$\alpha$-decay-correlated mass measurement of ${}^{206,207g,m}\mathrm{Ra}$ using an $\alpha$-TOF detector equipped multireflection time-of-flight mass spectrograph system

The atomic masses of the isotopes ${}^{206,207}\mathrm{Ra}$ have been measured via decay-correlated mass spectroscopy using a multireflection time-of-flight mass spectrograph equipped with an $\alpha$-TOF detector. The Ra isotopes were produced as fusion-evaporation products in the ${}^{51}\mathrm{V}+{}^{159}\mathrm{Tb}$ reaction system and delivered by the gas-filled recoil ion separator GARIS-II at RIKEN. The $\alpha$-TOF detector provides for high-accuracy mass measurements by correlating time-of-flight signals with subsequent $\alpha$-decay events. The masses of ${}^{206}\mathrm{Ra}$ and ${}^{207g,m}\mathrm{Ra}$ were directly measured using a multireflection time-of-flight mass spectrograph equipped with an $\alpha$-TOF detector. The mass excesses of ${}^{206,207g}\mathrm{Ra}$ and the excitation energy of ${}^{207m}\mathrm{Ra}$ were determined to be $\text{ME}=3540\left(50\right)\mathrm{keV}/c^2, 3538\left(15\right)\mathrm{keV}/c^2$, and $E_{\mathrm{ex}}=552\left(42\right)$ keV, respectively. The $\alpha$-decay branching ratio of ${}^{207m}\mathrm{Ra}, b_{\alpha }=0.26\left(20\right)$, was directly determined from decay-correlated time-of-flight signals, and the reduced $\alpha$ width of ${}^{207m}\mathrm{Ra}$ was calculated to be ${\delta }^2={43}_{-34}^{+68}$ keV from the branching ratio. The spin-parity of ${}^{207m}\mathrm{Ra}$ was confirmed to be $J^{\pi }=13/2^{+}$ from decay correlated mass measurement results.

Figure 1

Sketch of the experimental setup. Fusion-evaporation product ions are separated from the primary beam (projectile) using the gas-filled recoil ion separator GARIS-II. The ions are stopped and thermalized in the high-purity helium gas cell and subsequently transported and stored in the multiple radio-frequency ion traps before being sent to an MRTOF-MS for mass analysis. The ion detector at the end of the MRTOF-MS can detect both ion implantation and subsequent $\alpha$-decay.

Figure 2

The full-range time-of-flight singles spectra for two settings of the MRTOF-MS. (a) The upper panel shows the result of the six hour measurement wherein $A/q=103.5$ ions made 267 laps; (b) the lower panel shows the result of the one hour measurement wherein $A/q=103.5$ ions made 265 laps. The dashed blue lines indicate the positions of the Fr and Ra isotopes, demonstrating that their relative positions were largely constant under change of lap number between the two runs. Several peaks, presumed to be stable molecular ions extracted from the gas cell, were observed but did not correlate with $\alpha$-decay signals.

Figure 3

The $\alpha$ spectra obtained by the $\alpha$-TOF detector during the six hour measurement. (a) $\alpha$ singles spectrum. (b) The TOF-correlated $\alpha$-decay spectrum gated on ${}^{207}\mathrm{Ra}^{2+}$. (c) The TOF-correlated $\alpha$-decay spectrum gated on ${}^{206}\mathrm{Ra}^{2+}$.

Figure 4

Time-of-flight spectra from the two runs: (a) Run #1 with 267 laps, (b) Run #2 with 266 laps. The TOF singles spectra are drawn in black, while the ${}^{206}\mathrm{Ra}\alpha$-decay correlated spectra are shaded blue. The coincidence period of the gate was 1 s, corresponding to approximately four half-lives. The reduced ${\chi }^2$ value (${\chi }_{\nu }^2$) for each of the fits is indicated below the number of laps.

Figure 5

Decay time spectrum as a histogram of the time interval between the ion implantation signal used to determine the time of flight and the subsequent $\alpha$-decay signal of appropriate energy: (a) full range up to 5 s; (b) the first 1.2 s. The half-life of ${}^{206}\mathrm{Ra}$ was determined to be 260(55) ms by fitting these data as an exponential decay with constant background (solid red curve).

Figure 6

The time-of-flight spectrum around the ${}^{207}\mathrm{Ra}^{2+}$ region along with the fitting results. The black and blue filled histograms are the singles and decay-correlated TOF events, respectively. The solid lines indicate the fitting curves of the singles histogram for ground state (green), isomeric state (blue), and summing of both states (red). The dashed lines similarly represent the fitting curves for the decay-correlated TOF spectrum.

Figure 7

The decay time distribution of ${}^{207m}\mathrm{Ra}$, determined with an energy gate of $E_{\alpha }\ge 7.32$ MeV. The solid red lines indicate the fitted decay curve of ${}^{207m}\mathrm{Ra}$, while the dashed green lines indicate the constant background level and the long dashed dotted blue line represents the background-subtracted decay curve. (a) Full range spectrum up to 5 s, linear scale. (b) Focus on the first 500 ms, logarithmic scale.

Figure 8

The single particle level diagram for odd Ra and Rn nuclei. The closed circle indicates the data from this experiment.