A direct determination of the proton electron mass ratio

The cyclotron frequencies of free protons and electrons in a magnetic field of 5.81 Tesla with superimposed electrostatic quadrupole field have been measured. The increase of energy connected with a transition at cyclotron frequency is detected by the measurement of the time of flight through an inhomogeneous magnetic field. From the ratio of the measured cyclotron frequencies of both particles the proton electron mass ratio is deduced. The resultm p /m e =1,836.1527(11) agrees within the limits of error (0.6 ppm) with the value of the indirect determination.