NN2012 Slide 1

Exploring the neutron dripline two neutrons at a time:Observations of 26O and 16Be ground state resonances

Zach Kohley for the MoNA Collaboration

National Superconducting Cyclotron LaboratoryMichigan State University, E. Lansing, MI

International Conference on Nucleus-Nucleus CollisionsMay 31, 2012

26O

16Be

NN2012 Slide 2

Outline

• Motivation

• Experiments / Analysis

• 26O Results

• 16Be Results

• Summary

• Acknowledgements

26O

16Be

NN2012 Slide 3

Motivation

Explore nuclear structure at extreme neutron-to-proton ratios.

(Evolution of Shell Model)

Otsuka et al. PRL. 87, 082502 (2001).

N=1624O

30Si

Decay mechanismsand

3-body correlations

http://www.cenbg.in2p3.fr/desir/Beta-delayed-charged-particle

Sequential

3-body

dineutron(proton)

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Experiment

82 MeV/u 27F

Be production target

Producing 26O and 16Befrom 1-proton knockout rxns:

140 MeV/u 48Ca120 MeV/u 22Ne

82 MeV/u 27F53 MeV/u 17B

A1900 fragmentseparator

Coupled Cyclotrons

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Experiment

RIBs from A1900

27F(-p) 26O 24O + n + n17B(-p) 16Be 14Be + n + n

4 Tm Sweeper

Modular Neutron Array(MoNA)

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Experiment

Invariant mass spectroscopy:

3-body: 1st and 2nd hit in MoNA

Edecay = M24O+n+n – M24O – M2n

Neutron 4-vectors

Fragment 4-vector

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Experiment

n1,hit1

n1,hit2

2n-condition

• R1,2 > 50 cm

• v1,2 > vbeam

100 cm 100 cm 80 cm

Need to remove false 2n hits from true 2n hits

n2,hit1

v1,2 = R1,2 / (t2 – t1)

Effic. with cuts: 10-20% 90-95% true 2n

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26O

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Motivation

Baumann et al. Rep. Prog. Phys. 75, 036301 (2012).

Understanding drastic change in neutron dripline between Z=8 and Z=9

Z=8 dripline: 24O16 neutrons bound

Z=9 dripline > 31F> 22 neutrons bound

26O

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Motivation

Experiments show 26O to be unbound:

Tarasov et al. PLB. 409, 64 (1997).

slimit = 0.7 pb

Schiller et al. PRC. 72, 037601 (2005).

slimit = 3.0 mb

D. Guillemaud-Mueller et al. PRC. 41, 937 (1990).

slimit = 7 pb

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Motivation

Predictions for 26O: Bound or Unbound?

S2n = M26O – M24O – 2Mn

S2n > 0 unbound

S2n < 0 bound

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Motivation

Otsuka et al. PRL. 105, 032501 (2010).

Predictions for 26O: Bound or Unbound?

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Experiment

Previous MoNA Experiment:Hoffman et al. PRL 2008Measured g.s. of 25O @ 770 keV

Current work:Presence of low-energy neutrons

Dashed-lines CSM predictionsVolya & Zelevinsky PRL 2005

NN2012 Slide 15

Results and Discussion

c2 minimization 26O(g.s.) resonance

Edecay = 150+50-150 keV

G = insensitive (5 keV shown)

2-body

3-body

Fit experiment with 3 decay channels, using Monte Carlo simulation (with Geant4).

Lunderberg, DeYoung, Kohley, et al, PRL.108(2012)102501

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Results and Discussion

Confirm 3-body signal through removal of false 2n events.

Lunderberg, DeYoung, Kohley, et al, PRL.108(2012)102501

26O unbound by < 200 keV

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Outlook

Grigorenko et al. PRC 84, 021303(R) (2011)

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16Be

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Motivation

A. Spyrou, J.K. Smith et al, Phys Rev C84(2011)044309

NuShell, WBP interaction

16Be predicted to be: - unbound with respect to 2n decay - bound with respect to 1n decay

Scenario for “true” 2n emission

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Motivation

n1

n2

16Be

15Be+n

14Be+2n

n1n2

16Be14Be+2n

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Motivation - Dineutron

Halo neutrons on the same side of 6He

P. Mueller, PRL 99, 252501 (2007)

Nakamura, PRL 96, 252502 (2006)

Strong low-lying dipole strength 11Li indicating strong nn correlation.

n-n correlations from 11Li decay are reproduced by 3-body phase space decay

Ieki et al., PRC54(1996)1589 Zinser et al., NPA619(1997)151

Theory

Hagino et al., PRL 99(2007)022506

Strong dineutron component in11Li wavefunction

NN2012 Slide 24

Results: Decay Energy

Sequential

14Be+n1+n214Be+n1

14Be+n2

A. Spyrou, Z. Kohley et al, Phys Rev Lett. 108(2012)102501

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Results: Decay Energy

Sequential

14Be+n1+n214Be+n1

14Be+n2

A. Spyrou, Z. Kohley et al, Phys Rev Lett. 108(2012)102501

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Results: Correlations

E(n1+n2) θ(n1+n2) θ(14Be+n1)

Sequential

A. Spyrou, Z. Kohley et al, Phys Rev Lett. 108(2012)102501

NN2012 Slide 27

Summary

Observation of 26O ground state, which is unbound by < 200 keV.

S2n = M26O – M24O – 2Mn

S2n > 0 unbound

S2n < 0 bound

16Be ground state at 1.35 MeV,showed strong dineutron signatures

NN2012 Slide 28

MoNA-LISA

• LISA detectors were built and tested at 10 undergraduate institutions

• LISA installed successfully at NSCL in Spring 2011

• Commissioning experiment in June 2011 was successful.

• New experiment coming up summer 2012

Augustana College, ILCentral Michigan University, MIConcordia College, MNFlorida State University, FLGettysburg College, PAHope College, MIIndiana University at South Bend, INMarquette University, WIMichigan State University, MIOhio Wesleyan University, OHRhodes College, TNWabash College, INWestern Michigan University, MIWestmont College, CA

MRI-Consortium: Development of a Neutron Detector Array by Undergraduate Research Students for Studies of Exotic Nuclei.

NN2012 Slide 29

Acknowledgments

Thomas BaumannDaniel BazinGreg ChristianShea MosbyMichelle MosbyJenna SmithJesse SnyderArtemis SpyrouMichael StrongmanMichael Thoennessen

TheoryAlex BrownJeff TostevinAlexander Volya

Hope College: E. Lunderberg P. A. DeYoung

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Acknowledgments

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Backup

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Hagen et al. arXiv:1202.2839v1 [nucl-th] (2012)Accepted to PRL.

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27F and 26O

p n

27F

p n

26O

-p

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Backup