Semiconductor radiation detector with enhanced charge...

Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system

Reexamination Certificate

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C250S370130

Reexamination Certificate

active

06333504

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to a device and method for detecting ionizing radiation, and more particularly to a semiconductor radiation detector with enhanced charge collection for reducing low-energy tailing effects.
2. Description of Related Art
High-resistivity semiconductor radiation detectors are widely used for detecting ionizing radiation due to their ability to operate at room temperature, their small size and durability, and other features inherent in semiconductor devices. Such detectors are used in a wide variety of applications, including medical diagnostic imaging, nuclear waste monitoring, industrial process monitoring, and space astronomy. Ionizing radiation includes both particulate radiation, such as alpha or beta particles, and electromagnetic radiation, such as gamma or x rays.
Conventional semiconductor radiation detectors are generally referred to as “planar” detectors. As shown in
FIG. 1
, the architecture of such planar detectors
100
typically consists of a slab of semiconductor crystal
102
with metal covering two opposing surfaces of the slab to form two electrodes, a cathode
104
and an anode
106
. In one configuration, the anode
106
is connected to external signal conditioning circuitry
108
and to ground
110
, and the cathode
104
is connected to an external voltage source
111
. A bias voltage across the electrodes
104
,
106
creates an internal electric field. Electron and hole “charge clouds” generated within the semiconductor crystal
102
by an ionizing radiation
112
absorbed within the slab of semiconductor crystal
102
are swept toward the anode
106
and cathode
104
electrodes, respectively. These moving electron and hole clouds create charge-pulse signals in the external signal conditioning circuitry
108
.
If all the electrons and holes generated by the ionizing radiation
112
reach their respective electrodes (i.e., the electrons reach the anode
106
and the holes reach the cathode
104
), the output charge signal will exactly equal the charge from the energy deposited within the crystal
102
. Because the deposited charge is directly proportional to the energy of the ionizing radiation
112
, the semiconductor radiation detector
100
provides a means for measuring the energy of the ionizing radiation
112
. The ability to measure this energy is an important function of radiation detectors.
Planar radiation detectors, however, suffer from a serious drawback: because of limitation in the transport properties of the bulk semiconductor crystal
102
, some of the electrons and holes are generally lost by being trapped as they sweep toward their respective electrodes. Thus, the amplitude of the output charge signal becomes dependent on the position within the crystal at which the ionizing radiation is absorbed. Generally, the amplitude is less than the charge deposited by the ionizing radiation
112
, resulting in a corresponding reduction of energy measurement accuracy as well as poor resolution and reduced peak efficiency. This loss (or trapping) of charge in a radiation detector results in asymmetrical spectral peak shapes known as “low-energy tailing.”
As stated above, in a semiconductor radiation detector, when an ionizing event occurs, electrons are swept toward the anode
106
and holes toward the cathode
104
. In a typical experimental arrangement, with the cathode
104
facing the source of the radiation, many ionization events occur over some accumulation period, and the resulting charge signal pulses are detected and then displayed in a histogram. In an ideal detector, in which there is no low-energy tailing, all the pulses would be directly proportional to the energy of the ionizing radiation
112
. This would result in a histogram like that of
FIG. 2
, in which counts per channel are plotted versus charge signal pulse amplitude. As can be seen in
FIG. 2
, the energy histogram exhibits no tailing, because the energy peak (or “photopeak”)
202
appears as a straight vertical line at a single energy level, E, equal to the energy level of the ionizing radiation
112
. Thus, all the charge signal pulses have an amplitude equal to the energy level E of the ionizing radiation
112
, and no charge is lost in any single pulse.
Curves A and B of
FIG. 3
illustrate two idealized cases of low-energy tailing in a non-ideal detector. Curve A represents the histogram distribution that would result if the ionizing radiation were absorbed uniformly throughout the crystal, as would occur with a very low absorption coefficient of the crystal. Curve B represents the more typical situation, where absorption is large near the cathode and drops off exponentially as the ionizing event moves in a direction away from the cathode within the crystal. In both Curves A and B, there is a maximum signal
302
corresponding to full charge collection (at amplitude “E”) and pronounced low-energy “tails”
304
,
306
.
FIG. 4
shows an energy histogram exhibiting pronounced low-energy tailing for an actual semiconductor radiation detector made from Cadmium-Zinc-Telluride (CdZnTe) irradiated with gamma rays from a cobalt-57 (“
57
Co”) radiation source. This detector had area dimensions of 6.1 mm by 6.1 mm and a thickness of3 mm. Its bias voltage was −500 volts. The data values in
FIG. 4
are spread-out by electronic noise, an effect that was not considered in plotting the idealized curves of FIG.
3
. As with Curves A and B of
FIG. 3
, the histogram of
FIG. 4
has a pronounced low-energy tail
404
.
Because of the deleterious effects of low-energy tailing in semiconductor detectors, much effort has gone into attempting to solve this problem. One approach to reducing the tailing effect in semiconductor detectors is to reduce the dependence of the signal pulse-charge amplitude on the position at which the ionizing radiation is absorbed. This can be accomplished, in principle, by contriving to limit to a small distance the region in which charge is induced on one electrode by a charge cloud in front of that electrode. If this is accomplished, a charge cloud generated by an ionizing event induces little charge on the electrode until it becomes very near the electrode, after which the charge cloud induces essentially all of its charge on that electrode. This approach is especially useful for semiconductors in which the transport properties of one carrier type (e.g., electrons) are much better than those of the other type (holes in this example). These transport properties are expressed by a “mobility-lifetime product.” The ratio of the transport properties of one type carrier (e.g., holes) to those of the other type carrier (e.g., electrons) is expressed as the “mobility-lifetime-product ratio.” Thus, the general approach described above is useful for all mobility-lifetime-product ratios, but is most useful for semiconductors having a large ratio of the larger mobility-lifetime product divided by the smaller. Semiconductors for which the mobility-lifetime-product ratio is greater than 10 include cadmium-zinc-telluride, cadmium-telluride, and mercury-iodide.
An early effort aimed at minimizing low-energy tailing using the above approach employed a semiconductor detector having a hemispherical configuration. See, e.g, H. L. Malm, et al., “Gamma-Ray Spectroscopy with Single-Carrier Collection in High Resistivity Semiconductors, ” Appl. Phys. Lett., vol. 26, at 344-46 (1975). In Malm's detector, a large hemispherical surface of the cadmium-telluride was metallized to form the cathode. The anode formed a small circle at the center of the flat cross-section of the hemisphere. A bias voltage applied across these electrodes produced an internal electric field that varied from a low value near the cathode to a high value near the small anode. The electric field lines were thus concentrated near the central point by the spherical geometry. A result of this electric field concentration is that electrons move much faster in the close vicinity of the anode than in the remainder of the detector. Because the charge i

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