Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-02-14
2002-07-16
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370130
Reexamination Certificate
active
06420710
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to the domain of &ggr; radiation detection, the detector used being a semiconductor. It also relates to the field of spectrometric measurements in &ggr; imagery.
2. Discussion of the Background
Many types of detectors have been designed for detection of &ggr; radiation. The main innovation in &ggr; radiation detection techniques over the last 30 years has been the use of solid detectors based on semiconductors.
Detectors based on semiconductors convert &ggr; radiation in the material into energy directly without using any intermediate steps such as the emission of visible photons in the case of scintillators. This overcomes coupling problems that introduce loss of efficiency. The energy necessary to create an electron-hole pair in a semiconductor is much lower than in a gas or in a scintillator (about 4 eV for semiconductors compared with 30 eV in gases and 300 eV in photo-multiplier scintillator systems). Consequently, the number of free charges created for each detected photon is much higher, which can give better energy resolutions with low noise. Furthermore, the high atomic number and the high density of semiconductor materials make it possible to use detection volumes significantly smaller than the volumes of gas detectors or scintillators, while keeping the same quantic detection efficiency.
The use of these semiconductor materials as X or &ggr; radiation detectors implies the deposition of two electrical contacts on the surface of the material, at the terminals of which a polarization voltage is applied. Charge carriers, in other words electron-hole pairs created by the interaction of a &ggr; photon with the material, will separate under the action of the electrical field, the electrons migrating towards the positive electrode and the holes migrating towards the negative electrode. The capability of these charge carriers to migrate towards the electrodes without getting trapped by defects present in the semiconductor material will affect the energy resolution of the measured spectrum. This capability, also called the charge carrier transport property, is measured by the mobility and the life of the electrons and the holes.
The spectrometric measurement of incident photons consists of detecting the maximum number of photons within the detector volume, which requires a high thickness for better quantic detection efficiency, and precisely measuring the energy deposited by the photon, which requires an excellent efficiency for the collection of holes and electrons migrating towards the negative and positive electrodes respectively. These two parameters (quantic detection efficiency and charge carrier collection efficiency) are contradictory, since the former is proportional to the detector thickness, and the latter is inversely proportional to the detector thickness.
The quantic detection efficiency can be improved by optimizing the detector thickness compared with the field of application, since it only depends on the density and atomic number of the detector (for a given energy of incident photons).
However, the performances of current &ggr; detectors are limited by the presence of native defects in semiconductors which trap charge carriers during their migration towards electrodes, and correspondingly reduce their life and thus deteriorate the energy resolution of the detector. These native defects systematically appear during crystallogenesis of the semi-conducting material. There is a very abundant bibliography on the study of these defects that shows that the crystallogenesis of all high resistivity semiconductors that can operate at ambient temperature is not controlled sufficiently well to eliminate these defects.
The collection efficiency of charge carriers (holes and electrons) may be improved in different ways described in document EP-763 751.
This document also describes a process for the correction of the spectrometric measurement in the field of &ggr; photon detection.
This process consists of measuring the amplitude of the electronic contribution alone as a function only of its rise time (called the “electron” correction method) rather than measuring the total amplitude of the total integrated signal (electron+hole) as a function of its rise time (called the “hole” correction method described in FR-2 738 693). The measurement, and consequently mathematical knowledge of the function correlating the amplitude of the electron signal and its rise time, is used to correct the measured amplitude associated with the interaction of each photon throughout the volume of the detector.
The advantage of the “electron” correction is that the existing relation between the amplitude of the electron signal and its rise time only depends on the mobility of the electrons that varies only slightly within an ingot and between two different ingots; the mobility depends mainly on the crystalline network. However, the “holes” correction depends on the life of the holes, and variations in this life can vary significantly within a single ingot. This life is imposed by the different defects created during crystallogenesis which is not well controlled. One consequence of these differences is that the “electron” correction can correct measurements in most semi-conducting materials, but this is not the case for the “hole” correction.
The two documents mentioned above each describe a device for making or for correcting spectrometric measurements.
In both cases, a conventional preamplifier is used just at the detector output, followed by special electronics to measure the signal amplitude (electronic component in EP-763 751 and global signal in FR-2 738 693) and its rise time.
In both cases, the signal is measured using a conventional commercially available charge preamplifier well known to the expert in the subject (for example a 5093 preamplifier purchased from eV-Products).
FIG. 1
diagrammatically shows a detector
2
, for example a CdTe detector and its preamplifier
4
.
The charge deposited by the photon absorbed by detector
2
is integrated at the terminals of a capacitor C
i
8
called the “integration” capacitor. This capacitor counter-reacts with a resistance R
i
10
(through the use of an operational preamplifier) that “discharges” the integrated charge at the capacitor terminals. The time constant R
i
, C
i
may be adapted as a function of the nature of the semiconductor and the charge to be measured.
In all cases, the darkness current associated with the detector is not measured and is therefore integrated at the terminals of the capacitor C
i
, since its absolute value is still much too high. It is greater than the charge deposited by the photon interacting in the detector. It depends on the resistivity of the semiconductor and also on the polarization voltage applied to the detector terminals. However, the polarization voltage must be sufficient to collect charges deposited by each photon and thus measure their energy. The low charge transport properties (mobility and life) require polarization voltages (between 100 and 500 Volts depending on the detector thickness) for which the darkness current remains high. Consequently, a “decoupling” capacitor
12
is inserted between the detector and the capacitor. This capacitor
12
eliminates the DC component of the darkness current and only transient pulses corresponding to the various interactions of photons in the detector are considered. This measurement principle is now the only possible configuration from the “electronic” point of view that can be used to measure the deposited charge without it being “drowned” by the darkness current.
Unfortunately, this configuration is not electronically optimized in terms of the signal
oise ratio, since inserting the “decoupling” capacitor
12
has the disadvantage of introducing a “parasite capacitance” between the input of the preamplifier and the electrical ground (voltage reference). The effect at the preamplifier output is to multiply the preamplifier noise voltage in the ratio &Sgr;C
in
/C
i
, whi
Bonnefoy Jean-Paul
Rostaing Jean-Pierre
Verger Loïck
Commissariat a l'Energie Atomique
Epps Georgia
Hanig Richard
LandOfFree
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