Unveiling the Two-Proton Halo Character of 17Ne: Exclusive Measurement of Quasi-free Proton-Knockout Reactions
C. Lehr, F. Wamers, F. Aksouh, Yu. Aksyutina, H. Alvarez-Pol, L. Atar, T. Aumann, S. Beceiro-Novo, C.A. Bertulani, K. Boretzky, M.J.G. Borge, C. Caesar, M. Chartier, A. Chatillon, L.V. Chulkov, D. Cortina-Gil, P. Diaz Fernandez, H. Emling, O. Ershova, L.M. Fraile, H.O.U. Fynbo, D. Galaviz, H. Geissel, M. Heil, M. Heine, D.H.H. Hoffmann, M. Holl, H.T. Johansson, B. Jonson, C. Karagiannis, O.A. Kiselev, J.V. Kratz, R. Kulessa, N. Kurz, C. Langer, M. Lantz, T. Le Bleis, R. Lemmon, Yu.A. Litvinov, B. Loeher, K. Mahata, J. Marganiec-Galazka, C. Muentz, T. Nilsson, C. Nociforo, W. Ott, V. Panin, S. Paschalis, A. Perea, R. Plag, R. Reifarth, A. Richter, K. Riisager, C. Rodriguez-Tajes, D. Rossi, D. Savran, H. Scheit, G. Schrieder, P. Schrock, H. Simon, J. Stroth, K. Suemmerer, O. Tengblad, H. Weick, C. Wimmer
UUnveiling the Two-Proton Halo Character of Ne:Exclusive Measurement of Quasi-free Proton-Knockout Reactions
C. Lehr, F. Wamers,
1, 2
F. Aksouh, ∗ Yu. Aksyutina, H. Álvarez-Pol, L. Atar,
1, 2
T. Aumann,
1, 2, 4, † S. Beceiro-Novo, ‡ C.A. Bertulani, K. Boretzky, M.J.G. Borge, C. Caesar,
1, 2
M. Chartier, A. Chatillon, L.V. Chulkov,
2, 8
D. Cortina-Gil, P. Díaz Fernández,
3, 9
H. Emling, O. Ershova,
2, 10
L.M. Fraile, H.O.U. Fynbo, D. Galaviz, H. Geissel, M. Heil, M. Heine, D.H.H. Hoffmann, M. Holl,
1, 2, 9
H.T. Johansson, B. Jonson, C. Karagiannis, O.A. Kiselev, J.V. Kratz, R. Kulessa, N. Kurz, C. Langer,
2, 10, § M. Lantz, ¶ T. Le Bleis, R. Lemmon, Yu.A. Litvinov, B. Löher,
2, 1
K. Mahata,
2, 16
J. Marganiec-Galązka,
1, 2, ∗∗ C. Müntz, T. Nilsson, C. Nociforo, W. Ott, †† V. Panin,
2, 1
S. Paschalis,
2, 7, ‡‡ A. Perea, R. Plag,
2, 10
R. Reifarth,
10, 2
A. Richter, K. Riisager, C. Rodriguez-Tajes, D. Rossi,
1, 2, 13
D. Savran, H. Scheit, G. Schrieder, P. Schrock, H. Simon, J. Stroth,
10, 2
K. Sümmerer, O. Tengblad, H. Weick, and C. Wimmer
10, 2 Technische Universität Darmstadt, Department of Physics, D–64289 Darmstadt, Germany GSI Helmholtzzentrum für Schwerionenforschung GmbH, D–64291 Darmstadt, Germany Instituto Galego de Física de Altas Enerxias, Universidade deSantiago de Compostela, ES–15782 Santiago de Compostela, Spain Helmholtz Research Academy for FAIR, D–64289 Darmstadt, Germany Texas A&M University-Commerce, Commerce, USA Instituto de Estructura de la Materia, CSIC, ES–28006 Madrid, Spain Department of Physics, University of Liverpool, Liverpool L69 3BX, United Kingdom NRC Kurchatov Institute, RU–123182 Moscow, Russia Department of Physics, Chalmers Tekniska Högskola, SE–41296 Göteborg, Sweden Goethe Universität Frankfurt, Department of Physics, D–60438 Frankfurt am Main, Germany Department of Atomic, Molecular and Nuclear Physics,Universidad Complutense de Madrid, ES–28040 Marid, Spain Department of Physics and Astronomy, University of Aarhus, DK–8000 Aarhus, Denmark Institut für Kernchemie, Johannes Gutenberg-Universität Mainz, D–55122 Mainz, Germany Instytut Fizyki, Uniwersytet Jagelloński, PL–30-059 Krakóv, Poland Nuclear Physics Group, STFC Daresbury Lab, Warrington WA4 4AD, Cheshire, UK Nuclear Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India (Dated: January 28, 2021)The proton drip-line nucleus Ne is investigated experimentally in order to determine its two-proton halo character. A fully exclusive measurement of the Ne ( p, p ) F ∗ → O + p quasi-freeone-proton knockout reaction has been performed at GSI at around 500 MeV/nucleon beam energy.All particles resulting from the scattering process have been detected. The relevant reconstructedquantities are the angles of the two protons scattered in qusi-elastic kinematics, the decay of Finto O (including γ decays from excited states) and a proton, as well as the O + p relative-energyspectrum and the F momentum distributions. The latter two quantities allow an independentand consistent determination of the ratio of l = 0 and l = 2 motion of the valence protons in Ne. With a resulting relatively small l = 0 component of only around 35(3)%, it is concludedthat Ne exhibits a rather modest halo character only. The quantitative agreement of the twovalues deduced from the energy spectrum and the momentum distributions supports the theoreticaltreatment of the calculation of momentum distributions after quasi-free knockout reactions at highenergies by taking into account distortions based on the Glauber theory. Moreover, the experimentaldata allow the separation of valence-proton knockout and knockout from the O core. The latterprocess contributes with 11.8(3.1) mb around 40% to the total proton-knockout cross section of30.3(2.3) mb, which explains previously reported contradicting conclusions derived from inclusivecross sections.
Introduction .—Atomic nuclei at the limits of nuclearbinding, located close to the neutron and proton driplines, exhibit unusual and often unexpected propertieswhen compared to expectations from theoretical models,which describe known properties of ordinary stable orvery long-lived nuclei well. Such exotic nuclear propertiescan originate from a strong imbalance in the neutron-to-proton ratio amplifying subtle properties of the nuclearinteraction, which are not decisive for the understanding of stable nuclei. Because of the closeness of the con-tinuum, such nuclei are called ‘open quantum systems’,often dominated by correlations [1]. The experimentalstudy of properties of drip-line nuclei is thus key for de-veloping and testing modern nuclear theory and the in-teractions used. The knowledge of properties of exoticnuclei, either reliably theoretically predicted or directlymeasured, is also a basis for the understanding of nu-cleosynthesis processes in astrophysics, such as the rapid a r X i v : . [ nu c l - e x ] J a n proton- and neutron-capture processes.Particular attention in this direction was dedicatedto halo nuclei since their experimental discovery at theBerkeley BEVALAC by Tanihata et al. [2, 3] and the fol-lowing interpretation and name coining nuclear ‘halo’ byHansen and Jonson [4]. Halo nuclei exhibit a matter-density distribution with a pronounced, far-extending,low-density tail (halo), which is caused by the wavefunc-tion of the last weakly-bound valence nucleons, whichreaches far into the classical forbidden region. This gen-erates an almost pure neutron or proton low-density en-vironment at the surface of the nucleus. On the neutrondrip-line, several such nuclei have been studied in greatdetail, thanks to the tremendous progress in rare-isotopebeam accelerator facilities and experimental instrumen-tation. A prime example is Li with two loosely-boundneutrons forming the halo, where the two neutrons arestrongly correlated in a mixed ground-state wavefunctioncomprising components with angular momenta l = 0 and l = 1 [5, 6]. A sizable low-angular-momentum componentis essential for the formation of a halo-like structure.On the much-less studied proton drip-line side, Ne isthe most promising candidate for such a structure. In athree-body model, Ne can be described as a well bound O core plus two protons loosely bound with a two-proton separation energy of only S p = 933 . keV [7].According to the standard nuclear shell model, the oxy-gen core has a closed proton shell, with the next availablestates in the sd shell. This suggests the possibility of haloformation with the two protons in l = 0 motion [8]. More-over, like Li, with both sub-systems ( F and p − p )being unbound, Ne is a
Borromean nucleus. Albeitnumerous experimental efforts, a firm conclusion on thestructure of Ne has not yet been reached. In this Letter,we present experimental results from an exclusive mea-surement of the ( p, p ) proton knockout reaction. Theratio of the s and d components in the valence-nucleonmotion has been determined by two independently mea-sured quantities, providing a clear answer to this long-standing discussion. With a resulting s to d ratio ofaround to , it is concluded that Ne exhibits only arather moderate halo character.
Summary of the Ne puzzle .— Ne ( J π = / − ) hasseven neutrons and ten protons, decays ( β + ) towards F( T / = 109 ms), and is loosely bound with a two-protonseparation energy of S p = 933 . keV, while its neu-tron separation energy is S n = 15557(20) keV [7]. Evi-dence for a proton halo in Ne , i.e. , a dominance of the ( s / ) configuration was claimed from measurements oftotal interaction cross sections for A = 17 high-energybeams [9]. However, calculations of the interaction crosssection based on Hartree-Fock-type wave functions andthe Glauber model result in the opposite conclusion witha dominance of the (1 d / ) configuration [10]. An ( l =0)-dominating two-proton halo structure of Ne was alsoinferred from an analysis of the O momentum distribu- tion and cross section for the inclusive p -removal reac-tion at 66 MeV/nucleon [11, 12], while a shell-model in-terpretation of the magnetic-moment measurement [13]arrives at the opposite conclusion. Furthermore, themeasured charge radii for , , Ne of 3.04(2), 2.97(2),and 3.01(1) fm [14] do not support a pronounced halo for Ne.Results from theoretical predictions span the full rangefrom s dominance to d dominance. Calculations withinthe framework of a three-cluster generator-coordinatemodel resulted in a dominant (1 s / ) configuration [15],while calculations using a density-dependent contactpairing interaction predicted values of P ((1 s / ) ) =15 . %, P ((0 d / ) ) = 75 . %, and P ((0 d / ) ) =3 . % [16, 17]. A O+ p + p three-body model was used tocalculate Thomas-Ehrman shifts for Ne and N [18–20]resulting in P ((1 s / ) ) values of 40-50%. As mentionedin Ref. [20], the computed three-body Thomas-Ehrmanshifts are relatively inaccurate. Coulomb energies formirror nuclei Ne and N were computed in other mod-els [21–23]. While the predominance of a (1 s / ) config-uration was stated in Ref. [21], the two other publicationsagree on P ((1 s / ) ) = 24(3) %.Clearly, a more exclusive measurement of observableswith high sensitivity and selectivity to distinguish the l = 0 and l = 2 contributions for the two weakly-boundvalence protons is mandatory to conclude on the halocharacter of Ne . The quasi-free proton-knockout reac-tion described in this Letter provides this sensitivity. Thepopulated resonances with known structure are identifiedvia invariant-mass spectroscopy of the residual fragment.The shape of the measured momentum distribution issensitive to the slope of the nuclear density distributionat the surface, which is dominated by the exponentialdecay of the least-bound nucleon’s wavefunction, whichstrongly depends on the angular momentum due to theangular-momentum barrier. The different shapes of the s and d density distributions at the surface result in dif-ferent shapes of the measured momentum distributions.The extraction of a s to ( s + d ) ratio is thus rather directand less model dependent. Experiment .—The primary Ne beam was extractedfrom the GSI synchrotron SIS18 with an energy of630 MeV/nucleon and directed to the production tar-get at the fragment separator FRS. The secondary Nebeam entered the experimental area Cave C at GSI withan average energy of 498 MeV/nucleon in the middleof the CH target. The average intensity of the sec-ondary beam amounted to /s with a Ne contentof more than 90% before selection. The beam energyof 500 MeV/nucleon was chosen to minimize secondaryreactions of protons in the nuclear medium after the pri-mary pp scattering process. The energy of outgoing pro-tons is in average 250 MeV at scattering angles of around ◦ , for which the nucleon-nucleon ( N N ) cross sectionis minimal. The experimental setup is schematically x p T a r g e t p r o t on ppp O F *17 Ne - P l a s ti c s c i n till a t o r- P I N d i od e - Scintillating fibers - D r i f t c h a m b e r s Crystal Ball
Reaction chamber inside Crystal Ball
FIG. 1. Schematic drawing of the experimental setup (notto scale). The upper part indicates the detection systemsand measured quantities to track and identify projectiles andforward-emitted reaction products. The lower frame providesa more detailed view of the detection systems around the tar-get and the reaction studied. Photons are detected with NaICrystals, protons with double-sided Si-strip detectors (DSSD)and NaI crystals. shown in Fig. 1, and is identical to the one described inRefs. [24–26]. The quasi-free one-proton knockout reac-tion Ne ( p, p ) F ∗ → O + p has been analyzed. Mea-surements with 213 mg/cm CH and 370 mg/cm Ctargets have been performed as well as a measurementwithout target in order to determine background fromreactions in other materials outside the target.Identified Ne incoming ions are tracked withposition-sensitive silicon PIN diodes towards the target.After the reaction target, outgoing fragments are de-flected in the large-gap dipole magnet ALADIN and iden-tified. Their angles, velocity, and magnetic rigidity aredetermined by double-sided silicon micro-strip detectors(DSSD), scintillating fibre detectors, and a time-of-flight(ToF) wall. For the reaction channel of interest, identi-fied O fragments have been selected.The ( p, p ) reaction channel is further characterized bythe measurement of the angular distribution of the twoscattered protons (including the target proton), and theforward emitted proton from the decay of unbound Fstates populated after proton knockout. The scatteredprotons are detected by a box of four DSSDs covering anangular range of 15 ◦ to 72 ◦ , and the Crystal Ball (CB)consisting of 162 individual NaI crystals surrounding thetarget (see lower part of Fig. 1). The resulting angulardistributions of scattered protons are displayed in Fig. 2 in deg op α ( m b / d e g ) op α / d σ d in deg ϕ
100 200 300 i n d e g ϕ in deg θ
20 30 40 50 60 70 i n d e g θ FIG. 2. Angular correlations of the two scattered protons inthe ( p, p ) knockout reaction. The insets display the corre-lations between the polar (left) and azimuthal (right) angles.The main figure shows the distribution as function of the pro-jected opening angle α op = θ + θ of the two protons. (see lower part of Fig. 1 for the definition of angles). Theyexhibit a back-to-back scattering with an opening anglepeaking at around ◦ as expected for quasi-free N N scattering. Deviations from elastic pp scattering like thewidth of the distributions and the slightly reduced aver-age opening angle have their origin in the internal motionof the proton in Ne and its binding energy relative tothat of the the final state.Protons originating from the decay in flight of F areemitted in the laboratory frame at forward angles andtracked through the dipole magnet by two DSSDs lo-cated before the magnet, and drift chambers plus a ToFwall after the magnet. γ decays from excited states aredetected by the CB spectrometer and calorimeter. Results .—The resulting differential cross section dσ/dE fp as function of the relative-energy E fp betweenthe O fragment and the decay proton for the reaction Ne ( p, p ) F ∗ → O + p is shown in Fig. 3, where con-tributions of around 10% associated with additional γ -decays were subtracted. The spectrum exhibits two clearstructures. The peak structure at higher energies reflectsthe population of high-lying excited states in F afterknockout of a proton from the O core in Ne. Togetherwith the decays accompanied by additional γ decays,the core knockout reaction contributes with 11.8(3.1) mbaround 40% to the total one-proton knockout cross sec-tion σ ( p, p ) = 30 . . mb. The low-lying peak resultsfrom the population of single-particle states of F whichare not resolved. The black solid line shows a fit to thedata consisting of a sum of Breit-Wigner curves withknown energies and widths of the resonances adoptedfrom the literature [27]. The two low-lying − and − states (see level scheme shown in Fig. 3) are s -wave reso- FIG. 3. Differential cross section dσ/dE fp as function of therelative energy E fp between the O fragment and the decayproton for the reaction Ne ( p, p ) F ∗ → O + p . Contribu-tions involving γ -decays have been subtracted. The promi-nent peak below 2 MeV results from four low-lying resonancesin F populated after knockout of a valence proton from Ne, while the cumulation of events between 3 and 6 MeVcorresponds to knockout of a proton from the O core. Theinset shows the populated states with their energy and quan-tum numbers. nances populated after the knockout of a valence protonfrom the s configuration, while the two higher lying − and − states are the d -resonances in F populated af-ter knockout from the d configuration. The low-energypart of the spectrum below 2 MeV corresponds thus tothe valence-proton or ‘halo’ knockout. The fit resultsin a cross section of 18.5(2.1) mb for the halo knockoutwith a relative contribution for the s -states of 42(5)%The high-energy positive-parity states of the spectrumabove 2 MeV are populated in core knockout reactions.The grey curves indicates the fit result, wehre the twogroups of non-resolved resonances are approximated bytwo Breit-Wigner curves.The experimental cross sections are compared to the-oretical ( p, p ) quasi-free-scattering cross sections com-puted using the Glauber theory [28]. Inputs to the cal-culations are the O core density distribution, single-particle wave functions for the valence protons, and thefree
N N cross sections. A Hartree-Fock density is usedfor the core with a radius r rms = 2 . fm. The single-particle wave functions were obtained by solving theSchrödinger equation for a Woods-Saxon mean-field po-tential with radius parameter r = 1 . fm and diffuse-ness a = 0 . . Cross sections were computed individuallyfor the angular momenta l = 0 and l = 2 and effectivebinding energies according to the resonances populated.The obtained single-particle cross sections σ sp for the s and d states are 11.65 mb and 9.16 mb, respectively.The cross section for l = 0 is somewhat larger due to (MeV/c) y p - - - - ( m b / ( M e V / c )) y / dp s d FIG. 4. Momentum distribution dσ/dp y of F projected ontothe cartesian coordinate y perpendicular to the beam for thereaction Ne ( p, p ) F ∗ → O + p with the condition E fp < MeV. The solid curve represents the theoretical result afteradjusting the l = 0 (long-dashed) and l = 2 (short-dashed)contributions to the experimental data (symbols). the surface-dominated reaction probability and the longhalo-like tail of the s wavefunction. This results in spec-troscopic factors of C S = 0 . for the s configura-tion and C S = 1 . for the d configuration, where C S = σ exp/ σ sp . The probability P ( s s + d ) to find thetwo valence protons in the s configuration in the Neground state amounts thus to 36(5)%.The shape of the momentum distribution dσ/dp of theresidual fragment after one-nucleon knockout is charac-teristic for the angular momentum of the knocked-out nu-cleon. The transverse momentum distribution dσ/dp y of F, projected onto the cartesian coordinate y , is shownin Fig. 4. The distribution was reconstructed from themeasured momenta of O plus the forward-emitted pro-ton from the decay of F, with the condition that therelative energy E fp < MeV, i.e. , with a selection on‘halo’ knockout. The data clearly indicate a superposi-tion of two shapes. The solid curve represents a fit ofthe calculated distributions to the data, using the above-described theoretical description, for l = 0 (long-dashedcurve) and l = 2 (short-dashed curve) resulting in arelative contribution to the cross section of 39(4)% for l = 0 , corresponding to a probability of 34(3)% for the s configuration of the valence protons. This is in perfectagreement with the independent result derived from therelative-energy spectrum discussed above.The dominance of the l = 2 configuration is furthersupported by the proton-proton angular correlations inthe Ne ground state shown in Fig. 5. The p − p relativeangle θ pp in the Ne frame has been constructed underthe assumption that the relative motion of the fragment O and the proton p (from the decay of F, see in- MeV < E fp < 0.8 MeV p p = − p F MeV < E fp < 2.0 MeV θ pp p p p p p F p O FIG. 5. Proton-proton angular correlation θ pp of the momentaof the two halo protons in Ne, represented by the anglebetween the recoil momentum of F and the direction ofthe O − p relative motion (see text). The left frame showsthe distribution for halo knockout with population of F inthe energy region E fp < . MeV with overlapping s and d resonances, while the right frame displays the correlation for E fp < . MeV, where the l = 0 configuration dominates. set of Fig. 5) remains undisturbed after sudden knockoutof the proton p . The angular correlation between theprotons in the Ne ground state is accordingly reflectedby the angle between the momentum of the knocked outproton p , given by the F recoil with p p = − p F (inthe rest frame of the projectile), and the O − p relativemomentum p fp = µ ( p p /m p + p f /m f ) , where m p , m f ,and µ are the masses of proton, O, and the reducedmass of the F system, respectively. The distributionexhibits a strong asymmetry caused by the d -wave con-tribution with preferred back-to-back or parallel motion(Fig. 5 left frame), while for a pure s -state, a symmet-ric and isotropic distribution is expected [29, 30]. Thiscan be seen in the right frame of Fig. 5, where the dis-tribution is shown for the condition E fp < . MeV, forwhich the l = 0 contribution dominates. Only a slightasymmetry is visible caused by remaining d admixtures. Conclusion .—The structure of the proton-halo can-didate Ne has been investigated by performingand analyzing an exclusive measurement of the Ne ( p, p ) F ∗ → O + p reaction at high beam energyof around 500 MeV/nucleon. The data allowed for theidentification of quasi-free ( p, p ) knockout from the va-lence states. The analysis of two independent observ-ables, the O − p relative-energy spectrum and the Fmomentum distribution, results in a consistent interpre-tation of the structure of the Ne ground-state configu-ration, where the two valence protons occupy dominantly s and d configurations with a rather small s compo-nent of 35(3)%. The dominance of the d contributionsuppresses the halo character of Ne. The large totalspectroscopic factor of C S = 1 . indicates no or onlyminor contributions due to more complex configurations,and supports a description of Ne in a O + p + p three-body model with an inert O core. The quantitative agreement of the analysis of two in-dependently measured quantities, the population of finalstates and the momentum distributions, give confidenceon the accuracy of treating the ( p, p ) reaction in theGlauber theory based on eikonal wavefunctions as devel-oped in Ref. [28]. The extraction of ratios of different con-figurations from momentum distributions relies heavilyon the calculated shape of the distributions, which is af-fected by distortions due to the reaction mechanism. Theperfect agreement of the theoretical shape with the mea-sured distribution in conjunction with a perfect agree-ment of the extracted s ratio provides a solid basis forthe investigation of exotic nuclei using quasi-free scat-tering at the upcoming radioactive-beam facilities FAIRand FRIB.The controversial conclusions in the literature on thehalo structure of Ne can be resolved. We discuss hereonly the least indirect methods based on the measure-ment of observables exhibiting a pronounced sensitiv-ity on the s -wave character of the valence-nucleon wavefunction: the measurement of the magnetic dipole mo-ment [13] and the inclusive proton-removal reaction [11].The result presented in this Letter is in agreement withthe shell-model interpretation of the magnetic-momentmeasurement. The disagreement with the interpretationof the inclusive proton-removal reaction can also be un-derstood, since core-knockout contributions have beenassumed to be small in Ref. [11]. The separation of thecontributions of the core and valence-nucleon knockoutin the exclusive experiment, however, reveals a signifi-cant contribution to the total proton-removal cross sec-tion of proton knockout from the core of around 40%.The search for a well-developed halo nucleus at the pro-ton dripline, comparable to neutron-halo states, still re-mains an open challenge.This work is supported by the German Federal Min-istry for Education and Research (BMBF) under con-tract No. 05P15RDFN1, the ExtreMe Matter Insti-tute (EMMI), HIC for FAIR, and the GSI-TU Darm-stadt cooperation agreement. Financial support from theSwedish Research Council, from the Russian Foundationfor Basic Research (RFBR Grant 12-02-01115-a), andfrom the Spanish grants from MICINN AEI FPA2017-87568-P, PGC2018-0099746-B-C21, FPA2015-646969-P,PID2019-104390GB-I00 and Maria de Maeztu Units ofexcellence MDM-2016-0692 are also acknowledged. Oneof us (B. J.) is a Helmholtz International Fellow. C.A.B.acknowledges support by the U.S. DOE grant DE-FG02-08ER41533 and the U.S. NSF Grant No. 1415656. ∗ Present address:
Department of Physics and Astronomy,College of Science, King Saud University, P.O. Box 2455,11451 Riyadh, KSA † E-mail: [email protected] ‡ Present address:
NSCL, Michigan State University, EastLansing, Michigan 48824, USA § Present address:
FH Aachen University of Applied Sci-ence, D–52066 Aachen, Germany ¶ Present address:
Department of Physics and Astronomy,Uppsala University, SE–751 20 Uppsala, Sweden ∗∗ Present address:
National Centre for Nuclear Re-search, Radioisotope Centre POLATOM, 05-400 Otwock,Poland †† deceased ‡‡ Present address:
Department of Physics, University ofYork, York, YO10 5DD, UK[1] N. Michel, W. Nazarewicz, M. Płoszajczak, andT. Vertse, Journal of Physics G: Nuclear and ParticlePhysics , 013101 (2008).[2] I. Tanihata, H. Hamagaki, O. Hashimoto, S. Nagamiya,Y. Shida, N. Yoshikawa, O. Yamakawa, K. Sugimoto,T. Kobayashi, D. Greiner, N. Takahashi, and Y. Nojiri,Physics Letters B , 380 (1985).[3] I. Tanihata, H. Hamagaki, O. Hashimoto, Y. Shida,N. Yoshikawa, K. Sugimoto, O. Yamakawa,T. Kobayashi, and N. Takahashi, Phys. Rev. Lett. , 2676 (1985).[4] P. G. Hansen and B. Jonson, Europhysics Letters (EPL) , 409 (1987).[5] H. Simon, D. Aleksandrov, T. Aumann, L. Axelsson,T. Baumann, M. J. G. Borge, L. V. Chulkov, R. Collatz,J. Cub, W. Dostal, B. Eberlein, T. W. Elze, H. Emling,H. Geissel, A. Grünschloss, M. Hellström, J. Holeczek,R. Holzmann, B. Jonson, J. V. Kratz, G. Kraus, R. Ku-lessa, Y. Leifels, A. Leistenschneider, T. Leth, I. Mukha,G. Münzenberg, F. Nickel, T. Nilsson, G. Nyman, B. Pe-tersen, M. Pfützner, A. Richter, K. Riisager, C. Schei-denberger, G. Schrieder, W. Schwab, M. H. Smedberg,J. Stroth, A. Surowiec, O. Tengblad, and M. V. Zhukov,Phys. Rev. Lett. , 496 (1999).[6] Y. Kubota, A. Corsi, G. Authelet, H. Baba, C. Caesar,D. Calvet, A. Delbart, M. Dozono, J. Feng, F. Flavi-gny, J.-M. Gheller, J. Gibelin, A. Giganon, A. Gillib-ert, K. Hasegawa, T. Isobe, Y. Kanaya, S. Kawakami,D. Kim, Y. Kikuchi, Y. Kiyokawa, M. Kobayashi,N. Kobayashi, T. Kobayashi, Y. Kondo, Z. Korkulu,S. Koyama, V. Lapoux, Y. Maeda, F. M. Marqués,T. Motobayashi, T. Miyazaki, T. Nakamura, N. Nakat-suka, Y. Nishio, A. Obertelli, K. Ogata, A. Ohkura, N. A.Orr, S. Ota, H. Otsu, T. Ozaki, V. Panin, S. Paschalis,E. C. Pollacco, S. Reichert, J.-Y. Roussé, A. T. Saito,S. Sakaguchi, M. Sako, C. Santamaria, M. Sasano,H. Sato, M. Shikata, Y. Shimizu, Y. Shindo, L. Stuhl,T. Sumikama, Y. L. Sun, M. Tabata, Y. Togano, J. Tsub-ota, Z. H. Yang, J. Yasuda, K. Yoneda, J. Zenihiro, andT. Uesaka, Phys. Rev. Lett. , 252501 (2020).[7] M. Wang, G. Audi, A. Wapstra, F. Kondev, M. Mac-Cormick, X. Xu, and B. Pfeiffer, Chinese Physics C ,1603 (2012).[8] M. V. Zhukov and I. J. Thompson, Phys. Rev. C , 3505(1995).[9] A. Ozawa, T. Kobayashi, H. Sato, D. Hirata, I. Tani-hata, O. Yamakawa, K. Omata, K. Sugimoto, D. Olson,W. Christie, and H. Wieman, Physics Letters B , 18(1994).[10] H. Kitagawa, N. Tajima, and H. Sagawa, Zeitschrift fürPhysik A , 381 (1997). [11] R. Kanungo, M. Chiba, S. Adhikari, D. Fang, N. Iwasa,K. Kimura, K. Maeda, S. Nishimura, Y. Ogawa, andT. Ohnishi, Physics Letters B , 21 (2003).[12] R. Kanungo, M. Chiba, B. Abu-Ibrahim, S. Adhikari, D.Q. Fang, N. Iwasa, K. Kimura, K. Maeda, S. Nishimura,T. Ohnishi, A. Ozawa, C. Samanta, T. Suda, T. Suzuki,Q. Wang, C. Wu, Y. Yamaguchi, K. Yamada, A. Yoshida,T. Zheng, and I. Tanihata, European Physics Journal A , 327 (2005).[13] W. Geithner, B. A. Brown, K. M. Hilligsøe, S. Kap-pertz, M. Keim, G. Kotrotsios, P. Lievens, K. Marinova,R. Neugart, H. Simon, and S. Wilbert, Phys. Rev. C ,064319 (2005).[14] W. Geithner, T. Neff, G. Audi, K. Blaum, P. Dela-haye, H. Feldmeier, S. George, C. Guénaut, F. Herfurth,A. Herlert, S. Kappertz, M. Keim, A. Kellerbauer, H.-J.Kluge, M. Kowalska, P. Lievens, D. Lunney, K. Marinova,R. Neugart, L. Schweikhard, S. Wilbert, and C. Yazid-jian, Phys. Rev. Lett. , 252502 (2008).[15] N. Timofeyuk, P. Descouvemont, and D. Baye, NuclearPhysics A , 1 (1996).[16] T. Oishi, K. Hagino, and H. Sagawa, Phys. Rev. C ,024315 (2010).[17] T. Oishi, K. Hagino, and H. Sagawa, Phys. Rev. C ,069901 (2010).[18] L. V. Grigorenko, I. G. Mukha, and M. V. Zhukov, Nu-clear Physics A , 372 (2003).[19] L. V. Grigorenko, I. G. Mukha, and M. V. Zhukov, Nu-clear Physics A , 401 (2004).[20] E. Garrido, D. V. Fedorov, and A. S. Jensen, Phys. Rev.C , 024002 (2004).[21] S. Nakamura, V. G. aes, and S. Kubono, Physics LettersB , 1 (1998).[22] H. Fortune and R. Sherr, Physics Letters B , 70(2001).[23] H. T. Fortune, R. Sherr, and B. A. Brown, Phys. Rev. C , 064310 (2006).[24] F. Wamers, J. Marganiec, F. Aksouh, Y. Aksyutina,H. Álvarez-Pol, T. Aumann, S. Beceiro-Novo, K. Boret-zky, M. J. G. Borge, M. Chartier, A. Chatillon, L. V.Chulkov, D. Cortina-Gil, H. Emling, O. Ershova, L. M.Fraile, H. O. U. Fynbo, D. Galaviz, H. Geissel, M. Heil,D. H. H. Hoffmann, H. T. Johansson, B. Jonson, C. Kara-giannis, O. A. Kiselev, J. V. Kratz, R. Kulessa, N. Kurz,C. Langer, M. Lantz, T. Le Bleis, R. Lemmon, Y. A.Litvinov, K. Mahata, C. Müntz, T. Nilsson, C. Nociforo,G. Nyman, W. Ott, V. Panin, S. Paschalis, A. Perea,R. Plag, R. Reifarth, A. Richter, C. Rodriguez-Tajes,D. Rossi, K. Riisager, D. Savran, G. Schrieder, H. Si-mon, J. Stroth, K. Sümmerer, O. Tengblad, H. Weick,C. Wimmer, and M. V. Zhukov, Phys. Rev. Lett. ,132502 (2014).[25] V. Panin, J. Taylor, S. Paschalis, F. Wamers,Y. Aksyutina, H. Alvarez-Pol, T. Aumann, C. Bertu-lani, K. Boretzky, C. Caesar, M. Chartier, L. Chulkov,D. Cortina-Gil, J. Enders, O. Ershova, H. Geissel,R. Gernhäuser, M. Heil, H. Johansson, B. Jonson,A. Kelić-Heil, C. Langer, T. Le Bleis, R. Lemmon,T. Nilsson, M. Petri, R. Plag, R. Reifarth, D. Rossi,H. Scheit, H. Simon, H. Weick, and C. Wimmer, PhysicsLetters B , 204 (2016).[26] F. Wamers, J. Marganiec, F. Aksouh, Y. Aksyutina,H. Alvarez-Pol, T. Aumann, S. Beceiro-Novo, C. A.Bertulani, K. Boretzky, M. J. G. Borge, M. Chartier, A. Chatillon, L. V. Chulkov, D. Cortina-Gil, H. Emling,O. Ershova, L. M. Fraile, H. O. U. Fynbo, D. Galaviz,H. Geissel, M. Heil, D. H. H. Hoffmann, J. Hoffman,H. T. Johansson, B. Jonson, C. Karagiannis, O. A.Kiselev, J. V. Kratz, R. Kulessa, N. Kurz, C. Langer,M. Lantz, T. Le Bleis, C. Lehr, R. Lemmon, Y. A.Litvinov, K. Mahata, C. Müntz, T. Nilsson, C. Nocif-oro, W. Ott, V. Panin, S. Paschalis, A. Perea, R. Plag,R. Reifarth, A. Richter, K. Riisager, C. Rodriguez-Tajes,D. Rossi, D. Savran, G. Schrieder, H. Simon, J. Stroth,K. Sümmerer, O. Tengblad, S. Typel, H. Weick, M. Wi-escher, and C. Wimmer, Phys. Rev. C , 034612 (2018).[27] D. Tilley, H. Weller, and C. Cheves, Nuclear Physics A , 1 (1993).[28] T. Aumann, C. A. Bertulani, and J. Ryckebusch, Phys.Rev. C , 064610 (2013). [29] L. V. Chulkov and G. Schrieder, Zeitschrift für Physik A , 231 (1997).[30] L. V. Chulkov, T. Aumann, D. Aleksandrov, L. Axels-son, T. Baumann, M. J. G. Borge, R. Collatz, J. Cub,W. Dostal, B. Eberlein, T. W. Elze, H. Emling, H. Geis-sel, V. Z. Goldberg, M. Golovkov, A. Grünschloss,M. Hellström, J. Holeczek, R. Holzmann, B. Jonson,A. A. Korsheninnikov, J. V. Kratz, G. Kraus, R. Ku-lessa, Y. Leifels, A. Leistenschneider, T. Leth, I. Mukha,G. Münzenberg, F. Nickel, T. Nilsson, G. Nyman, B. Pe-tersen, M. Pfützner, A. Richter, K. Riisager, C. Schei-denberger, G. Schrieder, W. Schwab, H. Simon, M. H.Smedberg, M. Steiner, J. Stroth, A. Surowiec, T. Suzuki,and O. Tengblad, Phys. Rev. Lett.79