First observation of 28O

Abstract

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric N/Z ratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10^-21 s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of ²⁸O and ²⁷O through their decay into ²⁴O and four and three neutrons, respectively. The ²⁸O nucleus is of particular interest as, with the Z = 8 and N = 20 magic numbers^1,2, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called 'doubly magic' nuclei. Both ²⁷O and ²⁸O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of ²⁸O from a ²⁹F beam is consistent with it not exhibiting a closed N = 20 shell structure.

Fig. 1. Nuclear chart and shell structure.

a, Nuclear chart up to Z = 18 showing the stable and short-lived (β-decaying) nuclei. The experimentally established neutron drip line is shown by the thick blue line. Known doubly magic nuclei are also indicated. b, Schematic illustration of the neutron configuration for a nucleus with a closed N = 20 shell. c, The neutron configuration involving particle–hole excitations across a quenched N = 20 shell gap.

Fig. 2. Decay-energy spectra and decay scheme.

a, Five-body decay-energy (E₀₁₂₃₄) spectrum for ²⁴O+4n events. The solid red histogram shows the best-fit result taking into account the experimental response function. The dotted red histogram shows the contribution arising from residual crosstalk that survives the rejection procedures (see Methods). b, Four-body decay-energy (E₀₁₂₃) spectrum for ²⁴O+3n events. c, Same as b but gated by the partial decay energy E₀₁₂ < 0.08 MeV. The dashed red histograms represent the contributions from ²⁸O and ²⁷O events and the solid red histogram shows the sum. d, Definition of the partial decay energies. e, Decay scheme of the unbound oxygen isotopes. The newly observed resonances and their decays are shown in red. Source Data

Fig. 3. Ground-state energies with respect to ²⁴O.

Experiment is shown by the black circles, in which the values for ^27,28O are the present results and those for ^25,26O are taken from the atomic mass evaluation. The experimental uncertainties are smaller than the symbol size. Comparison is made with predictions of shell-model calculations using the EEdf3 (refs. ^,), USDB and SDPF-M (see text for ²⁷O) interactions, the coupled-cluster method with the statistical approach (CC) and shell-model calculations incorporating continuum effects (CSM and GSM). Also shown are the predictions of ab initio approaches (VS-IMSRG, SCGF and Λ-CCSD(T)). The vertical bars for CC denote 68% credible intervals. The shaded band for GSM shows the uncertainties owing to pf-continuum couplings.

Extended Data Fig. 1. Schematic view of the experimental setup.

The insets show the overall efficiency as a function of decay energy for detecting ²⁴O and four and three neutrons.

Extended Data Fig. 2. Partial decay-energy spectra.

a, The filled grey histogram is the three-body decay energy E₀₁₂ gated on the total decay energy E₀₁₂₃₄ < 1 MeV for the ²⁴O+4n coincidence events. The red and blue histograms are the results of simulations of sequential decay through the ²⁶O ground state (A and B in Fig. 2e) and five-body phase-space decay, respectively. b, Same as a but for the three-body decay energy E₀₃₄. c, The filled grey histogram is the partial decay-energy spectrum E₀₁₂ gated by 1.0 < E₀₁₂₃ < 1.2 MeV for the ²⁴O+3n coincidence events. The red and blue dashed histograms are the results of simulations assuming ²⁷O sequential (B and C in Fig. 2e) and four-body phase-space decay, respectively. The green hatched histogram represents the contribution from the decay of ²⁸O. The red (blue) solid histogram is the sum of the contributions from ²⁸O and ²⁷O for sequential (phase-space) decay. d, Same as c but for the two-body decay energy E₀₃. e, Decay-energy spectrum of ²⁴O+2n events from the ²⁹Ne beam data. The grey histogram represents events with E₀₁ < 0.08 MeV. The red histogram shows the results of the simulation for the decay of the ²⁷O resonance. The excess observed near-zero decay energy is interpreted as arising from direct population of the ²⁶O ground state from ²⁹Ne. f, Decay-energy spectrum of ²⁴O+2n events from the ²⁹F beam. The grey histogram represents events with E₀₁ < 0.08 MeV. The red histogram shows the best fit in the region of the peak arising from the decay of the ²⁷O resonance (dashed histogram) and an exponential distribution (dotted curve) arising from all other contributions that come primarily from the decay of ²⁸O. Source Data

Extended Data Fig. 3. Probability distribution of the calculated energy differences.

Survived non-implausible calculations are shown by blue dots as functions of energy differences ΔE(^28,24O) and ΔE(^27,28O). The black circle shows experiment. The dashed curves indicate 68% and 90% highest probability density regions. The top and right distributions are the one-dimensional probability density distributions. The values given by the other theories are plotted as squares: green, USDB, GSM and CSM; red, SDPF-M and EEdf3; purple, VS-IMSRG.

Extended Data Fig. 4. Transverse momentum distribution of the ²⁴O+3n system in the rest frame of the ²⁹F beam.

Events corresponding to the population of the ²⁸O ground state (E₀₁₂ < 0.08 MeV and E₀₁₂₃ < 0.8 MeV) are shown by the data points. The blue and red solid lines represent the DWIA calculations, including the experimental effects for s_1/2 and d_5/2 proton knockout, respectively, whereby the distributions have been scaled to best fit experiment. Source Data

Extended Data Fig. 5. Cross-validation of emulators.

Upper-left panel, total energies of ²⁴O computed with the coupled-cluster method in the CCSDT-3 approximation versus the SP-CC emulator for a validation set of 100 parameter samples. Upper-right panel, distribution of residuals in percent. Lower-left panel, 2⁺ excitation energies of ²⁴O computed with the coupled-cluster method in the EOM-CCSDT-3 approximation versus the SP-CC emulator for a validation set of 40 parameter samples. Lower-right panel, distribution of residuals in percent.

Extended Data Fig. 6. History-matching waves and Bayesian posterior sampling.

Lower-left triangle, the panel limits correspond to the input volume of wave 1. The domain is iteratively reduced and the input volumes of waves 2, 3 and 4 are indicated by the green/dash-dotted, blue/dashed and black/solid rectangles, respectively. The optical depths of non-implausible samples in the final wave are shown in red, with darker regions corresponding to a denser distribution of non-implausible samples. Upper-right triangle, parameter posterior pdf from MCMC sampling with the non-implausible samples of the history-matching analysis as starting points. We use an uncorrelated, multivariate normal likelihood function and a uniform prior bounded by the first wave initial volume. Note that the relevant posterior regions are small in some directions but larger in others, such as c_D and c_E.

Extended Data Fig. 7. ppds for ^16,22,24O.

MCMC samples of the ppd for selected oxygen observables. The black (maroon) histogram shows results obtained with an uncorrelated, Gaussian likelihood (including a discrete probability p(E_np.1S0 > 0|θ) = 1). The red histogram illustrates a low-statistics sample. The 68% credible regions and the medians are indicated by dashed lines on the diagonal, whereas the solid, vertical grey (blue) lines show the experimental target (prediction with the ΔNNLO_GO(394) interaction).