The present disclosure relates to room-temperature semiconductor radiation detectors and materials used for detection of ionizing radiation, e.g., x-rays, gamma rays, neutron, alpha particles, beta particles, free neutrons and others.
Radiation detectors are ubiquitous; they are used in such fields of endeavor as medical imaging, scientific research, security, combat theater awareness, etc. However, existing radiation detection systems rely heavily on technologies that were developed decades ago. Such relatively old technologies include scintillator based detectors such as thallium-doped sodium iodide (NaI(Tl)) or thallium-doped cesium iodide (CsI(Tl)) detectors; those that exhibit low performance; e.g., silicon (Si) semiconductor detectors; and/or those that require cryogenic cooling, e.g., high-purity germanium (HPGe) detectors.
Over the past 20 years, the demand for room-temperature semiconductor detectors (RTSDs) has steadily grown, particular in the fields of security and defense, space research and medicine. The general requirement for room-temperature operation of a semiconducting material as a nuclear radiation detector and spectrometer is relatively large band gap energy so that thermal generation of charge carriers is kept to a minimum. At the same time, high detector resolution requires small band gap energy so that a large number of electron-hole pairs are created for an absorbed quantum of ionizing radiation. Materials under consideration should also have a relatively high average atomic number, if used in gamma ray spectroscopy, to increase the gamma ray interaction probability. High charge carrier mobility and long charge carrier lifetime are also needed to ensure efficient charge carrier extraction and minimal effects from position dependent charge collection.
Research in recent years has led to some new RTSD materials, such as cadmium zinc telluride (CdZnTe or CZT) and thallium bromide (TlBr). However, widespread use of these new materials is impeded by high cost, low production yields, crystal growth constraints, e.g., single crystals of high volume, and, in the case of certain materials such as TlBr, reliability and health hazard issues.
Accordingly, engineering, research and development efforts are ongoing to produce RTSDs that not only overcome the limitations discussed above, but also have wideband detection characteristics, e.g., can be used for both gamma ray and neutron detection.
A nuclear radiation detector includes a solid-state detector material of formula Hg2X2, where X is a halogen. The material is formulated to produce an analytically measurable electrical response to nuclear radiation at room temperature. One or more electrodes are disposed on the detector material at which an electrical response is obtained.
The present inventive concept is best described through certain embodiments thereof, which are described in detail herein with reference to the accompanying drawings, wherein like reference numerals refer to like features throughout. It is to be understood that the term invention, when used herein, is intended to connote the inventive concept underlying the embodiments described below and not merely the embodiments themselves. It is to be understood further that the general inventive concept is not limited to the illustrative embodiments described below and the following descriptions should be read in such light.
Additionally, the word exemplary is used herein to mean, “serving as an example, instance or illustration.” Any embodiment of construction, process, design, technique, etc., designated herein as exemplary is not necessarily to be construed as preferred or advantageous over other such embodiments.
The present disclosure is directed to room-temperature semiconductor detector (RTSD) materials and detectors fabricated from such material. Materials embodying the present invention is a mercurous halide Hg2X2 crystal material, where X is a halogen such as iodine (I), chlorine (Cl) or bromine (B), expressed herein as “X∈{I, Cl, Br}.” RTSD material embodiments of the present invention address most of the issues faced by current room-temperature nuclear radiation detection technologies.
Nuclear radiation is made up of energetic subatomic particles, ions or atoms moving at high speeds (greater than 1% of the speed of light), and electromagnetic waves on the high-energy end of the electromagnetic spectrum. The nuclear radiation targeted by embodiments of the present invention include x-rays, gamma rays, alpha particles, beta particles and neutrons; however, it is to be understood that the present invention is not limited to these particular manifestations of nuclear radiation. As will be described below, the RTSD materials embodying the present invention are wideband materials capable of detecting, for example, gamma rays and alpha particles concurrently.
Detector material 100 may be sectioned from a boule of mercurous halide Hg2X2, X∈{I, Cl, Br} and electrodes may be disposed on surfaces of detector material 100. For example, a cathode 115 may be disposed on obverse surface 110 and one or more anodes 125a-125n, representatively referred to herein as anode(s) 125, may be disposed on reverse surface 120.
An electric field 105 may be established between cathode 115 and anode(s) 125 and an analytically measureable electrical response may be obtained from the material via the electrodes. As used herein, an “analytically measurable response” is a reaction of the material to a suitable stimulus, e.g., nuclear radiation 107, that can be resolved for purposes of analysis. An “analytically measurable electrical response,” refers to the case where the reaction is manifested in electrical energy output, as opposed to, for example, optical energy output, which is the case for scintillation detector materials. The analytically measurable electrical response of embodiments of the present invention is of sufficient resolution to distinguish separate ionizing events without conversion equipment, e.g., scintillation counters. Electrical energy arising from such ionizing events may be collected at the electrodes in a conventional manner. In certain embodiments, the electrodes, e.g., anodes 125 are disposed at intervals across the reverse surface 120, where each anode 125 collects the electrical energy of ionization occurring in its neighborhood, e.g., a pixel.
As illustrated in
Detector materials may be characterized by the electron mobility-lifetime product (μτ)e, or, as used herein, simply μτ. As the name suggests, the symbol μτ represents the product of an electron's mobility μ=E/νd (where E is electrical field strength V/D1, V is the bias voltage, D1 is the material thickness, and νd is the average drift velocity of an electron under the electrical field) and its lifetime τ, the average drift time before the electron is recombined with a hole in the valence band. As described below, increasing purity levels of the RTSD material embodying the present invention manifests itself as an increase in μτ.
Another characterizing parameter of radiation detectors is the detector resolution, i.e., the amount of separation between peaks of neighboring spectra. The width of spectrographic peaks is determined by the resolution of the detector; high resolution allows one to distinguish separate spectral lines that are close to each other. The peak shape is usually considered as following a Gaussian distribution where the central position of the peak is determined by the energy of the incoming nuclear radiation, and the area under the peak is determined by the intensity of the nuclear radiation and by the efficiency of the detector.
A common figure used to express detector resolution is full width at half maximum (FWHM), i.e., the width of the spectral peak at half of the highest point on the peak distribution. Resolution figures are given with reference to specified nuclear radiation energies. Resolution can be expressed in absolute (i.e., eV or MeV) or relative terms. For example, a sodium iodide (NaI) detector may have a FWHM of 9.15 keV at 122 keV, and 82.75 keV at 662 keV. These resolution values are expressed in absolute terms. To express the resolution in relative terms, the FWHM in eV or MeV is divided by the energy of the incident nuclear radiation and multiplied by 100. Using the preceding example, the resolution of the NaI detector is 7.5% at 122 keV, and 12.5% at 662 keV. A germanium detector may have a resolution of 560 eV at 122 keV, yielding a relative resolution of 0.46%.
In operation 220, it is determined whether a purity criterion has been met, e.g., whether the purified mercurous halide material is at least 6N pure. If the criterion has not been met, purified mercurous halide material 215 is re-introduced to purification process 210. Re-purification is repeated until the purity criterion is met, as determined in operation 220, at which point purity-specified mercurous halide material 225 is provided.
Purity-specified material 225 may be provided to crystal growth process 230 by which a single mercurous halide crystal 235. Crystal growth process 200 may include physical vapor transport (PVT) techniques, although the present invention is not so limited. Mercurous halide crystal 235 may undergo device-specific processing 240, such as slicing, polishing, passivation and application of conductive contacts.
Purity is an important factor in wide bandgap semiconductor materials, as those skilled in the semiconductor arts will appreciate. The difference in detector performance due to different material qualities can clearly be seen in
Device-specific processing 240 is, of course, dependent upon the form of the detector being fabricated.
In addition to the electron mobility-lifetime product, resistivity is a parameter that establishes the potential performance of a semiconductor nuclear detector. It is important to separate bulk resistivity from that of device (apparent) resistivity. Bulk resistivity can generally be derived from measuring the IV curve at very low bias voltage around the zero point where the impact of surface and metal contacts are negligible.
The descriptions above are intended to illustrate possible implementations of the present inventive concept and are not restrictive. Many variations, modifications and alternatives will become apparent to the skilled artisan upon review of this disclosure. For example, components equivalent to those shown and described may be substituted therefore, elements and methods individually described may be combined, and elements described as discrete may be distributed across many components. The scope of the invention should therefore be determined not with reference to the description above, but with reference to the appended claims, along with their full range of equivalents.
This application claims priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 62/107,586 entitled “DETECTION OF GAMMA AND PARTICLE RADIATION VIA MERCUROUS HALIDES,” filed Jan. 26, 2015. The disclosure of this provisional patent application is incorporated herein by reference in its entirety.
The invention disclosed herein was made with government support under contract numbers W911 SR-14-C-0065, W911QX-06-C-0074 and W911QX-06-C-0074-P0006 awarded by the United States Army; number NNX15CP7OP by NASA/JPL and number FA8051-15-P-0011 by DOD/US Air Force. The government has certain rights in the invention.
Number | Date | Country | |
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62107586 | Jan 2015 | US |