Accelerator Mass Spectrometry

AMS is continuously evolving into an even more powerful and sophisticated technique. New isotopes, new techniques, and new applications are being developed. The AMS radionuclides can be used for a wide variety of dating and tracing applications in environmental and biological sciences, in the study of ocean circulation patterns, in radioactive waste, nuclear safeguards and nuclear physics. One of the best known applications is radiocarbon ¹⁴C dating. Two features make a tandem accelerator the preferred instrument for AMS. First, the requirement of negative ion beams for injection eliminates the interference of some stable isobars (¹⁴N, ³⁶Ar, ¹²⁹Xe) which do not form negative ions that would impair the detection of the radioisotopes of interest (¹⁴C, ³⁶Cl, ¹²⁹I). Second, the stripping of electrons at the high voltage terminal eliminates molecular interference.

The most interesting radioactive ion beams (RIBs) produced for astrophysics and nuclear physics involve generally rather short-lived species such as ¹⁷F (T_1/2=64.5 s) and ¹⁸F (T_1/2=110 m). By contrast, one of the primary objectives of AMS is the measurement of long-lived radioisotopes produced in natural materials by the interaction of cosmic rays (half-lives from a few years to millions of years). From this perspective it would seem at first sight that AMS and RIB production are very different fields. However, there are a number of similarities between these two fields [3, 4]. The removal of interfering isobars is one of the several common challenges of both AMS and RIB production. A facility such as HRIBF has a variety of equipment choices for beam transport and analysis and for rejection of unwanted species. Both AMS and RIBs will benefit from the use of the most efficient techniques for production, isobar separation, transport, and detection.

The HRIBF Tandem Accelerator is the highest operating voltage electrostatic accelerator in the world. Tandem voltages up to 25.5 MV have been used in experiments. The HRIBF 25 MV Tandem Accelerator is capable of producing beams of 0.1-10 MeV per nucleon for light nuclei and up to 5 MeV per nucleon for mass 80. There are a number of isotopes of interest for AMS where the high energies achieved represent a very important advantage for their detection. For example, the beam energies are sufficiently high to strip all electrons from a good portion of the ions up to about mass 50. Isobars are then produced in different charge states that can be separated by beam line analyzers. This guarantees a strong suppression of background events originating from lighter isobars. Furthermore, the HRIBF offers a variety of equipment such as high-resolution momentum analyzers (including one internal to the machine), electrostatic analyzers, plus a vast array of particle detection devices. We have developed additional detection systems oriented to heavy isotopes such as gas Bragg Detectors. The higher energies achieved will facilitate the isobar separation for the heavy species. The various research tasks include the establishment of beam diagnostic procedures for the low- and high-energy ends of the machine and the study of the stability and reliability of operation of the tandem for extremely low beam intensities. In particular, the ion source, beam optics, terminal voltage stabilization, and detection systems are extremely important in AMS.

Recently we demonstrated the unique AMS capabilities of HRIBF by measuring ultra-low ratios of 36Cl/Cl in seawater samples (i.e., some of the lowest levels on Earth!) [5,6]. We have successfully measured ³⁶Cl/Cl ratios as low as few times 10^-16, pushing the limits of sensitivity of this important isotope by an order of magnitude over that of any other facility in the world and proving that ³⁶Cl can be measured at levels required for a tracer in oceanography. Chlorine-36, with a half-life of 301,000 years, is a by-product of reactor operations, and nuclear tests. It is produced by neutron activation of ³⁵Cl, which is present in trace quantities in all reactor and natural materials. A promising application of AMS is in the area of nuclear safeguards. We would like to further explore the capabilities of the 25-MV Tandem accelerator to do AMS at the lowest concentration levels by applying the technique for simultaneously determining the absolute exposure age and erosion rate of a rock based on the special depth dependence of ³⁶/Cl produced from neutron capture of ³⁵Cl. This application is expected to have low concentrations of ³⁶Cl and therefore the need for the highest sensitivity provided by the HRIBF AMS setup. Cosmogenic nuclides, both stable (e.g., ³He) and radioactive (e.g., ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl), are produced in rocks at the surface of the Earth by interaction of secondary cosmic rays, mainly neutrons, protons, and muons, with constituent nuclei in rocks. In the top meter, production is mainly by neutron-induced spallation and neutron-capture reactions.

We believe we can push even further the limits of ³⁶Cl sensitivity by applying a technique based on selective nonresonant laser photodetachment of unwanted negative ion species to efficiently suppress isobaric contaminants in negative ion beams [7, 8]. The use of a CW laser beam collinear with a low-energy ion beam in a gas-filled radio frequency quadrupole (RFQ) ion guide will help remove interfering isobaric ions at an early stage of an AMS system very efficiently by dramatically increasing the interaction time of the ions with the laser. The main idea is to remove electrons from the negative ion of ³⁶S (EA=2.08 eV) without affecting the beam of ³⁶Cl (EA=3.62 eV). If successful this method of photodetachment will facilitate AMS measurements of ³⁶Cl with much smaller tandem accelerators. It would also allow the measurements of ³⁶Cl samples with the 25-MV Tandem to be shortened by enabling us to go to lower (and more abundant) charge states. We believe we can further extend the limits for this and for other isotopes of relevance to the areas of nuclear forensics and nuclear safeguards [9].

Long-lived by-products of neutron capture on uranium isotopes such as ²³⁶U, ²³⁷Np, and ^239,240Pu are of particular interest because they are only present in the environment at detectable levels as a result of human activities, and their relative abundances leave a detailed and lasting record of the nature of their production. These nuclides are particularly amenable to counting atoms (by AMS) rather than detecting decays (by alpha counting) because

1. there are no stable or long-lived isobars which will give a background at the same mass;
2. when decay counting, their half-lives are long so the fraction that decays in a reasonable counting time such as one day is very small; and
3. when decay counting, the alphas from the decay of ^239,240Pu are difficult to distinguish because their energies are almost identical.

1. use of a sputter source which creates ions by a surface effect and is much less susceptible to contamination by previously measured samples than sources which create ions in a plasma volume;
2. a high-resolution magnet before injection into the accelerator which does not "leak in" the neighboring mass, even at mass 300;
3. a magnet at the high-voltage terminal of the folded geometry tandem ensures that a single charge state is accelerated on the high energy side, eliminating backgrounds from molecular fragments which might otherwise undergo charge-changing collisions and end up with the right magnetic rigidity.

The 25 MV Tandem Accelerator of the HRIBF offers a powerful tool for AMS. The necessary elements exist at ORNL for an AMS program: 25 MV Tandem, negative ion-source technology, accelerator physics, mass spectrometry, detector development, and research interests. We will focus on areas where the high voltage of the tandem plus the specialized instrumentation we have (or will develop) can play a unique role in AMS. Our results indicate that it is possible to use HRIBF as a prototyping facility to aid in the development of new AMS methods.

References

[1] H.W. Lee, A. Galindo-Uribarri, et al., Nucl. Instr. And Meth. B 5 (1984)208.
[2] A. Galindo-Uribarri et al., J. Chem. Phys. 83 (1985) 3685.
[3] A. Galindo-Uribarri et al., Nucl. Instr. and Meth. B 172 (2000) 647.
[4] P.A. Hausladen, et al., International Journal of Mass Spectrometry 251, 2-3(2006)119.
[5] J. P., Doupe, J.P. (2004) PhD Thesis, Department of Geology, University of Toronto.
[6] A. Galindo-Uribarri et al., Nucl. Instr. and Meth. B 259 (2007) 123.
[7] Y. Liu et. al., Appl. Phys. Lett. 87 (2005) 113504.
[8] T. Lewis, A. Galindo-Uribarri, Y. Liu and C. Havener, http://meetings.aps.org/link/BAPS.2007.DNP.BC.8
[9] DOE/SC-0062 Report on the Workshop on the Role of the Nuclear Physics Research Community in Combating Terrorism available online at http://www.sc.doe.gov/np/homeland/CombatTerrorismFinal110602.pdf where the potential of AMS in forensics is clearly recognized.

Collaborators

A. Galindo-Uribarri (ORNL), J. Beene (ORNL), P. Hausladen (ORNL), C. Havener (ORNL), Y. Liu (ORNL), M. Meigs (ORNL), P.E. Mueller (ORNL), D.W. Stracener (ORNL), R. Vane (ORNL), B. Fuentes (FC-UNAM, Mexico), E. Padilla-Rodal (ICN-UNAM, Mexico), J.P. Doupe (Alberta, Canada), A.E. Litherland (IsoTrace, Toronto, Canada), M. G. Clark (Geology, Tennessee)