Patent GB-1514563-A (GB)
Title: Atomic beam apparatus
IPC: H01S1/00
Filed: 1975-10-02
Abstract: 1514563 Atomic resonance apparatus FREQUENCY & TIME SYSTEMS INC 2 Oct 1975 [9 Oct 1974] 40403/75 Heading G6P Relates to apparatus which includes a radiofrequency transition section downstream from a first state selector selecing a portion of molecular particles in a beam, means producing a weak and generally homogeneous magnetic field in the transition section, a second state selector downstream from the latter section for selecting a further portion of the beam particles which have undergone resonance transitions, and detecting means responsive to the further portion. According to the invention the molecular beam source includes an ampoule containing liquid caesium, and a heater for raising the temperature of the ampoule to vapourize the caesium and cause failure of a eutetic seal between a base and an enclosure forming the ampoule, whereby the vapour pressure of the caesium causes the base to move outwardly, opening the ampoule and affording communication therefrom through a reservoir space to a beam collimator. The apparatus forms part of an atomic frequency standard. General arrangement, Fig. 2. A source includes an oven 10 which causes evaporated liquid caesium to emit a beam of neutral caesium atoms distributed between two stable energy states. The first state selector magnet 12 selects stated atoms for passage through radio-frequency interaction section 14 provided with a weak homogeneous magnetic field- "C-field"- by a winding 22. Microwave energy is supplied at the resonance frequency and beam atoms are further selected by a magnet 16. An electron is stripped from each atom by a hot-wire ionizer 20, the caesium ions being accelerated through a mass spectrometer 207 into an electron multiplier 18 which provides an output current proportional to the number of atoms raised to the second state in section 14. Oven-ampoule assembly, Fig. 3. The oven includes a reservoir 29 containing an ampoule 27. A eutectic material 32 seals a base 34 into an opening 49 and is designed to fail mechanically at 600‹C (e.g. an alloy of 45% copper and 55% indium). After filling with liquid caesium, tube 38 is closed. A wire screen mesh 36 retains and supports ampoule 27 within reservoir 29, together with a member 43, see also Fig. 4 (not shown). Tantalium heaters (90, 92, Figs. 5, 6 neither shown) in a ceramic support structure (88) are inserted into a collimator assembly (42), and cause the caesium in the ampoule to be released. Later on the heaters warm the oven assembly to operating temperature (90‹C). Arrangements are included for securing the oven to the first state selector magnet-"A-magnet"-, and for reducing heat loss. A-magnet and ion pump. A C-shaped permanent magnet (111, Figs. 9 to 12, none shown) is shared by the A-magnet (12) and a pump (110) which removes undesired gases and maintains vacuum. C-field/mcirowave module. Conductors of the C-field solenoid 22 are etched by printed circuit techniques, Fig. 15 (not shown), and are assembled into a rectangular loop, Fig. 14 (not shown). The solenoid is mounted on the inner surface of a magnetic shield, Fig. 18 (not shown). The arrangement approximates the equivalent of the uniform field of an infinitely long solenoid, with flux lines normal to the caesium beam path. Microwave radiation by a waveguide structure in known manner, Fig. 18 (not shown). An outer magnetic shield is added. Second state selector ("B-magnet")/detector module. Horseshoe permanent magnets (198, 199, Figs. 20 to 23, none shown) are assembled to provide two gaps about 180 degrees apart, one gap being on the beam axis. Soft iron pole pieces (200, 201) are provided in the one gap and act as the B- magnet 16 of Fig. 2. A pole piece assembly in the second gap functions as a mass spectrometer. A table (196) has tabs for securing the hot-wire ionizer assembly (21), and an electron multiplier is secured beneath the table. Outer package, Fig. 24. The outer package is a single vacuum-tight envelope composed of a rigid base 210 and a thin, flexible cover 212. The assembly of the modules described above is described, see also Fig. 25 (not shown).