Patent GB-1195535-A (GB)
Title: Improvements in or relating to Beam-Choppers for use in Spectroscopes and like Instruments
Inventor: ORMROD GEORGE THOMAS WALLACE
IPC: G01J1/36
Filed: 1966-07-22
Abstract: 1,195,535. Molecular beams. IMPERIAL CHEMICAL INDUSTRIES Ltd. 23 Oct., 1967 [22 July, 1966], No. 33071/66. Heading G6P. [Also in Divisions G1 and H1] A device for modulating a beam of matter or radiation comprises a solenoid-driven vibrating reed beam chopper, a ceramic pick-up being provided to transduce the vibrations of the reed into an alternating electric current which is amplified, phase shifted if necessary to bring it into phase with the vibrations of the reed, and applied at the same or half the frequency of the reed vibrations, to the solenoid. In this way the reed is kept vibrating at its natural resonance frequency. The alternating current may also be fed at the same frequency as the reed vibrations and in phase therewith, to a phase-sensitive detector or lock-in amplifier forming part of the detector circuit of the measuring instrument, such as a spectroscope, in which the beam chopper is being used. The use of beam chopper and phase-sensitive detector allow separation of a signal representing matter or radiation of interest from signals due to unwanted matter or radiation i.e. back-ground noise. Beam-chopper construction. Fig. 1.-The chopper comprises a pair of parallel polepieces 3, 4 connected by a bar 5 on which is wound the solenoid coil 6. The poles and bar are of a magnetizable alloy or nickel. The reed which has a beam affecting extension 2 at one end is attached at its other end to the top of polepiece 3. A strip of piezo-electric ceramic material 7 coated with conductive material on opposite faces, is clamped to the polepiece 4 and is coupled to the reed by means on a sensor 9. This is a metal strip clamped between reed and polepiece 3 having its free end bearing on or attached to the strip 7. Alternatively, the piezoelectric strip may be attached to the feed itself. Various alternative insulation constructions are described. Circuitry, Figs. 3, 4 (not shown).-The output of the transducer is fed to a transformer for providing a limited form of phase correction. Since a ceramic piezo-eleotric transducer is used, a transistorized impedance converter precedes the phase corrector. The signal is then rectified to half its frequency and amplified before application to the solenoid coil. A meter and capacitor to offset the back emf of the coil may be provided in the coil circuit. The signal from the impedance converter or the phase converter may also be applied to a phase sensitive detector as referred to above. Applications.-The beam chopper may be used in a mass spectrometer or similar instrument, Fig. 6 (not shown). The chopper is mounted in the second of three collimating chambers for a beam of matter, the third chamber containing means to produce an ionized beam. This instrument may require to be baked before use, in which case the coil is removed from the chopper beforehand. Other applications are examination of plasma beams or beam of hot ions. The beam chopper may also be used in a radiation spectrometer using light, ultra-violet or infra-red radiation.