Abstract
Precise knowledge of the energy and lifetime of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">229m</sup> Th isomeric state has notable importance as a basis for a nuclear clock. Such a clock would be capable to extend precision on the oscillator frequency by up to four orders of magnitude compared to the presently best atomic clocks. However, the technique proposed for the clock requires that the isomeric state energy is accessible with existing laser systems. Previous measurement placed this state at ∼8 eV (150 nm), in the Vacuum Ultra Violet (VUV) range of the electromagnetic spectrum. A precise direct measurement of the energy of this state is necessary to determine whether the nuclear clock can be made using existing laser technology. We are developing a cryogenic microcalorimeter to measure the energy and lifetime of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">229m</sup> Th isomeric state directly. The experiment will use a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">233</sup> U source whose alpha-decay will populate the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">229m</sup> Th isomeric state with 2% probability. The subsequent decay of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">229m</sup> Th will be measured by a Transition Edge Sensor (TES) with <1 eV resolution. Such a technique will allow to observe all possible types of decays of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">229m</sup> Th in the range of energy from 3 to 50 eV and lifetimes >5 microseconds. The single-photon TES has sufficient resolving power combined with high efficiency in the whole energy band for this experiment. Here we present a prototype of TES based on a 200 nm thick iridium-gold (Ir/Au) film which was tested with a pulsed laser source and demonstrated ∼0.8 eV energy resolution and 5.8 ± 2.1 μs signal recovery time.
Highlights
T HE recent direct observation of the isomeric state 229mTh makes 229Th nucleus to be a realistic and the only candidate known to the date for nuclear clock developments [1], [2]
In this work we reported about the development and test of a single Ir/Au Transition Edge Sensor (TES) prototype for the experiment for direct detection of 229mTh isomeric transition
Such a technique will allow to observe all possible types of decays of 229mTh in the range of energy from 3 to 50 eV and lifetimes of >5 μs
Summary
T HE recent direct observation of the isomeric state 229mTh makes 229Th nucleus to be a realistic and the only candidate known to the date for nuclear clock developments [1], [2]. Precise direct measurement of 229mTh transition to the ground state can be achieved with the use of cryogenic microcalorimeters which, unlike conventional detectors, would be sensitive to radiative, and to the dominant internal electron conversion (IC) channel. We will use a 233U source deposited as a thin film on the active surface of a silicon detector for tagging the alpha-decay that produces 229mTh ions recoiling outwards the U film itself. These ions, occupying the isomeric state with 2% probability, will implant in the TES microcalorimeter [6]. We report on development of a first prototype of TES detector based on an iridium-gold bi-layer
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