Combined application of alpha-track and fission-track techniques for detection of plutonium particles in environmental samples prior to isotopic measurement using thermo-ionization mass spectrometry

The fission track technique is a sensitive detection method for particles which contain radio-nuclides like ²³⁵U or ²³⁹Pu. However, when the sample is a mixture of plutonium and uranium, discrimination between uranium particles and plutonium particles is difficult using this technique. In this study, we developed a method for detecting plutonium particles in a sample mixture of plutonium and uranium particles using alpha track and fission track techniques. The specific radioactivity (Bq/g) for alpha decay of plutonium is several orders of magnitude higher than that of uranium, indicating that the formation of the alpha track due to alpha decay of uranium can be disregarded under suitable conditions. While alpha tracks in addition to fission tracks were detected in a plutonium particle, only fission tracks were detected in a uranium particle, thereby making the alpha tracks an indicator for detecting particles containing plutonium. In addition, it was confirmed that there is a linear relationship between the numbers of alpha tracks produced by plutonium particles made of plutonium certified standard material and the ion intensities of the various plutonium isotopes measured by thermo-ionization mass spectrometry. Using this correlation, the accuracy in isotope ratios, signal intensity and measurement errors is presumable from the number of alpha tracks prior to the isotope ratio measurements by thermal ionization mass spectrometry. It is expected that this method will become an effective tool for plutonium particle analysis. The particles used in this study had sizes between 0.3 and 2.0 μm.     

Five Decades Ago: From the “Transuranics” to Nuclear Fission

The discovery of nuclear fission is one of the most outstanding episodes in the history of chemistry: It starts in the spring of 1934 when Enrico Fermi and his group irradiate uranium with neutrons and seem to succeed in going beyond uranium, the then heaviest known element, reaching the first transuranic element; it continues with Otto Hahn, Lise Meitner and Fritz Strassmann who believe to have found additional transuranic elements; with Irène Curie and Paul Savitch who observe an activity which somehow does not fit into that scheme; again with Otto Hahn and Fritz Strassmann who first identify this activity as radium but then on the 17th of December 1938 after rigorous chemical tests realize that the activity is instead barium, thus discovering the fission of the uranium atom into two lighter nuclei; and with Lise Meitner and Otto Robert Frisch who explain nuclear fission on the basis of an already known nuclear model; Otto Robert Frisch finally performs a physical experiment on the 13th of January 1939 which corroborates the fission of uranium. This discovery of nuclear fission is not only an event of historic dimensions, it is also an excellent example of how science evolves, not by successive logical steps but rather through strange detours.     

Identification of a new heavy neutron-rich isotope238Th

A new heavy neutron-rich thorium isotope was produced via a multinucleon transfer reaction by irradiation of natural uranium targets with 60 MeV/u¹⁸O ions. The Th activities was radiochemically separated from the uranium and mixture of reaction products by the rapid solvent extraction method. Measurements of the gamma-ray spectra were performed with HPGe detectors. The new neutron-rich isotope²³⁸Th was identified for the first time based on the observation of the growth and decay of the gamma-ray with energy of 1060.5 keV from the decay of²³⁸Pa. The half-life of²³⁸Th has been determined to be 9.4±2.0 minutes. Further, a gamma-ray of 89.0 keV with the 8.9-minute half-life was found, which may be due to the decay of the new isotope²³⁸Th.     

Systematical calculations on the ground state properties of heavy and superheavy nuclei

The synthesis of superheavy elements is now a hot topic in nuclear physics. Alpha-decay and spontaneous fission are two main decay modes in heavy and superheavy regions. Theoretical studies on alpha radioactivity and spontaneous fission can provide useful information for experiments. We investigate the alpha-decay and spontaneous fission of heavy and superheavy nuclei with different models. This includes the alpha-decay energies, alpha decay half-lives, and half-lives of spontaneous fission. The theoretical alpha-decay half-lives are in good agreement with experimental ones. The calculated half-lives of spontaneous fission are in reasonable agreement with present data. The properties of unknown nuclei are predicted.     

ICP–MS with hexapole collision cell for isotope ratio measurements of Ca, Fe, and Se

To avoid mass interferences on analyte ions caused by argon ions and argon molecular ions via reactions with collision gases, an rf hexapole filled with helium and hydrogen has been used in inductively coupled plasma mass spectrometry (ICP–MS), and its performance has been studied. Up to tenfold improvement in sensitivity was observed for heavy elements (m > 100 u), because of better ion transmission through the hexapole ion guide. A reduction of argon ions Ar⁺ and the molecular ions of argon ArX⁺ (X = O, Ar) by up to three orders of magnitude was achieved in a hexapole collision cell of an ICP–MS (“Platform ICP”, Micromass, Manchester, UK) as a result of gas-phase reactions with hydrogen when the hexapole bias (HB) was set to 0 V; at an HB of 1.6 V argon, and argon-based ions of masses 40 u, 56 u, and 80 u, were reduced by approximately four, two, and five orders of magnitude, respectively. The signal-to-noise ratio ⁸⁰Se/ ⁴⁰Ar₂ ⁺ was improved by more than five orders of magnitude under optimized experimental conditions. Dependence of mass discrimination on collision-cell properties was studied in the mass range 10 u (boron) to 238 u (uranium). Isotopic analysis of the elements affected by mass-spectrometric interference, Ca, Fe, and Se, was performed using a Meinhard nebulizer and an ultrasonic nebulizer (USN). The measured isotope ratios were comparable with tabulated values from IUPAC. Precision of 0.26%, 0.19%, and 0.12%, respectively, and accuracy of 0.13% 0.25%, and 0.92%, respectively, was achieved for isotope ratios ⁴⁴Ca/ ⁴⁰Ca and ⁵⁶Fe/⁵⁷Fe in 10 μg L^–1 solution nebulized by means of a USN and for ⁷⁸Se/⁸⁰Se in 100 μg L^–1 solution nebulized by means of a Meinhard nebulizer. Received: 15 December 2000 / Revised: 26 March 2001 / Accepted: 27 March 2001     

Neutron generation in massive Cu-targets during the irradiation with 22 and 44 GeV carbon ions

An extended Cu-target was irradiated with 22 and 44 GeV carbon ions for about 11.3 and 14.7 hours, respectively. The upper side of the target was in contact with a paraffin-block for the moderation of secondary neutrons. Small holes in the moderator were filled with either lanthanum salts or uranium oxide. The reaction was studied via the decay of¹⁴⁰La(40h) using radiochemical methods, as has been published. The reaction was studied via the decay of²³⁹Np(2.3 d) as well as the reaction U(n,f) using radiochemical methods. In addition, solid state nuclear track detectors were used for fission studies in gold. The yields for the formation of (n,) products agree essentially with other experiments on extended targets carried out at the Dubna Synchrophasotron (LHE, JINR). To a first approximation, the breeding rate of (n, ) products doubles when the carbon energy increases from 22 to 44 GeV. If, however, results at 44 GeV are compared in detail to those at 22 GeV, we observe an excess of (37±9)% in the experimentally observed²³⁹Np-breeding rate over theoretical estimations. Experiments using solid state nuclear track detectors give similar results. We present a conception for the interpretation of this fact: There is the evident connection between anomalies we observe in the yield of secondary particles in relativistic heavy ion interactions above a total energy of approximately 30–35 GeV and increased yield of neutrons in this energy region.     

Analysis of glass by simultaneous spectrometry of scattered alpha particles and prompt protons

The energy spectrum of alpha particles scattered from a beam of 4 MeV was used to determine heavy elements and oxygen and silicon in glass. Simultaneously, but with a separate detector, the energy spectrum of prompt protons from (α, p) reactions were used to determine boron, sodium and aluminium. Both detectors were placed at 135° to the direction of the beam, but the proton detector was covered with an absorber to stop backscattered alpha particles, Glass samples analysed by other methods and standard glass powders from the U. S. Bureau of Standards were used as references.     

低浓缩铀靶辐照后溶液中铀的化学种态及主要裂变元素的影响

利用化学种态分析软件CHEMSPEC计算了低浓缩铀靶辐照后溶液中铀(U)的化学种态分布及其主要裂变元素对U化学种态的影响。结果表明,在单组分体系中,pH值和铀酰浓度都会显著影响U的化学种态分布。随着铀酰浓度的增大,溶液中将会生成多核配合物;在较高的NO₃^-浓度下,U在溶液中主要以UO₂²⁺和UO₂NO³⁺的形式存在。CO₂对不同浓度铀的种态分布影响结果表明,当铀酰浓度较低时,铀的化学种态多以碳酸铀酰的形式存在;当铀酰浓度较高时,铀的化学种态多以氢氧铀酰或柱铀矿沉淀的形式存在。计算发现,当裂片元素Tc、I、Mo的浓度小于0.01mol·L^-1并分别以TcO₄^-、I^-、MoO₄^2-的种态存在时,这些裂片元素不改变铀的各化学种态的分布。     

Discrimination ofn=2→2 UV transitions of he-like ions in a recoil light source

Neon and argon recoil ions were produced by collisions with 5.9 MeV/amu uranium projectiles andΔn=0 ultraviolet transitions from the metastable 1s2p ³ P _0,2 levels of the target ions were measured in delay to the pulsed projectile beam.     

Nuclear recoil induced in238U by high-energy protons

Recoil-range measurements of the products of²³⁸U, bombarded by protons with energies greater than 100 MeV, are surveyed. Values of 2W(F+B) and F/B from thick-target thick-catcher experiments are tabulated, where W is the thickness of the uranium target and F and B are the fraction of a given product which recoils into the forward and backward catchers, respectively. The mass number of the recoiling nucleus is given by A_REC. For A_REC<50, the values of 2W(F+B) and F/B peak at E_P=1 GeV to 3 GeV and drop at higher energies. For A_REC>50, the neutron-deficient nuclei generally have recoil properties which are noticeably different from those of other types of nucleus. Their recoil ranges are smaller and the values of F/B are larger. The values of 2W(F+B) for these nuclei drop by a factor of approximately 2 above E_P≅1 GeV. The values of F/B peak, as noted above. These quantities for the other types of nucleus decrease slightly with proton energy. Recoil ranges from a thin²³⁸U target into aluminium and mylar are also given.     

低浓缩铀靶辐照后溶液中铀的化学种态及主要裂变元素的影响

利用化学种态分析软件CHEMSPEC计算了低浓缩铀靶辐照后溶液中铀(U)的化学种态分布及其主要裂变元素对U化学种态的影响。结果表明,在单组分体系中,pH值和铀酰浓度都会显著影响U的化学种态分布。随着铀酰浓度的增大,溶液中将会生成多核配合物;在较高的NO₃^-浓度下,U在溶液中主要以UO₂²⁺和UO₂NO₃⁺的形式存在。CO₂对不同浓度铀的种态分布影响结果表明,当铀酰浓度较低时,铀的化学种态多以碳酸铀酰的形式存在;当铀酰浓度较高时,铀的化学种态多以氢氧铀酰或柱铀矿沉淀的形式存在。计算发现,当裂片元素Tc、I、Mo的浓度小于0.01 mol·L^-1并分别以TcO₄^-、I^-、MoO₄^2-的种态存在时,这些裂片元素不改变铀的各化学种态的分布。     

Fission track–secondary ion mass spectrometry as a tool for detecting the isotopic signature of individual uranium containing particles

A fission track technique was used as a sample preparation method for subsequent isotope abundance ratio analysis of individual uranium containing particles with secondary ion mass spectrometry (SIMS) to measure the particles with higher enriched uranium efficiently. A polycarbonate film containing particles was irradiated with thermal neutrons and etched with 6 M NaOH solution. Each uranium containing particle was then identified by observing fission tracks created and a portion of the film having a uranium containing particle was cut out and put onto a glassy carbon planchet. The polycarbonate film, which gave the increases of background signals on the uranium mass region in SIMS analysis, was removed by plasma ashing with 200 W for 20 min. In the analysis of swipe samples having particles containing natural (NBL CRM 950a) or low enriched uranium (NBL CRM U100) with the fission track–SIMS method, uranium isotope abundance ratios were successfully determined. This method was then applied to the analysis of a real inspection swipe sample taken at a nuclear facility. As a consequence, the range of ²³⁵U/²³⁸U isotope abundance ratio between 0.0276 and 0.0438 was obtained, which was higher than that measured by SIMS without using a fission track technique (0.0225 and 0.0341). This indicates that the fission track–SIMS method is a powerful tool to identify the particle with higher enriched uranium in environmental samples efficiently.     

ICP-MS with hexapole collision cell for isotope ratio measurements of Ca, Fe, and Se

To avoid mass interferences on analyte ions caused by argon ions and argon molecular ions via reactions with collision gases, an rf hexapole filled with helium and hydrogen has been used in inductively coupled plasma mass spectrometry (ICP-MS), and its performance has been studied. Up to tenfold improvement in sensitivity was observed for heavy elements (m > 100 u), because of better ion transmission through the hexapole ion guide. A reduction of argon ions Ar+ and the molecular ions of argon ArX+ (X = O, Ar) by up to three orders of magnitude was achieved in a hexapole collision cell of an ICP-MS ("Platform ICP", Micromass, Manchester, UK) as a result of gas-phase reactions with hydrogen when the hexapole bias (HB) was set to 0 V; at an HB of 1.6 V argon, and argon-based ions of masses 40 u, 56 u, and 80 u, were reduced by approximately four, two, and five orders of magnitude, respectively. The signal-to-noise ratio 80Se/ 40Ar2+ was improved by more than five orders of magnitude under optimized experimental conditions. Dependence of mass discrimination on collision-cell properties was studied in the mass range 10 u (boron) to 238 u (uranium). Isotopic analysis of the elements affected by mass-spectrometric interference, Ca, Fe, and Se, was performed using a Meinhard nebulizer and an ultrasonic nebulizer (USN). The measured isotope ratios were comparable with tabulated values from IUPAC. Precision of 0.26%, 0.19%, and 0.12%, respectively, and accuracy of 0.13% 0.25%, and 0.92%, respectively, was achieved for isotope ratios 44Ca/ 40Ca and 56Fe/57Fe in 10 microg L(-1) solution nebulized by means of a USN and for 78Se/80Se in 100 microg L(-1) solution nebulized by means of a Meinhard nebulizer.     

Radiation from metastable highly-charged ions extracted from a gas target

Few-electron ions of argon have been produced by heavy ion impact in a gas target. The recoil ions have been extracted into a region with high vacuum, and the K radiation of long-lived states has been observed by means of a position sensitive proportional counter oriented along the flight path of the ions. Fast ion beams are frequently used to determine the lifetimes τ of excited levels by time-of-flight techniques [1]. Typical ion velocities are v=1 cm/ns (beam energy 0.5 MeV/amu) or higher. The usual length l of experimental chambers rarely exceeds l=1 m. This decay path corresponds to roughly 100ns, and very few experiments on fast ion beams have been done on longer-lived states. Notable examples are the Berkeley measurements on the Ar¹⁶⁺ 1s2s³S₁ lifetime (calculated around τ=210 ns [2, 3], decay path ～8m) which resulted in a lower lifetime result (172±30ns) than expected and which were later shown to suffer from various systematic errors, the most important being the lack of spectral resolution of radiation from the helium-like or from core-excited lithiumlike ions. An alternative approach, a coincidence measurements of x-rays and projectile ions to associate the detected quanta with ions of a given charge state is difficult for high energy and high current ion beams.     

Contribution a l'etude de la determination de Be,B, C,N, O et F par activation au moyen de p,d,3He et α

The activation of these elements, homogeneously contained in a medium thicker than the path length, of slowing-down capacity close to that of aluminium, was determined as a function of energy by irradiating standards with p, d,³He and α particles of energies between: 5 and 15 MeV for protons, 5 and 20 MeV for deuterons and³He particles, 25 and 42 MeV for α particles. The detection sensitivities intrinsically possible in these energy ranges are deduced from the curves obtained. The results apply to all materials through a simple relationship taking into account the path lengths. They may be used in particular to foresee the possibilities offered by activation with these charged particles, from the viewpoint of sensitivity, when the samples to be analysed are irradiated with a beam of given intensity and energy lying between the above limits.     

Determination of the 238U spontaneous fission decay constant without neutron irradiation

We have developed a methodology for measuring the decay constant of the spontaneous fission of ²³⁸U, l_f, using nuclear particle track detectors where thermal neutron irradiation is unnecessary. This methodology is based on the fact that the radiation damage caused by spontaneous fission of trans-uranium elements bearing a mass number close to 238 are similar to ²³⁸U spontaneous-fission ones. Loading a thick source of uranium (thickness greater than the fission fragment range) with a small amount of a suitable trans-uranium element (for instance, ²⁴²Pu, which presents a spontaneous fission half-life of 6.75^.10¹⁰ y), it is possible to determine the observation efficiency of a particle-track detector for fission fragments. Procedures concerning our thick source manufacture and uniformity tests of the trans-uranium distribution are also presented. These results make it possible for the exposure of thick uranium sources (without trans-uranium element) to lead to a l_f value.     

Trace Analysis by Heavy Ion Induced X-Ray Emission

 Various K-, L- and M-shell X-ray production cross sections are measured for heavy ion impact on elements in the range Z ₂ = 13 to 83. The ion species range from Z ₁ = 10 to 36, and ion energies from 1 to 16 MeV are used. Enhanced cross sections are observed when the projectile K- or L- binding energy is similar to the energy of the target K-, L- or M-shell. This effect is used to improve the analysis sensitivity for selected elements. As an example trace analysis of Fe in glass with V, Mn, Co and Ni ions is investigated. Results are compared with proton induced X-ray emission analysis on the same samples. In these samples Fe-K_α X-ray production is similar for irradiation with 3 MeV protons and 14 MeV Ni ions. However the signal to background ratio is four times higher for the irradiation with Ni ions as compared to irradiation with protons. Advantages and drawbacks of heavy ion induced X-ray emission for quantitative analysis compared to proton induced X-ray emission analysis are discussed.     

On-stream nuclear activation of trace uranium and thorium using a 14 MeV neutron generator

A method for the continuous on-stream determination of trace concentrations of uranium and thorium in flowing streams is developed. The 14 MeV neutron generator was used for irradiation and the delayed neutron counting technique was employed in counting the induced neutron activity. The dependence of the minimum detectable concentration on the irradiation, decay and counting times, liquid flow-rate and the background was studied. At optimal conditions, uranium and thorium concentrations were determined down to 20 and 100 ppm, respectively. The interference of the neutron emitting nuclide^17m O was reduced to an insignificant level by optimizing the decay time.     

Highly asymmetric binary fission

In this article, a review of the work on the low-yield products formed in the highly asymmetric binary fission (70A160) of heavy elements (HABF) is made. HABF may be viewed as an extension of the usual asymmetric fission process. A few interesting features of this mode of fission which are apparent from the data available in the literature are identified: The process (a) is rare, (b) seems to depend on the excitation energy of the fissioning system, (c) is highly influenced by shell effects. A closely related phenomenon, viz the recent discovery of spontaneous emission of heavy ions (A>4, Z>2) from heavy nuclei is also examined briefly. It is speculated that HABF may be an important decay mode for the very heavy and the yet-undiscovered superheavy elements. Suggestions for future experiments to further pin down this fission mode are also given in this article.     

In-line determination of heavy elements by gamma ray-induced energy-dispersive K-line XRF

In-line determination of heavy elements, especially in nuclear fuel reprocessing solutions by means of XRF analysis and -emitting radionuclides as excitation sources for the K-lines has been investigated.⁵⁷Co,¹³³Ba and¹⁹²Ir are used as radionuclide sources. U, Np, and Pu can be determined from the lower ppm range up to the saturation concentration of about 400 g/l. In case of Pu concentrations >100 mg/l the detection limits for U and Np increase. A matrix effect due to the composition of the solution is observed, which depends only on the density of the solution. At higher activities of fission products in the solution, an increase of the background count rate but no interferences are observed. This allows determination of heavy elements in fission product solutions.