from an oscillator via a connector the frequency of the oscillator is slaved to the transition frequency by slave means comprising a photoelectric cell which receives the light from the lamp through the cell application_number US 31683589 A description the arrangement according to the invention as shown on fig2 comprises essentially a lamp 40 a gas cell 42 a helicoidal resonator 44 and a photoelectric cell 46 lamp 40 contains mainly rb 87 as well as argon at a pressure of 2 torr serving as triggering gas it is placed within an excitation coil 48 coupled to a coaxial cable 50 which receives from a radio frequency oscillator a signal having a frequency comprised between about 100 and 150 mhz a grid 52 is arranged about the assembly in order to avoid radiation of the signal toward the gas cell and the photoelectric cell a standard heating circuit not shown comprising a heating resistance and a temperature sensor is employed to maintain the lamp at its normal operating temperature the gas cell 42 is placed facing lamp 40 in the example as shown this cell contains rb 85 and rb 87 as well as a buffer gas which may be a mixture of nitrogen at a partial pressure of 8 torr and methane at a partial pressure of 5 torr it has a diameter of 16 mm and a length of 25 mm i e a volume of about 5 cm 3 this cell constitutes at the same time the isotopic filter and the absorption cell of the optical pumping arrangement of fig1 the atoms of rb 85 situated in the portion of cell 42 closest to lamp 40 absorb the spectral component a of the latter which brings about filtering and the atoms of rb 87 located in the.