Radiant energy – Invisible radiant energy responsive electric signalling – Including ionization means
Reexamination Certificate
2000-04-14
2004-11-09
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Including ionization means
C250S330000, C359S630000
Reexamination Certificate
active
06815687
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods and systems for high-speed, 3D imaging of optically-invisible radiation and detectors and arrays of such detectors for use therein.
BACKGROUND ART
One of the fundamental problems involving work with radioactive materials is that radiation is invisible to the human eye and thus poses an invisible hazard. The hazard is compounded when one considers that these materials can be present in an environment when not expected such as with radioactive contamination or leaking radioactive waste storage tanks. To make the concern even more valid, these sources of radiation can be moving, as can be the case with airborne contamination. Thus, it is clear that there is a need for a way to localize radioactive sources, preferably in real-time.
Much work has been done on ways to image various forms of radiation to provide the user with a “picture” of the radiation present in an environment. Currently available gamma-ray cameras are capable of providing two-dimensional information about the location and spectroscopy of a radioactive source similar to taking a snapshot with a standard camera. However, these cameras are not independently capable of providing information to locate the source in three dimensions. There have been cameras built that are capable of obtaining real-time information, which is useful for viewing changing sources. However, based on current designs, the performance of some tasks in radiation environments precludes simultaneous monitoring of the radiation field by the individual worker, possibly resulting in increased radiation exposures. 3D detection systems are available for medical and other environments, but these involve different geometries and source distributions than those considered here. Also, these methods rely on complex mathematical reconstruction making them cumbersome and time-consuming.
A new problem arises if one considers the complex environments that these sources can exist in since even when radiation images are blended with light images three-dimensionality is lost, real-time manipulation of the images becomes complex, and difficulties arise with time-varying source distributions. Only three-dimensional source location truly allows for accurate position determinations of radioactive materials. Furthermore, real-time simultaneous display of the physical and radiation environments is essential for observing moving or redistributing radiation sources.
Augmented Reality
Both virtual reality (VR) and augmented reality (AR) provide real-time interactivity which requires 3D registration. VR and AR require a motion tracker to determine the user's position in the virtual environment (VE), a computer to coordinate the user's relative location, and a display. VR and AR are currently being used in various fields including research and development, design and testing, navigation and targeting, training, and visualization (Azuma, 1997). There exists a wide variety of hardware and software capable of displaying VEs. Virtual Reality Modeling Language (VRML) 2.0 is the current industry standard for programming with many large software packages, such as AutoCAD and 3D Studio Max (Autodesk, Inc.), exporting to this file format. The display of VR is achieved by a head-mounted device (HMD), head-coupled display (HCD), or a Cave Automatic Virtual Environment (CAVE). AR display is limited to HMDs with modifications that allow the user to see the real world through the display.
With an VE application, there are always certain limitations that current researchers are trying to overcome. Those who program for VR or AR applications must achieve a high level of realism while not slowing down the computer system to intolerable speeds. Designers of VR and AR hardware must always consider problems arising from concerns of simplicity, spatial resolution, and safety. For AR, one must also be concerned with using reasonable separation for data collection and display so as to simulate the user's interpupillary distance. Focus also presents a current field of AR research since the human eye, when observing real objects, must match virtual object focus at the same distance as the physical objects. Finally, current research is being conducted into how to increase the field of view of HMDs and HCDs to most accurately match that of the user (Azuma, 1997).
Semiconductor Technology
Semiconductor devices typically operated by measuring the number of electrons and holes excited by ionizing radiation (gamma rays or charged particles) within the detector. The number of excited charge carriers is remarkably linear with respect to the absorbed energy from an ionizing event. The excited charge carriers are drifted across the semiconductor detector by an externally applied electric field, which, in turn, produces an image charge or induced charge on the output circuit. Electrons are drifted toward the device anode and holes are drifted toward the device cathode. For a planar detector, the Shockley-Ramo (Shockley, 1938; Ramo, 1939) theorem describes the relationship between the induced charge (Q*) and the displacement distance of the free electrons and holes:
Q
*
=
Q
0
&LeftBracketingBar;
Δ
x
e
&RightBracketingBar;
+
&LeftBracketingBar;
Δ
x
h
&RightBracketingBar;
W
D
(
1
)
where Q
0
is the initial magnitude of free charge liberated, &Dgr;x refers to the distance traveled by the electrons or holes from their point of origin toward their respective electrode, W
D
is the width of the planar detector, and e and h subscripts refer to electrons and holes, respectively. If the charge carriers are removed completely from the device, in which case they reach their respective electrodes, then the solution to Equation (1) is simply Q*=Q
0
. The importance of this result is that gamma-ray spectroscopy can be performed by simply measuring the total induced charge measured from electrons and holes drifted to the detector electrodes. In the presence of charge carrier trapping (caused by imperfections in the semiconductor), charge carriers often do not reach their respective electrodes, and the induced charge observed becomes very dependent on the location of the gamma-ray interaction (Day, Dearnaley and Palms, 1967; Knoll and McGregor, 1993). The Hecht relationship (Hecht, 1932) describes the expected induced charge for a planar detector with charge trapping:
Q
*
=
Q
0
{
ρ
e
(
1
-
exp
[
x
i
-
W
D
ρ
e
W
D
]
)
+
ρ
h
(
1
-
exp
[
-
x
i
ρ
h
W
D
]
)
}
(
2
)
where x
i
represents the interaction location in the detector as measured from the cathode. The electron or hole carrier extraction factor (Knoll and McGregor, 1993) is described by:
ρ
e
,
h
=
v
e
,
h
τ
e
,
h
*
W
D
(
3
)
where &ngr; is the charge carrier mobility and &tgr;* is the carrier mean free drift time before a trapping event occurs. As can be observed from Equations (2) and (3) the induced charge becomes a function of the interaction location within the detector. High &rgr; values (above 50) for both electrons and holes are desirable for high resolution gamma-ray spectroscopy. Unfortunately, the value of &rgr;
h
for most compound semiconductors is generally much lower than the value of &rgr;
e
. Largely differing values of &rgr; for electrons and holes are not conducive to high resolution gamma-ray energy spectroscopy when using simple planar semiconductor detector designs (Day, Dearnaley and Palms, 1967; Knoll and McGregor, 1993).
Recent results with novel geometrically weighted Frisch grid CdZnTe detectors demonstrate dramatic improvements in gamma-ray resolution. The devices no longer require signals from hole transport, hence the higher carrier extraction factor values of the electrons can be manipulated while ignoring the difficulties imposed by hole trapping. The device uses the geometric weighting effect, the small pixel effect and the Frisch grid effect to produce high gamma-ray energy resolution. The design is simple and easy to construct. The device performs as
Branch-Sullivan Clair J.
Kearfott Kimberlee J.
McGregor Douglas S.
Stojadinovic Bozidar
Brooks & Kushman P.C.
Epps Georgia
Harrington Alicia M.
The Regents of the University of Michigan
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