Radiative Capture Reactions Research Articles

Design of the Notre Dame recoil separator

The study of low energy radiative capture reactions is of prime interest for many nuclear astrophysics scenarios. While many measurements have been made using various setups in direct kinematics, the very small cross section of these reactions at astrophysically interesting energies, along with the beam induced background, limit the possible range of measurements. A facility for measuring cross sections of radiative 4He capture in inverse kinematics has been designed. This 20m long recoil mass separator (St. George) will be coupled to the KN Van de Graaff accelerator at the University of Notre Dame. The goal of St. George is to separate a low mass (A⩽40), low energy (E<0.5MeV/A), high intensity (∼100μA) beam from the few reaction products emerging from a windowless 4He jet gas target. The desired acceptance is Δθ=±40mrad and ΔE/E=±7.5%.

Nuclear Data Sensitivity Analyses of OKTAVIAN Shielding Benchmark Experiments

Nuclear data validations through the fusion neutronics shielding benchmarks for the OKTAVIAN nickel, iron, aluminum, silicon, and tungsten spheres were carried out with the MCNPX-2.5.0 code. The measured neutron and/or gamma-ray leakage spectra were compared with the calculation results using the ACE format libraries processed based on ENDF/B-VII.0, JEFF-3.1, JENDL-3.3, and FENDL-2.1. These benchmarks were suitable for the nuclear data sensitivity analyses through comparisons of the major reaction rates depending on the neutron energy. The neutron and gamma-ray leakage spectra were dominated by the productions from the (n,2n) and inelastic scattering reactions rather than those from the radiative capture reaction.

Low-energy radiative-capture reactions within two-cluster coupled-channel description

The formalism that describes radiative-capture reactions at low energies within an extended two-cluster potential model is presented. Construction of the operator of single-photon emission is based on a generalisation of the Siegert theorem with which the amplitude of the electromagnetic process is constructed in an explicitly gauge-independent way. While the starting point for this construction is a microscopic (single-nucleon) current model, the resulting operator of low-energy photon emission by a two-cluster system is expressed in terms of macroscopic quantities for the clusters and does not depend directly on their intrinsic coordinates and momenta. The multichannel algebraic scattering (MCAS) approach has been used to construct the initial- and final-state wave functions. We present a general expression for the scattering wave function obtained from the MCAS T matrix taking into account inelastic channels and Coulomb distortion. The developed formalism has been tested on the He 3 ( α , γ ) Be 7 reaction cross section at astrophysical energies. The energy dependence of the evaluated cross section and S factor agrees well with that extracted from measurement though the calculated quantities slightly overestimate data.

Insights into nuclear clusters in 28Si via resonant radiative capture measurements

The heavy-ion radiative capture reaction 12C(16O,γ)28Si has been studied at three energies on- (ELab = 20.0 and 21.2 MeV) and off- (ELab = 20.7 MeV) resonance at Triumf (Vancouver) using the state-of-the-art Dragon 0° spectrometer and its very efficient associated BGO γ array. Intermediate states around Ex = 11.5 MeV, carrying a large part of the resonant flux have been observed for the first time in this system. The nature of those doorway states is discussed in terms of recently calculated cluster bands in 28Si. The results are compared to a recent similar investigation of the 12C(12C,γ)24Mg reaction.

Breakup reaction models for exotic nuclei

Breakup reactions are one of the main tools for the study of exotic nuclei. In particular, Coulomb breakup is expected to provide information on spectroscopic properties of halo nuclei and on astrophysical S factors for radiative-capture reactions. The simplest studies are based on perturbation theory and especially on its first order. However the validity of the first-order approximation may be limited for extended systems such as halo nuclei and its conditions are not always satisfied in existing experiments. More elaborate reaction models are available: resolution of the semi-classical time-dependent Schrodinger equation, eikonal and dynamical eikonal approximations, method of coupled discretized-continuum channels, adiabatic approximation. These methods are reviewed, summarized and illustrated. Their interest and limitations are discussed.

A hybrid surface arc discharge ion source to produce ultra pure Ca+2 beams for Ca40(α,γ)Ti44 reaction studies at ISAC/TRIUMF

ISAC is an accelerator facility primarily dedicated to astrophysical studies. Off-line and online ion sources provide up to 65 keV of stable and radioactive beams to the ISAC accelerators. Initial acceleration is done via a constant velocity radio frequency quadrupole that requires 2 keV/u. Then the beam is further accelerated to 1.5 MeVu at ISAC-I and 6.5 MeV/u at ISAC-II. To study radiative capture reactions relevant for astrophysics, the recoil mass spectrometer DRAGON was built in the experimental area. (40)Ca(alpha,gamma)(44)Ti is identified as one of the key reactions in supernovae to produce (44)Ti and is given highest priority. For this experiment, an ultrapure Ca(+2) beam was requested from the off-line ion source. Initial tests showed that, when using conventional ion sources, (40)Ar and (40)K are the impurities that are most difficult to eliminate. In order to overcome this problem, a new concept was needed and the hybrid surface arc discharge ion source was born. The hybrid surface ion source consists of a small surface ionizer and an arc discharge placed in a solenoid field. A very low ratio of (40)Ar/(40)Ca=8 x 10(-5) was achieved with this new source and the experiment was completed successfully. The source is described in detail and its performance is discussed in this article.

P+D radiative capture and mesonic effects

We have measured precise data for the vector- and tensor-analyzing powers of the radiative capture reaction p+d→ 3He+γ at deuteron energies of 29 MeV and 45 MeV. These data are compared to modern three-body calculations that treat both the bound and the continuum state exactly. We show that the analyzing powers exhibit very large effects of meson exchange currents, despite the fact that the momentum transfers involved are very small. The MEC are well described by theory. We also find that, at extreme angles, relativistic effects are important.

Coulomb dissociation of 8B and 6Li

The analysis of high-energy Coulomb dissociation experiments with 8B and 6Li beams in order to extract low-energy S factors of the 7Be(p,γ)8B and 4He(d,γ)6Li radiative capture reactions relies on a precise knowledge of the reaction mechanism. A comparison of various experimental cross sections with predictions from theory allows us to identify the relevant ingredients that are necessary for a successful theoretical description. The cases of 8B and 6Li Coulomb breakup show substantial differences related to the structure of the nuclei.

Decay strength distributions inC12(C12,γ) radiative capture

The heavy-ion radiative capture reaction, {sup 12}C({sup 12}C,{gamma}), has been investigated at energies both on- and off-resonance, with a particular focus on known resonances at E{sub c.m.}=6.0, 6.8, 7.5, and 8.0 MeV. Gamma rays detected in a BGO scintillator array were recorded in coincidence with {sup 24}Mg residues at the focal plane of the DRAGON recoil separator at TRIUMF. In this manner, the relative strength of all decay pathways through excited states up to the particle threshold could be examined for the first time. Isovector M1 transitions are found to be a important component of the radiative capture from the E{sub c.m.}=6.0 and 6.8 MeV resonances. Comparison with Monte Carlo simulations suggests that these resonances may have either J=0 or 2, with a preference for J=2. The higher energy resonances at E{sub c.m.}=7.5 and 8.0 MeV have a rather different decay pattern. The former is a clear candidate for a J=4 resonance, whereas the latter has a dominant J=4 character superposed on a J=2 resonant component underneath. The relationship between these resonances and the well-known quasimolecular resonances as well as resonances in breakup and electrofission of {sup 24}Mg into two {sup 12}C nuclei are discussed.

Current status of the α + d → 6Li + γ reaction

The current experimental and theoretical situation for the radiative capture reaction α + d → 6Li + γ is reviewed. Particular emphasis is placed upon the low-energy region important for astrophysical and thermonuclear applications.

Excitation functions for complete-fusion and transfer reactions in 4He interaction with 197Au nuclei

Excitation functions are measured for the fusion reactions {sup 197}Au({sup 4}He, xn){sup 201-xn}Tl that are induced by alpha-particle interaction with gold nuclei in the energy range 14-36 MeV and in which x neutrons (0 {<=} x {<=} 3) are evaporated. The stack-activation technique was used to record and separate reaction products. Experimental data on the fusion reactions followed by evaporation of one to three neutrons agree with results of previous studies. For the radiative-capture reaction {sup 197}Au({sup 4}He,{gamma}){sup 201}Tl, the upper limit on the cross section proved to be much lower. The excitation functions for the reactions subjected to measurements are compared with the results of calculations based on the statistical model and with the results of an experiment performed previously in a {sup 6}He beam.

CNO hydrogen burning studied deep underground

In stars, four hydrogen nuclei are converted into a helium nucleus in two competing nuclear fusion processes, namely the proton-proton chain (p-p chain) and the carbon-nitrogen-oxygen (CNO) cycle. For temperatures above 20 million kelvin, the CNO cycle dominates energy production, and its rate is determined by the slowest process, the 14N(p,γ)15O radiative capture reaction. This reaction proceeds through direct and resonant capture into the ground state and several excited states in 15O. High energy data for capture into each of these states can be extrapolated to stellar energies using an R-matrix fit. The results from several recent extrapolation studies are discussed. A new experiment at the LUNA (Laboratory for Underground Nuclear Astrophysics) 400kV accelerator in Italy's Gran Sasso laboratory measures the total cross section of the 14N(p,γ)15O reaction with a windowless gas target and a 4π BGO summing detector, down to center of mass energies as low as 70keV. After reviewing the characteristics of the LUNA facility, the main features of this experiment are discussed, as well as astrophysical scenarios where cross section data in the energy range covered have a direct impact, without any extrapolation.

Calculation of the nuclear vertex constant (asymptotic normalization coefficient) for the virtual decay 6Li → α + d on the basis of the three-body model and application of the result in describing the astrophysical nuclear reaction d(α, γ)6Li at ultralow energies

Within the (α, n, p) three-body model, the s-wave 6Li → α + d vertex constant G01 and the asymptotic normalization coefficient C01 (these two quantities are proportional to each other) for the wave function describing the 6Li nucleus in the α + d channel are calculated by solving Faddeev equations for various sets of nucleon-nucleon and αN potentials. The resulting values of G01 and C01 are used to calculate the astrophysical factor S(E) for the radiative-capture reaction d(α, γ)6Li at energies in the range E = 0–600 keV. The calculated values of G01 and C01 appear to be sensitive to the form of the pair potentials used. It is shown that, upon correctly taking into account the contributions of the E2 and E1 multipolarities, the values of S(E) in the region E < 150 keV are determined primarily by the values of C01 and of the 6Li binding energy in the α + d channel.

Description of the 17F(p, γ) 18Ne radiative capture reaction in the continuum shell model

The shell model embedded in the continuum is applied to calculate the astrophysical S-factor and the reaction rate for the radiative proton capture reaction 17F(p, γ) 18Ne. The dominant contribution to the cross-section at very low-energies is due to M1 transitions J i π = 2 + → J f π = 2 1 + whose magnitude is controlled by a weakly bound 2 2 + state at the excitation energy E x = 3.62 MeV .

Commissioning the DRAGON facility at ISAC

The performance of the DRAGON recoil spectrometer at TRIUMF-ISAC has been studied using radiative capture reactions with stable beams of 12C, 20Ne, 21Ne, 23Na, 24Mg and 26Mg. Calibration of the deflection magnet measuring the beam energy was established and the beam suppression factors of the separator were investigated. Yields from six narrow resonances were measured and compared with previous results. For the 1,112.6 keV resonance in Ne 20 ( p , γ ) Na 21 our result is in disagreement with the NACRE database assignment but agrees with another previous result.

Three-body resonant radiative capture reactions in astrophysics

We develop the formalism based on the S-matrix for $3 \to 3$ scattering to derive the direct three-body resonant radiative capture reaction rate. Within this formalism the states, which decay only/predominantly directly into three-body continuum, should also be included in the capture rate calculations. %The results obtained indicate that some theoretical calculations are %incomplete. Basing on the derivation, as well as on the modern experimental data and theoretical calculations concerning $^{17}$Ne nucleus, we significantly update the reaction rate for $^{15}$O($2p$,$\gamma$)$^{17}$Ne process in explosive environment. We also discuss possible implementations for the $^{18}$Ne($2p$,$\gamma$)$^{20}$Mg, $^{38}$Ca($2p$,$\gamma$)$^{40}$Ti, and $^4$He($n\alpha$,$\gamma$)$^{9}$Be reactions.

Multichannel study of theC13(α,n)O16andO16(n,γ)O17reactions

The $^{13}\mathrm{C}(\ensuremath{\alpha},n)^{16}\mathrm{O}$ reaction is investigated in a microscopic two-cluster model with an effective interaction recently developed for transfer reactions. The wave functions are defined in the generator coordinate method. The basis includes all $^{13}\mathrm{C}$+\ensuremath{\alpha} and $^{16}\mathrm{O}$+n channels with $^{13}\mathrm{C}$ and $^{16}\mathrm{O}$ described in the p shell. Particle-hole excitations in the sd shell are also taken into account in order to include the $^{16}\mathrm{O}$$({3}^{\ensuremath{-}})$+n channel. The $^{17}\mathrm{O}$ spectroscopy is in good agreement with experiment. In particular, the width of the astrophysically relevant $1/{2}_{2}^{+}$ state is well reproduced ($\ensuremath{\Gamma}=121\phantom{\rule{0.3em}{0ex}}\text{keV}$ as compared to the experimental value $\ensuremath{\Gamma}=124\ifmmode\pm\else\textpm\fi{}12\phantom{\rule{0.3em}{0ex}}\text{keV}$). The calculation of the $^{13}\mathrm{C}(\ensuremath{\alpha},n)^{16}\mathrm{O}$ S factor shows the importance of the $3/{2}_{3}^{+}$ resonance, and provides $S(0.2\phantom{\rule{0.3em}{0ex}}\mathrm{MeV})=2.1\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}\text{MeV}$ b, slightly lower than the NACRE recommended value ($2.5\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}\text{MeV}$ b). The same model is applied to the $^{16}\mathrm{O}(n,\ensuremath{\gamma})^{17}\mathrm{O}$ radiative-capture reaction at thermal and astrophysical energies.

Electromagnetic structure of A=2 and 3 nuclei and the nuclear current operator

Different models for conserved two- and three-body electromagnetic currents are constructed from two- and three-nucleon interactions, using either meson-exchange mechanisms or minimal substitution in the momentum dependence of these interactions. The connection between these two different schemes is elucidated. A number of low-energy electronuclear observables, including (i) np radiative capture at thermal neutron energies and deuteron photodisintegration at low energies, (ii) nd and pd radiative capture reactions, and (iii) isoscalar and isovector magnetic form factors of $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$, are calculated to make a comparative study of these models for the current operator. The realistic Argonne ${v}_{18}$ two-nucleon and Urbana IX or Tucson-Melbourne three-nucleon interactions are taken as a case study. For $A=3$ processes, the bound and continuum wave functions, both below and above deuteron breakup threshold, are obtained with the correlated hyperspherical harmonics method. Three-body currents give small but significant contributions to some of the polarization observables in the $^{2}\mathrm{H}$($p,\ensuremath{\gamma}$)$^{3}\mathrm{He}$ process and the $^{2}\mathrm{H}$($n,\ensuremath{\gamma}$)$^{3}\mathrm{H}$ cross section at thermal neutron energies. It is shown that the use of a current that did not exactly satisfy current conservation with the two- and three-nucleon interactions in the Hamiltonian was responsible for some of the discrepancies reported in previous studies between the experimental and theoretical polarization observables in pd radiative capture.

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.

Behaviour of radiative-capture reactions near zero energy

The Taylor expansion of the astrophysical S factor for radiative-capture reactions between charged particles is calculated in a direct way near zero energy in the potential and R matrix models. The 14 N(p, γ ) 15 O reaction is treated as an example. A similar expansion derived for neutron capture provides simple low-energy parametrizations of the 12 C(n, γ ) 13 C cross sections.