Because of the large number of different gamma-ray detectors available, including both scintillation and semiconductor types, extensive analysis may be required to determine which detector system is optimal for a given application. In the selection of detectors for remote monitoring of nuclear materials, a methodology has been developed to assess which detectors are best suited for this application. The analysis provides a numerical ranking of the performance of each detector thereby reducing the large set of all potential detectors to a small tractable set of most promising candidates. The basis for the evaluation will be discussed, along with the application of the methodology to a wide range of scintillator and semiconductor detector materials. The most promising scintillator and semiconductor materials are identified for remote monitoring applications.
Radiation detectors have been fabricated from Cd1-xZnxTe crystals and their performance has been studied with respect their Cd1-xZnxTe surface preparation. Energy resolution, leakage current, resistivity, ohmicity as well as surface roughness of the detectors were measured and compared for various chemical treatments. Etching in lactic acid has been found to lead to better ohmic contract, a rougher surface and higher leakage current. Bromine is effective in removing the damaged layer and when added to 20 percent lactic acid in ethylene glycol leads to a lower leakage current and consequently a better energy resolution. The best detector performance was obtained with a two step treatment consisting of a standard etchant (5 percent Br in methanol) followed by 2 percent Br in'20 percent lactic acid in ethylene glycol'. The results clearly demonstrate the important of surface chemical treatments in the fabrication of Cd1-xZnxTe radiation detectors.
We exposed a CdZnTe detector to MeV neutrons from a 252Cf source and found no performance degradation for fluences below 1010 neutrons cm-2. Detector resolution did show significant degradation at higher neutron fluences. There is evidence of room temperature annealing of the radiation effects over time. Activation lines were observed and the responsible isotopes were identified by the energy and half-life of the lines. These radiation damage studies allow evaluation of the robustness of CdZnTe detectors in high neutron and radiation environments.
We have applied several techniques, including photoluminescence, proton-induced x-ray emission, photocurrent, and alpha particle response mapping, for mapping micron- and millimeter-scale variations in cadmium zinc telluride. We have correlated the degree of inhomogeneity determined by these techniques with performance of gamma-ray spectrometers fabricated from the material.
In nuclear medicine, a gamma-ray-emitting radiotracer is injected into the body, and the resulting biodistribution is imaged using a gamma camera. Current gamma cameras use a design developed by Anger '. An Anger camera makes use of a slab of scintillation detector that is viewed by an array of photomultiplier tubes and uses an analog position estimation technique to determine the position ofthe gamma ray's interaction. The image-forming optics is usually a multi-bore collimator made of lead. Such cameras are characterized by poor, system spatial resolution (-1 cm) due to poor detector resolution (-0.4 cm) and poor collimator performance. The energy resolution ofcurrent gamma cameras is also limited (-11% FWHM @ 140 keV). Energy resolution is useful for suppressing the effects ofCompton scattering in tissue. In single photon emission computed tomography (SPECT), a number of images are taken ofthe patient from different viewing directions and these images are used to reconstruct a representation ofthe three-dimensional source distribution. Another technique, positron emission tomography (PET), images the annihilation radiation from a positron emitter, but it is more costly than SPECT and less widely available.
We report progress in ongoing measurements of the performance of a sub-millimeter pitch CdZnTe strip detector developed as a prototype for astronomical instruments. Strip detectors can be used to provide two-dimensional position resolution with fewer electronic channels than pixellated arrays. Arrays of this type are under development for the position-sensitive image plane detector for a coded-aperture telescope operating in the hard x-ray range of 20-200 keV. The prototype is a 1.5 mm thick, 64 by 64 orthogonal stripe CdZnTe detector of 0.375 mm pitch in both dimensions, approximately one square inch of sensitive area. In addition to energy and spatial resolution capabilities, as reported last year, we demonstrate the imaging capabilities and discuss uniformity of response across an 8 by 8 stripe, 64 'pixel', segment of detector. A technique for determination of the depth of photon interaction is discussed and initial results related to depth determination are presented. Issues related to the design and development of readout electronics, the packaging and production of strip detectors and the production of compact strip detector modules, including detector and readout electronics, are also discussed.
The spatial resolution of the modern x-ray and neutron detectors is due to emission secondary particles. For fast x-rays this limit is connected with a range of secondary electrons and usually resolution is 20 microns; for neutrons, limit is connected with a range of (alpha) - particles and a resolution of 50 microns. It was shown that polycapillary Kumakhov optics can avoid this limit. We have carried out experiments on realization of this idea.
A ray-tracing technique based on the radiation transfer equation is used to describe the spontaneous emission gain in active media. Using this approach analytical solutions for the intensity distribution and coherence function in the output plane of an active medium with parabolic transverse profiles of dielectric constant and gain coefficient are presented. Applicability of the approximation when contribution into output emission is made by only spontaneous sources adjacent to the far face region of an active medium is analyzed. This approximation is seen not to be applicable for many real situations and it is necessary to take into account the sources in the whole active medium. The effect of dielectric constant fluctuations on output coherence is treated by using the phase approximation of Huygens-Kirchhoff method.
This work is devoted to the theoretical investigation of the multiplication processes in a channel of microchannel plate (MCP). Different experimental modes, time, space, energy and other characteristics of the output signal can be investigated by means of analytical technique of generating functionals and computer simulation. In the analytical model of MCP channel, we assume that every electron in a channel is characterized by the vector coordinate of birth with the set of components: space, time, energy, etc. If generating functional of the electrons, emitted by the incident radiation, and generating functional of the secondary electrons, emitted by a primary electron, interacting with the channel surface, are known, generating functional of the output signal can be obtained. It provides all statistical characteristics of the output electron flow. Computer simulation lets us investigate different, sometimes unusual, schemes of the single electron impulse formation, which can result in the definite improvements of functional abilities of the MCP. The channel with the resistance changing along the channel surface was considered. It was established that such channel provides not only the smaller recovery time, but also has the peaked pulse-height distribution.
Large area silicon avalanche photodiodes have been fabricated with maximum avalanche gains exceeding 10,000 and excellent signal to noise ratios. A model of device performance has been developed in which previously developed general expressions are numerically integrated using actual fabrication parameters. The gain, statistical fluctuations in the gain, electronic noise, and total peak broadening have been computed using this model. The results are in good agreement with measurements. The parameter keff was found to be 7.2 X 10-4, allowing a high signal to noise ratio at gains of several thousand.
In the recent years it has been demonstrated that computed tomography represents the most advanced imaging technique in the field of non-destructive testings. In this field, although x-ray based facilities still play a dominant role, neutron tomography can provide very good results for specific applications, such as the early detection of corrosion in metallic structures, due to the peculiarities of the neutron-to-matter interaction. Thus, x-ray tomography and neutron tomography can be considered as complementary rather than alternative techniques. In this aim, a multipurpose detection system which is able to perform x-ray as well as neutron examinations requiring no or little modifications has been developed. It is based on a multi- channel plate image intensifier, which is coupled to a thin radiation-to-light converter layer as the sensitive element and to a self-constructed CCD camera. The performances of the system are theoretically evaluated in terms of the modulation transfer function of its components, while experimental tests have been carried out by the visualization of sample objects when using either neutron or x-ray sources.
Calculation procedures and experimental results form glancing angle x-ray fluorescence from thin films on a flat substrate are presented. A new x-ray tube unit with a super smooth-surface anode and a built-in waveguide collimator is described. The unit makes it possible to obtain narrowly- collimated beams of x-ray radiation with a microfocus line.
Cobalt 60 is a particularly attractive candidate for marking explosives because not only does it have a useful half-life of 5.3 years, but it produces tow highly penetrating gamma rays, at 1.173 and 1.332 MeV, in coincidence. The fact that two strong, monoenergetic gamma rays are produced simultaneously makes it possible to detect a much smaller amount of this element in the presence of terrestrial background radiation than is possible for other radioactive elements that produce only one gamma ray. We have built a test-bed system comprising six large plastic scintillators and their associated hardware to investigate the performance of this technique. Experiments have shown that reliable detection can be done with a sufficiently small amount of cobalt 60 so as not to raise safety concerns.
The signal generated in pixel array detectors for gamma-ray imaging can be strikingly different from the signal seen in single-element detectors. When the pixel size is small compared with the detector thickness, the signal induced in the readout circuit becomes dominated by one charge carrier. If the small pixel is biased positive with respect to the continuous electrode the sensed signal will be due mostly to electron motion while the degrading effects of hole trapping and variation in interaction depth become far less important. To confirm experimentally the predictions of charge transport theory we fabricated CdZnTe test pixels of various sizes. Using a source of alpha radiation we measured the pulse timing properties for electron and hole transport as a function of pixel size. Pulse-height spectra were taken with gamma radiation from 99mTc. The results are in good agreement with the model for signal induction.
High-pressure-Bridgman grown CdZnTe x-ray detectors 1.25 approximately 1.7 mm thick were tested using monochromatic x-rays of 30 to 100 keV generated by a high energy x-ray generator. The results were compared with a commercially available 5 cm thick Nal detector. A linear dependence of the counting rate versus the x-ray generator tube current was observed at 58.9 keV. The measured pulse height of the photopeaks shows a linear dependence on energy. Electron and hole mobility-lifetime products were deduced by fitting bias dependent photopeak channel numbers at 30 keV x-ray energy. Values of 2 X 10-3 cm2/V and 2 X 10-4 cm2/V were obtained for (mu) (tau) e and (mu) (tau) p, respectively. The detector efficiency of CdZnTe at a 100 V bias was as high as, or higher than 90 percent compared to a Nal detector. At x-ray energies higher than 70 keV, the detection efficiency becomes a dominant factor and decreases to 75 percent at 100 keV.
The scientific objectives and future requirements of high energy x-ray astronomy are discussed and concepts for imaging instruments based on CdZnTe detectors and coded masks are reviewed. An instrument concept based on CdZnTe strip detectors, HEXIS, is described in detail. Technical requirements for large area CdZnTe strip detectors are discussed and recent work at UCSD and WU on the capabilities of CdZnTe strip detectors is described in detail. Studies with small, approximately 50 micron beams demonstrate that crossed strip detectors have good properties for both spatial and spectral measurements.
A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy is being developed for a space flight gamma-ray burst instrument. Two dimensional strip detectors with 100 micrometers pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 22 to 122 keV photons to less than 100 micrometers. In addition, good spectral resolution has been achieved. The uniformity of response and relative efficiency of the strip detector will be discussed. Results form electrical characterization which include strip leakage current and strip capacitance will be presented.
A 2D pixel array image sensor module has been designed for time resolved Protein Crystallography. This smart pixels detector significantly enhances time resolved Laue Protein crystallography by two or three orders of magnitude compared to existing sensors like films or phosphor screens coupled to CCDs. The resolution in time and dynamic range of this type of detector will allow to study the evolution of structural changes that occur within the protein as a function of time. This detector will also considerably accelerate data collection in static Laue or monochromatic crystallography and make better use of the intense beam delivered by synchrotron light sources. The event driven pixel array detectors, based on the column architecture, can provide multiparameter information (energy discrimination, time), with sparse and frameless readout without significant dead time. The prototype module consists of a 16 by 16 pixel diode array bump-bonded to the integrated circuit. Different detector materials (Silicon, CdZnTe) are evaluated. The detection area is 150 by 150 micrometers2 connected to the readout electronics. The individual pixel processor consists of a low-noise amplifier shaper followed by a differential threshold comparator which provides the counting of individual photons with an energy above a programmable threshold. To accommodate the very high rates, above 5 by 108/cm2/s, each pixel processor has a 3 bit pre-scaler which divides the event rate by 8. Overflow from the divider which defines a pseudo fourth bit will generate a readout sequence providing the pixel address. Addresses, generated locally as analog signals, are converted off-chip and used to increment a location in an histogramming memory to generate the computerized image of the Laue diagram.
Thick segmented scintillating converters coupled to optical imaging detectors offer the advantage of large area, high stopping power sensors for high energy x-ray digital imaging. The recent advent of high resolution and solid state optical sensors such as amorphous silicon arrays and CCD optical imaging detectors makes it feasible to build large, cost effective imaging arrays. This technology, however, shifts the sensor cost burden to the segmented scintillators needed for imaging. The required labor intensive fabrication of high resolution, large area hard x- ray converters results in high cost and questionable manufacturability on a large scale. We report on recent research of a new segmented x-ray imaging converter. This converter is fabricated using vacuum injection and crystal growth methods to induce defect free, high density scintillating fibers into a collimator matrix. This method has the potential to fabricate large area, thick segmented scintillators. Spatial resolution calculations of these scintillator injected collimators show that the optical light spreading is significantly reduced compared to single crystalline scintillators and sub-millimeter resolution x- ray images acquired with the segmented converter coupled to a cooled CCD camera provided the resolution to characterize the converter efficiency and noise. The proposed concept overcomes the above mentioned limitations by producing a cost-effective technique of fabricating large area x-ray scintillator converters with high stopping power and high spatial resolution. This technology will readily benefit diverse fields such as particle physics, astronomy, medicine, as well as industrial nuclear and non-destructive testing.
Associated particle imaging (API) is a technique that employs a small deuterium accelerator to accelerate deuterons into a tritium-impregnated target, producing coincident neutrons and alpha particles that travel in opposite directions. By detecting the arrival of an alpha particle on a phosphor, and its position in two dimensions, the time and direction of the neutron emission id determined. Neutrons that interact with material in their paths produce a gamma ray whose energy is characteristic of that material. From the arrival time and energy of these gammas, the elemental composition of an object can be deduced as a function of position. We have built a portable API system and conducted field tests. In these tests, we have identified a quantity of fertilizer 'hidden' in a closed vehicle and the contents of some common military munition casings.
In this paper, we present the experimental validation of the wide signal measurement range of a high sensitivity gateable camera based on a multi step parallel plate avalanche chamber (MSPPAC) for MeV pulsed radiography applications. The MSPPAC was tested with a metallic converter. Thus, we have demonstrated that the MSPPAC is a potentially high- performance imager, even for the high hard x-ray dose rates used in radiography and radiotherapy.
The state of the capillary optics is now characterized by the transition from solution of the purely research problems to the creating of new devices on the base of these optics: diffractometers, x-ray analyzers, systems for focusing and bending of the synchrotron and neutron beams, etc. A very important event is creating new medical imaging systems. These systems have a large area, high spatial resolution, and wide dynamical range. In this paper the results of experimental investigations on polycapillary optics performed by our Institute are analyzed. A new interference effect of 8 keV photons reflected from a single capillary of 0.1 mm, 0.5 mm, 1 mm diameters is discussed. At this stage technology of the capillary optics manufacturing is a decisive factor. Technology now is developed in a two directions. On one hand, systems with the more smaller capillary diameters are created. On the other hand area of these systems are increased already mounting to hundreds square centimeters. The wide use in the near future of devices based on the polycapillary optics not only in scientific instruments but also in medicine, microelectronics, biology, space, and so on may be forecast.
The results of an experimental investigation of transporting soft x-rays through quartz capillaries are presented. These results are used to design a capillary focusing system in the soft x-ray range. Monolithic and assembled x-ray lenses for AlK(alpha )- and MoL(alpha )-radiation are also investigated. These result are compared to those obtained with multilayer structures.
A brief review of x-ray focusing effects in different capillary structures is presented. Possible applications of focusing effects in non-imaging capillary optics are considered. The so-called 'interference effect in Kumakhov lens' is discussed on this basis. This effect is completely explained in terms of geometrical optics.
A monolithic polycapillary focusing optic, consisting of hundreds of thousands of small tapered glass capillaries, can collect a large solid angle of x-rays from a point source and guide then through the capillaries by multiple total reflections to form an intense focused beam. Such a focused beam has many applications in microbeam x-ray fluorescence (MXRF) analysis. Two monolithic polycapillary focusing optics were tested and characterized in a MXRF set- up using a microfocusing x-ray source. For the CuK(alpha ) line, the measured focal spot sizes of these optics were 105 micrometers and 43 micrometers full-width-half-maximum, respectively. When the source was operated at 16W, the average CuK(alpha ) intensities over the focal spots were measured to be 2.5 X 104 photons/s/micrometers2 and 8.9 X 104 photons/s/micrometers2, respectively. When we compared the monolithic optics to straight monocapillary optics with approximately the same output beam sizes, intensity gains of 16 and 44 were obtained. The optics were applied to the MXRF set-up to analyze trace elements in various samples and a minimum detection limit of about 2 pg was achieved for the transition elements (V, Cr, Mn, and Fe). The optics were also used to map the distributions of trace elements in various samples.
Recent developments in the design, fabrication and applications of polycapillary optics for x-rays and neutrons have produced impressive new results and possibilities. These include x-ray and neutron probe measurements; diffraction based structural analysis of thin metal and magnetic films, powders, and protein crystals; x-ray lithographic patterning of integrated electronics and microstructures; and materials and medical imaging applications. Advances in the understanding, control, and reproducibility of fabrication parameters along with recent commercial availability of these optics have increased their accessibility and broadened their application base. This will likely lead to the incorporation of polycapillary optic components in a variety of commercial x-ray and neutron analysis systems. Expansion of the effective x-ray energy range downward to 0.3 and upward to 80 keV opens new possibilities, especially for medical and astronomical applications. Quantitative agreement between computer simulations and observed properties of both individual capillaries and complete lens structures provides not only a reliable basis for design of custom optics for a particular source or application, but also provides a firm basis for defining the limitations of polycapillary optics. The broad range of demonstrated applications, as well as limitations and prospects for polycapillary optics will be discussed.
Usually the results of the experiment on transmitting x-rays through polycapillaries depend strongly on the geometry of the experiment: effective source size, source-capillary distance etc. A new method of investigating polycapillaries is suggested. This method reduces to minimum the role of exterior geometrical factors and allows to obtain essential transmissional properties of polycapillaries in the x-ray range. The results of measurements provide vast information concerning glass composition, quality of reflecting surfaces, presence of polycapillary small bents etc.
A geometrical optics simulation program which includes roughness and waviness effects has been developed to analyze the performance of polycapillary x-ray optics in the energy regime 10-80 keV. The simulation was in excellent agreement with previous experimental results. The calculations showed that low energy x-ray performance is sensitive to roughness while high energy x-ray performance is affected by waviness and profile error (bending). Despite the low critical angle for total external reflection at high energies, capillary x- ray optics appear promising for many hard x-ray applications. Transmission measurements at high energies have also proven to be a very sensitive tool in capillary quality analysis.
X-ray focusing based on Bragg reflection at curved crystals allows collection of large solid angles of incident radiation, subsequent monochromatization, and an efficient condensation of the reflected beam into a small spatial region in a pre-selected focal plane. Thus, for the Bragg- reflected radiation, one can achieve higher intensities than for the radiation passing directly to the same small area in the focal plane. The first-order reflection of x-rays at highly oriented pyrolytic graphite (HOPG) crystals offers a very high intensity of the reflected radiation. Furthermore, a special deposition procedure ensures the production of doubly-curved HOPG crystals with local curvature radii down to even less than 5 mm. By means of appropriate variations of both the local curvature radii and the crystal thickness not only efficient x-ray focusing is achievable, but also a desired modeling of the energetic band-pass characteristics of the device. A new HOPG device was designed to ensure a rectangular efficiency shape in the energy range from 9 keV to 16 keV. Furthermore, a high amount of the radiation emitted by a source with 1 mm diameter had to be focused into a 10 square-mm Si(Li) detector by this HOPG crystal. Both the theoretical construction of this crystal based on ray tracing calculations and the experimental investigation of its performance is presented. Furthermore, its use in the detection line of an EDXRF set-up for a drastic repartition of the detected photon distribution is demonstrated.
Polycapillary x-ray optics have found potential application in many different fields, including antiscatter and magnification in mammography, radiography, x-ray fluorescence, x-ray lithography, and x-ray diffraction techniques. In x-ray diffraction, an optic is used to collect divergent x-rays from a point source and redirect them into a quasi-parallel, or slightly focused beam. Monolithic polycapillary optics have been developed recently for macromolecular crystallography and have already shown considerable gains in diffracted beam intensity over pinhole collimation. Development is being pursued through a series of simulations and prototype optics. Many improvements have been made over the stage I prototype reported previously, which include better control over the manufacturing process, reducing the diameter of the output beam, and addition of a slight focusing at the output of the optic to further increase x-ray flux at the sample. We report the characteristics and performance of the stage I and stage II optics.
Kumakhov halflenses was used in a classical diffraction scheme. The angular divergence and transmittance of a drawn halflens are considered. The efficiency of polycapillary optics use in x-ray diffractometry is shown.
The results of computer simulation and experimental investigation of different capillary-based filters are presented. Filters with multiple reflections in polycapillaries, filters with single reflections, combinations of capillary structures with crystals and conventional x-ray absorption filters are considered. Capillary-based x-ray filters effectively suppress the high- energy part of radiation while the supplementary elements suppress the soft part of spectrum. The possibility of application of such filters in laboratory set-up and in synchrotron beam line is discussed.
Three problems in medical imaging may be solved by use of Kumakhov optics: suppression of the radiation scattered in a phantom for improving the image contrast; increasing the spatial resolution for early diagnostic of a cancer; decrease of the patient irradiation dose. For solutions to these problems we used the polycapillary raster of d equals 5 cm, consisting of capillaries (Pb-containing) with diameter of channels 10 microns. Such systems permit to solve above mentioned problems.
The experimental and theoretical results on bending of thermal neutron beams by means of polycapillary structures are presented. It is shown that neutron beams may be bent on large angles (up to 30 degrees) through short distances due to the use of multichannel systems. Creating and testing of the first bender of neutral particles is described.
Use of Kumakhov optics in the element analysis permits to increase sharply its sensitivity owing to the scattered radiation suppression. We used the capillary lenses in several schemes. The sensitivity of x-ray analysis is increased in all these schemes. On the basis of investigations, a new x-ray spectrometer with a capillary lens was created by the Institute for Roentgen Optics.
We have tested a new concept of segmented scintillating block in order to realize a low cost fast 2D integrating imager for hard x-ray radiography and for neutron radiography. COntrary to other scintillating camera for high energy imaging, our scintillating materials is not made up of heavy crystal; it is composed of less expensive scintillating plastic fibers. However, in order to insure a higher detection quantum efficiency, these fibers are surrounded by a lead x-ray electron converter (lead coating). This hybrid scintillating block is then adjusted upon a microchannel plate image intensifier. We present experimental results about the performances of this kind of scintillating device as a hard x-ray imager. We can notice that the detection quantum efficiency is very high, and simultaneously the spatial resolution is preserved by this kind of pixellization. We have demonstrated that such a hybrid scintillating set-up is already interesting in order to constitute a NeV radiography detector. It reaches the quantum limit of detection and can also provide images at high dose rate. On the same principle, with a light metal replacing the lead it is possible to make an efficient fast neutron imaging device.