Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
1998-05-06
2002-05-21
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S307000
Reexamination Certificate
active
06392408
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a bulk substance detection system employing nuclear quadrupole resonance (NQR) for detecting concealed explosives and drugs, and more particularly to a pulse sequence for use in NQR for cancellation of, among others, magneto-acoustic ringing, piezoelectric ringing, electronic circuit ringing (pulse ringdown), and baseline offset.
2. Description of the Related Art
In the fight against terrorism, and for other purposes, there is a great need for a system that can quickly and reliably detect hidden explosives. Plastic explosives, for example those containing RDX and PETN, such as C-4 and Semtex, are highly stable, have clay-like malleability, and are deadly in relatively small quantities. Plastic explosives have an almost infinite variety of possible shapes and uses for terrorist bombing tactics. For example, a small piece of plastic explosive, a detonator, and a trip wire inside a large mailing envelope can cause a deadly explosion. Unfortunately, without close and potentially dangerous visual inspection, plastic explosives can be made virtually undetectable. For example, plastic explosives that have been formed into thin sheets have not been effectively detectable with prior technologies.
Similarly, in the fight against the illegal drug trade, there is a great need for a system that can quickly and accurately detect drugs inside sealed containers, mail it, parcels, and other small packages. Conventional drug detection methods are time-consuming, costly, and have had only marginal reliability.
NQR is a branch of radio frequency spectroscopy that has been used for the to detection of explosives and drugs. NQR exploits the inherent electrical properties of atomic nuclei. Nuclei with non-spherical electric charge distributions possess electric quadrupole moments. In solid materials, electrons and atomic nuclei produce electric field gradients. These electric field gradients interact with the nuclear quadrupole moments of quadrupolar nuclei, producing energy levels for the quadrupolar nuclei, and hence their characteristic transition frequencies. Measurements of these frequencies, or relaxation time constants, or both, can indicate not only which nuclei are present but also their chemical environment.
The phenomenon of NQR is possible because, when a quadrupolar atomic nucleus is within an electric field gradient, the quadrupoles experience a torque, which causes the nucleus to precess about the electric field gradient. This precessional motion causes the nuclear magnetic moment to also precess. An externally applied radio frequency (RF) magnetic field having a frequency at or near the quadrupole's precessional frequency results in resonance between the applied RF magnetic field and the nuclei's magnetic moment. This RF magnetic field, which is applied as a pulse, tips and momentarily synchronizes the orientations of the magnetic moments of these quadrupolar nuclei as they precess. As a result of this tipping of the orientations of the nuclei, which can also be described as the excitation of tile nuclei to higher energy levels, the energy level within each nucleus is briefly not in equilibrium. After application of the external RF magnetic field has ceased, the magnetic moments of the nuclei move out of synchronization with each other as the nuclei begin to return to equilibrium. As the nuclei move out of synchronization, they cumulatively generate an RF signal known as the free induction decay (FID) signal. The transmission of additional appropriate RF pulses can cause the generation of additional FID signals, and can also cause the generation of additional signals known as echoes, also referred to as spin-echoes. The spin-echoes are generated as the orientations of the magnetic moments of the nuclei pass through a synchronized orientation. A pick-up coil is used to detect both the FID and echo signals generated by the nuclei, which are subsequently received and amplified by a sensitive receiver.
Any chemical element's nucleus that has a spin quantum number greater than one-half has a non-spherical charge distribution, and can therefore exhibit quadrupolar resonance. Many nuclei have been identified that exhibit quadrupolar resonance, including:
7
Li,
9
Be,
14
N,
17
O,
23
Na,
27
Al,
35
Cl,
37
Cl,
39
K,
55
Mn,
75
As,
79
Br,
81
Br,
127
I,
197
Au, and
209
Bi. A significant factor in contraband detection by means of NQR is that quadrupolar nuclei that are commonly present, and potentially readily observable, in explosives and drugs include nitrogen (
14
N), chlorine (
35
Cl,
37
Cl), sodium (
23
Na), and potassium (
39
K). The hydrochloride forms of drugs, such as cocaine hydrochloride, contain the quadrupolar nuclei
35
Cl and
37
Cl.
14
N is the most studied quadrupole nucleus for explosives and narcotics detection, because
14
N is a constituent element of both explosives and drugs, including 2,2-bis[(ni-troxy)methyl]-1,3-propanediol, dinitrate (commonly referred to as PETN), cocaine base, cocaine hydrochloride, and heroin based drugs.
The frequencies of the NQR signals generated by quadrupolar nuclei are an important distinguishing feature of the NQR signals. An NQR frequency of a nucleus is equal to the precessional frequency of the nucleus, which is determined both by the characteristics of the quadrupolar nucleus and by the local electronic environment of the nucleus. Importantly, due to differences between the local crystalline environments of different substances, the resonant frequencies of an element that may be present in many substances,
14
N for example, are different in each substance. The resonant frequencies of an element may also be different in electronically inequivalent sites within a given substance. However, each resonant frequency is unique, well defined, and consistent.
In addition to the resonance frequencies, relaxation times of quadrupolar nuclei are other distinguishing features of an NQR response. Resonance frequencies and relaxation times are compound-, molecular site-, temperature-, and pressure-specific. The repetition rate and timing of RF pulses required for exciting and detecting a specific NQR signal are determined by relaxation times. Relaxation times can be as short as a few microseconds or as long as many seconds. The fundamental relaxation times are designated as follows: T
1
, the spin-lattice relaxation time; T
2
*, the FED decay time constant; T
2
, the spin-spin relaxation time; and T
2eff
(T
2
effective), the echo train decay time constant. The spin-lattice relaxation time T
1
is indicative of the time required for the nuclear spin system to return to equilibrium with its surroundings following a disturbance such as a resonant RF pulse. The FID decay time constant T
2
* is the time constant of the decay of FID signals. The spin-spin relaxation time T
2
is the time constant of the decay of a two-pulse echo sequence. For spin-lock spin-echo (SLSE) pulse sequences (discussed below), the peak echo amplitudes decrease exponentially with a time constant T
2eff
.
The presence and identity of explosives or drugs in a container are determined with an NQR system by irradiating the container with RF signals at or near a known NQR frequency of a target substance, and then detecting any quadrupolar resonance signals created by the application of the RF signal. The quadrupolar resonance signals, if any, are received by a receiver, and the output of the receiver is digitized and inputted to a computer. The computer compares the received signals with known parameters of a target substance, thereby determining whether the target substance is present. The process is then repeated by transmitting RF signals at NQR frequencies of additional target substances. Previous work in this area is reflected in U.S. Pat. No. 5,592,083 to Magnuson et al, (“Magnuson '083”) for a “System and Method for Contraband Detection Using Nuclear Quadrupole Resonance including a Sheet Coil and RF Shielding Via Waveguide Below Cutoff.”
It is w
Barrall Geoffrey A.
Burnett Lowell J.
Sheldon Alan G.
Arana Louis
Quamtum Magnetics, Inc.
The Maxham Firm
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