Accelerator Mass Spectrometry Analysis of Ultra-Low-Level ¹²⁹I in Carrier-Free AgI-AgCl Sputter Targets

Abstract

Abstract. Separation of carrier-free iodine from low-level iodine samples and accurate measurement of ultra-low-level 129 I in microgram iodine target are essential but a bottleneck in geology and environment research using naturally produced 129 I. This article presents a detection technique of accelerator mass spectrometry (AMS) for accurate determination of ultra-low-level 129 I in carrier-free AgI-AgCl sputter targets. Copper instead of aluminum was selected as the suitable sample holder material to avoid the reaction of AgI-AgCl powder with aluminum. Niobium powder was selected as thermally and electrically conductive matrix to be mixed with AgI-AgCl powder, in order to obtain and maintain a stable and high iodine ion current intensity, as well as less memory effect and low background level of 129 I. The most optimal ratio of the Nb matrix to the AgI-AgCl powder was found to be 5:1 by mass. The typical current of 127 I 5+ using AgI-AgCl targets with iodine content from 5 to 80 μg was measured to be 5 to 100 nA. Four-year AMS measurements of the 129 I/ 127 I ratios in standards of low iodine content and the machine blanks showed a good repeatability and stability.

Preliminary Investigation on the Rapid and Direct AMS Measurement of 129I in Environmental Samples without Chemical Separation

“…In this approach, 127 I carrier solution was added to the Nb powder, the mixture was then homogenized, dried, and mixed with the reference samples for measurement. According to our previous work on AMS measurement of 129 I in microgram-level iodine targets (Liu et al 2015), when the iodine concentration reaches 7 µg/mg in the target, an 127 I 5+ current of 30~80 nA can be measured at the high-energy end of the AMS; this 127 I 5+ current is sufficient to stabilize the 127 I 5+ beam position, and in turn to stabilize the terminal voltage by the slit stabilization function. In this work, the iodine concentration in the targets was increased by factors of 60, 213, and 4233 for NIST-SRM-4359, IAEA-446, and IAEA-410, respectively, with the addition of 127 I carrier into the Nb powder.…”

“…All targets were measured on the Xi’an 3.0MV AMS with the terminal voltage of 2.5MV and the charge state of +5, following the same steps as for the measurement of micro-iodine targets by the AgI-AgCl coprecipitation sample processing method, as described in Hou et al (2010) and Liu et al (2015). The ion source parameters and status are given in Table 2.…”

Radioanalysis of ultra-low level radionuclides for environmental tracer studies and decommissioning of nuclear facilities

“…No visible reaction was observed when they were pressed in holders made of copper for both AgI and AgI-AgCl precipitate, even after exposure to air 825 for more than 7 days. This might be attributed to a solid phase reaction: 3AgI + Al = 3Ag + AlI3 and 3AgCl + Al = 3Ag + AlCl3, these reactions take place when water vapor and water soluble iodide/chloride are present [45].…”

Determination of 129I in environmental solid samples using pyrolysis separation and accelerator mass spectrometry measurement

“…In the AMS measurement of 129 I, AgI precipitate is often mixed with niobium or silver power and pressed in a target holder made of aluminum, copper or stainless steel. A solid phase reaction of AgCl and aluminum has been reported when water vapor presence, which will convert the pressed AgCl to liquid form, all samples in the target will be splashed to the ion source chamber when bombarding with Cs+ source, consequentially contaminated the ion source, cause a serious cross contamination during AMS measurement [30]. Therefore, target holder made of copper is always applied for AMS measurement of I, because chlorine is always present in environmental samples in a large concentration compared to iodine, and some amount of AgCl is often presented in the prepared AgI target.…”