A Laser Excitation Scheme for Th229m

Abstract

Direct laser excitation of the lowest known nuclear excited state in 229 Th has been a longstanding objective. It is generally assumed that reaching this goal would require a considerably reduced uncertainty of the isomer's excitation energy compared to the presently adopted value of (7.8 ± 0.5) eV. Here we present a direct laser excitation scheme for 229m Th, which circumvents this requirement. The proposed excitation scheme makes use of already existing laser technology and therefore paves the way for nuclear laser spectroscopy. In this concept, the recently experimentally observed internal-conversion decay channel of the isomeric state is used for probing the isomeric population. A signal-to-background ratio of better than 10 4 and a total measurement time of less than three days for laser scanning appear to be achievable.Direct nuclear laser excitation has come closer into reach within the past years, partly due to the development of free-electron-laser technology. Such ideas are typically dealing with the excitation of nuclear states in the energy range of at least a few keV or above [1,2]. However, there is one exceptional nuclear state known for the last 40 years with a significantly lower energy of presumably below 10 eV [3,4]. An excitation energy of (7.8 ± 0.5) eV, corresponding to (159 ± 11) nm wavelength or ∼ 1900 THz, is by today the most accepted value for the isomeric first excited state of 229 Th [5,6]. This state conceptionally allows for direct nuclear laser excitation using solid-state laser technology and was proposed for the development of a nuclear clock of extremely high stability, due to an expected high resilience against external influences and a radiative lifetime in the range of minutes to hours [7][8][9][10]. It is generally assumed that direct nuclear laser excitation of 229m Th requires a considerably improved knowledge of the isomeric transition energy (see e.g. Ref.[11] and references therein). The reasons are that, first, the present uncertainty in the knowledge of the isomeric energy value is still rather large. The currently best energy value of 7.8±0.5 eV leads to an energy range of at least 1 eV (corresponding to 2.4·1014 Hz) to be scanned when searching for the isomeric excitation. Second, the radiative lifetime of 229m Th was theoretically predicted to be in the range of hours [12][13][14], leading to long required detection times when searching for a radiative decay channel. This has led to the assumption that the required time for laserbased scanning of the large energy range of 1 eV would be prohibitively long. In order to shorten the required scanning times, there are worldwide efforts ongoing to decrease the uncertainty of the transition energy, which would bring the isomeric state into realistic reach of direct laser excitation (see e.g. Refs. [15-18]). A recent review is found in Ref. [11].Here we propose a different approach, which allows for a direct laser excitation of 229m Th without the requirement of an improved knowledge of the transition energy. As the requi...

The concept of laser-based conversion electron Mössbauer spectroscopy for a precise energy determination of 229mTh

“…The concept of laser-based conversion-electron Mössbauer spectroscopy (CEMS) was first proposed in 2017 for direct nuclear laser excitation of 229m Th [27]. The central idea of this concept is to perform nuclear laser spectroscopy of neutral 229 Th atoms instead of ions, as usually considered.…”

“…In Refs. [27,39] it was shown that the number of excited nuclei Nexc of a nuclear two-level system, when resonantly irradiated with a broad-band and low-intensity source of light, can be calculated as a function of excitation time texc via…”

“…The first and most direct approach is to perform internal-conversion (IC) electron spectroscopy of the IC electrons emitted in the isomer's ground-state decay [24,25]. A second approach is to use a superconducting nanowire single-photon detector (SNSPD) [26] to investigate the isomeric energy based on a transition-edge detection technique and the third approach is to perform laser-based conversion-electron Mössbauer spectroscopy (CEMS) in a thin layer of neutral 229 Th atoms [27].…”

“…Such measurement could be further confirmed by complementary techniques with a microcalorimetric γ-ray spectrometer or an SNSPD. In a second step direct laser spectroscopy on 229 Th atoms can be performed, offering the potential to pin down the isomeric energy value by additional three orders of magnitude to 40 µeV, limited by the bandwidth of the laser system used for nuclear excitation, for which ∼10 GHz is assumed [27]. The energy would then be sufficiently constrained to allow for nuclear laser excitation of 229 Th ions in a Paul trap and thus for the development of a single-ion nuclear clock.…”

Laser-induced electronic bridge for characterization of the229mTh $ \rightarrow $229gTh nuclear transition with a tunable optical laser

“…This opens the possibility for exciting applications such as a very precise nuclear frequency standard [4][5][6] or a nuclear laser [7]. The extremely low energy may render nuclear spectroscopy and even coherent control of the nuclear excitation with vacuum ultraviolet (VUV) lasers possible [8,9]. Moreover, strong coupling of the nuclear transition to the electronic shell in 229 Th ions may allow usage of the outer electrons to excite the nuclear isomeric state with optical lasers in a two-photon electronic bridge (EB) process [10].…”