Sputter-pits casting to measure AMS sample consumption

Increased 14C AMS Efficiency from Reduced Competitive Ionization

“…We confirm the effect discovered by Shanks and Freeman (2015) that sample wells in metals with low EA increase C – ion currents at small well ⌀ over Al. We used solid metal inserts, while they press powder in a recess into which they drill the sample well.…”

“…The amount of competitive ionization from Al 2 depends on the primary Cs focus, ~2 mm behind the sample holder surface in our case, and the width of the primary beam, found to be 1.2 mm by the fits in Figure 5a. The “pit casting” done by Shanks and Freeman (2015) show their focus to be 1 mm deeper than the surface, allowing excess Al sputtering. Location of the primary focus at the well entrance (Han et al 2007) has three advantages: maximal impact of the Cs + on the sample for all sample ⌀, erosion of the sample in a ring pattern (keeping the maximum of the ion beam on axis), and minimizing sputtering of the holder material.…”

“…Sputtered Cu 0 atoms are in direct competition with C 0 for capturing the Cs(7s,p) electrons and halve the C – ionization. Shanks and Freeman (2015) confirm this halving of current by Cu. Such “competitive” ionization explains many mysteries of ion “sorcery” of the last 3 decades.…”

“…Demonstration of 35% ionization efficiency in reaching C – beams of >300 µA shows that a much more efficient physical process is at work in most CSIS used in accelerator mass spectrometry (AMS) for 14 C measurements (Fallon et al 2007). The recent report of a 80 µA/mm 2 C – current density from a 0.5-mm ⌀ (diameter) sample compared to 20 µA/mm 2 from 1-mm ⌀ confirms that anion production does not scale as surface area (Shanks and Freeman 2015). This example shows an equal C – current from a 1-mm ⌀ sample well in an aluminum holder as from a sample in a 0.5-mm-diameter well that had 0.25-mm-thick iron coat over the aluminum walls of the sample recess.…”

“…Shanks and Freeman (2015) demonstrate that small C samples, recessed in 0.5-mm ⌀ wells having Fe-coated walls, produce as much C – current as larger samples in 1-mm ⌀ wells in Al holders. We attribute this to competitive ionization for Cs(7s) by sputtered Al 2 and by Al ionizing states above Cs(7p), reducing its production from cascade radiation, as indicated in Figure 2 (Vogel 2015).…”