Isobar Attenuation Using Anion–Gas Reactions for Accelerator Mass Spectrometry and Application to ³⁶Cl

Abstract

Preliminary tests of a prototype radio‐frequency quadrupole (RFQ) collision cell system, known as an isobar separator for anions (ISA), for the removal of isobaric interferences for accelerator mass spectrometry (AMS) and for studies of anion–gas interactions are reported. The ISA decelerated a mass‐analysed beam of anions from an energy (∼ 20 keV) typically generated by an AMS ion source to < 10 eV. RFQs and electrostatic lenses then guided the ions through the collision cell where ion‐gas collisions reduced both the energy and energy spread of the ion beam (cooled the ions) and ion‐gas reactions attenuated most of the unwanted isobars. The anions were then re‐accelerated to their original energy for injection into the rest of the AMS system. With the ISA installed on a full 3 MV AMS system, attenuations of 32S‐, 12C3‐ and 39K‐ by six, seven and greater than ten orders of magnitude, respectively were achieved using 0.7–1 Pa NO2 gas in the collision cell, while maintaining approximately 10–30% of the chlorine anion transmission. A further measurement of a 36Cl/Cl = 4.1 × 10‐11 RM is also described. The results suggest that the 36Cl/Cl lower detection limit of the current system was 10‐14–10‐15 for samples that could be prepared with S/Cl ratios below 10 μg g‐1.

On‐line HfF5−/WF5− separation in an O2‐filled radiofrequency quadrupole gas cell

“…Other system details and experimental procedures were identical to those described earlier. [13][14][15][16] RESULTS AND DISCUSSION Figure 2 shows the Hf and W beams transmitted through the test isobar separator containing varying concentrations of NO 2 gas. NO 2 is both an electronegative gas and a radical.…”

“…The total attenuation was close to 3 orders of magnitude compared with the 4-6 orders of magnitude achieved for several other pairs. [13][14][15][16] The absolute transmission of HfF 5 À corresponding to the highest WF 5 À attenuation was~75%. This is encouraging, as it indicates that the losses of HfF 5 À in O 2 could be prevented completely, if the ion energy throughout the RFQ gas cell could be much more finely controlled than was possible with the test isobar separator used.…”

“…It is the first such integrated ISAMS setup, and it has been successfully used to demonstrate large separations of a number of isobar pairs. [13][14][15][16] Various configurations of the original isobar separator have also been investigated to study the ion transport efficiency and to provide some ion energy control, which led to a temporary test configuration of the isobar separator [16] used in this work. To minimize losses of the fragile analyte anions by electron detachment or collisional fragmentation, it is an objective to prepare the anions captured by the RFQ ion guide with a small average energy and also a small energy spread, prior to significant gas collisions.…”

“…This procedure was proven to be successful for lighter superhalogen anions and has now been tested for HfF 5 À / WF 5 À separation using a variant of the apparatus under (2) an RFQ gas cell wherein isobars are separated by aniongas reactions at eV energies; and (3) a constant voltage reaccelerator to extract the separated analyte anions and accelerate them to the original keV energies for the subsequent AMS analysis. In recent years, this experimental device with various configurations has been shown to have great power for separating isobars for AMS measurement of 36 Cl, [12][13][14] 90 Sr and 135 Cs, [15] and recently 41 Ca. [16] In this report, following a brief description of the particular setup used for these initial HfF 5 À /WF 5 À studies, the preliminary results obtained using two different gases, NO 2 and O 2 , will be presented, as these results have relevance to the further development of such apparatus for the detection of 'live' 182 Hf in nature.…”

The ILIAS project for selective isobar suppression by laser photodetachment

“…Information on isobar suppression by anion-gas reactions in reaction cells and their application in AMS can be found e.g. in [4,5]. This work deals with the latter case, i.e.…”