Radioactive Iron - An Astrophysical Clock for Nucleosynthesis
Author: Anton Wallner1,2
1Dept of Nuclear Physics, Australian National University, Canberra, Australia,
2VERA Laboratory, Faculty of Physics, University of Vienna, Austria.
Massive stars may end their life in a supernova explosion - one of the most violent events in our galaxy. Supernovae are thus massive exploding stars that return a large fraction of the star’s material back to the interstellar medium. Nucleosynthesis in massive stars shapes therefore the elemental abundance pattern and the galactic chemical evolution, e.g. our solar system is the product of many preceding star generations .
Extraterrestrial material in the form of interstellar dust can also enter the solar system and may be deposited on Earth . Their nucleosynthetic history is locked in its isotopic signatures. Interstellar matter will contain stable isotopes but also freshly produced radionuclides. Thus, the existence of fresh radionuclides in the interstellar medium serves as radioactive clocks for their recent production.
Radioactive iron-60 (Fe-60) is a radionuclide with a half-life of about 2 million years. It is predominantly formed in massive stars at the end of their lives just before and during a supernova and then distributed by the explosion into the interstellar space. Fe-60 is thus an ideal candidate to monitor supernova explosions and recent element synthesis.
Since this radioactive iron is not naturally present on Earth, trace amounts of this isotope are a particularly sensitive astrophysical marker. Supernova-produced iron from the interstellar medium can be captured by the Earth on its way through the Milky Way. If one finds this radioactive iron-60 in the terrestrial environment (apart from artificial production), it must come from cosmic explosions; more precisely from the last few million years, otherwise it would have long since decayed.
With its half-life in the million year range, Fe-60 is suitable for dating astrophysical events, such as supernova explosions. The usability of this isotope, in particular as an astrophysical clock, was however limited, because the lifetime of this nuclide was not exactly known - an important prerequisite to serve as a chronometer. There were two measurements so far, one from 1984  and another very precise one from 2009 , but both were almost a factor of 2 different.
Iron-60 – a monitor for element synthesis and nearby supernova explosions
This isotope has a variety of applications in astrophysics. The main reason is, it is observed in space through its radioactive decay and it is not naturally present on Earth.
Researchers can virtually monitor live nucleosynthesis in massive stars, e.g. active regions of element formation and also the distribution of ejected stellar material in the Milky Way. Iron-60 can be observed directly in our Milky Way via space-born satellites through its decay and the characteristic radiation emitted (similar to another radioactive isotope, Al-26) [5,6]. These observations clearly demonstrate its presence in the interstellar medium. Such radionuclides were produced 'recently", i.e. within a few half-lives. As their decay is observed, one needs the half-life to calculate the number of atoms present in the interstellar medium.
Knie et al., in a pioneering work at the Technical University of Munich, Germany, found Fe-60 at the ocean floor in a manganese crust indicating a possible near-Earth supernova activity about 2 to 3 million years ago [7,8]. Iron-60 was present at the birth of our solar system, more than four billion years ago. This is evidenced today in pre-solar material by overabundances of Fe-60’s decay products .
Establishing a connection between these observations of the radioactive decay of Fe-60 and the number of iron-60 atoms, however, requires a precise knowledge of its life-time, that is, its half-life.
How to measure a half-life of millions of years?
Firstly, one needs a sufficient number of atoms. We, a team of scientists from Australia, Switzerland and Austria  used artificially produced iron-60 extracted from nuclear waste of an accelerator facility in Switzerland. This iron fraction was separated by specialists in Switzerland and then analyzed for its Fe-60 content. The number of radioactive atoms must be measured in absolute terms, and this is a difficult task and was probably the reason for the discrepancy in earlier measurements.
We used a very sensitive method to accurately determine the low number of Fe-60 atoms in their sample: accelerator mass spectrometry (AMS) [11,12], a technique that counts atoms directly and that is used for example, also for radiocarbon dating. The Fe-60 measurements were carried out at the Heavy Ion Accelerator Facility at the Australian National University in Canberra, one of the world's most sensitive facilities to detect tiny traces of rare elements in our environment. With this extremely sensitive facility no background could influence our results. Further, we counted Fe-60 relative to another radioactive iron isotope, namely Fe-55. Fe-55 is well known and easier to measure. By using the same measurement setup for Fe-60 and Fe-55, we are confident that potential unknown errors were minimized in our work.
The new value for the half-life of Fe-60  shows a good agreement with the precise measurement by Rugel et al. from the year 2009 . According to our result, they had done a very good job! Combining both measurements, this allows now the use of Fe-60 as a precise cosmic clock. It eliminates a long-standing discrepancy and thus establishes this radionuclide as a precise astrophysical chronometer.
As another additional outcome we encourage other groups to repeat such kind of measurements. With respect to the difficulty of performing measurements of long half-lives, independent and complementary techniques are essential for settling open and difficult-to-solve questions.
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