Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1998-07-06
2001-04-24
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
Reexamination Certificate
active
06222192
ABSTRACT:
FIELD OF THE INVENTION
The invention herein described relates generally to a scintillation detector and to a method of manufacturing a scintillation detector. The scintillation detector and method are particularly useful for borehole logging applications, but may, however, have use in other applications.
BACKGROUND OF THE INVENTION
Scintillation detectors have been employed in the oil and gas industry for well logging. These detectors have used thallium-activated sodium iodide crystals that are effective in detecting gamma rays. The crystals are enclosed in tubes or casings to form a crystal package. The crystal package has an optical window at one end of the casing which permits radiation-induced scintillation light to pass out of the crystal package for measurement by a light sensing device such as a photomultiplier tube coupled to the crystal package. The photomultiplier tube converts the light photons emitted from the crystal into electrical pulses that are shaped and digitized by associated electronics. Pulses that exceed a threshold level are registered as counts that may be transmitted “uphole” to analyzing equipment or stored locally.
The ability to detect gamma rays makes it possible to analyze rock strata surrounding the bore holes, as by measuring the gamma rays coming from naturally occurring radioisotopes in down-hole shales which bound hydrocarbon reservoirs. Today, a common practice is to make measurements while drilling (MWD). For MWD applications, the detector must be capable of withstanding high temperatures and also must have high shock resistance. At the same time, there is a need to maintain performance specifications.
As new MWD tools are developed, the need for smaller detectors that meet or exceed larger detector performance is paramount. Current geophysical detectors that use hygroscopic crystals, such as thallium-activated sodium iodide crystals, require that the crystal be hermetically sealed in a stainless steel container. In order to maintain that seal under operating conditions, typically a soda lime glass window is hermetically sealed to the stainless steel housing by means of a glass to metal seal. The window is required to transmit the scintillated light produced in the crystal to a light sensing device such as a photomultiplier tube. This window assembly, along with the multiple optical interfaces needed, degrades the light transmitted to the photomultiplier. It follows, if the window and the associated interface can be removed, a gain in optical performance can be realized. This translates into a smaller crystal that has increased system nuclear performance of a larger crystal having an interface/window assembly. Therefore, it is desirable to have the photomultiplier tube directly coupled to the crystal and hermetically sealed in the housing.
However, there are many problems that must be addressed in the construction of such a windowless detector. These problems include the hermeticicity of the electrical pass-throughs, the off-gassing of volatile components that may degrade the hygroscopic crystal, and the survivability of the device under extreme environmental conditions.
Accordingly, it will be understood from the above that it would be desirable to have a scintillation detector without an optical window which overcomes the above problems.
SUMMARY OF THE INVENTION
The present invention provides a scintillation detector wherein a scintillation crystal is directly coupled to a photomultiplier tube (PMT). The crystal/PMT subassembly is attached to a voltage divider and the entire device is hermetically sealed in a stainless steel outer case. Conductors are passed through the hermetic package from the voltage divider via a high temperature metal to ceramic pass-through. The crystal and PMT are longitudinally loaded within the outer case by springs in order to minimize vibrations in the crystal and PMT and to accommodate thermal expansion and contraction of the crystal/PMT subassembly. A thermoplastic support sleeve circumscribes the crystal and the PMT to protect the crystal and PMT from excessive longitudinal and bending loads. The support sleeve and the crystal have similar coefficients of thermal expansion so that the crystal and the support sleeve experience similar dimensional changes due to temperature fluctuations, allowing the support sleeve to best maintain its stress-limiting function and avoiding damage to the crystal/PMT, solid reflector or optical interface as temperature within the detector changes. The support sleeve is radially compressible and expandable, preferably by means of a longitudinal slot in it.
According to an aspect of the invention, a scintillation detector includes a sleeve supporting a light sensing device against longitudinal and/or bending is loads.
More particularly, according to another aspect of the invention, a scintillation detector includes a hygroscopic scintillation crystal; a light sensing device, such as a PMT, optically coupled to the crystal; a resilient biasing device which loads the crystal and the light sensing device longitudinally; and a support sleeve circumscribing the crystal and the light sensing device which limits the longitudinal load on the light sensing device and/or associated electronics.
According to yet another aspect of the invention, a method of manufacturing a scintillation detector includes the steps of optically coupling a hygroscopic scintillation crystal to a photomuitiplier tube; forming an equipment assembly by inserting the crystal and the photomultiplier tube in a support sleeve which limits the longitudinal loading on the photomultiplier tube; inserting the equipment assembly in a housing; longitudinally loading the equipment assembly; and sealing the housing while maintaining a longitudinal load on the equipment assembly.
According to a further aspect of the invention, a scintillation detector includes means for optically coupling a hygroscopic scintillation crystal and a light sensing device; means for longitudinally loading the crystal and the light sensing device; and means for limiting the longitudinal load on the light sensing device.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
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Grodsinsky Carlos
Sekela William D.
Bulson Don W.
Hannaher Constantine
Saint-Gobain Industrial Ceramics, Inc.
Ulbrich Volker R.
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