基于激发态寿命测量研究N=20与N=82核区的单粒子与集体自由度的竞争

The interplay between single-particle and collective degrees of freedom is one of the hottest research focuses in the field of the nuclear structure.The intrinsic structural information of nuclei can be derived from the reduced electromagnetic transition probability which can deduce via the excited-state lifetime measurement. In the Si-Ca region around N=20, the research focus of this program will be on the competition between the spherical and deformed structure properties resulting from the evolution of N=20 shell closure. In the Eu region around N=82, this program will emphasize the properties of the L-forbidden M1 transition and also consider the likely existing magnetic rotation. The lifetime of the excited state will be measured using β-γ and γ-γ coincidences, where the β particles and γ rays will be detected by plastic scintillators and LaBr3(Ce) detectors. The preliminary experiment plans are shown below: (1) As for N=20 region, at HIRFL-RIBLL1, the projectile fragmentation reaction of 40Ar primary beam with 9Be target will produce the secondary beam of 39Cl, which can populate the excited states of 39Ar via β decay. Then the lifetime will be measured with β-γ and γ-γ coincidences. In addition, at NSCL, the projectile fragmentation reaction of 48Ca primary beam with 9Be target will produce the secondary beam of 39P, which can populate the excited states of 39S via β decay. Then the coincidence relations of β-γ and γ-γ can deduce the level scheme of 39S and lifetimes of some states..(2) At HIRFL-SHANS, the excited states of 145Eu will be populated with the 123Sb(26Mg, 4n) fusion-evaporation reaction. Then the 145Eu residues at long-lived isomeric state will be separated and purified by the gas-filled recoil separator. The lifetime of the state below the isomer will be measured with γ-γ coincidence.

原子核的单粒子与集体自由度的竞争一直是核结构研究的热点。通过测量激发态的寿命，能得到约化跃迁概率，从而透视原子核的内部结构。在N=20的Si-Ca核区，着重研究N=20壳演化带来的球形与形变结构特征的竞争；在N=82的Eu核区，着重研究L禁戒的M1跃迁特性，并考虑可能存在的磁转动特征。本项目使用β-γ与γ-γ符合来测量寿命，β粒子由塑闪探测，γ射线由LaBr3(Ce)探测。初步实验设想如下：（1）对于N=20核区，计划在HIRFL-RIBLL1使用40Ar初级束或在NSCL使用48Ca初级束，与9Be靶的弹核碎裂反应产生次级束39Cl或39P，经其β衰变布居目标核39Ar或39S的激发态，然后搭建能级纲图并测量寿命。（2）对于N=82核区，计划在HIRFL-SHANS通过123Sb(26Mg, 4n)熔合蒸发反应布居145Eu的激发态，使用充气谱仪筛选出同核异能态，然后测量低位能级的寿命。

在课题11805243的支持下，我们成功地完成了塑料闪烁体探测器的研制，并开展了145Eu附近核素的激发态寿命测量实验。..基于束流测试，我们得到了塑闪探测器的最佳时间分辨σ可以达到8 ps以下，因此可以极好地测量带电粒子的时间信息；根据探测器周围耦合的四个光电倍增管探测的信号时间差，我们得到了探测器上的束流位置分布，提取的位置分辨σ为0.7 mm；另外，我们还测试了数字化仪CAEN-DT5742对于塑闪探测器的快时间信号的处理能力，得到了与传统电子学相当的时间分辨；以上测试结果发表在NIMA上。..在兰州重离子加速器国家实验室的充气谱仪终端，我们利用124Sn(27Al, xn)145,146,147Eu熔合蒸发反应，使用LaBr3探测器和γ-γ符合技术测量了目标核素在同核异能态以下的低激发态寿命；通过实验首次测量了145Eu的7/2_1^+态的寿命为100 ps，以及146Eu的6_1^-和6_2^-态的寿命分别为3±5 ps和41±6 ps，同时还得到了147Eu的7/2_1^+态的寿命为167 ps，并于前人对147Eu的结果进行了比较；146Eu的相关结果已经投稿至Physical Review C，对其余两个核素的结果正在撰写论文中。..在本课题执行期间，发表SCI论文2篇，培养博士研究生1名，取得的研究成果为下一步的研究工作奠定了比较坚实的硬件和软件基础。

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