The muonic helium atom, ofju"^", has been formed by stopping polarized negative muons in He gas at 14 atm with a Xe admixture of 2%, and observed through its characteristic Larmor precession frequency of 1.4 MHz/G. In addition a nonzero residual polarization of P =0.06± 0.01 for ju" stopped in pure He gas has been measured for the first time, which corresponds to a depolarization factor of 18 Ĵ^ 3.The muonic helium atom aii'e" is the simple basic atom in which one of the two electrons in a normal helium atom is replaced by a negative muon. In its structure the muonic helium atom is similar to hydrogen with the relatively small muonic helium "nucleus" (a/i")"^ corresponding to the proton.^'^ It is the simplest prototype of an electronic atom with a muonic nucleus^ and provides an interesting and potentially useful system for study of a very different type of atomic structure, and also for study of the ii' -e' interaction and for the precise determination of the magnetic moment and mass of the negative muon.When a negative muon is stopped in He gas, it is captured by a He atom in an Auger process, and then as a result of further Auger and radiative processes will form {aii'y in its ground IS state. In a collision with He at thermal energy {aii'y cannot capture an electron to form aii'e' because of the 11.0 eV difference in binding energy of an electron in He and in a^'e" ^ Hence to form aii'e' an electron donor with an ionization potential less than that oi a^x'e' is needed; xenon is chosen and the reaction will be^'^ {a^'Y -hXe-^aii'e' +Xe"^.(1)The residual polarization of /JL" in a/i" (IS) is an essential factor for our experiment. The theory® of the depolarization of a negative muon in its capture and cascade to the IS state due to the jLt" spin-orbit interaction predicts a residual polarization P = 0.17, when the initial polarization is 1 and a spin-0 nucleus is involved. For C and many other materials, this value of residual polarization has been observed,*^ but in experiments done thus far in liquid helium^ and in gaseous helium at 50 atm ^ the residual polarizations observed were about P = 0.01 ± 0.02 and P = 0.035 ±0.024, respectively. 1i the a^'e" atom were formed by Reaction (1), the resulting /i" polarization in a weak external magnetic field would be i that of j u, ** in a/x" because of the hfs interac-1417
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