利用18O(6Li,d)22Ne反应研究22Ne Eα=470 keV共振态的性质

慢速中子俘获过程（s过程）是合成比铁重元素的重要途径之一。22Ne（α,n）25Mg反应是大质量AGB星中s过程主要的中子源,其中的22Ne主要通过14N（α,γ）18F（β+）18O（α,γ）22Ne反应链合成。该反应链中关键反应18O（α,γ）22Ne在天体物理感兴趣能区的截面非常低,其天体反应率主要来自于22Ne α分离阈附近低能共振态的贡献,但目前相关能级的共振参数严重缺失。在HI-13串列加速器的Q3D磁谱仪上,通过测量18O（6Li,d）22Ne反应的角分布,利用DWBA分析确定了22Ne分离阈附近共振能级E_α=470 keV的自旋宇称为0+,为后续计算18O（α,γ）22Ne的天体反应率打下了基础。About a half of the abundances of elements heavier than iron comes from the so-called slowneutron capture process (s-process) in Asymptotic Giant Branch (AGB) stars, with the 22Ne(α, n)25Mg reaction as one of the main neutron sources. In the beginning phase of AGB thermal pulse, 22Ne is produced by the 14N(α, γ)18F(β+)18O(α, γ)22Ne reaction sequence, in which the 18O(α, γ)22Ne reaction plays a key role. While the reaction rate of the 18O(α, γ)22Ne is mainly affected by several resonant states lying closely to the α threshold in 22Ne, up to now, the relevant 22Ne parameters are fragmentary in the energy region corresponding to the typical temperatures of s-process. The direct measurement of the 18O(α, γ)22Ne reaction rate is extremely difficult due to the very low cross section. In this work, we investigated the 22Ne resonant states via the 18O(6Li, d)22Ne reaction at the Beijing HI-13 tandem accelerator of China Institute of Atomic Energy. Based on the DWBA analysis, preliminary results showed that the spin-parity of 22Ne E_α=470 keV resonant states was assigned as 0+, which would make contributions to subsequent calculation for the reaction rate of the 18O(α, γ)22Ne.     

The Mo(p, n)Tc reaction cross section and its use as a normalization for shuttle-system measurements

The ¹⁰⁰Mo(p, n)¹⁰⁰Tc reaction cross section was determined from 2 to 11 MeV (c.m.), by counting the delayed γ-rays from the ¹⁰⁰Tc decays (half-life = 15.8 s). These yields were measured with Ge(Li) and NaI(Tl) detectors and thick natural Mo targets. The use of this reaction as a normalizing reaction for cross-section measurements with a rapid target-transfer system is discussed. This technique was applied to the measurement of the astrophysically interesting ²³Na(p, n)²³Mg cross section (²³Mg HALF-LIFE = 11.57s). The results agree well with an independent ²³Na(p, n)²³Mg cross-section measurement.     

Hydrogen burning of O in the CNO cycle

The direct capture process in the ¹⁷O(p, γ)¹⁸F reaction has been used to determine the reduced proton widths of the states in ¹⁸F near the proton threshold. The observed upper limits on these widths are up to factors of 60 smaller than the previously assumed values implying a corresponding reduction of the stellar reaction rate for the ¹⁷O(p, )¹⁴N reaction. These results together with results from previous work lead to the conclusion that the CNO cycle is tricycling with the reactions ¹⁷O(p,γ)¹⁸F(β⁺ν)¹⁸O(p, )¹⁵N as the third cycle. The implications of these results on the equilibrium abundances of the elements synthesized in this cycle are discussed.     

Heavy-ion reactions induced by O, O and F ions

Excitation curves have been determined from γ-ray yield measurements for heavyion reactions induced by E_lab = 12–30 MeV ¹⁶O, ¹⁸O and ¹⁹F ions incident upon thick targets of ⁹Be, ¹⁰B, ¹¹B, ¹²C and ²³Na. The yields of radioactive decay products with half-lives greater than one second were measured; hence a large number of the outgoing reaction channels could be observed. The preponderance of heavy reaction products suggests compound-nucleus formation as the dominant reaction mechanism. Statistical-model calculations with a spin-dependent level density have been performed, in which the nuclear moment of inertia was treated as a parameter. Many of the results can be explained satisfactorily with a nuclear moment of inertia 0.55 to 0.7 of the rigid body value.     

Identification of the Li ground state

The proton-stripping reaction from a ¹¹Be radioactive beam incident on a beryllium target demonstrates that only (7±3)% of the ⁹Li residues in the reaction are in coincidence with the 2.7 MeV γ-ray corresponding to the ⁹Li first excited state. This implies that the previously observed low-energy neutrons from the decay of the unbound nucleus ¹⁰Li represent a direct l=0 transition to the ⁹Li ground state. Consequently, neutron-unbound ¹⁰Li is proven to have the same parity inversion as occurs in the case of ¹¹Be with a  intruder state below the natural parity state.     

恒星氦燃烧关键反应12C(α,γ)16O天体物理S因子及其反应率

恒星氦燃烧阶段3α反应和12C（α,γ）16O反应相互竞争,两者的反应率共同决定了氦燃烧结束后12C与16O的丰度比,该比值是大质量恒星后继演化以及伴随的元素核合成过程的初始条件。目前,氦燃烧12C（α,γ）16O反应起始T₉=0.2处,天体物理模型要求的反应率的精确度要低于10%,然而尚未有实验或理论给出满足要求的结果。最为直接和可靠地获取12C（α,γ）16O反应率的方法,就是尽可能往低能区测量其天体物理S因子,然后通过理论外推到感兴趣的能区。为此基于经典的R-矩阵理论,建立了适用于低能核反应的多道、多能级的约化R-矩阵理论来拟合几乎所有可用的16O系统的实验数据。配合使用协方差统计和误差传播理论,拟合外推得到了客观的、内部自恰的和唯一性好的12C（α,γ）16O反应天体物理S因子。总的外推S因子STOT（0.3 MeV）=162.7±7.3 keV·b,理论上首次给出达到恒星演化与元素核合成模型的最低要求的S因子。基于计算给出的全能区的S因子,数值积分给出了温度位于0.04 6 T₉ 6 10的12C（α,γ）16O天体物理反应率。在T₉=0.2处,推荐的反应率为（7.83 ±0.35）×10-15 cm3mol-1s-1。During stellar helium burning, the rates of 3α and the 12C(α,γ)16O reaction, in competition with one another, determine the relative abundances of 12C and 16O in a massive star. The abundance ratio is the beginning condition of the following nucleosynthesis and star evolution of massive stars, which are extremely sensitive to the rate of 12C(α,γ)16O reaction at T₉=0.2. The most direct and trustworthy way to obtain the reaction rate of the 12C(α,γ)16O reaction is to measure the S factor for that reaction to as low energy as possible, and to extrapolate to energies of astrophysical interest. Based on a new multilevel and multichannel reduced R-matrix theory for applications in nuclear astrophysics, we have obtained an accurate and self-consistent astrophysical S factor of 12C(α,γ)16O, by a global fitting for almost all available experimental data of 16O system, with the coordination of covariance statistics and error-propagation theory. The extrapolated S factor of 12C(α,γ)16O was obtained with a recommended value STOT (0.3 MeV)=162.7±7.3 keV·b. And the reaction rates of 12C(α,γ)16O for stellar temperatures between 0.04 6 T₉ 6 10 are provided. At T₉=0.2, the reaction rate is (7.83 ±0.35)×10-15 cm3mol-1s-1, where stellar helium burning occurs.     

Gamma decay of K levels

Electromagnetic transition rates between states in ⁴⁰K up to 2.6 MeV excitation were investigated. The reaction ³⁷Cl(, nγ)⁴⁰K was used for lifetime measurements with the DSAM, and the reaction ⁴⁰Ar(p, nγ)⁴⁰K for the derivation of spins and mixing ratios from angular distributions. The transition strengths between states of predominant particle-hole character are well described by a rather simple shell-model structure with empirical effective charges and moments.     

Excitation function of (p, pn) reactions at high energy

A closed theoretical formula for the (p, pn) reaction cross section at high energies in light, medium and heavy nuclei has been derived, and a comparison of the calculated cross sections with the experimental datais made for ¹²C, ¹⁹F, ⁵⁴Fe, ⁵⁸Ni, ⁵⁹Co, ⁶⁴Zn, ⁶⁵Cu, ¹⁰⁰Mo, ¹²⁷I, ¹⁴²Ce, ¹⁸²Ta and ²³⁸U at incident energies above 50 MeV. Good agreement between experimental and calculated cross sections is obtained in the entire energy range considered. In deducing the formula a direct reaction with a pure knock-out mechanism has been assumed.     

COULD 7Be BE DESTROYED BY NEUTRON ABSORPTION IN THE SUN? -A NOTE ON THE SOLAR NEUTRINO PUZZLE

A comparison is made between the rate of the ⁷Be(p,y) ⁸B reaction and the rate of the neutron absorption reaction ⁷Be(n,p) ⁷Li which might have been important in. destroying ⁷Be in the solar interior.     

Vector analysing power of the pick-up reaction C(d, t)C

The vector analysing power has been measured for the reaction ¹³C(d, t)¹²C initiated by 12.1 MeV polarized douterons and proceeding to the ground and 4.43 MeV states of ¹²C. Large analysing powers were observed especially for the ground state reaction. The results have been compared with the predictions of the distorted waves Born approximation and with data for the ¹²C(d, p)¹³C reaction     

贫化铀/聚乙烯球壳交替系统中铀-238中子俘获率的测量与分析

为校验次临界能源堆的概念设计,建立了贫化铀/聚乙烯球壳交替系统, 采用活化法测量²³⁸U的中子俘获率. 贫化铀片置于系统内与入射D离子束成90^o的方向上活化 ,用HPGe探测器测量²³⁸U俘获中子衰变产生的²³⁹Np 衰变产生的277.6 keV特征γ射线计数,实验修正了贫铀片对277.6 keV γ射线的自吸收, 得到了交替系统中²³⁸U (n, γ)反应率的径向分布,反应率的相对不确定度为3.5%-3.7%, 并计算得到系统上整个贫铀区中²³⁸U的总中子俘获率为2.24 ± 0.09. 用MCNP5程序在常用ENDF库下进行了模拟计算, ²³⁸U (n, γ)反应率分布计算与实验一般在5%以内符合, 总俘获率在1%以内符合.     

The C(Li, t)O reaction at 20 MeV

States in ¹⁶O op to an excitation energy of 16.9 MeV were observed from the ¹³C(⁶Li, t)¹⁶O reaction at 20 MeV. Differential cross sections were obtained from θ_lab = 15° to 105° for the triton groups corresponding to the states in ¹⁶O at 6.13, 6.92, 7.12, 8.87, 9.85, 10.35 and 11.09 MeV.     

Decay Analysis of Ge Isotopes Formed in Reactions Induced by Tightly and Loosely Bound Projectiles

The dynamical cluster-decay model (DCM) is employed to investigate the decay of ^68,70Ge^* compound nuclei formed respectively via tightly (⁴He) and loosely (⁶He) bound projectiles, using ⁶⁴Zn target. The study is carried out over a wide energy range (E_c.m.~5 MeV to 16 MeV) by including the quadrupole deformations (β2i) and optimum orientations (θ_i^opt) of the decaying fragments. The fusion cross-sections, obtained by adding various evaporation channels show nice agreement with the experimental data for ⁴He+⁶⁴Zn reaction. The contribution from competing compound inelastic scattering channel is also analyzed particularly for ⁶⁸Ge^* nucleus at above barrier energies. On the other hand, the decrement in the fusion cross-sections of ⁷⁰Ge^* nuclear system is addressed by presuming that ⁶⁵Zn ER is formed via two different modes:(i) the αn evaporation of ⁷⁰Ge^* nucleus, and (ii) 1n-evaporation of ⁶⁶Zn^* nuclear system, formed via breakup and 2n-transfer channels due to halo structure of the ⁶He projectile. Besides this, the suppression in 2np evaporation cross-sections suggests the contribution of another breakup and transfer process of ⁶He i.e. ⁴He+⁶⁴Zn. The contribution of breakup+transfer channels for ⁶He+⁶⁴Zn reaction is duly addressed by applying relevant energy corrections due to the breakup of " ⁶He" projectile into 2n and ⁴He. In addition to this, the barrier lowering, angular momentum and energy dependence effects are also explored in view of the dynamics of chosen reactions.     

Studies on the Ne(p, d)Ne and Ne(d, p)Ne reactions

Angular distribution measurements have been performed on the ²¹Ne(p, d)²⁰Ne and ²¹Ne(d, p)²²Ne reactions at E_p = 20 MeV and E_d = 10.2 MeV, respectively. In the ²¹Ne(p, d) ²⁰Ne reaction, the prolific formation of the J^π = 2⁺, 1.63 MeV state was characterized by l_n = 2 pickup, and the distribution associated with the 4⁴, 4.25 MeV state was suggestive of a weak l_n = 2 pickup. All of the observed l_n = 1 pickup strength is associated with formation of the 2⁻, 4.97 MeV ²⁰Ne level. The ²¹Ne(d, p)²²Ne results indicate that l_n = 2 transfer is involved in the formation of the 1.28, 3.36, 5.52, 5.63 and 6.65 MeV ²²Ne states. The angular distribution observed for the 2⁺, 4.46 MeV state and also the unresolved 5.33, 5.36 MeV composite of states required both l_n = 0 and l_n = 2 components in the associated distorted-wave Born approximation fits. The spectroscopic factors extracted from the present results are compared with those predicted by the Nilsson model without mixing: Applications of the angular momentum projection rule to the ²¹Ne(d, p)²²Ne reaction are considered.     

The two-neutron transfer reactions (O, O) with O, C and Mg targets

Excitation functions, angular distributions and differential ranges were measured for the ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg reaction at ¹⁸O beam energies of 20–45 MeV. Excitation functions only were measured for the reactions ¹⁴C(¹⁸O, ¹⁹O)¹³C, ¹⁴C(¹⁸O, ¹⁶O)¹⁶C, ¹⁴C(¹⁸O, ²⁰O)¹²C, ¹⁴C(¹⁸O, ¹⁵N)¹⁷N and ¹⁸O(¹⁸O, ¹⁹O)¹⁷O, ¹⁸O(¹⁸O, ¹⁶O)²⁰O, ¹⁸O(¹⁸O, ¹⁵N)²¹F at ¹⁸O beam energies of 13–41 MeV. We have identified these as direct reactions in which a single neutron, a two-neutron cluster, a deuteron and a triton are transferred between projectile and target.

The cross sections for two-neutron transfer reactions were found to be relatively high and those for the ¹⁸O+¹⁸O and the ¹⁴C+¹⁸O reactions were higher than the ones of single-neutron transfers over most of the energy range.

Attempts were made to apply the theory of Buttle and Goldfarb for single-neutron transfer to the case of two-neutron transfer in the ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg reaction below the Coulomb barrier. It is shown that for those reactions for which the assumptions, implicit in the model, are valid, good agreement is obtained with experiment. We also tried to apply the diffraction model of Dar and Kozlovsky to the calculation of the angular distribution of these reactions. A good fit to the experimental results could be obtained if quite different sets of parameters were used in the calculations for the two bombarding energies.     

A study of the Li(p,n)Be reaction by the neutron threshold technique

The reaction ⁶Li(p,n)⁶Be has been studied by slow-neutron detection, with particular attention to the ⁶Be ground state threshold. The detector response was calculated by a Monte Carlo technique and verified with a measurement of the ⁷Li(,n)¹⁰B threshold. An analysis of the shape of the ⁶Be threshold indicated that both s- and p-wave neutrons contribute significantly to the reaction. The results depend on what mode of decay is assumed for the ground state of ⁶Be. If a two-stage process is assumed, with either an alpha particle and an unbound diproton or ⁵Li and a proton as intermediate state, the width is found to be 95±28 keV and the Q-value obtained for the ⁶Li(p,n)⁶Be reaction is −5074±13 keV. It was not possible to detertmine which process is predominant. No higher thresholds in the slow neutron yield up to 4 MeV of excitation in ⁶Be were observed. The data above the ground state threshold are consistent with a broad excited state of ⁶Be or with the occurrence of other neutron-producing reactions.     

Cross sections for several (n, 2n) reactions at 14 Mev

Precision measurements of the cross sections of the (n, 2n) reactions to produce ^44mSc, ⁸⁸Y, ⁸⁹Zr, ⁹²Nb, ¹⁵⁰Eu(35y), ¹⁶⁸Tm, ^174m+gLu and ¹⁹⁶Au were made for a range of neutron energies from 13.7 to 15.0 MeV. The samples were irradiated at various positions on the surface of a 20 cm radius sphere centered at the ²H–³H neutron source. This allowed a range of neutron energies to be obtained with good energy resolution. The ²⁷Al(n, )²⁴Na reaction was used to monitor the neutron fluence in the target samples. Particular attention was given to the accurate measurement of the (n, 2n) products, using calibrated Ge(Li) and NaI(Tl) detectors. An uncertainty of about ±5% is assigned to the results, except those for ¹⁵⁰Eu and ¹⁷⁴Lu, which have an additional uncertainty in their decay schemes.     

20Mgβ衰变谱学与15O(α,γ)19Ne反应(英文)

15O（α,γ）19Ne（p,γ）20Na反应链是高温CNO循环向快速质子俘获过程突破的一条关键路径,相关的反应率输入量可通过20Mg的β衰变可布居19Ne共振态并测量其衰变性质来获得。通过高效率高精度地测量20Mg β衰变中产生的质子与γ射线得到了20Mg衰变的详细信息,并构建了完整的衰变纲图,还进行了19Ne 4 033 keV共振态衰变性质的探索,获得了该态在20Mg β衰变中被布居的分支比上限。通过比20Mg和20O镜像能级跃迁的结果确认了同位旋非对称性,为检验相关理论模型提供了精确的实验数据。对于突破路径中有重要影响的19Ne 4 033 keV共振态的性质,有待更高统计的实验进一步研究。The breakout from the hot CNO cycle to the rapid proton capture process can occur via the 15O(α,γ)19Ne(p,γ)20Na reaction sequence, and the β decay of 20Mg can be used as an alternative method to characterize some specific resonances, which will provide detailed nuclear structure input for reaction rate calculations. The reliable information on the decay properties and structure of 20Mg was obtained by measuring the emitted particles and γ-rays in the β decay with high efficiency and high resolution. Attempt was also made to search for the decay channels associated with the 4 033 keV resonance in 19Ne. To test fundamental symmetries, the transitions in the mirror decays of 20Mg and 20O were compared. The precise experimental data presented here would be important to constrain the theoretical calculations. It is desirable to clarify the astrophysically relevant problem by further experiments with improved statistics on the basis of the present work.     

OSCILLATIONS DRIVEN BY 3He IN THE NUCLEAR REACTIONSYSTEM DISTURBED BY CONVECTIVE MOTION

The nuclear reactions in the solar interior are always influenced by the effects from convection, diffusion,pressure,gravitation,etc.,among which the convection is primarily responsible for driving the instability.On the basis of the charact eristic with which the particle number density of chemical compositions changes, we investigate the dynamical behaviors driven by ³He in the proton prot on nuclear reaction chain disturbed by convective motion with the aid of nonequi librium dynamics.The new supercritical oscillations of ³He are found in producing and destroying ³He in the nuclear reaction zone of the sun.The energy generation is therefore oscillating,and then this might change the predicted ⁸B and ⁷Be neutrino fluxes.