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The dipole strength distribution of $^8$He and decay characteristics

C. Lehr, M. Duer, A. T. Saito, T. Nakamura, N. L. Achouri, D. Ahn, H. Baba, S. Bacca, C. A. Bertulani, M. Böhmer, F. Bonaiti, K. Boretzky, C. Caesar, N. Chiga, D. Cortina-Gil, C. A. Douma, F. Dufter, Z. Elekes, J. Feng, B. Fernández-Domínguez, U. Forsberg, N. Fukuda, I. Gasparic, Z. Ge, R. Gernhäuser, J. M. Gheller, J. Gibelin, A. Gillibert, K. I. Hahn, Z. Halász, M. N. Harakeh, A. Hirayama, M. Holl, N. Inabe, T. Isobe, J. Kahlbow, N. Kalantar-Nayestanaki, D. Kim, S. Kim, T. Kobayashi, Y. Kondo, D. Körper, P. Koseoglou, Y. Kubota, P. J. Li, S. Lindberg, Y. Liu, F. M. Marqués, S. Masuoka, M. Matsumoto, J. Mayer, K. Miki, M. Miwa, B. Monteagudo, A. Obertelli, N. A. Orr, H. Otsu, V. Panin, S. Y. Park, M. Parlog, S. Paschalis, P. M. Potlog, S. Reichert, A. Revel, D. M. Rossi, R. Roth, M. Sasano, H. Scheit, F. Schindler, T. Shimada, S. Shimoura, H. Simon, S. Storck Dutine, L. Stuhl, H. Suzuki, D. Symochko, H. Takeda, S. Takeuchi, J. Tanaka, Y. Togano, T. Tomai, H. T. Törnqvist, J. Tscheuschner, T. Uesaka, V. Wagner, H. Yamada, B. Yang, L. Yang, Z. H. Yang, M. Yasuda, K. Yoneda, L. Zanetti, J. Zenihiro

Abstract

The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus $^8$He is the most neutron-rich bound nucleus with a mass-to-charge ratio of $A/Z=4$. We measure the dipole response of $^8$He, including for the first time the four-neutron decay channel. A total dipole strength of $\sum B(E1)(E^*<15$~MeV$)=0.95(16)~e^2$fm$^2$ and a dipole polarizability of $α_D = 0.61(1)$~fm$^3$ are extracted from the differential Coulomb-excitation cross section and compared to state-of-the-art theoretical calculations employing coupled cluster and three-body approaches. We find that the dipole continuum is dominated, even at high excitation energies well above the $4n$ decay threshold, by two-neutron emission, pointing to a $^6$He$+2n$ structure of the excited dipole mode. No indication was found for a $4n$ final-state correlation, while pronounced $nn$ and $^6$He-$n$ final-state correlations are apparent.

The dipole strength distribution of $^8$He and decay characteristics

Abstract

The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus He is the most neutron-rich bound nucleus with a mass-to-charge ratio of . We measure the dipole response of He, including for the first time the four-neutron decay channel. A total dipole strength of ~MeVfm and a dipole polarizability of ~fm are extracted from the differential Coulomb-excitation cross section and compared to state-of-the-art theoretical calculations employing coupled cluster and three-body approaches. We find that the dipole continuum is dominated, even at high excitation energies well above the decay threshold, by two-neutron emission, pointing to a He structure of the excited dipole mode. No indication was found for a final-state correlation, while pronounced and He- final-state correlations are apparent.

Paper Structure

This paper contains 2 equations, 5 figures.

Figures (5)

  • Figure 1: Two-body relative-energy spectrum of the $^7$He resonance populated by $^8$He(-1$n$) removal reaction induced by a carbon target. The dark purple curve represents a fit to an energy-dependent Breit-Wigner line shape, convoluted with the experimental response. The dotted-dashed curve displays the overall detection efficiency.
  • Figure 2: Coulomb excitation cross section spectra for three- (dark green) and five-body (light green) breakup channels of $^8$He measured with lead target. The inset shows the corresponding dipole strength distributions, $dB(E1)/dE^*$, as a function of the excitation energy.
  • Figure 3: Total $B(E1)$ spectrum measured with lead target. Theory predictions folded with the experimental response are shown in blue based on CC Bonaiti2022 (band), HH Grigorenko2009 (solid curve) and COSM Myo2022 (dotted-dashed curve).
  • Figure 4: (a) Fragment-neutron ($fn$) two-body relative energy ($E_{fn}$) for $^8$He$\rightarrow$$^6$He+2$n$ breakup channel. Shown is the measured spectrum divided by a simulated one assuming phase-space decay. (b) Same for the neutron-neutron sub-system.
  • Figure 5: Top: 4$n$ relative-energy spectrum ($E_{4n}$) for $^8$He$\rightarrow$$^4$He+4$n$ breakup channel. The red curve represents a simulated distribution assuming 5-body phase-space decay. Bottom: measured $E_{4n}$ distribution divided by the simulated one.