIOACTIVE NUCLEI NO!

24 Production of exotic nuclei using a driver

25 ISAC-I and TRIUMF Radioactive beam Target - Source 500 MeV proton Up to 100µA DRIVER

26 11 Li β-decay of 11 Li: a very complex decay scheme S.Landowne and S.C.Pieper, PRC 29 (1984) 1352 I.Mukha et al., PLB 367 (1996) 65 M.J.G.Borge et al., PRC 55 (1997) R8 11 " Li # 8 $ % 11 Be * 7 + e # +& e # 3 $ 4 ~0.01% ~0.01% 9 Li + d ; 17.9 MeV 8 Li + t ; 15.7 MeV 11 4 Be * 7 " 10 4 Be * n 1.9% 1.0% 4.2% 2α +3n / 8 Be + 3n; 8.9 / 9.0 MeV 6 He + α + n ; 7.9 MeV 9 Be + 2n ; 7.3 MeV 10 4 Be * 6 " 10 4 Be 6 +# Q β = 20.6 MeV Can we discriminate the halo neutrons in the β-decay process? 87.6% 6.3% 11 Be 10 Be + n ; 0.5 MeV One additional recent work: M.Madurga et al., NPA 810 (2008) 1

27 β-decay of 11 Li at ISAC/TRIUMF 8PI: F.Sarazin et al., PRC70 (2004) , C.M.Mattoon et al., PRC80 (2009) Target/Source + Mass Separator (Underground) TRIUMF Cyclotron p MeV ; I<100µA

28 The 8pi spectrometer Low Energy Line (E<60 kev) 8pi: 20 HPGe Compton Suppressed Absorbers Additional equipments: 20 Plastic Scintillators Tape system (not shown here) N detected/s = N part/s N target dσ dω (E,θ) ΔΩ(θ) ε

29 The 8pi spectrometer Parent γ singles γ β γ coincidences γ γ coincidences β Beam? β γ γ coincidences γ γ Daughter Bremstrahlung rejection

30 11 " Li # 8 $ % 11 Be * 7 + e # +& e # 3 $ 4 γ-ray spectrum following the β-decay of 11 Li 11 4 Be * 7 " 10 4 Be * n 10 4 Be * 6 " 10 4 Be 6 +# γ-singles β γ coincidences β γ coincidences + bremstrahlung veto Be 0

31 A typical γ-ray spectrum with the 8pi C.M.Mattoon et al., PRC75 (2007) Beam intensity: Na atoms/s!

32 11 " Li # 8 $ % 11 Be * 7 + e # +& e # 3 $ 4 γ-ray spectrum following the β-decay of 11 Li 11 4 Be * 7 " 10 4 Be * n 10 4 Be * 6 " 10 4 Be 6 +# Be 0 Really bad resolution HPGe γ-ray spectrum? No Doppler broadening!

33 Doppler broadening following the β-n decay 1. Implantation of 11 Li 11 Li (30 kev) 11 Be* β γ 320 kev 2. β-decay of 11 Li Some excited states of 11 Be de-excites by γ-decay or one-neutron emission n 3. β-delayed one-neutron emission De-excitation of 10 Be by γ-ray emission 10 Be*

34 Understanding the γ-lineshapes 10 Be* n Doppler broadening neutron emitted isotropically in the lab frame n-γ correlation could affect lineshapes full Doppler broadening yield (max) neutron energy Slowing down of the recoil lineshape depends on lifetime of 10 Be excited states Very short T 1/2 Short T 1/2 Long T 1/2 E γ E γ E γ Simple case: single neutron emission, no HPGe resolution effect

35 10.6 The to keV transition γ-emission from 0 + state --> n-γ correlation isotropic 8.82 Half-life: T 1/ 2 (0 + 2 ) = 983 ± 27(stat) +200 "120 (syst) fs Be χ 2 /ν=1.240, ν= MeV Be 0

36 Final decay scheme C.M.Mattoon et al., PRC80 (2009) Caleb Mattoon (PhD, 2007) Now: scientist at LLNL

37 3/2-11 Li Halo: 45(10)% s-wave 55(10)% p-wave H.Simon et al., PRL 83 (1999) Li Be(8.81 MeV) shares some similarities with 9 Be+2n 11 Li β-delayed 2n-emission via the 8.81 MeV state of 11 Be R.Azuma et al., PRL 43 (1979) 1652 ; M.Marques, Private Communication (2004) The 8.81 MeV states is strongly populated by 2n-transfer reactions: (t,p), ( 13 C, 11 C), ( 14 N, 12 N), ( 16 O, 14 O) See for example H.G.Bohlen et al., Phys.Atom.Nuclei 65 (2002) 635 Q-Values differences Q( 11 Li 11 Be(8.81MeV) ) = MeV Q( 9 Li - 9 Be(gs ; 3/2 - ) ) = MeV? 11 Be Focus on one decay path F.Sarazin et al., PRC 70 (2004) Extended structure of the 2 - state Y.Ogawa et al., NPA 673 (2000) 122 gs r m =2.28 ; r p =2.17; r n =2.35 (fm) r m =2.58 ; r p =2.36; r n =2.72 (fm) 2 - r m =3.17 ; r p =2.50; r n =3.50 (fm) K.Arai, PRC 69 (2004) & J.Al-Khalili & K.Arai, PRC 74 (2006) r m =2.36 fm; 9 Be(3/2 -,5/2 - ) x νp 1/2 2 - r m =2.90 fm; mostly 9 Be(3/2 - ) x νs 1/2 9 Be core 10 Be s-wave Excited halo p-wave

38 Exotic structures: halo, cluster or both! P.J.Haigh et al., Phys. Rev. C79 (2009) ? 2α + ν(1p 3/2 ) [(2s 1/2 ) 2 or (1p 1/2 ) 2 ] s-states: σ-orbit or halo p-states: π-orbits (or halo) --> Coexistence? One suggestion M.Freer et al., Nucl. Phys. A834 (2010) 621c

39 Understanding the decay path 3/2-11 Li 8.81 Study in more details the structure of this excited state NUCLEAR REACTION: 9 Be( 6 He, 4 He) 11 Be* Very challenging because this state is very unbound Test with 12 C( 6 He, 4 He) 14 C* D.Smalley et al., PRC89 (2014) σ π 11 Be σ and this one. π 10 Be NUCLEAR REACTION: 11 Be(p,d) 10 Be* (Experiment performed May 2013) σ 2 - Duane Smalley (PhD, 2012) Now: scientist at NSTec (Los Alamos)

40 Studying the 2 - excited state in 10 Be Beam Ejectile Excited state 11 Be(p, d) 10 Be * π 10 Be σ 2 - Target Recoil 11 Be 10 Be + + A fraction of the ground-state wavefunction should look like this. 2 - state

41 Number of particles detected at angle θ per second Target content Solid angle covered by the detector at angle θ N detected/s (!) = N part/s! N target! d" (E,!)! #"(!).# d" Beam intensity Differential cross section Detection efficiency

42 Reaction kinematics Beam Ejectile Excited state 11 Be(p, d) 10 Be * Target Recoil gs, 0 MeV 2 + 1, 3.37 MeV (2 + 2,1-,0 + 2,2- ), ~6 MeV 2-body kinematics = (E,p) for d and 10 Be* can be predicted knowing 11 Be(E,p) and all the rest masses

43 γ-ray detection Experimental setup Charged particle detection

44 Kinematics ALL CHARGED PARTICLES Expected deuteron kinematics

45 Charged particle Identification ΔE E ΔE α MZ 2 /E tot with E tot = ΔE+E O (Z=8) N (Z=7) H (Z=1) He (Z=2) C (Z=6) B (Z=5) Be (Z=4) Li (Z=3)

46 Charged particle Identification He (Z=2) Deuterons 1 H (p) 3 H (t) 2 H (d) H (Z=1)

47 Deuteron kinematics DEUTERONS ONLY Expected deuteron kinematics

48 Deuteron-gated excitation spectrum (2 + 2, 1-, 0 + 2, 2- ) Excitation spectrum Ground state Be 0

49 γ-ray detection γ + charged particle background γ-lines

50 γ-ray detection γ + charged particle Deuteron-γ coincidences background γ-line (correlated with d)

51 γ-ray detection Deuteron-γ coincidences Doppler Shift Beam Target Recoil

52 γ-ray detection Deuteron-γ coincidences Doppler corrected

53 γ-ray detection Keri Kuhn (PhD student) Be 0 Analysis still in progress!

54 Summary β-delayed one-neutron emission of 11 Li investigated through the analysis of the Doppler-broadened lineshapes of γ transition in 10 Be. Many quantities deduced including half-lives, branching ratios and intensities. Decay path proceeding through the 8.82MeV state in 11 Be suggests that the β-decay is sometime occurring in the 9 Li core with the 11 Li halo neutrons surviving in their original configuration. The 8.82MeV state in 11 Be is probably not a pure excited halo state, but possibly a rather complex mix of halo and molecular configurations with the two original halo neutrons also acting as valence neutrons. Further investigation of the 2 - excited state in 10 Be is in progress. Uncovering the complex nature of this excited state will be challenging!

55 What is low-energy nuclear physics? Halo neutron survival in the β-decay of 11Li The complex structure of the 2- state in 10Be FRIB: Facility for Radioactive Ion Beams Target: operational by 2020

56 THE END