【作者】 孙大鹏；

【导师】 王守宇；

【作者基本信息】 山东大学 ， 粒子物理与原子核物理， 2023， 博士

【摘要】 本论文用在束伽马谱学研究了 119Sn和76As的能级结构。首先介绍了半幻核119Sn的能级结构和形状共存,然后介绍了 80核区奇奇核76As的高自旋态的首次观测。质子数Z=50幻数的Sn同位素在低激发态原子核核结构研究中发挥着重要作用,它们的低激发态能够用壳模型理论方法重现。近来实验发现Sn同位素部分原子核表现出球形和长椭形的共存。球形状态被认为中子准粒子激发占主导地位。变形结构被认为是通过Z=50壳层的质子粒子-空穴激发,价核子耦合到两粒子两空穴侵入状态。在偶偶核106-118Sn和奇A核109-117Sn中,系统地观察到了球形能级与形变集体带的共存现象。更重的Sn同位素是否仍然存在这样的形状共存现象,是一个开放的科学问题。在稳定线附近的原子核119Sn的高自旋态难以通过常规的熔合蒸发反应布居,本工作采用非完全熔合反应7Li+116Cd反应中的(7Li,1p3n)、(7Li,1d2n)和(7Li,1t1n)反应道布居了 119Sn的激发态,实验在中国原子能科学研究院HI-13串列加速器国家实验室完成,束流能量为42MeV,探测阵列由九个带反康的HPGe探测器和两个小平面(LEPS)探测器组成,使用152Eu和133Ba标准源置于靶的位置对探测器进行了能量刻度和效率刻度。实验中共获得约1.2× 108二重符合事件。本次实验共发现8条新跃迁,通过提取伽马射线的ADO值,确定了跃迁的多极性;发现一个新的能级序列,通过其实验特征的系统学对比和绝热的组态固定的三轴相对论平均场理论计算,表明119Sn中这一能级系列是个长椭球形变带,从而将Sn同位素链的形变集体带的实验观测推广到了质量数为119。自2011年在80核区发现首例手性原子核80Br后,80核区的手性原子核研究引起了人们的关注,陆续实验上在这个核区发现了好几例候选手性原子核,形成了一个“手性原子核岛”,这个手性原子核岛有多大,也就是它的边界在哪里?此外,这个质量最轻的手性原子核岛是否有表现出什么特殊性质?这都需要对80核区的原子核结构展开更深入的研究。80核区奇奇核76As的激发态是通过74Ge(4He,1p1n)76As熔合蒸发反应布居的,实验在南非iThemba国家实验室完成,束流离子由分离扇形回旋(SSC)加速器提供,束流能量为58.6MeV和62.6MeV,靶的厚度为2.85mg/cm2,碳衬的厚度为10.7mg/cm2,生成核退激的γ射线是由AFRODITE(African Omnipurpose Detector for Innova-tive Techniques and Experiments)探测阵列进行探测,探测阵列包含8个Clover探测器和2个低能光子探测器(LEP:low-energy photon spectro-meters),最终实验获得的3×109个γ-γ符合事件。经过数据分析,共发现27个新能级,42条新跃迁,形成了 5条转动带,两条正宇称转动带组态被建议为πg9/2 ?vg9/2,三条负宇称转动带组态分别被建议为πf5/2?vg9/2、πp3/2?vg9/2和πg9/2?v(p3/2/f5/2)。其中两条正宇称转动的实验能谱、S(I)和电磁跃迁概率比值符合手征带的实验判据,被解释为手征双重带。通过系统的比较80核区候选手征带的能级劈裂值,发现76As双带的能级劈裂最小,预示着76As很可能是80质量区已发现的手征几何最稳定的原子核。更多还原

【Abstract】 The level structures of 119Sn and 76As have been studied via in-beam spectroscopy.We first introduced the level structures and shape coexistence of the semimagic nucleus 119Sn,and then introduced the first observation of the high spin state of the odd-odd nucleus 76As in the A～80 mass region.Sn isotopes near the magic number of proton Z=50 play an important role in the study of low-energy nuclear structure.Their low energy states can be well reproduced by using shell model-calculations.Recently,experiments have found that some nuclei of Sn isotopes exhibit coexistence of spherical and prolate shapes.The spherical state is considered to be dominated by neutron quasiparticle excitation.The deformed structure is thought to be excited by the protons in the shell of Z=50,and the valence nucleons is coupled to the two-particle two-hole intrusion state.The coexistence of spherical and deformed collective bands has been systematically observed in even-even nuclei 106-118Sn and odd-A nuclei 109-117Sn.Whether the heavier Sn isotopes still have the shape coexistence phenomen-on is an open question.The high spin state of atomic nucleus 119Sn near the stability line is difficult to population through conventional fusion evaporation reactions,excited states in 119Sn were populated in the incompete fusion 7Li+116Cd reaction via the(7Li,1p3n),(7Li,1d2n),and(7Li,1t1n)channels.A 7Li beam of 42MeV was provided by the HI-13 Tandem Accelerator at the China Institute of Atomic Energy in Beijing(CIAE).The target was self-supporting 116Cd with a thickness of 2.5mg/cm2.The γ rays of the product nuclei were detected with an array consisting of nine Compton-suppressed HPGe detectors and two low-energy photon spectrometer(LEPS)detectors.The detectors were calibrated for both energy and efficiency using 152Eu and 133Ba sources placed at the target position.A total of approximately 1.2 ×108 γ-γ coincidence events were accumulated during the experiment.A total of 8 new transitions were discovered in this experiment,and the multipolarity of the transitions was determined by extracting the ADO values of gamma rays;A new energy level sequence is found.Through the systematics comparison of its experimental characteristics and the theoretical calculation of the adiabatic and configuration-fixed three-axis relativistic mean field method,it is shown that this energy level sequence in 119Sn is a prolate shape deformation band,thus extending the experimental observation of the deformation collective band of Sn isotope chain to the mass number of 119.Since the discovery of the first 80Br chiral atomic nucleus in 2011,the study of the chiral atomic nucleus in the A～80 mass region has aroused great interest of researchers.Several candidate atomic nuclei have been discovered in this nuclear region,forming a "chiral atomic island".How large is this chiral atomic island,and where is its boundary?In addition,does this lightest chiral atomic island exhibit any special properties?This requires us to further study the nuclear structure of the 80 nuclear region.The excited state of odd-odd nucleus 76As in the A～80 mass region was populated by the fusion evaporation reaction of 74Ge(4He,1p1n)76As.The experiment was performed in the iThemba National Laboratory of South Africa.The beam was provided by the separated sector cyclotron(SSC)accelerator,the beam energies are 58.6MeV and 62.6MeV,the thickness of the target is 2.85mg/cm2,the thickness of the carbon backing is 10.7mg/cm2,and the y rays were detected by AFRODITE(African Omniposition Detector for Innovative Technologies and Experiments)detection array,including 8 Clover detectors and 2 low-energy photon detectors(LEP).Totally 3×109 y-y coincidence events were obtained from this experiment.After data analysis,a total of 28 new energy levels,42 new transitions and 5 rotational bands were found,and the configuration of two positive parity rotational bands were suggested as πg9/2?vg9/2,the three negative parity bands configuretions were suggested as πf5/2?vg9/2,πp3/2?vg9/2 and πg9/2?v(p3/2/f5/2),respectively.The pair of positive parity bands were interpreted as chiral doublet bands by studying their experimental energy spectra,S(Ⅰ),and electromagne-tic transition probability ratios.By systematically comparing the energy level splitting values of candidate chiral bands in the 80 mass region,it was found that the energy level splitting of the 76As double band is the smallest,indicating that 76As is likely to be the most stable chiral geometrically discovered atomic nucleus in the A～80 mass region.更多还原