High resolution plutonium-239/240 mixture alpha spectroscopy using centrifugally tensioned metastable fluid detector sensor technology

Abstract

This paper presents a novel and rapid, wet chemistry technique for spectroscopically detecting trace (∼10⁻³ Bq mL⁻¹) level alpha emitting radionuclides mixtures with under 10 keV alpha energy resolution – with 100% gamma–beta rejection. The Centrifugally Tensioned Metastable Fluid Detector (CTMFD) sensor technology with a ∼16 mL sensitive volume was utilized and adapted for the ability to decipher trace level Pu-239 and Pu-240 content in mixtures of these two isotopes ranging in content from 1 : 0 to 0 : 1 in relative proportions with gamma–beta rejection, and ∼100% (4π) alpha detection sensitivity validated for accuracy (within ±5%) against NIST standards. Pu-239 and Pu-240 isotopes emit closely spaced (<10 keV separated) energetic alpha particles and constitute a known challenge to decipher without resorting to microcalorimetry or mass spectrometry. For the work presented in this paper, a relatively rapid (<1 h) sampling protocol was developed to create mixtures of these isotopes for CTMFD based examination and to derive the mixture's characteristic response function, viz., alpha decay detection rate over a range of tensioned metastable state negative pressures spanning the detection thresholds for the two isotopes. An accompanying methodology and algorithm were developed to analyze-deconvolute the Pu isotope bearing mixture's response curves, and to determine the composition of each isotope within the mixture. Results based on experiments revealed this technique identified the Pu-239 : Pu-240 isotopic activity ratio combinations within <±12% for each of the samples ranging from 1 : 0, 5 : 1, 1 : 1, to 0 : 1 ratios – enabling alpha spectroscopy within few hours of counting, for an arbitrary Pu-239 : Pu-240 (∼10⁻³ Bq mL⁻¹) mixture, using a single ∼16 mL CTMFD apparatus.