Nuclear Structure Studies at Medium to High Spins with REA6/12 M.P. - - PowerPoint PPT Presentation

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

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Predicted ground state deformations from HFB calculations using Gogny force

see www-phynu.cea.fr

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.

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Nilsson Diagram

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.

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The Ti story: N=32 shell gap, N=34 no gap.

32 28 34

1129

f7/2 p3/2 p3/2 p1/2 p1/2 f5/2 48Ca + 238U DIC with Gammasphere R.V.F. Janssens et al., PLB 546, 22 (2002)

• B. Fornal et al., PRC 70, 064304(2004)

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 106 with 1010). 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 Ebeam ~ 5-6 MeV/u.  REA6 is necessary LARGEST and DOMINANT CROSS-SECTION is DIRECT EXCITATION OF BEAM

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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

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Phoswich Wall (Wash. Univ.)

• No. Pixels: 256 (4 PMT’s)
• Thin fast-plastic + CsI(Tl)
• Angle range: 9º ≤ θ ≤ 72º
• Preferred position: downstream

CHICO II (Rochester, LLNL)

• Angular coverage: 12o ≤ θ ≤ 85o, 95o ≤ θ ≤ 168o;

280o in φ

• Angular resolution: Δθ = 1o, Δφ = 1.4o
• Solid angle: ~69% of 4π
• Time resolution: Δt ≤ 500ps
• Mass resolution: Δm/m ≤ 5%
• Q-value resolution: ≤ 20 MeV

Expected Efficiency of GRETINA

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Singles Calorimeter Tracked

Should be here soon

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 mb and beam intensity at 1x1011 particles/sec in ~ 1 week.

• GRETINA at REA6 will require ~ 5x105 particles/sec to collect similar

statistics with s ~ 500 mb .

• GRETA with ~25% efficiency would give similar statistics with beam

intensities ~ 1x104 particles/sec at s ~ 500 mb .

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Unsafe Coulex of Actinide Targets

• K. Abu Saleem, Ph D. Thesis, Argonne Nat. Lab.

232Th

Double Gamma Beta Gamma K=0- Octupole K=1- Octupole

 208Pb beam at ~ 1.3 GeV ( 6.25 MeV/A)  232Th target (thick)

• Separation of 144Ba from

contaminants other than isobars made by the measured quasi-two- body kinematics

– Time-of-flight difference vs. scattering angle

• Major contaminant is 134Xe 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.

Coulomb Excitation of Reaccelerated 144Ba at ATLAS

• C. Y. Wu et al.

1 4

64Ni 208Pb

• W. Królas, R. Broda, B. Fornal , T. Pawłat, H. Grawe, K.H. Maier M. Schramm, R. Schubart, Nucl. Phys.

A724 (2003) 289.

• Effective method for identifying excited states in neutron rich nuclides.
• Target should have large N/Z ratio – 208Pb, 238U
• Many nuclei populated – not optimal for RIB’s.

Deep Inelastic Reactions

Alternative Methods to DIC (transfer, ICF)

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642 455 627 579 524

Eγ (keV)

1000 counts 100 counts

particle gate #1 red = 139Ba particle gate #2 green = 142Ce * * * * * * contamination (144Nd)

138Ba + 13C, Elab = 550 MeV (ATLAS)

Recent experiment with GRETINA and Phoswich Wall (W. Reviol et al.)

• 139Ba + 12C products (one-neutron transfer)
• 142Ce + 9Be products (incomplete fusion)

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Fusion Evaporation: the case of 100Sn

58Ni+46Ti g104Sn g100Sn+4n STABLE BEAM 58Ni 56Ni: 1011/s

Cross section: ~1 nb (out of 500 mb) Yield:60/day Channel selection: Recoil-Decay Tagging?

56Ni+50Cr g106Te g100Sn+a+2n PROTON RICH RADIOACTIVE 56Ni BEAM 56Ni: 108/s

Cross section: ~1 mb (out of 500 mb) Yield:60/day Channel selection: Recoil-Decay Tagging?

63As+40Cag103Ig100Sn+3p PROTON RICH HEAVY RADIOACTIVE BEAM 63As: 1000/s

Cross section: ~100 mb (out of 500 mb) Yield: 60/day Channel selection: Z from DE measurement

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68Ni has high E(2+)

In contrast, Fe and Cr E(2+) decrease as

• ne 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.

Current State of the Art: Evidence for the Disappearance of the N=40 Gap

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Compression of Levels also observed at higher spins

56Cr32 58Cr34 60Cr36

• S. Zhu et al., Phys. Rev. C 74 (2006) 064351

From inelastic excitation of 48Ca + 238U and 48Ca + 208Pb at ATLAS + Gammasphere

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Fusion Evaporation Provides Access to Highest Spins

56Cr

DIC limit

14C(48Ca, α2n)56Cr

(g9/2xf5/2) Fusion Evaporation at ATLAS with Gammasphere + FMA

• S. Zhu et al., Phys. Rev. C 74 (2006) 064351

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Rotational Bands at higher spins in Cr-Fe Isotopes

• A. Deacon et al., Phys. Rev. C, 73 (2007) 054303.

Fusion Evaporation at ATLAS with Gammasphere (48Ca + 14C) Configuration of all rotational bands have been assigned at least one g9/2 neutron.

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(g9/2)2

} yrast bands

• All yrast bands are approaching (g9/2)2 aligned structure at 8+
• 8+ levels in Fe isotopes decrease in energy 60Fe (5.3 MeV), 62Fe (4.25 MeV) and

64Fe (3.6 MeV).

F.R. Xu, Peking University

60Cr

where ħω ~ Eγ/2

What Is the Nature of the Enhanced Collectivty at N=40 in Fe and Cr?

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Shape co-existence in Hg isotopes

Systematics of Yrast Bands in Hg Isotopes M.P. Carpenter et al., Phys. Rev. Lett. 78 (1997) 3650.

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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 66Fe at ~250 keV.
• 62,64Cr the deformed 0+ is the ground state but strongly mixed with excited 0+.

M.P. Carpenter et al., PRC 87 (2013) 041305(R)

Robert V. F. Janssens, Zakopane 2014

• Y. Tsunoda et al., PRC 89, 031310(R)

Monte Carlo SM pf-g9/2-d5/2

• S. Suchyta et al., PRC 89, 021301(R) and F. Recchia et al., PRC 88, 041302(R)

0+

2  0+ 1

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68Ni is Even More Intriguing than We first Thought

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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. Ebeam ~ 6 MeV/A.

• With GRETINA, Imin ~ 5 x105 to perform g-g.
• With GRETA, Imin ~ 1x104 to perform g-g.