Systematics of isomeric configurations in $N=77$ odd-Z isotones near the proton drip line

The systematics of the $\pi h_{11/2}\otimes \nu h_{11/2}$ and $\pi h_{11/2}\otimes \nu s_{1/2}$ isomeric configurations was studied for the odd-Z $N=77$ isotones near the proton drip line. The isomeric decays in ${}^{140}\mathrm{Eu}$, ${}^{142}\mathrm{Tb}$, ${}^{144}\mathrm{Ho}$, and ${}^{146}\mathrm{Tm}$ were measured by means of x-ray, γ-ray, and charged particle spectroscopy at the Recoil Mass Spectrometer at the Holifield Radioactive Ion Beam Facility (ORNL). The spin and parity of $I^{\pi }=8^{+}$ and $5^{-}$ were deduced for the isomers in ${}^{140}\mathrm{Eu}$ and ${}^{142}\mathrm{Tb}$. New decay schemes were established, and the half-lives of the $8^{+}$ isomers were measured to be 302(4) ns for ${}^{140m2}\mathrm{Eu}$ and 25(1) μs for ${}^{142m2}\mathrm{Tb}$. No evidence for the expected $1^{+}$ ground-state was found in the ${}^{144}\mathrm{Ho}$ decay data. The proton-emission from ${}^{146}\mathrm{Tm}$ was restudied. Five proton transitions were assigned to two proton-emitting states. The half-lives of 198(3) ms and 68(3) ms and the spin and parity values of $I^{\pi }={10}^{+}$ and $5^{-}$ were established for ${}^{146m}\mathrm{Tm}$ and ${}^{146\mathrm{gs}}\mathrm{Tm}$, respectively. For the first time for an odd-odd nucleus, the interpretation of the observed decay properties and structure of the proton-emitting states was made by accounting for deformation and proton and neutron coupling to the core excitations. A complex wave-function structure was obtained, with dominating components of $\pi h_{11/2}\otimes \nu h_{11/2}$ for the ${10}^{+}$ isomer and $\pi h_{11/2}\otimes \nu s_{1/2}$ for the $5^{-}$ ground state.

Figure 1

Systematics of the $\pi h_{11/2}\otimes \nu h_{11/2}$ and the $\pi h_{11/2}\otimes \nu s_{1/2}$ states in the $N=77$ odd-Z isotones and $Z=69$ odd-N thulium isotopes around the proton drip line. The spin and parity assignments are from this work and from the literature [1, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16].

Figure 2

Schematic diagram of the setup used in the study of odd-Z $N=77$ isotones ${}^{142}\mathrm{Tb}$ and ${}^{144}\mathrm{Ho}$ at the HRIBF. The setup was placed at the final focus of the RMS.

Figure 3

Schematic diagram of the recoils passing through the two MCPs and the two-slit three-blocker system to be finally implanted in the DSSD at the final focus of the RMS in the ${}^{146}\mathrm{Tm}$ proton-emitter study. The large MCP was used for $A/Q$ identification of the recoils. The small MCP was used as an additional timing reference.

Figure 4

Photographs of the detection setup used during the ${}^{146}\mathrm{Tm}$ experiment. The 65-μm-thick DSSD was surrounded by four 700-μm-thick Si detectors mounted in a box geometry. The left-hand picture shows a side view of the setup while the right-hand picture shows an angled top view. Below the DSSD is a Si(Li) detector, 45 mm × 45 mm, with an active measuring thickness of 3.9 mm.

Figure 5

(a) Energy spectrum of protons and α particles from the DSSD with no veto applied. (b) Energy spectrum of the particles detected in the DSSD in coincidence with any of the Si box detectors. (c) Energy spectrum of protons and α particles from the DSSD in anticoincidence with the Si box detectors and the SiLi detector. A rigid-mass gate was also applied here and a condition that the front and back energy spectra of the DSSD be in agreement with each other within 2%.

Figure 6

(a) Spectrum of γ rays collected within 1 μs of an implant minus γ rays collected between 1 and 2 μs during the $A=140$ measurement. The 79-, 128-, and 275-keV lines shown in the figure are long-lived impurities and are not in coincidence with the Eu x-rays. (b) Spectrum of γ rays in coincidence with the 170.4-keV line in the decay of ${}^{140m2}\mathrm{Eu}$.

Figure 7

Decay pattern of ${}^{140m2}\mathrm{Eu}$, $T_{1/2}=$ 302(4) ns, from the sum of 98.1-, 170.4-, 252.0-, and 422.5-keV γ lines after the appropriate background is subtracted. “Time” refers to the time difference between the implant signal and the recorded γ ray.

Figure 8

Proposed ${}^{140}\mathrm{Eu}$ level scheme from this work. The 125-ms, $I^{\pi }=5^{-}$, isomer in the decay scheme was taken from Ref. 1 and was not directly studied in this work. The energy of this isomer, X, was reported in Ref. 1 to be $<234$ keV.

Figure 9

γ rays following the decay of the 302-ns isomer (${}^{140m2}\mathrm{Eu}$) within the first 200 ms, less those from the 200–400 ms time window after the isomeric decay. The presence of the 175- and 185-keV lines indicates that the 302-ns ${}^{140m2}\mathrm{Eu}$ isomeric decay feeds the 125-ms ${}^{140m1}\mathrm{Eu}$ level. All other lines are believed to be the result of random accidental correlation.

Figure 10

Spectrum of γ rays measured by (a) two clover detectors and (b) LOAX detector appearing within 90 μs after implantation of mass 142 recoils.

Figure 11

Decay pattern of the sum of the 137.7-, 165.4-, 219.4-, and 302.9-keV γ-ray transitions after the appropriate background is subtracted. The fitted curve yields a half-life of 25(1) μs for ${}^{142m2}\mathrm{Tb}$.

Figure 12

The γ-ray spectrum in coincidence with the 197.6-keV line. Also shown is the decay pattern of the 197.6-keV line.

Figure 13

The γ spectrum in coincidence with (a) the 32.2-keV line, and (b) the 302.9-keV line. The 24- and 27-keV lines are the In K x rays. The 178-keV line represents the strongest line in ${}^{142}\mathrm{Eu}$ that contaminates our data.

Figure 14

Conversion electron data measured by the BESCA in the first 90 μs after the implantation of mass 142 recoils. The 108-keV line is the L-conversion lines for the 115.8-keV γ ray. The 146- and 188-keV lines are the K and L lines for the 197.6-keV γ-ray transition. The 140-keV line is just the 137.7-keV γ ray shifted by 2–3 channels. The 204-keV line is the L-conversion line for the 211.5-keV γ-ray. The lines labeled “i” are impurities from ${}^{142}\mathrm{Eu}$. They are the 273- and 277-keV L-conversion lines for the 280- and 284-keV γ-ray transitions in ${}^{142}\mathrm{Eu}$ that are among the strongest lines in ${}^{142}\mathrm{Eu}$.

Figure 15

${}^{142}\mathrm{Tb}$ level scheme.

Figure 16

Conversion electron data measured by the BESCA within the first 5 ms after a mass 142 recoil implantation.

Figure 17

γ spectrum associated with the decay of ${}^{144m}\mathrm{Ho}$ measured by the LOAX detector within 1500 ns after implantation of mass $A=144$ recoils. The upper panel shows the x-ray and low-energy γ-ray part of the spectrum.

Figure 18

γ spectrum measured by the clover detectors within 1500 ns after implantation of mass $A=144$ recoils.

Figure 19

Decay pattern of the sum of the 56.4-, 148.1-, and 208.9-keV γ rays in ${}^{144m}\mathrm{Ho}$ after the appropriate background is subtracted. The fitted curve yields a half-life of 564(60) ns for ${}^{144m}\mathrm{Ho}$.

Figure 20

${}^{144}\mathrm{Ho}$ level scheme.

Figure 21

Mass spectrum for the ${}^{146}\mathrm{Tm}$ experiment measured by the large MCP detector at the focal plane of the RMS. The RMS was optimized for $A/Q=146/27$ and $A/Q=146/28$ recoils. A two-slit three-blocker system was used before the large MCP to allow $A=146$ ions to reach the DSSD while stopping or reducing the implantation of other masses.

Figure 22

Energy spectrum of protons correlated with implanted ${}^{146}\mathrm{Tm}$ ions. The correlation time window for this spectrum was 200 ms. The spectrum was also gated by anticoincidence with the Sibox.

Figure 23

Determination of the proton half-lives in ${}^{146}\mathrm{Tm}$ decay by use of the exponential rise to maximum least-squares-fit function for the four proton energies: (a) 1.120 MeV, (b) 1.191 MeV, (c) 1.016 MeV, and (d) 0.938 MeV. The 1.016- and 0.938-MeV lines show bigger background components than the 1.120- and 1.191-MeV lines because of the close proximity of longer-lived lines, which are the 1.05-MeV line in ${}^{147}\mathrm{Tm}$ and the 0.89-MeV line in ${}^{146m}\mathrm{Tm}$.

Figure 24

The decay scheme of ${}^{146}\mathrm{Tm}$ established in the present work.

Figure 25

Decay pattern of the 0.889-MeV proton line deexciting ${}^{146m}\mathrm{Tm}$ after the appropriate background is subtracted. The fitted curve yields a half-life of 200(40) ms.

Figure 26

Synopsis of the experimentally determined level systematics for the lightest $N=77$ even-Z isotones. This figure is an extension of Fig. 1 from Ref. 2. The level energies are labeled in kilo-electron-volts. The half-lives of the $\nu {\mathcal{h}}_{11/2}$ isomeric states are also given.

Figure 27

Comparison of the energy differences between the $\pi h_{11/2}\nu s_{1/2}$ and $\pi h_{11/2}\nu h_{11/2}$ configurations with the energy difference of the $\nu s_{1/2}$ versus $\nu h_{11/2}$ configurations in the $N=77$ isotones. The even-$Z\hspace{1em}N=77$ isotones decay by means of β emission only while the odd-$Z\hspace{1em}N=77$ isotones decay by means of γ-ray and particle emission.