High-spin states above the isomers in neutron-rich iodine nuclei near $N=82$

Excited states of neutron-rich iodine isotopes ${}^{130\text{–}134}\mathrm{I}$ above the high-spin isomers have been identified using prompt-delayed $\gamma$-ray spectroscopy. The iodine isotopes were produced as fission fragments of fusion-fission and transfer induced fission of ${}^9\mathrm{Be}({}^{238}\text{U},f$) at a beam energy of 6.2 MeV/u. The complete $(A,Z)$ identification was obtained using the large acceptance magnetic spectrometer $\mathrm{VAMOS}++$. The AGATA $\gamma$-ray tracking array was used to detect the prompt $\gamma$ rays while the delayed $\gamma$ rays (in the time range of 100 ns to 200 $\mu \mathrm{s}$) from the isomeric states were identified by the EXOGAM segmented clover detectors, placed behind the focal plane of the $\mathrm{VAMOS}++$ spectrometer. The high-spin states above the $\left(8^{-}\right)$ isomers in ${}^{130,132}\mathrm{I}$ were populated for the first time, and a new isomer in ${}^{132}\mathrm{I}$ was identified. A new $\gamma$-ray transition was also assigned to the level structure of ${}^{134}\mathrm{I}$. Prompt transitions above the $19/2^{-}$ isomer were identified in ${}^{131,133}\mathrm{I}$, for the first time. The level structures are interpreted in terms of the systematics of odd-$Z$ nuclei above the $Z=50$ shell closure and large-scale shell model calculations.

Figure 1

Charge state ($Q$) as a function of the mass over charge ($M/Q$) after the selection of iodine ($Z=53$). The iodine isotopes are obtained from various charge states and the identification of $A=130$ and 134 are marked for charge state $Q=40$.

Figure 2

Level scheme of ${}^{130}\mathrm{I}$ above the ($8^{-}$) isomeric level, as obtained in the present work. The newly observed $\gamma$ rays are displayed in red. The thickness of the $\gamma$ rays represents their relative intensities.

Figure 3

(a) The prompt singles $\gamma$-ray spectrum (${\gamma }_P$) of ${}^{130}\mathrm{I}$. (b) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with added gates of ${\gamma }_P=748+922+971$ keV. (c) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with ${\gamma }_P=588$ keV. (d) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with ${\gamma }_P=922$ keV. (e) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with ${\gamma }_P=630$ keV. The new $\gamma$ transitions are marked with “$*$” in all the cases.

Figure 4

Level scheme of ${}^{131}\mathrm{I}$ as obtained in the present work. The newly observed $\gamma$ rays are displayed in red. Isomeric states are indicated with a thick line and the half-life remeasured in this work is underlined by a red line. The thickness of the $\gamma$ rays represents their relative intensities.

Figure 5

(a) The prompt singles $\gamma$-ray spectrum of ${}^{131}\mathrm{I}$. (b) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with added gates of ${\gamma }_P=773+782$ keV. The inset shows the expanded energy range of $300\text{–}400$ keV. (c) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with added gates of ${\gamma }_P=201+491+564+692$ keV. (d) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with ${\gamma }_P=1028$ keV. The new $\gamma$ transitions are marked with “$*$”.

Figure 6

(a) The delayed singles $\gamma$-ray (${\gamma }_D$) spectrum of ${}^{131}\mathrm{I}$ for $t_{\mathrm{Decay}}<75\mu \mathrm{s}$. The inset shows the decay curve of ${\gamma }_D=122+331$ keV delayed transitions with exponential fit. (b) Delayed $\gamma$ rays (${\gamma }_D$) for $t_{\mathrm{Decay}}<75\mu \mathrm{s}$ in coincidence with ${\gamma }_P=949$ keV. (c) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with ${\gamma }_D=122$ keV. The new $\gamma$ transitions are marked with “$*$”.

Figure 7

Level scheme of ${}^{132}\mathrm{I}$ above the ($8^{-}$) isomeric level, as obtained in the present work. The newly observed $\gamma$ rays are displayed in red. The isomeric state is indicated with a thick line and the newly measured half-life has been marked with a red box. The thickness of the $\gamma$ rays represents their relative intensities with respect to the 762 keV doublet.

Figure 8

(a) The prompt singles $\gamma$-ray spectrum of ${}^{132}\mathrm{I}$. (b) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with ${\gamma }_P=762$ keV. (c) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with ${\gamma }_P=605$ keV. The new $\gamma$ transitions are marked with “$*$”.

Figure 9

(a) The delayed singles $\gamma$-ray (${\gamma }_D$) spectrum of ${}^{132}\mathrm{I}$ for $t_{\mathrm{Decay}}<3\mu \mathrm{s}$. The inset shows the decay pattern of the delayed $\gamma$ ray of 762 keV with exponential fitting. (b) Delayed $\gamma$ rays (${\gamma }_D$) for $t_{\mathrm{Decay}}<3\mu \mathrm{s}$ in coincidence with ${\gamma }_P=605$ keV. (c) Delayed $\gamma$ rays (${\gamma }_D$) for $t_{\mathrm{Decay}}<3\mu \mathrm{s}$ in coincidence with ${\gamma }_D=762$ keV. (d) Delayed $\gamma$ rays (${\gamma }_D$) for $t_{\mathrm{Decay}}<3\mu \mathrm{s}$ in coincidence with ${\gamma }_D=912$ keV of ${}^{133}\mathrm{I}$. The new $\gamma$ transitions are marked with “$*$”.

Figure 10

Level scheme of ${}^{133}\mathrm{I}$ as obtained in the present work. The newly observed $\gamma$ rays are displayed in red. The isomeric state is indicated with a thick line and the half-life remeasured in this work has been underlined by a red line. The thickness of the $\gamma$ rays represents their relative intensities.

Figure 11

(a) The prompt singles $\gamma$-ray spectrum of ${}^{133}\mathrm{I}$. The higher energy part of this spectrum is shown in an expanded scale in the inset. (b) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with the added gate of ${\gamma }_P=647+874+912$ keV. (c) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with added gate of ${\gamma }_P=614+629+714$ keV. (d) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with added gate of ${\gamma }_P=536+729+956$ keV. The new $\gamma$ transitions are marked with “$*$” in all the cases.

Figure 12

(a) Delayed $\gamma$-ray (${\gamma }_D$) spectrum of ${}^{133}\mathrm{I}$ for $t_{\mathrm{Decay}}<3\mu \mathrm{s}$. The inset of the figure shows the decay pattern of delayed $\gamma$ rays ${\gamma }_D=353+874+1168$ keV with exponential fitting. (b) Delayed $\gamma$ rays (${\gamma }_D$) in coincidence with the added prompt gate of ${\gamma }_P=614+629+714+786$ keV.

Figure 13

Level scheme of ${}^{134}\mathrm{I}$ as obtained in the present work. The newly observed $\gamma$ rays are displayed in red. The thickness of the $\gamma$ rays represents their relative intensity.

Figure 14

(a) The prompt singles $\gamma$-ray spectrum of ${}^{134}\mathrm{I}$. (b) Prompt $\gamma$ rays (${\gamma }_P$) in coincidence with added gate of ${\gamma }_P=244+640+752+785+952$ keV. The higher energy parts of the spectra are shown in insets. The new $\gamma$ transitions are marked with “$*$” in all the cases.

Figure 15

Systematics of experimental energy differences for the lowest states with $\mathrm{\Delta }I=2$ for (a) even-$N$ isotopes of Sn $[E\left(2^{+}\right)-E\left(0^{+}\right)]$, Sb $[E(11/2^{+})-E(7/2^{+})]$, Te $[E\left(2^{+}\right)-E\left(0^{+}\right)]$, and I $[E(11/2^{+})-E(7/2^{+})]$, and. (b) odd-$N$ isotopes of Sn $[E(15/2^{-})-E(11/2^{-})]$, Sb $[E\left({10}^{-}\right)-E\left(8^{-}\right)]$, Te $[E(15/2^{-})-E(11/2^{-})]$, and I $[E\left({10}^{-}\right)-E\left(8^{-}\right)]$. The new states observed in the present measurements are shown by filled symbols.

Figure 16

The comparison of experimental (Exp, shown in black) and theoretical (SM, shown in red) level schemes for even-$A{}^{130,132,134}\mathrm{I}$ (a)–(c) and odd-$A{}^{131,133}\mathrm{I}$ (d)–(e) isotopes for both positive and negative parities. The experimental and shell model states of the same spin-parity have been joined with dotted lines to guide the eye.

Figure 17

(a) Probability of neutron hole occupancy in the $\nu h_{11/2}$ orbital for ${}^{130,132}\mathrm{I}$ as a function of the total spin ($J$) of the negative parity states. (b) Average spin contributions of proton ($2J_{\pi }$) and neutron ($2J_{\nu }$) to the total spin ($J$) of the negative parity states in ${}^{130}\mathrm{I}$ for different numbers of hole occupancy in the $\nu h_{11/2}$ orbital. (c) Same as (b), but for ${}^{132}\mathrm{I}$.

Figure 18

(a) Probability of neutron hole occupancy in the $\nu h_{11/2}$ orbital for ${}^{131,133}\mathrm{I}$ as a function of the total spin ($2J$) of the negative parity states. (b) Average spin contributions of proton ($2J_{\pi }$) and neutron ($2J_{\nu }$) to the total spin ($2J$) of the negative parity states in ${}^{131}\mathrm{I}$ for different numbers of hole occupancy in the $\nu h_{11/2}$ orbital. (c) Same as (b), but for ${}^{133}\mathrm{I}$.