Nuclear Structure Studies at Medium to High Spins with REA6/12 M.P. Carpenter Physics Division, Argonne National Laboratory With Thanks to S. Zhu, D. Seweryniak and R.V.F. Janssens REA3 Upgrade Workshop Michigan State University Aug. 20, 2015
Why Study Nuclei to High Angular Momentum? • A variety of nuclear properties can be described by the shell model, where nucleons move independently in their average potential, in close analogy with the atomic shell model. • The nucleus often behaves collectively, like a fluid - even a superfluid, in fact the smallest superfluid object known in the nature and there are close analogies both to condensed matter physics and to familiar macroscopic systems, such as the liquid drop. • A major thrust in the study of nuclei at high angular momentum is to understand how nucleon-nucleon interactions build to create the mean field and how single-particle motions build collective effects like pairing, vibrations and shapes • The diversity of the nuclear structure landscape results in the fact that the the small number of nucleons leads to specific finite-system effects, where even a rearrangement of a few particles can change the “face” of the whole system.
Nuclear Structure Varies as a Function of N and Z
Nuclear Landscape Predicted ground state deformations from HFB calculations using Gogny force see www-phynu.cea.fr 4
Why Study Nuclei to Higher Spins? Identification of high-spins states which are inaccessible from beta-decay or reaction studies provide a more complete knowledge base for understanding the nuclear structure not only at high-angular momentum but near the ground state as well. Nilsson Diagram 5
Measure Nuclear Levels and Properties • Level sequences determined by measuring de-excitation g rays in coincidence (2-fold, 3- fold, …). • Only Ge detectors can offer the required efficiency and energy resolution to perform high precision spectroscopy. • Lifetime information is often crucial to characterize state and can be measured using RDDM or DSAM. • Spins and parities of levels can be determined from gamma-ray angular distributions, angular correlations and polarizations. • Highest spins reached using fusion evaporation reactions.
The Ti story: N=32 shell gap, N=34 no gap . 32 34 28 p 3/2 f 5/2 p 1/2 f 7/2 p 1/2 p 3/2 1129 48 Ca + 238 U DIC with Gammasphere R.V.F. Janssens et al., PLB 546, 22 (2002) B. Fornal et al., PRC 70, 064304(2004) 7
Possible reactions to populate high-spins at REA6 Fusion-like reactions will have higher probabilities of neutron evaporation as compound systems becomes more neutron-rich – limits the accessibility to neutron rich isotopes – BUT – don’t forget proton rich isotopes. Deep-Inelastic Reactions are in principle but are not very selective – i.e. reaction channels can encompass hundreds of nuclei and this can be a show- stopper using RIB’s with reduced intensities (compare 10 6 with 10 10 ). Transfer is probably better. But – CARIBU at ATLAS and now REA at NSCL, we have access to reaccelerated neutron rich RIBs which offers the opportunity for direct excitation of the beam i.e Unsafe Coulex – excitation of beam using where E beam ~ 5-6 MeV/u. REA6 is necessary LARGEST and DOMINANT CROSS-SECTION is DIRECT EXCITATION OF BEAM 8
What Detector do we require? High efficient, segmented gamma-ray array – GRETINA/GRETA. Highly segmented particle detectors – CHICO II/Phoswich Array Mass Separator – ISLA/FMA/AGFA CHICO II (Rochester, LLNL) Phoswich Wall (Wash. Univ.) • Angular coverage: 12 o ≤ θ ≤ 85 o , 95 o ≤ θ ≤ 168 o ; • No. Pixels: 256 (4 PMT’s) 280 o in φ • Thin fast-plastic + CsI(Tl) • Angular resolution: Δθ = 1 o , Δφ = 1.4 o • Angle range: 9º ≤ θ ≤ 72º • Solid angle: ~69% of 4 π • • Preferred position: downstream Time resolution: Δt ≤ 500ps • Mass resolution: Δm/m ≤ 5% • Q-value resolution: ≤ 20 MeV 9
Expected Efficiency of GRETINA Singles Calorimeter Tracked Should be here soon 10
What Beam Rates are Required for g-g Analysis GRETINA will soon have ~10% efficiency for 1 MeV gamma-rays. Gammasphere has ~ 10% efficiency for 1 MeV gamma-rays. Gammasphere can produce enough gamma-ray coincidences to perform a proper g-g correlation analysis for s ~ 1 m b and beam intensity at 1x10 11 particles/sec in ~ 1 week. GRETINA at REA6 will require ~ 5x10 5 particles/sec to collect similar statistics with s ~ 500 mb . GRETA with ~25% efficiency would give similar statistics with beam intensities ~ 1x10 4 particles/sec at s ~ 500 mb . 11
Unsafe Coulex of Actinide Targets 208 Pb beam at ~ 1.3 GeV ( 6.25 MeV/A) 232 Th target (thick) K=0 - K=1 - Double Beta Gamma Gamma Octupole Octupole 232 Th K. Abu Saleem, Ph D. Thesis, Argonne Nat. Lab .
Coulomb Excitation of Reaccelerated 144 Ba at ATLAS • Separation of 144 Ba from contaminants other than isobars made by the measured quasi-two- body kinematics – Time-of-flight difference vs. scattering angle • Major contaminant is 134 Xe in the final Doppler-shift corrected g spectrum g energy resolution is ~ 5.5 keV for • 847 keV, nearly a factor of 2 improvement compared to that of GS/CHICO • Beam Intensity ~ 5000/sec. Need more beam intensity for g-g. • C. Y. Wu et al.
Deep Inelastic Reactions • Effective method for identifying excited states in neutron rich nuclides. • Target should have large N/Z ratio – 208 Pb, 238 U • Many nuclei populated – not optimal for RIB’s. 208 Pb 64 Ni W. Królas , R. Broda, B. Fornal , T. Pawłat , H. Grawe, K.H. Maier M. Schramm, R. Schubart, Nucl. Phys. A724 (2003) 289. 1 4
Alternative Methods to DIC (transfer, ICF) Recent experiment with GRETINA and Phoswich Wall (W. Reviol et al.) 138 Ba + 13 C, E lab = 550 MeV (ATLAS) • 139 Ba + 12 C products (one-neutron transfer) • 142 Ce + 9 Be products (incomplete fusion) 1000 counts 627 particle gate #1 455 red = 139 Ba 642 * 100 counts 579 particle gate #2 524 green = 142 Ce * * * * * contamination ( 144 Nd) E γ (keV) 15
Fusion Evaporation: the case of 100 Sn 58 Ni+ 46 Ti g 104 Sn g 100 Sn+4n STABLE BEAM 58 Ni 56 Ni: 10 11 /s Cross section: ~1 nb (out of 500 mb) Yield:60/day Channel selection: Recoil-Decay Tagging? 56 Ni+ 50 Cr g 106 Te g 100 Sn+ a +2n PROTON RICH RADIOACTIVE 56 Ni BEAM 56 Ni: 10 8 /s Cross section: ~1 m b (out of 500 mb) Yield:60/day Channel selection: Recoil-Decay Tagging? 63 As+ 40 Ca g 103 I g 100 Sn+3p PROTON RICH HEAVY RADIOACTIVE BEAM 63 As: 1000/s Cross section: ~100 mb (out of 500 mb) Yield: 60/day Channel selection: Z from D E measurement 16
Current State of the Art: Evidence for the Disappearance of the N=40 Gap 68 Ni has high E(2 + ) In contrast, Fe and Cr E(2 + ) decrease as one approaches N =40. Measured B(E2) for Fe and Cr isotopes also support increase in collectivity A. Gade et al. , Phys. Rev. C, 81 (2010) 051304(R) and ref. therein. 17
Compression of Levels also observed at higher spins From inelastic excitation of 48 Ca + 238 U and 48 Ca + 208 Pb at ATLAS + 56 Cr 32 Gammasphere 58 Cr 34 60 Cr 36 S. Zhu et al ., Phys. Rev. C 74 (2006) 064351 18
Fusion Evaporation Provides Access to Highest Spins 14 C( 48 Ca, α 2 n ) 56 Cr Fusion Evaporation at ATLAS with Gammasphere + FMA DIC limit (g 9/2 xf 5/2 ) 56 Cr S. Zhu et al ., Phys. Rev. C 74 (2006) 064351 19
Rotational Bands at higher spins in Cr-Fe Isotopes Fusion Evaporation at ATLAS with Gammasphere ( 48 Ca + 14 C) Configuration of all rotational bands have been assigned at least one g 9/2 neutron. A. Deacon et al. , Phys. Rev. C, 73 (2007) 054303. 20
What Is the Nature of the Enhanced Collectivty at N=40 in Fe and Cr? 60 Cr } yrast bands (g 9/2 ) 2 where ħ ω ~ E γ /2 F.R. Xu, Peking University All yrast bands are approaching (g 9/2 ) 2 aligned structure at 8 + • • 8+ levels in Fe isotopes decrease in energy 60 Fe (5.3 MeV), 62 Fe (4.25 MeV) and 64 Fe (3.6 MeV). 21
Shape co-existence in Hg isotopes Systematics of Yrast Bands in Hg Isotopes M.P. Carpenter et al., Phys. Rev. Lett. 78 (1997) 3650. 22
Results of simple two band mixing calculations: • For Fe isotopes, deformed 0 + never drops below spherical 0 + . • The deformed 0 + is the first excited state in 66 Fe at ~250 keV. • 62,64 Cr the deformed 0 + is the ground state but strongly mixed with excited 0 +. M.P. Carpenter et al., PRC 87 (2013) 041305(R) 23
68 Ni is Even More Intriguing than We first Thought Y. Tsunoda et al. , PRC 89, 031310(R) Monte Carlo SM pf-g 9/2 -d 5/2 S. Suchyta et al. , PRC 89, 021301(R) and F. Recchia et al., PRC 88, 041302(R) 2 0 + 0 + 1 24 Robert V. F. Janssens, Zakopane 2014
Summary • Prospect for studies of both proton-rich and neutron-rich nuclides to spins in excess of 10 h with REA3 are exciting. • In order to have access to the entire nucleonic chart, REA6 is required i.e. E beam ~ 6 MeV/A. • With GRETINA, I min ~ 5 x10 5 to perform g-g . • With GRETA, I min ~ 1x10 4 to perform g-g . 25
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