Stopping power measurement for low-velocity heavy ions in a high-temperature, high-density plasma target.

我们实验研究了低能重离子在高温、高电离等离子体靶中的停止能力。抛射离子由1.7 MV串联静电加速器输送。目标等离子体是用调q钕玻璃激光器照射小LiH颗粒产生的。研制了一种用于时间分辨能量损耗测量的飞行时间系统。我们发现，在等离子体中测量到的225 keV/u ^<16>O和180 keV/u ^<28>Si的阻挡功率比在冷中性靶中的阻挡功率大3-4倍。为了确定抛射离子的有效电荷，我们测量了^<16>O抛射体通过等离子体后的电荷态分布。为此，我们研制了一种传输效率高的宽量程磁谱仪。利用3个塑料闪烁探测器同时测量了4+、5+和6+的电荷态强度。对于等离子体靶，与冷当量相比，测得的分布曲线向高电荷态显著偏移。冷物质最可能的电荷态是4+，而等离子体靶最可能的电荷态是5+ - 6+。实验结果表明，低原子序数、高电离的等离子体比相同密度的冷物质能产生更高的电荷态。这种影响主要是由于可用电子捕获通道数量的减少。结果表明，特别是对于低速重离子，有效电荷的增加是等离子体靶中停止功率大幅增强的原因。

We have experimentally investigated the stopping power of low-energy heavy ions in a high temperature, highly ionized plasma target. The projectile ions were delivered from a 1.7 MV tandem electrostatic accelerator. The target plasma was produced by irradiating a small LiH pellet with a Q-switched Nd glass laser. A time-of-flight system was developed for the time-resolved energy loss measurement. We found that the measured stopping power of 225 keV/u ^<16>O and 180 keV/u ^<28>Si in the plasma were larger than these in the cold neutral target by a factor of 3-4. In order to determine the effective charge of the projectile ions we measured the charge state distribution of the ^<16>O projectiles after the passage through the plasma. For this experiment we developed a broad range magnetic spectrograph with high transmission efficiency. The intensity of the charge states 4+, 5+ and 6+ were measured simultaneously by using three plastic scintillation detectors. For the plasma target, compared with the cold equivalent, the measured distribution curve shifted remarkably toward higher charge states. The most probable charge state for the cold matter is 4+, while that for the plasma target is 5+ - 6+. This experimental result shows that a low atomic number, highly ionized plasma can yield higher charge states than cold matter of the same density. This effect is principally attributable to the reduction in the number of available electron capture channels. As a result we conclude that, especially for low-velocity heavy ions, the increase in the effective charge is responsible for the large enhancement of the stopping power in the plasma target.