Vatulin et al. (2017) reported that irradiating a rhenium target containing the long-lived 186m^{186m}Re isomer with a 300 J, 0.3 ns laser pulse at the ISKRA-5 facility appeared to disturb the radioactive equilibrium between the 2×1052\times10^5-year isomer and the 3.7-day 186^{186}Re ground state, consistent with stimulated de-excitation of roughly 10−710^{-7} of the isomeric nuclei; the effect was statistically significant in only one of several shots, and control irradiations with bremsstrahlung photons up to 46 keV and with 10-keV electrons gave null results. The paper leaves open whether the effect is real and reproducible, by what plasma-specific mechanism the de-excitation could be stimulated, and whether the delayed time structure of the daughter 137-keV gamma line requires a previously unknown long-lived isomeric state ('isomer X') in 186^{186}Re. The follow-up literature - a picosecond-laser null result for 186m^{186m}Re, an inconclusive dedicated search for isomer X, and several untested mechanism hypotheses (epithermal electrons, nuclear excitation by electron capture, resonant NEET in a two-component plasma) - confirms that all three questions remain unresolved. The problem is to settle them: establish or refute the effect under controlled conditions, identify the operative mechanism with rates consistent with all observations, and confirm or exclude the second isomer.

Background

A nuclear isomer is a metastable excited state of a nucleus whose decay to lower states is hindered by angular-momentum or shape selection rules. The subject here is the isomer 186m^{186m}Re: an 8+8^+ state at about 149 keV excitation in the odd-odd nucleus 186^{186}Re, with a half-life of 2×1052\times10^5 years, decaying essentially 100% by isomeric transition (IT), i.e., by electromagnetic decay within the same nucleus, either by gamma emission or by internal conversion, in which the decay energy is transferred to an orbital electron that is ejected instead of a photon being emitted. The 186^{186}Re ground state (1−1^-, half-life 3.7 d; the older literature value 90.64 h is also used) decays by β−\beta^- emission and electron capture to 186^{186}Os, whose de-excitation includes a characteristic 137-keV gamma line. Because the isomer feeds the ground state through its IT cascade, an aged 186m^{186m}Re sample reaches a radioactive (secular) equilibrium in the 186m^{186m}Re-186^{186}Re-186^{186}Os chain: production and decay of each member balance, so the 137-keV line intensity is constant in time. Any process that suddenly moves isomeric nuclei to the ground state, or to states that decay faster, breaks this equilibrium and produces a time-dependent 137-keV signal that relaxes on the ground-state half-life. Such a disturbance would be the signature of stimulated (triggered) de-excitation - the controlled release of energy stored in an isomer - a long-sought capability motivated by the gamma-ray laser concept and by isomer-based energy storage. Claims of triggered isomer decay have a contested history: reports of X-ray-induced accelerated decay of 178m2^{178m2}Hf were later put into doubt, and the decades-old claim of laser-plasma excitation of the low-lying 181m^{181m}Ta isomer was recently answered by a combined experimental and theoretical reinvestigation that found no effect (Gargiulo et al., 2024). Candidate microscopic channels for coupling a plasma to a nucleus include nuclear excitation by electron capture (NEEC - resonant capture of a free electron into an atomic-shell vacancy that deposits the transition energy into the nucleus) and nuclear excitation by electron transition (NEET - resonant transfer between an atomic and a nuclear transition); both have very small predicted rates. 186m^{186m}Re was proposed as a favorable test case because the spacing between the 8+8^+ isomer and the nearest known level (a 3−3^- level at 146.275 keV) is only a few keV - about 3 keV in the level evaluation used by the authors - comparable to the electron temperature of a nanosecond laser plasma, so plasma electrons or photons could in principle drive a 'trigger' transition from the isomer to a level from which the nucleus decays rapidly to the ground state. In the experiment of Vatulin et al. (2017) at the ISKRA-5 facility (a 12-channel iodine photodissociation laser at 1.315 μ\mum delivering 300 J, 0.3 ns pulses), a metallic rhenium layer 4 mm in diameter and 500 μ\mug cm−2^{-2} thick, with a specific isomer activity of only ~0.5 Bq, on a tungsten substrate was irradiated by the laser plasma. In one of several shots, the 137-keV γ\gamma-line of 186^{186}Os first rose and then decayed back to equilibrium with a period of 112±10112\pm10 h, with a peak variation of (1.7±0.2)(1.7\pm0.2) relative to the equilibrium level - interpreted as stimulated de-excitation of about 10−710^{-7} of the 186m^{186m}Re nuclei. Control irradiations of similar targets with bremsstrahlung (photon energies up to 46 keV and an integrated flux of ∼4×1017\sim4\times10^{17} photons keV−1^{-1} cm−2^{-2} near 3 keV) and with 10-keV electrons produced no disturbance even though their integrated photon and electron fluences exceeded those delivered by the laser plasma, so a purely radiative or electron-impact origin is disfavored. The rise-then-decay time structure could not be explained by a simple step disturbance of the equilibrium; to accommodate it, the authors hypothesized a previously unknown long-lived isomeric state in 186^{186}Re ('isomer X'), populated in the stimulated process and decaying to the ground state with a half-life of order ten days. Circumstantial support came from a cyclotron test in which 186^{186}Re produced via the 186^{186}W(p,np,n)186^{186}Re reaction showed an anomalously fast initial decay of the 137-keV line, suggesting a short-lived isomeric state of order one day that is preferentially populated in charged-particle reactions. No such second isomer is known: the level scheme of 186^{186}Re in the Evaluated Nuclear Structure Data File (ENSDF, maintained by the US National Nuclear Data Center) contains only the 148.2-keV (8+8^+, 2×1052\times10^5 y) isomer, and all other known levels below 1 MeV have half-lives below 30 ns.

Problem Statement

Determine whether - and by what physical mechanism - irradiation by laser-produced plasma stimulates the de-excitation of the 186m^{186m}Re isomer, and settle the associated nuclear-structure question this raises. The single scientific objective is a verified (or experimentally excluded) plasma-induced disturbance of the 186m^{186m}Re-186^{186}Re radioactive equilibrium together with a mechanism that quantitatively accounts for it. Three linked results jointly address this objective, and their relationship is intrinsic: confirming the effect requires understanding why it appears only under some plasma conditions and what produces its observed time structure, while refuting it requires equally controlled evidence. (1) Reproducibility: establish or refute, under controlled conditions (targets of higher specific 186m^{186m}Re activity, systematically characterized plasma parameters), the reported disturbance of the 186m^{186m}Re-186^{186}Re-186^{186}Os equilibrium - the time-varying 137-keV γ\gamma-line intensity attributed to stimulated de-excitation of ~10−710^{-7} of the isomeric nuclei - taking into account that it appeared significantly in only one of several nanosecond-laser shots and not at all in a picosecond-laser plasma. (2) Mechanism: identify the stimulation channel and its quantitative rate; any proposed mechanism must reproduce the observed de-excitation fraction under the ISKRA-5 nanosecond-plasma conditions (electron temperature ~1 keV, plasma lifetime ~0.3 ns) while remaining consistent with the null controls - no effect under bremsstrahlung up to 46 keV at ∼4×1017\sim4\times10^{17} photons keV−1^{-1} cm−2^{-2} near 3 keV, no effect under 10-keV electron irradiation, and no 186m^{186m}Re effect in the 1.5-ps, ~10-keV picosecond-laser plasma. Candidate channels to be distinguished include direct photon or electron impact, nuclear excitation by electron capture (NEEC), resonant nuclear excitation by electron transition (NEET) during ion-atom collisions, resonant stimulation of conversion transitions, collective atomic processes, and charge-state-induced kinematic shifts. (3) Isomer X: confirm or refute the existence of the hypothesized additional long-lived isomeric state in 186^{186}Re (inferred half-lives between roughly one and ten days at low excitation energy) - for example by γ\gamma-γ\gamma coincidence spectroscopy (detecting two gamma rays within a narrow time window to identify transitions belonging to the same decay sequence), by population routes that favor high-spin states, or by a demonstrated alternative explanation of the delayed intensity maximum - and reconcile the outcome with the evaluated 186^{186}Re level scheme, which contains no such state. How the de-excitation probability depends on the plasma parameters (electron temperature and density, plasma lifetime, pulse energy, duration, and focal-spot size) is relevant context for the mechanism question rather than an independent required result: in the cited literature it appears as a design tool within proposals that presuppose the effect (for example, Koltsov (2021) derives a simple formula relating the estimated de-excitation probability to the plasma parameters in order to select efficient stimulation regimes). An answer is complete when an expert reviewer can decide from the submitted evidence (i) whether the plasma-induced de-excitation of 186m^{186m}Re is real, (ii) which mechanism operates at what rate, and (iii) whether the second isomeric state exists; an experimentally or theoretically rigorous refutation at a stated sensitivity is an equally acceptable way to settle each of these questions, and all three must be settled.

References

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