US1955608A - Photoelectric device - Google Patents

Description

April 17, 1934- H. c. RENTSCHLER PHOTOELECTRICIDEVICE Filed Jan. 14, 1931 2700 WAVE-50670 //v fl/vjrfiom 01/035.

INVENTOR HCFE/VT5C/7L 6 ATTORNEY Patented Apr. 17, 1934 NITE D STATES PATENT PHOTOELECTRIC DEVICE Application January 14, 1931, Serial No. 508,570

5 Claims. (Cl. 250-275) This invention relates to photoelectric tubes and more particularly-to photoelectric tubes which are sensitive to radiation in the ultraviolet region of the spectrum.

In a copending application Serial No. 360,667 filed May 6, 1929, I have described and claimed a photoelectric tube in which the sensitivity starts in the blue region of the spectrum and extends intothe ultraviolet region.

In copending applications Serial No. 360,666 filed May 6, 1929 and Serial No. 370,223, filed June 12, 1929 I have disclosed photoelectric tubes sensitive to restricted portions of ultraviolet radiation. Application Serial No. 370,223 has issued into Patent #1,815,073, dated July 21, 1931. Such devices are useful in the measuring of preferred or of restricted bands of radiation, and particularly useful in the measurement of radiation lying in the so-called erythema range of radiation having wave lengths approximating 2900Angstrom units.

One of the objects of the present invention is to provide a photoelectric tube which is non-sensitive to the entire visible spectrum but which is particularly sensitive to radiation 'in the erythema range of radiation.

Another object is to produce a photoelectric tube which is sensitive only to ultraviolet light havinga wave length below about 3200 Angstrom units.

A further object is to provide an improved method of producing a photosensitive electrode.

Another object of this invention is to provide a photosensitive cathode for a photoelectric device which is strongly sensitive to radiation of wave lengths less than about 3200 Angstrom units.

Other objects and advantages will appear as the invention is more fully disclosed.

Heretofore in the production of photoelectric tubes for measuring radiation in the ultraviolet region, cadmium or zinc have been generally used as the photosensitive cathode, but the photoelectric current obtained from cadmium is extremely feeble, requiring for its detection a sensitive electroscope or electrometer or special vacuum tube amplifying means. Moreover the photosensitivity of cadmium does not extend to radiation'of wave lengths greater than about 3000 Angstrom units.

In accordance with my invention I have produced a photoelectric tube which is sensitive only to ultraviolet light and in which the photoelectric effect is relatively strong.

I have discovered that the'metal titanium exhibits a normal photoelectric effect which starts at about 3200 Angstrom units, which is well within the ultraviolet range, and is substantially non-sensitive to light within the visible region.

When the photosensitive cathode of a photoelectric device is comprised of titanium the envelope of the tube must be composed at least in part of a material, such as quartz, which is permeable to radiation of wave lengths less than about 3200 Angstrom units. If it is desired to restrict the measurement to a narrow band of radiation one envelope may be composed of a glass which is transparent only to such radiation. By properly selecting the composition of the envelope practically any band of radiation from 3200 Angstrom units down may be measured.

Thus for instance for erythema treatment, the

range of useful rays is restricted to a narrow band each side of about 2900 Angstrom units and therefore in the production of a dosimeter employing a photoelectric tube for determining the length of treatment from a given source of ultraviolet light, it is necessary'that the tube be sensitive to light principally in the region between about 2700 and 3100 Angstrom units.

By comprising the envelope of the tube of material which is opaque to radiation of wave lengths below about 2400 Angstrom units and utilizing a titanium photosensitive cathode, a photoelectric device having a maximum sensitivity at about 2900 Angstrom units, may be obtained. Such a device would have only a low order of sensitivity to wave lengths appreciably above and below this value. The percent transmission of the envelope for various wave lengths may be varied by changing the thickness of a glass of definite composition and in this manner the peak of the output curve of the tube may, to a limited extent, be varied.

I have found that by employing very thin glass windows for the tube, there is a very great increase in the current output of the tube, particularly to radiation of the lower Wave lengths.

When employing relatively thin windows comprised of glass having the composition of Pyrex, which is a commercially known glass, that ordinarily is not considered transparent to ultraviolet light, the ultraviolet transmission of the glass is comparable with that of quartz, at the thickness ordinarily employed except for the short wave length end of the spectrum. The thickness of the Pyrex glass window should be of the order of .01 inches or less for best results.

In producing a photoelectric tube in accordance with my invention I preferably employ titanium for both the anode and cathode although it is to be understood that the anode may be comprised of another metal, such as nickel. The cathode may consist of a sheet of titanium or it may take the form of a relatively thin coating deposited on the wall of the envelope in contact with a leading-in conductor or upon the surface of an electrical conductor. When employed as a coating on the glass envelope, I prefer to first coat the glass over the portion which is to constitute the cathode with a noble metal, such as gold, in order to obtain a better contact with the cathode leading-in wire. When employed as a coating upon the surface of an electrical conductor the conductor should preferably be comprised of a metal, such as nickel, and should have a surface thoroughly free of impurities, such as oxides, carbon, nitrides and the like which are deleterious to the photosensitivity of the titanium coating.

In order to render the titanium cathode photosensitively active it is essential to obtain there on a surface substantially free of all impurities, such as oxides, carbides, nitrides, etc. This can be most conveniently accomplished by electrically sputtering away the surface of a solid cathode by creating a glow discharge in an inert gas between an anode and a cathode of titanium. Where a solid cathode is desired the titanium may be deposited by such an electrical sputtering means upon the surface of an electrical conductor. In such an electrically sputtered condition the metal appears to have its greatest photosensitivity.

The construction of the tube and the method of treating and producing the active cathode surface will appear more fully in connection with a detailed description of the accompanying drawing in which Fig. 1 is a side elevational view in section of a photoelectric tube embodying the present invention;

Fig. 2 is a cross sectional view taken along plane II--II Fig. 1;

Fig. 3 is a cross sectional view taken along plane III-III Fig. 1;

Fig. 4 is a cross sectional view as taken along plane II-II Fig. 1 showing a modification of the present invention utilizing a relatively thin raypermeable window; and

Fig. 5 is a curve showing the current output of a tube plotted against the wave length of the incident ultraviolet light.

Referring to Fig. 1, I have shown a photoelectric tube comprising an envelope 1 composed of a material which is transparent to ultraviolet light, such as quartz or special ultraviolet transmitting glasses having the composition of those glasses known to the trade as Corex, Vitaglass, etc.

Within the envelope 1 is mounted a rod shaped anode 2 and a cathode 3. composed of titanium. The anode 2 is mounted upon a leading-in conductor 4 extending through press 5 to an external conductor 6. The conductor a is enclosed in a dielectric shield member '7 comprised of quartz, glass, porcelain or similar material and restricts the active area thereof to the end portion 2. Only the exposed end of the anode may be comprised of titanium, if desired.

The cathode 3, preferably comprises a film of titanium which is deposited upon an electrical conductor, preferably comprised of a sheet of nickel 21 which is electrically connected to lead- The electrode 2 is ing in conductor 8 passing through the envelope 1 in the manner shown.

A frame-work consisting of two parallel longitudinally extending wires 9 and 10 and a bracing cross wire 11 is mounted above the press 5 by means of collar 12 frictionally engaging the press 5. Supported on the frame-work is a cylindrical shield 13 enclosing the anode 2 and the electrical conductor 21 serving as the base for cathode 3. Means, comprising inner surface straps or eyelets 14 (Fig. 3) through which the vertical wires 9 and 10 have sliding engagement, are provided to permit a movement of shield 13 along the wires 9 and 10. By inverting the tube the shield 13 may be moved from one end of the frame to the other. The function of the shield 13 is to protect the walls of the envelope from vaporized or sputtered electrode material during the electrical sputtering of the anode to form the cathode 3.

To obtain the photosensitive cathode the electrode assembly is sealed into the envelope, the envelope is baked out and exhausted in the usual manner heretofore employed to obtain high vacuo and the shield 13 is degasified by high frequency induction heating. A filling of a gaseous medium, such as argon, neon or helium is then introduced within the envelope in any convenient manner as through the exhaust tubing 15. Argon gas at a pressure of from about 1 to 6 mm. is preferably employed.

The tube is then inverted to position the shield 13 about the electrode 2 and the glow discharge is created between the electrode 2 and leading-in conductor 8 of sufficient current density to effect positive ion bombardment of the electrode and a sputtering or electrical vaporization of the electrode material.

A direct current discharge is employed and the electrical conductor 21 should serve as the anode for the glow discharge. The glow discharge should be continued until a sufficient deposit of the titanium is obtained upon the surface of the conductor 21 to render the same photoelectrically sensitive. I usually continue the discharge for about two hours. This, however, is arbitrary depending upon conditions. The sputtered material, other than that deposited upon the conductor 21 collects on the interior of the shield 13 and the envelope is maintained clear and free from light obscuring deposits. After the cathode is thus formed, the shield 13 is moved back to the position shown in Fig. 1.

If a gas filling is desired within the tube, the tube may be employed immediately following the glow discharge otherwise the gas must be pumped out completely. Or if the gas pressure is too high part may be pumped out to obtain the proper pressure.

In Fig. 2 I have shown a cross sectional view of the photoelectric tube shown in Fig. l, with the shield 13 in position as shown in Fig. 1. Fig. 3 is a cross section view taken along plane IIIIII illustrating the position of shield 13 when inverted during the forming of the cathode 3.

Fig. 4 is a contemplated modification of the present invention wherein the enclosing envelope is provided with a radiation permeable window 17 of relatively great thinness which is formed by heating up the glass wall of the envelope and drawing it in a bulbous portion having a relatively thin dome 18 and gradually tapering walls 19. The thickness of the window may be about .01 inches although windows of a diameter of about 1 inch and a thickness of only .0001 inches will withstand full atmospheric pressure.

The thin window may be comprised of material which is more transparent to ultraviolet light than the remainder of the envelope or may be comprised of material of the same degree of permeability as the envelope. Thus, for instance, the envelope may be composed of an ultraviolet transmitting glass, such as CoreX having a thickness of about one thirty-second of an inch and a window of a thickness of about .01 inches. There is a lesser absorption of the ultraviolet light of short wave lengths in the thin window, than in the glass of full wall thickness, and consequently the current output of the tube is materially increased.

A thin window of the type shown when formed in envelopes of glass ordinarily deemed opaque to ultraviolet light, such as ordinary lime glass or Pyrex, readily transmit ultraviolet light so that it is unnecessary to employ expensive special ultraviolet transmitting glasses or quartz envelopes unless very short ultraviolet rays are to be used.

Fig. 5 shows the relation between the wave length of the incident light and the current output of a tube employing an envelope which absorbs the ultraviolet light of short wave lengths and a photosensitive cathode, which is sensitive only to a restricted band of such radiation. Three types of cathodes are illustrated, zirconium, titanium and vanadium, each indicated by their respective chemical symbol.

The short wave portion of each curve designates approximately the relationship of the per cent transmission of the ultraviolet radiation through the Wall of the envelope, the long wave portion of each curve the total current of photoelectric sensitivity of the cathode, to that radiation which is transmitted by the envelope. Each curve, therefore, is a resultant curve indicating the eifect of the energy absorption of the glass on the current output of the tube. The particular glass composition comprising the envelope employed in obtaining these curves is known to the art as Corex D glass. The variation in the short wave portion of the three curves is due to unavoidable differences in the thicknesses of the respective glass envelopes.

The form of the curves will vary somewhat with different glass composition and may be modified materially by employing glass of different thicknesses. In the curves shown the maximum sensitivity of the tubes employing the zirconium and titanium cathodes occurs at about 2900 Angstrom units. This region of the spectrum is most suitable for erythema treatment and is most desirable to measure. The peak of senstivity of the vanadium curve with Corex glass lies below this 2900 Angstrom wave length.

The peak of each curve may be shifted to the left by using a thinner envelope and thus decreasing the amount of absorption of the lower wave length radiation or it may be shifted to the right to narrow up the band to which the tube is sensitive by increasing the thickness of the envelope. The sensitivity of the device may also be increased by the use of a relatively thin glass window in the envelope.

The envelope may also be comprised of glass having a permeability to radiation that is different to that of Corex glass, if desired, in order to obtain diiferent ranges of sensitivity in the device. It is apparent that where a titanium cathode is employed the envelope must be permeable to radiation of wave lengths less than about 3200 Angstrom units in order to utilize this cathode.

While I have shown several embodiments of my invention it is to be understood that the invention is not limited to such specific embodiments but is susceptible of many changes and modifications and such embodiments are to be considered merely as illustrative of the invention and not in a limiting sense, except in accordance with the appended claims.

What is claimed is:

l. A photoelectric tube comprising an envelope transparent to radiation of wave lengths less than about 3200 Angstrom units, an anode and a photosensitive cathode therein, said cathode being composed of titanium and a window in said envelope opposite said cathode more transparent to said radiation than the remainder of the envelope, said cathode being sensitive to substantially all radiations between about 2500 and 3100 Angstrom units and substantially insensitive to radiations above about 3200 Angstrom units.

2. An electron discharge device comprising an envelope, permeable to radiation of wave lengths below approximately 3200 Angstrom units, an anode and a photosensitive cathode therein, said cathode being composed of titanium and a thin bulbous window formed in said envelope opposite said cathode, said cathode being sensitive to substantially all radiations between about 2500 and 3100 Angstrom units and substantially insensitive to radiations above about 3200 Angstrom units.

3. An electron discharge device comprising an envelope transparent to ultraviolet light of wave lengths between about 2700 and 3200 Angstrom units, an anode and a photosensitive cathode therein, said cathode being composed of titanium and sensitive to substantially all radiations between about 2500 and 3100 Angstrom units.

4. A photoelectric tube comprising an envelope having a portion thereof permeable to radiation below 3200 Angstrom units, a cathode sensitive to substantially all radiation between 2700 and 3100 Angstrom units and insensitive to radiation above about 3200 Angstrom units, said cathode including a photosensitive surface containing titanium.

5. A photoelectric tube comprising an envelope having a portion thereof permeable to radiation below 3200 Angstrom units, a cathode sensitive to substantially all radiation between 2700 and about 3200 Angstrom units and insensitive to radiation above about 3200 Angstrom units, said cathode including a photosensitive surface film of titanium.

HARVEY CLAYTON RENTSCHLER.

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