Investigation on Periodic Oscillation in Orbital Electron Capture Decay in $^{140}$Pr

According to recent experimental data at GSI , periodic oscillation have been observed in orbital electron capture(EC) of Hydrogen-like ion $^{140}$Pr$^{58+}$. They reported that this phenomenon can be explained by the neutrino mass difference . In order to test the existence of such a periodic oscillation in the decay of $^{140}$Pr under the normal condition, we carried out an experiment to observe the EC decay of $^{140}$Pr at the Van de Graaff accelerator facility in Osaka University. A 4.7-MeV proton beam was used to produce $^{140}$Pr through the $^{140}$Ce(p,n)$^{140}$Pr reaction. Observing K-X rays of Ce emitted just after the EC decay by using a Ge detector, a decay curve of $^{140}$Pr was obtained. We could accumulate the time spectrum of the K-X rays with a good statistics under the low background condition. As a result, a finite-size component of the periodic oscillation could not be observed. We will discuss the experimental details and the result comparing with the GSI result.