Accelerator Mass Spectrometry
Accelerator mass spectrometry (AMS) is an experimental method for detecting the presence of rare isotopes of exceedingly small concentration within a macroscopic-sized sample of material. The method of measurement takes advantage of well-known techniques of charged particle identification developed in the long history of experimental nuclear physics. There are three essential ingredients within an AMS facility:
- An ion source, that ionizes the atoms within the sample material into a beam of charged partilcles
- An accelerator that receives the charged particles and accelerates them to high kinetic energy
- A charged particle detector that detects, counts and uniquely identifies each charged particle it receives from the accelerator
The types of isotopes sought in AMS measurements are typically those that have sufficiently long half lives to be of use for archaeological, geophysical, and astrophysical significance. If a natural process creates enough quantity of an isotope of sufficiently long half life, then dating of geological events and processes that occurred millions of years ago is possible.
Wissenschaftliche Beispielen
As an example in geophysics and climate science, such techniques can be used to determine when the ice sheets presently covering Antarctica began their growth in the Earth's long distant past. The isotopes 10Be and 53Mn can be used for such purporses because their half lives are 1.39 Myr (doi:10.1016/j.nimb.2009.09.020) and 3.7 Myr, respectively. Both half lives are much shorter than the age of the Earth (about 4500 Myr), so one normally would not expect either to exist on Earth. They are produced, however, by nuclear reactions induced by cosmic rays impinging on iron atoms contained in rocks. When the rocks lay bare to the atmosphere, the cosmic rays are able to induce these nuclear reactions as they pass through the rocks. If the rate of the reactions producing these isotopes is constant over time, eventually an equilibrium abundance of these istopes in the rocks will occur (the number decaying per unit time is equal to the number of new isotopes being produced per unit time). However, should these rocks become covered in thick layers of ice, such as the case in Antarctica, then the cosmic rays are unable to penetrate the thousands of meters of overlying rock. At that point, the production of these isotopes in the rocks ceases. Those atoms of 10Be and 53Mn in the rocks, however, continue to undergo radiactive decay. If the production rate before ice coverage is known, and if we can measure the amount of 10Be and 53Mn in those rocks today, then the known half lives allow us to determine when the rocks became covered in ice. Through AMS, the concentrations of 10Be and 53Mn in the rocks can be determined, thus providing the age of the onset of ice coverage in the Antarctic.
The AMS facility here at TUM was also instrumental in having discovered a direct supernova-Earth interaction that occurred ≈3 Myr ago. The exploding star was close enough to our solar system to have deposited the isotope 60Fe on our planet; it was also far enough away that the evolutionary ancestors to our species were not wiped out by it, allowing us to still be here. Radioactive 60Fe has a newly revised half life of 2.62 Myr [G. Rugel at al., Phys. Rev. Lett. 103, (2009) 072502; doi:10.1103/PhysRevLett.103.072502], and it is produced at levels of 10-5 solar masses within massive stars that end their lives as core collapse supernovae and by Type 1a supernovae, which are white dwarf remnants that exceed the Chandrasehkar mass limit, causing a thermonuclear runaway in their core that consumes the entire white dwarf. Indeed, so far as we know in astrophysics, the only site in the cosmos responsible for producing 60Fe in large quantities are these supernovae, and direct observational studies have confirmed the presence of 60Fe in the plane of our galaxy; a known site for massive stars. The atoms of 60Fe were discovered, using our AMS facility, in a ferromanganse crust pulled up from 4000 m down in the Pacific ocean. The measaured 60Fe atom concentration was 60Fe/Fe1 ≈ 2×10-15 [K. Knie et al., Phys. Rev. Lett. 93, (2004) 171103; doi:10.1103/PhysRevLett.93.171103], and demonstrates the extremely high sensitivity of this AMS facility. It also proves that our planet was subjected to the debris of one of the most violently explosive and energetic phenomena that occur in our universe.
Building on these discoveries, we are now beginning a research program to search for this 60Fe supernova signature in the micro-fossil record of our planet. You can read more about this here.
A more complete publications history of the AMS group can be found here.