基于磁光阱中^(6)Li冷原子的真空度测量

Vacuum pressure measurement based on ^(6)Li cold atoms in a magneto-optical trap

2018年第26届国际计量大会召开后,伴随着国际单位制的重新定义,真空量值加速了其量子化进程.在超高/极高真空测量领域,可基于囚禁在磁光阱中的冷原子与背景气体碰撞的损失率以及损失率系数反演真空度.本文从磁光阱中冷原子真空测量的基本原理出发,基于量子散射理论小角近似和冲激近似计算了^(6)Li冷原子与背景气体碰撞的损失率系数,并利用光缔合法测定了在一定磁场和光场条件下的磁光阱阱深,基于两级磁光阱装置通过拟合冷原子数的衰减曲线精确提取出了碰撞损失率.最后在1×10^(-8)-5×10^(-6)Pa压强范围内将真空反演量值与电离计示数对比,分析了制约测量精度提高的因素并提出了改进措施.

Ultra-high vacuum measurement and extremely high vacuum(UHV/XHV)measurement play an important role in high-tech fields such as deep space exploration,particle accelerators,and nanoscience;with the continuous extension of the lower limit of measurement,especially when it reaches the order of 10-10 Pa,higher requirements are placed on the accuracy of the measurement.At present,in the field of UHV/XHV measurement,ionization gauges based on the principle of neutral gas ionization are commonly applied to the vacuum measurement.However,traditional ionization vacuum gauges during use can create electronic excitation desorption effects,soft X-rays,and the effect of hot cathode out gassing,thereby affecting the accuracy of measurement and limiting the lower limit of measurement.Compared with the traditional measurement technology,this method uses the relationship between the loss rate and pressure caused by the collision of cold atoms trapped in the trap depth with the background gas to calculate the gas density and inversely calculate the vacuum pressure.Based on the intrinsic quantum mechanical properties of cold atom collisions,this method is expected to be developed into a new vacuum traceability standard.In this paper,based on the small-angle approximation and impulse approximation under the quaitum scattering theory,the loss rate coefficient of the collision of 6 Li cold atoms with background gas molecules is calculated.According to the ideal gas equation,the pressure inversion formula is obtained.The collision loss rate is extracted by accurately fitting the loss curve of the cold atom.In order to improve the accuracy of vacuum inversion and reduce the influence of quantum diffractive collision on loss rate measurement,the trap depth under the conditions of a certain cooling laser intensity,detuning,and magnetic field gradient is determined by the photoassociation method.Finally,in a range of 1×10^(-8)-5×10^(-6)Pa,the inverted pressure value is compared with the measured value of the ionization meter,proving that this method has good accuracy and reliability in the inversion of vacuum pressure.At present,the main factor restricting the improvement of accuracy is the influence of the collision between the excited atoms in the magneto-optical trap and the background gas on the loss rate measurement.In the future,with the proportion of excited atoms and the excited state C6 coefficient to be precisely determined,the uncertainty of vacuum pressure measurement can be further reduced.