Accelerator Mass Spectrometry at Purdue University PRIME Lab
Prof. Marc Caffee1,2, Dr. Greg Chmiel1, Dr. George Jackson1, Prof. Darryl Granger2, Prof. Nat Lifton1,2, Dr. Lan Luo1, Prof. Paul Muzikar1, Dr. Thomas Woodruff1
1Department of Physics and Astronomy, Purdue University, West Lafayette, United States, 2Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, United States
The Purdue Rare Isotope Measurement Laboratory (PRIME Lab) is a dedicated AMS research facility measuring ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ¹²⁹I. PRIME Lab is an NSF facility providing measurement and sample preparation services and expertise to the US NSF community. In addition to performing AMS measurements of the radionuclides ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, and ¹²⁹I in terrestrial materials we provide chemical extraction techniques for these radionuclides. PRIME Lab measurements address many scientific issues, including polar ice sheet stability, landform evolution, solar activity, and human evolution. We also perform AMS measurements of extra-terrestrial materials for NASA researchers, obtaining cosmic ray exposure ages of meteorites and investigating lunar regolith processes. For extra-terrestrial samples we measure ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca; we anticipate running ¹²⁹I in meteorites in the near future. We are looking forward to measuring radionuclides in Hayabusa and Osiris returned samples.
The PRIME Lab accelerator is an FN accelerator that has been upgraded with a pelletron charging system. The low-energy transport system has a high-intensity ion source for AMS; the sample changer is our own design and holds 64 cathodes. The high-energy transport system consists of an analysis magnet and a switching magnet. A Wien filter is between the analysis magnet and switching magnet. There are two AMS beam-lines: a 45° port (R45) off the switching magnet and a 30° port (R30) off the switching magnet. The R45 beamline has two Wien filters and a gas-filled-magnet (GFM) detector system. The R30 beamline has a 15° ESA and a dE/dx detector.
Be-10 (injected as BeO-), ¹⁴C (C-), ²⁶Al (AlO-), ³⁶Cl (Cl-), and ⁴¹Ca (CaF-) are all measured using the GFM system on the R45 beamline. Iodine-129 (I-) measurements are made on the R30, although we occasionally measure ¹⁴C (+3) on this beamline. Pure gas stripping is used for Be charge state (+3) and I (+5) and foils are used for Al (+7), Cl (+8), and Ca (+8). We are currently experimenting with combined gas/foil stripping for Al. Preliminary measurements indicate a higher transmission.
Much of our recent work has focused on continued improvements in ²⁶Al measurements. The GFM has produced substantial improvements in ²⁶Al measurement precision. Before the GFM we injected Al- and typical currents were .5 to 2 μA. With the GFM we now inject AlO-, MgO- is also injected but the GFM suppresses Mg in the detector. The Al currents measured at the high-energy cup are variable, but range from 5 to 15 μA. We have also made improvements for Al in the ion source. Using Nb as a binder for Al cathodes results in less ion source memory; typical backgrounds are < .5 x 10-¹⁵. We are also experimenting with gas plus foil stripping for Al; preliminary measurements indicate increased yield. The GFM has enabled measurements of ²⁶Al that were previously not possible some years ago, it is likely that additional development will produce additional improvements.
This work has been funded primarily by NSF and NASA.
Marc Caffee is the director of the PRIME Lab. He has studied cosmogenic nuclides in solar systems materials for over 40 years, starting with cosmogenic noble gases in meteorites. He has worked in AMS for over 30 years. He has applied cosmogenic radionuclides to a variety of solar system materials, including meteorites, lunar soils, and terrestrial rocks. During this time he has been involved in both development of instrumentation and applications.