Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-04-23
2002-03-12
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S36100C
Reexamination Certificate
active
06355932
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to highly ruggedized electro-optical devices for detecting radiation within a harsh environment. More particularly, the present invention relates to a radiation detecting device having a spring suspension system that provides for adjusting the resonant frequency and dynamic isolation characteristics while allowing the size of the device to be maximized within the available space in which it must operate.
The present invention also relates to the support of light detecting elements within the radiation detection device and to the optical coupling between the light detecting element and the scintillation element.
Radiation detectors are well known in the drilling industry and are often incorporated into drilling tools for oil wells and into the tools used to log the geologic formations. Such detectors are also used for mining operations, such as coal mining or uranium mining. Radiation detectors typically include a light detecting and quantifying device, such as a photo-multiplier tube, and a scintillation element that may be a crystal or suitably compounded element. The scintillation element functions by capturing radiation from the formation and converting that energy into light. The radiation may be ambient radiation emitted by radioactive materials in the formation, or radiation emitted in response to bombardment of the formation by radiation sources within the tools or equipment in which the detectors are operating.
Light generated within a scintillation element, as a result of intercepting radiation, is transmitted through an optical window into the photo-multiplier tube. The light impulses are transformed into electrical impulses that are transmitted via a data stream to an instrumentation system. Optical coupling elements are normally used between the scintillation element and the light-detecting element in order to achieve better light transmission, and may be used to provide dynamic isolation between the scintillation element and the light-detecting element.
A radiation detector may be incorporated into a variety of instrumentation and/or control systems where harsh environments exist. The process of logging typically is accomplished by lowering the detector into oil wells or survey holes while remaining connected to instrumentation systems by wires. Rapid movement through the holes produces considerable vibration and shock while the device is passing through temperature extremes, in some cases above 200 degrees Centigrade. Measurement While Drilling (MWD) operations or Logging While Drilling (LWD) operations utilize the detectors to help guide the drills and/or to help evaluate the formation, concurrent with the drilling operation, thereby subjecting the detector to extreme vibration and shock, while at temperatures up to 175 degrees Centigrade, or higher. Other drilling applications that subject the radiation detectors to extreme environments include environmental evaluations, geologic surveys and construction projects. Radiation detectors may also be used in coal mines to detect the boundary between the coal and shale or fire clay in order to control the mining equipment and to monitor the operation of the equipment.
In all the above-noted instances, a highly ruggedized detector is essential so that the detector will not fail and will not produce noise as a result of the vibration and shock. Elements that are likely to be damaged or to produce noise due to vibration, such as a scintillation crystal or a photo-multiplier tube, need to be isolated from the induced vibrations.
There is a need to be able to design ruggedized nuclear detectors in a systematic manner rather than using a trial-and-error approach as has been characteristic of the industry. Resonant frequencies can be selected to optimize the dynamic characteristics of the detector and its suspension system for most any vibration environment. There has been a tendency in the industry to design nuclear detectors to withstand shock levels; yet, it is more likely that the most severe punishment to the hardware is a result of the elements resonating with the induced vibrations or repetitive shock, which behaves much like a vibration. Shock parameters do not lend themselves very well to analytical methods required to design a dynamic support system. Consistent with a prevailing view that the environment is characterized by shock which requires cushioning the sensitive element, the general approach often used is to add more cushioning to protect the elements and to add more force to prevent movement. This is generally a poor approach and leads to the trial-and-error design approach based on low resonant frequencies that correspond too closely with the induced vibrations. Typically, soft elastomeric materials are used to provide cushioning, the greater the anticipated shock, the thicker the elastomer to be used. This material can be shaped in the form of boots, or sheaths, and may be achieved by potting the vibration sensitive element in an elastomer. Elastomers tend to change shape faster during large temperature changes due to their high coefficient of thermal expansion or due to high mechanical loading. This change in shape factor, in turn, changes the stiffness properties of the element made with the elastomer. The high thermal expansion also frequently results in high forces when operating at high temperatures. Elastomers tend to permanently deform under high pressure and high temperature. Designs which rely on elastomers for the dynamic support of vibration-sensitive elements seldom can be analytically derived; i.e., they are fundamentally trial-and-error, not easily optimized for best performance.
A major obstacle faced in the design of radiation detectors relates to space constraints. Very little space is available in the tools used in well bores and the industry trend is toward smaller tools, providing even less space. Detectors used on continuous miners need to be small in order for them to be strategically placed. The result of these factors is that it is very important to maximize the use of the space available for radiation detectors.
The inventions described in U.S. Pat. No. 5,962,855 provide means for maximizing the size of the scintillation element within its housing. A larger scintillation element increases the cross-section and therefore increases the probability that a gamma ray or neutron will pass into the element. Also, the greater thickness increases the probability that the gamma ray or neutron will produce a scintillation, rather than just pass through the element. They also provide for a highly effective support system that provides for a high resonant frequency of the scintillation element and provides for thermal compliance needed at high temperatures. However, neither the '282 application, nor any other previous art, provide a fully suitable means for maximizing the volume of the light detecting element, such as a photo-multiplier tube. In addition, other prior devices do not present a suitable means for providing dynamic isolation for highly ruggedized detectors while, at the same time, maximizing the volume available to the detector. In the typical application, when significant levels of isolation are provided, comparative large amounts of space are occupied by the support system, typically consisting of potting or boots made from elastomeric material.
There are problems associated with supporting light-detecting devices in radiation detectors that are similar to those problems which are solved for scintillation elements as described in the '855 patent.
Prior constructions utilize elastomers in the space between the light-detecting device and the surrounding housing in order to provide for cushioning to protect against shock and vibration. Use of elastomers in this space, particularly potting, causes other problems, however, that must be solved, some of which are usually not fully satisfactory solutions. First, prior boots or potting made from elastomers are thicker than required for the present invention.
General Electric Company
Hannaher Constantine
Lee Shun
Nixon & Vanderhye P.C.
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