US3389578A

US3389578A - Refrigerator for multiplier phototubes

Info

Publication number: US3389578A
Application number: US583285A
Authority: US
Inventors: Slava A Pollack
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1966-09-30
Filing date: 1966-09-30
Publication date: 1968-06-25
Anticipated expiration: 1985-06-25

Description

June 25, 1968 s. A. POLLACK 3,

REFRIGERATOR FOR MULTIPLIER PHOTOTUBES Filed Sept. so, 1966 Slova A. Pollock,

INVENTOR.

AGENT.

United States Patent Office 3,389,578 Patented June 25, 1968 3,389,578 REFRIGERATOR FOR MULTIPLIER PHOTOTUBES Slava A. Pollack, Palos Verdes Estates, Calif., assignor to TRW Inc., Redondo Beach, Calif., a corporation of Ohio Filed Sept. 30, 1966, Ser. No. 583,285 5 Claims. (Cl. 62-514) ABSTRACT OF THE DISCLOSURE The measurement of extremely small values of light flux by a multiplier phototube is limited by the thermionic dark current originating at the photocathode. This dark current can be reduced to negligibly small values by refrigerating the tube. When the ambient temperature is lowered by 80-100 C., the dark current drops by oneto-two orders of magnitude for the blue sensitive tubes and by four-to-five orders of magnitude for red sensitive tubes.

The conventional refrigerating procedure consists of placing the tube in a double-walled vacuum Dewar flask which is then filled with a refrigerant such as Dry Ice or liquid nitrogen. This technique is cumbersome because it restricts the use of the multiplier phototube to one position; it also requires the construction of an expensive, specially designed Dewar flask.

A simpler and more practical method of cooling consists of circulating cold nitrogen gas around the multiplier phototube. This method permits the multiplier phototube to be used in a horizontal or any other position. Commercially available multiplier phototube assemblies, which employ cold nitrogen gas for cooling, house the tube in an expensive high vacuum metallic Dewar flask.

The present invention is based on the realization that an insignificant reduction in the dark current is gained by cooling a multiplier phototube below temperatures in the range of -50 C. to l00 C. Most commercially available multiplier phototubes exhibit saturation in the signal-to-noise ratio in the aforementioned temperature range. Accordingly, it has been found unnecessary to house a multiplier phototube in an expensive high vacuum Dewar flask.

According to the invention, a refrigerator is provided which includes an outer casing of light shield material, having a thermally insulating window sealed at one end thereof. The outer casing is internally lined with an inner jacket of thermal insulation material, which in its preferred form is made of molded polystyrene foam.

The insulation jacket houses a multiplier phototube within its interior, with an annular space or chamber provided between the jacket and the multiplier phototube. Two longitudinally spaced, angularly bent passageways provide communication between the exterior of the outer casing and the interior of the insulating jacket. The passageways serve as inlet and outlet ports of the flow of a coolant gas into the annular space surrounding the multiplier phototube.

The single figure of the drawing is a cross-sectioned elevational view of a refrigerator for a multiplier phototube according to the invention.

Referring to the drawing, a cylindrical outer casing 10, made of light-weight material, such as aluminum, has a double window 12 sealed at one end thereof. The window 12 is formed of two glass or

quartz disks

14 and 16 spaced from each other by a vacuum to thermally insulate the regions on opposite sides of the

disks

14 and 16 from each other. The window 12 may be sealed to the outer casing 10 by epoxy cement.

The end of the outer casing .10 opposite the window 12 is closed by a cover 18, which may be fastened by screws 20. A multiplier phototube 22, having its base 24 disposed within an opening in the cover 18, extends longitudinally within the outer casing 10. The phototube 22 is mounted with its photocathode 26 spaced adjacent to the window 12 for reception of light to be detected.

An electrical socket 28 is fastened to the tube base 24 to provide electrical conduction means for operating potentials applied to the phototube 22 as well as for the output signal generated by the phototubev 22. A voltage divider network 30 fastened to the socket. 28 and a high voltage connector 32 is used to furnish the operating potentials. An anode connector 34 serves as an output terminal. The high voltage and

anode connectors

32 and 34 are insulatively mounted in a perforated can 36 which surrounds the voltage divider network 30. The perforated can 36 is fastened to the cover 18 by means of screws 38. The perforations 40 provide cooling vents for the voltage divider network 30 which generates substantial heat during tube operation.

The inner surface of the outer casing 10 is lined with a jacket 42 of thermal insulation material, which is preferably made of moldable material such as polystyrene foam or the like. In addition, the inside of the cover 18 is lined with a ring 44 of similar thermal insulation material which fits around the tube base 24 to thermally isolate the voltage divider network 30.

The insulation jacket 42 is dimensioned to leave an annular space or chamber 45 surrounding the main body of the phototube 22. For cooling the phototube 22 to its low operating temperature, the annular chamber 45 surrounding the phototube 22 is filled with a coolant gas. The coolant gas enters the annular chamber 45 through an inlet port 46 or angularly bent passageway extending between a lower opening 48 inside the jacket 42 and an upper opening 50 outside the outer casing 10. The inlet port 46 may be made of stainless steel or other low thermally conductive tubing and is bent to prevent light from leaking into the annular chamber 45.

A conduit 52 is connected between the inlet port 46 and a source 54 of coolant gas. The source 54 is preferably a container of liquid nitrogen 56 having a. heating coil 58 immersed therein to evolve nitrogen gas. The heating of the liquid nitrogen 56 may be adjusted to cause nitrogen gas to flow at a temperature between -50 C. and -100 C.

For circulating the coolant gas about the phototube 22, an outlet port 60 is spaced longitudinally from the inlet port 46 in the upper region of the gas chamber 45. The outlet port 60 is similar to the inlet port 46 and may comprise a bent stainless steel tube extending between an upper opening 62 inside the insulation jacket 42 and a lower opening 64 outside the outer casing 10. An open conduit 56 connected to the outlet port 60 expels the coolant gas into the atmosphere. If desired the conduit 66 may be connected to the gas source 54 to return the gas to the source 54.

A temperature monitoring device 68, such as a thermistor, is disposed within the gas chamber 45 adjacent to the photocathode 26 to monitor the temperature of the latter. The temperature monitoring device 68 is brought out to an electrical connector 70 for connection to a temperature indicating means, not shown.

The insulation jacket 42 is most conveniently made by molding it to the inside of the outer casing 10 around a removable core having the same size as the outside dimensions of the gas chamber 45. For this purpose a liquid such as urethane Nopcofoam or Eccofoam FP, made by Emerson-Cuming, Inc., Canton, Mass, may be mixed with a catalyst to form a solid foamy material. If desired a light absorbent coating 72 such as a mat black paint or a carbon coating may be applied to the inside surface of the insulation jacket 42 to prevent internal light reflections. It has been found that an insulation jacket 42 formed with a thickness of about an inch or more provides satisfactory insulation for cooling a red sensitive phototube such as a Du Mont type 6911 to -50 C.

It is now apparent that the above-described refrigerator of the invention provides a light-weight, easy to manufacture, economical structure for cooling multiplier phototu'bes to suificiently low temperatures to reduce the dark current to negligible levels. The simplicity of the structure is due largely to the avoidance of high vacuum construction other than the vacuum window 12.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination:

(a) an outer casing;

(b) a window mounted in the wall of said outer casing;

(c) a jacket of thermal insulation material lining the inner surface of said outer case;

(d) a pair of longitudinally spaced angular bent passageways communicating between the exterior of said outer casing and the interior of said jacket;

(e) a multiplier phototube within the interior of said jacket and spaced from the latter to form an annular chamber surrounding said phototube;

(f) means for mounting said phototube with its photocathode adjacent to said Window for reception of light to be detected;

(g) and means for introducing a coolant gas into said chamber through one of said passageways and for exiting said gas out of said chamber through the other passageway.

2. The invention according to claim 1, and further including a light absorbent coating on the inner surface of said insulation jacket.

3. The invention according to claim 1, wherein said jacket is made of molded polystyrene foam.

4. The invention according to claim 1, wherein said coolant gas means includes nitrogen gas at a temperature in the range of about to C.

5. The invention according to claim 1, wherein the structure in which said phototube is disposed is devoid of any hermetic seals except for said window.

References Cited UNITED STATES PATENTS 2,948,127 8/1960 Carter 62-5l4 3,006,157 10/1961 Haettinger et al 62514 3,258,602 6/1966 Promish 62514 3,261,180 7/1966 Porter et a1. 62-45 3,293,877 12/1966 Barnes 62-5l4 LLOYD L. KING, Primaty Examiner.