S- and P-wave neutron spectroscopy. XII. Higher resolution techniques in the keV region with windowless gas targets
Abstract
The relative total yield of the O18(p, n) reaction has been studied from threshold to 3.290 MeV with a proton resolution of about . A previously unreported resonance was observed and improved values of widths and energies were obtained for the other resonances. The Q value for the reaction was determined to be −2439 ± 1 keV which places threshold at 2576 ± 1 keV.
The reaction took place in the windowless cryogenic target chamber. A gas handling system was developed to allow one liter of oxygen gas, 65% enriched in O18, to be cycled through the target chamber and recovered with a minimum of loss or dilution.
The O18(p, n) neutrons were used in a series of transmission measurements to test the resolution capability of the gas target as a neutron source. Results were in good agreement with resolution predictions. Total neutron energy spreads were 250 eV and 500 eV at neutron energies of 30 keV and 525 keV respectively.
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Cited by (13)
Radionuclide production for positron emission tomography: Choosing an appropriate accelerator
1989, Nuclear Inst. and Methods in Physics Research, BThe appropriate accelerator for producing 18F, 15O, 13N and 11C depends upon the existing conditions at the intended installation site. The existence of limited resources (e.g. financial, space, etc.) require that the relationship between the accelerator beam energy (E) and beam current (I), the yield (Y) and the external radiation burden be known for each of the reactions leading to the above end products. The interdependence of these parameters is calculated using published cross section data. Isoexposure curves I = D(E) trace the locus of points (I, E) that cause the radiation exposure, outside a concrete shield of given thickness, to equal a set value. Similarly, isoyield curves I = Y(E) trace the production of the desired radionuclide. The appropriate accelerator must have operating parameters within the region of (I, E) space bounded by the critical yield and exposure isocontours. The final choice among the candidates within this region is then governed by the particular constraints of an institution (e.g. technical support, manpower requirements, cost, etc.). Factors leading to the purchase of an accelerator at the University of Wisconsin-Madison are presented.
Oxygen diffusion in niobium and NbZr alloys
1977, Acta MetallurgicaThe tracer diffusion of oxygen-18 in pure Nb and NbZr alloys (≤ 1% Zr) has been measured from 550 to 1100°C. After the diffusion anneal, the specimens were bombarded with protons to cause the reaction 18O(p,n)18F. The oxygen profile was then measured by autoradiographic techniques which detected the 18F. The temperature dependence of the oxygen diffusivity in pure niobum is . Oxygen additions to the niobium (≤1.5 at.% O) did not significantly affect the oxygen diffusivity. As the zirconium content of the niobium increased the oxygen diffusivity decreased. As zirconium atoms are dissolved substitutionally onto the niobium lattice, they form trapping sites at which OZr clusters may be formed. The binding entropy, AS of the clusters was determined to be about −3.3 k e.u.
On a mesuré de 550 à 1100°C la diffusion du traceur 18O dans le Nb pur et dans des alliages NbZr de teneur en Zr inférieure ou égale à 1%. Après le recruit de diffusion, on a bombardé les échantillons avec des protons pour provoquer la réaction 18O(p,n)18F. On mesurait ensuite le profil de la teneur en oxygène par autoradiographie, en détectant le18F. La diffusivité de l'oxygène dans le niobium pur en fonction de la température, est donnée par . L'addition de moins de 1,5 at.% d'oxygène dans le niobium ne change pratiquement pas la diffusivité de l'oxygène. Lorsque la teneur en zirconium du niobium croît, la diffusivité de l'oxygène décroit. Lorsque les atomes de Zr se dissolvent substitutionnellement sur les sites du Nb, ils forment des sites de piègeage ou des amas OZr peuvent se former. On a déterminé l'entropie de liaison AS de ces amas; elle est voisine de −3,3 k e.u.
Die Tracerdiffusion von Sauerstoff-18 wurde in reinem Nb und in Nb-Zr-Legierungen (≤1% Zr) zwixchen 550 und 1100°C gemessen. Nach der Diffusionswärmebehandlung wurden die Proben mit Protonen beschossen, um eine Reaktion 18O(p,n)18F zu verursachen. Danach wurde das Sauerstoffprofil mittels autoradiographischer Techniken, die 18F nachweisen, gemessen. Die Temperaturabhängigkeit der Sauerstoffdiffusivität ist in reinem Niob: . Sauerstoffzugaben zum Niob (⩽1,5 At.%) änderten die Sauerstoffdiffusivität nicht nennenswert. Mit ansteigendem Zirkongehalt nimmt die Sauerstoffdiffusivität ab. Da Zirkonatome substitutioneil im Niobgitter gelöst werden, stellen sie Einfangstellen dar, an denen O-Zr-Agglomerate gebildet werden können. Die Bindungsentropie dieser Agglomerate AS wurde zu etwa − 3,3 k bestimmt.
Oxygen diffusion in β-Zircaloy
1977, Journal of Nuclear MaterialsThe diffusion of oxygen in β-Zircaloy-4 has been studied from 900 to 1500°C with survey measurements for β-zirconium and β-Zircaloy-2. The tracer diffusivity was measured over the entire temperature range and the chemical diffusivity from 1100 to 1450°C. The experiments were performed by using oxygen-18 as the tracer and activating it by proton bombardment. Some complementary measurements were made using Auger Electron Spectroscopy. The results indicated that the tracer and chemical diffusivity of oxygen in β-Zircaloy-4 are statistically identical, and that there is no oxygen concentration dependence over the oxygen concentration range studied, 0.1 to 0.6 wt.%. The temperature dependence of the diffusivity of 18O from 1000 to 1500°C is given by D = 2.48 × 10−2 exp(−28200/RT) cm2/sec. The results for the β-zirconium and β-Zircaloy-2 indicated that the compositional differences between the three host materials exert no influence upon the oxygen diffusivity. An examination of the activation entropy, calculated assuming that Snoek's model describes the diffusion process, indicates that this model probably is not appropriate.
La diffusion de l'oxygène dans le zircaloy-4 en phase β a été étudié de 900 à 1500°C en comparant ces mesures à celles dans le zirconium β et le zircaloy-2 β. La diffusivité du traceur a été mesurée dans l'intervalle complet de température et la diffusivité chimique de 1100 à 1450°C. Les expériences ont été réalisées en utilisant l'oxygène 18 comme traceur et en l'activant par bombardement par des protons. Quelques mesures complémentaires ont été faites en utilisant la spectroscopie d'électrons Auger. Les résultats ont montré que la diffusivité des traceurs et la diffusivité chimique de l'oxygène dans le Zircaloy-4 en phase β sont statistiquement identiques et qu'il n'y a pas de variation de la diffusivité avec la concentration en oxygène dans le domaine de concentrations en oxygène étudié (de 0,1 à 0,6% en poids). La variation de la diffusivité de 18O avec la température entre 1000 et 1500°C obéit à la relation: D = 2,48 × 10−2 exp(−28 200/RT) cm2 sec−1. Les résultats pour le zirconium β et le zircaloy.2. en phase β indiquent que les différences de composition entre les 3 matériaux n'exercent pas d'influence sur la diffusivité de l'oxygène. Un examen de l'entropie d'activation, calculée en supposant que le modèle de Snoek décrit le processus de diffusion, indique que ce modèle n'est probablement pas approprié.
Die Sauerstoffdiffusion in β-Zircaloy-4 wurde zwischen 900 und 1500°C zusammen mit Übersichtsmessungen in β-Zr und β-Zircaloy-2 untersucht. Die Tracer-Diffusion wurde im gesamten Temperaturbereich, die chemische Diffusion zwischen 1100 und 1450°C gemessen. Die Experimente wurden mit Sauerstoff-18 als Tracer und durch Aktivierung durch Protonenbeschuss durchgeführt. Einige komplementäre Messungen erfolgten durch Auger-Elektronenspcktroskopie. Die Ergebnisse zeigen, dass die Tracer- und die chemische Diffusion von Sauerstoff in β-Zircaloy-4 statistisch identisch sind und keine Abhängigkeit der Sauerstoffkonzcntration im untersuchten Sauerstoffkonzentrationsbereich zwischen 0,1 und 0,6% vorliegt. Die Temperaturabhängigkeit der Diffusion von 18O ist zwischen 1000 und 1500°C durch D = 2,48 · 10−2 exp(−q/RT) cm2/s mit q = 28 200 cal/mol gegeben. Die Ergebnisse an β-Zr und β-Zircaloy-2 zeigen, dass der Unterschied in der Zusammensetzung der drei Wirtsmaterialicn keinen Einfluss auf die Sauerstoffdiffusion ausübt. Eine Überprüfung der Aktivierungsentropie, die unter der Annahme berechnet wurde, dass das Snoeksche Modell den Diffusionsvorgang beschreibt, weist darauf hin, dass dieses Modell wahrscheinlich nicht geeignet ist.
s- and p-wave neutron spectroscopy Xf(i). Intermediate structure in the <sup>28</sup>Si + n reaction: R-matrix interpretation of experimental data
1977, Annals of PhysicsA multilevel R-matrix analysis of Si neutron cross-section data measured at NBS has been performed up to about 4.5 MeV neutron energy. Only a small fraction of the p- and s-wave s.p. strength is observed, but both exhibit local concentrations of strength indicative of doorway structure around 1 and 0.2 MeV, respectively. Besides the well-known 180 keV, strong, resonance, the s-wave resonance structure is of moderate strength and widely distributed. The f- and d-wave assignments are not unambiguous, but J > 3/2 resonances show strong signs of intermediate structure for d-waves. A possible correlation between neutron and gamma decay channels and the connection between the states observed in (n, n), (d, p), (n, γ), and (γ, n) channels is discussed. A coreparticle doorway interpretation for s and p- waves is presented.
Fine structure of analogue states
1976, Physics ReportsThis article reviews all the high-resolution data on fragmented analogue states taken at the Triangle Universities Nuclear Laboratory, over a period of ten years. There are fifty analogue states observed by proton scattering on targets of masses between 40 and 64, and mass 92. The number of fragments may be only two or three, or as many as fifty. Of the total, only 17 states have a sufficient fine structure pattern where an analysis is attempted. In a few cases, inelastic widths and photon and neutron widths are observed besides proton elastic ones. The article discusses the optimum method of analysis of the data with a view to extracting the physical parameters of the analogue: the energy, proton spectroscopic factor, spreading width, shift and asymetry parameter. The last three quantities are discussed from the viewpoint of the Robson model based on one-channel external mixing of the analogue by Coulomb forces. This model has qualitative success, particularly in describing the large asymmetry sometimes seen, but it is not quantitatively adequate. When the proton spectroscopic factors are compared to the neutron counterparts for the parent (as measured by (d,p) studies), after allowing for Coulomb effects, the proton ones are smaller by an amount which increases to 30% in the heavier nuclei (Ni,Mo). This is in line with the situation on 2 0 8Pb. Thus it seems that the inadequacy of a Coulomb explanation of the analogue-parent energy shift (the Nolen-Schiffer anomaly) has a counterpart in the spectroscopic factors.
s- and p-wave neutron spectroscopy. Xe. Intermediate structure in <sup>90</sup>Y and the generalized R-matrix theory
1976, Annals of PhysicsThe resonance structure observed in the 89Y(n, n)89Y total cross section measurements in the range of 0.3 to 1.2 MeV incident energy was investigated using the generalized R-matrix theory of nuclear reactions and the doorway interpretation of intermediate structure. The energies and wave functions of the doorway resonances were calculated in a 2-particle and 3p-1h basis of the shell model. The model space and the parameters of the model calculation chosen were consistent with other shell model calculations in the mass-90 region. Several strong p-wave doorways with Jπ = 0+, 1+, and 2+ were predicted by the model in the energy range studied. This is due to proximity of p-wave giant resonance. The escape widths Γ↑ and the spreading widths Γ↓ for these states were evaluated using the model wave functions and the R-matrix formalism. The calculated energy dependence of the total cross section shows that most of the predicted doorways are in general agreement with the observed anamolies with similar relative strength. More significantly, the underlying p-wave gross structure representing a grand average is of very similar shape in both theory and experiment. As expected in the mass 90-region, the s- and d-wave doorways contribute less significantly to the calculated resonance structure.
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Now at U. S. Naval Base, Charleston, S. C.
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Now at E. I. duPont, Savannah River Laboratory, Aiken, S. C. 29801