pre-PAC Workshop for AGATA@LNL

5 Oct 2022, 07:15 → 7 Oct 2022, 15:00 Europe/Rome

INFN-LNL

Jose' Javier Valiente Dobon (Istituto Nazionale di Fisica Nucleare)

We are pleased to announce the second pre-PAC Workshop for the AGATA (https://www.agata.org/) physics campaign at LNL. The workshop will take place on October 5-7th, 2022 at LNL. 

The aim of the workshop is to assist the spokespersons in putting the strongest cases for their proposals forward through a discussion of the physics to be investigated, and to assess the feasibility of the experiments they intend to propose. This includes all experiments planning to use stable beams from the Tandem-ALPI-PIAVE complex for studies involving AGATA in a possible combination with PRISMA and/or ancillary detectors that are compatible with PRISMA (see technical-prePAC-final.pdf). All such proposals will have to be discussed at this workshop before being submitted to the LNL PAC. This applies also to the projects that were discussed at the PAC meeting in February 2022, but have not been granted beamtime in 2022.  By applying such a procedure, the collaboration hopes to avoid their potential overlaps with new projects.

The next PAC meeting will be held at LNL from the 5th to the 7th of December. The deadline for the submission of the proposals will be one month before, i.e. 4th of November. 

AGATA_2ndPrePAC_Workshop_LNL_First_circular.pdf

AGATA_2ndPrePAC_Workshop_LNL_Second_circular.pdf

pre-PAC_2022_timetable.pdf

Tech. information for proposal preparation

• A180euler_LNL_2022.list
• tandem_piave_alpi_beams_agata.pdf
• Technical-prePAC-final.pdf

Wednesday, 5 October

09:00 → 09:30 Registration

09:30 → 11:00 Status and facility updates

Convener: Andres F. Gadea Raga (IFIC CSIC-University of Valencia)

09:30 Welcome and introduction by the organisers

Speakers: Jose' Javier Valiente Dobon (Istituto Nazionale di Fisica Nucleare), Magda Zielinska (CEA Saclay)

09:40 Acceleratus Status

Speaker: Enrico Fagotti (Istituto Nazionale di Fisica Nucleare)

10:00 AGATA

Speaker: Emmanuel Clement (GANIL)

PrePAC-AGATA-Week-2022.pptx

10:20 Installation of AGATA @LNL

Speaker: Jose' Javier Valiente Dobon (Istituto Nazionale di Fisica Nucleare)

10:40 Physics addressed in the first AGATA@LNL campaign

Speaker: Magda Zielinska (CEA Saclay)

11:00 → 11:30 Coffee break

11:30 → 11:50 Status and facility updates

11:30 Three-foil plunger

Speaker: Franziskus Von Spee (IKP Cologne)

11:50 → 13:30 Session: LoI 1

Convener: Suzana Szilner (Ruder Boskovic Institute)

11:50 Lifetime measurements in 50-52Ca , 46-48Ar, 43-45Cl

Lifetimes in Ca isotopes: Large matter radii and halo-like neutron p-shells hypothesis in 50-52Ca: do we see evidence from spectroscopy?

Physics motivation

The large charge radii measured in 50-52Ca [Garcia] have proven a surprise from the experimental point of view, and a challenge for theoretical description. Not only laser spectroscopy has determined a large charge radius for 50-53Ca, but also the matter radii are very large compared to the standard R=1.2·A1/3 formula, and indeed the discrepancy is even more striking than for the charge radii [Tanaka]. Until now, two different hypotheses have been made to explain such a behavior [Nowacki]. A. Zuker and collaborators postulated the existence of large, halo-like neutron 1p3/2 and 1p1/2 orbitals in 50-54Ca which would in turn determine a large charge radius by isovector interaction [Bonnard]. In Ref. [Tanaka], the authors noted indeed that the orbitals should be as large as 6-7 fm (similar to the 11Li halo) to justify their measurement of matter radii. A second hypothesis is that the filling of the neutron 1p3/2 and 1p1/2 orbitals, which have a node and thus have a larger density of the center of the nucleus compared to 0f7/2 orbital, engenders a swelling of the 48Ca core when going from 48Ca to 54Ca [Horiuchi]. This would justify both the large charge and matter radii without the need of halo-like p-orbitals.

Discriminating between these two very different hypotheses is not easy, yet the difficulty in the description of the saturation properties of nuclei not so far from the stable doubly magic 48Ca demands prompt investigation. In particular, one may wonder if there are spectroscopic observables which could be sensitive to large neutron radii. We note here, that, at the first order, the electromagnetic transition in low lying 50-52Ca states involve the supposedly neutron-halo orbitals. The E2 operator connects (at leading order) different configurations:

        50Ca   B(E2; 2+->0+): νp3/2-νp3/2

        50Ca   B(E2; 4+->2+): νp3/2-νp1/2

        52Ca   B(E2; 2+->0+): νf5/2-νp3/2

Since B(E2) strengths scale with r4, a significant increase of the B(E2)s is expected. However, this effect is compensated for by opposite change in the E2 neutron change in effective charges. Nevertheless, the measurement of the 50,52Ca 4+ and 2+ half lives, respectively, will provide an essential benchmark for our understanding of the N=32 shell closure in terms of the shell model predictions.

While “neutron” E2 transition are not sensitive to nuclear radii, proton transitions are, as well as M1 transitions. For protons, their bare electric charge of 1e will preserve the r4 dependence of the B(E2): only the renormalization charge of 0.5e will scale as 1/ r4. For M1 transitions, a large orbital radius will mean M1 spin and orbital effective charges close to their bare value, and thus an increase of the B(M1) transition.

In 50Ca, the most interesting state is the second 2+, which is predicted to come from the p3/2-p1/2 coupling and it decays to the p3/2 first 2+ state. This is an allowed M1 p3/2->p1/2 transition which should be increased by 50% if the neutron spin effective charge has a value close to its bare one as a result of the large radius.

In 51Ca, the lifetimes of interest are those of the 1/2-, 5/2- and second 3/2- levels. Also the 7/2- ->5/2+ E1 transition is expected to be sensible to a large matter radius.

The physics case for the 46-47Ar nuclei : development of collectivity

The 46Ar isotope, located between the doubly magic 48Ca and the collective 44S nucleus, has challenged the existing shell-model description of nuclei in this region. While some observables related to the neutron contribution are well described by the theory, others, where the role of the proton is relevant, are not. In 46Ar, a shallow nuclear potential results in a moderate oblate ground-state deformation. The B(E2:2+ -> 0+) value was measured using intermediate Coulomb excitation [Gade] as well as extracted from the measured lifetime [Men], giving conflicting results, see Fig. 1 on the left part. The former smaller value is in agreement with time-dependent Hartree-Fock-Bogoliubov calculations that predict a strong N=28 shell gap in the 46Ar isotope. Conversely, shell-model calculations favor a larger B(E2), obtained from lifetime measurements, linking it to a quenching of the N=28 shell gap. Significantly, a Coulomb excitation measurement in 47Ar [Win] gives a B(E2:5/2+ -> 3/2+) value which is also not reproduced by shell model calculations, although the B(E2:2+ -> 0+) in 48Ar [Win] is well reproduced. The failure in predicting the B(E2) values is counterbalanced by the striking agreement with other obervables related to the neutron wave function, as detailed in Ref. [Mei]. In that work, the authors state that the neutron gap at N=28 is well reproduced by their shell-model calculations performed with SDPF interaction, since they predict Sn values in perfect agreement with mass measurements. In addition, the energy of the 2+ in 48Ar state is well reproduced and the state is interpreted to derive mainly from the valence neutrons and N=28-core breaking.

Recently, the first excited 0+ state has been measured in 46Ar using a (t,p) reaction [Now]. The observed state, as with other states involving a proton-neutron interaction, cannot be reproduced by shell model and there is a large difference among the predictions of the different interactions. The authors relate this problem to the monopole part of the tensor interaction nf7/2-πd3/2, which is the strongest among the T=0 cross-shell monopole terms. Indeed, the tensor part of the nuclear interaction, by causing a change in shell splitting, also plays a fundamental role for the quadrupole correlation of the protons in the sd shell and the neutrons in the pf shell. Subsequently, quadrupole correlations, together with the large fraction of neutron excitation across the N=28 gap, trigger the rapid transition from spherical to deformed shapes in 42Si, as pointed out in Ref. [Bha]. It is apparent that there is a need for measurements to clarify the role of protons in the excitation spectra and in the evolution of the shell below N=28.

The physics case for the 43-45Cl nuclei: towards 44S

The overestimation of the measured B(E2) by the SDPF-U Hamiltonian is not only present in 46Ar, but also in its lighter isotone 44S [Long]. In this regard, a measurement lifetime of excite states in 43-45Cl, particularly the 1/2+, 3/2+ first excited states as well se the 2+ core-coupled state, can provide a useful benchmark on the evolution of collectivity approaching the N=28 island of inversion.

Proposed Measurement

Ca isotopes

We propose to measure the lifetimes of the 50Ca 4+ , second 2+ states (and remeasure the 2+ as a cross-check) , the 52Ca 2+ state. In 51Ca, we propose to measure the lifetimes of the 1/2-, 5/2- and second 3/2- states. Also the 7/2- ->5/2+ transition will be measured. Nuclei will be populated by multi-nucleon transfer reproducing the same experimental conditions as in Ref. [Rejmund]. A 208Pb beam at 1.31 GeV will impinge on a 1 mg/cm2 48Ca target, mounted on a plunger device. PRISMA will be placed at the grazing angle for the inverse kinematics, i.e. 35 degrees as in the GANIL run. The 48Ca target will be sandwiched between two Au layers of 2 mg/cm2 each to prevent oxidation and enable stretchability for the plunger device. The differential plunger degrader will be made of 22Mg (93Nb is also a possibility).

Fig. 1: 50-52Ca populated in a multi-nucleon transfer reaction from Ref. [Rejmund]

Figure 1 show the level populated in the GANIL experiment. The 50Ca 4+ state is predicted to have lifetime of 0.3 ps, while the 52Ca 2+ state should have a lifetime of 0.9 ps (according to KB3G predictions with harmonic-oscillator wave functions). Since these lifetimes are at the limit of lunger capabilities, and they could turn out shorter if the p1/2 orbital has a large radius, we will add a thicker Au backing on the back of the 48Ca target in order to be able to perform DSAM measurements together with the differential plunger. Feeding to the 50Ca 4+ state is via a 595 keV E1: if one takes a similar 5- state in 48Ca (B(E1)=0.00012 Wu), a feeding lifetime of 0.3 ps is predicted. This will be measured and dealt with Bateman equations. A similar situation holds for the 52Ca lifetimes measurement, where the 3- state can feed the 2+ with a lifetime of 1.4 ps (rescaling from 48Ca 3-): also in this case the lifetime of the 3- state will be measured and a Bateman equation decay sequence reconstructed.

Concerning the expected statistics, AGATA will have an efficiency larger than that of EXOGAM in Ref. [Rejmund] at large gamma-ray energies (2-4 MeV), and similar at energies of 1 MeV and below. The P/T ratio will be very superior to EXOGAM thanks to the better Doppler reconstruction.

Considering the expected 50-52Ca production yield, obtained from the GANIL experiment, we propose to measure 14 days to have about 500 events in the 52Ca 2+ gamma-ray peak, and a thousand in the feeding transition. A similar estimate holds for the gamma-ray depopulating the 50Ca 4+ , second 2+ states: 800-1000 events should be collected for each level. This should allow one to measure the B(E2) of the states of interest with a 20% error at the maximum. Finally, the 51Ca lifetimes measurements will have on the order of few hundred counts per gamma-ray line, enabling the measurement of their lifetime.

46-48Ar

We plan to (re)measure the lifetime of the 2+ states in 46,48Ar, and of the single-particle and core-coupled states in 47Ar.

Cross sections from GRAZING are of the order of 1.4 mb and 0.25 mb for 46,48Ar, respectively. A hundred of gamma-ray events should be collected per day, allowing one to measure the lifetimes of the 2+ states and the 47Ar states within 14 days with the plunger technique.

44-45Cl

We plan to measure the 1/2+, 3/2+ first excited states as well se the 2+ core-coupled states 5/2+, 7/2+. Those states are typically well populated by multi nucleon transfer reactions.

Cross sections from GRAZING are of the order of 0.03 mb and 0.06 mb for 4,45Cl, respectively.

Several tens gamma-ray events shpuld be collected per day, allowing one to measure the lifetimes of the 2+ states and the 47Ar states within 14 days with the plunger technique.

Direct kinematics

Owing to the difficulty of producing a plunger 48Ca target, a direct kinematics is also considered. In this case, several different Brho setting of PRIMSA will be needed, since past measurement s at LNL show that the charge states of 52Ca and 50Ca cannot be accepted at the same time into PRISMA without suppressing important components of the charge state distribution of either of the two isotopes.

References:

[Bha] S. Bhattacharyya et al., Phys. Rev. Lett. 101, 032501 (2008).

[Bonnard] J. Bonnard, S.M. Lenzi, A.P. Zuker, Phys. Rev. Lett. 116 (2016) 212501.

[Gade] A. Gade et al., Phys. Rev. C 68, 014302 (2003).

[Garcia] F.R. García Ruiz, et al., Nat. Phys. 12 (2016) 594

[Horiuchi] W. Horiuchi, T. Inakura, Phys. Rev. C 101 (2020) 061301, (R).

[Long] B. Longefellow et al, Phys. Rev. C 103 (2021) 054309.

[Mei] Z. Meisel et al., Phys. Rev. Lett. 114, 022501 (2015).

[Meng] D. Mengoni, Phys. Rev. C 82, 024308 (2010).

[Now] K. Nowak et al. Phys. Rev. C 93, 044335 (2016).

[Nowacki] F. Nowacki, A. Obertelli and A. Poves, Prog. in Part. and Nucl. Phys. 120, 2021, 103866

[Rejmund] M. Rejmund at al., Phys. Rev. C 76, 021304(R) (2007)

[Tanaka] M. Tanaka, et al., Phys. Rev. Lett. 124 (2020) 102501.

[Valeinte] J.J. Valiente et al., Phys. Rev. Lett. 102, 242502 (2009)

[Win] R. Winkler et al., Phys. Rev. Lett. 108, 182501 (2012).

Speaker: Andrea Gottardo (Istituto Nazionale di Fisica Nucleare)

11:50 Shell evolution in neutron-rich nuclei south-east of doubly-magic 48Ca

The aim of this work is an investigation of the structural evolution
in neutron rich nuclei ``south and south-east'' of doubly-magic $^{48}$Ca,
especially with respect to rapidly changing nuclear properties. Here, we want
to investigate $^{46}$Ar, $^{48}$Ar and neighboring odd-A $^{47}$Ar and
$^{49}$K that are accessible in the same experiment. We will deduce unique
structural information from absolute transition strengths determined from
level lifetimes that will be measured with the Recoil Distance Doppler-shift
technique employing a new Cologne compact plunger device coupled to AGATA
and PRISMA and employing multinucleon transfer reactions. The investigation
of this region is particular interest: These nuclei are located along the
path from doubly magic $^{48}$Ca to collective nuclei, e.g., neutron-rich S
and Si isotopes. Especially, an erosion of the N=28 shell closure is in the
focus of this work. Such was observed in lighter isotopes in this region,
but sparse and partly ambiguous data exists for neutron rich argon isotopes
so far, even though newer results support a weakening of the N=28 closure
already in argon. Thus this region is critical for the understanding of shell
evolution and structural changes where
shell model descriptions already turned out to be a challenge. Further, recent
experimental findings in the neutron-rich Ti isotopes, which are valence
proton partners for Ar with respect to Z=20, also give evidence for rapid
structural changes for increasing neutron number. The results of this work
will allow to judge a possible valence proton symmetry with respect to Z=20.

Speaker: Christoph Fransen (Institut für Kernphysik, Universität zu Köln)

12:15 Shape coexistence and mixing in the N=Z=34 68Se

The nucleus 68Se presents two rotational band structures that have been identified, on the basis of the moments of inertia, as oblate and prolate. The ground state band, presenting the characteristics of an oblate rotational band, is crossed at spin 8+ by a prolate-deformed band that becomes yrast. The second excited state in this nucleus is a 2+ state, allowing to study experimentally the two structures in a single experiment. Theoretical calculations performed with state-of-the-art large scale shell model predict utterly pure configurations with no mixing between the two bands. On the other hand, mean field Vampir calculations predict a large mixing of the two bands. This puts a very interesting question on the nature of these bands, the phenomenon of the shape coexistence and mixing. To answer to this question we propose to measure the lifetime of several states in this nuclei, mainly in the low-spin region via the DSAM technique. Excited states in 68Se will be populated using the 36Ar+40Ca reaction, with a PIAVE beam of 36Ar at an energy of 140 MeV and a gold-backed 40Ca target. The proposed set-up will require AGATA coupled to the EUCLIDES Si-ball. The beamtime request is 8 days.

Speaker: Silvia Monica Lenzi (Istituto Nazionale di Fisica Nucleare)

12:40 Isospin Mixing in the N=Z=36 72Kr: Lifetime measurement of E1 isospin forbidden transitions

The investigation of mirror nuclei along the N = Z line is of considerable interest since it directly addresses the validity of the charge symmetry of nuclear forces and the role of Coulomb effects in determining nuclear structure. Isospin symmetry breaking effects are important to probe theoretical corrections to the superallowed Fermi  decay matrix elements I = 0+, T = 1 → I = 0+, T = 1 between the isobaric analogue states, which are critical for precise determination of the leading element Vud of the Cabibbo-Kobayashi-Maskawa flavour-mixing matrix and, in turn, for further stringent tests of its unitarity, violation of which may signal new physics beyond the Standard Model of particle physics. In the limit of long wavelengths, where the Siegert theorem holds, the E1 transition operator is purely isovector. If the charge symmetry of the nuclear force is exact, E1 transitions between states of equal isospin are forbidden in N = Z nuclei and have equal strength in Tz=1/2 mirror nuclei. Experimental deviations from the two rules above can, therefore, be used to investigate isospin symmetry breaking. The aim of the present proposal is the measurement of the electromagnetic transition matrix elements for two E1 (forbidden) transitions in the N=Z=36 72Kr nucleus. This knowledge would allow us to probe the validity of the models used for the determination of the isospin mixing corrections in one of the heaviest nuclear system presently accessible. Excited states in 72Kr will be populated using the 40Ca+40Ca reaction, the LNL Tandem accelerator providing the 40Ca beam at an energy of 177 MeV. The proposed set-up will require AGATA coupled to the EUCLIDES Si-ball and the Plunger.

Speakers: Aysegul Ertoprak (LNL), Giacomo De Angelis (Istituto Nazionale di Fisica Nucleare)

13:05 Test of the CKM unitarity and the existence of Fierz interference through the measurement of superallowed β decay of light nuclei

Superallowed 0+→ 0+ β decay in light nuclei gives the most accurate Vud, the up-down mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. In the Standard Model (SM), the matrix is supposed to have unitarity. The unitarity of the CKM matrix ensures that the three generations of quarks form a complete set of quark elements. In other words, the non-unitarity of the matrix implies the existence of hidden physics, such as the existence of the fourth-generation quarks [Ma86]. The unitarity can be tested through the examination of the square sum of the first column elements: |Vud|2 + |Vus|2 + |Vub|2. It should be unity in the regime of SM, but the current value is slightly smaller than one. Among the matrix elements, Vud is critical because the value dominates the sum. Historically, uncertainties inherited from hadronic interactions take a large fraction of the total uncertainty of Vud. Recently, several works successfully reduced hadronic uncertainty. Ref. [Se18] re-estimated the radiative correction to be 0.02467(22), and it led to the 4-sigma difference, 0.9984(4), of the squared sum of the column elements in the CKM matrix from the unitarity.

The reduction of the theoretical uncertainty and a crack of solidity on the unitarity provides the new motivation for re-measuring Ft values, especially on the light nuclei, which can impact the overall results. In particular, the Ft value of 10C β decay slightly deviates from the average Ft value keeping large uncertainty [Ha20]. Thus, a new measurement of Ft for the 10C β decay deserves to have a priority. Moreover, the new measurement for 10C can have an impact on the search for Fierz interference term as well.

For the last three decades, only three measurements [Sa95, Fu99, Bl20] have been reported. Among them the two measurements [Sa95, Fu99] have a similar setup but their uncertainty budget was very different. These non-compatible results put a question mark on the systematic error analysis of the measurements.

We suggest a new experiment with the AGATA. A Tendem beam of 20-pnA proton at an energy of 15 MeV will impinge on a 2-um-thick, self-supporting 10B target to produce 10C nuclei through (p,n) reaction. The reaction 10B(p,p’)10B* will be exploited to measure the in-beam efficiency ratio 𝜀(718𝑘𝑒𝑉)/𝜀(1022𝑘𝑒𝑉) from the transition at 2.154 MeV. The ratio is directly estimated from the γ-ray spectrum gated on the 414-keV γ-ray peak. The compact configuration of AGATA and capability for dealing with the pileup events is expected to significantly reduce the background coming from the 511-keV γ rays. In total 10 days of beam time will be requested.

References
[Ma86] W. J. Marciano and A. Sirlin, Phys. Rev. Lett. 56, 22 (1986).
[Se18] C.-Y. Seng et al., Phys. Rev. Lett. 121, 241804 (2018).
[Ha20] J. C. Hardy and I. S. Towner, Phys. Rev. C 102, 045501 (2020).
[Sa95] G. Savard et al., Phys. Rev. Lett. 74, 1521 (1995).
[Fu99] B. K. Fujikawa et al., Phys. Lett. B 449, 6 (1999).
[Bl20] B. Blank et al., Eur. Phys. J. A 56, 156 (2020).

Speakers: Jeongsu Ha (KU Leuven), Francesco Recchia (University of Padova, Italy)

13:30 → 15:00 Lunch

15:00 → 16:40 Session: Zero degree workshop: Targets and detectors

Convener: Giacomo De Angelis (Istituto Nazionale di Fisica Nucleare)

15:00 Zero degree introduction

Speakers: Jose' Javier Valiente Dobon (Istituto Nazionale di Fisica Nucleare), Magda Zielinska (CEA Saclay)

15:10 SUGAR

Speaker: LUIS ARMANDO ACOSTA SANCHEZ

15:40 CTADIR

Speaker: Andrea Gottardo (Istituto Nazionale di Fisica Nucleare)

16:10 GRIT

Speaker: Daniele Mengoni (Istituto Nazionale di Fisica Nucleare)

16:40 → 17:10 Coffee break

17:10 → 19:10 Session: Zero degree workshop: Detectors and discussion

Convener: Giacomo De Angelis (Istituto Nazionale di Fisica Nucleare)

17:10 NEDA

Speaker: Andres F. Gadea Raga (IFIC CSIC-University of Valencia)

17:40 PARIS

Speakers: Adam Maj, Franco Camera (Istituto Nazionale di Fisica Nucleare)

18:10 Discussion

Thursday, 6 October

09:00 → 09:15 Welcome address

09:00 Welcome by the director of LNL and vice director of UNIPD physics department

Welcome by directors

Speakers: Fabiana Gramegna (Istituto Nazionale di Fisica Nucleare), Giulio Peruzzi (Università di Padova)

09:15 → 10:55 Session: LoI 2

Convener: Silvia Monica Lenzi (Istituto Nazionale di Fisica Nucleare)

09:15 High Spin Evolution of the doubly midshell nucleus 170Dy

A.J. Boston, H.C. Boston, L. Harkness-Brennan, E.S. Paul, University of Liverpool, UK
J. Nyberg, Uppsala University, Sweden
P.H. Regan, Zs. Podolyàk, University of Surrey, UK
P-A. Söderström, ELI-NP, Romania
J. Simpson, M. Labiche, STFC Daresbury Laboratory, UK
M. Bentley, University of York, UK
J. Smith, University of the West of Scotland, UK

We propose to investigate the high spin evolution in the doubly midshell nucleus 170Dy and its near neighbours in an experiment with AGATA + PRISMA + DANTE. This proposal is the continuation of a series of experiments which have furthered our knowledge and understanding of the physics of nuclei in this mass region. The aim is to maximise the potential offered by the increase in γ-γ efficiency provided by the AGATA array and the unique supporting infrastructure available at LNL for this campaign.

The nucleus 170Dy has the largest value of the proton-neutron valence product, NpNn, of all nuclei with A < 208. Theoretical predictions suggest that 170Dy may be one of the stiffest axially deformed nuclei in nature [1], which has significant consequences for the robustness of the K quantum number. In addition, 170Dy may represent the best case of the SU(3) dynamical symmetry of the interacting boson approximation of all nuclei.

The structure of 170Dy is challenging to study experimentally. A number of attempts have been made using projectile fragmentation of a lead beam [2]; multi-nucleon transfer reactions between 82Se and 170Er, where a 4+ -> 2+ ground-state band transition candidate at 163 keV was first reported [3], and in-flight fission from where an isomeric state was observed [4]. The most recent work was published [5] in 2016 by Söderström et. al. Nuclei in the 170Dy region were produced by in-flight fission of a 345 MeV/u 238U beam on a Be target at RIBF in RIKEN. Utilising the BigRIPS separator, gamma-ray transitions were identified using a time window of 0.3 – 0.6μs after 170Dy implantation. The three lowest-lying excited states identified were assigned as the 2+, 4+ and 6+ members of the yrast ground-state rotational band, confirming the earlier assignment as the 4+ -> 2+ .

In this proposal we suggest populating 170Dy by +2p-xn (x>1) transfer reactions using a 900 MeV 136Xe beam on a self-supporting 0.5 mg/cm2 thick 170Er target. According to results of GRAZING calculations 170Dy can be populated with a cross-section of 1.3 mb for this reaction. The beam-like fragments will be identified by the PRISMA spectrometer complemented by the DANTE detector array for additional channel selection based on isomer tagging. By gating on the strongly populated delayed γ rays in 134,136Ba, it will be possible to identify decays in the binary partners 168,170Dy. The advantages of using AGATA coupled to PRISMA and DANTE in this experiment is the high γ-ray efficiency (particularly the γ- γ efficiency), excellent Doppler correction and isomer tagging capabilities of AGATA, the very good A, Z identification and velocity vector determination of PRISMA and the high efficiency and precise determination of the angle of the target-like fragments of DANTE. The efficacy of this approach was illustrated by the tentative identification of excited states in 170Dy achieved in an earlier work [3] with the CLARA + PRISMA setup and a 82Se beam. The use of a 136Xe beam will give a large increase of the yield of neutron-rich reaction products as well as of the angular momentum transferred to the fragments, compared with the 82Se induced reaction.

In the proposed experiment, we estimate an increase of the number of counts in the 4+->2+ transition in 170Dy by a factor of about 100 compared with the earlier work. In addition, we expect to considerably increase the knowledge of the high-spin structure of 168Dy up to and beyond the backbending region. Estimates of yields based on the previous CLARA + PRISMA experiment and on GRAZING calculations indicate that we also may be able to identify higher lying excited states for the in the nuclei 166Gd , 168Gd , 172Dy and 174Dy.

[1] P. Regan et al., Phys Rev. C 65, 037302
[2] Zs. Podolyák, et al., in: J.H. Hamilton, W.R. Phillips, H.K. Carter (Eds.), Proc. of the Second Int. Conf. on Fission and Properties of Neutron-Rich Nuclei, St. Andrews, Scotland, 1999, World Scientific, 2000, p. 156.
[3] P.-A. Söderström, et al., Phys. Rev. C 81 (3) (2010) 034310.
[4] D. Kameda, et al., RIKEN Accel. Prog. Rep. 47 (2014)
[5] Söderström et.al. Physics Letters B 762 (2016) 404–408

Speaker: Andrew Boston

Boston-dy170-LOIv2.pdf

09:40 Study of the evolution of deformation and collectivity in tungsten isotopes through lifetime measurements in 190W

The N=116 chain from Z=78 towards Z=70 shows a clear indication of unexpected nuclear shape behavior. The drop of the 4$^{+}$ to 2$^{+}$ excitation energy ratio, E(4$^+$)/E(2$^+$) seen from $^{190}$Os to $^{190}$W doesn't follow the systematic expectations in the neighboring nuclei. Experimental and theoretical efforts have been made to understand the structure of $^{190}$W that remains mostly unexplained. The lifetime of the 4$^{+}$ state in $^{190}$W will provide relevant information on the collectivity of this state and an insight into the nuclear structure in this mass region.

Speaker: Julgen Pellumaj (Istituto Nazionale di Fisica Nucleare)

10:05 Investigating the nature of the low-lying states of 196Os via lifetime measurements

The neutron-rich region of the nuclear chart with Z between 70 to 78 has been long known as a region which shows a wide variety of nuclear structure phenomena\iffalse properties \fi. In particular, the shape evolution in the neutron-rich Os isotopic chain from prolate to oblate to spherical has been pointed out by theoretical calculations and explored in an extensive experimental effort. Results suggest $^{196}$Os as an almost perfect $\gamma$-unstable/triaxial rotor character.

Information on the B(E2) values of the low lying states of $^{196}$Os will provide a deeper understanding of its nuclear structure and of the shape evolution when going towards N=126 magic number.\

We aim to measure the lifetimes of the $2^+$ and $4^+$ states of $^{196}$Os, exploiting for the first time the plunger setup in its reverse configuration.

The states of interest will be populated via a multinucleon-transfer reaction with a beam of $^{136}$Xe impinging on a $^{198}$Pt target. The beam-like reaction fragments will be identified by the PRISMA spectrometer while the AGATA array will detect the emitted gamma rays.

Speaker: Daniele Brugnara (Istituto Nazionale di Fisica Nucleare)

10:30 Lifetime measurements of 202Pt and 206Hg: Shape evolution towards N=126 closed shell

The aim of the proposed experiment is to perform lifetime measurements and in-beam spectroscopy of $^{202}$Pt for the first time. We also intend to carry out lifetime measurements of $^{206}$Hg in order to expand the systematics of excited levels of even-even isotopes and get a better insight into the shape evolution towards N = 126 closed shell since this area of isotopes is experimentally very challenging and very little is known. These nuclei are expected to be more spherical near shell closure, but there is a lack of data to support this. Excited states of $^{202}$Pt and $^{206}$Hg will be populated simultaneously by multi-nucleon transfer (MNT) reactions with a beam of $^{136}$Xe impinging on an enriched $^{204}$Hg target. We intend to employ the dedicated plunger device in reversed configuration, which was commissioned at LNL, together with the AGATA advanced $\gamma$-ray tracking array and the PRISMA separator. The PRISMA separator will identify beam-like reaction fragments, and AGATA will detect $\gamma$ rays of their binary partners. We ask for $^{136}$Xe beam with an energy of 1110 MeV and 7 days of beam time with the PIAVE - ALPI accelerator complex.

Speaker: Matúš Sedlák (Istituto Nazionale di Fisica Nucleare)

10:55 → 11:25 Coffee break

11:25 → 13:30 Session: LoI 3

Convener: Andrew Boston

11:25 Lifetime measurements in low-spin states for neutron-rich 28,30Mg via multi-nucleon transfer reactions

We plan to measure the lifetime of the $2^+$ in $^{28}$Mg and perform spectroscopy for low spin states in $^{30}$Mg via multi-nucleon transfer reactions from $^208$Pb at 7.2 MeV/A on a 30 Si target. In the same reaction we expect to populate $^{32}$Si to revisit the $2_{1,2}^+$ lifetimes. Similar experiments have been performed in this region, such as a $^{40}$Ar beam impinged upon a lead target, with the differential cross-section distributions for the +2n, -2p and -2p+2n channels peaking at about 70 mb, 10 mb and 1 mb respectively. We expect similar cross-sections for this experiment.

Speaker: Pablo Antonio Aguilera (Istituto Nazionale di Fisica Nucleare)

11:50 Intruder configurations toward the Island of Inversion

see attached

Speaker: Kathrin Wimmer

12:15 Study of shape coexistence in 60Fe via lifetime measurement of excited 0+ states

The neutron-rich nuclei below the nickel isotopic chain manifest a sudden onset of collectivity going toward $N=40$, despite the proximity of the magic $Z=28$ number. This is interpreted as a result of the change in the underlying shell structure allowing correlation energies to dominate the monopole energy. In other words, structures built on Np-Nh configurations become yrast: we are in the Island of Inversion (IoI). The configurations that are yrast in the IoI can often be found as non-yrast structures in neighboring nuclei and give rise to shape-coexistence. Therefore, further nuclear structure studies devoted to the exploration of shape-coexistence phenomena in nuclei near to the IoI at $N=40$ will provide valuable data to constrain and test nuclear theory.

To study the presence of coexisting shapes, the measurement of experimental accessible nuclear properties, such as the lifetime of the $0_j^+$ excited states, is essential. We propose to measure the lifetime of the first excited $0_j^+$ states in $^{60}$Fe with the Recoil Distance Doppler Shift technique by populating low-lying yrast and non-yrast states in $^{60}$Fe via the multi-nucleon transfer reaction $^{58}$Fe($^{18}$O,$^{16}$O)$^{60}$Fe at energy below the Coulomb barrier. De-excitation $\gamma$ rays will be detected in coincidence with the back-scattered ions of $^{16}$O by using the $\gamma$-rays tracking array AGATA and the SPIDER detector for heavy ions.

Speaker: Giorgia Pasqualato

12:40 Emergence of the enhanced collectivity near magic nuclei: Coulomb excitation of 60Ni

In the proposed experiment, we plan to investigate the properties of nuclei close to the doubly-magic $^{56}$Ni isotope ($N = Z = 28$) using the Coulomb excitation method. Specifically, we propose to study the electromagnetic structure of $^{60}$Ni with the AGATA spectrometer coupled to the SPIDER heavy-ion %detection system. A $^{60}$Ni beam of 240 MeV energy and 1 pnA from the TANDEM-XTU accelerator will be used, impinging on a $^{208}$Pb target.

The results of the proposed experiment, along with those from the $^{58}$Ni Coulomb-excitation experiment already performed at INFN LNL with the GALILEO+SPIDER setup, will allow us to bring an important experimental input to investigate fundamental aspects of the structure of doubly-magic nuclei and in their vicinity. In particular, with the proposed Coulomb-excitation project we will provide new inputs to discuss some open questions, such as:

• Are nuclei nearby doubly-magic isotopes spherical in their ground states?
• What is the reason for the emergence of collectivity close to shell closures? And what is the potential role of triaxiality in forming deformed shapes in these regions?
• How "good" the 28 magic number is?

This experiment will be complemented by the data from the Coulomb-excitation experiment of a $^{60}$Ni beam on the medium-mass $^{110}$Cd target performed with the AGATA+SPIDER setup in June 2022. The simultaneous analysis of these two datasets will provide sensitivity to second-order effects influencing the Coulomb-excitation cross sections.

Speakers: Dr Kasia Hadynska-Klek (University of Warsaw), Marco Rocchini

13:05 Evolution of deformation along the calcium isotopic chain: Coulomb excitation of 44Ca

We propose to perform a dedicated Coulomb excitation of the $^{44}$Ca at INFN LNL using the AGATA $\gamma$-ray array coupled with the charged-particle array SPIDER. The aim of the experiment is to study for the first time the deformation of the low-lying structures in $^{44}$Ca in order to construct a more systematic view on the deformation of calcium isotopes. Data from this experiment should be sufficient to study the mixing of the states with different deformation. The obtained data could also allow for unique insight into the nature of the, presumably, super-deformed structure in $^{44}$Ca in an analogous way to our previous work on $^{42}$Ca, if the matrix elements connecting this structure to the normally-deformed structures are favorable or are similar to those determined for $^{42}$Ca.

Speaker: Matus Balogh (Istituto Nazionale di Fisica Nucleare)

13:30 → 15:00 Lunch

15:00 → 16:25 Session: LoI 4

Convener: Andrea Gottardo (Istituto Nazionale di Fisica Nucleare)

15:00 Towards the 132Sn nucleus via lifetime measurements

Properties of low-lying states in Z≈50 nuclei have been in the focus of both experimental and theoretical communities to systematically investigate the evolution of nuclear features, from neutron-deficient to neutron-rich cases.
While in the neutron-rich Sn and Te nuclei the excitation energy of the $2^+_1$ and $4^+_1$ levels and the B(E2;$2^+_1 \to 0^+_{g.s.}$) values have traditionally been described in the context of the generalized seniority scheme, several experimental results put in doubt this interpretation. The generalized-seniority interpretation is also challenged by large-scale shell-model theoretical calculations, which predict a completely different trend of the B(E2;$4^+_1 \to 2^+_1$) values.
Furthermore, due to the proximity to Z=50 and N=82 shell closures, the structure of both Te and Sn isotopes presents peculiar features that make these nuclei the perfect test bench to investigate the single-particle excitations of both protons and neutrons, and also their coupling in the nuclear Hamiltonian. Unfortunately, precise experimental information, which could shed light on the structure of nuclei in this mass region, is missing.

For these reasons, we propose a direct measurement of the lifetimes of low-lying excited states in $^{126,128,130}$Sn and $^{128,130,132}$Te aiming for an uncertainty around $10\%$. Additionally, the lifetime of other low-lying states will be measured in $^{130}$Te, providing crucial complementary information for the neutrino-less double $\beta$-decay investigation performed by the collaboration via Coulomb-excitation measurement.
In order to achieve these goals, we propose a multi-nucleon transfer reaction of a $^{197}$Au beam at 1122 MeV on a $^{130}$Te target: the reaction products and the emitted $\gamma$ rays will be detected by PRISMA and AGATA spectrometers, respectively, while the dedicated plunger will be used for the lifetime measurements.

Speaker: Dr Marco Siciliano (Argonne National Laboratory)

15:25 Emergence of collectivity in neutron-rich Sb isotopes

See attachment

Speaker: Dr Simone Bottoni (Università degli Studi di Milano and INFN)

15:35 Anomalously-low B4/2 values in neutron-rich Cd isotopes: lifetime measurement of the first 2+ and 4+ states in 120,122,124,126Cd

The long isotopic chains with $Z$ around 50 serve as good testing grounds for nuclear models studying the evolution of shell structure and interplay between pairing and quadruple correlations. The Cd isotopes have only two proton holes below the Z = 50 shell, being expected to show similar features with Sn nuclei except for the larger collectivity. The comparison of B($E2;2^+_1 \rightarrow 0^{+}_{g.s.}$) values between Sn ($\times$ 1.9) and Cd isotopic chains display quite similar trends in the neutron-deficient side. However, on the neutron-rich side, an unexpected bump is observed for Cd isotopes [see Fig. 1 in Phys. Rev. C 104, 034320 (2021) ]. Recently, large-scale-shell-model calculation performed by A. P. Zuker in Phys. Rev. C 103, 024322 (2021) suggest that the inclusion of N $ = $ 82 core excitation is essential to explain the bump of B($E2;2^+_1 \rightarrow 0^{+}_{g.s.}$) for Cd isotopes when increasing neutron number from 64. Meanwhile, the prediction power decrease quickly beyond $^{120}$Cd~(N$=72$), demanding a carefully modification of the interplay between pairing and quadruple interaction which is more sensitive to B($E2;4^+_1 \rightarrow 2^{+}_1$) values.

The experimental information on lifetime are available only with large uncertainties for the 2$^+$ states and still missing for the 4$^+$ states in Cd isotopes with N $>72$. It should be notices that, even though with a large error bar, an anomalous sharp decrease ($<$1) in the B$_{4^ +/2^+}$ ratio is also observed at $^{120}$Cd. In order to investigate the evolution of collectivity and the anomalous B$_{4^ +/2^+}$ ratio in the neutron-rich Cd isotopes, the current proposal aims at precisely measuring the lifetimes for the first 2$^+$ and 4$^+$ states in $^{120-126}$Cd. A multi-nucleon transfer reaction of a $^{124}$Sn beam at 900 MeV on a $^{198}$Pt target is proposed: the projectile-like fragments and the emitted $\gamma$ rays will be detected by PRISMA and AGATA spectrometers, respectively, and a dedicated plunger will be used for the lifetime measurements.

Speaker: Guangxin Zhang (Istituto Nazionale di Fisica Nucleare)

16:00 Lifetime measurements in A ∼ 90 nuclei between the N=Z line and the N=50 shell closure: Limits of seniority symmetry, isoscalar correlations and emergence of collectivity

Abstract: We propose to measure lifetimes of excited states in A~90 nuclei between N=Z and the closed neutron shell at N=50 using the Recoil Distance Doppler Shift (RDDS) technique. Level lifetimes of low-lying yrast states in this region of the nuclear chart are sensitive to the competition between isovector pairing forces resulting in seniority symmetry, isoscalar correlations, and the emergence of collective strength. Furthermore, electromagnetic transition strengths are often more sensitive than level energies to small components of the nuclear wave function and may therefore provide critical tests of nuclear models.
The lifetimes of states below the seniority isomers are difficult to access due to delayed feeding from the seniority isomers themselves when investigated in-beam in standard fusion-evaporation reactions and therefore poorly known. This warrants alternative approaches, such as fast-timing measurements following isomer decays or via controlled state feeding as addressed in the present proposal.
Recently, new and partly conflicting results on seniority symmetry in 94Ru, near the proton g9/2 mid-shell have been reported and received widespread attention due to the implications for state-of-the art theory. This proposal addresses this issue with a high-precision measurement using AGATA in conjunction with the PRISMA spectrometer. The experiment will focus primarily on remeasuring the lifetimes of the first excited 4+ and 2+ states in the semimagic nucleus 94Ru and measuring lifetimes of low-lying excited states in several neighboring N=48-50 isotones with high precision. In the N=48 isotones, experimental information on the excited-level lifetimes for several nuclides is crucially missing or lacks sufficient precision below the 8+ seniority isomers. The experiment also aims to investigate previously obtained evidence for octupole collectivity in this region of the nuclear chart.
The experiment will combine the use of a multi-nucleon transfer reaction mechanism that produces significant direct feeding of the states of interest with a detection system, AGATA+PRISMA+Differential plunger, that has optimal performance for RDDS measurements. The experiment will focus primarily on remeasuring the lifetimes of the 4+ and 2+ states in 94Ru and measuring lifetimes of low-lying excited states in the N=48 nuclides 92Ru and 90Mo and intermediate-spin states in the N=49 nucleus 92Tc nucleus with high precision. In the N=48 isotones, experimental information on the excited-level lifetimes for several nuclides are crucially missing below the 8+ seniority isomers. In the 92Tc nucleus lifetime measurements of intermediate-spin states will enable us to fine tune the neutron-proton interaction in large-scale shell model calculations by comparing measured and calculated M1 and E2 transition strengths. The experiment also aims to investigate previously obtained evidence for octupole collectivity in this region of the nuclear chart. The neutron deficient A~90 nuclei will be produced by multi-nucleon transfer reactions induced by a 410 MeV 92Mo beam (3 pnA intensity) impinging on a ~2 mg/cm2 thick 58Ni target. Eight days of beam time are requested including one day for tuning the detectors.

Speaker: Bo Cederwall (Royal Institute of Technology)

16:25 → 16:55 Coffee break

16:55 → 18:10 Session: LoI 5

Convener: Franco Galtarossa (Istituto Nazionale di Fisica Nucleare)

16:55 Nuclear structure studies in the vicinity of the Z = 28 neutron-rich isotopes with AGATA and PRISMA

In this letter of intent, we propose four experiments for nuclear structure studies of neutron-rich isotopes in the vicinity of $Z=28$ via lifetime measurements. The production of the nuclei of interest was optimized to efficiently collect data for the proposed physics cases by multi-nucleon transfer reactions of a $^{208}$Pb beam impinging on an innovative $^{70}$Zn(80\%)/$^{64}$Ni(20\%) alloy target (in-beam test of the alloy scheduled in November 2022). The target-like products will be identified in the PRISMA spectrometer in coincidence with the de-excitation gamma-rays in the AGATA array. The lifetimes of the excited states in the isotopes of interest will be measured by the so-called Recoil-Distance Doppler-Shift Method with a differential plunger. The total amount of beam time requested with PIAVE+ALPI is 20 days (16 for lifetime measurements and 4 for gamma spectroscopy studies).

Speaker: Rosa Maria Perez Vidal (Istituto Nazionale di Fisica Nucleare)

17:20 The fusion dynamics far below the barrier for 12C + 24Mg by gamma-particle coincidences with AGATA+Si-detectors

Abstract

Fusion hindrance in 12C + 24Mg was observed in a recent experiment where the excitation function was measured down to ~4mub and over-estimated by standard CC calculations. An S-factor maximum vs energy shows up. This system is slightly heavier than those of astrophysical interest, like e.g. 12C + 12C,16O. The cross-section at hindrance threshold is remarkably large.
The S-factor maximum is nicely  fitted using both an empirical adiabatic model, and the hindrance
parametrisation. Discriminating between the two models requires further measurements at lower
energies, that would be very relevant also for astrophysics. This LoI is the proposal of a further
experiment on 12C + 24Mg aiming at the measurement of fusion cross sections below the  mub range
with the combined set-up of AGATA and silicon detectors. The fusion events will be identified
by coincidences between the prompt
gamma-rays and the light charged particles (p,alpha) evaporated from
the compound nucleus. We present the preliminary results of the test carried out on 12C + 30Si,
demonstrating the feasibility of the proposed experiment. The required beam time is 12 days.

Speaker: Mirco Del Fabbro (UNIPD and INFN)

17:45 Probing nucleon-nucleon correlations in the 48Ca+208Pb system below the Coulomb barrier

We propose to measure the transfer probability for multi-neutron and multi-proton transfer channels in the system $^{48}$Ca+$^{208}$Pb at energies close to and below the Coulomb barrier. In the chosen system stripping and pick-up of both neutrons and protons are open with comparable cross sections. This gives the opportunity to investigate nucleon-nucleon correlations simultaneously for a complete set of transfer channels, involving both addition and removal of neutron and proton pairs.

The measurement will be performed in inverse kinematics by using a $^{208}$Pb beam onto a $^{48}$Ca target, employing the superconducting PIAVE-ALPI accelerator complex of LNL. We will determine the transfer probability P$_{tr}$ for the open transfer channels, identifying the light partner of the reaction in PRISMA. We intend to measure an excitation function from above to below the Coulomb barrier in such a way to cover a wide range of distances of closest approach between the two interacting nuclei $D$, from $\sim$ 12 fm up to $\sim$ 17 fm. At selected energies, in longer runs, we will measure the fragment angular distribution (PRISMA), and the fragment-$\gamma$ coincidences (PRISMA+AGATA).

At small $D$ we will define the shape and the size of the form factors in the absorption region, important to consistently follow the evolution of quasi-elastic processes. At far distances we will define their exponential behaviour, in slope and absolute value.

In this measurement we will focus in particular on proton transfer channels in the energy region below the Coulomb barrier, where very few data are available.

We ask for a total of 10 days of beam time with PIAVE+ALPI.

Speaker: Tea Mijatovic (Ruder Boskovic Institute)

20:30 → 22:30 Workshop dinner

Friday, 7 October

09:15 → 10:55 Session: LoI 6

Convener: Simone Bottoni (Università degli Studi di Milano and INFN)

09:15 Lifetime of the 6.793 MeV state in 15O

The $^{14}$N($p,\gamma$)$^{15}$O is the slowest reaction in the CNO cycle and thus regulates both the rate of the energy production and the nucleosynthesis in stars more massive than our Sun. Additionally, it has a crucial impact on the flux of the solar CNO neutrinos, recently measured at Borexino. Nevertheless, the reaction rate at astrophysical energies is poorly constrained due to the uncertainty in the width of the subthreshold resonance located at $E_{r} = -504$ keV, corresponding to the $E_{x} = 6.793$ MeV state in $^{15}$O, which dominates the extrapolations at lowest energies.

One of the methods to obtain the width of a subthreshold resonance is the measurement of its lifetime. Since the expected lifetime of the $6.793$ MeV state is of the order of $1$ fs, the only technique that can be employed is the Doppler Shift Attenuation Method (DSAM). Several studies have used this approach in the past, but none of them were able to obtain a lifetime precise enough to properly constrain the extrapolations. The lifetime, in fact, lies at the edge of DSAM applicability. Nevertheless, by using a HPGe tracking array alongside a segmented silicon detector, the DSAM sensibility can be pushed down to $0.1$ fs. This would permit a precise lifetime measurement of the $6.793$ MeV state and put a hard constrain on the reaction rate of the $^{14}$N($p,\gamma$)$^{15}$O at astrophysical energies.

For these reasons, we propose to use the AGATA array coupled with a highly segmented silicon detector (TRACE,SPIDER, or annular). The $^{16}$O($^{3}$He,$^{4}$He)$^{15}$O reaction in inverse kinematics will be used to produce the $^{15}$O in the desired excited state. The targets will be made by implanting $^{3}$He in Au backings and the beam will be provided by the TANDEM accelerator at LNL.

Speaker: Jakub Skowronski (Istituto Nazionale di Fisica Nucleare)

Skowronski_prePAC22.pdf

09:40 Lifetime measurements of excited states in neutron-rich 16C and 18C isotopes: a test of the three-body forces

M. Ciemała$^1$, B. Fornal$^1$, A. Maj$^1$, P. Bednarczyk$^1$, N. Cieplicka-Oryńczak$^1$,
M. Kmiecik$^1$, Ł.W. Iskra$^1$, M. Matejska-Minda$^1$, et al.
F. Crespi$^{2,3}$, S. Leoni$^{2,3}$, G. Benzoni$^3$, S. Bottoni$^2$, A. Bracco$^{2,3}$, F. Camera$^{2,3}$,
G. Corbari$^{2,3}$, E. Gamba$^{2,3}$, B. Million$^3$, O. Wieland$^3$, et al.
D. Mengoni$^4$, R. Menegazzo$^4$, S. M. Lenzi$^4$, F. Recchia$^4$, D. Bazzacco$^4$, G. Montagnoli$^4$, et al.
A. Goasduff$^5$, A. Gottardo$^5$, G.deAngelis$^5$, D.R. Napoli$^5$, J.J. Valiente-Dobon$^5$,
I. Zanon$^5$, L. Corradi$^5$, E. Fioretto$^5$, et al.

$^1$ Nuclear Physics Institute, Polish Academy of Sciences, Krakow, Poland
$^2$ Università degli Studi di Milano, Via Celoria 16, 20133, Italy
$^3$ INFN, sezione di Milano, Italy
$^4$ University of Padova and INFN sez. Padova, Italy
$^5$ Legnaro National Laboratory, Italy

Spokespersons:
M. Ciemala$^1$ (Michal.Ciemala@ifj.edu.pl)
F. Crespi$^{2,3}$ (University of Milan and INFN, Fabio.Crespi@mi.infn.it)

1. The physics case

We propose an experiment with the AGATA array coupled to the PRISMA spectrometer, to investigate excited states in n-rich B, C, N, O and F nuclei, populated by deep-inelastic reactions induced by an $^{18}$O beam on a $^{198}$Pt target. For these nuclei, limited spectroscopic information is available in terms of the lifetimes of the excited states.

The focus of the experiment will be on exotic C isotopes. In these nuclei, ab initio calculations provide a description of the excited states and predict a strong sensitivity of the electromagnetic transition probabilities to the details of the nucleon-nucleon interactions, especially in connection with the role played by the three-body (NNN) forces [For13,Vos12]. Among the most interesting cases are $^{16}$C and $^{18}$C, for which ab initio calculations without and with inclusion of the NNN term predict B(E2) and B(M1) of the second 2$^+$ states to vary by a factor of 2 to 5. For example, for $^{16}$C, the calculated lifetime of second 2$^+$ state with two-body (NN) forces, only, is equal to 230 fs versus 80 fs with NN+NNN forces included. Similar calculations for 18C provide a lifetime of ~1 ps versus ~2 ps considering NN or NN+NNN, respectively. Therefore, precise measurements of the lifetimes of the second 2$^+$ states in these systems are needed, which require techniques able to extract information in the time intervals from one hundred femtosecond to few picoseconds.

The present experiment is a follow up of a 2017 measurement performed at GANIL with the AGATA+VAMOS+PARIS setup, employing an 18O-induced reaction (at 7 MeV/A) on a thick $^{181}$Ta target (6.6 mg/cm$^2$). After developing a novel Doppler Shift Attenuation Method for assessing tens-to-hundreds femtoseconds state lifetimes, using deep-inelastic reactions, the lifetime of the 2$^+$ state was determined in $^{20}$O and an estimate was provided for the second 2$^+$ state lifetime in $^{16}$C [Cie20,Cie21]. It was shown that this type of experimental setup and analysis approach are well suited for these measurements, which could be further pursued at LNL, exploiting a better angular coverage of AGATA (including 90° HPGe detectors, which are crucial for a precise gamma-ray energy determination, not Doppler affected) [Cie21]. At LNL, a thick 198Pt target will be employed, and the PRISMA spectrometer will be placed at the most forward possible angles (between 20° and 35°, depending on the distance of AGATA from the target), in order to take advantage of the enhanced cross sections of deep-inelastic processes, with respect to the grazing angle. Furthermore, the DSAM method developed for the GANIL experiment [Cie21] will be extended in order to cover the 10’s fs – few ps lifetime interval. For this purpose, a thick degrader will be used at close distance from the target, as discussed later.

2. Experimental details and beam time estimation

The main aim of the present proposal is to measure the second 2$^+$ state lifetime in the $^{16}$C and $^{18}$C, which are located at 3979 and 25157 keV, respectively, as illustrated in Fig. 1.

Figure 1. Partial level schemes for $^{16}$C [Cie20]) and $^{18}$C [Vos12].

The nuclei of interest will be populated in deep-inelastic processes induced by a $^{18}$O beam (at ~7 MeV/A, i.e., about 50% above the Coulomb barrier) on a $^{198}$Pt target, 10 mg/cm$^2$ thick. The reaction grazing angle is $\theta$g ~54° and the projectile-like fragments will have v/c~10%. State lifetimes of the order of tens-to-hundreds fs, corresponding to decays in flight inside the target, will be assessed employing the DSAM method developed for the GANIL experiment [Cie21]. This is the case of the second 2$^+$ state of $^{16}$C. Sensitivity to state lifetimes of the order of 1 to 2 ps (which will partially decay after the target, as expected for the second 2$^+$ state of $^{18}$C) will be gained by placing a $^{93}$Nb degrader, 10 mg/cm$^2$, at ~15 $\mu$m from the target.

The evaluation of the expected reaction cross-section for $^{16}$C and $^{18}$C can be based on Deep Inelastic Transport (DIT) model calculations, reported in Ref. [Ste18] and illustrated in Fig. 2. Such calculations refer to a similar reaction employing a $^{18}$O (8.5 MeV/A) on a 238U target, and give a reasonable account for the statistics collected in the previous $^{18}$O+$^{181}$Ta experiment of GANIL. According to the DIT model, at slightly forward angles with respect to the grazing angle, the cross section is ~1 mbarn/sr for $^{16}$C, and ~0.01 mbarn/sr for $^{18}$C ions. A 10% probability can be assumed for the population of the second 2$^{+}$ in both cases. The PRISMA solid angle coverage is 80 msr, and the transmission efficiency for the light ion of interest is about 10%. Based on the previous 2017 experiment, performed in GANIL, the beam current should not exceed 1 pnA, in order to limit the pileup in the AGATA crystals. With the abovementioned conditions and with AGATA placed in close geometry (i.e., 18 cm from the target), the statistics collected in 10 days in the main gamma transition depopulating the 2$^+_2$ state of 16C and 18C (i.e., the 2217-keV and 932-keV gamma rays shown in Fig. 1) should be of the order of 9000 and ~200 counts, respectively. Such a statistics will be sufficient to precisely determine the lifetime in the case of $^{16}$C, while for $^{18}$C the sensitivity will probably be limited to distinguish between lifetimes of ~1 ps and 2 ps, as predicted by ab initio calculations assuming NN+NNN or NN interactions.

Figure 2. Cross section calculations, as a function of angle, for the deep-inelastic reaction $^{18}$O (8.5 MeV/A)+$^{238}$U target, green for the NNCLE +NRV model (green) and in DIT +Gemini++approach (blue) [Ste18]. The red symbol corresponds to the experimental point measured close to 0° with the LISE spectrometer at GANIL.

[For13] C. Forssen et al., J. Phys. G: Nucl. Part. Phys. 40, 055105 (2013).
[Vos12] P. Voss et al., Phys. Rev. C 86, 011303(R) (2012)
[Cie20] M. Ciemała et al., Phys. Rev. C 101, 021303(R), (2020).
[Cie21] M. Ciemała et al., Eur. Phys. J. A 57, 156, (2021).
[Ste18] I. Stefan et al., Phys. Lett B 779, 456 (2018).

Speaker: Michal Lukasz Ciemala

10:05 Lifetime measurements for the study of intruder states towards the island of inversion along the N = 20 shell closure

The study of lifetimes of nuclear states offers a unique picture of the structure of nuclei. In particular, transition probabilities are a common tool to disentangle the properties and the nature of different nuclear states. The aim of this proposal is to study the interplay of spherical ($0 \; \hbar \omega$) and intruder deformed ($2 \; \hbar \omega$) configurations in the low-lying states of isotopes on the edge of the island of inversion.
In particular, we aim to study the lifetime of the first two $2^+$ states of $^{34}$Si and the $5/2^+_1$ state of $^{35}$P using the Doppler Shift Attenuation Method.
The low-lying states of the isotopes of interest will be populated via multinucleon-transfer reaction using a beam of $^{36}$S impinging on a $^{208}$Pb target.
The beam-like partner will be identified by the PRISMA spectrometer, while the $\gamma$~rays will be detected by the AGATA array.

Speaker: Irene Zanon (Istituto Nazionale di Fisica Nucleare)

10:30 Development of deformation in the Island of inversion at N=20 AGATA-PRISMA

By bombarding a 232Th target with a 36S beam we aim to populate, via multi-nucleon transfer
and deep-inelastic reactions, yrast and yrare states in neutron-rich isotopes in order to explore the
new region of deformation and the doorway states that give rise to the island of inversion (IoI)
at N = 20. The coupling of the AGATA array to the Prisma spectrometer will allow to map the
boundaries of the IoI at N = 20 and its possible extension towards N = 28. It will be possible to
extend up to 6 units of angular momentum above the g.s.. Moreover, it will be possible to cover
some notable gaps left from previous experiments such as 35P, to be extended from 7/2− up to
13/2− and 34Si for which we will observe for the first time the 4+ and the 6+ states. With this
experiment we will investigate in this exotic region the shape coexistence and the robustness of the
quadrupole collectivity with increasing excitation energy and angular momentum.

Speaker: Silvia Monica Lenzi (Istituto Nazionale di Fisica Nucleare)

10:55 → 11:25 Coffee break

11:25 → 12:40 Session: LoI 7

Convener: Simone Bottoni (Università degli Studi di Milano and INFN)

11:25 Lifetime measurements in neutron-rich Pb isotopes. The role of the effective three-body forces

Abstract: This proposal aims to investigate the shell evolution in the region of the double
magic 208Pb isotope involving the neutron g9/2 shell through the lifetime determination of the
lowest lying yrast excited states in 212Pb and 208Hg isotopes. In particular, the main goal will
be to determine the B(E2 : 4+ → 2+) and B(E2 : 2+ → 0+) transition strengths in 212Pb and
208Hg isotopes and to compare with recent large-scale shell-model calculations to evaluate the
performance of the effective three-body forces in heavy systems.
The neutron-rich isotopes will be produced by a multinucleon-transfer reaction in direct
kinematics, employing a 136Xe beam, impinging onto a 208Pb target. The target will be mounted
with an Nb degrader foil in a compact inverse Plunger device, which was successfully tested at
LNL in the summer of 2022. The lifetime of the excited states of interest will be determined
using the recoil distance Doppler shift method. The AGATA array will be used to measure the
γ rays while the beam-like recoil nuclei will be identified with the PRISMA spectrometer.

Speaker: Benito Gongora (INFN, Laboratori Nazionali di Legnaro)

11:50 Octupole corelations in the neutron-deficient plutonium isotopes

In this Letter of Intent, we propose an experiment to study the onset of octupole correlations in the neutron-deficient plutonium ($Z=94$) isotopes. Specifically, we will identify and study excited states in the isotopes $^{232,234}$Pu nuclei, for which there is no known information. The nuclei of interest will be populated using multinucleon transfer reactions. One possibility is to use a beam of $^{112}_{~50}$Sn at 550 MeV on a target of $^{238}_{~92}$U; the cross sections for these reaction channels have been calculated to be 0.02 ($^{232}$Pu) and 0.4 mb ($^{234}$Pu). We will use the AGATA $\gamma$-ray spectrometer, in conjunction with the DANTE charged-particle detector array and the PRISMA spectrometer. We will search for low-lying negative-parity states and E1 transitions in the level schemes of the nuclei of interest, which are indicative of octupole correlations. The experiment will require 14 days of beamtime.

Speaker: John Smith (University of the West of Scotland)

12:15 Search for octupole structures in the light U, Th and Pa isotopes via Multinucleon Transfer

Octupole deformation can be inferred in even-even nuclei by two signatures: the presence of low-lying negative parity states and strong E3 transition strengths. In odd nuclei, this deformation gives rise to opposite parity doublets.
The aim of this experiment is to exploit the potentiality of the AGATA + PRISMA setup, implemented with an array of fast scintillators, to identify the collective states in 226Th, 228,230U populated using multi-nucleon transfer reactions. The proposed reaction is the 129Xe + 232Th at 950 MeV of beam energy. It is also important to highlight here the relevance of the measurements of E3 moments with on-going searches for non-zero Electric Dipole Moments (EDMs) in atoms with odd-A nuclei, whose observation would indicate CP violation much larger than that predicted by the Standard Model. Octupole-deformed nuclei have enhanced nuclear Schiff moments that induce the atomic EDM due to the presence of nearly degenerate parity doublets and large reflection-asymmetric octupole deformations

Speaker: Giacomo De Angelis (Istituto Nazionale di Fisica Nucleare)

12:40 → 13:05 Closing discussion

12:40 Closing remarks

Speakers: Jose' Javier Valiente Dobon (Istituto Nazionale di Fisica Nucleare), Magda Zielinska (CEA Saclay)