Ionizing radiation detection apparatus using high-resistivity semiconductor

The present invention relates to a low- and high-energy ionizing radiation detection apparatus composed of a semiconductor material provided with a cathode, an anode and at least one grid to form a radiation detector with a very high energy resolution rate that operates at ambient temperature.

DESCRIPTION 
1. Field of the Invention 
The present invention relates to an ionizing radiation detection apparatus 
based on a high-resistivity semiconductor with a high energy resolution 
level (20 to several MeV). 
2. State of Prior Art 
Many types of detectors have been designed for detecting X or .gamma. 
radiation. Over the last three decades the use of semiconductor-based 
solid detectors has been the chief advance in techniques for detecting X 
or .gamma. radiation; most of these apparatuses have been gas or 
scintillation detectors. 
Semiconductor-based detectors directly convert the X or .gamma. radiation 
in the material into energy without going through intermediate steps such 
as emitting visible photons in the case of scintillators. It is thus 
possible to eliminate the sort of coupling problems that result in loss of 
efficiency. The energy needed to produce an electron-hole pair in a 
semiconductor is considerably less than in a gas or scintillation detector 
(approximately 4 eV in semiconductors compared with 30 eV in gas and 300 
eV in photomultiplying scintillator systems). Consequently the number of 
free charges created per photon detected is higher, making it possible to 
obtain better energy resolution with low noise. Moreover their high atomic 
number and great density make it possible to use considerably lower 
detection volumes than those required by gas or scintillation detectors 
while retaining the same quantum efficiency of detection (see reference 
[1] at end of Description). 
All these advantages have made it possible to use semiconductor detectors 
in the following three fields (see reference [2]: 
nuclear detection in which the aim is to measure the energy deposited by a 
.gamma. photon emitted by a source of nuclear radiation, 
detection of X radiation in which the aim is to produce a radiological 
image of an object irradiated by a generator of X rays, 
scientific instrumentation wherever there is a need to detect short X 
radiation pulses and measure their development over time and their 
intensity. 
The use of semiconductor materials as detectors of X or .gamma. radiation 
involves depositing two electrical contacts on the surface of the material 
with terminals to which a polarizing voltage is applied. The charge 
carriers, i.e. the electron-hole pairs created by the interaction of the 
.gamma. photon with the material, separate under the effect of the 
electrical field, the electrons migrating towards the positive electrode 
and the holes towards the negative electrode. The ability of these charge 
carriers to migrate towards the electrodes without being trapped by 
defects present in the semiconductor determine the energy resolution of 
the 30 spectrum (in .gamma. detection) or the signal (in X detection) 
measured. This ability, which is also referred to as "transport properties 
of charge carriers" is measured as the mobility and lifetime of the 
electrons and holes (see article reference [3]). 
The performances of present-day .gamma. detectors are limited by the 
presence of inherent defects in semiconductor which, by trapping charge 
carriers during migration to the electrodes, reduce their lifetime and 
thereby diminish the energy resolution of detectors. The influence of 
these defects on the trapping of charge carriers increases with the 
thickness of the detector and the consequently higher the rate of energy 
required (&gt;100 keV). These intrinsic defects appear systematically during 
the crystal-growth phase of semiconductor material production. There is 
considerable literature available on these defects, demonstrating that no 
crystal-growth techniques for any high resistivity semiconductor operating 
at ambient temperature is capable of eliminating them (see article 
reference [4]). 
There are several ways of compensating for the trapping of charge carriers 
during migration (see reference [1]): 
Using diode structures which, by reducing dark current by a factor of 
approximately 10, make it possible to apply a powerful electric field, 
thereby limiting trapping of free carriers. However, improvements in 
detector performance have only been seen at low thicknesses. Moreover, 
some semiconductors like CdTe have a polarization effect (reduction of the 
signal over time) which makes diode structures unusable. Even though this 
effect can be overcome by subjecting CdTe diodes to low temperatures (see 
reference [5]), implementation is fairly clumsy. 
Selecting the photon interactions with the material for which carrier 
trapping is limited. This means selecting interactions close to the 
irradiated electrode for which the electron signal is predominant. Since 
the transport properties of electrons are considerably better than those 
of holes, very good energy resolution can be obtained. Unfortunately this 
is to the detriment of the quantum detection efficiency of the detector 
since this selection eliminates interactions occurring remote from the 
irradiated electrode. The useful thickness of the detector is therefore 
electronically limited. 
Compensating for the poor transport properties of the holes by measuring 
the relation that exists between the amplitude of the integrated signal of 
each interaction and its rise time. This new method shows that this 
relation is linear for certain CdTe detectors, making it possible to 
correct poorly-collected "hits" from low amplitudes and long rise times 
(see reference [6]). At the present time this appears to be the only 
method capable of producing CdTe-based .gamma. detectors with good energy 
resolution and quantum detection efficiency. Unfortunately it will not 
operate with all types of CdTe material; some crystal-growing methods 
producing no correlation between amplitude and rise time and irrespective 
of the type of CdTe detector, thickness must be limited to 2 nm since 
above this measurement amplitude-time correction cannot be corrected. The 
detector also needs to have certain physical properties (good 
hole-transfer properties). 
Using hemispherical detectors, but these are difficult to implement. 
The aim of the present invention is to provide a new detection structure 
based on a high-resistivity semiconductor that makes it possible to 
produce a .gamma. detector with a very high energy resolution that 
operates at ambient temperature. It is assumed that the said structure 
overcomes the poor transport properties of the charge carriers in thick 
detectors. 
The problem of efficient collection of charge carriers is not new and was 
already known in gas detectors. One of the solutions was the introduction 
of the Frisch strip (see reference [7]), the principle of which is to 
insert a transparent grid between the two electrodes. This grid has a 
potential intermediate between that of the anode and that of the cathode. 
The useful signal of the detector is measured between the potential of the 
anode and the potential of the Frisch strip disposed close to the anode. 
Following irradiation of the cathode with .gamma. radiation, charge 
carriers are created between the cathode and the Frisch strip; electrons 
migrate easily towards the Frisch strip while the ions have difficulty 
migrating towards the cathode. During migration of the charge carriers no 
signal is measured at the anode since the potential of the Frisch strip 
constitutes an electrostatic screen. A signal corresponding to migration 
of electrons between the Frisch strip and the anode is only measured when 
electrons pass through the grid. Because the transport properties of the 
electrons are excellent no electrons are trapped during migration between 
their interaction site and the Frisch strip. Any trapping may be corrected 
by adding a variable gain to the anode charge preamplifier. Consequently 
the signal measured at the anode does not depend on the interaction site 
of the photon between the cathode and the Frisch strip. The ion signal 
component is thereby eliminated. 
In order to increase the carrier collection efficiency, semiconductor 
detectors in the prior art comprise interdigitized comb electrodes to 
produce a Frisch strip effect. These apparatuses required two 
preamplifiers per pixel, thereby complicating the electronic design of a 
gamma imager and causing other technical problems such as resistivity 
between electrodes on the same surface. 
Examples of embodiments of this type of semiconductor structure may include 
interdigitized combs known as "Coplanar Frisch strip Structures" (CFS), 
the names "common mode" and "strip mode" being used to refer to the 
acquisition electronics. 
P. N. Luke (see reference [8]) provides the construction of a structure 
similar to Frisch strips for CdTe semiconductors. This consists of 
depositing standard contacts by gold or platinum evaporation on the entire 
surface for the cathode (irradiated electrode) and in strip form for the 
anode. One half of these strips are interconnected with a potential weaker 
than that of the other half, which are also interconnected. Each of these 
two potentials is connected to a load preamplifier in which the charge due 
to the migration of the electrons to the anode is integrated. This charge 
is identical for the two preamplifiers until the electrons reach the 
anode. In contrast, due to the difference in the two potentials of the 
anode, the charge integrated on each preamplifier is different when the 
electrons are very close to the anode. This geometrical peculiarity makes 
it possible, by using the difference in the two signals integrated on the 
two preamplifiers, to make this difference (useful signal) independent of 
the photon interaction site since the difference only appears when the 
electrons are very close to the anode. Although fairly remarkable results 
can be obtained, this method has the drawback of requiring two 
preamplifiers per pixel. When constructing a .gamma. imager composed of 
thousands of pixels, the electronics required are very clumsy. It is also 
sensitive to the surface state of the anode, i.e. its polishing quality, 
the sensitive area being close to the anode; it also causes technical 
problems due to the fact that the resistance between the combs must be 
high. 
The purpose of the invention is therefore to produce this type of Frisch 
strip detecting structure suitable for semiconductors. 
DISCLOSURE OF THE INVENTION 
The invention relates to an ionizing radiation detection apparatus 
comprising a block of high-resistivity semiconductor material capable of 
detecting an electric signal obtained by the interaction of radiation 
striking the semiconductor material, provided with a cathode at a first 
potential and an anode at a second potential, characterized in that it 
includes at least one grid located between the cathode and the anode, said 
grid being polarized at a third potential greater than the first and 
slightly less than the second so that the signal generated between the 
grid and the anode is independent of the site at which the photon 
interacts with the semiconductor material and in that the Frisch strip 
thus constituted is formed of a single piece without requiring any 
specific contact geometry or depositing of diodes. 
In a first alternative embodiment the metallic grid or grids are obtained 
by inserting semiconductors into the crucible before growing the crystals 
so that said grids are an integral part of the bar before the crystals are 
grown. The bar is then cut into detectors and standard contacts applied to 
either surface of the detector to produce true Frisch strips. 
In a second alternative this grid is produced by forming a comb whose upper 
sections are at the second potential and the lower sections are at the 
third potential. A toothed structure is made in one of the surfaces of the 
block of semiconductor material. The lines of contact, polarized at the 
third potential, constitute an electrostatic screen with contact lines 
polarized at the second potential when the charges migrate from the 
cathode to the strip. The signal measured corresponds to the migration of 
the charges from the grid to the anode. 
The invention may be applied to thick detectors (between two and ten 
millimeters) of .alpha., .beta., .gamma. and X radiation. In the case of X 
rays the photon flow rate should be slow enough for the detector to 
distinguish each photon. 
The main application of the invention is to the field of nuclear detection 
but it can also be applied to X-ray detection and scientific instruments 
where less exacting detection requirements apply.

DETAILED DISCLOSURE OF AN EMBODIMENT 
The ionizing radiation detection apparatus shown in FIG. 1 comprises a 
block of high-resistivity semiconductor material capable of detecting an 
electric signal obtained by the interaction of radiation 10 striking the 
semiconductor material 11. The apparatus which due to its described 
structure can be considered to be monolithic, is provided with a cathode 
12 at a first potential V1, an anode 13 at a second potential V2, a grid 
14 and a diode zone 15 located between the cathode 12 and the anode 13. 
Said grid 14 is polarized at a third potential V3 that is greater than V1 
and slightly less than V2 so that the signal generated between the grid 
and the anode is independent of the site at which the photon interacts 
with the semiconductor material. Therefore V1&lt;&lt;V3&lt;V2 where V3&gt;0 and V2&gt;0. 
This technique can be applied to thick detectors (i.e. of width between 2 
and 10 mm) of .alpha., .beta., .gamma. and X radiation. In the case of X 
rays the photon flow rate should be slow enough for the detector to 
distinguish each photon. 
This apparatus makes it possible to produce a grid that is transparent to 
electrons and whose potential screens the signal measured at the anode and 
to deposit a diode sensitive to electrons on the resistive semiconductor 
material. ("Diode" is understood to mean a material that gives a signal 
when an electron enters its electrode (electron detector)). 
For example, the semiconductor material may be chlorine-doped CdTe, 
although any other type of CdTe may be used, preferably oriented (111). 
This (111) orientation makes it possible to adapt a first surface of the 
block of semiconductor material (surface B or white surface revealed using 
lactic acid or deposition of a HgCdTe-based heterojunction). A metal 
contact (gold, platinum, silver, copper, etc.) is deposited on the entire 
surface to constitute the cathode 12. A second surface opposite the first 
is chemically stripped using, for example, brominated methanol. A 
lithographic stage is used to deposit grid 14 that has a mesh optimized to 
achieve a compromise between being transparent to electrons and creating a 
potential, but also the quality of the diode layer 15 to be deposited. 
A compromise is needed when making the grid: 
if the mesh is fine, transparency is increased to the detriment of 
electrostatic screening (i.e. a signal resulting from electron migration 
may be detected), 
if the detector is too fine, the photons pass through without being 
converted to electron-hole pairs; when it is too thick, the electrons may 
be trapped and there is a loss in collecting efficiency. 
This apparatus offers a compromise between good detection efficiency (a 
thick detector enabling the generation of an electron-hole pair for each 
photon received) and good energy resolution (good charge collection). It 
offers a semiconductor detector in which the Frisch strip effect is 
obtained not because of the geometrical structure of the electrodes but 
because of the geometrical structure of the detecting material itself. 
It is important to stress that the screening effect and the useful 
detection zone are due to a geometrical effect associated with the 
detector and not with the electric contact. This makes better use of the 
Frisch strip effect and makes it possible to use a single preamplifier per 
detector. 
Diode layer 15 is sensitive to electrons and therefore of limited thickness 
(a few tens of microns). This limited thickness encourages increased 
sensitivity. This layer may be a heterojunction (HgCdTe-n, HgCdTe-p, CdS, 
ZnS, etc.) or a junction (MBE, EJM, LPE deposits embedded in the resistive 
material) formed by the curvature of the strip related to differences in 
potential in the Fermi levels of the substrate (resistive CdTe) and the 
deposited layer. The very limited thickness required to detect electrons 
opens the way to many deposition possibilities. The technology used in 
solar collectors may be used to advantage. Anode 13 consists of the 
deposition of a contact suitable for its function on the entire surface of 
the diode. This produces a semiconductor-based detection structure with a 
Frisch strip capable of giving high energy-resolution performances while 
only using a single charge preamplifier per detection cell (pixel). The 
operating principle is the same as that of a gas detector fitted with a 
Frisch strip. The thickness of CdTe detectors may reach six to ten 
millimeters thereby giving high resolution spectrometry for high energy 
levels (.apprxeq.1 MeV). 
In order to produce semiconductor detectors that give improved detection 
efficiency and energy resolution, many laboratories offer Frisch strips 
based on special contact geometries (semiconductor+special contact 
geometries) or on deposition of a diode on a grid previously deposited on 
the semiconductor (semiconductor+diodes). The originality of the present 
invention consists in producing a Frisch strip composed of a single piece, 
the semiconductor only, without using special contact geometries or 
deposition of diodes. 
In a first embodiment the present invention consists of placing one or more 
metallic grids (made, for example, of silicon carbide) in the crucible 
before growing semiconductor crystals. Said grid, whose mesh is optimized 
to give a compromise between transparency and screening, is thus 
incorporated into the bar before growth (the grid must be suitable for use 
at high frequencies) . Special techniques for cutting the bar into 
detectors and deposition using standard techniques of contacts on both 
surface of the detector thereby produces a true Frisch strip as shown in 
FIG. 1. 
A second alternative consists in producing a toothed structure on one of 
the surfaces of a CdTe detector as shown in FIG. 2. Grid 14 may be 
produced by forming a comb whose upper sections are at a second potential 
V2 and whose lower sections are at a third potential V3. The polarized 
contact lines at the first potential V1 constitute an electrostatic screen 
with contact lines polarized at the second potential V2 when the charges 
migrate from cathode 12 to grid 14. The signal measured corresponds to the 
migration of charges from grid 14 to anode 13. If the upper lines are 
shortcircuited the Frisch strip operates in common mode. For better 
performances a preamplifier per line may be used (strip mode). 
The toothed structure may, for example, be formed using a cutting machine 
with a diamond saw. 
The principle of the Frisch strip used in gas detectors and applied to the 
semiconductor-based detector of the invention gives higher performance 
characteristics, both in terms of detection efficiency and energy 
resolution, the higher the screening effect and the larger the useful 
electron detection zone: 
the strip structure uses electrode geometries to produce the screening 
effect and the useful detection zone. This zone is dependent on the pitch 
and bandwidth of the comb which, given the technical difficulties of 
producing a comb, limits the possibilities. Moreover, a preamplifier must 
be provided for each band in order to observe the screening effect. For 
example, eight to sixteen preamplifiers are required per 4.times.4.times.5 
mm.sup.3 detector (one pixel in a matrix system), 
the interdigitized comb structure also has an electron-sensitive zone 
dependent on the pitch and bandwidth. Moreover, the useful signal 
corresponds to the subtraction of the signal associated with each comb. 
Two preamplifiers per detector are therefore required and the resulting 
signal (subtraction of the two signals measured by the preamplifiers) 
remains weak with a poor signal-to-noise ratio. 
FIG. 3 shows a comparison between the function of the screen and that of 
the toothed Frisch strip of a CdTe detector in plane mode (I), SF strip 
mode (II), CFDC common mode (III) and GFDS strip mode (IV). The detector 
has the following measurements: 
Thickness=5 mm 
Geometry=5*5 mm.sup.2 
Tooth height=0.8 mm 
Wc (width of collector electrode (V2))=0.56 mm 
Wnc (width of non-collector electrode (V3))=0.04 mm 
Pixel width=0.56 mm 
Interpixel distance=0.04 mm 
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