Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2000-03-27
2002-06-18
Mack, Ricky (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S370110, C250S370150
Reexamination Certificate
active
06407390
ABSTRACT:
The invention herein described relates generally to a scintillation detector and more particularly to a circuit and method for temperature compensating temperature dependent components of a scintillation detector.
BACKGROUND OF THE INVENTION
Scintillation detectors have been employed in several fields such as the oil and gas industry for well logging, in the nuclear industry for radiation detection, as well as in many other industries. A typical scintillation detector employs a scintillator, such as NaI(Tl), and a photo-detector, such as a photomultiplier tube (PMT), for detecting ionizing radiation, e.g., x-rays, gamma rays and particles such as electrons and alpha particles.
The response of the aforesaid scintillation detector usually is temperature dependent, i.e., varies as the ambient temperature changes. This temperature dependence is primarily the result of the scintillator and the PMT being temperature dependent. For example, the scintillation light yield of a NaI(Tl) crystal changes with temperature at a rate of about −0.3% per °C., and the gain of a bialkali PMT changes with temperature at a rate of about −0.4% per °C. Thus, a scintillation detector comprised of a NaI(Tl) crystal and a bialkali PMT can have a total pulse height change of about 40% for a temperature change of 60° C. (from 0° C. to 60° C.). This means that in a scintillation detector that is doing gross counting and experiencing a 60° C. temperature change, a count of 100 times at 0° C. would occur for every 60 times at 60° C. In a system that is doing spectroscopy, the spectral peaks will shift in position. This broadens the peak widths causing movement of the peaks to the wrong spectral locations or complete loss of the peaks due to smearing.
This temperature dependency may or may not be acceptable according to the application for which the scintillation detector is to be used. For those applications where the temperature dependent variation in the signal is disadvantageous or unacceptable, prior art solutions have relied on active real time hardware and/or software corrections to keep the system gain in calibration (i.e., temperature independent) or within in some limited range that is acceptable. One such solution has been to control the temperature of the scintillation detector with a cooling apparatus, for example a thermo-electric cooler. Another solution has been to adjust the signal according to the temperature. For example, any one of a known radioactive source, a NaI(Tl)+Am
241
light pulser, a light pulsed LED, or a lamp may be used as a reference for adjusting the signal, or the signal may be adjusted in accordance with the ratio of the yields of several components of the scintillation pulse.
In addition to the active systems above, there are also passive systems which use a thermistor to alter the gain of the PMT so as to effect temperature compensation. Passive systems have the advantage of not requiring the special hardware or software demanded by the active temperature compensation systems. These prior art thermistor-based passive systems, however, only provide temperature compensation over a limited range or in a limited amount which is not sufficient for many applications. Thus resort must be had in those situations to active temperature compensation techniques.
Each of the above active solutions to the problem of temperature variation requires additional equipment such as a cooling system or reference system. This significantly increases the cost of the scintillation detector. Additionally the use of a radioactive source may require a license for the radioactive material. The above passive solutions only provide a limited amount of compensation for variations due to temperature which is often insufficient or disadvantageously inaccurate. Thus, there is a need in the prior art to overcome the above problems associated with active and passive scintillation detectors.
SUMMARY OF THE INVENTION
The present invention provides a passive temperature compensation circuit and technique for scintillation detectors that improves temperature compensation performance. The invention enables the use of passive compensation where active temperature compensation scintillation detectors previously were required to obtain an acceptable level of temperature compensation. Moreover, benefit can be gained by combining the passive temperature compensation technique of the invention with other techniques, even active temperature compensation techniques, for more improved performance.
The present invention improves the precision and/or temperature range over which useful scintillation detection may be performed by compensating the scintillation detector for temperature dependency without significant additional costs associated with active compensation techniques. The compensation is achieved by incorporating one or more elements into the circuit associated with the photo-detector. The one or more elements offset the variation resultant from temperature dependency of the components of the scintillation detector. Specifically, the one or more elements provide offsets that vary at different rates at different temperatures. The different rates at different temperatures create an offset that more accurately matches and thus more accurately compensates for the temperature dependency. This increases the useful temperature range over which the scintillation detector may be utilized and/or enhances the precision of the scintillation detector.
According to one aspect of the invention, a temperature compensated scintillation detector comprises a scintillator, a photo-detector optically coupled to the scintillator and operative to convert photons emitted by the scintillator into an electrical signal, a first circuit for providing an offset to compensate the electrical signal for variations due to temperature, the offset varying with temperature, and a second circuit coupled to the first circuit for altering the amount of the offset when the temperature exceeds a first predetermined temperature.
In an embodiment, a third circuit is coupled to the first circuit for altering the amount of the offset when the temperature exceeds a second predetermined temperature.
In an embodiment, the second circuit includes a switching device for controlling the extent to which the second circuit functions to provide temperature compensation.
In an embodiment, the first circuit includes a thermistor, the second circuit includes a resistive element in series with a switching element, the photo-detector is a photomultiplier tube, and/or the second circuit includes a diode. Preferably, the diode is a zener diode or Schottky barrier diode.
In an embodiment, the second circuit may include a metal-insulator-metal (MIM) device.
According to another aspect of the invention, a temperature compensated scintillation detector comprises a scintillator, a photo-detector optically coupled to the scintillator and operative to convert photons emitted by the scintillator into a photo-detector electrical signal, and an associated photo-detector circuit electrically coupled to the photo-detector. The associated photo-detector circuit includes a primary temperature compensating circuit, a secondary temperature compensating circuit, and a switching device for selectively connecting the secondary temperature compensating circuit with the primary temperature compensating circuit.
In an embodiment, the primary temperature compensating circuit includes a thermistor, the secondary temperature compensating circuit includes a resistive element in series with a switching element, the photo-detector is a photomultiplier tube, and/or the secondary temperature compensating circuit includes a diode as the switching element.
According to a further aspect of the invention, a temperature compensated scintillation detector comprises a scintillator, a photo-detector optically coupled to the scintillator and operative to convert photons emitted by the scintillator into a photo-detector electrical signal, and associated photo-detector circuit electrically couple
Bulson Don W.
Mack Ricky
Saint-Gobain Industrial Ceramics, Inc.
Ulbrich Volker R.
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