Photomultiplier tube having an improved centering and cathode contacting structure

A photomultiplier tube comprises an evacuated envelope including a faceplate and a sidewall. A conductive coating is disposed annularly around an interior portion of the sidewall adjacent to the faceplate and on a longitudinally extending portion of the sidewall as a strip. A photoemissive cathode is disposed on an interior surface of the faceplate and on the conductive coating adjacent thereto. A shield cup is spaced from the cathode and centered within the envelope by a plurality of bulb spacers. An electron multiplier cage assembly abuts the shield cup and is attached thereto. A cathode contact assembly is in contact with the strip on the sidewall. A plurality of locating slots are formed in the base of the shield cup to orient the bulb spacers in contact with the sidewall of the envelope and spaced from the longitudinally extending strip portion of the conductive coating thereon. The bulb spacers include stop shoulders which retain the bulb spacers within the locating slots. The cathode contact assembly includes a cathode contact support member and a resilient cathode contact member. The cathode contact support member includes an electrical contact tab struck from the body thereof and a cathode support tab which includes an extruded area to circumferentially locate the resilient cathode contact member between two consecutively spaced bulb spacers and to align the resilient cathode contact member with the longitudinally extending strip portion of the conductive coating on the sidewall of the envelope.

BACKGROUND OF THE INVENTION 
The present invention relates to photomultiplier tubes and, more 
particularly, to a photomultiplier tube having a conductive strip disposed 
on the envelope sidewall which is connected to a photoemissive cathode. 
Electrical connection to the conductive strip is provided by a resilient 
cathode contact member which is attached to a cathode contact support 
member. The cathode contact support member includes alignment means which 
aligns the resilient cathode contact member with the conductive strip. The 
tube includes a shield cup which has attached thereto an electron 
multiplier cage assembly. The shield cup is centered by means of a 
plurality of bulb contacts which are disposed within the bulb locating 
means formed in the shield cup. The bulb locating means are oriented such 
that the bulb contacts can contact the sidewall of the tube envelope 
without contacting the conductive strip connected to the photocathode. 
In U.S. Pat. No. 4,370,585, issued to G. N. Butterwick on Jan. 25, 1983, 
shows a photomultiplier tube having an evacuated glass bulb with a shield 
affixed to one end of a dynode assembly and having a plurality of 
prong-like bulb contact members attached to the outer surface of the 
shield. The first dynode of the dynode assembly is affixed to the shield. 
An aluminum coating is disposed on the sidewall of the bulb, and a portion 
of the coating extends longitudinally along the sidewall and is contacted 
by a tab-type cathode contact member which comprises one element of an 
evaporator structure affixed to one side of the dynode assembly. A 
photoemissive cathode is formed on an interior surface of the bulb and on 
a portion of the aluminum coating. In the structure described in the 
foregoing Butterwick patent, the shield operates at a potential different 
from that applied to the cathode; thus, it is necessary that the bulb 
contact members, which center the dynode assembly, do not contact the 
aluminum coating on the envelope. If the bulb contact members are 
improperly positioned on the shield so that they contact the aluminum 
coating, an electrical short results between the cathode and the first 
dynode of the dynode assembly which is attached to the shield. 
U.S. Pat. No. 4,426, 596, issued to G. N. Butterwick on Jan. 17, 1984, 
discloses a photomultiplier tube comprising a glass bulb having a 
plurality of circumferentially disposed strip-like bulb spacers attached 
to the outer surface of the shield cup. The shield cup supports a dynode 
cage assembly having a first dynode which is electrically connected to the 
shield cup. An aluminum coating is disposed around the faceplate of the 
glass bulb and in a longitudinally extending strip along a portion of the 
sidewall of the bulb. A photoemissive cathode is formed on the interior 
surface of the bulb and on a portion of the aluminum coating. A tab-like 
cathode contact is attached to a stem lead to apply cathode potential to 
the aluminum strip. The bulb spacers must be attached to the outer surface 
of the shield cup in such a manner that when inserted into the glass bulb, 
the bulb spacers do not contact the aluminum strip, or the shield cup and 
the first dynode will be shorted to the cathode rendering the tube 
inoperable. 
The requirement that the bulb spacers do not contact the aluminum strip on 
the bulb sidewall requires careful fabrication and inspection of the tube 
before sealing to prevent loss of tubes due to electrical shorts. In order 
to improve efficiency and reliability of the tube, it is desirable that 
the tube structure provide means for accurately positioning the bulb 
spacers relative to the aluminum strip to prevent contact therebetween. It 
also is imperative that the cathode contact is accurately located with 
respect to the aluminum strip on the sidewall of the bulb to ensure that a 
potential can be applied to the strip and, thus, to the cathode. 
SUMMARY OF THE INVENTION 
A photomultiplier tube comprises an evacuated envelope including a 
faceplate and a sidewall. A conductive coating is disposed annularly 
around an interior portion of the sidewall adjacent to the faceplate and 
on a longitudinally extending portion of the sidewall as a strip. A 
photoemissive cathode is disposed on an interior surface of the faceplate 
and on the conductive coating adjacent thereto. A shield cup is spaced 
from the cathode and centered within the envelope by bulb contacts. An 
electron multiplier cage assembly abuts the shield cup and is attached 
thereto. A cathode contact assembly is in contact with the strip on the 
sidewall. The shield cup has formed therein bulb contact locating means 
which are oriented to position the bulb contacts in contact with the 
sidewall of the envelope and spaced from the longitudinally extending 
strip portion of the conductive coating. The bulb contacts each include 
stop means which retain the bulb contacts within the bulb contact locating 
means. The cathode contact assembly includes a cathode contact support 
member and a resilient cathode contact member. The cathode contact support 
member includes electrical contact means and a cathode support tab which 
has alignment means formed therein to circumferentially locate the 
resilient cathode contact member between two consecutively spaced bulb 
contacts and to align the resilient cathode contact member with the 
longitudinally extending strip portion of the conductive coating on the 
sidewall of the envelope.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A photomultiplier tube 10, shown in FIG. 1, comprises an evacuated envelope 
12 having a sidewall 14. The envelope 12 is closed at one end by a 
transparent faceplate 16 and at the other end by a stem portion (not 
shown). A conductive layer 18 is vapor deposited annularly around an 
interior portion of the sidewall 14 adjacent to the faceplate 16 and on a 
longitudinally extending portion of the sidewall as a strip 18a. A 
photoemissive cathode 20 is formed on an interior surface of the faceplate 
16 and also along a portion of the conductive layer 18 on the sidewall 14. 
The photoemissive cathode 20 may comprise any of the alkali-antimonide 
materials known in the art and is preferably a potassium-cesium antimonide 
structure. The photoemissive cathode 20 emits photoelectrons in response 
to radiation incident thereon. 
A shield cup 22 is provided in spaced relation to the photoemissive cathode 
20. The shield cup 22 is a cup-shaped field forming electrode having a 
substantially flat base portion 24 and an annular wall portion 26 disposed 
perpendicular to the base portion 24. In the present structure, the shield 
cup 22 is electrically isolated from the cathode 20 and operates at a 
potential different therefrom. A centrally disposed substantially 
rectangular aperture 28 extends through the base portion 24 of the shield 
cup 22. The shield cup 22 is centered within the envelope 12 by a 
plurality of novel bulb contacts or spacers 29. A primary dynode 30 is 
disposed within the aperture 28 and is spaced therefrom. The primary 
dynode 30 has a cross-sectional contour substantially identical to that 
described in U.S. Pat. No. Re. 30,249, issued to R. D. Faulkner on Apr. 1, 
1980, and comprises the first electrode of an electron multiplier cage 
assembly 32. A substantially flat primary field mesh member 34 is affixed 
to an input aperture 36 of the primary dynode 30. The primary dynode 30 
preferably comprises a nickel substrate with an alkali-antimonide 
secondary emission coating 38 formed thereon. Alternatively, the primary 
dynode may be formed of a beryllium-copper material having a 
beryllium-oxide secondary emissive surface. The primary dynode 30 has an 
output aperture 40. 
The electron multiplier cage assembly 32 further includes a box-like 
secondary dynode 42 which acts as a receiving member for secondary 
electrons emitted from the secondary emission coating 38 of the primary 
dynode 30. The dynode 42 has an input end 44 and an output end 46. A 
secondary field mesh member 48 extends across the input end of the 
secondary dynode 42. A plurality of additional substantially-identical 
box-like secondary dynodes 50 are disposed between the secondary dynode 42 
and an ultimate secondary dynode 52. The ultimate secondary dynode 52 
encloses an anode 54. Each of the secondary dynodes 42, 50 and 52 is 
preferably formed of nickel and has an alkali-antimonide coating (not 
shown) formed on the inside surface thereof so that the dynode can 
propagate electron emission from the cathode 20 to the anode 54. Each of 
the secondary dynodes 50 includes a field mesh 56 disposed across the 
input end thereof. The primary dynode 30, the secondary dynodes 42, 50, 52 
and the anode 54 are disposed between a pair of substantially parallel, 
spaced apart insulating support plates 58 (only one of which is shown in 
FIG. 1). A novel cathode contact assembly 60 extends transversely between 
the support plates 58 and is electrically connected to the cathode 20 
through the conductive strip 18a on the sidewall 14. A light shield 62 
closes the lower end of the cage assembly 32. The cage assembly 32 is 
attached to the shield cup 22 by means of a pair of connecting tab members 
64 (only one of which is shown in FIG. 1). The connecting tab members are 
described in a copending U.S. patent application Ser. No. 611,873, filed 
concurrently herewith, assigned to the assignee of the present invention, 
entitled, "SHIELD CUP TO CAGE ASSEMBLY CONNECTING TAB MEMBER", by A. F. 
McDonie et al., which is incorporated by reference herein for the purpose 
of disclosure. A base 66, having a plurality of electrical contact pins 68 
therein, is attached to the stem end of the envelope 12. A plurality of 
electrical leads 70, only some of which are shown, are connected between 
the photoemissive cathode 20, the shield cup 22, the dynodes 30, 42, 50, 
52, the anode 54 and the pins 68 in the base 66. 
As shown in FIGS. 2 and 3, the novel cathode contact assembly 60 includes a 
cathode contact support member 72 and a resilient cathode contact member 
74 which is affixed thereto, for example by welding. Details of the 
cathode contact support member 72 are shown in FIGURES 4, 5 and 6. The 
cathode contact support member 72 includes an L-shaped cathode support tab 
76 which extends from the body of the member 72. The surface of the tab 76 
includes an extruded area 78, which is extruded to a depth of about 0.2 to 
0.3 mm (0.008 to 0.012 inches), approximately equal to the thickness of 
the material which is used to form the member 72. The extruded area 78 
provides alignment means for the resilient cathode contact member 74, 
which is nested in the extruded area 78 and welded therein to align with 
and contact the longitudinally extending strip portion 18a of the 
conductive coating 18, which is electrically connected to the 
photoemissive cathode 20. To facilitate electrical connection to the pins 
68 in the base 66, an electrical contact tab 80 is struck from the body of 
the cathode support tab 76. One of the electrical leads 70 is welded 
between the tab 80 and one of the pins 68 in order to apply a potential to 
the cathode 20. The cathode contact support member 72 includes oppositely 
disposed attachment tabs 82, extending from the sides thereof, which 
provide means for attaching the cathode contact assembly 60 between the 
insulating support plates 58. As shown in FIG. 3, the attachment tabs 82 
are disposed within a support slot 84. 
FIGS. 3, 7 and 8 show the novel bulb spacers 29 which comprise the bulb 
contact means for centering the shield cup 22 and the attached cage 
assembly 32 within the envelope 12. As shown in FIG. 7, the shield cup 22 
includes four locating slots 86 formed in the base portion 24 thereof 
adjacent to the wall portion 26. The locating slots 86 are oriented to be 
aligned radially along the diagonals connecting the oppositely facing 
corners of the rectangular aperture 28 formed in the shield cup 22. In the 
preferred embodiment, only four locating slots 86 are disposed at 90 
degree intervals circumferentially around the shield cup 22 to provide 
bulb contact locating means for the bulb spacers 29 disposed therein. Each 
of the locating slots 86 has a dimension of about 3.18.+-.0.05 mm 
(0.125.+-.0.002 inches) long and 1.02.+-.0.13 mm (0.040.+-.0.005 inches) 
wide. Each of the bulb spacers 29 comprise an arcuately shaped stop 
shoulder portion 88, which conforms to the annular wall portion 26 of the 
shield cup 22 to facilitate attachment thereto, and an offset bulb contact 
portion 90. The stop shoulder portion 88 has a chordal length of 
5.08.+-.0.25 mm (0.200.+-.0.010 inches) which is wider than the width 
(transverse dimension) of the locating slot 86 so that the bulb spacers 29 
are retained within the locating slots 86. The stop shoulder portion 88 is 
welded to the wall portion 26 of the shield cup 22. The thickness of the 
bulb contact portion 90 of the bulb spacer 29 is preferably about 0.25 mm 
(0.010 inches), and the transverse dimension of the bulb contact portion 
90 is about 3.05.+-.0.05 mm (0.120.+-.0.002 inches) which is substantially 
less than the width (transverse dimension) of the locating slot 86 so that 
the bulb contact portion 90 of the bulb spacer 29 extends freely through 
the locating slot 86 and can flex radially without contacting the base 
portion 24 of the shield cup 22. To facilitate contact with the envelope 
sidewall 14, the lower part of the bulb contact portion 90 is angled 
outwardly toward the sidewall about 20 degrees. The orientation of the 
locating slots 86 position the bulb spacers 29 such that the resilient 
cathode contact member 74 is circumferentially located between two 
consecutive bulb spacers 29 and is aligned with the conductive strip 18a 
on the sidewall 14. This orientation ensures that the bulb spacers 29 
cannot contact the conductive strip 18a. Thus, the present structure 
eliminates the possibility of inadvertently shorting the shield cup 22 to 
the cathode 20.