Radhika M. Rao

Comparative study of plutonium-239, plutonium-240 and plutonium-242 spikes for determining plutonium concentration by isotope dilution-thermal ionization mass spectrometry

Abstract An experimental evaluation of 239Pu, 240Pu and 242Pu spikes in terms of precision and accuracy for determining the plutonium concentration by isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) in irradiated fuels is presented. Results obtained on different plutonium samples covering a wide range of burn-up values are given. Mean precision values of 0.1–0.2% are achieved by using any of the three spikes and factors close to unity are obtained for the ratios ID-TIMS(240Pu)/ID-TIMS(242Pu) and ID-TIMS(239Pu)/ID-TIMS(242Pu). This study shows that there is no significant advantage in using 240Pu spike when compared to 239Pu spike. Further, it is shown that the overall error (internal+external) obtained in replicate determinations of 242Pu/239Pu atom ratio is larger due to enhanced isotope fractionation compared to that in the case of 240Pu/239Pu atom ratio.

Experimental evaluation of plutonium-239 spike for determining plutonium concentration by isotope dilution-thermal ionization mass spectrometry

Abstract The use of 239 Pu as a spike in isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) for determining the plutonium concentration in irradiated fuels is demonstrated. The method is based on the high precision better than 0.1% determination of the 240 Pu/ 239 Pu atom ratio in the sample, the spike, and the spiked mixture. The precision and accuracy of the results obtained on different plutonium samples using a 239 Pu spike are compared with those obtained using a 242 Pu spike. A mean precision of 0.1–0.2% is achieved using a 239 Pu or 242 Pu spike and a factor close to unity is obtained for the ratio ID-TIMS ( 239 Pu)/ID-TIMS ( 242 Pu). This study provides a viable alternative spike material to those laboratories which do not have access to enriched 242 Pu, which is generally used in determining the plutonium concentration by ID-TIMS, and at the same time, offers certain advantages over the use of a 242 Pu or 244 Pu spike.

High precision isotope ratio measurements on boron by thermal ionization mass spectrometry using Rb2BO2+ ion

Studies were carried out to determine the 10B/11B isotope ratio by positive thermal ionization mass spectrometry (P-TIMS) analyzing boron as rubidium borate ions. Of the 36 different ionic species formed, the boron and rubidium isotope ratios were obtained from the ion intensity ratios of the most suitable ion pairs corresponding to masses 212, 213 and 215, 213 respectively. The investigations were carried out to explore the possibility of correcting the observed isotope ratio of boron by using a modified internal normalization technique based on the observed Rb isotope ratio. The method is based on the relation of isotopic fractionation of boron as a function of the rubidium (natural isotopic composition) isotopes fractionation obtained during TIMS analysis from the same filament loading. The application of the methodology to improve the precision of the observed 10B/11B isotope ratio during analysis of irradiated boron alloy samples is demonstrated. Improvement in precision from 0.25% to better than 0.05% was demonstrated using this approach.

Experimental evaluation of the characteristic features of passivated ion implanted and surface barrier detectors for alpha spectrometry of plutonium

Experimental evaluation of a commonly used silicon surface barrier detector and of the recently introduced passivated ion implanted detector for alpha spectrometry is reported in terms of FWHM, peak to valley ratio, tail parameter and % tail contribution per unit alpha activity ratio using electrodeposited sources of plutonium prepared on platinum backing material. For this purpose, detectors of nearly the same diameter were employed (100 mm2 silicon surface barrier detector with a diamater of 1.13 cm and 80 mm2 passivated ion implanted silicon detector with a diameter of 1.01 cm). It is shown that the recently introduced passivated ion implanted detectors give smaller tailing effects. But there is no significant difference between the two detectors used in the present work w.r.t. FWHM and peak to valley ratios. Further, it is observed that the peak to valley ratio can be used to get an idea about240Pu/239Pu and241Am/238Pu alpha-activity ratios in the sample.

Determination of boron at sub-ppm levels in uranium oxide and aluminum by hyphenated system of complex formation reaction and high-performance liquid chromatography (HPLC).

Boron, at sub-ppm levels, in U3O8 powder and aluminum metal, was determined using complex formation and dynamically modified reversed-phase high-performance liquid chromatography (RP-HPLC). Curcumin was used for complexing boron extracted with 2-ethyl-1,3-hexane diol (EHD). Separation of complex from excess reagent and thereafter its determination using the online diode array detector (DAD) was carried out by HPLC. Calibration curve was found to be linear for boron amounts in the sample ranging from 0.02 microg to 0.5 microg. Precision of about 10% was achieved for B determination in samples containing less than 1 ppmw of boron. The values obtained by HPLC were in good agreement with the data available from other analytical techniques. The precision in the data obtained by HPLC was much better compared to that reported by other techniques. The present hyphenated methodology of HPLC and complex formation reaction is interesting because of cost performance, simplicity, versatility and availability when compared to other spectroscopic techniques like ICP-MS and ICP-AES.

Simultaneous analysis of uranium and plutonium using thermal ionization mass spectrometry

Simultaneous isotopic analysis of uranium and plutonium using thermal ionization mass spectrometer coupled to a multi-collector detection assembly with 9 Faraday cups has been reported earlier. Subsequently investigations have been carried out (1) to understand the applicability of correction methodologies available to account for the contribution of238Pu at238U and (2) to evaluate the effectiveness of these methodologies on the accuracy of235U/238U atom ratio being determined, particularly when samples containing different U/Pu atom ratios. Isotopic fractionation for both U and Pu in the simultaneous isotopic analysis has been compared with the results of the individual analysis of these elements. The different isotopic fractionation factors observed for U were attributed to different conditions of analysis. There was no significant difference in the isotopic fractionation patterns for Pu. The consideration to extend this method to actual samples from our observations on synthetic samples with diferent U/Pu atom ratios containing U and Pu isotopic reference standards is described.

Simultaneous isotopic analysis of uranium and plutonium by thermal ionisation mass spectrometry coupled to a variable multicollection detection system

Simultaneous isotopic analysis of uranium and plutonium employing a thermal ionisation mass spectrometer coupled to a variable multicollection Faraday cup detector system is reported. Factors such as the U/Pu ratio in the sample, filament currents during sample de-gassing and data acquisition have been investigated to arrive at optimum conditions for analysis. A simple correction to the observed 235/238 peak ratio is necessary to account for the 238Pu isotopes contribution to 238U. The precision and accuracy achievable in the measurement of isotopic ratios is better than 0.2%. An attempt was also made to show that uranium and plutonium vaporise predominantly through their oxides and not through direct metal evaporation when the samples are loaded from dilute nitric acid medium. This might explain the earlier appearance of U+ ions in preference to Pu+ ions in the mass spectra of all the U/Pu mixtures investigated.

Precision and accuracy in the determination of plutionium-239 /uranium-233, americium-241/ uranium-233 and curium-244 /uranium-233 alpha activity ratios by alpha spectrometry

Determination of239Pu/233U,241Am/233U and244Cm/233U alpha activity ratios is required when using233U as a tracer for the determination of plutonium, americium and curium by alpha spectrometry. Precision and accuracy in the determination of these alpha activity ratios was evaluated by preparing synthetic mixtures from solutions of enriched isotopes of239Pu,241Am,244Cm and233U. Separate synthetic mixtures were prepared for each of the three alpha activity ratios. The sources from the synthetic mixtures were prepared by direct evaporation method using tetra ethylene glycol /TEG/ as a spreading agent, alpha spectra were recorded by employing solid state silicon surface barrier detectors coupled to a 4 K analyzer and the alpha spectra were evaluated by a method based on the geometric progression decrease for the far tail of the spectrum. Large area detector /i.e. 450 mm2/ was observed to reduce the effect of nonhomogeneous distribution, if any, of the two elements present in the source. Precision and accuracy of about 1% is demonstrated for the determination of239Pu/233U,241Am/233U and244Cm/233U alpha activity ratios using large area silicon surface barrier detector.

The preparation and use of synthetic isotope mixtures for testing the accuracy of the PTIMS method for 10B/11B isotope ratio determination using boron mannitol complex and NaCl for the formation of Na2BO2+

The development of a method for determination of isotopic composition requires calibration of the method over a wide range of isotopic ratios. Further, validation of data obtained by the method requires isotopic reference materials spanning a wide range of ratios. In the present studies, gravimetric synthetic mixtures of boron with 10% 10B to 95% 10B were prepared by mixing solutions of enriched boron isotopes. The solutions of 10B and 11B enriched isotopes were calibrated using isotopic reference material NIST SRM-951 as a spike for isotope dilution. 10B/11B atom ratios in all the blends were determined by Positive Thermal Ionization Mass Spectrometry (PTIMS) using NaCl with mannitol for the formation of Na2BO2+. A measurement precision of about 0.03% was obtained from repetitive P-TIMS analyses of the mixtures with 10B content ranging from 10 to 70%. An instrument isotope fractionation factor (K-factor) of 0.99935 ± 0.00030 (1 s) was obtained for 10B/11B atom ratios of the different blends and this agreed with the K-factor determined from P-TIMS analysis of NIST SRM-951. The different isotopic blends prepared were also used in Inductively Coupled Plasma Quadrupole Mass Spectrometry, (ICP-QMS) employing NIST SRM-951 (10B/11B ratio of 0.2473) as a bracketing standard. An accuracy and precision of about 0.5% was obtained during ICP-QMS analyses of these mixtures. A novel and simple method for validation of analysis methodology with synthetic mixtures prepared using enriched isotopes is described in the present work.

Accuracy in the Isotope Dilution Mass Spectrometry of Uranium in Rubidium Uranium Sulphate Rb2U(SO4)3

Abstract Problems encountered in the determination of uranium in rubidium uranium sulphate (Rb2U(SO4)3) employing isotope dilution thermal ionisation mass spectrometry (ID-TIMS) are discussed. The positive bias of 0.2 to 0.3% in the determination of uranium in Rb2U(SO4)3 by ID-TIMS with respect to the stoichiometric composition has been resolved by modifying the chemical exchange procedures. The concentration of uranium in Rb2U(SO4)3 could be determined with an accuracy better than 0.1% employing the HClO4 treatment for proper isotopic exchange between the spike and sample isotopes.