Radiant energy – Invisible radiant energy responsive electric signalling – Ultraviolet light responsive means
Reexamination Certificate
1999-07-29
2002-09-10
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Ultraviolet light responsive means
C250S358100
Reexamination Certificate
active
06448562
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to remote sensing of materials and more particularly to a remote sensor for fissile or nuclear material. Even more particularly, the present invention relates to a remote sensor for fissile material utilizing optical filters for filtering out light outside a pre-determined radiation spectral band selected according to certain naturally occurring properties of the selected band up to a certain earth altitude, with a relatively high mean free path in air, and a sufficiently short radiative emission rate with photons up to the earth altitude.
Proliferation of weapons of mass destruction has generated a need to detect and locate fissile material that may be fabricated into nuclear weapons and to detect nuclear weapons themselves (hereinafter collectively referred to as nuclear materials). Unfortunately, these nuclear materials are difficult to detect with available nuclear material detectors (such as gamma ray detectors) because these detectors, in practice, must be held a few tens of meters or less from a nuclear source in order for detection to occur.
It would greatly improve the effectiveness of a nuclear materials detector if the detector could be mounted on an aircraft or flown on a satellite and could reliably and remotely detect fissile material from distances on the order of kilometers, rather than meters. Advantageously, this would allow detection of nuclear materials without the need to have an inspector on site to carry out the inspection.
The inventors are not aware of any heretofore available and practical technologies that allow remote detection of fissile material, such as at distances on the order of kilometers.
The current state of the art in nuclear material detection (i.e., detection of fissile material) has been summarized in a Los Alamos report entitled “Final Report: Scoping Study of SNM Detection and Identification for Adjunct On-site Treaty Monitoring.”
Nuclear material detectors are currently categorized as three types of detectors, Gas filled, scintillation or solid state detectors. Gas filled detectors have a sensitive volume of gas contained within a sealed chamber between two electrodes. The chamber allows ionizing radiation from outside the chamber to enter the chamber, and may be, for example, glass. There are three types of gas filled detectors. They may be: 1) an ionization chamber; 2) a proportional counter; or a 3) Geiger-Mueller tube (GMT).
In each of these three types of gas filled detectors, the electrodes are biased with a biased power supply. An ionization event within the gas is caused by the ionizing radiation entering the gas. This causes the generation of electron hole pairs that are, in turn, collected by the two electrodes.
In an ionization chamber only primary charge created from a first ionizing event with the ionizing radiation are collected due to a low voltage in the ionization chamber. As voltage on the electrodes is increased due to the collection of electron hole pairs, the primary charge attains enough energy to ionize additional molecules. (This creates a mechanism called avalanche amplification, also used in both “proportional counters” and Geiger-Mueller tubes.)
In proportional counters, the avalanche amplification that occurs when the primary charge attains enough energy to ionize additional molecules is used to generate a “count”. The number of “counts” generated over time provides an indication of the amount of ionizing radiation present and thus an indication as to whether, and how much, nuclear material is present.
Both ionization chambers and proportional counters collect charge generated as a result of ionization events, with the amount of charge over time being proportional to energy deposited in the gas as a result of ionization events. Both measure ionizing radiation by measuring collected charge from electron hole pairs collected by the electrodes, represented by voltage. However, because of their efficiency level they are limited to detecting x-rays rather than gamma rays generated by fissile materials. And further, because they directly detect ionizing radiation (through ionization events) they must be used within close proximity of the nuclear materials. (This is because ionizing radiation is consumed naturally by ionization events as the ionizing radiation travels through space, particularly through an atmosphere, such as at the surface of the Earth.) Thus, the amount of measured ionizing radiation quickly fades into background radiation levels varying as a function of distance (on the order of meters generally) from the particular nuclear materials from which the ionizing radiation is being emitted.
If the electrode voltage on a “proportional counter” detector is increased further, the ionization within the gas becomes space-charge limited and the charge produced is independent of the initial deposition of energy in the gas. This type of detector is termed a Geiger-Mueller tube (GMT) and cannot differentiate between the energy level of the particle it detects. Thus, in addition to being unable to detect nuclear material at larger distances, Geiger-Mueller tubes are unable to differentiate between different types of radiation sources.
A further type of nuclear material detector, a scintillation detector, uses scintillation, which occurs when ionizing radiation strikes a luminescent material (“scintillator material”) such as, NaI, BGO, CsI, ZnS or LiI. A scintillation detector is a device that detects gamma ray induced scintillation emissions (“scintillators”). For example, gamma rays cause scintillations by exciting atoms that emit optical photons (light) as the atoms decay back to a ground state. Optical photons have energies corresponding to 2000-15,000 A.
In a scintillation detector, isotropically emitted photons are optically coupled to a photocathode of a Photo-Multiplier Tube (PMT), which transforms the photons into electrical pulses measured by a sensor circuit. Image detection in this type of detector depends upon energy of the gamma ray, statistical fluctuations in light production and quality of the Photo-Multiplier Tube (PMT). Problematically, as with gas filled detectors, scintillation detectors directly detect the effects of ionizing radiation. And, as a result, scintillation detectors must also be used within a close proximity to the nuclear materials being detected, as detected ionizing radiation quickly fades into background levels as a function of distance from the source, especially in an atmosphere environment, such as on the surface of the Earth.
Yet another type of nuclear material detector, solid state detectors, directly detect the interaction of a gamma ray within an active region of a semi-conductor. As with gas filled detectors, gamma rays generate electron hole pairs that are collected by electrodes attached to a semiconductor crystal. Solid state detectors dramatically improve resolution over scintillation detectors.
Unfortunately, like gas filled detectors and scintillation detectors, solid state detectors also only directly detect the effects of ionizing radiation and therefore must operate in close proximity to the source of the ionizing radiation, e.g., the nuclear material, if the ionizing radiation is to be detected above background levels. Thus, because all of the prior state of the art nuclear material detectors require that the x-ray or gamma ray penetrate an active volume of the detector, all of these prior detectors must be used within meters of the source of the ionizing radiation to be useful.
To the knowledge of the inventors, there heretofore has not been a detector that remotely detects (e.g., on the order of kilometers) the few gamma rays that penetrate a typical radiation shield surrounding nuclear material.
The present invention advantageously addresses the above and other needs.
SUMMARY OF THE INVENTION
The present invention advantageously addresses the needs above as well as other needs by providing an optical system for remotely detecting (e.g., from a surface platform, an aircraft up to about 20 km or
Blackwell William
Fry Edward L.
Hill Charles H.
Seidler William A.
Zukic Muamer
Fitch Even Tabin & Flannery
Gabor Otilia
Hannaher Constantine
Jaycor
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