亞穩態反質子原子生成反應的碰撞截面積

Collision cross section for metastable antiprotonic atom formation

袁學平(Xue-Ping Yuan)

指導教授 ： 寺西慶哲

國立交通大學/理學院/物理研究所/碩士(2018年)

摘要

反質子是質子的反粒子，在真空中其壽命據實驗估算至少為6.6×〖10〗^33年。但在物質中由於與原子的碰撞會導致其在極短的時間內與質子發生湮滅反應，在物質中反質子的預期壽命甚至只有數皮秒。另一方面，當它在氦介質中時，3％的反質子可以有更長的壽命，長達一微秒。
造成壽命差異的原因是反質子被捕捉到的量子態。當反質子被捕捉到角量子數l較低的量子態時，俄歇效應(Auger effect)隨即發生使電子從反質子原子中迅速解離，解離後的結構增強了斯塔克混合效應(Stark mixing effect)，結果導致反質子與原子核中的質子湮滅。如果反質子被捕捉到l較高的亞穩態時，因為俄歇很難發生在那樣的量子態，需經過一系列輻射衰變過程，直到其衰變至俄歇態(Auger state)。
在這項研究中，我們把重點放在反質子被捕獲到亞穩狀態上的條件和發生的機率上。我們提出了辨別俄歇和亞穩態的一個簡單的標準，廣泛適用於不同的靶原子。我們也利用古典軌跡蒙地卡羅法(CTMC)討論捕捉過程，得到被捕獲反質子的能量和角動量分佈。結合這二個理論所得的結果，我們計算氦，鋰和鈉捕獲至亞穩狀態的概率，我們發現被裡捕捉的反質子有超過60%是處在亞穩態上，多於氦的10倍。

關鍵字

反質子原子 ； 俄歇效應 ； 古典軌跡蒙地卡羅法

並列摘要

Antiproton, the antiparticle of proton has an extremely long lifetime, and only the lower limit can be experimentally estimated as 6.6×〖10〗^33 years. It can decay, however, in much shorter time if it is embedded in media due to the collision with atoms and the pair annihilation with proton. The typical lifetime of antiproton in media is known as pico seconds. When it is in helium media, on the other hand, 3% of antiprotons can have much longer lifetime as long as μsec.
The difference between the long and short lifetime is in the captured state of the antiproton. When it is captured into a low l state, the Auger transition removes one electron from the antiprotonic atom promptly, giving rise to enhance the Stark mixing, which cause the pair annihilation between the antiproton and a proton in the atomic nucleus. On the other hand, if antiprotons captured into large l states, it is metastable because the Auger transition hardly takes place. It decays after a cascade of radiative transitions before captured in the Auger state.
In this study, we focus on the antiproton capture into metastable states. We propose a simple criterion of Auger and metastable states, which is applicable to general atomic targets. The capture process is discussed using the CTMC method to obtain the energy and angular momentum distribution of captured antiproton. Combining the two theories, we calculate the probability of capturing into metastable state for helium, lithium and sodium to discuss which atom yields higher meta-stability. We find that more than 60% of antiprotons captured by lithium are metastable, which is more than 10 times larger than the helium case.

並列關鍵字

antiprotonic atom ； Auger effect ； CTMC method

參考文獻

Reference

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