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Protactinium-231 ( 231 Pa) Measurement for Isotope Chronometry in Nuclear Forensics

Protactinium-231 ( 231 Pa) Measurement for Isotope Chronometry in Nuclear Forensics. Liz Keegan , Attila Stopic and Grant Griffiths. Talk outline.

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Protactinium-231 ( 231 Pa) Measurement for Isotope Chronometry in Nuclear Forensics

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  1. Protactinium-231 (231Pa) Measurement for Isotope Chronometry in Nuclear Forensics Liz Keegan, Attila Stopic and Grant Griffiths

  2. Talk outline This presentation will give an overview of ANSTO’s experience in producing Pa-233 for isotope dilution measurement of Pa-231 for age dating of uranium materials using the U-235 → Pa-231 chronometer. • Brief background on ANSTO’s NFRF • Production of Pa-233 by neutron irradiation of Th-232 • Separation of Pa-233 and Th-232 using ion chromatography • Evaluation of ICP-qMS for Pa-233/Pa-231 measurements OPAL Research Reactor

  3. Nuclear Forensics Research Facility Central hub for nuclear forensics in Australia

  4. ‘Model’ Age – a valuable NF signature • In 2010 ANSTO took part in an inter-laboratory ITWG NF exercise. • Decay of U-234 to Th-230 is the most widely employed isotope chronometric system in nuclear forensics. • Decay of U-235 to Pa-231 (via Th-231) is a useful complementary system. Using two (or more) chronometric pairs gives increased confidence in the determined age value • ‘Model ages’ based on assumptions: • Complete separation of daughter from U during sample formation • Closed-system behaviour of the sample after formation

  5. Comparison of chronometers U-234 → Th-230 t1/2 = 2.45 x 105 years t1/2 = 75,690 years U-235 → Pa-231 t1/2 = 7.04 x 108 years t1/2 = 32,760 years • Minimum measureable age depends on: • Amount of sample available • Separation efficiency • Detection limit of instrument used to measure daughter isotope

  6. Solution chemistry of protactinium • Actinide element (Z=91) between Th (Z=90) and U (Z=92) • Preferred oxidation state is 5. (compared to preferred oxidation state 4 and 6 for U and 4 for Th) • Separation of Pa is notoriously difficult. Hydrolyses readily. • Insoluble: HF critical for keeping Pa in solution; almost completely insoluble in all common aqueous media except hydrofluoric acid and sulphuric acid Affinity for inorganic ligands: F- > OH- > SO42- > Cl- > Br- > I- > NO3- ≥ ClO4- Pa has strong affinity for organic complexing ligands

  7. Measurement of Pa Alpha spectrometry Pa-231: 5.03 (20%) 5.01 (25%), 4.95 (23%), MeV Gamma spectrometry Pa-233: 311.8 (39%) keV Pa-231: 27.3 (10%), 300.1 (2%), 302.7 (2%), 330.1 (1%) keV Mass spectrometry • Thermal ionisation mass spectrometry (TIMS) • Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) • Accelerator mass spectrometry (AMS) ANSTO Centre for Accelerator Science new AMS facility • Quadrupole ICP-MS

  8. Production of Pa-233 spike (n,γ) Irradiation of Th-232 in OPAL reactor 232Th 233Th (t½ = 22.3 min) β 233Th 233Pa (t½ = 26.967 days) β 233Pa 233U (t½ = 1.59 x 10 5 yrs) • Procedure: • Deposit Th-232 on Al foil • Place foil in HDPE ‘snap-cap’ vial • Irradiate in neutron flux of 7.5 x 1012 n. cm-2.s-1 for 9 hours (20 hours) • Allow to cool (ANSTO procedure 4 days)

  9. Gamma spec measurement of irradiated target Neutron fluence 7.5 x 1012 n. cm-2.s-1

  10. Separation of Th and Pa for Pa-233 spike production Dissolved sample (9M HCl) Load onto column Condition 9M HCl Wash 9M HCl Condition 9M HCl Wash 9M HCl Condition 9M HCl Wash 9M HCl AG1-X8 AG1-X8 AG1-X8 Elute Pa 9M HCl/0.1M HF Elute Pa 9M HCl/0.1M HF Add H3BO3, dry down, take back up to ~15 mL 9M HCl Add H3BO3, dry down, take back up to ~15 mL 9M HCl Elute Pa 9M HCl/0.1M HF Column 3 Separation factor ~50 Chemical yield ~92 % Column 1 Separation factor ~2 x 104 Chemical yield ~100 % Column 2 Separation factor ~50 Chemical yield ~94 %

  11. Separation of Th and Pa from U sample U-233/U-234,U235 measurement Uranium sample dissolution (in 8 M HNO3) Spike addition (Th-229 + Pa-233) Evaporate and re-dissolve in 9M HCl Th, Pa, U separation using BioRad AG1x8 Th elution in 9 M HCl Th-229/Th-230 measurement Pa elution in 9 M HCl/ 0.1 M HF Pa-233/Pa-231 measurement

  12. Evaluation of quadrupole ICP-MS for Pa-233/Pa-231 measurement • Advantages of quadrupole ICP-MS: • less expensive • easier to operate • easier maintenance • more widely available Bruker 820 ICP-MS (active area) • PFA sample introductory system • Pt sampler and skimmer cones • Sensitivity ~ 3.0 x 105 cps/ppb • Background ~20 cps • Rinse solution 2% HNO3/0.4% HF • Limit of detection ~ 0.5 pg/g • Mass bias correction (using NBS U500) Measure the day after separation (Pa-233 (t ½ = 27 days) → U-233)

  13. Evaluation of quadrupole ICP-MS for Pa-233/Pa-231 measurement Initial result for ITWG NF sample (HEU) • Precision of Pa-231/Pa-233 measurements (at ~10 pg/g > 1000 c/s) RSD ~ 0.5% • Calibrate the Pa-233 solution using CRM NBS U-100 (10% enriched – known production date) [Eppich et al., 2013; Williams et al., 2011]

  14. Planned Work Verification of Results • Validation of the separation and measurement procedure using previously measured NBS uranium standards (Williams and Gaffney, 2011; Eppich et al., 2013) • Establish and validate procedures for MC-ICP-MS and AMS Example Application ANSTO legacy fuel pellet sample. Natural U, UO2 ceramic, not irradiated The fuel pellet was typical of those manufactured at ANSTO (then known as the Australian Atomic Energy Commission, AAEC) as part of a fuel development research program in the late 1970s to early 1980s

  15. Questions?

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