Uranium age dating by gamma spectrometry

Uranium age dating by gamma spectrometry C. T. Nguyen, J. Zsigrai1 , L. Lakosi Centre for Energy Research (EK), Hungarian Academy of Sciences,Budapest, Hungary 1Present address: Institute for Transuranium Elements, JRC, EC, 76125 Karlsruhe, Germany Int. Conf. Advances in Nuclear Forensics - IAEA CN-218 Vienna, 7-10 July, 2014 ID 13

OUTLINE 1. Introduction 2. Principle of U age dating 3. HRGS measurements 4. Applications and results 5. Detection performance, lower age limits 6. Summary

1/1. Introduction • Nuclear forensic analysis:comprehensive nuclear forensic characterization of most types of NM intheCentre for Energy Research (EK), Budapest • Provenance of seized samples; Relevance to nuclear forensics: tracing seized material back to its origin • One of the characteristics of nuclear material of unknown origin: Age of the sample • Destructive methods (mass spectrometry, α-spectrometry) Drawbacks: sample preparation; preservation of evidences; sample cannot be takenfrom some of the materials • Non-destructive gamma-spectrometry (HRGS) in EK Full analysis: Isotopic composition, total U content,age, reprocessed material, … “Age”:time elapsed since last chemical separation/enrichment

Gamma-spectrometric U age dating: First usein 2001 New methodfor U samples of arbitraryshape: Invented at our laboratory on the occasion of an intercomparison exercise for characterizing U - samples: „Round Robin” (RR)sample: ITWG, 2001 (Oxide powder ~ 2 g, > 90 % enrichment) Development of γ-spectrometric age dating of U-samples at our laboratory: Applicationsfor - low-,medium-, and high-enriched seized and reference samples; - VVR-SM fresh fuel rods; broken fuel rod; - E > 90% 235U layer built in a fission ionization chamber 1/2. Introduction 1/2. Bevezetés

234U 230Th 226Ra 222Rn 218Po 214Pb 214Bi 2/1. Principle of U age dating • Mass spectrometry: 230Th/234Uatom ratio • Gamma-spectrometry:226Ra/234U • (= 214Bi/234U) activity ratio • (226Rain radioactive equilibrium with214Bi) Measurements: • 234U/238U by planar HPGe • 214Bi/238U by coaxial HPGe in low background iron chamber Chronometer: 214Bi/234U Activity ratios determined by relative (“intrinsic”) efficiency calibration • Most relevant gamma peaks: • 121 keV of 234U • 609 keV of 214Bi, in low background

2/2. Principle of U age dating Correlation ofU age(T) withtheatom ratio230Th/234U(mass sp.)andwith theactivity ratio 214Bi/234U (gamma sp.) Mass spectrometry (m: mass, M: molar mass) Gamma spectrometry Atom ratio 230Th/234U and activity ratio 214Bi/234U as functions of the U age

3/1. HRGS measurements • with the following nuclear materials: Material, enrichment, method • HEU metal (RR), 93 %,DA & NDA • HEU metal (RR),91 %, DA & NDA • HEU oxide powder (RR), 90 %, DA & NDA • HEU oxide powder, 36 %, DA & NDA • HEU fuel rods,36 %, NDA • LEU fuel rod, 10 %, NDA • Fission ionization chamber, >90 %, NDA • LEU oxide powder, 10 %, DA & NDA • LEU oxide powder, 5 %, DA & NDA • LEU pellet, 4,4 %, DA & NDA Non-destructive techniques (NDA) may be needed even though very accurate destructive methods (DA) are available

3/2. HRGS measurements Equipment Sample container Sample supporter Detector Low-background iron chamber (20 cm wall thickness) with a 150 cm3 coaxial HPGe detector for measuring the 609 keV line of 214Bi Planar (20 cm2) HPGe detector: for measuring 121 keV of 234U x

3/3. HRGS measurements Various approaches to measuring 214Bi/234U • Using the absolute efficiency of the detector • self-absorption correction • efficiency-calibrated geometry needed • calibration by point-like sources • Using relative (intrinsic) efficiency calibration • independent of the geometry of the measurement • applicable to samples of arbitrary shape, chemical form • activity ratios are measured: • 214Bi/238U • 234U/235U • 235U/238U

4/1. Applications and results Application 1: HEU oxide powder • Method development and first application: 90 % 235U: sample from a Round-Robin Exercise (2001) (inter-laboratory comparison organized by ITWG) • Measured age consistent with results of other labs • 90 % and 36 % 235U: test samples • 90% sample: Measured age: (43 ± 2)y from records: > 41y • 36% sample: Measured age: (43 ± 6)y (by rel. eff. calibr.): (43 ± 5)y (42 ± 4)y (using abs. det. eff.): 45 ± 4y

4/2. Applications and results Application 2: Research reactor fuel rods • “VVR-SM” and “EK-10” • can only be assayed as a whole (item analysis) • no calibration standards available “VVR-SM”, E=36% broken pieces of “EK-10”, E=10%

4/3. Applications and results Application 2: Researchreactor fuel rods • VVR-SM: 36 % 235U • EK-10: 10 % 235U • Cannot be dismantled! • Nocalibration standards available • By a coaxial Ge detector (relative efficiency: 34%) • In a low-background iron chamber • 609 keV line of 214Bi • 1 measurement ~ 6 hours fuel rod 10 cm wall thickness

4/4. Applications and results Application 2: Researchreactor fuel rods Results>

4/5. Applications and results Application 2: Researchreactor fuel rods

4/6. Applications and results Application 3: LEU • CRM with 10 % 235U • measured in ITU,Karlsruhe, and inBudapest • results agree with each other and consistent with certificate • CRM with 5 % 235U • result consistent with certificate • LEU pellet 4.4 % 235U • result confirmed by LA-ICP-MS

4/7. Applications and results Application 3: LEU pellets • Difficult samples: young LEU • Young LEU Low 234U Low 214Bi • Problems • Compton background of 238U • Fluctuations of 222Rn background • Some solutions • use a better detector • keep 222Rn away Polystyrene(“Styropor”) N2 flushing Measurement of the 214Bi activity in the iron chamber

4/8. Applications and results Application 4: HEU metal Intrinsic calibration: Relativeefficiency is determined from the same spectrum: , , 235U/238U:MGAU, f(keV): rel. eff. function ~5 g metalsamplesRR 2010 (E=93%): Sample A, 150 cm3 coax. detector, acquisition time: 3.9 d Spectrum of the sample takenfor relative efficiency calibrationby the 150 cm3coaxial detectorin the iron chamber

4/9. Applications and results Application 4: HEU metal Measurements with a well-type HPGe detector Type GCW2023, active volume 110 cm3, rel. eff. 20 %, well diam. 16 mm, depth 40 mm Iron chamber wall thickness: 10 cm Compton-background (238U) limits the sensitivity Spectra of U-samples enriched to 4.5 %and 92 % taken by a well-type HPGe crystal (1.25 % abs. eff. at 609 keV)

4/10. Applications and results Application 4: HEU metal Age dating of the two HEU Round Robin (RR of 2010)samples by HRGS (150 cm3 coax. and 110 cm3 well Ge detectors)

4/11. Applications and results Summary of U age resultsin EK Results measured by gamma spectrometry Results obtained so farby γ-spectrometric age datingin ourlaboratory

5/1. Detection performance, lower age limits • Calculated detection lower limits of specific 214Bi activities by various types of detectors as the function of 235U enrichment: • Detector types: • Coax.150cm3 (sample on the surface) - Well type 110 cm3 (20% rel. eff.) • Well type 200 cm3 (40% rel. eff) • - Well type 260 cm3 (50% rel. eff) • Well type 300 cm3 (60% rel. eff.) • Well type 300 cm3 (60% rel. eff.), small (1g) • sample placed on the bottom of the well Compton tail of 238U lines decreases sensitivity of detecting the 609 keV214Bi line toward low enrichment: lower limits of age dating Continuous lines apply above the point series according to the corresponding detector Calculated specific activity of 214Bi (226Ra) as a function of age of 5 g samples enriched to 1 – 90%

5/2. Detection performance, lower age limits Minimum detectable ages: Assessment of lower limit of age dating as a function of enrichm. for various detectors, for 5 g U • HRGS Lower limit decreases with increasing enrichment and detector efficiency ●ICP-MS: NU:0.15 yr. LEU, HEU: even younger(it depends also on the amount of mat.) ●LA-ICP-MS:Not as sensitive, as the destructive method, but it may be the only applicable method in certain cases.

5/3. Detection performance, lower age limits Estimated lower limits of age dating for various HPGe detectors based on the data of the previous table, for 5 g U samples Lower limit of age dating as a function of enrichment for various detector types (CGW6023* detector: sample mass of 1 g, abs. eff. 10 %)

Advantages of gamma-spectrometry: non-destructive no sample preparation preservation of evidence relatively simple equipment provides a faster result in general no dismantling (e. g. fuel) suitable for in-field analysis HRGS for uranium age dating: results are in good agreement with mass spectrometric measurements altogether 13 samples of various ages and enrichments were dated so far • the youngest: age 6.7±0.7 yr, E=91 % • the less enriched: E=4.4 %, age of 54±7 yr with the new well detector the lowest age limit assessed to ~6 yr for samples of E=5%, 5 g U, whereas to ~1 yr for samples of E=90 %, 5 g U for NU it is expected to ~15 yr “Difficult” samples: low-enriched young samples Further development: higher efficiency detector suppression of background fluctuations 6. Summary

Acknowledgments This work was supported by the International Atomic Energy Agency in the frame of the Coordinated Research Project “Application of Nuclear Forensics in Illicit Trafficking of Nuclear and Other Radioactive Material” under contract No. 13839, as well as by the Hungarian Atomic Energy Authority Thanks for your attention!

Uranium age dating by gamma spectrometry