The atomic nucleus and its electrons are often thought of as independent systems that are held together in the atom by their mutual attraction. Their interaction, however, leads to other important effects, such as providing an additional decay mode for excited nuclear states, whereby the nucleus releases energy by ejecting an atomic electron instead of by emitting a γ-ray. This 'internal conversion' has been known for about a hundred years and can be used to study nuclei and their interaction with their electrons. In the inverse process-nuclear excitation by electron capture (NEEC)-a free electron is captured into an atomic vacancy and can excite the nucleus to a higher-energy state, provided that the kinetic energy of the free electron plus the magnitude of its binding energy once captured matches the nuclear energy difference between the two states. NEEC was predicted in 1976 and has not hitherto been observed. Here we report evidence of NEEC in molybdenum-93 and determine the probability and cross-section for the process in a beam-based experimental scenario. Our results provide a standard for the assessment of theoretical models relevant to NEEC, which predict cross-sections that span many orders of magnitude. The greatest practical effect of the NEEC process may be on the survival of nuclei in stellar environments, in which it could excite isomers (that is, long-lived nuclear states) to shorter-lived states. Such excitations may reduce the abundance of the isotope after its production. This is an example of 'isomer depletion', which has been investigated previously through other reactions, but is used here to obtain evidence for NEEC.

Partial γ-ray production cross sections and the total radiative thermal-neutron capture cross section for the 185 Re(n, γ) 186 Re reaction were measured using the Prompt Gamma Activation Analysis facility at the Budapest Research Reactor with an enriched 185 Re target. The 186 Re cross sections were standardized using well-known 35 Cl(n, γ) 36 Cl cross sections from irradiation of a stoichiometric nat ReCl3 target. The resulting cross sections for transitions feeding the 186 Re ground state from low-lying levels below a cutoff energy of Ec = 746 keV were combined with a modeled probability of ground-state feeding from levels above Ec to arrive at a total cross section of σ0 = 111(6) b for radiative thermal-neutron capture on 185 Re. A comparison of modeled discrete-level populations with measured transition intensities led to proposed revisions for seven tentative spin-parity assignments in the adopted level scheme for 186 Re. Additionally, 102 primary γ-rays were measured, including 50 previously unknown. A neutron-separation energy of Sn = 6179.59(5) keV was determined from a global least-squares fit of the measured γ-ray energies to the known 186 Re decay scheme. The total capture cross section and separation energy results are comparable to earlier measurements of these values.

The spallation neutron source at the Los Alamos Neutron Science Center Weapons Neutron Research facility was used to populate excited states in 186 Re via (n, 2nγ) reactions on an enriched 187 Re target. Gamma rays were detected with the the GErmanium Array for Neutron Induced Excitations spectrometer, a Compton-suppressed array of 18 HPGe detectors. Incident neutron energies were determined by the time-of-flight technique and used to obtain γ-ray excitation functions for the purpose of identifying γ rays by reaction channel. Analysis of the singles γ-ray spectrum gated on the neutron energy range 10 ≤ En ≤ 25 MeV resulted in five transitions and one level added to the 186 Re level scheme. The additions include the placement of three γ rays at 266.7, 381.2, and 647.7 keV which have been identified as feeding the 2.0 × 10 5 yr, J π = (8 + ) isomer and yield an improved value of 148.2(5) keV for the isomer energy. These transitions may have astrophysical implications related to the use of the Re/Os cosmochronometer.

We appreciate the interest of Guo et al., the points that they raise, and the opportunity that we have to provide additional details that are not included in ref. 1 . This allows us to strengthen our experimental case 1 while, in parallel, recent developments are improving our theoretical understanding of nuclear excitation by electron capture (NEEC), such as the exploration of a substantial increase in predicted NEEC probability when considering capture by an ion in an excited state (S. Gargiulo et al., submitted) or the impact of the momentum distribution of target electrons (J.R. et al., submitted). In the accompanying Comment 2 , Guo et al. focus on whether potential background contributions were underestimated in our analysis. As discussed below, these concerns are mostly unwarranted; aside from a small systematic uncertainty that could

Abstract. The neutron-capture reaction is fundamental for identifying and analyzing the γ -ray spectrum from an unknown assembly because it provides unambiguous information on the neutron-absorbing isotopes. Nondestructive-assay applications may exploit this phenomenon passively, for example, in the presence of spontaneous-fission neutrons, or actively where an external neutron source is used as a probe. There are known gaps in the Evaluated Nuclear Data File libraries corresponding to neutron-capture γ -ray data that otherwise limit transport-modeling applications. In this work, we describe how new thermal neutron-capture data are being used to improve information in the neutron-data libraries for isotopes relevant to nonproliferation applications. We address this problem by providing new experimentally-deduced partial and total neutroncapture reaction cross sections and then evaluate these data by comparison with statistical-model calculations.