Average Capture Cross Sections Research Articles

Measurement of the radiative neutron capture cross section ofPb206and its astrophysical implications

The (n, gamma) cross section of 206Pb has been measured at the CERN n_TOF facility with high resolution in the energy range from 1 eV to 600 keV by using two optimized C6D6 detectors. In the investigated energy interval about 130 resonances could be observed, from which 61 had enough statistics to be reliably analyzed via the R-matrix analysis code SAMMY. Experimental uncertainties were minimized, in particular with respect to (i) angular distribution effects of the prompt capture gamma-rays, and to (ii) the TOF-dependent background due to sample-scattered neutrons. Other background components were addressed by background measurements with an enriched 208Pb sample. The effect of the lower energy cutoff in the pulse height spectra of the C6D6 detectors was carefully corrected via Monte Carlo simulations. Compared to previous 206Pb values, the Maxwellian averaged capture cross sections derived from these data are about 20% and 9% lower at thermal energies of 5 keV and 30 keV, respectively. These new results have a direct impact on the s-process abundance of 206Pb, which represents an important test for the interpretation of the cosmic clock based on the decay of 238U.

Measurement of the neutron capture cross section of thes-only isotopePb204from 1 eV to 440 keV

The neutron capture cross section of Pb204 has been measured at the CERN n_TOF installation with high resolution in the energy range from 1 eV to 440 keV. An R-matrix analysis of the resolved resonance region, between 1 eV and 100 keV, was carried out using the SAMMY code. In the interval between 100 keV and 440 keV we report the average capture cross section. The background in the entire neutron energy range was reliably determined from the measurement of a Pb208 sample. Other systematic effects in this measurement were investigated and precisely corrected by means of detailed Monte Carlo simulations. We obtain a Maxwellian average capture cross section for Pb204 at kT=30 keV of 79(3) mb, in agreement with previous experiments. However our cross section at kT=5 keV is about 35% larger than the values reported so far. The implications of the new cross section for the s-process abundance contributions in the Pb/Bi region are discussed.Received 18 October 2006DOI:https://doi.org/10.1103/PhysRevC.75.015806©2007 American Physical Society

Proton Source Efficiency for Heterogeneous Distribution of Actinides in the Core of an Accelerator-Driven System

The distribution of actinides in the core of an accelerator-driven system loaded with plutonium, americium, and curium has been studied in order to optimize the proton source efficiency ψ*. The optimization of ψ* was performed by keeping some important characteristics of the system, e.g., the radial power profile and the reactivity of the core, constant. One of the basic assumptions of the study, that the magnitude of ψ* is sensitive primarily to the composition of actinides in the inner part of the core, whereas only marginally to that in the outer part, has been confirmed. It has been shown that the odd-N nuclides (those nuclides with an even number of neutrons) in general and 241Am and 244Cm in particular have favorable properties with respect to improving ψ* if they are placed in the innermost part of the core. The underlying reason for this phenomenon is that the energy spectrum of the source neutrons in the inner part of the core is harder than that of the average fission neutrons. Moreover, it has been shown that loading the inner part of the core with only curium increases ψ* by ˜7%. Plutonium, on the other hand, in particular high-quality plutonium consisting mainly of 239Pu and 241Pu, was found to be a comparatively source inefficient element and is preferably located in the outer part of the core. The differences in ψ* are due to combined effects from relative changes in the average fission and capture cross sections and in the average fission neutron yield.

New measurement of neutron capture resonances inBi209

The neutron capture cross section of Bi-209 has been measured at the CERN n_TOF facility by employing the pulse-height-weighting technique. Improvements over previous measurements are mainly because of an optimized detection system, which led to a practically negligible neutron sensitivity. Additional experimental sources of systematic error, such as the electronic threshold in the detectors, summing of gamma-rays, internal electron conversion, and the isomeric state in bismuth, have been taken into account. gamma-Ray absorption effects inside the sample have been corrected by employing a nonpolynomial weighting function. Because Bi-209 is the last stable isotope in the reaction path of the stellar s-process, the Maxwellian averaged capture cross section is important for the recycling of the reaction flow by alpha decays. In the relevant stellar range of thermal energies between kT=5 and 8 keV our new capture rate is about 16 presently accepted value used for nucleosynthesis calculations. At this low temperature an important part of the heavy Pb-Bi isotopes are supposed to be synthesized by the s-process in the He shells of low mass, thermally pulsing asymptotic giant branch stars. With the improved set of cross sections we obtain an s-process fraction of 19 +/- 3 residual of 81 +/- 3 is also of relevance for the design of accelerator driven systems based on a liquid metal Pb/Bi spallation target.

Neutron capture cross section ofTh232measured at the n_TOF facility at CERN in the unresolved resonance region up to 1 MeV

We have measured the neutron capture reaction yield of Th-232 at the neutron time-of-flight facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The average capture cross section has been extracted in the energy range from 4 keV up to 1 MeV with an overall accuracy better than 4 data. The average cross section has been expressed in terms of average resonance parameters using the partial waves l=0,1, and 2.

Stellar neutron capture cross sections of the Lu isotopes

The neutron capture cross sections of $^{175}\mathrm{Lu}$ and $^{176}\mathrm{Lu}$ have been measured in the energy range 3--225 keV at the Karlsruhe 3.7 MV Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}$($p,n$)$^{7}\mathrm{Be}$ reaction by bombarding metallic Li targets with a pulsed proton beam, and capture events were registered with the Karlsruhe 4\ensuremath{\pi} barium fluoride detector. The cross sections were determined relative to the gold standard using isotopically enriched as well as natural lutetium oxide samples. Overall uncertainties of \ensuremath{\sim}1% could be achieved in the final cross section ratios to the gold standard, about a factor of 5 smaller than in previous works. Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT = 8 and 100 keV. These values are systematically larger by \ensuremath{\sim}7% than those reported in recent evaluations. These results are of crucial importance for the assessment of the s-process branchings at A = 175/176.

Stellar neutron capture cross section of the unstables-process branching pointSm151

The neutron capture cross sections of the radioactive isotope $^{151}\mathrm{Sm}$ and of natural samarium have been measured in the energy range from 3 keV to 225 keV at the Karlsruhe 3.7 MV Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}$($p,n$)$^{7}\mathrm{Be}$ reaction by bombarding metallic Li targets with a pulsed proton beam and capture events were registered with the Karlsruhe 4\ensuremath{\pi} Barium Fluoride Detector. The cross sections were determined relative to the gold standard using a 206 mg sample of samarium oxide with 90% enrichment in $^{151}\mathrm{Sm}$. Over most of the measured energy range uncertainties of ~2--3% could be achieved for the $^{151}\mathrm{Sm}$/$^{197}\mathrm{Au}$ ratio. Maxwellian averaged neutron capture cross sections of $^{151}\mathrm{Sm}$ were calculated for thermal energies between kT = 8 keV and 100 keV with due consideration of the stellar enhancement factor and were found to be systematically larger than all previous theoretical predictions used in the analysis of the s-process branching at $^{151}\mathrm{Sm}$. In the context of the branching analysis, an experimental determination of the stellar enhancement factor due to captures in thermally excited states is proposed, and the tentative determination of the p-process residual of $^{152}\mathrm{Gd}$ and a few other cases is discussed.

Co57(n,γ)Co58reaction cross section: Thermal and resonance integral measurements and energy dependence

The {sup 57}Co(n,{gamma}){sup 58}Co thermal and resonance integral cross section were measured as 51(5) b and 20.0(19) b, respectively, by irradiating aliquots of {sup 57}Co solution sealed inside quartz bottles near the core of the IEA-R1 IPEN research reactor and counting the gamma-ray residual activity. The irradiations were monitored using Au-Al alloy foils, with and without Cd cover. The gamma-ray measurements were performed with a shielded HPGe detector. Westcott formalism was applied for the average neutron flux determination. The cross section energy dependence was evaluated using the multilevel Breit-Wigner expression considering the first two resonances and the statistical model for energies above the second resonance. Maxwellian averaged neutron capture cross section with neutron temperatures between 5 and 100 keV were also evaluated.

04/02094 Estimating the price of tradable permits for greenhouse gas emissions in 2008-12: Springer, U. and Varilek, M. Energy Policy, 2004, 32, (5), 611–621

Absolute average capture cross sections of gold, thorium, tantalum, molybdenum, copper and strontium in 252Cf spontaneous fission neutron spectrum were simulated for two types of experiment setups preformed by Z. Dezso and J. Csikai and by L. Green. The experiments were simulated with MCNP5 using cross section data from the ENDF/B-VII.0 library. The determination of neutron backscattering was calculated with the use of neutron flagging. Correction factors to experimentally measured values were determined to obtain average cross sections in a pure 252Cf spontaneous fission spectrum. Influence of concrete wall thickness, air moisture and room size on the average cross section was analyzed. Correction factors amounted to about 30%. Corrected values corresponding to average cross sections in a pure 252Cf spectrum were calculated for 197Au, 232Th, 181Ta, 98Mo, 65Cu and 84Sr. Average cross sections were also calculated with the RR_UNC software using IRDFF-v.1.05 and ENDF/B-VII.0 libraries. The revised average radiative capture cross sections are 75.5±0.1 mb for 197Au, 87.0±1.6 mb for 232Th , 98.0±4.5 mb for 181Ta, 21.2±0.5 mb for 98Mo, 10.3±0.3 mb for 63Cu, and 34.9±6.5 mb for 84Sr.

Stellar neutron capture onTam180. I. Cross section measurement between10keVand100keV

The neutron capture cross section of $^{180}\mathrm{Ta}^{m}$ has been measured at energies between $10\phantom{\rule{0.3em}{0ex}}\text{keV}$ and $100\phantom{\rule{0.3em}{0ex}}\text{keV}$ in a time-of-flight experiment at the Karlsruhe $3.7\phantom{\rule{0.3em}{0ex}}\text{MV}$ Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ reaction by bombarding metallic Li targets with a pulsed proton beam, and gold was used as a cross section standard. Though the world supply of enriched $^{180}\mathrm{Ta}^{m}$ was available for this experiment, the sample consisted of only $150\phantom{\rule{0.3em}{0ex}}\text{mg}$ ${\text{Ta}}_{2}{\mathrm{O}}_{5}$ with a $^{180}\mathrm{Ta}^{m}$ content of only $5.5%$. The difficult separation of the comparably few capture events in $^{180}\mathrm{Ta}^{m}$ from the much larger background due to captures in $^{181}\mathrm{Ta}$ could be achieved by means of the Karlsruhe $4\ensuremath{\pi}$ barium fluoride detector, taking advantage of its combination of high efficiency, good energy resolution, and high granularity. The cross section was determined with an overall uncertainty of better than $10%$ in the energy range from $30\phantom{\rule{0.3em}{0ex}}\text{keV}$ to $100\phantom{\rule{0.3em}{0ex}}\text{keV}$ and could be used for renormalizing statistical model calculations in the entire energy range of astrophysical interest, which had predicted about two times larger values. Based on these first experimental data, Maxwellian averaged neutron capture cross sections were calculated for thermal energies between $kT=8\phantom{\rule{0.3em}{0ex}}\text{keV}$ and $100\phantom{\rule{0.3em}{0ex}}\text{keV}$.

Neutron capture cross section of139La

The neutron capture cross section of ${}^{139}\mathrm{La}$ has been measured relative to that of ${}^{197}\mathrm{Au}$ by means of the activation method. The sample was irradiated in a quasistellar neutron spectrum for $kT=25\mathrm{keV}$ generated via the ${}^{7}\mathrm{Li}{(p,n)}^{7}\mathrm{Be}$ reaction with the proton energy adjusted 30 keV above the threshold. Maxwellian averaged neutron capture cross sections were calculated for energies $kT=5--100\mathrm{keV}.$ The new value for $kT=30\mathrm{keV}$ is found to be $31.6\ifmmode\pm\else\textpm\fi{}0.8\mathrm{mb},$ 18% lower and considerably less uncertain than the previously recommended value of $38.4\ifmmode\pm\else\textpm\fi{}2.7\mathrm{mb}.$ With these results the s- and r-process components could be more accurately determined, making lanthanum a reliable s- and r-process indicator in stellar spectroscopy.

The 208Pb( n,γ) cross section

The Maxwellian average neutron capture (MAC) cross section of 208Pb has been reinvestigated. With the activation technique cross section measurements were performed at 25, 30, 52, 104, 149, and 215 keV. At 30 keV we succeeded to measure the direct p-wave capture part of the 208Pb capture cross section. The s-process at the termination of the synthesis has been analyzed. Independent calculations of s- and r-process yield a consistent decomposition for the Pb and Bi isotopic s- and r-contributions.

Neutron Data Measurements for Waste Transmutation at Gelina

During the last decade there has been a growing interest in Partitioning and Transmutation technologies as a means of strongly reducing the long-term radiotoxic inventory of nuclear waste. Main emphasis has been placed on the burning of minor actinides (MA) and of long-lived fission products (LLFP). In order to contribute building a sound data base for the neutron induced reactions of the concerned isotopes, a collaboration has been started in 1995 between the Service de Physique Nuclaire of CEA France and the Institute for Reference Materials and Measurements of Geel, Belgium. In the frame of this co-operation, a mixed group of scientists carries out measurements and analysis of neutron capture and transmission cross sections at the GELINAtime-of-flight facility of the IRMM. An important preparatory activity is also the procurement of such radioactive isotopes and the production of gram-quantity samples, a work that is also carried out at the IRMM, Geel. The isotopes investigated so far are 99Tc, 129I and 237Np. Work on 99Tc, which is considered the most offending LLFP, is practically completed: a total of 659 resonances have been analyzed up to 10 keV neutron energy making use of both capture and transmission data. Spin has also been assigned to 51 s-wave levels. Quantities such as the average level spacing, the average capture width and the strength function for s- and p-wave levels have been derived from such a large sample of resonances. Average capture and transmission cross sections have also been determined in the unresolved region from 10 to 150 keV. The analysis of 237Np transmission data has allowed to determine the neutron widths, and at low energy also the capture widths, of some 570 resonances below 500 eV and preliminary data up to 1 keV are available. Preliminary results of 129I transmission measurements below 1 keV energy are presented at this conference in a separate contribution. 1)

Reinvestigation of 85Kr Branching in the Light of New 82, 84, 86Kr Neutron Capture Cross Section Measurements

High resolution neutron capture cross section measurements of 3 stable krypton isotopes, 82Kr, 84Kr and 86Kr, have been performed at the Geel Linear Accelerator (GELINA) on both resolved and unresolved resonance region. In addition, a total cross section measurement on 84Kr has also been performed in the energy range from 1 eV up to 120 kev. Accurate values of the Maxwellian–averaged capture cross section have been derived for all isotopes for stellar temperatures ranging from kT=5 up to 100 keV. A re–investigation of the s–process in the mass region around the prominent branching at 85Kr has been undertaken in the light of our new results.

Measurement of the 232Th Neutron Capture Cross Section in the Region 5 keV–150 keV.

The average capture cross-section of 232Th has been measured at the 14.37 m flight path of GELINA, IRMM-Geel, in the energy range from 5 to 150 keV. The capture events were detected by two C6D6 liquid scintillators and the neutron flux was measured with a 10B-loaded ionisation chamber. The data, corrected with the pulse-height weighting technique, have been normalised to the well-isolated and nearly saturated 232Th(n, γ) resonances at 21.8 eV and 23.5 eV. Below 15 keV neutron energy, we do not observe the discrepancies, up to 40%, with the evaluated ENDF/B-VI data as reported by Wisshak et al.. Between 5 and 80 keV our results are about 10% systematically above the ENDF/B-VI data and approach the evaluated data between 80 and 100 keV.

Measurement of neutron capture on50Tiat thermonuclear energies

At the Karlsruhe and T\"ubingen 3.75 MV Van de Graaff accelerators the thermonuclear ${}^{50}\mathrm{Ti}(n,\ensuremath{\gamma}{)}^{51}\mathrm{Ti}(5.8\mathrm{min})$ cross section was measured by the fast cyclic activation technique via the 320.852 and 928.65 keV $\ensuremath{\gamma}$-ray lines of the ${}^{51}\mathrm{Ti}$ decay. Metallic Ti samples of natural isotopic composition and samples of ${\mathrm{TiO}}_{2}$ enriched in ${}^{50}\mathrm{Ti}$ by 67.53% were irradiated between two gold foils which served as capture standards. The capture cross section was measured at the neutron energies of 25, 30, 52, and 145 keV, respectively. The direct capture cross section was determined to be $0.387\ifmmode\pm\else\textpm\fi{}0.011\mathrm{mb}$ at 30 keV. We found evidence for a bound state s-wave resonance with an estimated radiative width of 0.34 eV which destructively interferes with direct capture. The strength of a suggested s-wave resonance at 146.8 keV was determined. The present data served to calculate, in addition to the directly measured Maxwellian averaged capture cross sections at 25 and 52 keV, an improved stellar ${}^{50}\mathrm{Ti}(n,\ensuremath{\gamma}{)}^{51}\mathrm{Ti}$ rate in the thermonuclear energy region from 1 to 250 keV. The new stellar rate leads at low temperatures to much higher values than the previously recommended rate; e.g., at $kT=8\mathrm{keV}$ the increase amounts to about 50%. The new reaction rate therefore reduces the abundance of ${}^{50}\mathrm{Ti}$ due to s processing in asymptotic giant branch stars.

Neutron capture of46Caat thermonuclear energies

The measurement of the neutron capture reaction ${}^{46}\mathrm{Ca}(n,\ensuremath{\gamma}{)}^{47}\mathrm{Ca}$ is of astrophysical interest, because ${}^{46}\mathrm{Ca}$ is bypassed by charged-particle reactions. The nucleus ${}^{46}\mathrm{Ca}$ is produced and destroyed by neutron-induced nucleosynthesis in hydrostatic helium, carbon, and neon burning through the reaction chain ${}^{45}\mathrm{Ca}(n,\ensuremath{\gamma}{)}^{46}\mathrm{Ca}(n,\ensuremath{\gamma}{)}^{47}\mathrm{Ca}.$ At the Karlsruhe and T\"ubingen 3.75 MV Van de Graaff accelerators the thermonuclear ${}^{46}\mathrm{Ca}(n,\ensuremath{\gamma}{)}^{47}\mathrm{Ca}(4.54d)$ cross section was measured by the activation technique via the 1297.09 keV $\ensuremath{\gamma}$-ray line of the ${}^{47}\mathrm{Ca}$ decay. Samples of ${\mathrm{CaCO}}_{3}$ enriched in ${}^{46}\mathrm{Ca}$ by 5% were irradiated between two gold foils which served as capture standards using the ${}^{7}\mathrm{Li}(p,n)$ and $T(p,n)$ reactions. The capture cross section was measured at the mean neutron energies 30, 104, 149, 180, and 215 keV, respectively. Maxwellian averaged capture cross sections were measured at the quasithermal neutron energies $kT=25$ and 52 keV, respectively. It was found that the ${}^{46}\mathrm{Ca}(n,\ensuremath{\gamma}{)}^{47}\mathrm{Ca}$ cross section in the thermonuclear energy region and at thermal energy is dominated by an s-wave resonance at 28.4 keV with a neutron width ${\ensuremath{\Gamma}}_{n}{=(17.4}_{\ensuremath{-}2.8}^{+3.6})$ keV and a radiation width ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}=(2.4\ifmmode\pm\else\textpm\fi{}0.3)$ eV. The stellar reaction rate is determined in the temperature range from $kT=1$ to 250 keV and is compared with previous investigations using Hauser-Feshbach calculations or experimental cross section data. The astrophysical consequences of the new stellar reaction rate with respect to the nucleosynthetic abundance of ${}^{46}\mathrm{Ca}$ are discussed.

Stellar neutron capture cross sections of Pr and Dy isotopes

The neutron capture cross sections of ${}^{141}\mathrm{Pr}{,}^{160}\mathrm{Dy}{,}^{161}\mathrm{Dy}{,}^{162}\mathrm{Dy}{,}^{163}\mathrm{Dy},$ and ${}^{164}\mathrm{Dy}$ have been measured in the energy range from 3 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the ${}^{7}\mathrm{Li}{(p,n)}^{7}\mathrm{Be}$ reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4$\ensuremath{\pi}$ Barium Fluoride Detector. The cross sections were determined relative to the gold standard. For the first time the correction for undetected capture events was completely obtained from experimental information, using capture cascades derived from measurements with an analog-to-digital converter system. The cross section ratios could be determined with an overall uncertainty of 1--1.5 %, an average improvement compared to previous measurements by a factor 4. Maxwellian averaged neutron capture cross sections were calculated for thermal energies between $kT=8 \mathrm{keV}$ and 100 keV. For most of the isotopes there is reasonable agreement with recent evaluations, but discrepancies of $\ensuremath{\sim}20%$ were obtained for ${}^{160}\mathrm{Dy}$ and ${}^{164}\mathrm{Dy}.$ The experimental data were complemented by statistical model calculations in order to describe the cross section enhancements in the stellar environment. The astrophysical implications of the present data include the quest for the origin of ${}^{160}\mathrm{Dy},$ the decomposition of the Dy abundances in the respective s- and r-process contributions, branchings of the s-process chain at the terrestrially stable isotopes ${}^{157}\mathrm{Gd}$ and ${}^{163}\mathrm{Dy},$ and the isotopically anomalous dysprosium in meteoritic inclusions.

Neutron capture of26Mgat thermonuclear energies

The neutron capture cross section of 26Mg was measured relative to the known gold cross section at thermonuclear energies using the fast cyclic activation technique. The experiment was performed at the 3.75 MV Van-de-Graaff accelerator, Forschungszentrum Karlsruhe. The experimental capture cross section is the sum of resonant and direct contributions. For the resonance at E(n,lab) = 220 keV our new results are in disagreement with the data from Weigmann et al. An improved Maxwellian averaged capture cross section is derived from the new experimental data taking into account s- and p-wave capture and resonant contributions. The properties of so-called potential resonances which influence the p-wave neutron capture of 26}Mg are discussed in detail.

R-Matrix Analysis of the 240Pu Neutron Cross Sections in the Thermal to 5700-eV Energy Range

A Bayesian R-matrix-fitting code called SAMMY is used in a new analysis of the 240Pu neutron cross sections in the 0.02- to 5700-eV energy range. This work aims to resolve the discrepancies revealed by the JEF2 validation studies. A set of experimental data, suitable for the analysis, is determined from preliminary SAMMY analyses of the available experimental data treated individually. Finally, the sequential analysis of the selected transmission, total, and fission cross-section measurements gives an accurate set of resonance parameters. For the 1.056-eV resonance, the analysis of a recent transmission measurement gives parameters very close to those adopted in ENDF/B-VI. In the energy range above 200 eV, the average capture cross section calculated from the present evaluation is in agreement with ENDF/B-VI but is 25% lower than the values of JEF-2 and JENDL-3. A rigorous study of the statistical properties of the resonance parameters is done during this work. Although 158 new resonances are identified by the careful examination of the experimental fission cross-section and transmission data, the value of the average spacing above 2750 eV (D0 = 16.10 eV) shows a 25% loss of resonances compared with the lower energy region (D0 = 12.06 eV). Finally, for the s-wave resonance parameters, this work recommends the following average values: resonance spacing, D0 = 12.06 ± 0.60 eV; strength function, S0 = (1.032 ± 0.071) × 10−4 eV; and capture width, .