Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2002-08-08
2004-08-24
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370010, C250S336100
Reexamination Certificate
active
06781132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to collimated radiation detector assemblies, arrays of collimated radiation detectors and collimated radiation detector modules.
2. Background Art
The material requirements for a room temperature operated high resolution semiconductor gamma ray spectrometer include large free charge carrier mobilities (&mgr;), or alternatively, high achievable free charge carrier velocities (&ngr;), long mean free drift times (&tgr;*), a relatively large energy band gap (E
g
) generally between 1.4 eV to 2.5 eV, high representative values of atomic number (Z), and availability of large volumes. Presently, no semiconductor has all of the listed ideal material properties desired for the “perfect” room temperature operated semiconductor radiation spectrometer, although many have a considerable fraction of the required properties. Some wide band gap compound semiconductors that offer promise as room temperature operated gamma ray spectrometers include GaAs, HgI
2
, PbI
2
, CdTe, and CdZnTe. One difficult problem to resolve with these materials is gamma ray energy resolution degradation from charge carrier trapping losses.
The general planar detector design that is used for compound semiconductor radiation detectors consists of a block of material with contacts fabricated on either side of the block. Spectroscopic measurements of gamma radiation interactions require that both electrons and holes be extracted efficiently from a conventional planar detector, hence the device dimensions are usually tailored to reduce trapping effects from the most effected charge carrier (usually holes). Generally, compound semiconductors have notable differences between the mobilities and mean free drift times of the electrons and holes. For instance, CdZnTe material has reported mobility values of 120 cm
2
/V-s for holes and 1350 cm
2
/V-s for electrons. Additionally, the reported mean free drift times are 2×10
−7
s for holes and 10
−6
s for electrons. Hence, the effect of trapping losses is much more pronounced on holes than on electrons, and the device dimensions would have to be designed to compensate for the problem.
A similar situation is experienced with gas filled ion chambers, in which electron-ion pairs are produced by gamma ray interactions in the gas. The electron mobilities are much higher than the positive ion mobilities, hence the extraction times of the electrons are considerably less than the extraction times of the ions. For typically used integration times, the measured pulse amplitude becomes dependent on the initial gamma ray interaction location in the ion chamber. As a result, wide variations in pulse amplitude are possible. The problem was significantly reduced by Frisch with the incorporation of a grid in the ion chamber near the anode. The measured pulses from the detector corresponded to only the movement of mobile charges in the region between the grid and the anode, hence ion movement in the bulk of the device no longer affected the signal output.
The Frisch grid concept has been demonstrated with semiconductor detectors using a “co-planar” design. The devices work well, but unlike the true Frisch grid, they generally require more than one output signal or a circuit capable of discerning the different grid signals.
A simple planar semiconductor detector is operated by applying a bias voltage across the bulk of the material. Ionizing radiation excites electron-hole pairs that are drifted apart by the device electric field. Electrons are drifted towards the anode and holes are drifted towards the cathode. An induced charge is produced at the terminals of the device by the moving free charge carriers, and the induced charge can be measured by an externally connected circuit. Shockley and Ramo derived the dependence of the induced current and induced charge produced by point charges moving between electrodes, which was later shown to apply to semiconductor detectors as well.
The Shockley-Ramo theorem shows that the induced charge appearing at the terminals of a planar device from moving point charges is proportional to the distance displaced by the moving point charges, regardless of the presence of space charge. Hence, the change in induced charge Q* can be represented by
Δ
Q
*
=
Q
o
|
Δ
x
e
|
+
|
Δ
x
h
|
W
D
,
(
1
)
where Q
o
is the initial charge excited by the interacting gamma ray, W
D
is the detector length, &Dgr;x is the distance traveled by the electrons or holes, and the e and h subscripts refer to electrons or holes, respectively. With trapping, the total induced charge from a single gamma ray event in a planar semiconductor detector can be represented by
Q*=Q
o
{&rgr;
e
(1−exp[(x
i
−W
D
)/&rgr;
e
W
D
])+&rgr;
h
(1−exp[−x
i
/&rgr;
h
W
D
])}, (2)
where x
i
represents the interaction location in the detector as measured from the cathode and p is the carrier extraction factor represented by
ρ
e
,
h
=
v
e
,
h
τ
e
,
h
*
W
D
,
(
3
)
where v is the charge carrier velocity and &tgr;* is the carrier mean free drift time. From equations 2 and 3, it becomes clear that the induced charge (Q*) will be dependent on the location of the gamma ray interaction. Small values of &rgr; for either holes or electrons will cause large deviations in Q* across the detector width. The induced charge deviation can be greatly reduced if a detector is designed such that the carrier with the longer mean free drift time and highest mobility contributes to all or most of the induced charge.
A Frisch grid gas ion chamber is designed to measure the induced charge primarily from electrons, and the general configuration and operation of a Frisch grid ion chamber is shown in
FIGS. 1
a
and
1
b
. A gamma ray interaction occurring in the main volume of the detector excites electron-ion pairs. An externally applied electric field drifts the carriers in opposite directions, in which the electrons drift through the grid and into the measurement region of the device. From the Shockley-Ramo theorem, the induced charge produced at the anode results from charge carriers moving between the grid and the anode and not from charge motion between the cathode and the grid. As a result, the detector is primarily sensitive to only the electron charge carriers.
A simple semiconductor Frisch grid detector can be built using the design shown in FIG.
2
. As shown, a semiconductor block is cut and polished with metal electrodes fabricated at the ends. These electrodes serve as the anode and cathode. Parallel metal contacts are fabricated on opposite faces of the device, which serve to act as the Frisch grid. The region between the cathode and the parallel Frisch grid is the interaction region, the region underneath the parallel grid is the pervious region, and the region between the parallel grid and the anode is the measurement region. The device is a three terminal device, with the electrodes biased such that electrons are drifted from the interaction region, through the pervious region between the parallel grid, and into the device measurement region.
The different regions and their designations are again shown in
FIG. 3. A
gamma ray event occurring in the interaction region will excite electron-hole pairs. Electrons are swept from the interaction region towards the parallel grid, however some trapping will occur as the electrons drift across the interaction region and the measurement region. Including the effect of trapping, the measured induced charge from electrons excited in the interaction region by a gamma ray event at a distance x
i
from the cathode will be
Q
*
=
K
(
x
,
y
)
Q
o
ρ
e
m
(
1
-
exp
[
-
1
ρ
e
m
]
)
exp
[
x
i
-
W
p
2
-
W
i
v
e
τ
*
e
]
,
(
4
)
where K(x,y) is a correction factor for deviations in the weighting potential across the device and
ρ
e
&emsp
Brooks & Kushman P.C.
Porta David
Sung Christine
The Regents of the University of Michigan
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