Radiant energy – Radiation tracer methods
Reexamination Certificate
2000-02-11
2004-05-25
Lee, John (Department: 2881)
Radiant energy
Radiation tracer methods
C250S303000, C250S363010, C250S363040, C250S370090, C250S370100
Reexamination Certificate
active
06740875
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the creation and use of generalizations of classical watermarks for object identification, and more specifically, it relates to a relatively covert, “watermark” expressed in gamma-ray-emitting materials affixed to objects and employed for object identification. The “gamma watermark” of the present invention is a type of steganography, or “hidden writing,” which employs tiny quantities of material containing radionuclides to encode and continuously express a digital bit-string which may, for instance, be used to connote ownership of, or some type of prior contact with, an object whose provenance is in some manner contested or doubted.
2. Description of Related Art
A need exists for greatly improved means for general-purpose object identification. For example, a need exists for apprehending those responsible for theft of rare human artifacts and paleontological specimens, as a rapidly growing to problem is posed by escalating fossil and artifact thefts worldwide. A need exists for a broadly applicable means of labeling all such objects with a physically essentially-invisible, zero-hazard and relatively inexpensive ‘tag’ which could be discerned and then ‘read’ unequivocally only by well-equipped and expert individuals (e.g., law enforcement officials). Familiar tagging means is such as bar-codes, while eminently readable, also are readily detectable and often may be easily removed or altered. Digital watermarking of collections-of-bits encoding audio or graphics information is applicable only to bit-strings whose low-order bits may be manipulated for encoding purposes without damage to the perceived content of the collection-of-bits, a quite scope-limited though increasingly important type of property.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a gamma watermark containing a unique digital signature comparable in salient qualities to that of the digital watermark, but which may be applied to identify essentially, all items implemented as greater than microscopic-sized material objects.
It is another object, albeit an optional one, to provide, within a gamma watermark, a built-in ‘clock’ providing a date-stamp representation of the time of gamma watermark creation relative to the time at which the gamma watermark is being read, i.e., an age of the watermark.
Another object of the invention is to provide a gamma watermark that is undetectable by ordinary technical-inspection means (e.g., use of UV-fluorescence-stimulating illumination, magnified visual inspection, acoustic scanning, chemical treatment of an object's surface, x-ray inspection, etc.),
Still another object of the invention is to provide a gamma watermark having an ultra-low radiation ‘signature’ hidden in the ubiquitous natural background radiation due to cosmic radiation and natural plus man-made radioactivity in the environment (e.g., that due to decay of potassium 40, a billion-year half-lived isotopic component of natural potassium).
An object of the invention is provide a gamma watermark having an effectively microscopic physical size in order to enable second-level covertness-of-tagging and to confer sweep-resistance and counterfeit-robustness by owner-determined selective positioning on or within an object.
The gamma watermark is a new type of very low-level (i.e., nanoCurie-scale) gamma-ray-emitting tag or “watermark”, comprised of a sufficiently precisely metered, typically unique mixture-ratio of very small (of the order of 1 nanoCurie, or 10
−9
Curie) quantities of radioisotopes of appropriately long half-lives, none of which occur naturally (at levels as high as 1 nanoCurie) in the object to be tagged. The tag's location may be variable, ranging from surface emplacement to cm-scale depth inside a full-density object (composed, e.g., of plastic, wood, stone, etc.) because MeV-energy gamma-rays are quite penetrating. In creating the tag, the ratios of the quantities of radioisotopes selected to comprise any given tag are made to be sufficiently precise to encode a binary bit-string with adequate “noise margin” for unequivocal read-out at all subsequent times-of-interest and, if required by the particular tagging application, to be sufficiently unique among the tags applied to the class of objects that will ever be identified with such watermarks. Selected radionuclides package a huge amount of energy per gram and release it at a known rate for decades, so that gamma watermarks may be created which are fully useful over multi-decade intervals.
Radionuclides chosen for constituting a gamma watermark are either not present in the environment or are present at very low levels, so that a gamma watermark signatures may be very ‘clean’ in the signal-to-noise sense. It is also possible to use radionuclides which are quite difficult to prepare, for example one which have unique production signatures betraying their means of generation. A number of radionuclides (e.g.,
44
Ti) of interest from these perspectives may be produced only, by spallation or charged-particle bombardments. Others have unique isotopic purity by virtue of using mass-separated target material or mass-separation after production. Use of such nuclides in gamma watermarks can drastically raise the threshold of endeavor for a would-be watermark counterfeiter, due to their uniqueness or the difficulty of obtaining them.
The nature of the gamma watermark is particularly convenient in many applications because gamma rays are peculiarly penetrating electromagnetic radiations. Many normal structural materials (e.g., wood, common plastics) have density &rgr;~1 gm/cc, while fossilized bone can have &rgr;=2-3 gm/cc and paper typically has &rgr;=1 gm/cc. Since the mass absorption coefficient of light elements such as carbon, nitrogen, oxygen, magnesium, aluminum and silicon for photons of 1 MeV energy, hereinafter referred to as 1 MeV &ggr;s, is ≦0.04 cm
2
/g, the transport mean free path for such MeV-energy gamma-rays in all such materials is 25 g/cm
2
, e.g., 25. cm in 1 gm/cc material. Therefore, a gamma watermark could be implanted at a one-inch depth in low-to-moderate Z material and 90% of the emitted gamma-rays would still travel without scattering or absorption to a detector positioned over the material's surface.
Similarly, a gamma watermark can be easily detected through modest stack-heights (a few cm) of paper. In fact, the activity level of the gamma watermark on any given paper-sheet could be made exceedingly small (picoCurie level), if working with stacks of paper all of which were so watermarked in a (nearly) identical manner; a detector used to examine a sheaf of such individually watermarked paper-sheets would “see” all the separate-but-identical watermarks superimposed into one which could be readily read out.
The salient components of standard physical theory underlying the gamma watermark include nuclear beta-decay and gamma-ray spectroscopy, semiconductor-based detection of ionizing radiation and viscous fluid-mechanical theory underlying ink-jet printers leveraged to enable high precision, swift creation of tokens in the direct gamma-watermarking of sheets of material such as plastic and paper. The gamma watermark (typically, redundantly) encodes its age (i.e., the time-elapsed since its creation) and a unique digital signature in the sufficiently-precisely-metered relative quantities of several different species of long lived, gamma ray-emitting radioisotopes. Because the photonic output of the beta-decay of a single atomic nucleus may be recorded with high efficiency and high precision, the amount of beta radioactivity needed to continuously express a unique digital signature may be made to be exceedingly small, at most 1 nanoCurie in many applications.
From a communications engineering perspective, the gamma watermark utilizes very low effective radiated power and very high spectral brightness at certain very narrowly defined energies/frequencies to “narrow-cast” a low-probab
Ishikawa Muriel Y.
Lougheed Ronald W.
Moody Kenton J.
Wang Tzu-Fang
Wood Lowell L.
Lee John
Souw Bernard
The Regents of the University of California
Thompson Alan H.
Wooldridge John P.
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