The Gas-Filled-Magnet at PRIME Lab: Increased Sensitivity of Cosmogenic Nuclide Measurements

Abstract:

Abstract: Using accelerator mass spectrometry (AMS), radionuclides produced either by cosmic-ray interactions or by nucleogenic means can be measured. Typical isotopic abundance ratios range from 1 x 10^-10 to 1 x 10^-15. The routinely measured radionuclides are ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, and ¹²⁹I. Be-10, ²⁶Al, and ³⁶Cl have isobaric interferences that cannot be eliminated mass through mass analysis, but dE/dx techniques suppresses these isobars enough to allow successful measurements. There are compromises, the isobar for ²⁶Al, ²⁶Mg, precludes successful measurement of ²⁶Al if AlO^- is injected into the accelerator. Mg^- doesn’t form a stable negative ion so a ²⁶Al measurement requires injection of ²⁶Al^-. But the Al^‑ ion is formed inefficiently; secondary ion currents using Al^- are ~ 10 times less than an AlO^- secondary ion beam. Precision scales with count rate so precise measurement of the ²⁶Al/Al for all but higher ratio samples is difficult. It has long been recognized that a gas-filled-magnet (GFM) could potentially improve the measurement of those radionuclides with intractable isobar interferences. A GFM works on the principle that each element of an isobar pair, e.g. ²⁶Mg and ²⁶Al, has a different average charge state as it traverses a gas (3-4 Torr of N₂) contained within the vacuum jacket of a magnet. The magnet steers each species with its own momentum-to-charge ratio on its own distinct radius of curvature. The magnet can be tuned to allow the isotope of interest into a dE/dx detector; most of the isobar doesn’t make it into the detector. Using the PRIME Lab GFM we are now able to routinely run ²⁶Al with a precision that is comparable to that obtained with ¹⁰Be. We are also using the GFM for routine measurements of ¹⁰Be and ³⁶Cl. Although the improvement for these nuclides is not as pronounced as it is for ²⁶Al, the GFM has improved the detection sensitivity for both. Our ¹⁰Be background is now ~ 5 x 10^-16 and for ³⁶Cl we can now run the source more efficiently since we reject ³⁶Cl. In addition to improving ¹⁰Be, ²⁶Al, and ³⁶Cl measurements the GFM may allow the measurement of radionuclides not traditionally measured, most notably ⁵³Mn.