Observation of the Deexcitation of theTh229mNuclear Isomer

Abstract

The (229)Th nucleus possesses the lowest-energy nuclear isomeric state. Two widely accepted indirect measurements of the transition energy place it within reach of existing laser capabilities. Direct searches for the isomer deexcitation have proven elusive despite extensive effort over the past couple of decades. There is now a growing interest in finding this unique transition because of its potential applications in nuclear, atomic, condensed matter, and optical physics, quantum information, metrology, and cosmology, including the development of a new type of clock based on this nuclear transition. In this Letter we report the first direct observation of the deexcitation of the lowest-lying isomeric state in (229)Th. By collecting (229)Th recoils following the alpha decay of (233)U into MgF(2) plates and measuring the subsequent light emission, we have isolated the isomer deexcitation and measured the transition's half-life to be 6±1 h. Through comparison measurements with (235m)U isomer, we found that the observed (229m)Th deexcitation signal originates from photon emission rather than internal conversion electron emission. This discovery lays the groundwork for optical and laser spectroscopy of (229m)Th nuclear isomer and the development of a (229)Th nuclear clock.

Radiative lifetime and energy of the low-energy isomeric level inTh229

“…The internal conversion process could also have strong implications for the experiments reported in Refs. [24] and [35]. Though crystalline material is used as host in these experiments, the charge state of the thorium atom is not known, since the thorium atoms are either implanted into [24] or chemically adsorbed onto the surface of [35] the crystal.…”

“…[16,30,31] the requirements and characteristics of the requisite crystals were made rigorous, showing that 229 Th:LiCaAlF 6 and 229 Th:LiSrAlF 6 were likely good choices; other efforts focus on CaF 2 [32,33], or ThF 4 and Na 2 ThF 6 [34]. Several experiments using this solid-state approach have been carried out in recent years [2,24,35], though as described in Sec. III, care must be taken to interpret the results of these experiments.…”

“…Therefore, new approaches to determine the isomeric energy are required. While there are some ongoing attempts to better measure the isomeric transition energy (see for example [24]), directly driving the nuclear transition inside an insulator with a large band gap (i.e. a crystal), first proposed in the works [25,26], or in sample of trapped ions [27][28][29] appear to be the most promising routes forward in the short term.…”

Nuclear clocks based on resonant excitation of γ-transitions

“…A spectral selectivity can be provided by PMT properties or by VUV bandpass filters. Groups at Physikalisch-Technische Bundesanstalt [33], the Lawrence Livermore National Laboratory [34] and the Los Alamos National Laboratory [35] have performed related investigations. A report about an observation of the deexcitation of the isomeric state in the experiment at LANL was published [35].…”

“…Groups at Physikalisch-Technische Bundesanstalt [33], the Lawrence Livermore National Laboratory [34] and the Los Alamos National Laboratory [35] have performed related investigations. A report about an observation of the deexcitation of the isomeric state in the experiment at LANL was published [35]. However, it is not clear from this publication that the deexcitation of the isomer was observed [36].…”