简单的原子极端的核--对奇异氦的激光捕捉与探测

Simple atom,extreme nucleus——Laser trapping and probing of exotic helium isotopes

氦-8(8He)是丰中子核.它由2个质子与6个中子组成,平均寿命为0.2s,其中子丰度是在已知的所有原子核中最高的.氦-8的结构中有一个紧密束缚的α粒子,环绕于α粒子有4个松散的中子,其分布范围较大,形成中子晕.因为氦-8有这特殊的结构,它为研究基础核作用力提供了一个有意义的系统.文章作者近期使用原子阱及激光冷却技术成功地捕捉到这个奇异的氦同位素,并对阱内单原子做了精密的激光光谱测量,由此首次得到氦-8的核电荷半径.这个结果可与不同的核结构理论计算值相比较,有助于了解在极端丰中子环境中的核作用力以及核结构.

Helium-8 （^8 He） is the most neutron-rich matter to have been synthesized on the Earth： it consists of two protons and six neutrons, and remains stable for an average of 0.2 seconds. It is often viewed as a ^4 He core with four additional neutrons orbiting at a relatively large distance, forming a halo. Because of its intriguing properties, ^8 He has the potential to reveal new aspects of the fundamental forces among the constituent nucleons. We have recently succeeded in laser trapping and cooling this exotic helium isotope, and have performed precision laser spectroscopy on individual trapped atoms. Based on the atomic frequency differences measured along the isotope chain ^3He-^4He-^6He-^8He, the nuclear charge radius of ^8He has now been determined for the first time. Comparing this result with the values predicted by a number of nuclear structure calculations will test our theoretical understanding of the nuclear forces in an extremely neutron-rich environment.