Radiative Capture Data Research Articles

Neutron Capture and Transmission Measurements of 54Fe at the RPI LINAC

54Fe radiative capture cross section and transmission measurements were conducted at the Rensselaer Polytechnic Institute (RPI) Gaerttner Linear Accelerator (LINAC) Center using an enriched 54Fe sample in the keV energy region. 54Fe is a constituent of natural iron, which is present in a large variety of nuclear grade materials. Therefore, it is important to have an accurate understanding of the cross sections of 54Fe, which can be measured experimentally. In the time-of-flight measurements conducted at the LINAC, an array of four C6D6 detectors surrounded the sample and radiative capture data were collected using a digital data acquisition system. Additionally, a Li-glass detector was used to collect transmission data using an analog data acquisition system. The radiative capture yield of the 54Fe measurements were normalized to saturated resonances observed in Au and Ta to obtain an absolute capture yield. The preliminary capture yield and preliminary transmission obtained can be compared to evaluations and existing experimental data. Some disagreements were observed in prominent d-wave capture resonances observed in 54Fe in the low-keV neutron energy region. Both sets of experimental data along with pre-existing datasets will greatly enhance RPI’s ability to perform resonance evaluation for 54Fe up to roughly 1 MeV.

Halo effective field theory constrains the solar 7Be + p → 8B + γ rate

We report an improved low-energy extrapolation of the cross section for the process Be7(p,γ)B8, which determines the 8B neutrino flux from the Sun. Our extrapolant is derived from Halo Effective Field Theory (EFT) at next-to-leading order. We apply Bayesian methods to determine the EFT parameters and the low-energy S-factor, using measured cross sections and scattering lengths as inputs. Asymptotic normalization coefficients of 8B are tightly constrained by existing radiative capture data, and contributions to the cross section beyond external direct capture are detected in the data at E<0.5 MeV. Most importantly, the S-factor at zero energy is constrained to be S(0)=21.3±0.7 eVb, which is an uncertainty smaller by a factor of two than previously recommended. That recommendation was based on the full range for S(0) obtained among a discrete set of models judged to be reasonable. In contrast, Halo EFT subsumes all models into a controlled low-energy approximant, where they are characterized by nine parameters at next-to-leading order. These are fit to data, and marginalized over via Monte Carlo integration to produce the improved prediction for S(E).

Description of dipole strength in heavy nuclei in conformity with their quadrupole degrees of freedom

In conformity to new findings about the widespread occurrence of triaxiality arguments are given in favor of a description of the giant dipole resonance in heavy nuclei by the sum of three Lorentzians. This TLO parameterization allows a strict use of resonance widths {\Gamma} in accordance to the theoretically founded power law relation to the resonance energy. No additional variation of {\Gamma} with the photon energy and no violation of the sum rule are necessary to obtain a good agreement to nuclear photo-effect, photon scattering and radiative capture data. Photon strength other than E1 has a small effect, but the influence of the level density on photon emission probabilities needs further investigation.

The 7Be(p, γ) 8B Reaction and its Future

The experimental landscape for the 7 Be+p radiative capture reaction is rapidly changing as new high precision data become available. We present an evaluation of existing data, detailing the treatment of systematic errors and discrepancies, and show how they constrain the astrophysical S factor ( S 17 ), independent of any nuclear structure model. With theoretical models robustly determining the behavior of the sub-threshold pole, the extrapolation error can be reduced and a constraint placed on the slope of S 17 . Usin only radiative capture data, we find S 17 ( 0 ) = 20.7 ± 0.6 ( stat ) ± 1.0 ( syst ) eV b if data sets are completely independent, while if data sets are completely correlated we find S 17 ( 0 ) = 21.4 ± 0.5 ( stat ) ± 1.4 ( syst ) eV b . The truth likely lies somewhere in between these two limits. Although we employ a formalism capable of treating discrepant data, we note that the central value of the S factor is dominated by the recent high precision data of Junghans et al., which imply a substantially higher value than other radiative capture and indirect measurements. Therefore we conclude that future progress will require new high precision data with a detailed error budget.

Watson’s theorem for low-energyp−dradiative capture

The use of Watson's theorem in the analysis of $p\ensuremath{-}d$ radiative capture measurements at low energies is discussed. The principle of Watson's theorem is outlined, and a detailed description of how the theorem can be used in a matrix element analysis of radiative capture data is presented. It is shown that with Watson's theorem it is possible to reduce the number of unknown parameters in a matrix element analysis by essentially a factor of 2. This is done by employing a representation in which the capture matrix elements are all real. The phase information needed to construct the reaction amplitudes is then obtained from a separate phase shift analysis of elastic scattering data. Details concerning the extension of Watson's theorem to situations in which there is mixing between angular momentum states are given. The paper presents a consistent formulation which facilitates the simultaneous analysis of the elastic scattering and radiative capture channels.

Present experimental constraints on the total S-factor of 12C(α,γ) 16O and proposals for future experiments

Because of the critical importance of the 12 C 16 O ratio resulting from helium burning to the later evolution of massive stars, R-matrix fits have been made to the available angular distribution data from radiative α-capture and elastic α-scattering on 12C to estimate the total 12C(α,γ) 16O rate at stellar energies. Largely primary data, i.e. energy-dependent differential cross sections, are used in the analysis with all relevant partial waves being fitted simultaneously (surface fits). It is shown that while the E1 part of the reaction is well constrained by a recent experiment on the β-delayed α-particle decay of 16N only upper limits can be placed on the E2 ground state transition which we take conservatively as S E2 (300) <140keV b. Monte-Carlo simulations were subsequently carried out to explore what kind of future data could lead to better restrictions on S E2 (300). We find that improved elastic scattering data may be the best candidate for such restrictions while improving S(300) with new radiative capture data seems to be much more difficult.

Off-shell effects in the energy dependence of the 7Be( p, γ) 8B astrophysical S factor

I show that off-shell effects, like antisymmetrization and 7Be distortions, can significantly influence the energy dependence of the nonresonant 7Be( p, γ) 8B astrophysical S factor at higher energies. The proper treatment of these effects results in a virtually flat E1 component of the S factor at E cm = 0.3 – 1.5 MeV energies in the present eight-body model. The energy dependence of the nonresonant S factor, predicted by the present model, is in agreement with the low-energy radiative capture data and the existing high-energy Coulomb dissociation data. Irrespective of whether or not the present energy dependence is correct, off-shell effects can cause 15–20% changes in the value of S(0) extrapolated from high-energy ( E cm > 0.7 MeV) data.

Analysis of the total 12C( alpha, gamma )16O cross section based on available angular distributions and other primary data.

Because a knowledge of the 12C/16O ratio is crucial to the understanding of the later evolution of massive stars, new R- and K-matrix fits have been completed using the available angular distribution data from radiative α capture and elastic α scattering on 12C. Estimates of the total 12C(α,γ)16O rate at stellar energies are reported. In contrast with previous work, the analyses generally involve R- and K-matrix fits directly to the primary data, i.e., the energy- and angle-dependent differential yields, with all relevant partial waves fitted simultaneously (referred to here as surface fits). It is shown that, while the E1 part of the reaction is well constrained by a recent experiment on the β-delayed α-particle decay of 16N, only upper limits can be placed on the E2 ground state cross section factor which we take conservatively as SE2(300)<140 keV b. Simulations were then carried out to explore what kind of new data could lead to better restrictions on SE2(300). We find that improved elastic scattering data may be the best short-term candidate for such restrictions while significantly improving S(300) with new radiative capture data may require a longer-term effort. Theoretical models and estimates from α-transfer reactions for the E2 part of 12C(α,γ)16O are then discussed for comparison with the R- and K-matrix fits of the present work.

Reaction pi -p--> gamma n below the Delta resonance.

The reaction ${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}\ensuremath{\gamma}n has been studied at ${T}_{\ensuremath{\pi}}$=45.6, 62.2, 76.4, 91.7, 106.8, and 121.9 MeV for nine laboratory angles between 30\ifmmode^\circ\else\textdegree\fi{} and 140\ifmmode^\circ\else\textdegree\fi{}. The differential cross section measurements have angle to angle errors of about 4% with an additional 3% normalization error. The results are typically a factor of 2 more accurate than previous radiative capture data and are more reliable than cross sections obtained from the reverse reaction \ensuremath{\gamma}d\ensuremath{\rightarrow}${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$${\mathrm{pp}}_{s}$. The data are compared with recent multipole analyses, various calculations, as well as previous data, and the best agreement is with the Tokyo multipole analysis.

Direct E2 neutron capture in light nuclei.

We present an analysis of the E2 strength data recently available from thermal radiative neutron capture within the context of direct capture theory. The radiative capture data require a neutron E2 effective charge identical to the one used in bound state shell model calculations, in marked contrast to the E1 situation. We suggest that this difference has its origin in the nature of the coupling governing the particle-core interaction in E2 and E1 radiative capture processes.

Limits on the Isotensor Electromagnetic Current in Pion Photoproduction

Recent data on the π- / π + ratio for photoproduction in the first resonance region are analyzed using a model developed previously. Within the model, the data set an upper limit of about 3 percent on the ratio of isotensor to isovector resonance excitation, assuming T invariance. The pi radiative-capture data are investigated again within the model, to assess the evidence for this latter assumption.

Neutron Capture in Fluorine Below 1500 keV

Neutron time-of-flight radiative capture data taken at the Oak Ridge electron linear accelerator have been analyzed for single-level resonance parameters. Eleven resonances were found, with parameters as indicated, listing ${E}_{0}$ (keV), $J$ value assumed, and ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$ (eV) in order: 27.07 (2) 1.4 \ifmmode\pm\else\textpm\fi{} 0.3, 48.7 (1) 1.7 \ifmmode\pm\else\textpm\fi{} 0.4, 97.0 (1) \ensuremath{\le}6.0 \ifmmode\pm\else\textpm\fi{} 1.8, 269 (2) 3.5 \ifmmode\pm\else\textpm\fi{} 0.8, 270 (1) \ensuremath{\le} 4.4, 386 (1) \ensuremath{\ge} 7\ifmmode\pm\else\textpm\fi{}2, 490.5 (0) \ensuremath{\ge} 10\ifmmode\pm\else\textpm\fi{}3, 595 (2) \ensuremath{\ge} 7 \ifmmode\pm\else\textpm\fi{} 2, 1460 (1) \ensuremath{\ge} 11\ifmmode\pm\else\textpm\fi{}3. Values of total width were also found for these resonances. Two resonances are very narrow and their capture areas yield estimates of $g{\ensuremath{\Gamma}}_{n}$. At 43.5 keV, $g{\ensuremath{\Gamma}}_{n}=0.086\ifmmode\pm\else\textpm\fi{}0.02$ eV if $J\ensuremath{\ge}1$ or ${\ensuremath{\Gamma}}_{n}=0.42\ifmmode\pm\else\textpm\fi{}0.1$ eV if $J=a$, and, at 173.5 keV, $g{\ensuremath{\Gamma}}_{n}=0.35\ifmmode\pm\else\textpm\fi{}0.10$ eV. The increase of nearly an order of magnitude in radiative width with increasing energy up to 600 keV is notable. Twelve large resonances between 1600 and 5000 keV were not analyzed for capture because of detector sensitivity to the inelastic scattering channels which open in that energy region.

Discussion of Isotensor Terms andTViolation in Pion Photoproduction near the First Resonance

An analysis is made of the presently available experimental and theoretical information for pion photoproduction and ${\ensuremath{\pi}}^{\ensuremath{-}}$ radiative capture near the first resonance with regard to the possibility of a violation of the $\ensuremath{\Delta}I\ensuremath{\le}1$ rule, and the possibility of a $T$-violating electromagnetic current. An isotensor term is only one of the several possible explanations of the recent ${\ensuremath{\pi}}^{\ensuremath{-}}$ photoproduction data. The radiative-capture data near the first resonance support a substantial $T$ violation in the magnetic dipole amplitude. However, it is not established that this $T$ violation may be ascribed to a single isospin part of the amplitude. Some predictions for experimental tests are given which could clarify the situation.