As much I enjoy getting out in the field with my compass, most of my work involves using machines with funny acronyms. The coolest of these machines, in my opinion is the accelerator mass spectrometer or AMS.
What is an AMS?
|This is part of the 3 million volt accelerator mass spectrometer at the University of Toronto IsoTrace lab. I analyse my samples on this machine. (Photo: Matt Herod)|
|A typical mass spectrometer. This is one of the newer models of Agilent ICP-MS's. Computer and keyboard for scale. (www.agilent.com)|
For anyone not familar with mass spectrometry the principle is relatively straight forward. The sample is placed into the machine and is ionized. The charged atoms are then acceleration from the ion source and bent by a magnet. Once they have been deflected by the magnet they enter a detector where it is counted and then translated into usable data. The key part of mass spectrometry is the deflection of the ion by the magnet. The deflection of the particles is based on the mass difference between isotopes of the elements in question. For example, if I want to analyze a water sample for oxygen isotopes I would have to deal with three main isotopes of oxygen: oxygen-18, oxygen-17 and oxygen 16, named thus because that is how much they all weigh. This weight difference is caused by differing numbers of neutrons in the nucleus of each oxygen isotope. e.g. 18O has one more neutron than 17O. This weight difference is what causes the deflection by the magnet as the ion flies by. Oxygen 18 is heaviest so it gets bent the least whereas oxygen 16 is lightest so it gets bent the most. We then set up a detector and can detect how much of each isotope there is.
|Basic mass spectrometry diagram illustrating the principle of mass separation by an electromagnet. (www.physics.utoronto.ca/~istotrace)|
In principle AMS is very similar to this although it has a few more steps...
The first part of an AMS machine, as with any mass spec is the ion source. The sample is prepared in the lab and then the "target" is loaded into the ion source. AMS systems use a cesium ion source that is essentially a large gun that fires cesium ions at the target. Some of these cesium ions collide with the target, knock atoms of the sample off and ionize them. The ionized sample ions then fly out of the ion source and are then bent by an electromagnet. This removes some unwanted atoms. The ions are then enter the high voltage particle accelerator which contains a tube full of gas that pulls electrons off the ions and breaks apart molecules that could interfere with the detection. The, now positively charge ions, leave the accelerator and are then bent by another magnet into the high sensitivity detectors.
|Schematic of the AMS machine at IsoTrace in Toronto. This diagram is representative of all AMS systems. (www.physics.utoronto.ca/~istotrace)|
|The sample holder where the targets go. (Photo: M. Herod)|
|The 3 million volt accelerator with the stripper canal inside. The flight tube is entering on the left and is all negatively charged. (Photo: M. Herod)|
|Flight tube coming out of the accelerator. Everything is positively charged on this side. (Photo: M. Herod)|
|Ion source (Photo: M. Herod)|
|Flight tube with the rare isotope detector at the end. (Photo: M. Herod)|
AMS has a wide variety of applications in many fields of science. The primary one is the measurement of carbon-14, which is usually used for carbon dating. However, AMS has a wide variety of applications making it an extremely useful instrument in the geologist's arsenal. Up until recently most AMS machines were located in university physics departments and were used almost exclusively for particle physics research. Recently most physicists have lost interest in AMS's and the machines are starting to end up in geology departments around the world. This is opening new doors in applied AMS research and is turning decades old technology into a cutting edge field.
The reason that AMS is different and useful when so many other smaller, and cheaper mass spectrometry systems exist (a new AMS is ~7-8 million dollars) is that it allows for the analysis of rare isotopes that other systems cannot detect due to interferences from other elements. It also allows us to measure much smaller quantities than any other method and allows for the analysis of radioisotopes that cannot be detected using other methods such as decay counting.
Some of the major isotopes that most AMS machines around the world analyse for are: carbon-14, beryllium-10, aluminum-26, chlorine-36, calcium-41, iodine-129 and isotopes of uranium and plutonium. I'll talk about the uses of some of the major ones.
Beryllium-10 and aluminum-26 are isotopes that are used in the field of exposure age dating. Basically, when cosmic rays interact with the mineral quartz they produce 10Be and 26Al. The amount of each isotope and the amount of its radioactive decay products can tell us how long a rock has been exposed at the surface of the Earth. This is useful for estimating erosion rates, dating glacial events, dating landslides, and other stuff like that.
Carbon-14, the dating isotope. Everyone has heard about 14C dating. Basically anything on Earth that contains carbon has some radioactive carbon-14 as well. This means that we can date anything that contains carbon, but only if it less than 50,000 years. However, that encompasses almost all of human history so a lot of really interesting things can be dated this way. Everything from ancient trees and bones to picture frames and historical artifacts can all be dated with 14C. One way to measure 14C is decay counting in which the beta particles coming off the sample are counted and then dated, however, AMS provides a much faster and more sensitive way to detect 14C. This has made it the method of choice for anyone doing carbon dating.
Iodine-129, I could write a whole thesis about this one...oh wait. I'll just give the highlights. 129I has a long half life and is produced naturally and by human nuclear activities. Nuclear fuel reprocessing and nuclear bomb testing are the two major sources, but nuclear accidents such as Fukushima, Tomsk-7, etc. have contributed lots to the environment. The reason that we/I study is that understanding its movement in the environment is crucial for the future storage of nuclear waste. Therefore, we need to establish current levels, determine how it travels, and where it comes from. Also, as an emerging contaminant it is not a health risk yet, however, if concentrations continue to increase it could become worth regulating. AMS is the only way to analyse for 129I reliably at the moment.
Finally, the newest advances in applied AMS detection are to analyse for uranium and plutonium isotopes. These are such heavy and rare elements that it has always been problematic to analyse for them. However, new techniques are making it possible to detect Pu and U. This has applications in the burgeoning field of nuclear forensics. Basically, say a terrorist organization were to get its hands on some enriched uranium. It is very difficult to tell where the got it from, however, if the isotopic signature of the material can be ascertained it makes it much easier for investigators to determine where it came from.
To sum up I have explained only one small part of the rapidly growing intersection between geology and technology, however, I hope that you now have more of an appreciation for AMS technology and the powerful tool that it can be to solve real world problems. If you have any questions or comments please add them below.
Thanks for reading,
Ragnar Hellborg and Goran Skog (2008). Accelerator Mass Spectrometry Mass Spectrometry Reviews (27), 398-427 DOI: 10.1002/mas.20172
IsoTrace Laboratory: http://www.physics.utoronto.ca/~isotrace/
PRIME Lab: http://www.physics.purdue.edu/primelab/