Patent GB-931825-A (GB)
Title: Method and apparatus for measuring low density atmospheres
IPC: G01N23/201
Filed: 1962-04-04
Abstract: 931,825. Gas density measurment using particle radiation. ONTARIO RESEARCH FOUNDATION. April 4, 1962 [April 5, 1961], No. 13057/62. Class 40 (3). [Also in Group XL (c)] The invention relates to a method and apparatus for measuring the density of a low density atmosphere having a substantially uniform density and comprises directing a beam of charged particles, for instance alpha or beta particles, electrons or ion beams from a source and through the atmosphere towards a detector, preventing unscattered ones of the charged particles from being detected by the detector, detecting and counting scattered particles at an acute forward angle relative to the direction of travel of the beam and relating the number of scattered particles counted to the density of the atmosphere. In one embodiment shown (Fig. 2) the source of particles is an electron accelerating system having a thermionic cathode 10, a focusing electrode 18, an anode 19 and a pin-hole aperture plate 11. Unscattered charged particles 16<SP>1</SP> travel straight through a tube 24 in a detector 12, but particles scattered by the atmosphere between the source and the detector strike a scintillator 21 in a window 25 of the detector housing 20. The scintillations are reflected by a mirror 23 and pass through a light guide 21 to a photo-multiplier 22 where they are counted. In another embodiment (Fig. 1, not shown), charged particles from an isotope source or an electron gun are directed down a long tube surrounded by a detector such as a Geiger counter, unscattered particles passing straight down the tube but particles scattered by the atmosphere passing through the walls of the tube and being detected and counted by the Geiger counter. In an alternative form, the tube is surrounded by a photo-multiplier and the tube is covered with a layer of fluorescent material or other suitable scintillator. In another embodiment which is used to determine the density of the earth's atmosphere at very high altitudes (Fig. 4), a radio-active source 42 such as Kr-85 is contained in an annular channel 43 in the nose of a satellite, rocket or the like. Charged particles 59 from the source 42 are directed by means of ring-shaped beam stops 46 and 47 and adjustable beam stop 48 generally in the direction of a detector 52 comprising a ring 53 having an annular detector chamber 54 containing a probe or anode wire 58 connected to a known counting circuit (not shown). The cone-shaped beam of charged particles 59 is arranged to just miss the detector 52, but scattered particles 17 from the beam will pass through a ring-shaped aperture 57 on to the detector. In an embodiment shown in Fig. 7, the densities of different gases in an atmosphere may be determined, and collimators formed for instance of one hundred small tubes of 1 mm. diameter and 100 mm. long are placed in front of a mono-energetic source 10 and a detector 12 capable of discriminating between particles of different energies, so that only those particles scattered through the angle 0 can reach the detector. From elementary collision theory the energy of the particles scattered through angle # will be different for each of the gases in the atmosphere investigated and a discriminating detector will enable the densities of the gases to be determined. In a further embodiment, Fig. 6, the density of an atmosphere and the velocity or Mach number of a rocket or the like through that atmosphere may be determined. Two separate radiation sources 42 and two separate detectors 52 are mounted on supports 41 and the charged particles from the sources 42 are directed perpendicularly to the direction of motion of the rocket and just miss the detectors. The detector nearest the rocket 40 and gives the density of the atmosphere between the rocket and a shock wave 70 set up in the atmosphere by the rocket, whereas the detector furthest from the rocket gives the density of the undisturbed atmosphere. The ratio of the readings of the two detectors is related to the relative speed of the rocket and the atmosphere. In an alternative version of this embodiment (Fig. 5, not shown), a source similar to that shown in Fig. 4 is mounted on one support at one end of a rod and a pair of detectors are mounted at the other end of the rod and in this case the shock wave set up by the rod is utilized to determine the velocity as in the embodiment described with reference to Fig. 6.