Lifetime measurement of the 167.1 keV state in ${}^{41}\mathrm{Ar}$

The Advanced-Time-Delayed method was used to measure lifetimes of the states in ${}^{41}\mathrm{Ar}$ populated in the β decay of ${}^{41}\mathrm{Cl}$. The nuclei ${}^{41}\mathrm{Cl}$ were produced at ISOLDE by 1.4-GeV proton bombardment of a thick UC${}_x$ target and mass-separated as molecular ions, ${\mathrm{XeCl}}^{+}$. Our measured half-life of the 167.1-keV state, $T_{1/2}=315(15)$ ps, is significantly lower than the previously measured value of 410(30) ps. We have also determined $T_{1/2}=260(80)$ ps and $T_{1/2}⩽46$ ps for the 515.9- and 1867.7-keV states, respectively. These are the shortest lifetimes measured so far with the ultrafast timing method using the new ${\mathrm{LaBr}}_3$(Ce) crystals for γ-ray detection.

Figure 1

The level scheme from the decay of ${}^{41}\mathrm{Cl}$ into ${}^{41}\mathrm{Ar}$ as determined in this work. The half-lives for the 1033.8- and 1353.6-keV levels are taken from Ref. [1].

Figure 2

Partial $\gamma \gamma$ coincidence energy spectra recorded in the (a) Ge, (b) ${\mathrm{LaBr}}_3$(Ce), and (c) BaF${}_2$ detectors, respectively, when the same gate was set on the 1186.6-keV γ peak in the coincident Ge detector. Note a superior energy resolution for ${\mathrm{LaBr}}_3$(Ce) in comparison to the BaF${}_2$ spectrum. The insert shows a simplified γ-decay scheme including only the 514.2-1186.6-167.1-keV γ cascade.

Figure 3

Time-delayed spectra sorted from the $\beta \gamma \gamma (t)$ data. Transitions selecting the decay path were chosen in the Ge detector; the time-delayed spectra (a), (c), and (d) were measured between β events and γ rays recorded in the BaF${}_2$ detector. No significant asymmetry is observed in spectrum (a); thus only an upper limit on the half-life is determined for the 1867.7-keV level. Spectrum (c) shows a mixture of a semiprompt (semi-Gaussian) spectrum and a slope that is due to the 515.9-keV level. Time-delayed $\beta \gamma \gamma (t)$ spectra (b) and (d) are gated by the de-exciting 167.1-keV transition in the ${\mathrm{LaBr}}_3$(Ce) and BaF${}_2$ detectors, respectively. Note the almost identical time spectra and slope determinations in (b) and (d). The slope is due to the lifetime of the 167.1-keV level in ${}^{41}\mathrm{Ar}$, with an average half-life of $T_{1/2}=315(15)$ ps.