First evidence for the J$>$1 components of the pygmy dipole resonance in neutron-rich nuclei

Abstract

Gamma ($γ$) decay shapes the synthesis of heavy elements in neutron-rich nuclear environments of neutron star mergers, supplying the Universe with heavy elements. The low-energy pygmy dipole resonance (PDR) influences nuclear reaction rates of the rapid nucleosynthesis through enhanced $γ$ transitions. However, since it is difficult to reproduce astrophysical conditions in laboratories, PDR was previously observed only in $J = 1$ spin states. Here we report the first experimental observation of $J > 1$ components of PDR, identified in the $β$-delayed $γ$ decay of the J$^π$ = 3$^{-}$ spin-parity isomer of $^{80}$Ga. The data analysis, combined with decay information and theoretical calculations allows the identification of resonant structures below the neutron emission threshold of the neutron-rich germanium $^{80}$Ge as J$^π = (2,3)^-$ components of the PDR built on the low-lying J$^π$ = 2$^+$ quadrupole state. Our findings extend the concept of PDR beyond dipole states, with implications for nuclear structure theory and experiment, as well as the element production in the cosmos.

Figures (11)

• Figure 1: (a) Characteristic scales of the electromagnetic spectrum of space, astronomical objects, and laboratory nuclear astrophysics nasa; (b) Schematic illustration of atomic nuclei embedded in a stellar environment vs (c) selective probing nuclei in a laboratory; (d) Schematic spectra of transitions between the nuclear energy levels: E1 excitations from J$^{\pi}$ = 0$^{+}$ ground or excited states, only possible to J$^{\pi}$ = 1$^{-}$ levels (left), vs E1 excitations from J$^{\pi}$ = 2$^{+}$ excited states, ending in a multiplet of J$^{\pi}$ = {1,2,3}$^{-}$ levels (right); (e) A typical dipole spectrum of a medium-mass neutron-rich nucleus (stable $^{120}$Sn) dominated by the low-energy pygmy and high-energy giant resonances, from left to right: the lowest two-phonon $[2^+\otimes3^-]^{1-}$ state and the discrete dipole states pertaining to the lower part of PDR and observed in the gamma ($\gamma,\gamma'$) scattering experiments (black bars) OezelTashenov2014, overlapping with the data on the proton ($p,p'$) scattering (purple error bars) Bassauer2020 which span both the PDR and GDR energy domains. The insets show the proton (red) and neutron (blue) transition densities characterizing the E1 transitions from the ground to J$^{\pi} = 1^{-}$ states in the respective energy intervals as functions of the radial distance $r$ from the nuclear center. The vertical lines mark the nuclear radius.
• Figure 2: Schematic design of the ALTO ISOL facility (see text for details). The inset displays the scheme of BEDO with two HPGe detectors for the low-energy $\gamma$-ray detection with high energy resolution and three PARIS clusters for high-energy $\gamma$-ray detection with high efficiency, mounted with a cylindrical plastic detector for $\beta$-tagging (not shown).
• Figure 3: (a): High-energy $\beta$-gated $\gamma$-ray spectrum from PARIS, black in mode 1, red in mode 2, and blue in mode 3. Inset: spectrum on a linear scale for the energy region from 7.0 to 7.5 MeV; (b): $\gamma$-rays of the spectrum shown in red in panel (a), in coincidence with the 659 keV $\gamma$-ray. The $\gamma$-rays with E1 multipolarities are marked with vertical dashed lines at 3664.6, 3818.6, 3920.4, 4225.1, 4412.8, 4665.3, 4678.5, 5387.4, 7181, and 7337 keV. Inset: spectrum on a linear scale for the energy region from 6.5 to 8.5 MeV and the fitting curves; (c): The inverse coincidence: $\gamma$-rays registered in HPGe in coincidence with the 7181 keV (red color) and 7337 keV (blue color) $\gamma$-rays from PARIS, gated on the red histogram of panel (a). Inset: simplified level scheme showing the E1 transitions from the excited states at 7840 keV and 7996 keV, located below S$_n$ and identified with $J^{\pi} = (2,3)^-$, to the lowest 2$_1^+$ state; (d): The measured B(GT) distribution of $^{80m,g}$Ga versus the excitation energy of the states in $^{80}$Ge decaying by E1 transitions; (e): The decay branching ratio to 2$_1^+$ and 4$_1^+$ for the states from panel (d); (f): Schematic representation of the shell structure involved in the $^{80m,g}$Ga $\beta$-decay, wherein particles (holes) are shown as filled (open) circles, and $\beta$ and $\gamma$ transitions of unspecified complexity are associated with blue and red arrows, respectively.
• Figure 4: Left: REOM$^3$ strength $S(E)$ in $^{80}$Ge for the isoscalar (IS) electric dipole $1^-$ (a), magnetic quadrupole $2^-$ (spin-dipole) (b) and electric octupole $3^-$ (c) transitions from the ground state (g.s.) (solid light blue curves) with the squared reduced matrix element $|{\bar{F}}_{mn}|^2$ [e$^2$ fm$^2$] of E1 transitions between the obtained excited states $(1,2,3)^-$ and the lowest quadrupole state $2_1^+$ (dark blue histograms). Panel (d) displays the IVGDR computed in REOM$^{1-3}$ with correlated$2q$, $4q$, and $6q$ configurations, respectively. Right: the corresponding neutron ($\nu$) and proton (${\pi}$) transition densities of the characteristic states from the lower (l) and upper (u) parts of the (e,f,g) PDR$^{\ast}(2^+)$ ($\delta\rho^{mn}(1^-) \equiv \delta\rho^{\ast}$); (l): 8.0 -- 9.0 MeV, (u): 9.0 -- 10.0 MeV and (h) IVGDR ($\delta\rho^{0n}(1^-) \equiv \delta\rho$); (l): 17.5 -- 18.5 MeV, (u): 18.5 -- 19.5 MeV.
• Figure 5: Schematic diagram of the "X" equation: the parameters involved in building the "X" equation for a given state, which is marked in purple.