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QST measures the half-life of nuclear cosmochronometer Lutetium-176: Determined to be 37.19 billion years


Radioactive isotopes with long half-lives ranging from tens of millions to tens of billions of years are called cosmic nuclear clocks and are used to measure the age of cosmic events and the time between their occurrence and the present. Ruthenium-176 is used as a cosmic clock owing to its half-life of approximately 40 billion years before it decays into Hafnium-176 via beta decay. However, there has been an issue with differing half-lives observed in multiple experiments.

A research team led by Senior Principal Researcher Takehito Hayakawa, Senior Principal Researcher Toshiyuki Shizuma of the Kansai Institute for Photon Science at the National Institutes for Quantum Science and Technology (QST) and Associate Professor Tsuyoshi Iizuka of the Graduate School of Science at the University of Tokyo, has measured the most accurate half-life of the long-lived radioactive isotope Lutetium-176, a cosmic nuclear clock, using a new experimental method. This team solved the inconsistency in half-lives measured in the past. In the future, it is expected that research on the formation ages not only of the Earth but also of various celestial bodies in the solar system and the formation ages of the Earth's crust will be advanced. This study was published in Communications Physics.

In previous half-life measurement methods, the number of times beta decay occurred was determined by counting the number of gamma and beta rays emitted from a pre-prepared Lutetium sample within a certain time period. Generally, the number of measured radiations is less than the number of beta decays because the detector cannot measure all the radiation emitted from a sample. Therefore, it is necessary to obtain a coefficient for determining the number of beta decays from the number of measured radiations through simulation calculations or calibration experiments.

In the past, nuclear physics and nuclear engineering experts performed such measurements of radiations with varied results, ranging from approximately 32 to 42 billion years. Unable to rely on these results, specialists in astro-planetary science conducted measurements using meteorites and earth rocks. However, these outcomes still exhibited a degree of variability, with estimations in the range of 34 to 38 billion years.

The research team devised a new measurement method and used it to measure Lutetium-176 for the first time. In this method, a Lutetium sample is placed inside a scintillation crystal, which constitutes a radiation detector, and the total energy of the radiation emitted from the sample is measured. When beta decay occurs, gamma and beta rays are always converted into light in the scintillation crystal, regardless of their directions, and all the energy of the incident radiation can be measured by detecting the light using a photomultiplier tube. Therefore, calibration from the number of radiations to the number of beta decays is not required.

Hayakawa commented, "This measurement method has existed as a concept since around 1950, but it hasn't applied because of technical difficulties. The measurement is difficult to perform because the sample must be assembled every time it is placed inside the scintillation crystal, which must be shielded to prevent external radiation from entering."

Using this measurement method, the half-life of Lutetium was determined to an unprecedented accuracy of 37.19 billion years (with an error of 0.07 billion years). There are other radioactive isotopes used as cosmic nuclear clocks, and some of them have large half-life errors. "This detector is not usable; however, I will accurately measure the half-life of either Rubidium or Uranium in the future. As it takes approximately 10 years to measure just one, I think this research project will last my lifetime as a researcher," said Hayakawa.

By measuring the amount of Lutetium-176, and its daughter nucleus Hafnium-176, contained in samples such as meteorites, the time elapsed from the formation of the meteorite's parent body to the present can be determined.

Iizuka added, "Now that we know the exact half-life, we can more precisely discuss the evolution of the solar system and terrestrial planets."

This article has been translated by JST with permission from The Science News Ltd. ( Unauthorized reproduction of the article and photographs is prohibited.

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