The existence of a new isomeric state in ${}_{}{}^{110}{}_{}{}^{}\mathrm{Ag}$ at ∼ 1 keV excitation has been established in two experiments using a new instrument, the inner-shell-vacancy (ISV) detector. In the first experiment, a transition with a half-life of 660 ± 40 ns was observed to follow the well-known 116-keV $M4\mathrm{}$ transition that depopulates the $6^{+}$ 250-day isomeric level in ${}_{}{}^{110}{}_{}{}^{}\mathrm{Ag}$; the energy of the new transition was deduced to be <3.3 keV. In the second experiment, a half-life consistent with 660 ns was observed in the ${}_{}{}^{109}{}_{}{}^{}\mathrm{Ag}(n, \gamma ){}_{}{}^{110}{}_{}{}^{}\mathrm{Ag}$ reaction to follow $\gamma$ transitions previously assigned by others to populate a 1-keV excited state. In combination the two results indicate the existence of a $2^{-}$ 660-ns isomer at 1.11 keV, although the (remote) possibility of more than one state near 1 keV cannot be excluded by direct experimental evidence. Under the assumption that the newly observed transition is from a $2^{-}$ 1.11-keV state to the $1^{+}$ ground state, its hindrance factor with respect to the Moszkowski estimate is ∼ 2.6 × ${10}^3$, within the range expected from systematics. Possible chemical-state perturbations of the measured half-life are estimated to be much smaller than the measurement error. In both experiments the ∼ 1-keV transition was detected with the ISV detector, a new device based on the well-established atomic effect that within ∼ ${10}^{-14\mathrm{}}$ s after the formation of an inner shell vacancy an atom will undergo a multiple loss of electrons ranging from several to 20 or more, the number being a function of $Z$ and subshell. The emitted electrons, which are very soft, are collected with an accelerating and focusing electrostatic lens and detected with a plastic scintillator and a photomultiplier tube. Nuclear transitions that cause ISVs can thus be sensed. Experiments are described that show the detector is fast, sensitive, selective, and efficient in responding to ISVs. It is primarily a timing device with very restricted suitability for determining the energy of an initiating nuclear transition.

RADIOACTIVITY ${}_{}{}^{110}{}_{}{}^{}\mathrm{Ag}_{}^m{}_{}{}^{}$; measured ce-ISV delayed coin; deduced new isomer $T_{\frac{1}{2}}$, $J$, $\pi$. Inner-shell-vacancy (ISV) detector.

NUCLEAR REACTIONS ${}_{}{}^{109}{}_{}{}^{}\mathrm{Ag}(n, \gamma )$, $E=\mathrm{thermal}$; measured $E_{\gamma }$, $\gamma$-ISV delayed coin; deduced new isomer level. Inner-shell-vacancy (ISV) detector.

• Received 22 April 1975

DOI:https://doi.org/10.1103/PhysRevC.12.595