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2Physics Quote:
"The exchange character of identical particles plays an important role in physics. For bosons, such an exchange leaves their quantum state the same, while a single exchange between two fermions gives a minus sign multiplying their wave function. A single exchange between two Abelian anyons gives rise to a phase factor that can be different than 1 or -1, that corresponds to bosons or fermions, respectively. More exotic exchanging character are possible, namely non-Abelian anyons. These particles have their quantum state change more dramatically, when an exchange between them takes place, to a possibly different state." -- Jin-Shi Xu, Kai Sun, Yong-Jian Han, Chuan-Feng Li, Jiannis K. Pachos, Guang-Can Guo
(Read Full Article: "Experimental Simulation of the Exchange of Majorana Zero Modes"

Sunday, March 15, 2015

Radioactive Iron - An Astrophysical Clock for Nucleosynthesis

Anton Wallner (photo credit: Stuart Hay, ANU)

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 [1].

Extraterrestrial material in the form of interstellar dust can also enter the solar system and may be deposited on Earth [2]. 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 [3] and another very precise one from 2009 [4], 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 [9].

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 [10] 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.

Figure caption: Identification spectra with a clear separation of the main background Ni-60 from Fe-60: single atom counting of Fe-60 at the ANU - each point represents a single atom. Combining up to 5 different detector signals results in an unsurpassed sensitivity of Fe-60/Fe = 4 X 10-17 (A. Wallner et al., [10]).

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 [10] shows a good agreement with the precise measurement by Rugel et al. from the year 2009 [4]. 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.

[1] R. Diehl, D.H. Hartmann and N. Prantzos (eds.), "Astronomy with Radioactivities", Lecture Notes in Physics, vol. 812, Springer, Berlin (2011). Google Books Preview.
[2] A. Wallner, T. Faestermann, C. Feldstein, K. Knie, G. Korschinek, W. Kutschera, A. Ofan, M. Paul, F. Quinto, G. Rugel, P. Steier, "Abundance of live 244Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis", Nature Communications, 6:5956; DOI: 10.1038/ncomms6956 (2015). Full Article.
[3] Walter Kutschera, Peter J. Billquist, Dieter Frekers, Walter Henning, Kenneth J. Jensen, Ma Xiuzeng, Richard Pardo, Michael Paul, Karl E. Rehm, Robert K. Smither, Jan L. Yntema, Leonard F. Mausner, "Half-life of 60Fe", Nuclear Instruments and Methods in Physics Research, Section B, 5, 430 (1984). Abstract.
[4] G. Rugel, T. Faestermann, K. Knie, G. Korschinek, M. Poutivsev, D. Schumann, N. Kivel, I. Günther-Leopold, R. Weinreich, and M. Wohlmuther, "New Measurement of the 60Fe Half-Life", Physical Review Letters, 103, 072502 (2009). Abstract.
[5] W. Wang, M. J. Harris, R. Diehl, H. Halloin, B. Cordier, A.W. Strong, K. Kretschmer, J. Knödlseder, P. Jean, G. G. Lichti, J. P. Roques, S. Schanne, A. von Kienlin, G. Weidenspointner, and C. Wunderer, "SPI observations of the diffuse 60Fe emission in the galaxy", Astronomy & Astrophysics, 469, 1005 (2007). Abstract.
[6] Roland Diehl, "Nuclear astrophysics lessons from INTEGRAL", Reports on Progress in Physics. 76, 026301 (2013). Abstract.
[7] K. Knie, G. Korschinek, T. Faestermann, E. A. Dorfi, G. Rugel, A. Wallner, "60Fe Anomaly in a Deep-Sea Manganese Crust and Implications for a Nearby Supernova Source", Physical Review Letters, 93, 171103 (2004). Abstract.
[8] C. Fitoussi, G. M. Raisbeck, K. Knie, G. Korschinek, T. Faestermann, S. Goriely, D. Lunney, M. Poutivtsev, G. Rugel, C. Waelbroeck, A. Wallner, "Search for Supernova-Produced 60Fe in a Marine Sediment", Physical Review Letters, 101, 121101 (2008). Abstract.
[9] A. Shukolyukov, G.W. Lugmair, "60Fe in eucrites", Earth and Planetary Science Letters, 119, 159 (1993). Abstract ; A. Shukolyukov, G.W. Lugmair, "Live iron-60 in the early solar system", Science, 259, 1138 (1993). Abstract.
[10] A. Wallner, M. Bichler, K. Buczak, R. Dressler, L. K. Fifield, D. Schumann, J. H. Sterba, S. G. Tims, G. Wallner, W. Kutschera, “Settling the half-life of 60Fe – fundamental for a versatile astrophysical chronometer”, Physical Review Letters, 114, 041101 (2015). Abstract.
[11] Hans-Arno Synal, "Developments in accelerator mass spectrometry", International Journal of Mass Spectrometry, 349–350, 192 (2013). Abstract.
[12] Walter Kutschera, "Applications of accelerator mass spectrometry", International Journal of Mass Spectrometry, 349–350, 203 (2013). Abstract.

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