Low-methoxyl pectin-Ca2＋ gelation as a sample solidification protocol for marine radioactivity monitoring by HPGe gamma spectrometry.

Marine radioactivity monitoring requires rapid, reliable sample preparation methods that remain functional aboard ships and other mobile platforms. Liquid seawater samples, however, are prone to geometric instability and spillage during vessel motion, which degrade HPGe gamma spectrometry measurement quality. We developed a solidification procedure based on low-methoxyl (LM) pectin and controlled Ca2+ supplementation, which converts seawater into a mechanically stable, leak-resistant gel through egg-box cross-linking. Under the optimised condition (1.4 g LM-pectin per 28.26 g seawater, 300 rpm, 120 min), the gel formed a mechanically stable cylindrical counting geometry (diameter 60 mm, fill height 10 mm) under the selected preparation condition. Paired activity retention, calculated from measurements of the same sample before and after solidification of Korea Institute of Nuclear Safety (KINS) proficiency-test samples, was 100.2±6.1% for 137Cs (n=8), 95.5±6.6% for 134Cs (n=7), 97.0±4.9% for 241Am (n=8), and 97.2±2.4% for 60Co (n=3) across a gamma-energy span of 59.5-1332.5 keV. All 26 paired determinations remained within the 80-120% screening range, and 24 of 26 were consistent with complete retention within their expanded uncertainty (k=2). The procedure uses food-grade reagents, requires no pre-evaporation or specialised equipment, and completes gelation within two hours. With minimum detectable activities of a few Bq kg-1 for a 28.26 g sample counted for 30,000 s, the protocol is intended for rapid shipboard screening of elevated-activity samples in emergency situations rather than for routine low-level monitoring. These results support further shipboard validation of gel-based sample preparation for environmental radioactivity surveys.