Towards a precise determination of the excitation energy of the Thorium nuclear isomer using a magnetic bottle spectrometer

Abstract

229 Th is the only known nucleus with an excited state that offers the possibility for a direct laser excitation using existing laser technology. Its excitation energy has been measured indirectly to be 7.8(5) eV (≈160 nm). The energy and lifetime of the isomeric state make it the presently only suitable candidate for a nuclear optical clock, the uncertainty of the excitation energy is, however, still too large to allow for a direct laser excitation in a Paul trap. Therefore, a major goal during the past years has been an improved energy determination. One possible approach is to measure the kinetic energy of electrons which are emitted in the internal conversion decay of the first isomeric state in 229 Th. For this reason an electron spectrometer based on a magnetic bottle combined with electrical retarding fields has been built. Its design, as well as first test measurements are presented, which reveal a relative energy resolution of 3 % and thus enable to measure the electrons' expected kinetic energy to better than 0.1 eV. This is sufficiently precise to specify a laser system able to drive the nuclear clock transition in 229 Th.

The $$^{229}$$Th isomer: prospects for a nuclear optical clock

“…A precise energy determination based on IC electron spectroscopy was achieved in 2019 [321][322][323]. The experimental setup used for this detection is shown in Fig.…”

“…In order to measure the electron's kinetic energy, the 229 Th atom beam was injected into an electron spectrometer. A magnetic-bottle type retarding field spectrometer was chosen for this purpose, as it provides a high acceptance and therefore leads to a reasonably high detection rate for the IC electrons [320,323].…”

“…Only the electron's velocity component perpendicular to the retarding grid is affected, which leads to the requirement of good electron collimation. Behind the retarding grid, the electrons are post-accelerated towards an MCP detector, where they are counted as a function of the applied retarding voltage, which results in an integrated electron spectrum [323].…”

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

“…A first time direct detection of the 229m Th groundstate decay in 2016 [22] and a subsequent lifetime determination of the internal conversion process on a nickel alloy surface [23] have laid the foundation for three new ways of investigating the 229m Th excitation energy. 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].…”

A Unique System for Registering One‐Photon Signals in the Ultraviolet Range from An Isomeric 229mTh Nucleus Implanted on Thin SiO2/Si Films

“…Another approach to measuring the 229 Th isomer energy was proposed by Stellmer and et al The experiment builds on the measurement of low‐energy electrons in coincidence with U‐233 α decays inside a stopping layer. In recent research, Seiferle et al used electron spectroscopy to determine the energy of internal conversion (IC) electrons. Measurements were conducted on neutral atoms in isomeric state populated via a 2% decay branch in the α decay of 233 U.…”