A Laue lens gamma-ray telescope represents an exciting concept for a future high-energy mission. The
feasibility of such a lens has been demonstrated by the CLAIRE lens prototype; since then various mission concepts
featuring a Laue lens are being developed. The latest, which is also the most ambitious, is the European Gamma-Ray
Imager (GRI). However, advancing from the CLAIRE prototype to a scientifically exploitable Laue lens requires still
substantial research and development. First and foremost, diffracting elements (crystals) that constitute the Laue lens
have to be optimized to offer the best efficiency and imaging capabilities for the resulting telescope. The characteristics
of selected candidate crystals were measured at the European Synchrotron Radiation Facility on the high-energy
beamline ID 15A using a beam tuned at 292 keV. The studied low mosaicity copper crystals have shown absolute
reflectivity reaching 30%. These crystals are promising for the realization of a Laue lens, despite the fact that they
produce a diffracted beam featuring a Gaussian intensity profile, which contributes to the spread of the focal spot. A
composition gradient Si<sub>1-x</sub>-Ge<sub>x</sub> crystal has been investigated as well, which showed a diffraction efficiency reaching
50% (disregarding absorption) - half of the theoretical maximum - that represents an absolute reflectivity around 39 %,
the best that we measured at this energy to date. This gradient crystal also showed a square-shaped rocking curve that is
almost the best case to minimize the spread of the focal spot. We also show that bending a gradient crystal could still
enhance the focusing. Thanks to the better focusing, a factor of 2 in sensitivity improvement may be achieved.
The mission concept MAX is a space borne crystal diffraction telescope, featuring a broad-band Laue lens optimized for the observation of compact sources in two wide energy bands of high astrophysical relevance. For the first time in this domain, gamma-rays will be focused from the large collecting area of a crystal diffraction lens onto a very small detector volume. As a consequence, the background noise is extremely low, making possible unprecedented sensitivities. The primary scientific objective of MAX is the study of type Ia supernovae by measuring intensities, shifts and shapes of their nuclear gamma-ray lines. When finally understood and calibrated, these profoundly radioactive events will be crucial in measuring the size, shape, and age of the Universe. Observing the radioactivities from a substantial sample of supernovae and novae will significantly improve our understanding of explosive nucleosynthesis. Moreover, the sensitive gamma-ray line spectroscopy performed with MAX is expected to clarify the nature of galactic microquasars (e<sup>+</sup>e<sup>-</sup> annihilation radiation from the jets), neutrons stars and pulsars, X-ray Binaries, AGN, solar flares and, last but not least, gamma-ray afterglow from gamma-burst counterparts.
CLAIRE is a balloon-borne experiment dedicated to validating the concept of a diffraction gamma-ray lens. This new concept for high energy telescopes is very promising and could significantly increase sensitivity and angular resolution in nuclear astrophysics. CLAIRE's lens consists of 556 Ge-Si crystals, focusing 170 keV gamma-ray photons onto a 3x3 matrix of HPGe detectors, each detector element being only 1.4x1.4x4 cm<sup>3</sup>. On June 14 2001, CLAIRE was launched by the French Space Agency (CNES)from its balloon base at Gap in the French Alps and was recovered near the Atlantic ocean (500 km to the west) after about 5 hours at float altitude. Pointing accuracy and gondola stabilization allowed us to select 1<sup><i>h</i></sup>12' of "good time intervals" for the data analysis. During this time, 33 diffracted photons have been detected leading to a 3σ detection of the source. Additional measurements made on a ground based 205 meters long test range are also presented. The results of this latter experiment confirm those of the stratospheric flight.
This paper describes a modification of a new imaging system developed at Argonne National Laboratory that has the potential of achieving a spatial resolution of 1 mm FWHM. The imaging system uses a crystal diffraction lens to focus gamma rays from the radioactive source. The medical imaging application of this system would be to detect small amounts of radioactivity in the human body that would be associated with cancer. The best spatial resolution obtained with the present lens at the time of the presentation made at the Medical Imaging Symposium 2001, was 6.7 mm FWHM for a 1-mm-diameter source. Since then it has been possible to improve the spacial resolution of the lens system to 3 mm FWHM. Experiments with the original lens system have led to a new design for a lens system that could have a spacial resolution of 1 mm FWHM. This is accomplished by: one, reducing the radial dimension of the crystals, and two, by replacing the small individual crystals with bent strips of single-crystalline material. Experiments are under way to test this approach.
We present the design and performance of the gamma-ray lens telescope CLAIRE, which flew on a stratospheric balloon on June 14, 2001. The objective of this project is to validate the concept of a Laue diffraction lens for nuclear astrophysics. Instruments of this type, benefiting from the dramatic improvement of the signal/noise ratio brought about by focusing, will combine unprecedented sensitivities with high angular resolution. CLAIRE's lens consists of Ge-Si mosaic crystals, focusing gamma-ray photons from its 505 cm<sup>2</sup> area onto a small solid state detector, with only 7.2 cm<sup>3</sup> volume for background noise. The diffracted energy of 170 keV results in a focal length of 279 cm, yet the entire payload weighed under 500 kg. CLAIRE was launched by the French Space Agency (CNES) from its balloon base at Gap in the French Alps (Southeast of France) and was recovered near Bordeaux in the Southwest of France after roughly 5 hours at float altitude. After presenting the principle of a diffraction lens, the CLAIRE 2001 flight is analyzed in terms of pointing accuracy, background noise and diffraction efficiency of the lens.
Design, fabrication, testing, and performance of an x-ray lens assembly are described. The assembly consists of a number of precisely stacked and aligned parts, each of which is a section of an extruded aluminum piece having 16 parabolic cavities. The wall thickness between adjacent cavities is 0.2 mm. By stacking a number of long, extruded parts and cutting the assembly diagonally, a variable-focus lens system is derived. Moving the lens horizontally allows the incident beam to pass through fewer or more cavities focusing the emerging beam at any desired distance from the lens.
The variable focus aluminum lens has been used at the Advanced Photon Source to collimate a monochromatic, 8 keV undulator beam. Results indicate collimation consistent with theoretical expectations.
A copper crystal lens designed to focus gamma ray energies of 100 to 200 keV has been assembled at Argonne National Laboratory. In particular, the lens has been optimized to focus the 140.6 keV gamma rays from technetium-99 m typically used in radioactive tracers. This new approach to medical imaging relies on crystal diffraction to focus incoming gamma rays in a manner similar to a simple convex lens focusing visible light. The lens is envisioned to be part of an array of lenses that can be used as a complementary technique to gamma cameras for localized scans of suspected tumor regions in the body. In addition, a 2- lens array can be used to scan a woman's breast in search of tumors with no discomfort to the patient. The incoming gamma rays are diffracted by a set of 828 copper crystal cubes arranged in 13 concentric rings, which focus the gamma rays into a very small area on a well-shielded NaI detector. Experiments performance with technetium-99 m and cobalt 57 radioactive sources indicate that a 6-lens array should be capable of detecting sources with (mu) Ci strength.
A new type of medical imaging system based on crystal diffraction is being developed at the Advanced Photon Source at Argonne National Laboratory. It is designed to image very small amounts of radioactivity in the human body. The system has very fine spatial resolution (1-3 mm) and very high sensitivity. By combining two or more lenses, one can generate a 3-D image of the cancer. Micro-Curie sources can be detected with relative ease. These features make the system very useful to confirm or reject possible sites for a cancer in the human body following a full body scan. It could also have considerable use as a method of checking for breast cancer.
A crystal diffraction lens for focusing energetic gamma rays has been developed at Argonne National Laboratory for use in medical imaging of radioactivity in the human body. A common method for locating possible cancerous growths in the body is to inject radioactivity into the blood stream of the patient and then look for any concentration of radioactivity that could be associated with the fast growing cancer cells. Often there are borderline indications of possible cancers that could be due to statistical functions in the measured counting rates. In order to determine if these indications are false or real, one must resort to surgical means and take tissue samples in the suspect area. We are developing a system of crystal diffraction lenses that will be incorporated into a 3- D imaging system with better sensitivity (factors of 10 to 20) and better spatial resolution (a few mm in both vertical and horizontal directions) than most systems presently in use. The use of this new imaging system will allow one to eliminate 90 percent of the false indications and both locate and determine the size of the cancer with mm precision. The lens consists of 900 single crystals of copper, 4 mm X 4 mm on a side and 2 - 4 mm thick, mounted in 13 concentric rings.
The use of refractive lenses for focusing x-ray beams has been the subject of publications since the early 1980s. Detailed calculations have been made for different shapes for the refractive lens: cylindrical, spherical, parabolic, and for a Fresnel-type refractive lens. The main drawback to the use of a single refractive lens to focus x-rays is that the index of refraction (n equals 1 - (delta) ) is very close to 1, which results in a lens with a very long focal length. Recently Snigerov and others have suggested and experimentally demonstrated, using cylindrical-shaped lenses, that this problem of long focal lengths can be overcome by using many lenses in series. Each lens refracts the photon through a small angle, but the sum of these sequential changes in direction can be moderately large. This increase in effective refraction angle reduces the focal length of the lens to a few meters or less and makes the multi-element lens a much more useful instrument for focusing x-rays. This paper, annualizes the expected performance of a lens consisting of a series of aligned hollow spheres in a beryllium substrate. The use of hollow spheres rather than hollow cylinders produces focusing of the x rays into a small focal spot in contrast to the single-directional focusing of the hollow cylinders, which produces a line focus. The use of beryllium as the substrate results in lower photo cross sections for both scattering and absorption relative to the value of the refractive index as compared to higher-Z materials and results in higher transmission values than for lenses with thin webs between the lens elements without distorting the surfaces of the neighbor lens element. This plus beryllium's low density, keep the absorption and scattering in the web at a minimum. The calculations suggest that one will be able to make Be lenses with short focal lengths (1 to 2 m) with useable transmissions (10 to 30%). Two multi-element lenses have been constructed: one with 20 1-mm-diameter hollow spheres in an aluminum substrate, and one with 50 hollow spheres, 1 mm in diameter, in a beryllium substrate. Some construction details and calculations of the expected performance, are given for these two multi-element lenses.
The Argonne/Toulouse collaboration is developing a crystal lens diffraction telescope for use as an astrophysical detector in the energy range of 200 keV to 1.3 MeV. The lens consists of 8 rings of diffraction crystals that all focus a narrow band of energies on a common HPGe detector. The inclination angle of these crystals controls the energy band being focused and will need to be adjusted over a range of 0.5 to 1.5 degrees with arcsecond precision to cover this energy band. At Argonne National Laboratory, a new lens frame was constructed and the inner ring was equipped with 16 Ge crystals of 1 cm<SUP>3</SUP> size. The orientation of each crystal could be adjusted using a piezo-based picomotor in combination with a noncontact eddy-current sensor. The sensors have 0.1 - 0.2 arcsecond resolution; the motors have a step size of 0.05 - 0.2 arcseconds. By changing the crystal inclination and the distance of the detector from the lens, we were able to focus the 662 keV radiation from a <SUP>137</SUP>Cs source at 24.75 m as well as line energies at 276, 303, 356, and 383 keV from a <SUP>133</SUP>Ba source at 24.45 m. The sensor and system stability were demonstrated by alternately focusing line energies. We were able to simulate scans in energy of a spaceborne instrument as well as the enlargening of the energy repone by a slight detuning of the lens crystals. At the Advanced Photon Source (APS) Facility, an experiment to directly measure the diffraction efficiency of lens crystals from 200 - 500 keV using a beam with 3 arcsecond divergence was carried out. A double-crystal monochromator using two 3-mm-thick Ge crystal in Laue geometry was realized. The experimental results imply diffraction efficiencies for an astrophysical point source of 38% to 41% over the energy range for the crystals used.
Until recently, focusing of gamma-radiation was regarded as an impracticable task. Today, gamma-ray lenses have become feasible and present promising perspectives for future instrumentation. For the first time in high energy astronomy the signal/noise ratio will be dramatically improved as gamma-rays are collected on the large area of a lens from where they are focused onto a small detector. Besides an unprecedented sensitivity, such instruments feature very high angular and energy resolution.
A crystal diffraction lens was constructed at Argonne National Laboratory for use as a telescope to focus nuclear gamma rays. It consists of 600 single crystals of germanium arranged in 8 concentric rings. The mounted angle of each crystal was adjusted to intercept and diffract the incoming gamma rays with an accuracy of a few arcsec. The performance of the lens was tested in two ways. In one case, the gamma rays were focused on a single medium size germanium detector. In the second case, the gamma rays were focused on the central germanium detector of a 3 multiplied by 3 matrix of small germanium detectors. The efficiency, image concentration and image quality, and shape were measured. The tests performed with the 3 by 3 matrix detector system were particularly interesting. The wanted radiation was concentrated in the central detector. The 8 other detectors were used to detect the Compton scattered radiation, and their energy was summed with coincident events in the central detector. This resulted in a detector with the efficiency of a large detector (all 9 elements) and the background of a small detector (only the central element). The use of the 3 multiplied by 3 detector matrix makes it possible to tell if the source is off axis and, if so, to tell in which direction. The crystal lens acts very much like a simple convex lens for visible light. Thus if the source is off to the left then the image will focus off to the right illuminating the detector on the right side: telling one in which direction to point the telescope. Possible applications of this type of crystal lens to balloon and satellite experiments are discussed.
The Hard X-Ray Telescope was selected for study as a possible new intermediate size mission for the early 21st century. Its principal attributes are: (1) multiwavelength observing with a system of focussing telescopes that collectively observe from the UV to over 1 MeV, (2) much higher sensitivity and much better angular resolution in the 10 - 100 keV band, and (3) higher sensitivity for detecting gamma ray lines of known energy in the 100 keV to 1 MeV band. This paper emphasizes the mission aspects of the concept study such as the payload configuration and launch vehicle. An engineering team at the Marshall Space Center is participating in these two key aspects of the study.
The Advanced Photon Source (APS) has embarked on a systematic program in high heat load x-ray monochromator optics to mitigate the thermal effects due to the powerful x-ray beams at third-generation synchrotron sources. This program includes both experimental and computational studies. The approaches being studied include the use of new coolants (cryogens and liquid gallium), new crystal geometries (inclined and variable asymmetric), and new materials (diamond). The paper summarizes the high heat load monochromator program at the APS.
The installation of insertion devices at existing synchrotron facilities around the world has stimulated the development of new ways to cool the optical elements in the associated x-ray beamlines. Argonne has been a leader in the development of liquid metal cooling for high heat load x-ray optics for the next generation of synchrotron facilities. The high thermal conductivity, high volume specific heat, low kinematic viscosity, and large working temperature range make liquid metals a very efficient heat transfer fluid. A wide range of liquid metals were considered in the initial phase of this work. The most promising liquid metal cooling fluid identified to date is liquid gallium, which appears to have all the desired properties and the fewest number of undesired features of the liquid metals examined.
Single crystal silicon has been the material of choice for x-ray monochromators for the past several decades. However, the need for suitable monochromators to handle the high heat load of the next generation synchrotron x-ray beams on the one hand and the rapid and on-going advances in synthetic diamond technology on the other make a compelling case for the consideration of a diamond monochromator system. In this paper, we consider various aspects, advantages and disadvantages, and promises and pitfalls of such a system and evaluate the comparative performance of a diamond monochromator subjected to the high heat load of the most powerful x-ray beam that will become available in the next few years. The results of experiments performed to evaluate the diffraction properties of a currently available synthetic single crystal diamond are also presented. Fabrication of a diamond-based monochromator is within present technical means.