Direct detection of the ≈ 8.4 eV internal conversion energy of
229m
Th embedded in a
superconducting nanowire
Galen O’Neil,
1
Kjeld Beeks,
2
Eric Hudson,
3
Justin Jeet,
3
David Leibrandt,
3
Marion Mallweger,
4
Sae Woo Nam,
1, ∗
Sayan Patra,
1, 5, †
Gil Porat,
6, ‡
Dileep Reddy,
1, 5
Thorsten Schumm,
7
Stephen B. Schoun,
6
Benedict Seiferle,
8
Christian Schneider,
3
Lars von der Wense,
9
Peter G. Thirolf,
8
Varun Verma,
1
Jun Ye,
6
and Chuankun Zhang
6
1
National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80303, USA
2
´
Ecole Polytechnique F´ed´erale de Lausanne, Rte Cantonale, 1015 Lausanne, Switzerland
3
University of California at Los Angeles, Department of Physics and Astronomy,
475 Portola Plaza Knudsen 1-129, Los Angeles, CA 90095, USA
4
Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden
5
Department of Physics, University of Colorado, Boulder, CO 80303, USA
6
JILA, National Institute of Standards and Technology and Department of Physics,
University of Colorado, Boulder, Colorado 80309, USA
7
TU Wien, Institute for Atomic and Subatomic Physics, Stadionallee 2, 1020 Vienna, Austria
8
Faculty of Physics, Ludwig-Maximilians-Universit¨at M¨unchen, Am Coulombwall 1, 85748 Garching, Germany
9
Institut f¨ur Physik, Johannes Gutenberg-Universit¨at Mainz, Staudingerweg 7, 55128 Mainz, Germany
(Dated: August 25, 2025)
We report on a direct measurement of the ≈ 8.4 eV nuclear excitation energy of the isomeric first
excited state
229m
Th via the internal conversion (IC) decay channel. Thermalized and mass-filtered
recoiling
229m
Th ions from
233
U α decay are delivered to the surface of a superconducting nanowire
sensor and become embedded. The ion is neutralized, triggering the IC decay, and the energy
released by the IC decay is detected with high quantum efficiency by the nanowire sensor. Energy
resolution is enabled by the current dependence of the internal quantum efficiency of the nanowire
sensor. The techniques presented here are complementary to light-based detection schemes. The IC
decay channel is about eight orders of magnitude faster than the photo-emission channel, thus the
ability to detect IC decays with high efficiency with superconducting nanowire sensors is likely to
be a valuable tool for future
229m
Th experiments.
I. INTRODUCTION
The isomeric first excited state of the actinide isotope
229
Th, known as the thorium isomer
229m
Th, is the
energetically lowest-lying isomeric excited nuclear state
in the known landscape of nuclear species, about five
orders of magnitude lower energy than typical nuclear
isomers. A proposed nuclear clock [1] based upon this
state could reach a fractional accuracy of 10
−19
[2, 3].
The use of a nuclear transition would enable the clock
to be both less sensitive to magnetic and electric
fields, while also being 10
4
− 10
5
more sensitive to
potential variations in the fundamental constants like
the fine structure constant or the scale parameter of
the strong interaction compared to conventional atomic
clocks [4–8]. Further applications proposed for a
229m
Th
based nuclear clock include (ultra-light) dark matter
research [9–13] or relativistic geodesy [14].
The
229m
Th thorium isomer decays to the ground
state primarily via internal conversion (IC) or photo-
∗
deceased
†
Present address: Lawrence Livermore National Laboratory, 7000
East Ave, Livermore, CA 94550, USA
‡
Present address: Department of Electrical and Computer Engi-
neering and Department of Physics, University of Alberta, Ed-
monton, Alberta T6G 1H9, Canada.
emission. These two decay channels have vastly dif-
ferent timescales. The photo-emission channel, which
emits vacuum ultraviolet (VUV) photons, has a half-life
of 1, 740(50) s [15]. This value for the vacuum half-life
is based upon lifetime measurements in CaF
2
, a VUV
transparent crystal that suppresses the IC decay chan-
nel. In these crystals the half-life is reduced due to the
increased photon density of states in a dielectric, by a
factor equal to the index of refraction cubed. The half-
life in CaF
2
is 630(15) s [7, 15, 16], and 568(24) s in
LiSrAlF
6
[17]. Internal conversion occurs when the ex-
cited nucleus transfers energy to the electron cloud, eject-
ing a conversion electron, similar to the Auger effect. The
IC half-life is 7(1) µs for neutral
229m
Th [18–20], about
eight orders of magnitude faster than photo-emission. A
third decay channel, bound internal conversion, does not
play a significant role in the experiments discussed here.
Several measurements[15, 17, 21] of the thorium iso-
mer transition frequency based upon the photo-emission
decay channel have been published recently. The most
accurate reported value of 2020407384335(2) kHz, cor-
responding to 8.355733554020(8) eV, was achieved with
laser excitation of the isomer in a CaF
2
crystal, with the
laser referenced to a
87
Sr based optical atomic clock [22].
Because these measurements rely on decay via photo-
emission, the measurement cycles are relatively long
(hundreds of seconds).
Here we present a method of direct detection of the en-
ergy released by the IC decay and show that this much
