Photomultiplier with reduced dark current

Abstract

A high sensitivity Dynamic Crossed Field Photomultiplier (DCFP) includes a photocathode for emitting electrons in response to a light input, an adjacent dynode for providing electron amplification by multiple secondary electron emissions in response to electrons emitted from the photocathode, a magnetic field for providing a component force to urge emitted electrons to move linearly down said dynode, a fixed-potential accelerating field over the dynode and photocathode for providing a component force for attracting electrons therefrom, a radio frequency field over the dynode and photocathode for providing a component force for urging emitted electrons to bombard the dynode for causing copious secondary emission therefrom, and a collector for collecting and producing an output signal in accordance with the final secondary electron current. The resultant effect of the component forces is that electrons ''''hop'''' down the dynode in multiplying fashion (multiple secondary emissions) to provide a highly amplified electronic version of the light input. At normal room temperature, electrons thermionically emitted by the cathode and adjacent cathode supporting structure are maximally amplified, causing a substantial random output signal or ''''dark current'''' to appear in absence of any light input to the device. This dark current; insofar as stemming from the cathode supporting structure, is a spurious signal which deleteriously obscures true signals produced by a light input. It is substantially eliminated by the provision of conductive shields maintained at accelerating potential and provided over the cathode support structure in those areas suitable for intercepting and attracting just the spurious thermionic electrons emitted by said support structure.

Claims

1. In a crossed-field photomultiplier of the type comprising a photocathode for emitting electrons in response to a light input, an elongated dynode structure, one end thereof being adjacent said photocathode, said dynode adapted to be maintained at the same potential as said photocathode, an elongated accelerating electrode above said photocathode and dynode and adapted to be maintained at an elevated potential with respect to said dynode and photocathode for attracting electrons emitted thereby, means for providing a magnetic field about said photocathode and dynode to urge electrons emitted thereby along said dynode in a direction parallel to the direction of elongation of said dynode, and a collector for receiving electrons from an end of said dynode remote from said photocathode, the improvement comprising shield means for preventing electrons thermionically emitted by cathode support structure adjacent said photocathode from reaching said dynode so as to prevent amplification of said thermionic electrons said shield means comprising a conductive shield spaced between said support structure and said accelerating electrode. 2. The photomultiplier of claim 1 wherein said conductive shield is maintained at the elevated potential of said accelerating electrode with respect to said dynode. 3. The photomultiplier of claim 1 whereIn said support structure includes a portion of said dynode which is wider than said photocathode and, in addition to extending from one side of said photocathode in its direction of elongation, extends from two further, opposed sides of said cathode, and wherein said means for preventing electrons thermionically emitted by structure adjacent said photocathode from reaching said dynode comprises shield electrode means for preventing electrons thermionically emitted by the portions of said dynode extending from said two further opposed sides of said photocathode from reaching any other portion of said dynode. 4. The photomultiplier of claim 3 wherein said shield electrode means comprises a pair of metallic plates mounted above the portions of said dynode extending from said two further opposed sides of said photocathode, respectively, said plates being electrically insulated from said photocathode and said dynode. 5. The photomultiplier of claim 4 wherein said plates are maintained at an elevated potential with respect to said photocathode and said dynode. 6. The photomultiplier of claim 5 wherein the confronting edges of said plates are spaced just far enough from said photocathode so as not to intercept electrons emitted thereby, but closely enough to intercept electrons emitted by closely adjacent portions of said dynode. 7. The photomultiplier of claim 1 further including means for providing a radio frequency field above said photocathode and said dynode for influencing electrons emitted thereby. 8. A crossed field photomultiplier comprising: a photocathode, elongated dynode means for secondary emission multiplication of electrons from said photocathode, accelerating electrode means spaced from said photocathode and said dynode means for drawing electrons from said photocathode and said dynode means, means for collecting electrons emitted from said dynode means, means for providing a magnetic field for directing electrons emitted from said photocathode and said dynode means in the direction of elongation of said dynode means, support means for said photocathode, shield means for preventing electrons emitted from said support means from reaching said dynode means, said shield means comprising a conductive shield member spaced between said support means and said accelerating electrode means. 9. The photomultiplier of claim 8 wherein said shield member is adapted to operate at a shield potential more positive than the potential of said support means. 10. The photomultiplier of claim 9 wherein said shield potential is the d.c. component of the potential of said accelerating electrode. 11. The photomultiplier of claim 8 wherein said support means comprises a portion of said dynode means adjacent said photocathode.
United States Patent [1 Buck, Jr. et al. [ PHOTOMULTIPLIER WITH REDUCED DARK CURRENT [75] Inventors: Richard S. Enck, Jr., Mountain View, Calif.; Ronald H. Goehner, Wayne, N.J.; Gordon Noel Guy, Campbell, Calif. [73] Assignee: Varian Associates, Palo Alto, Calif. [22] Filed: Mar. 18, 1974 [21] Appl. No.: 452,151 Primary Examiner-James W. Lawrence Assistant ExaminerT. N. Grigsby Attorney, Agent, or Firm-Stanley Z. Cole; D. R. Pressman; R. K. Stoddard [57] ABSTRACT A high sensitivity Dynamic Crossed Field Photomultiplier (DCFP) includes a photocathode for emitting electrons in response to a light input, an adjacent dynode for providing electron amplification by multiple livt\\\\\\\ [4 1 Oct. 21, 1975 secondary electron emissions in response to electrons emitted from the photocathode, a magnetic field for providing a component force to urge emitted electrons to move linearly down said dynode, a fixed-potential accelerating field over the dynode and photocathode for providing a component force for attracting electrons therefrom, a radio frequency field over the dynode and photocathode for providing a component force for urging emitted electrons to bombard the dynode for causing copious secondary emission therefrom, and a collector for collecting and producing an output signal in accordance with the final secondary electron current. The resultant effect ofthe compo nent forces is that electrons hop down the dynode in multiplying fashion (multiple secondary emissions) to provide a highly amplified electronic version of the light input. At normal room temperature, electrons thermionically emitted by the cathode and adjacent cathode supporting structure are maximally amplified, causing a substantial random output signal or dark current" to appear in absence of any light input to the device. This dark current; insofar as stemming from the cathode supporting structure, is a spurious signal which deleteriously obscures true signals produced by a light input. It is substantially eliminated by the provision of conductive shields maintained at accelerating potential and provided over the cathode support structure in those areas suitable for intercepting and attracting just the spurious thermionic electrons emitted by said support structure. 11 Claims, 4 Drawing Figures US. Patent Oct. 21, 1975 3,914,599 PHOTOMULTIPLIER WITH REDUCED DARK CURRENT FIELD OF INVENTION This invention relates to photomultipliers and particularly to photomultipliers of the crossed field type which employ a photocathode to generate electrons in response to light and a dynode to multiply said electrons by plural secondary emissions. The particular improvement of the present invention relates to increasing the purity of the output signal of such a device; the present invention is especially useful in connection with high sensitivity photomultipliers of the dynamic crossed field type, i.e., those which employ a radiofrequency signal to effect electron multiplication. PRIOR ART PI-IOTOMULTIPLIERS In prior art crossed field photomultipliers, it has been noted that in the absence of any light input, a substantial output or dark current is produced by the device. This dark current is harmful because it has a random frequency spectrum distribution and thus is sometimes seen as a true signal. It also persists during the presence of a light input to the device, thereby creating a spurious signal in the presence of the true signal such that the output of the device is obscured or has a poor signal-to-noise ratio. This problem has been sufficiently acute in many devices, especially those of high sensitivity, such as dynamic crossed field photomultipliers of the type shown in Holshouser U.S. Pat. No. 3,233,140, granted Feb. 1, 1966, that the dark current is often actually greater than the true output signal produced when the device receives a light input. Thus the dark current often is actually great enough to render the device useless. Accordingly several objects of the invention are to provide an improved photomultiplier which has reduced dark current and higher output signal-to-noise ratio. Also the invention seeks to provide a high sensitivity dynamic crossed field photomultiplier which is not rendered inoperable by the presence of dark current in the output thereof. Further objects and advantages of the invention will become apparent from a consideration of the ensuing description thereof. DRAWING FIG. 1 is a cross sectional side view of a dynamic crossed field photomultiplier of the invention; FIG. 2 is a cross sectional view of the structure shown in FIG. 1 taken along the bilevel line 22 together with a partial view of the magnet used therewith; FIG. 3 is a cross sectional view of the structure of FIG. 2 taken along the line 33; and FIG. 4 is an underside view of the cathode area as seen along the line 44 in FIG. 3. DISCUSSION OF INVENTION It has been determined that the deleterious dark current in prior art photomultipliers originates at the supporting structure adjacent the cathode of the device as well as from the cathode itself. In particular it has been noted that the cathode and its supporting structure, which is used to mount the cathode and hold it in a precise position, emit electrons at normal room temperatures due to thermionic emission. Since these electrons are emitted in the cathode area, they are maximally amplified by the device, thereby creating the spurious output signal aforenoted. According to the present invention, means are provided for preventing the deleterious electrons thermionically emitted by the cathode supporting structure from reaching the dynode or the output of the device, thereby eliminating substantially all of the dark current previously generated. While the cathode itself also generates spurious electrons through thermionic emission, these electrons cannot be eliminated without eliminating the true signal from the cathode itself. A reduction in the thermionic electrons emitted from the supporting structure was found to be extremely valuable even without the elimination of thermionic cathode emission, as will be discussed in more detail infra. DESCRIPTION OF INVENTION The photomultiplier according to the invention comprises an evacuated enclosure 10 (FIGS. l-3), preferably of metal, having a window 12 for admission of light 14 into the enclosure. The light passes through this gridded aperture 16 to the surface of a photocathode 18. The photocathode is preferably a III-V compound (i.e., a compound composed of elements selected from columns III and V of the periodic table) such as gallium arsenide which is heavily P-doped, e.g., with zinc, to a charge carrier concentration of about IO /cc. The photocathode is activated with cesium oxide in the conventional manner. Such a photocathode has a wavelength sensitivity range from aboutultraviolet to about 9,000 angstroms. Alternatively the photocathode may be fabricated of Zn-doped InGaAsP (CsO activated); such material has a wavelength sensitivity range from about ultraviolet to about 10,600 anstroms. Cathode 18 is mounted in an oblong recess 20 (FIG. 4) formed by the legs of a dynode 22 by means of spring clips 24 (FIG. 4). It is held down by an overlying metal plate 26 which has a narrower recess than recess 20 in dynode 22. Dynode 22 is maintained at about 400 volts with respect to the potential of enclosure 10 by means of a connection 28 (FIG. 2) to dynode 22. Dynode 22 is insulated from the floor of enclosure 10 by a ceramic insulator 50. Dynode 22 is held in position by means of spring clips such as 32. Since clips 32 are at ground potential, they are insulated from dynode 22 by means of ceramic insulating wafers such as 34. Spring clips 32 and ceramic insulators 34 are spaced along the length of dynode 22; at the left end of dynode 22, near cathode 18, spring clips 32 hold metal plate 26, and hence cathode 18, as well as dynode 22, in position. Bonded to the two spring clips 32 on each side of dynode 22 at the left end thereof (near cathode 18) are two metallic shield electrodes 36 and 38. l.e., each shield electrode is bonded to the first two spring clips 32 on one side of dynode 22. Shields 36 and 38 each extend over a portion of metal plate 26 near cathode 18 according to the invention. Near the right end of dynode 22 an aperture 38 (FIGS. 1 and 2) is provided to receive electrons and transmit them to a collector structure 40 on which an output signal is derived. A high intensity magnetic field is provided transverse to dynode 22 by means of a magnet 42 (shown in partial form). A radio frequency input signal, preferably one gHz at one watt, is applied to the tube at input 44 (FIG. 1 A tuning element 46 is provided to adjust the frequency of the tube cavity to resonate with that of the input signal. An accelerating electrode or rail 48 is maintained at ground potential and extends over dynode 22 and photocathode 18. Dynode electrode 22 is preferably formed of beryllium copper with a berryllia surface for enhanced secondary emission properties. The dimensions of the active portion of the dynode are preferably about 0.8 inch by about 1.8 inches. The dimensions of photocathode 18 are about mils thick by 0.25 inch in diameter; however due to the partial masking of the edges of photocathode 18 by metal plate 26, the exposed area is only about 0.2 inch in diameter. Magnet 42 provides a substantially uniform field of about 320 gauss. OPERATION OF INVENTION When light 14 impinges on photocathode 18, primary electrons are emitted in conventional fashion. These electrons are attracted up from photocathode 18 by the relatively positive potential on rail 48 and are urged to the right (FIGS. 1 and 2) under the influence of the magnetic field from magnet 42. The radio frequency field present in the tube and above dynode 22 causes the electrons to return to and impinge violently against dynode 22 at a location to the right of cathode 18. These impinging electrons generate an increased number of secondary electrons in conventional fashion. Due to the influence of the magnetic, static electric, and dynamic electric fields, these electrons are caused to hop a downstream location on dynode 22, causing a further increased number of secondary electrons to be generated. The secondary electrons execute multiple hops along the dynode until, at the end of the last hop, they pass through aperture 38 and onto collector 40 and out of the vacuum whereupon they are sent to further circuitry for conventional signal amplification. Further details of the operation of the present DCFP as discussed in the aforementioned Holshouser patent and in US. Pat. No. 3,757,157 to Enck and Abraham, granted Sept. 4, 1973, and assigned to the present assignee. It has been observed that in the absence of any light input 14 to the device, a substantial random output current appears on collector 40. This dark current has been traced to thermionically emitted electrons originating at cathode 18 and the cathode supporting structure, primarily metal plate 26, adjacent cathode 18. Although the number of thermionically emitted electrons is relatively small at room temperatures, the fact that such emission occurs at an area adjacent cathode 18, where maximum amplification is provided, makes such emission particularly harmful and causes the substantial dark current aforenoted even though no optical input signal is provided to the device. Even when a light input 14 is applied tothe device, causing a normal output signal in accordance with the modulation on the light, to appear on collector 40, the aforenoted dark current is present and, depending upon the material used for photocathode l8 and the other parameters of the tube, such dark current can range from about one-fifth the value of the normal signal to greater than the normal signal. For example, in one embodiment of the invention, a normal output signal was 50 microamperes and the dark current signal ranged from 10 microamperes to over 50 microamperes. The presence of this dark current makes detection of the normal signal difficult and sometimes impossible and also causes the tube to have a very low signal-tonoise ratio. According to the present invention shield electrodes 36 and 38 (FIG. 3) are provided over the cathode supporting structure and are maintained at accelerating potential (ground-or 400 volts more positive than dynode 22 and cathode 18). Thus electrons thermionically emitted by metal plate 26 are attracted to electrodes 36 and 38 and are prevented from returning to dynode 22 where amplification thereof would occur. The innermost, confronting edges of electrodes 36 and 38 are spaced away from the exposed edges of cathode l8 sufficiently that electrons emitted by cathode 18 will not be attracted thereto, yet closely enough that electrons emitted by the portion of plate 26 immediately adjacent cathode 18 will be attracted to electrodes 36 and 38. Through the use of electrodes 36 and 38, a dark current reduction ratio of to l was achieved in a tube having parameters similar to those aforedescribed. As will be appreciated by those skilled in the art, such a reduction is extremely valuable and makes possible a very great increase in the signal-to-noise ratio of the tube and converts those tubes in which dark current is so great as to render the tube inoperable into an operable device. While the above-description contains many specificities, these should not be construed as limitations upon the scope of the invention, but merely as an exemplification of several preferred embodiments thereof. Many other embodiments and ramifications of the invention are possible. For example, dynode 22 can be segmented with increasing potentials applied to the successive segments in order to enhance electron multiplication effects. Various other materials can be used for photocathode l8 and dynode 22 and various other configurations for the tube itself will be apparent. Also the structure of shield electrodes 36 and 38 according to the invention can be varied both as to shape and basic configuration. The true scope of the invention should accordingly be determined only by the appended claims and their legal equivalents. What is claimed is: 1. In a crossed-field photomultiplier of the type comprising a photocathode for emitting electrons in response to a light input, an elongated dynode structure, one end thereof being adjacent said photocathode. said dynode adapted to be maintained at the same potential as said photocathode, an elongated accelerating electrode above said photocathode and dynode and adapted to be maintained at an elevated potential with respect to said dynode and photocathode for attracting electrons emitted thereby, means for providing a mag netic field about said photocathode and dynode to urge electrons emitted thereby along said dynode in a direction parallel to the direction of elongation of said dynode, and a collector for receiving electrons from an end of said dynode remote from said photocathode, the improvement comprising shield means for preventing electrons thermionically emitted by cathode support structure adjacent said photocathode from reaching said dynode so as to prevent amplification of said thermionic electrons said shield means comprising a conductive shield spaced between said support structure and said accelerating electrode. 2. The photomultiplier of claim 1 wherein said conductive shield is maintained at the elevated potential of said accelerating electrode with respect to said dynode. 3. The photomultiplier of claim 1 wherein said support structure includes a portion of said dynode which is wider than said photocathode and, in addition to extending from one side of said photocathode in its direction of elongation, extends from two further, opposed sides of said cathode, and wherein said means for preventing electrons thermionically emitted by structure adjacent said photocathode from reaching said dynode comprises shield electrode means for preventing electrons thermionically emitted by the portions of said dynode extending from said two further opposed sides of said photocathode from reaching any other portion of said dynode. 4. The photomultiplier of claim 3 wherein said shield electrode means comprises a pair of metallic plates mounted above the portions of said dynode extending from said two further opposed sides of said photocathode, respectively, said plates being electrically insulated from said photocathode and said dynode. 5. The photomultiplier of claim 4 wherein said plates are maintained at an elevated potential with respect to said photocathode and said dynode. 6. The photomultiplier of claim 5 wherein the confronting edges of said plates are spaced just far enough from said photocathode so as not to intercept electrons emitted thereby, but closely enough to intercept electrons emitted by closely adjacent portions of said dynode. 7. The photomultiplier of claim 1 further including means for providing a radio frequency field above said photocathode and said dynode for influencing electrons emitted thereby. 8. A crossed field photomultiplier comprising: a photocathode, elongated dynode means for secondary emission multiplication of electrons from said photocathode, accelerating electrode means spaced from said photocathode and said dynode means for drawing electrons from said photocathode and said dynode means, means for collecting electrons emitted from said dynode means, means for providing a magnetic field for directing electrons emitted from said photocathode and said dynode means in the direction of elongation of said dynode means, support means for said photocathode, shield means for preventing electrons emitted from said support means from reaching said dynode means, said shield means comprising a conductive shield member spaced between said support means and said accelerating electrode means. 9. The photomultiplier of claim 8 wherein said shield member is adapted to operate at a shield potential more positive than the potential of said support means. 10. The photomultiplier of claim 9 wherein said shield potential is the dc. component of the potential of said accelerating electrode. 11. The photomultiplier of claim 8 wherein said support means comprises a portion of said dynode means adjacent said photocathode.

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