P. M. Shah

Studies on the evaporation and ionisation behaviour of uranium and plutonium in thermal ionisation mass spectrometry

Studies are reported on the evaporation and ionisation behaviour of uranium (U) and plutonium (Pu) under conditions of their simultaneous mass spectrometric analyses using a double rhenium filament assembly, a multicollector Faraday cup detector system and a synthetic mixture of SRM-200-U and SRM-947-Pu with the U/Pu ratio as 10. The U+ and UO+ ions start appearing at an evaporation filament temperature lower than that for appearance of Pu+. However, the ion current of Pu+ is 5-10 times greater than that of U+. The coexistence of oxides of Pu and U on the evaporation filament appears to enhance the stability of UO+. Two different approaches are compared to account for Pu-238 isobaric interference at U-238 during simultaneous isotopic analyses of U and Pu from the same filament loading. The K factor values for U and Pu during their simultaneous mass spectrometric analyses are lower compared with those observed previously when U and Pu are analysed from separate filament loadings with chemically pure fractions of each.

INVESTIGATIONS FOR ISOBARIC INTERFERENCE OF 238PU AT 238U DURING THERMAL IONIZATION MASS SPECTROMETRY OF URANIUM AND PLUTONIUM FROM THE SAME FILAMENT LOADING

Three different methodologies are reported for circumventing the isobaric interference of 238Pu at 238U during simultaneous thermal ionization mass spectrometric analysis of U and Pu. These are as follows: (i) obtaining the 235U238U amount ratio at lower evaporation filament current and hence eliminating the need for any isobaric interference correction due to 238Pu at 238U; (ii) determining the 240Pu239Pu amount ratio at lower evaporation filament current during the optimization of the ion current of U+ and Pu+ and calculating the 238Pu239Pu amount ratio of the unknown sample; (iii) optimizing the evaporation filament heating current for simultaneous mass spectrometric analysis of U and Pu and using the 240Pu239Pu amount ratio obtained from the first 2–3 blocks to arrive at the 238Pu239Pu atom ratio required in the interfering element correction (IEC) approach. The three methodologies described are useful for either (a) completely eliminating the need to know the 238Pu239Pu amount ratio or (b) determining the 238Pu239Pu amount ratio from the same filament loading prior to performing simultaneous mass spectrometric analysis. The drawbacks and advantages of each of the methodologies are discussed with respect to the accuracy of the data and the time of analysis. It is shown that the last methodology along with the IEC approach provides a universal solution to correct for the isobaric interference of 238Pu at 238U in all the unknown samples. The studies were carried out by using the conventional method of loading nitrate solutions of U and Pu, a double rhenium filament assembly, a multicollector Faraday cup detector system and employing synthetic mixtures of SRM-200-U and SRM-947-Pu with varying UPu ratios.

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.

An experimental evaluation of procedures used for quantitative analysis in RF-spark source mass spectrometry

Abstract The determination of trace constituents in materials of high purity using spark source mass spectrometry normally employs the sensitivity calibration technique. In this technique, either the matrix element is used as reference or certain selected elements known as internal standards are used as reference. The photoplate evaluation procedure adapted for quantitative analysis is a matter of choice from the various procedures available in the literature. Investigations were carried out on the adaptability of these methods and different photoplate evaluation procedures. This paper describes the precision and accuracy achievable, as well as the relative advantages and disadvantages of these methods when the matrix or an element closely resembling the element being determined is used as reference element.

Simultaneous determination of the 235U/238U isotope ratio and concentration at nanogram levels of uranium employing a mixed spike in thermal ionization mass spectrometry

Abstract A method for the simultaneous determination of the 235U/238U isotope ratio (at 235U abundances close to that of natural uranium) and the concentration of uranium in the sample at nanogram levels of uranium using a commercially available thermal ionization mass spectrometer with secondary electron multiplier (SEM) detector is presented. The method depends on the use of a spike of 233U + 235U with about 4 at.% of 235U which is added to the unknown sample solution. This increases the ion current of 235U+ in the mass spectrometer, thereby facilitating the use of the peak integration option with peak centring each time during data acquisition for the 235U peak. The 235U/238U isotope ratio and the concentration or total amount of uranium in the sample are calculated from the data obtained on the 238U/233U and 235U/233U isotope ratios in the spiked mixture. An accuracy of better than 1% is demonstrated for the determination of the 235U/238U isotope ratio. This methodology will be useful for determining the abundances of the minor isotopes of different elements using the recently available multi-Faraday cup detector system. From the analyses of isotopic reference materials of uranium, the overall mass discrimination factor determined using a Faraday cup detector is 0.09% per mass unit which is due to the mass-dependent isotope fractionation in the ion source using the double rhenium filament assembly. The overall mass discrimination determined using SEM in the current mode at an operating voltage of 1.87 kV is 0.33% per mass unit, which leads to a contribution of 0.24% (= 0.33−0.09)% per mass unit due to the electron multiplier.

Determination of Hydrogen in Zircaloy by Spark Source Mass Spectrometry

Abstract Application of Spark Source Mass Spectrometry(SSMS) for the determination of hydrogen in zircaloy clad material used in nuclear reactors is reported. The advantage of SSMS lies in the fact that along with the metallic trace constituents even the nonmetallics including the gaseous impurities can also be determined in a single analysis using the photoplate detection system. The value obtained for H2 using photoplate detection system is compared with that obtained in electrical detection system in SSMS and also with the value obtained employing the conventional inert gas fusion technique.

A comparative study of234U/238U alpha-activity ratios determined by alpha-spectrometry and calculated from mass spectrometric data

Abstract234U/238U α-activity ratios determined by α-spectrometry (AS) and those calculated from the atom ratio data using the half-life values are compared in some of the isotopic reference materials of uranium and a few other uranium samples. For α-spectrometry, electrodeposited sources were prepared and a large area passivated ion implanted (IPE) detector (450 mm2) was used for recording the α-spectra. The isotopic composition of U was determined by thermal ionisation mass spectrometry (TIMS) and the recommended half-life values of234U and238U were used to calculate the α-activity ratio. It is observed that234U/238U α-activity ratios calculated from the atom ratio data are consistently high, with a mean difference of about 5%, when compared to the α-spectrometry results. This discrepancy warrants confirmation by a few more laboratories and suggests redetermination of the half-life values of234U and238U.

Preparation and characterization of234U for mass spectrometry and alpha-spectrometry

Abstract234U of high isotopic purity (>99 atom%) as well as of high radiochemical, purity was separated from aged238Pu prepared by neutron irradiation of237Np. Methodologies based on ion exchange and solvent extraction procedures were used to achieve high decontamination factor from238Pu owing to the very high α-specific activity of238Pu (2800 times) in comparison to that of234U. Isotopic composition of purified234U was determined by thermal ionisation mass spectrometry. Alpha spectrometry was used for checking the radiochemical purity of234U with respect to concomitant α-emitting nuclides. The separated234U will be useful for different investigations using mass spectrometry and alpha spectrometry.

Electrodeposition of milligram amounts of uranium on electropolished stainless steel disks

Investigations have been carried out for the electrodeposition of milligram amounts of uranium on electropolished stainless steel disks with an objective of preparing good quality sources for α-spectrometric studies on uranium. The parameters studied include the variation of electrodeposition yield as a function of voltage, time, distance between the cathode and anode, and the volume of 0.2M ammonium oxalate solution. The conditions selected for preparing good quality sources with nearly 100% yield were: electrodeposition voltage 25 V, time of deposition 15 min, volume of 0.2M ammonium oxalate solution in the cell 4 ml and a distance of 2 cm between the cathode and anode. The sources prepared using this method have been used successfully for the α-spectrometric determination of234U/238U ratios in uranium samples.

Experimental evaluation of239Pu,238Pu and233U spikes for determining plutonium concentration by thermal ionization mass spectrometry and alpha-spectrometry

Experimental evaluation on the use of239Pu spike in Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS),238Pu spike in Isotope Dilution Alpha Spectrometry (IDAS) and233U as a Non-Isotopic Diluent in Alpha Spectrometry (N-IDAS), for determing plutonium concentration in samples with burn-up values in the range of 1,000–10,000 MWD/TU is done. Precision is determined by analyzing replicate aliquots from different samples using each of the three spikes. Accuracy is established by comparing the results with those obtained by using well recognized spike242Pu in ID-TIMS. It is shown that the use of239Pu spike with the latest generation thermal ionization mass spectrometers gives the best precision (0.2%), whereas the precision values of 0.5 and 1% can be obtained by using238Pu and233U spikes, respectively, on a routine basis. Reasons for the difference in the precision values are discussed, along with the merits and drawbacks on the use of different spike isotopes.