Understanding the advancement of the nuclear fee radius is just one of the long-standing obstacles for nuclear theory. Recently, thickness functional theory calculations utilizing Fayans functionals have efficiently reproduced the charge radii that a range of exotic isotopes. However, challenges in the isotope production have actually hindered testing these models in the immediate an ar of the nuclear chart listed below the heaviest self-conjugate doubly-magic cell core 100Sn, wherein the near-equal number of protons (Z) and neutrons (N) lead to magnified neutron-proton pairing. Here, we current an optical excursion right into this an ar by cross the N = 50 magic spirit number in the silver- isotopic chain with the measurement of the charge radius that 96Ag (N = 49). The results carry out a an obstacle for atom theory: calculations space unable come reproduce the pronounced discontinuity in the fee radii together one moves listed below N = 50. The technical advancements in this work open the N = Z region below 100Sn for more optical studies, which will lead to more comprehensive input because that nuclear theory development.

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The region of the atom chart below the most difficult self-conjugate doubly-magic nucleus, 100Sn, is a continuing subject for intense theoretical and experimental studies. The carefully equal variety of protons and also neutrons (N = Z) in the areas atomic nuclei leader to magnified neutron-proton pairing1,2, hence posing a productive ground for testing the validity the shell-model (SM) predictions and for improving our knowledge of the proton–neutron (p–n) interaction. The an ar is also rich in isomers3,4,5,6, few of which exhibit distinct features7, and the astrophysical rapid-proton capture procedure (rp-process), powering type I x-ray bursts8,9, traverses through these nuclei. Despite the interest, experimental data top top the ground-state properties of this nuclei room scarce10.

Laser spectroscopy is one efficient method for the decision of nuclear ground- and isomeric-state properties. Measurements of atomic hyperfine structure and isotope shifts administer nuclear-model independent access to the nuclear spin, magnetic dipole, and also electric quadrupole moments and changes in root-mean-square fee radii11. The nuclear fee radius gives insight into the shell and also subshell effects12, correlations and also mixing13, and nuclear deformation14. The recent demonstration that the charge radii’s sensitivity to facets of atom structure15 has enabled testing of abdominal initio methods, thickness functional approaches, and also SM calculations16.

In this article, we present a laser spectroscopic excursion near 100Sn and also below N = 50, with dimensions of the mean-squared charge radii the 96−104Ag through in-source resonance ionization spectroscopy (RIS) additionally by upright laser spectroscopy of 114−121Ag. The research of 96Ag (Z = 47, N = 49) to be made possible with a tailor-made method, combining effective in-source RIS in a hot-cavity catcher v sensitive ion detection via the phase-imaging ion-cyclotron resonance (PI-ICR) Penning catch mass spectrometry17 technique. The prestige of these outcomes is demonstrated v comparisons to thickness functional theory (DFT) calculations through a Fayans sensible coupled to a deformed implementation.


Experimental setup

The experiment took place at the Ion guide Isotope Separation On-Line (IGISOL) basic at the college of Jyväskylä Accelerator Laboratory18. Our approach is three-fold, realized v an experimental setup gift in Fig. 1. Firstly, for silver nuclei v N > 51, the 92Mo(14N, 2pxn)104−99Ag heavy-ion fusion–evaporation reaction offered the optimal manufacturing yields with respect to easily accessible primary beam intensity, together in a ahead study19. While the cross-sections for developing 96−98Ag to be lower contrasted to other possible reactions3,19, the advances in the production and also detection techniques, described below, allowed the research of also these highly an overwhelming cases v the very same reaction.


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A 148 MeV 14N beam from the K-130 cyclotron impinges top top a ~ 3 mg cm−2 92/nat.Mo target and also produces silver- nuclei in the an ar of N = 50 via fusion–evaporation reactions. The products recoil out of the target, implant into warm graphite, and promptly diffuse the end to the catcher cavity prior to effusing right into a carry tube. The atoms overlap v counter-propagating laser beams and also undergo a three-step resonance laser ionization process. The silver ions, in addition to surface ions and also sputtered target-like contaminants, room then guided right into the sextupole ion guide (SPIG) by a ~ 1 V every cm ar gradient along the tube, and also subsequently accelerated to 30 keV, fixed separated, and also introduced into the radiofrequency quadrupole (RFQ) cooler-buncher. The ions are released as bunches and injected right into the JYFLTRAP Penning catch for mass evaluation and detection via the PI-ICR method. In ~ the last stage, software processing of the laser frequency-tagged ion affect location data returns the hyperfine spectrum, presented here for 99Ag ground-state and isomer (99g and 99m, respectively) and also for the 96Ag ground-state (96g). The error bars show the statistical error.


Secondly, we imposed a hot-cavity catcher laser ion source20,21 because that fast, high-efficiency stopping, and also extraction of these reaction products22,23. The created silver nuclei, i m sorry recoil out of the target, implant right into a warm graphite catcher from wherein they promptly diffuse out right into the catcher cavity together atoms, before effusing into a carry tube. There, the silver atoms space selectively ionized v a three-step laser ionization scheme. Compared to the typical gas-cell method in usage at the IGISOL facility, this technique has the advantage of a an extremely high stopping performance while neutralizing the reaction products for a subsequent selective re-ionization.

Lastly, we utilize the well-established coupling the RIS to mass spectrometry. The ions are mass be separated in the JYFLTRAP double Penning trap24, making use of the PI-ICR method17, and tagged through the frequency of the an initial resonant step in the three-step scheme. The high mass addressing power available by the PI-ICR method enables simultaneously measurement that the hyperfine framework of many long-lived, (t1/2 > 100 ms), nuclear says with mass distinctions as low together ~10 keV, in an basically background-free manner. However, also though long-lived low-spin isomers have been observed in nearby silver isotopes3,25,26, the cross-section for producing these states with our reaction is order of magnitude smaller sized than for the higher-spin states. Thus, an isomeric state was just observed for the high-yield situation of 99Ag (Eex = 506.2 keV, I = (1/2−)) within the time constraints of the measurement. The RIS that 96Ag, with on-resonance signal rates as low together ~0.005 ions per second, demonstrates the instant potential the this PI-ICR aided in-source RIS technique. These rates correspond to an average cross-section that the order of one μbarn, approximated using the PotFus27 and also GEMINI++ fusion–evaporation codes28 as presented in ref. 29. Dimensions with together low occasion rates room only possible if the background price is an in similar way low, a condition which is satisfied through the PI-ICR method.

Laser spectroscopy that silver

A step dimension of ≈300 MHz was offered for scanning the laser frequency over the hyperfine framework of the proton-rich silver isotopes causing multiple data points in ~ the resonance full-width fifty percent maximum (FWHM) of about 5 GHz. The FWHM is written of the laser linewidth of about ~3 GHz in the third-harmonic, with an additional contribution the ~3 GHz because of the Doppler broadening caused by the hot-cavity temperature of around 1800 K. The laser strength was no optimized to minimize the power widening component due to a lack of power stabilization. This linewidth is sufficiently small to extract dependable magnetic dipole moments and the alters in mean-squared fee radii. Numerous sweeps across the whole frequency range were performed because that every hyperfine scan, accumulating around 10 s of measurement time every data point, every sweep, v the full measurement time varying from a few tens of minutes to 12 h in the instance of 96Ag. The approaches section describes how the hyperfine spectra are constructed from the PI-ICR data and also outlines the details that the data analysis. Indigenous the analysis, the isotope shift δν109,A = νA − ν109 between the centroid of the isotope of attention νA and the reference isotope 109Ag, ν109, was extracted. The magnetic moments determined for 97−104Ag in the PI-ICR helped in-source RIS agree well with the literary works values, and also we report \(\mu _exp\,=\,4.57_-0.18^+0.28\mu _N\) because that 96Ag\(^(8^+)\). A detailed discussion of the magnetic moment is, however, past the border of this work.

We additional these dimensions with high-precision researches using typical collinear laser spectroscopy. This technique, used to probe neutron-rich 114−121Ag created in proton-induced fission of uranium, is feasible for silver isotopes with manufacturing rates as low as 1000 ion per second, and features linewidths of around 75 MHz. The nuclear properties to be inferred from the same 328 nm optical change as the in-source measurements. Us restrict the high-precision dataset of these neutron-rich isotope to the atom ground-states; data obtained on the isomeric says are still experience analysis. By expanding the measurements performed from below N = 50 right into the mid-shell region, and towards N = 82, an ext stringent exam of the capabilities of the theoretical models are possible. Furthermore, through data extending 19 silver- isotopes, atom mass- and field-shift constants forced to extract nuclear charge radii native optical data have the right to be validated. Much more details top top the production, ion beam manipulation, measure up and evaluation protocol of upright laser spectroscopy deserve to be uncovered in the methods section. Table 1 presents the measure isotope shifts compared to literature values, wherein available.


The readjust in the mean-squared fee radii \(\delta \langle r^2\rangle ^109,A\) is regarded the isotope shift as


$$\delta \nu ^109,A\,=\,M\fracm_A\,-\,m_109m_A\,\times\, m_109\,+\,F\,\, K\delta \langle r^2\rangle ^109,A$$

where M and F room the atom mass and field-shift constants, respectively, and mA,109 room the atom masses because that the isotope that interest and also 109Ag. The factor K corrects for greater order radial moments, different from unified by only a couple of percent even for really heavy nuclei. In this fixed region, K = 0.97630. Exact values for the atomic factors M and F room not available from speculative data alone due to the fact that there are only two secure silver isotopes and also thus a King plot procedure is no possible; instead, atomic framework calculations are, in principle, required31. Use the values of M and also F supplied in vault work19,30 returns a fee radius trend that does not agree well v those observed in neighboring isotope chains. We thus performed an empirical calibration by retaining the previously provided value19 the F = −4300 (300) MHz fm−2, and also adjusting the worth of M to match the fee radius difference of the two steady silver isotopes to the literature difference32 of δR109,107 = 0.021 fm. This procedure yields M = 1956 (360) GHz u; the uncertainty is as result of the propagation of the hesitation in F. Literature data making use of the 547.7 nm shift from ref. 30 to be empirically calibrated in a comparable fashion for consistency. The compare with various other isotopic chains33,34,35, displayed in Fig. 2a, which to be made possible due come the extr data clues on the neutron-rich side derived with collinear laser spectroscopy, validates this empirical calibration.


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a speculative charge radii that Sn, In, Cd, Ag, Pd, Rh, and Ru isotopes as much as N = 7433,34,35. The derived empirical atomic parameters for the 328 nm shift aided in the recalculation of the charge radii that Ag in cases where literature isotope shifts to be available. Similar treatment surrendered empirical atom parameters for the 547.7 nm transition30. Whereby multiple worths were available, a weighted typical with the uncertainty offered by the standard error of the weighted typical is shown. Because that asymmetric errors, the bigger one is used. b Ground-state readjust in mean-squared fee radii because that 96−104Ag (in-source RIS) and also 114−121Ag (collinear laser spectroscopy). The data are contrasted to theoretical calculations with Fy(Δr, Hartree-Fock-Bogoliubov (HFB))43, UNEDF041 and also UNEDF242 EDFs. The error bars suggest the statistics error. The systematic error, due to the skepticism in the atomic parameters, is indicated by the shaded band. c The readjust in the fee radii because that Zn38, Mo39, and also Ag near N = 50 illustrate boosting trend in the magnitude of the kink together a role of proton number towards 100Sn. The error bars indicate the statistical error.


This work reveals a kink in the fee radius once crossing N = 50, together presented in Fig. 2, despite the rather big uncertainty ~ above the radius of 96Ag. Such discontinuities have frequently been it was observed at the crossing of magic numbers, for instance at N = 2816 and N = 8236, giving support because that the function of magicity in ~ N = 50 in the vicinity that 100Sn. Recently, magicity was also demonstrated on the far neutron-rich next of stability37 in 78Ni (N = 50). Together our data stand for the only crossing of N = 50 in the instant vicinity the 100Sn, the is instructive to compare the \(\delta \left\langle r^2\right\rangle ^N \,=\, 49,\,50\) values of silver with other isotope chain to investigate the loved one magnitude the the kink in the charge radius at N = 50, as presented in Fig. 2c. In ~ zinc38 (Z = 30), the lightest isotone with a measured fee radius at N = 50, the \(\delta \left\langle r^2\right\rangle ^49,50\) is essentially zero. In the molybdenum chain (Z = 42), the nearest aspect to silver with experimental data in ~ N = 5039, a minor increase in the radius is observed because that N = 49 family member to N = 50. Through five added protons compared to molybdenum, silver- exhibits a larger rise in charge radius, perhaps indicating an enhancing trend in magnitude in the direction of doubly-magic 100Sn.

In order come investigate this qualitative observations in much more detail, DFT calculations, utilizing Fayans (Fy)40 and also UNEDF41,42 Skyrme Energy density Functionals (EDF), to be performed. Recently, the effectiveness of the Fy(Δr, HFB)43 parameterization ~ above reproducing complete radii14,35,36,44 and smaller odd-even variations, such together the odd-even staggering in copper13 isotopes, to be demonstrated. The (Δr) parametrization has additionally demonstrated the ability to blee the changes of nuclear fee radii as soon as crossing shell closures because that both proton-rich calcium (Z = 20) isotopes44 and neutron-rich Sn (Z = 50)36 isotopes. This successes make the Fayans sensible a prime candidate to investigate the tendency of the silver- radii. Us note, in this work, the results with Fayans functionals were calculated with readjusted pairing strength due to differences in between DFT solvers offered in initial parameter convey (coordinate an are solver) and present calculations (harmonic oscillator basis).

Figure 2b compares every models with the experimental transforms in mean-squared charge radii. In general, the UNEDF EDFs predict a quite smooth behavior, when the Fayans EDF an ext closely complies with the neighborhood variation in fee radii. Every functionals administer a very great reproduction of the speculative trend because that the neutron-rich isotopes. The odd-even staggering acquired with Fy(Δr, HFB) is but too large, a phenomenon i beg your pardon was also observed to a similar extent in the potassium45 (Z = 19), copper13 (Z = 29), and cadmium35 (Z = 48) isotope chains. Much more striking is the deviation of all models near N = 50: none of the models can reproduce the pronounced boost of the measured radius of 96Ag below N = 50. The remarkable difference in between UNEDF and Fy(Δr, HFB) functionals in the vicinity the the shell closure, also present in calculated absolute values, might arise indigenous the distinctions in the limitless nuclear issue properties between the models. The nuclear issue saturation density is higher with Fy(Δr, HFB) than with the UNEDF models, resulting thus in a smaller nucleus.

It is an extremely unlikely the the rise of the fee radius watched in 96Ag can be reproduced with any reasonable EDF model which is based on a deformed mean-field. In this approach, the mean-field and also resulting wavefunction breaks, for example, rotational symmetry, which would be conserved by the underlying nucleon-nucleon interaction. The wavefunction is no longer an eigenstate of angular momentum. A possible solution towards a an ext accurate description of the fee radius can be v the repair of damaged symmetries. The symmetry-restored wavefunction becomes a linear mix of multiple symmetry-broken wavefunctions, incorporating correlations beyond the mean-field. This has actually the potential to far better describe the local variation the the charge radius.

Beyond silver, our combination of techniques have the right to be used to examine other facets in the N = Z region near 100Sn. Cross-section calculations, validated by experiment at the GSI Helmholtzzentrum für Schwerionenforschung7,21,46,47,48,49,50, together with high laser ionization efficiencies51,52 and also our main beam intensities imply 93Pd, 97Cd, 100In, and 101Sn are within reach, as highlighted in Fig. 3.


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The number presents the standing of optical dimensions in the 100Sn region of the atom chart and also the projected with of the PI-ICR-assisted RIS technique at IGISOL. The projections are based upon LISE++ simulations and also Gemini++ cross-section calculations. They i think a 0.5% effectiveness after mass separation and also a 10% efficiency through the RFQ and also Penning catch as watched in Fig. 1. The pure laser ionization performance for these facets ranges from below 9% in Sn51 to over 50% in Pd52. Just for In the absolute worth is not known, yet in this calculation we assume it to be of the order of 10%. Relying on the isotope the interest, the primary beam is either 40Ca or 58Ni, v a projected conservative 50 pnA intensity. Through these assumptions, similar statistics as for 96Ag deserve to be gathered for also the many exotic situations in 7,21,48–50.


Laser spectroscopy also as precise mass measurements have the right to be exploited in parallel, help to elucidate the structural development of nuclei follow me the N = Z line. Phenomena arising from enhanced correlations in between neutrons and protons occupying orbitals with similar quantum numbers remains a difficulty to understand. Because that example, a clear fingerprint because that spin-aligned neutron-proton pairing remains an open question53, the evidence for this effect in N = Z nuclei2,54 phone call for more probing of fee radii the self-conjugate nuclei in the region. The newly demonstrated treatment of p–n pairing in a symmetry-restored mean-field approach55 has actually opened a course for the development of DFT models come elucidate this phenomenon. Furthermore, while the charge radii actions in silver across N = 50 supports the magicity of N = 50, the robustness the the adjacent N = Z = 50 shell closure in Sn is quiet under debate56,57,58,59.

In conclusion, us performed in-source RIS measurements of the mean-squared fee radii of 96−104Ag. The in-source measurements were additional by high-precision upright laser spectroscopy of neutron-rich 114−121Ag. The in-source measurement compelled the advancement of a perceptible laser spectroscopy technique which combines the high mass solving power of the PI-ICR technique with the performance of a hot-cavity catcher laser ion source. This job-related represents a milestone for further studies in the region, namely towards 94,95Ag where the properties of the distinctive isomer of 94Ag(21+)7 in ~ the N = Z line room still under debate60. Furthermore, the charge radii the 94,95Ag are compelled to understand the trend throughout N = 50 and to provide a more stringent check of the different DFT models. The work-related presented here thus opens a home window of opportunity to laser spectroscopic researches of very proton-rich nuclei in the 100Sn region.


Laser setup

The three-step laser ionization scheme, gift in Fig. 1, was realized v pulsed titanium:sapphire lasers, created in-house, pumped through 10 kHz repetition price Nd:YAG lasers (Lee Laser LDP 200 MQG) operating at 532 nm. The 328.1624 nm first step change 4d105s2S1/2 → 5p2P3/2 was offered for the laser spectroscopy. Usage of a 0.3 mm thick etalon (LaserOptik 40% reflectivity at 950–1050 nm) merged with a 6 mm special etalon in the titanium:sapphire laser resonator diminished the laser linewidth close come the Doppler circulation in the hot cavity. The 6 mm special etalon was a polished, uncoated, and undoped YAG crystal with about 10% reflectivity61. Both that the etalons were placed in LiopTec V100R-100-1PCL-BU mounts thrust by closed-loop PiezoMike E-472 motors.

The synchronized etalon scanning, compelled for the wide scan range, was achieved with a lookup table generated manually through optimizing the etalons because that maximal output strength for a provided frequency within the range. A third-harmonic generation setup converted the an essential laser calculation to the compelled wavelength. After the third-harmonic generation, the output power differed from 20 come 5 mW across the scan variety due come walk-off accident in the thick etalon. Laser strength stabilization to be not available during the experiment; however, the scans that 96,97Ag provided two separate scan arrays with similar average powers in stimulate to maintain the family member intensities the the two resolved peaks.

A standard titanium:sapphire laser using intra-cavity 2nd harmonic generation produced the 2nd resonant step at 421.2142 nm. In order come avoid expanding effects ~ above the 328 nm shift due to heavy 2nd step saturation, the laser power was limited to ~150 mW. A an unified output of two titanium:sapphire lasers, operating at ~792 nm fundamental frequency, enabled us to accomplish the crucial power to saturate the ionization step. The choice of the wavelength because that the ionization step was guided by the highest possible output power, and the possibility to recover population from a state populated by a radiative decay from the second excited state, 6d2D5/2.

Wavemeters indigenous HighFinesse monitored the laser frequencies. A medium-precision model, WS-6, monitored the titanium:sapphire lasers provided for the second and non-resonant ionization step, and also a high-precision HighFinesse WSU-10 wavemeter measure up the frequency that the spectroscopy action with a in the name accuracy that 10 MHz62.

Ag production with the warm cavity

The hot-cavity catcher functions two sections; both operated at roughly 1800 K during the experiment. First, a catcher assembly consist of of a molybdenum crucible, graphite catcher63, and also a target stack is heated inductively utilizing an UltraFlex SM-2/200 induction heater system capable of providing up come 2 kW of absorbed heating power. A six revolve induction coil wound indigenous a 3 mm copper tube v a ~12 mm diameter opening about the catcher home window to allow the i of the main beam, it is provided the power to the crucible. The coil is separated by ~2 mm indigenous the crucible. Second, a graphite transfer tube, confining the atom for effective laser ionization, is cook resistively v ~80 A listed by a Delta Elektronika SM 30–100 D DC power supply. As result of the tiny cross-sectional area and the resistivity the graphite, the existing effectively heats the tube v Joule heating and simultaneously develops a couple of Volt potential gradient along the tube length64. The effectiveness of the system was determined to be of the bespeak of 1% after the mass separator, with a couple of tens of multiple sclerosis mean delay for the release of silver, through implanting a 107Ag beam from the K-130 cyclotron23.

A 14N beam, created at the Heavy-Ion Ion source Injector ion source65, was increased to 148 MeV by the K-130 heavy-ion cyclotron. The beam impinged top top a circular target silver paper stack placed in the molybdenum crucible of the hot-cavity catcher. The target foil stack, through an efficient area of around 64 mm2, closed the hot cavity to type a 220 mm3 cylindrical enclosure. Relying on the isotope the interest, the stack was created from either a single 2.6–3.3 mg cm−2 92/nat.Mo foil or it had actually a backing foil made from a 3 come 12.5 μm molybdenum. LISE++66 simulations guided the selection of the main beam energy, and also the target configuration for different isotopes that interest.

The reaction assets recoil indigenous the target and subsequently implant into the catcher. The radioactive silver isotopes promptly diffuse out as atoms right into the catcher volume23 from whereby they effuse onward right into the graphite deliver tube where a three-step laser ionization system selectively ionizes them. The ions room extracted quickly and efficiently into the sextupole ion guide (SPIG)67. The ions guided via the SPIG space electrostatically increased to 30 keV and transported come the dipole magnet. ~ the mass separation, the constant ion beam with a selected mass-to-charge ratio A/q is converted into a bunched-beam by a gas-filled radiofrequency quadrupole cooler and buncher68 before injection right into the JYFLTRAP dual Penning trap24 put in a 7-T superconducting solenoid.

PI-ICR-based RIS

RIS experiments have recently started to utilize multi-reflection time-of-flight gadgets for isobaric purification to accomplish high count sensitivity14,69. In this work, we developed a technique to harness the high mass fixing power the the PI-ICR17 method, recently imposed at JYFLTRAP70, for RIS.

The ions injected into the trap (see Fig. 1) were an initial cooled, centered, and in addition purified indigenous isobaric contaminants in the preparation trap via a mass-selective buffer gas cooling technique71 prior to being transported come the measurement trap wherein the ion’s mass-dependent cyclotron movement was excited. After a particular phase buildup time, various mass claims in the ions accumulate a step difference. Publication the ion at this suggest projected the radial ion activity in the catch onto a position-sensitive detector (MCP detector v a hold-up line anode), placed exterior the solid magnetic field70.

In suitable projection, the enhanced image top top the detector represents the ion movement in the trap and also preserves the phase angle in between the states. Thus, different ion landing regions on the detector exchange mail to different ion states (as an example, see inset in Fig. 1). The ions of one species, one of two people the ground state or the isomeric state, deserve to be preferred for analysis by using software entrances on ion landing coordinates. Furthermore, correlating the ion to the tagged laser frequency enables the building and construction of a hyperfine spectrum.

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Analysis procedure for in-source RIS

An in-house written evaluation software merged the different scans exported native the JYFLTRAP data-acquisition program, Pymasscanner. Furthermore, the software binned the data and converted the complete counts every bin to an ion rate as a duty of the laser frequency making use of the trap pattern duration, hence resulting in the final speculative hyperfine spectrum.

The SATLAS package72 was offered to right a calculated hyperfine spectrum come the experimental data with shift frequencies pertained to the fine-structure transition frequency given as