1. Experimental studies of the continuum in nuclei
Alexandra Gade
Professor of Physics
NSCL and Michigan State University

2. Outline
Introduction
One of the frontiers of rare-isotope science: unbound systems
Recent examples of experiments that highlight the nucleus as an open quantum system
An unprecedented view at correlations: Two-proton decays
Near threshold systems – 26O and a new form of radioactivity, maybe
Unusual topologies react – isoscalar dipole resonance in 11Li
The most recent study of 5H – when is a nucleus a nucleus?
Summary and outlook

3. Around 300 stable nuclei exist

4. Around 300 stable nuclei exist … and two Nobel Prizes have been awarded for modeling their properties
Maria Goeppert- Mayer 1963
Hans D. Jensen 1963
Leo James
Rainwater 1975
Ben Roy Mottelson 1975
Aage Niels Bohr 1975

5. More than 3000 nuclei have been made in laboratories …

6. 5000-12000 might be out there … One answer: 6900(500) – Nature 486, 509 (2012)
We do not know that limit
Proton dripline
An additional proton cannot be bound anymore
(Sp=0 MeV)
Neutron dripline
An additional neutron cannot be bound anymore
(Sn=0 MeV)

7. Nuclei near the threshold of binding
Highly excited nuclei – nuclear systems can be catapulted into the continuum by supplying enough excitation energy (Ex~Sn and/or Sp)
Rare isotopes – towards the driplines, the respective nucleon species becomes progressively less bound (lower nucleon separation energy) and continuum effects are important and low excitation energy since Sn/p are small
Proton dripline
Neutron dripline
Systems beyond the driplines may exist as resonances, characterized by and energy and width

8. The heaviest and the lightest two-proton emitters – Two different tales
Light two-proton emitters: break up in flight after formation within ~10-22 s (not enough Coulomb barrier to keep it together)
Heavier systems: longer-lived two-proton emitters (2p radioactivity T1/2 ~ few ms)

9. Two-proton decay (A-1)+p Energy A A (A-2)+2p (A-2)+2p
Sequential
Di-proton like, highly correlated
Direct 3-body decay or democratic breakup
(A-1)+p A
Energy A
(A-2)+2p
(A-2)+2p
Why is this interesting: The two protons are messengers of correlations from the interior of weakly-bound nuclei

10. Experiments – How? Produce them
Recent review: M. Pfuetzner, Phys. Scr. T152, (2013)
Produce them
All candidate nuclei for two-proton radioactivity have been produced by projectile fragmentation (GSI/Germany, GANIL/France, NSCL/USA, RIBF/Japan)
Experiments – either using Si detectors or Time Projection Chambers to measure global observables (T1/2, Q2p), proton energies and angular distributions
54Zn
45Fe
48Ni

11. Frontier: The heaviest two-proton radioactivity yet …
67Kr and neighbors were produced by fragmentation of a 345 MeV/u stable 78Kr beam, implanted into a DSSD Si stack and charged particle emission was measured following implantation
E(2p)=1690(17) keV, T1/2~7ms and 37(14)% 2p branch in competition with -delayed radiation
T. Goigoux et al., PRL 117, (2016)

12. … comes with a puzzle and a future challenge
The most successful model to describe 2p radioactivity has been a core-p-p 3-body cluster approach by Grigorenko et al. (2000)
The model has been very successful in describing the lifetimes and correlations for the three previously established long-lived 2p emitters (Fe, Ni, Zn) as well as complex correlations for short-lived light 2p emitters (6Be, 16Ne)
However, for 67Kr this approach predicted a half-live more than an order of magnitude longer
Grigorenko et al. reanalyzed the case and conclude the that the measured half-life is only possible if
67Kr has an unrealistically dominant p3/22 configuration and Er(66Br+p)= MeV
67Kr is in between true 2p emission and sequential decay (transitional) with 66Br+p Er= MeV
 energy sharing would tell
T. Goigoux et al., PRL 117, (2016)
L. V. Grigorenko et al. PRL 85, 22 (2000)
L.V. Grigorenko et al., PRC 95, (R) (2017)

13. Light two-proton emitters
Produced by fragmentation or transfer on a reaction target and the subsequent breakup is tagged with charged-particle detectors downstream of the reaction target
Typical half-lives T1/2~10-21s
R. A. Kryger et al., PRL 74, 860 (1995)

14. Express correlations in terms of angles between and energies of sub-systems
6Be was produced in-beam in the sudden collision of a 7Be beam at 70 MeV/u with a target
The +p+p breakup residues of 6Be were detected with the HiRA Si-CsI array
+p+p correlations were evaluated within two coordinate systems p
Epp
Efp k
I. A. Egorova et al., PRL 109, (2012)

15. Light two-proton emitters … it’s complicated
Correlation studies proved the idea of true and sequential 2p decay as too simplistic
It was thought that whenever there is a resonance available in the (A-1)+p system, the two-proton emission would proceed sequentially through the accessible intermediate state
The data is not in agreement with the sequential decay (dotted curve) even at a total energy that energetically allows for a decay through the 5Li resonance
Complex 3-body dynamics is at play rather than the simple energetics of intermediate states being accessible in the decay or not (complex pp and p final-state interactions)
I. A. Egorova et al., PRL 109, (2012)

16. Oddities in the continuum –The curious case of 26O: Two-neutron radioactivity?!… now with a puzzle
Unbound by only ~20 keV … life just above the threshold
Are there longer-lived two-neutron emitters of even two-neutron radioactivity?
See also C. Caesar et al., PRC 88, (2013) - GSI

17. 26,25O relative to stable 24O in 2012 – the possibility for two-neutron emission
Continuum shell model
measured 0+ 0+
keV
E. Lunderberg et al., PRL 108, (2012)

18. Oddities in the continuum –The curious case of 26O: Two-neutron radioactivity?!
Invariant mass spectrometry at NSCL
One lifetime limit for radioactivity is ~10-12 s
New technique developed to measure the lifetime of neutron emitting nuclei: Decay in Target (DiT)
Edecay= keV
Vrel=Vn-Vfrag
Vn=11.8 cm/ns
Immediate decay
Vn=10.9 cm/ns
“Long” lifetime = slow neutrons
Lifetime of two-neutron decay of 26O measured to be T1/2 = 4.5 ± 1.5 (stat.) ± 3 (sys.) picoseconds
Z. Kohley et al. PRL 110, (2013)

19. Oddities in the continuum –The curious case of 26O: Two-neutron radioactivity?!… now with a puzzle
3-body 24O+n+n model by Grigorenko et al. (PRL 111, (2013): “An upper limit of 1 keV for the decay energy of unbound 26O is inferred based on the recent experimental lifetime value”
High-statistics 27F-1p  26O  24O+2n measurement at RIBF/RIKEN with the SAMURAI spectrometer
Precise 26O ground-state decay energy Edecay=18±3(stat)±4(syst) keV
Spectroscopy in the continuum: 2+ energy of 26O
Interesting puzzle: Too high of a decay energy for the observed ps-range lifetime
J. Kondo et al., PRL 116, (2016)
See also C. Caesar et al., PRC 88, (2013) - GSI

20. Unusual topologies react – Dynamics Isoscalar dipole resonance in the 2n-neutron halo nucleus 11Li
R. Kanungo et al., PRL 114, (2015)

21. Unusual topologies react – Dynamics Isoscalar dipole resonance in the 2n-neutron halo nucleus 11Li
Halo nuclei
I. Tanihata et al., Phys. Rev. Lett. 55, 2676 (1985)
11Li
6He
8He Z

22. Unusual topologies react – Dynamics Isoscalar dipole resonance in the 2n-neutron halo nucleus 11Li
3-body models agree
Shell-model study shows that the monopole tensor interaction is important
First ab-initio CC calculations (2N only) disagree
Inelastic scattering 11Li+d (IRIS facility at ISACII/TRIUMF)
Resonance at 1.03(3) MeV (0.51 MeV FWHM) discovered with ℓ=1 angular distribution  isoscalar soft dipole resonance
Previous measurements found non-resonant enhancements or were unable to determine ℓ-value and/or width
R. Kanungo et al., PRL 114, (2015)

23. Ground-state properties of 5H when is a nucleus a nucleus?

24. Ground-state properties of 5H – extremely neutron-rich N/Z=4
A. Wuosmaa et al., PRC 95, (2017)
6He(d,3He)5H (3H+2n)
R. Kanungo et al., PRL 114, (2015)

25. Ground-state properties of 5H – experimental scheme
6He(d,3He)5H (3H+2n)
3H+2n
(CD2)n 5H
~190 MeV
6He d
330 MeV
3He
~10 MeV
Experimental challenge: detect charged particles over a large dynamic range
A. Wuosmaa et al., PRC 95, (2017)

26. Ground-state properties of 5H
Example spectrum of the 3He
Reconstructed missing mass of 5H
 T1/2~10-23s
When is it not a nucleus anymore?
A. Wuosmaa et al., PRC 95, (2017)

27. Ground-state properties of 5H … puzzling comparison to earlier results
A. Wuosmaa et al., PRC 95, (2017)

28. Ground-state properties of 5H – broader impacts
How do we know that the observed resonance is the ground state of 5H?
A. Wuosmaa et al., PRC 95, (2017)
Signals the overlap – how similar is 5H+p to 6He
Broader impacts
Lambda hyper nucleus 6H will be unbound with respect to 2n emission
7H width likely very broad or not observable
Failed search (J-PARC) just reported in arXiv:

29. Summary and outlook
One of the frontiers of rare isotope science is the investigation of the nucleus as an open quantum system as the driplines are approached
With sufficient excitation energy supplied, stable nuclei are laboratories for continuum effects (e.g. clustering in 12C, 24Mg, …)
Recent experimental studies of dripline systems have focused on correlations and dynamics
Clever detection schemes allow to investigate energy sharing and angular correlations in many-body decays or breakups
The future is bright with complementary major rare-isotope facilities in operation or coming online [or being envisioned] in North America (NSCL, TRIUMF, FRIB), Japan (RIBF), and Europe (ISOLDE, GANIL, GSI, FAIR, [EURISOL])
The nuclear many-body system is a sizable challenge …
…that is well worth it!

30. Examples were the choice of the presenter …
… and selected to be representative while at the frontier of what is (experimentally) possible
… and chosen to be (hopefully) complementary to what may be shown later in the meeting
I apologize if I missed your favorite case