Radiant energy – Ionic separation or analysis – Methods
Reexamination Certificate
2000-11-27
2002-11-26
Anderson, Bruce (Department: 2881)
Radiant energy
Ionic separation or analysis
Methods
C250S282000, C250S36100C, C250S374000, C250S382000
Reexamination Certificate
active
06486468
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
Devices for high-resolution gamma ray spectroscopy.
2. Description of the Related Art
Energy measurements of gamma rays in the energy range 100 keV to 10 MeV are of great use in many fields including; medical imaging, astrophysics, environmental monitoring and general nuclear spectroscopy.
High-pressure xenon (HPXe) has a potential to provide a medium with excellent spectroscopic characteristics. Excellent spectroscopic resolution of the xenon gas (0.6% full width half maximum (FWHM) energy resolution) is preserved up to a xenon gas density of 0.55 g/cm
3
. Above this density, resolution degrades, eventually reaching the levels found in liquid xenon (LXe) at very high densities. Therefore, HPXe at 0.55 g/cm
3
is ideal because of its excellent energy resolution (0.6% FWHM at 662 keV and 0.4% FWHM at 1 MeV) and high stopping power (density 0.55 g/cm
3
and high Z of 54). The expected energy dependence of xenon resolution at 0.55 g/cm
3
is shown in FIG.
1
. The characteristics of the leading media now in use, including liquid xenon, are contrasted with HPXe in Table 1.
TABLE 1
Properties of Detection Media for the 100 keV to 10 MeV Energy Range
Detection Medium
CdTe/CdZn
NaI
HPGe
Te
LXe
HPXe
Energy Resolution
at 662 keV
6.0%
0.2%
1.2%
7.3%
0.6%
at 1.8 MeV
3.6%
0.1%
0.9%
4.4%
0.33%
Stopping power
Atomic Number (Z)
11-53
32
48-52
54
54
Density (g/cm
3
)
3.67
5.33
5.83-6.06
3.06
0.55
Photoelectric/Compton
cross-section at 1 MeV
0.06
0.01
0.06
0.08
0.08
Because of the notable compressibility of xenon, a density of 0.55 g/cm
3
can be achieved at a reasonably modest pressure of about 55 atmospheres at room temperature (Bolotnikov A, Ramsey B.
Development of High
-
Pressure Xenon Detectors.
SPIE, 34446: 64-75, 1998). Other intrinsic characteristics of HPXe, including a photoelectric cross-section six times that of HPGe, make it an excellent detecting medium for gamma ray spectroscopy in the energy region of 50 keV to 10 MeV. Xenon gas itself is relatively cheap, and high levels of purity are easily achieved. Unlike other gamma ray spectroscopy detection media, notably HPGe, performance of HPXe is temperature insensitive, and a cryogenic cooling system is not required (Ulin S E, Dmitrenko A E, et. al.
Cylindrical High
-
Pressure Xenon Detector of Gamma Radiation. Instruments and Experimental Techniques,
37(2) part 1: 142-45, 1994). Also, unlike some semiconductor detectors, HPXe does not undergo radiation damage (Vlasik K F, Grachev V M, et. al.
High
-
Pressure Xenon Gamma
-
Ray Spectrometers. Instruments and Experimental Techniques,
42(5): 685-92, 1999).
Gamma ray detectors using HPXe have been designed in both parallel-plate and cylindrical configurations. In the parallel-plate HPXe ionization chamber, anode and cathode electrodes are separated by a volume of xenon detection medium and incoming gammas are recorded by their Compton and photoelectric interaction histories within this medium. Since the amplitude of the signal pulse caused by an ionizing interaction vertex in such a geometry is linearly dependent on the distance of the occurrence of such ionizing vertex from the anode plate, a shielding mesh must be placed parallel to and near the anode plate to facilitate precise energy resolution of such incident gammas. Incomplete shielding from the shielding grid, and the high capacitance of the parallel-plate geometry, and the nearby grid, significantly decreases the energy resolution of such an HPXe ionization chamber. While the intrinsic energy resolution of HPXe is 0.6% FWHM at 662 keV, the best parallel plate detectors have only achieved 2% FWHM resolution (Knoll G F.
Radiation and Detection and Measurement,
third edition. SPIE, 3446: 81-87, 1998). Furthermore, construction of a large HPXe parallel-plate ionization chamber is severely hampered by the bulk of the pressure vessel necessary to contain a large volume of the gas, which dictates a flat, not round, exterior surface. Additionally, the high electric field required to prevent electron-ion recombination in HPXe (2000-4000 V/cm) requires high voltage levels which are impractical for an electron drift distances over 10 to 15 cm. Finally, the mechanical instability of the shielding grid structure leads to serious microphonic noise sensitivity.
Cylindrical ionization chambers comprising a small cylindrical anode electrode surrounded by a concentric cathode electrode, are the geometry of choice for HPXe detectors (Tepper, supra, 1998) because they provide a much more efficient and safe containment of the required pressure and provide a simple and low cost construction free from fragility and microphonic sensitivity. However this seemingly simple cylindrical ionization chamber structure produces substantial geometric variation of the delivered signal. When a number N electrons are freed in the xenon gas by an ionizing interaction vertex, fewer than N electrons are actually sensed by the electronic means connected to the anode electrode because of electrostatic effects. The number of electrons delivered is only equal to N if the electrons are deposited very near the cathode electrode and these electrons then drift through the entire potential voltage difference between the anode and cathode electrodes. At any other radius of electron deposit, a fraction F are delivered which varies from one (100%) for a radius equal to the cathode radius down to zero (0%) for a radius equal to the anode radius. This fraction is in fact equal to the fraction of the total cathode to anode potential difference through which the electron charge falls during collection.
FIG. 2
shows the variation of pulse heights obtained from the anode electrode when an equal energy interaction occurs at different radial positions in a 5 cm diameter detector having an anode radius of 0.05 cm.
FIG. 3
shows the distribution of pulse heights observed on the anode wire when such a cylindrical detector is uniformly irradiated. Because the field is highly concentrated near the anode a strongly peaked distribution is seen, but the peak width is on the order of 5 to 8%, very much poorer than the intrinsic limit for a xenon detection medium of optimal density 0.55 g/cm
3
density. Therefore, to determine the deposited energy accurately, the radial position of the deposited energy relative to the anode electrode must also be determined with precision, and the pulse height must be corrected for this radial dependence. Without correction, relatively poor energy resolution is obtained.
So far, no method to accurately determine this radial coordinate has been reported. The only method that has achieved significant improvement of resolution in a cylindrical detector is use of a Fritch grid positioned as a concentric mesh around the anode wire, analogous to the grids used in planar devices. Cylindrical ionization chambers employing a Fritch grid have only achieved energy resolution figures of approximately 2% FWHM, similar to planar grid devices, and no group has been able to approach the intrinsic energy resolution of HPXe. Wiley & Sons, 2000; Tepper G, Loose J, Palmer R.
Development of High
-
Resolution. Room Temperature. Compressed
-
Xenon Cylindrical Ionization Chamber Gamma Radiation Detector.
Incomplete grid shielding, the microphonic sensitivity of the grid, the high capacitance, and the extreme dimensional tolerances and fragility of the structure of the grid may explain this.
SUMMARY OF THE INVENTION
This application presents a novel approach to the design and readout of a xenon detection medium based device, which can provide improved spectroscopic performance compared to currently commercially available devices. In order to achieve energy resolution improvement, a structure for and method to accurately measure radial spatial position for interacting events within the HPXe cylindrical detector is described using the a cylindrical ionization detector as shown in FIG.
4
. This is accom
Akin Gump Strauss Hauer & Feld L.L.P.
Anderson Bruce
Hashmi Zia R.
Proportional Technologies, Inc.
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