[en] Projectile-like fragments were detected and characterized in terms of A, Z, and energy for the reactions ³⁷Cl on ⁴⁰Ca and ²⁰⁹Bi at E/A = 7.3 MeV, and ³⁵Cl, on ²⁰⁹Bi at E/A = 15 MeV, at angles close to the grazing angle. Mass and charge distributions were generated in the N-Z plane as a function of energy loss, and have been parameterized in terms of their centroids, variances, and coefficients of correlation. Due to experimental problems, the mass resolution corresponding to the ³¹Cl on ²⁰⁹Bi reaction was very poor. This prompted the study and application of a deconvolution technique for peak enhancement. The drifts of the charge and mass centroids for the system ³⁷Cl on ⁴⁰Ca are consistent with a process of mass and charge equilibration mediated by nucleon exchange between the two partners, followed by evaporation. The asymmetric systems show a strong drift towards larger asymmetry, with the production of neutron-rich nuclei. It was concluded that this is indicative of a net transfer of protons from the light to the heavy partner, and a net flow of neutrons in the opposite direction. The variances for all systems increase with energy loss, as it would be expected from a nucleon exchange mechanism; however, the variances for the reaction ³⁷Cl on ⁴⁰Ca are higher than those expected from that mechanism. The coefficients of correlation indicate that the transfer of nucleons between projectile and target is correlated. The results were compared to the predictions of two current models based on a stochastic nucleon exchange mechanism. In general, the comparisons between experimental and predicted variances support this mechanism; however, the need for more realistic driving forces in the model calculations is indicated by the disagreement between predicted and experimental centroids

[en] From the late 1960s to about 1985, the Pulsed-Sphere Program at Lawrence Livermore National Laboratory (LLNL) was carried out to measure 14-MeV neutron leakage spectra from target spheres made out of various elements, compounds, and mixtures Data from these experiments have been and continue to be fundamental in the evaluation of neutron Monte Carlo transport codes and cross section data libraries In addition, the data provide important integral information for stockpile stewardship, fusion technology, neutron therapy, and other applications Therefore, comparisons between computer Monte Carlo simulations and the results of these experiments are pivotal for the integral testing of processed nuclear data libraries and transport codes Fortunately, a large subset of data from the pulsed-sphere program (some 70 experiments) is available as a computer file called disp93in Furthermore, in the past few years, there has been a remarkable improvement in computer performance that allows for more realistic simulations by Monte Carlo codes such as TART 4 Previous TART simulations of the pulsed-sphere experiments were performed using simplified models with relatively small numbers of histories and very large solid angle detectors to offset the limitations in computer power. Also, not all the TART input files were created with the same level of detail For example, some input files included the air around the sphere while others did not These factors prompted a study to simulate in more detail all of the available pulsed-sphere experiments using the Monte Carlo transport code, TART, and the LLNL evaluated neutron data library, ENDL The timing of this study is significant because many years have passed since those experiments were done, and only a few people who participated in them are still working at LLNL Their help has been essential for an accurate documentation of the experiments For the Stewardship Program it is important to preserve and make use of as much of the data as possible, because it represents a unique resource and an enormous effort that would be very costly to reproduce The initial part of this study consisted of an analysis of the experimental setup and neutron source characteristics necessary to create the foundation for a TART input file The next task was to create the input files for the various experiments by incorporating particular settings, such as flight path, sphere materials and dimension, etc , to the basic TART input The results of the TART simulations of these experiments will be used to help assess the quality of evaluated data currently in ENDL Further, other nuclear libraries, e g , ENDF/B-VI, JENDL-3, and JEF-2, will be used in the Monte Carlo simulations after being reformatted for TART and other Laboratory codes Finally, this work will be part of the integral check of the new evaluations being prepared for the Stewardship Barn Book by the LLNL Nuclear Data Group

[en] Methods for error analysis pertaining to isotopic ratio measures made by accelerator mass spectrometry (AMS) typically focus on accuracy/precision of detected counts of a single rare isotope (e.g. ¹⁴C), relative to a current strength indicating the abundance of an isotope present in far greater amounts (e.g. ¹³C). Because AMS analysis of the relative abundance of actinide isotopes involves comparing detected counts that each correspond to a relatively rarely occurring isotope, error analysis is complicated by correlations between isotope-specific counts and by isotope-specific dead times, that affect each pair of isotope-specific counts. A new method of error analysis described here was developed specifically to estimate and characterize error in AMS measures of actinide isotope ratios. The method models a series of detected pairs of isotope counts as correlated compound-binomial (censored trinomial contingency) data, provides an approximately unbiased moment-method estimator of a common isotopic proportion (or ratio) corresponding to any data-pair series, and provides a corresponding homogeneity test for isotopic proportions observed within any data-pair series. Applications of this method to AMS measures of the relative abundance of plutonium isotopes, in samples analyzed at the Lawrence Livermore National Laboratory Center for AMS, revealed that observed measurement errors were nearly all attributable to modeled counting errors

[en] Studies using ¹²⁹I as a tracer have increased significantly since the AMS measurement of ¹²⁹I became available. Determinations of ¹²⁹I and total iodine in soils can help understand fallout patterns from anthropogenic sources (e.g. nuclear reprocessing plant, nuclear accidents). Iodine is a trace element in soils with a concentration range generally between 0.1 and 10 ppm. Traces of iodine are normally determined using neutron activation analysis. In this talk a simple and reliable method will be described for the determination of total iodine in soils using gas chromatography. Also, the preparation of samples for ¹²⁹I determination will be discussed along with examples of these measurements in the case of soil samples related to the Chernobyl accident

[en] Two projects employing AMS are summarized and updated. One project employs AMS to measure ³⁶Cl in concrete and other mineral samples from Hiroshima and Nagasaki to help reconstruct neutron fluences received by the atom-bomb survivors. In this project, we have demonstrated a large discrepancy between the neutron activation measured in Hiroshima and predictions based on the current dosimetry system. This discrepancy has practical implications for radiation risk assessment and radiation protection standards. The other project employs AMS to measure ¹²⁹I in soil and other environmental samples from Belarus, Ukraine, and Russia. This is a proof-of-principle study to determine if the long lived ¹²⁹I isotope (half life, 16 x 10⁶ y) measured by AMS can be used to reconstruct deposition of the short lived ¹³¹I isotope from the 1986 Chernobyl reactor accident. This is required because ¹³¹I disappeared before adequate measurements could be made

[en] The charge and mass of the projectile-like fragments produced in the 12-MeV/nucleon ⁵⁶Fe + ¹⁶⁵Ho reaction were measured at a laboratory scattering angle of 16 degrees. The mass and charge distributions of the projectile-like fragments were generated as a function of total kinetic energy loss (TKEL), and characterized by their neutron and proton centroids and variances, and correlation factors. A weak drift of the system towards mass asymmetry, opposite to the direction which minimizes the potential energy of the composite system, was observed. The increase in the variances with energy loss is consistent with a nucleon exchange mechanism as a means for energy dissipation. Predictions of two nucleon exchange models, Randrup's and, Tassan-Got's models, are compared to the experimental results of the 672-MeV ⁵⁶Fe + ¹⁶⁵Ho reaction and to other Fe-induced reactions. The proton and neutron centroids were found to be generally better reproduced by Tassan-Got's model than by Randrup's model. The variances and correlation factor are well reproduced for asymmetric systems by both models

[en] The large neutron discrepancy in the DS86 dosimetry system for Hiroshima survivors may now be resolvable. This possibility is the result of a recent breakthrough in the ultra-separation of nickel from copper and the measurement of ⁶³Ni using accelerator mass spectroscopy (AMS). The reaction, ⁶³Cu(n,p)⁶³Ni, results in the production of ⁶³Ni (half-life, 100 y) by neutrons above ∼ 1 MeV. A laboratory test experiment demonstrates that this reaction can now be used to reconstruct the high energy neutron fluence in Hiroshima, which would eliminate the most serious obstacle that has prevented a credible solution to the DS86 neutron dosimetry problem. (Author)

[en] A deconvolution technique was used to try to reveal peaks in poorly resolved mass spectra resulting from heavy-ion nuclear reactions. Deconvolution presents the advantage of not requiring an initial guess of the position and number of centroids. The technique was tested using a known well-resolved spectrum that was purposely distorted and subsequently deconvolved. The technique was then applied to the experimental data. It is shown that deconvolution can be successfully applied to reveal peaks in poorly resolved spectra. (orig.)

[en] Radioiodine released to the atmosphere from the accident at the Chernobyl nuclear power station in the spring of 1986 resulted in large-scale thyroid-gland exposure of populations in Ukraine, Belarus, and Russia. Because of the short half life of ¹³¹I (8.04 d), adequate data on the intensities and patterns of iodine deposition were not collected, especially in the regions where the incidence of childhood-thyroid cancer is now increasing. Results are presented from a feasibility study that show that accelerator-mass-spectrometry measurements of ¹²⁹I (half life 16 x 10⁶y) in soil can be used to reconstruct ¹³¹I-deposition density and thus help in the thyroid-dosimetry effort that is now urgently needed to support epidemiologic studies of childhood-thyroid cancer in the affected regions. 32refs., 9 figs., 3 tabs

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