Muonium formation in liquid ammonia

Abstract

A study has been started on the chemical behaviors of positive muons in condensed phase ammonia and sodium/ammonia solutions. The sample was set in a styrofoam thermal insulator equipped with a nitrogen gas flow temperature controller at the mu port of KEK-MSL (High Energy Accelerator Research Organization-Meson Science Laboratory). Muonium precession signals were measured in 3 G transverse field at 178 K (solid) and 225 K (liquid). Diamagnetic muon yields and muonium yields (P(sub D0 = 0.67 and P(sub M) = 0.17) in solid ammonia are typical for compounds with saturated bonds. Muonium precession signals were not observed in the liquid, while the P(sub D) of 0.66 did not change. The reason can be explained as the following: the muonium formed in solid would diffuse and react with the stable solvated electrons produced and remain in the terminal track, and then the muonium rapidly undergoes spin exchange reactions, resulting in loss of polarization. The sodium/ammonia solutions exhibited larger P(sub D) than pure ammonia, and 1.5 MP solution (Na/NH3) had almost P(sub d) of 1.0. One explanation of the reason is presented as follows. Although muonium is formed in sodium solutions, there exist large amount of paramagnetic centers in the solution that give apparent diamagnetic signals.

凝縮相アンモニアおよびナトリウム/アンモニア溶液における正ミューオンの化学挙動の研究を開始した。試料はKEK-MSL(高エネルギー加速器研究機構-中間子科学研究所)のμポートで窒素気体流温度制御器を備えたスチロフォーム熱絶縁体中にセットし、ミューオニウム歳差信号を178K(固体)と225K(液体)で3G横磁場において測定した。固体アンモニアの反磁性ミューオン収量とミューオニウム収量(P(sub D)=0.65とP(sub M)=0.17)は飽和結合化合物に対する代表的な値である。液体試料ではミューオニウム歳差信号は観測されなかったが、P(sub D)=0.66は変化しなかった。このことは、固体中で生成したミューオニウムが拡散し、生成した安定な溶媒電子と反応し、末端跡に残り、急速にスピン交換反応し、偏極損失が生じるとして説明できる。ナトリウム/アンモニア溶液は純粋アンモニアより大きなP(sub D)を示し、1.5MP(Na/NH3)溶液はほぼPD=1.0であった。このことの1つの説明は、ミューオニウムがナトリウム溶液で生成されるが、溶液中に多くの常磁性中心が存在し、見かけの反磁性信号を出すからである。