Possible alpha-particle cluster absorption of pions by 16O

It is shown that ¹²C 4.44 MeV γ-rays result from π^? absorption at rest in ¹⁶O. The ¹²C recoil momentum distribution was determined from the Doppler-broadening and compared with (p,pα) measurements on ¹⁶O.     

Study of some two-nucleon pick-up reactions in the 1p shell with 39.8 MeV protons

Differential cross sections at a proton energy of 39.8 MeV (lab) have been measured for the following reactions (energies in MeV): ¹²C(p,τ)¹⁰B(g.s., 0.72, 1.74, 2.15, 3.59), ¹⁴C(p, t)¹²C(g.s., 4.43), ¹⁴N(p, τ)¹²C(g.s., 4.43), and ¹⁶O(p,τ)¹⁴N(g.s., 2.31, 3.95). A zero-range DWBA analysis of the data has been performed using the 1p shell wave functions of Cohen and Kurath. The fits we find are overall somewhat worse in shape than those found in the (p, t) survey of the 1p shell performed by Kahana and Kurath, the principal reason being that of the (p, τ) transitions which proceed with both L = 0 and L = 2 components we find several which occur with a much weaker L = 0 strength than the calculations predict. When ratios of experimental integrated cross sections to DWBA integrated cross sections are compared for all transitions, an rms deviation about the mean of 39% of the mean is found, whereas if only ratios for transitions from a given target nucleus are compared, then the rms deviations are considerably smaller.     

Light particle emission and projectile fragmentation in 14N + 12C

Quasi-complete in-plane correlations are measured between two heavy fragments (¹²C, ¹²C; ¹²C, ¹³C) which leave a missing mass such as n, p, d or p + n in ¹⁴N + ¹²C at 48 MeV. The use of two position-sensitive telescopes in coincidence allows an observation of data over 675 combinations of angles. The strongest coincidence yield concerns the ¹²C + ¹³C + p exit channel in which the target (¹²C) is left either in its ground state or in its first excited state ($\text{σ ? 40}\text{and}\text{15}\text{mb}$ respectively). No significant yield is found for the dissociations ¹²C + ¹²C + d, ¹²C + ¹³N + n or ¹²C + ¹⁰B + α at this energy.Using the reconstructed proton velocities, we obtain a source pattern in the rapidity plot which is roughly centered in the so-called mid-rapidity region. On the other hand, the Dalitz plot exhibits an enhancement of events which could be due to a projectile fragmentation $\text{(}{}^{14}\text{N}\text{→}{}^{13}\text{C}\text{+}\text{p}\text{)}$ via a real or virtual excitation energy of 8.7 MeV.     

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.     

The (n,p) reaction as a probe of Gamow-Teller strength

The ground state differential cross sections for the (n, p) reaction at 0° on ⁶Li, ¹²C and ¹³C have been measured for 198 MeV neutrons with a precision of about 4% relative to the corresponding value for the H(n, p) reaction. These data, together with the accurately known ft values for the β- decay of ⁶He, ¹²B and ¹³B provide three calibrations of the (n, p) reaction as a probe of Gamow-Teller strength for isospin raising transitions.     

Quasi-free scattering in 7(p,pd)5He and 12C(p,pd)10B reactions at 670 MeV

The reactions ⁷Li(p, pd)⁵He and ¹²C(p, pd)¹⁰ were investigated at 670 MeV in quasi-free scattering geometry at large angles. In transitions to low-lying as well as to highly excited states of the residual nuclei the energy distributions of the scattered protons are in agreement with the mechanism of quasi-free scattering on quasi-deuterons in the p- and s-shells of ⁷Li and ¹²C.     

Interference effects in the strong isospin dependence of the polarization asymmetry in 12C(p,p′)12C11+ transitions

The strong isospin-dependence observed in the polarization asymmetry data for the ¹²C(p,p′)¹²C¹ 1⁺ transitions has been successfully interpreted as a result of interference effects between one-step and two-step (pdp′) excitation mechanisms, in addition to the interference between the central and tensor terms in the one-step excitation process.     

High-resolution time-of-flight system for precise measurement of excitation energies and Q-values

A time-of-flight system has been installed for the measurement of accelerator beam energies with an absolute accuracy of 1.5 : 10⁵ as proved by determining excitation energies in ⁶⁰Ni. The first $\text{T=}\text{3}\text{2}$ resonance in ¹²C(p,p)¹²C was measured at E_p=14230.75±0. 20 keV.     

Photon polarisation asymmetries in (p,p′γ) experiments

The Stokes parameters characterising the polarization of the photon in theγ-decay of¹²C*(1⁺) are examined in correlation with the scattered proton in¹²C(p,p′)¹²C*(1⁺). This opens up the possibility of efficiently determining empirically the six inelastic scattering amplitudes utilizing only 13 measurements. We identify several alternative sets of such measurements to encourage experimental efforts in this direction.     

First study of the (14C, 12C) reaction: Selectivity of the reaction and the energy levels of 28Mg and 30Si

The first measurements are reported for (¹⁴C, ¹²C) two-neutron stripping reactions. Energy spectra up to an excitation energy of 12 MeV have been measured at 69 MeV for the ²⁶Mg(¹⁴C, ¹²C) and ²⁸Si(¹⁴C, ¹²C) reactions. A strong selectivity of this reaction is observed. Using this selectivity, the comparison of the spectra suggests J^π assignments for several ³⁰Si and ²⁸Mg states. The results from the other two-neutron stripping reactions, (t, p) and (¹⁸O, ¹⁶O) are compared with those of the (¹⁴C, ¹²C) reaction.     

Di-electron production in C+C and p+p collisions with HADES

The High Acceptance Di-Electron Spectrometer HADES at GSI, Darmstadt, has been designed for systematic studies of hadron properties inside nuclear matter. First measured mass and momentum distributions of e⁺e^- pairs produced in ¹²C+¹²C collisions at 1 and 2 AGeV are presented and compared with p+p collisions at 2.2 GeV incident energy. The spectrometer set-up and the analysis methods are briefly outlined and a comparison of the data with Monte-Carlo events from a generator based on known production cross-sections and branching ratios is shown and discussed.     

Streuwirkungsquerschnitte12C(p,p)12C undl2C(d,d)12C im EnergiebereichE p=350...550 keV,bzw.E d=600...1100 keV

Scattering Cross Sections¹²C(p, p)¹²C and^l2C(d, d)¹²C in the Energy Range E_p=350...550 keV resp. E_d=600...1,100 keV For an application in a work in nuclear solid state physics (elastic recoil detection with fast¹²C-ions) the scattering cross sections were measured. When adapting excitation functions with a Breit-Wigner formalism it was found that a satisfactory adaptation needed the reduction of one hard-sphere phase by the factor 0.4, and smaller, in order to maintain a reasonable value of the interaction radius.     

High energy proton-carbon scattering in the alpha-particle model

We investigate elastic and inelastic 0⁺–2⁺ high energy proton-¹²C scattering in the alpha-particle model. We use a rigid equilateral triangle nuclear wave function with a Gaussian dispersion function allowing theα-particles of the¹²C-nucleus to deviate from their most probable positions at the triangle vertices. Expressions for the differential scattering cross sections are deduced using Glauber multiple diffraction theory. Thus we need thep?α-particle scattering amplitude, which is calculated from a Gaussian nucleon-nucleon profile function. Numerical calculations show that the model reproduces the experimental results onp?α andp-¹²C scattering.     

Selective population of the lowest-lying negative parity states in23Na in the12C(12C,p)23Na reaction

The¹²C(¹²C,p)²³Na reaction was investigated in the vicinity of the Coulomb barrier. The lowest-lying negative parity states in²³Na were found to be preferentially populated at energies for which correlated resonances exist in the excitation functions of theα, p, n andγ exit channels of the¹²C+¹²C reaction. If these negative parity states are assumed to consist of ap _1/2 hole coupled to a²⁰Ne quartet state, this selectivity can be explained with the existence ofα-particle doorway states.     

The 12C + 12C sub-coulomb fusion cross section

The ¹²C + ¹²C fusion cross section has been studied over the energy range 2.46 ≦ E_c.m. ≦ 5.88 MeV. The yields of the γ-rays emitted from the first excited states of ²³Na and ²⁰Ne, following ²⁴Mg compound nucleus decay via proton- and α-emissions, were measured using a Ge(Li) detector. The fusion cross section was obtained by normalizing these yields to previously reported ¹²C(¹²C, p)²³Na and ¹²C(¹²C, α)²⁰Ne cross sections. The data indicate that the cross section below 3.5 MeV is dominated by two or more resonances, and that the average trend in this energy region does not show the absorption-under-the barrier features of the optical model. For astrophysical extrapolations to lower energies, the new results are consistent with the extrapolation proposed by Fowler, Caughlan and Zimmerman.     

Quasi-free electron scattering on light nuclei

The separation energy spectra and the momentum distributions obtained from the (e, e′p) reaction on ⁶Li, ⁷Li, ⁹Be, and ¹²C are presented. A simple DWIA analysis using the Elton-Swift wave functions gives good agreement with the ¹²C data, but does not explain the data for ⁶Li and ⁷Li.     

在独立α集团模型下对敲出反应16O(p,pα)12C和16 O(α,2α)12C的研究

在独立α集团模型下，利用扭曲波冲量近似(DWIA)，计算了实验室系下入射能量为101.5MeV 的敲出反应¹⁶O(p，pα)¹²C和实验室系下入射能量分别为90Me V和140MeV 的敲出反应¹⁶O(α,2α)¹²C.得到的反应截面和谱因子与实 验数据基本符 合，比用壳模型的结果有了一定的改进，表明独立α集团模型能较好地描述α集团敲出反应 的机理. 关键词： α集团模型 敲出反应 反应截面 谱因子     

Radiative pion capture in 12C and 16O

Radiative pion capture in ¹²C and ¹⁶O is investigated. An effective interaction Hamiltonian based upon the CGLN theory is employed. Capture rates in ¹²C and ¹⁶O from Is and 2p pionic orbits are calculated by using the nuclear model of Kamimura, Ikeda and Arima for the odd parity states of ¹²C, and that of Walker with strong ground state correlations, for the odd parity states of ¹⁶O. The results are compared with those of the 1p-1h nuclear model under the Tamm-Dancoff and random phase approximations of Gillet and Vinh Mau. Detailed examinations are made for the transitions to low-lying states with 1⁺ and 2⁺ for ¹²C and with 1^?, 2^? and 3^? for ¹⁶O by using the Cohen-Kurath and the Walker models, respectively. The results are in good agreement with the recent experimental data from Berkeley.