Kernradien von12C,13C,14N und16O aus Elektronenstreuung zwischen 30 und 60 MeV

Elastic electron scattering cross sections of¹²C,¹⁴N and¹⁶O relative to the proton, and of¹³C relative to¹²C have been measured. Using harmonic oscillator wave functions the followingrms charge radiiR _m were deduced by phaseshift calculations: 2.395 (28) fm for¹²C, 2.384 (47) fm for¹³C, 2.492 (33) fm for¹⁵N, 2.666 (33) fm for¹⁶O. The ratioR _m (¹³C)/R _m (¹²C) is 0.995±0.008. The errors given do not include uncertainties from the model dependence of the evaluation which may be of the same magnitude.     

Lifetime of the second excited state of K

Energy distributions of neutrons from the (d, n) reactions on ^12–14C leading to unbound states of ^13–15N have been measured at 6.3 or 6.5 MeV deuteron energy. Angular distributions have been extracted for ^{13, 14}C(d, n) transitions and analysed with DWBA using the extra-polation technique to give l-values and transition strengths for ten unbound states in ¹⁴N and six in ¹⁵N. For the ¹⁵N level at 10.541 MeV it is concluded that J^π is $\text{3}\text{2}$^?. A new ¹⁵N level is observed at 11.44 MeV. The 0° (d, n) cross sections have been set in proportion to (p, p₀) resonance cross sections, and a pronounced l-dependence of the ratio is obtained.     

Measurement and folding-potential analysis of the elastic α-scattering on light nuclei

Differential cross sections ofα-elastic scattering have been measured for the target nuclei¹¹B,¹²C,¹³C,¹⁴N,¹⁵N, and¹⁶O atE=48.7 and 54.1 MeV and for the nuclei¹⁷O,¹⁸O, and²⁰Ne atE=54.1 MeV. The experimental results were analysed in terms of the optical model using different complex potentials. Special emphasis is given to the application of the double-folding approach for the real part of the potential. The imaginary part is expressed in terms of Fourier-Bessel functions. Differential cross sections for theα-¹⁶O scattering over a wide energy range and for the elasticα-scattering for nuclei in the mass rangeA=11 up toA=24 atE=54.1 MeV are analysed by this method. A close correlation between the absorptive part of the potential and nuclear deformation is observed.     

The decay of the18O giant dipole resonance to excited residual nuclear states

The decay of the¹⁸O GDR to excited residual nuclear states has been examined by measuring spectra of prompt deexcitationγ-rays. The target was irradiated by bremsstrahlung with endpoint energies of 23.5 and 28 MeV. “Bremsstrahlung-weighted” integrated cross sections are given for the population of the residual nuclear states in¹⁷O,¹⁷N,¹⁶O and¹⁴C. Proton decay to excited states in¹⁷N is remarkably weak. It seems that 1p _3/2 excitations play only a minor role in the¹⁸O GDR.     

Scattering and reactions of 14C + 14C and 14C + 14C

We have studied elastic scattering, inelastic scattering and several transfer channels of the systems ¹⁴C + ¹⁴C and ¹⁴C + ¹²C over a wide range of energies up to E_c.m. = 35 eMeV and 32 MeV, respectively. The reaction channels were identified by means of kinematic coincidences between solid-state detectors, γγ coincidences were measured to determine cross sections for mutual inelastic scattering of ¹⁴C + ¹⁴C.Pronounced regular gross structures, similar to those found for ¹⁶O + ¹⁶O, are observed in the elastic excitation function of ¹⁴C + ¹⁴C at θ_c.m. = 90°, The angular distributions measured at the energies of the maxima and an optical-model analysis suggest that one or a few surface partial waves dominate the scattering behaviour. Correlated structure of narrower width is found in the inelastic channels and, to a lesser degree, in the transfer channels which appear with rather small cross sections.In ¹⁴C + ¹²C elastic scattering the gross structures are strongly fragmented, in contrast to ¹⁴C + ¹⁴C but similar to ¹²C + ¹²C. While the ¹²C(2⁺) excitation is very weak, the observed strengths of the ¹⁴C(3^?) excitation and of neutron transfer point to a substantial role of these channels as coupling partners to the elastic configuration and to their influence on the elastic scattering behaviour. A correlated intermediate structure is observed near 23.5 MeV, where a dominance of l = 18 is suggested by the elastic scattering angular distribution. This unexpectedly high l-value exceeds l_graz at this energy by at least two units of ?.     

Elastische Elektronenstreuung an14N,15N,16O und18O bei kleiner Impulsübertragung

Elastic electron scattering cross sections of¹⁴N and¹⁶O have been measured relative to the proton and of¹⁵N and¹⁸O relative to the lighter isotope (¹⁴N,¹⁶O resp.) using gas targets. The momentum transfer ranged from 0.22 to 0.48 fm^?1. The data were analyzed by phase shift calculations assuming harmonic oscillator shell model charge distributions. The following rms charge radii have been deduced: R_m(¹⁴N)=2.540±0.020 fm R_m(¹⁵N)=2.580±0.026 fm R_m(¹⁶O)=2.718±0.021 fm R_m(¹⁸O)=2.789±0.027 fm. The errors include statistical and systematic uncertainties and an estimate of effects due to the choice of the model. The radius differences of the isotopes are smaller than the values predicted by anA ^1/3 relation     

Cross section measurements for the 9Be+9Be System at subbarrier energies

The total reaction cross section and the characteristic y-ray cross sections have been measured for the ⁹Be+ ⁹Be reaction in the energy range E_cm = 1.4–3.4 MeV, detecting the prompt γ-rays emitted by the various residual nuclei with two Nal detectors in nearly 4π geometry and with a germanium detector, respectively. The differential elastic cross sections for the same system have also been measured at e_c.m.= 2.2, 2.7 and 3.2 MeV. The cross sections calculated with the “standard” and the proximity optical model potentials, which describe well the total reaction cross sections of the light nuclei, agree with the ⁹Be + ⁹Be elastic-scattering data, but underpredict the total reaction cross section by a factor of 2 to 3. The characteristic γ-ray measurements show that all two-particle emission channels, nα ¹³C, nn¹⁶O, np¹⁶N and αα¹⁰Be are enhanced by about that factor, while the single-particle emission channel, p¹⁷N, is not enhanced.     

Total reaction and transfer cross sections for 11B + 9Be and 13C + 9Be reactions at low energies

The total reaction cross sections for ¹¹B + ⁹Be and ¹³C + ⁹Be have been measured by the total γ-ray yield method over the energy intervals E_c.m. = 1.4–4.4 MeV and E_c.m. = 2.0–5.2 MeV, respectively. The cross sections for the neutron transfer reactions ¹¹B(⁹Be, ⁸Be)¹²B, leading to the ¹²B 0.953 and 1.674 MeV states, and ¹³C(⁹Be, ⁸Be)¹⁴C, leading to the ¹⁴C 6.094, 6.728 and 6.902 MeV states, have been obtained from the yields of the characteristic γ-rays. The α-transfer reaction ¹¹B(⁹Be, ⁵He)¹⁵N, leading to many unresolved ¹⁵N states, has been observed with large cross section. There is, however, no evidence for the ¹³C(⁹Be, ⁵He)¹⁷O transfer process in the ¹⁷O + nα channels. This different behaviour of the ¹¹B + ⁹Be and ¹³C + ⁹Be systems seems to indicate that the α-transfer reaction at sub-barrier energies is not a direct transfer process, and that it probably occurs via molecular state formation.     

The15N(p,α 0)12C reaction at stellar energies

The¹⁵N(ρ, α₀)¹²C reaction has been investigated in the energy range ofE _p (lab)=78-810keV. The measurement of the excitation functions and α-particle angular distributions involved solid targets as well as a quasi-point supersonic jet gas target. The determination of absolute cross sections has been carried out with the gas target. The observed energy dependence of the total cross sections can be described in terms of two-level Breit-Wigner shapes including the resonances atE _p (J ^π)=335(1^?) and 1,028(1^?)keV. The data lead to a zero-energy intercept of the astrophysicalS(E) factor ofS(0)= 65±4MeV-b. The angular distributions are asymmetric around 90° and require an additional amplitude in the reaction mechanism, which interferes predominantly with the 335 keV resonance. The origin of this background amplitude is discussed.     

The (γ, n) reaction on 16O between giant resonance and pion threshold

A new TOF facility has been built for measurements of differential (γ, n) cross sections to discrete final states of light nuclei in the photon energy range between giant resonance and pion threshold. The observed neutron angle θ_n can continuously be varied between 0° and 150°, and additionally measurements at 175° and 180° are possible. Differential cross sections for the reaction ¹⁶O(γ, n₀)¹⁵O are presented for E_γ = 60 MeV (40° ? θ_n ? 149°) and for θ_n = 90° (60 MeV ? E_γ ? 160 MeV). The results, combined with the corresponding (γ, p₀) cross sections, indicate an absorption mechanism of high energy photons by neutron-proton pairs.     

Structures in the excitation functions for compound reactions in the12C+14C system

Excitation functions for the elastic scattering of¹²C on¹⁴C and the reactions¹⁴C(¹²C, α)²²Ne,¹⁴C(¹²C,t)²³Na and¹⁴C(¹²C,d)²⁴Na have been measured in the vicinity of the Coulombbarrier. Strong fluctuations of the differential cross sections as a function of incident energy are observed in the α-particle, triton and deuteron channels. The total yield in the three different channels shows correlated structures at energiesE _c.m.=6.8, 7.2 and 8.3 MeV. This phenomenon is similar to the structures observed in the¹²C+¹²C reactions and indicates the possible presence of resonances in the entrance channel.     

Experimental study for the production cross sections of positron emitters induced from 12C and 16O nuclei by low-energy proton beams

In proton therapy, positron emitters are induced from ¹²C and ¹⁶O nuclei by protons on the beam path in the patient. Many studies for monitoring positron emitters with beam-induced PET technique have been performed by various groups to verify the proton beam range and the dose in the patient for quality assurance (QA). The QA methods proposed by some groups require accurate production cross sections of the positron emitters produced by protons, especially in the low-energy region. The aim of this study was to develop a method for measuring the production cross sections of positron emitters using standard equipment for proton therapy, and to measure the cross sections of positron emitters produced by low-energy protons and verify them in comparison with data of previous experiments. An 80-MeV proton beam was produced by a synchrotron, and the energy was degraded by polyethylene blocks to obtain various beam energies. The number of protons was estimated from the charge induced in a parallel-plate ionization chamber by protons. Low-energy protons of 14–70 MeV were used to bombard ¹²C-rich and ¹⁶O-rich target materials: namely, polyethylene and gelatinous water. The time-activity curve was then measured by a high-sensitivity PET scanner to extract the number of positron emitters produced in the target. The production cross sections for four reaction channels: ¹⁶O(p, pn)¹⁵O, ¹⁶O(p, 3p3n)¹¹C, ¹⁶O(p, 2p2n)¹³N, and ¹²C(p, pn)¹¹C were then measured. The cross sections for the ¹⁶O(p, pn)¹⁵O reaction channel were consistent with data of previous experiments within the uncertainties, while those of ¹²C(p, pn)¹¹C were generally lower than data of previous experiments. These results suggested that further measurements of the production cross sections will be necessary.     

Winkelverteilung der Polarisation der Neutronen aus der Reaktion14C(d,n 0)15N bei Deuteronenenergien zwischen 1,3 und 1,9 MeV

Polarization distributions of ground state neutrons emitted from the¹⁴C(d,n ₀)¹⁵N reaction were investigated over the angular range from 15 ° (lab) to 150 ° (lab) at bombarding energies of 1.28, 1.55 and 1.88 MeV. Scattering of neutrons from helium served as polarization analyzer. The experimental results show a large variation with energy of the polarization ranging betweenP(Θ_lab=130 °)=?21% andP(Θ_lab=130 °)=+50% at 1.28 and 1.88 MeV respectively.     

Comparison of one and two-neutron transfer near the coulomb barrier for the27Al(18O,16O)29Al,27Al(180,17O)28Al and27Al(13C,12C)28Al reactions

Total reaction cross sections for the transfer reactions²⁷Al(¹⁸O,¹⁶O)²⁹Al,²⁷Al(¹⁸O,¹⁷O)²⁸Al and²⁷Al(¹³C,¹²C)²⁸Al are reported for center-of-mass energies between 13 and 20 MeV for¹⁸O projectiles and between 11 and 17.5 MeV for¹³C projectiles. The reaction products,²⁹Al and²⁸Al, beta decay to²⁹Si and²⁸Si, respectively, and the subsequentγ decays of²⁹Si and²⁸Si were measured. Due to the relatively long beta decay half lives, data were taken in a beam-off mode, resulting in very clean spectra. Total cross sections were calculated and compared with a theoretical model for barrier penetration proposed by C.Y. Wong. Differences between¹⁸O induced one and two-neutron total transfer reaction cross sections are discussed.     

Energy damping feature in light heavy-ion reactions

The energy damped reaction products from³⁷Cl+¹²C,²⁷Al,⁴⁸Ti and¹⁶O+⁴⁸Ti were measured over a wide range of angles (typically 18°<θ _lab<70°), incident energies (160 <E _lab(³⁷Cl)<200 MeV,E _lab(¹⁶O)=118 MeV) and charges Z, including two systems (³⁷Cl+³⁷Al and¹⁶O+⁴⁸Ti) which lead to the same compound nucleus⁶⁴Zn with the same excitation energy and comparable angular momenta. The angular dependences of total kinetic energy (TKE) and dσ/dθ were decomposed into two components (forward peaked and nearly constant at backward angles), and the elemental TKE and cross sections were derived. The backward components of³⁷Cl+²⁷Al and¹⁶O+⁴⁸Ti exhibit very different Z-distributions, indicating that the fragments do not originate from compound nucleus decay. The results can be understood in terms of an energy damping process.     

Direct and compound nuclear contributions to the 13C(6Li,t) reaction at Elab = 25 MeV

The reaction ¹³C(⁶Li, t)¹⁶O has been studied in the incident energy range 24–26 MeV. Complete angular distributions have been measured at E_⁶Li, = 25 MeV in the angular range θ_lab = 8°–172°, with the reaction ⁶Li(¹³C, t)⁶O being used for the backward angle measurements. Cross sections for evaporation residues from the fusion of the ⁶Li + ¹³C system have been measured in the incident ⁶Li energy range 9.2–35.1 MeV. Compound nuclear contributions to the transfer cross sections have been calculated using the Hauser-Feshbach statistical theory with the assumption that the compound-nucleus formation cross section is equal to the measured fusion cross section. By comparison of the compound nuclear calculations with backward angle data it is found that the sharp cutoff approximation commonly used to represent the initial angular momentum distribution of the compound nucleus is not adequate for the ¹³C(⁶Li, t)¹⁶O reaction. Good fits to the backward angle data can be obtained by using a smooth cutoff approximation. The forward angle cross sections have been compared with exact finite-range distorted-wave Born approximation calculations to extract transferred angular momenta and spectroscopic strengths. The present results differ from those of an earlier study. These differences are due to the inclusion of forward angle data in the present study.     

Comparison of the fusion reactions12C+20Ne and16O+16O near the Coulomb barrier

The partial cross sections of heavy residual nuclei produced in the heavy ion fusion of¹²C+²⁰Ne have been measured atE _c.m.=6–15 MeV viaγ-ray spectroscopy with a Ge(Li) detector. Windowless and recirculating gas target systems have been used. The dominant residual nuclei are²⁴Mg,²⁷Al,²⁸Si,³⁰Si,³⁰P and³¹P, which arise from two- and three-body breakups in the exit channels. The observed excitation functions are smooth in their energy dependence and give no indications for the existence of pronounced resonance structures, in contrast to theoretical predictions. The Coulomb excitation of²⁰Ne served as an intrinsic calibration standard in the determination of absolute partial and total fusion cross sections. The same experimental set-up was also used in the reaction studies of¹⁶O+¹⁶O atE _c.m.=7–14 MeV, going through the same compound nucleus³²S at similar excitation energies. The observed energy dependence in the excitation functions is in good agreement with previous work. The total fusion cross section agrees fairly well with two sets of values reported previously, but deviates significantly from other reported absolute cross section values. The relative evaporation distributions of the residual nuclei are similar for both heavy ion reactions. However, the ratio of their total fusion cross sections deviates from model predictions and suggests that compound nucleus formation does depend on the microscopic structure of the colliding nuclei in the entrance channel. From the observed energy dependence of the above ratio, particularly at subcoulomb energies, geometrical effects in the entrance channel (due to deformed and spherical nuclei) appear to be weak. The astrophysical aspects of the data in the context of late stellar nucleosynthesis are discussed.     

The 14C(p, γ0)15N cross section below Ep = 12 MeV

The cross section for the ¹⁴C(p.γ₀)¹⁵N reaction has been measured up to E_p = 12 MeV. These measurements cover the region of the main component of the giant dipole resonance in ¹⁵N previously observed in the inverse ¹⁵N(γ, p₀)¹⁴C reaction. The structures seen are compared to recent measurements of the ¹⁴N(p, γ₀)¹⁵O cross section at corresponding energies in ¹⁵O, allowing clear identification of $\text{T =}\text{1}\text{2}\text{and}\text{3}\text{2}$ states. A comparison of the present experimental results is also made to recent bound and continuum shell model calculations for this region of ¹⁵N.