US3394280A - Ultraviolet detector tube having photoemissive cathode and gas filling - Google Patents

Description

23, 1953 T. M. TRUMBLE 3.394280 ULTRAVIOLET DETECTOR TUBE HAVING PHOTOEMISSIVE CATHODE AND GAS FILLING Filed Aug. 1, 1966 QJ JV Fig-f1 4 3 2 7 I Requmrco r L O we. Pawn: vvmnaruw 2 E sup/"L9 5 4 1 8 i ourror BY fan United States Patent 3,394,280 ULTRAVIOLET DETECTOR TUBE HAVING PHOTOEMISSIVE CATHODE AND GAS FILLING Terry M. Trumble, 2757 Horstman Drive, Kettering, Ohio 45429 Filed Aug. 1, 1966, Ser. No. 569,525 2 Claims. (Cl. 313--98) ABSTRACT OF THE DISCLOSURE An ultraviolet radiation detector of the wire electrode photoelectron triggered gaseous discharge type having two parallel closely spaced straight wire coextensive anode electrodes and a straight wire photoemissive cathode parallel to, equidistant from and coextensive with said anodes, all contained in a sealed envelope transparent to the radiation and filled with a gas ionizable by electron collision.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to ultraviolet detector tubes of the gas ionization or Geiger-Mueller type and has as its principal object an increase in the sensitivity of such tubes. Other objects are to increase the incident radiation acceptance angle over which high sensitivity is obtained in comparison with current tubes, and to make the directional sensitivity pattern of the tube symmetrical.

Current ultraviolet detector tubes of the gas ionization type, such as shown in Patents No. 3,207,903 to Abromaitis et al. and No. 3,209,190 to Cade, contain two similar parallel closely spaced wire electrodes, preferably of tungsten, in an envelope transparent to ultraviolet radiation and filled with an ionizable gas such as hydrogen. Either DC. or AC. energization may be used and either electrode may be the anode or the cathode, each electrode being alternately anode and cathode when A.C. energization is employed. The incidence of ultraviolet radiation on the cathode of a tube of this type results in a relatively heavy current flow between the electrodes due to cumulative ionization of the gas molecules triggered by the emission of one or more photoelectrons from the cathode.

In order for a tube of the above type to respond to ultraviolet radiation, the photons must impinge on the cathode in an area where the electric field is sufficiently strong to accelerate the photoelectrons to a velocity high enough to ionize the gas molecules with which they collide. This limits the active area of the cathode to that facing the anode and correspondingly limits the sensitivity of the tube. Also, as a result of this requirement, the direction of maximum sensitivity of the tube, in a plane normal 'to the electrode axes, is at an acute angle to the plane of the electrode axes, and the directional sensitivity pattern, in the normal plane, is limited in angular width and is unsymmetrical.

In accordance with the invention, the sensitivity of ultraviolet detector tubes of the gas ionization type is increased and the other objects of the invention obtained by increasing the area of the cathode over which the electric field is strong enough for the emitted photoelectrons to effect gas ionization. This is accomplished, in a tube intended for DC. energization, by providing two parallel wire electrodes which act as anodes and a third wire electrode acting as a cathode which is parallel to, equidistant from, and situated behind the plane of the anode axes relative to the radiation source. With this arrangement, the

3,394,280 Patented July 23, 1968 "ice angular distance around the cathode over which the electric field has the required strength, as defined above, is greatly increased. This results in an increase in sensitivity, an increase in the angular width of the directional sensitivity pattern in the normal plane, and asymmetrical configuration of this pattern.

The invention will be described in more detail with reference to the specific embodiment thereof shown in the accompanying drawings in which FIG. 1 schematically illustrates the construction of current two-electrode ultraviolet detector tubes, showing the electrostatic lines of force associated with the two electrodes,

FIG. 2 schematically illustrates a three-electrode detector tube in accordance with the invention, showing the electrostatic lines of force associated with the three electrodes,

FIGS. 3, 4 and 5 show plan, elevation and side views of an ultraviolet detector tube constructed in accordance with the invention,

FIG. 6 illustrates a suitable D.C. energization and quenching circuit for a three-electrode tube of the type shown in FIGS. 3, 4 and 5, and

FIG. 7 illustrates the use of ultraviolet optics with a tube construct-ed in accordance with the invention.

Referring to FIG. 1, there is shown, schematically and not to scale, a cross section, in a plane normal to the electrode axes, of a two-electrode gas ionization ultraviolet detector tube of the type currently in use. Electrodes 1 and 2 are parallel wires made preferably of tungsten, although any metal capable of photoemission at the Wavelengths of interest and suitable to the tube manufacturing process may be used. The electrodes are enclosed in a glass envelope 3 which is transparent at the wavelengths to be detected and which is filled with an ionizable gas such as hydrogen.

The construction and operation of tubes such as illustrated in FIG. 1 are Well known and understood in the art. With the polarization shown, electrode 1 is the anode and electrode 2 the cathode. This polarization is continuous for DC. energization. For A.C. energization it exists during one set of alternate half cycles, with the roles of anode and cathode being interchanged during the other set of alternate half cycles. The electrostatic lines of force between the two electrodes are substantially as shown, the shorter the line the greater the strength of the electric field at that point, i.e. the greater the potential gradient along the line of force. A photon striking the cathode liberates an electron which is accelerated toward the anode at a rate determined by the electric field strength at the point of liberation. If the velocity attained by the electron is suflicient, it ionizes a gas molecule by collision and the resulting ion is accelerated toward the oathode. This ion may in turn ionize another gas molecule by collision liberating an electron, or it may impinge on the cathode and liberate an electron from the cathode. In either case, the liberated electron behaves like the original photoelectron in producing further gas ionization. In this manner a cumulative ionization of the gas occurs which results in a relatively heavy discharge between anode and cathode that continues until the anode potential is lowered below a sustaining value. In a tube of the type shown in FIG. 1, the direction of greatest sensitivity in a plane normal to the electrode axes is at an acute angle 0, in the vicinity of 45 measured from the plane of the electrode axes, with the anode closer to the radiation source than the cathode. Also, the directional sensitivity pattern in the normal plane is not symmetrical.

A tube constructed in accordance with the invention is indicated schematically and not to scale in FIG. 2. In this tube a third wire electrode 4 is added to the electrodes 1 and 2 of FIG. 1. The electrode 4 is parallel to and equidistant from electrodes 1 and 2, and is situated beyond electrodes 1 and 2 with respect to the source of radiation. Therefore, in a plane normal to the electrodes, the electrode axes are 'at the vertices of an isosceles triangle which if desired may be equilateral. Direct current energization is used. Electrodes 1 and 2 are the anodes and have the same positive potential relative to electrode 4 which is the cathode. The electrostatic lines of force between the three electrodes are substantially as shown. It will be apparent by comparison with FIG. 1 that the area of the cathode in FIG. 2 over which the electrostatic lines of force are relatively short is roughly twice that in FIG. 1. Consequently, the area of the cathode over which incident ultraviolet radiation is able to trigger a discharge in the tube is roughly twice that in FIG. 1. This advantageous reshaping of the electrostatic field results in a 40 to 60 percent increase in the sensitivity of the tube over one of the type shown in FIG. 1. Also, the angle in a plane normal to the electrode axes within which relatively high sensitivity is obtained is greater than in FIG. 1 and the directional sensitivity pattern in this plane is symmetrical relative to the central direction 5. The acceptance angle may be varied by moving cathode 4 relative to the anodes.

A possible physical arrangement of a tube constructed in accordance with the invention is illustnated in FIGS. 3, 4 and 5, the reference numerals corresponding to those used in FIG. 2.

FIG. 6 illustrates a suitable D.C. energizing circuit for a tube such as that shown in FIGS. 3, 4 and 5. A regulated D.C. power supply 6 provides the normal operating voltage for the tube. In the absence of radiation the tube is nonconductive and the full power supply voltage lappears between the anodes, which are connected together, and the cathode. Incident radiation producing a discharge between the anodes and the cathode in the manner already described causes the anode voltage to fall below the sustaining value, due to the drop across resistor 7, and the discharge to be extinguished, producing in the process an output pulse across load resistor 8. This cycle repeats as long as the radiation is present.

A tube constructed in accordance with the invention is particularly suited to use with ultraviolet optics, as illustrated in FIG. 7. Here an ultraviolet lens such as one made of quartz concentrates the radiation on the cathode 4 thus producing an added increase in sensitivity.

I claim:

1. An ultraviolet radiation detector of the wire electrode photoelectron triggered gaseous discharge type comprising: a sealed envelope transparent to said radiation and filled with an ionizable gas; a pair of parallel closely spaced straight wire coextensive anodes in said envelope; a straight wire cathode in said envelope parallel to, equidistant from and coextensive with said anodes and lying outside the plane defined by the anode axes, said cathode being made of .a material that is photoemissive at the wavelengths of said radiation; means providing electrical connections from said anodes and cathode to the outside of said envelope; means directly connecting said anodes together for maintaining them at the same electrical potential; a source of direct potential and an output impedance connected in series between said anodes and said cathode, said source being poled to make said anodes positive relative to said cathode and the magnitude of said potential being such that the electric field established between said anode-s and said cathode over an extended area of said cathode is of sufiicient strength to accelerate photoelectrons emitted by said cathode to energy levels sufficient to ionize said gas by collision, the potential developed across said load impedance constituting the output of said detector.

2. Apparatus as claimed in claim 1 in which the wire diameter of said cathode equals the wire diameter of said anodes and in which the spacing between said cathode and said anodes approximately equals the spacing between said anodes.

References Cited UNITED STATES PATENTS 1,955,971 4/1934 Muse 313-192 X 1,998,884- 4/1935 Rockwood et al. 313-197 X 2,582,367 1/1952 Williams et al. 313192 X 3,209,190 9/1965 Cade 313-93 X ROBERT SEGAL, Primary Examiner.

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