High diffraction efficiency, broadband, diffraction crystals for use in crystal diffraction lenses

A major goal of the MAX program is to detect and measure gamma rays produced following the nuclear reactions that take place in a supernova explosion. To detect a reasonable number of supernovae, sensitivities of the order of a few times 10-7 γ cm-2sec-1 are needed – much better than possible with current instruments. The approach in the MAX program is to use a crystal diffraction lens to collect photons over a large area and concentrate them on a small well-shielded detector, greatly improving the signal to noise ratio. The crystals need to have both high diffraction efficiency and a relatively broad energy bandwidth. With mosaic crystals there is a trade-off between bandwidth and diffraction efficiency – one can have either high efficiency or large bandwidth, but not both without losing too much intensity through atomic absorption. A recent breakthrough in our understanding of crystal diffraction for high-energy gamma rays has made it possible to develop crystals that have both high diffraction efficiency and a relatively broad energy bandwidth. These crystals have near perfect crystal structure, but the crystalline planes are slightly curved. Such curved planes can be obtained in 3 different ways, by using mixed crystals with a composition gradient, by applying a thermal gradient, and by mechanically bending a near perfect crystal. A series of experiments have been performed on all three types of crystals using high-energy x-ray beams from the Advanced Photon Source at the Argonne National Laboratory. Experiments performed at 3 energies, 93 keV, 123 keV and 153 keV, with both the thermal gradient Si crystals and with the mechanically bent Si crystals, demonstrated that one can achieve diffraction efficiencies approaching 100% with moderate energy bandwidths (ΔE/E = 1.4%) and low atomic absorption (transmission = 0.65), in excellent agreement with theory. The use of this type of diffraction crystal is expected to increase the sensitivity of gamma ray telescopes by a factor of 5 over that possible with normal mosaic crystals.     

The X-Ray Facility of the Physics Department of the Ferrara University

We will report on the equipment and performance of the X-ray facility of the University of Ferrara. Initially developed to test the PDS (Phoswich Detection System) instrument aboard the BeppoSAX satellite and to perform reflectivity measurements of mosaic crystal samples of HOPG (Highly Oriented Pyrolytic Graphite), with time the facility has been improved and its applications extended. Now these applications include test and calibration of hard X-ray (> 10 keV) detectors, reflectivity measurements of hard X-ray mirrors, reflectivity tests of crystals and X-ray transparency measurements. The facility is being further improved in order to determine the optical axis mosaic crystals in Laue configuration within a project devoted to develop a hard X-ray (> 60 keV) focusing optics (Pisa, A. et al.: in press, Feasibility study of a Laue lens for hard X-rays for space astronomy, SPIE Proc., 5536).     

Mosaic and gradient SiGe single crystals for gamma ray Laue lenses

Both Ge_1−x Si _x mosaic crystals and Si_1−x Ge _x crystals with gradient of composition could be grown using the modified Czochralski technique to produce the diffracting elements for the MAX gamma ray telescope. Although many elements cut from the mosaic crystal and used before for CLAIRE gamma ray telescope had an efficiency up to 20%, the overall efficiency of the lens was about 8.1 ± 0.7 %, which is more than twice lower than the theoretically predicted efficiency. Some causes of this behaviour are discussed. In addition to mosaic crystals, the growth of Si_1−x Ge _x crystals with a gradient of composition and their properties are analysed. Such composition-gradient crystals could be a promising way to improve the diffraction efficiency of Laue lens for MAX.     

Self-standing bent silicon crystals for very high efficiency Laue lens

Silicon mono-crystals have been bent thanks to a series of parallel superficial indentations on one of the largest faces of the crystals. This technique relies on irreversible compression of the crystal beneath and beside the indentations. This latter causes deformation with no need for external device, resulting in a uniform self-standing curvature within the crystal. Indented Si crystals have been characterized at European Synchrotron Radiation Facility using a monochromatic beam ranging from 150 to 700 keV. Crystals exhibited very high diffraction efficiency over a broad range of energy, peaking 95% at 150 keV. Measured angular spread of the diffracted beam was always very close to the morphological curvature of the sample under investigation, proving that the energy passband of bent crystals can be controlled by simply imparting a selected curvature to the sample. The method of superficial indentations was found to offer high reproducibility and easy control of diffraction properties of the crystals. Moreover the method is cheap and simple, being based on mass production tools. A Laue lens made of crystals bent by superficial indentations can provide high-efficiency concentration of hard x-ray photons, leading significant improvement in many astrophysical applications.     

CLAIRE: First light for a gamma-ray lens

The objective of the R&D project CLAIRE was to prove the principle of a gamma-ray lens for nuclear astrophysics. CLAIRE's Laue diffraction lens has a diameter of 45 cm and a focal length of 277 cm; 556 germanium-silicon crystals are tuned to focus 170 keV photons onto a 1.5 cm diameter focal spot. Laboratory measurements of the individual crystals and the entire lens have been used to validate a numerical model that we use to estimate the lens performance for a source at infinity. During a stratospheric balloon flight on 2001 June 14, CLAIRE was directed at the Crab nebula by a pointing system able to stabilize the lens to within a few arcseconds of the target. In 72 min of valid pointing time, 33 photons from the Crab were detected in the 3 keV bandpass of the lens: CLAIRE's first light! The performance of CLAIRE's gamma-ray lens, namely the peak reflectivity for a polychromatic source (9±1%), has been confirmed by ground data obtained on a 205 meter long test range. CLAIRE's measured performance validates the principle of a Laue lens for nuclear astrophysics, opening the way for a space-borne gamma-ray lens telescope that will achieve one to two orders of magnitude improvement in sensitivity over present technologies.     

Thick self-standing bent crystals as optical elements for a Laue lens for applications in astrophysics

In this paper we report progresses in the realization of self-standing bent crystals, which are suitable as optical elements for Laue lenses, i.e. for optic to focus hard X-rays in the 100–1000 keV energy range. The curvature of the crystals is a key factor to enhance diffraction efficiency and energy bandpass for such an optic. In particular, two bent crystals featuring a thickness of 5 mm, made of Si and Ge respectively, were produced at the Sensor and Semiconductor Laboratory in Ferrara, Italy. The crystals were bent through the application of a carbon fibre composite. This proved to be a relatively low cost method for crystal bending, suitable for mass production. The manufactured samples were characterised via optical interferometry, and showed a fairly uniform curvature. Finally, the samples were tested exploiting hard X-ray diffraction at the ID11 facility of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. A careful analysis of the experimental data highlighted that the samples feature large energy bandpass, wide geometrical acceptance for incoming hard X-rays, and high diffraction efficiency. We therefore conclude that such self-standing crystals are good candidates as Laue lens components for astrophysics applications.     

Quantitative in situ XRD measurement of shock metamorphism in Martian meteorites using lattice strain and strain‐related mosaicity in olivine

All Martian meteorites have experienced shock metamorphism to some degree. We quantitatively determined shock‐related strain in olivine crystals to measure shock level and peak shock pressure experienced by five Martian meteorites. Two independent methods employing nondestructive in situ micro X‐ray diffraction (μXRD) are applied, i.e., (1) the lattice strain method, in which the lattice strain value (ε) for each olivine grain is derived from a Williamson–Hall plot using its diffraction pattern (peak width variation with diffraction angle) with reference to a best fit calibration curve of ε values obtained from experimentally shocked olivine grains; (2) the strain‐related mosaicity method, allowing shock stage to be estimated by measuring the streaking along the Debye rings of olivine grain diffraction spots to define their strain‐related mosaic spread, which can then be compared with olivine mosaicity in ordinary chondrites of known shock stage. In this study, both the calculated peak shock pressures and the estimated shock stages for Dar al Gani 476 (45.6 ± 0.6 GPa), Sayh al Uhaymir 005/8 (46.1 ± 2.2 GPa), and Nakhla (18.0 ± 0.6 GPa) compare well with literature values. Formal shock assessments for North West Africa 1068/1110 (53.9 ± 2.1 GPa) and North West Africa 6234 (44.6 ± 3.1 GPa) have not been reported within the literature; however, their calculated peak shock pressures fall within the range of peak shock pressures defining their estimated shock stages. The availability of nondestructive and quantitative μXRD methods to determine shock stage and peak shock pressure from olivine crystals provides a key tool for shock metamorphism analysis.     

Review of crystal diffraction and its application to focusing energetic gamma rays

The basic features of crystal diffraction and their application to the construction of a crystal diffraction lens for focusing energetic gamma rays are described using examples from the work performed at the Argonne National Laboratory. Both on-axis and off-axis performance are discussed. The review includes the use of normal crystals, bent crystals, and crystals with variable crystal-plane spacing to develop both condenser-type lenses and point-to-point imaging lenses.     

MAX, a Laue diffraction lens for nuclear astrophysics

The next generation of instrumentation for nuclear astrophysics will have to achieve a factor of 10–100 improvement in sensitivity over present technologies. With the focusing gamma-ray telescope MAX we take up this challenge: combining unprecedented sensitivity with high spectral and angular resolution, and the capability of measuring the polarization of the incident photons. The feasibility of such a crystal diffraction gamma-ray lens has recently been demonstrated with the prototype lens CLAIRE. MAX is a proposed mission which will make use of satellite formation flight to achieve 86 m focal length, with the Laue lens being carried by one satellite and the detector by the other. In the current design, the Laue diffraction lens of MAX will consist of 13740 copper and germanium (Ge_1−x Si _x , x ∼ 0.02) crystal tiles arranged on 36 concentric rings. It simultaneously focuses in two energy bands, each centred on one of the main scientific objectives of the mission: the 800–900 keV band is dedicated to the study of nuclear gamma-ray lines from type Ia supernovae (e.g. ⁵⁶ Co decay line at 847 keV) while the 450–530 keV band focuses on electron-positron annihilation (511 keV emission) from the Galactic centre region with the aim of resolving potential point sources. MAX promises a breakthrough in the study of point sources at gamma-ray energies by combining high narrow-line sensitivity (better than 10⁻⁶ cm⁻² s⁻¹) and high energy resolution (E/dE ∼ 500). The mission has successfully undergone a pre-phase A study with the French Space Agency CNES, and continues to evolve: new diffracting materials such as bent or composite crystals seem very promising. PACS: 95.55.Ka, 29.30.Kv, 61.10.-i     

NEUTRON DIFFRACTION STUDY OF MUNGINDI IRON NICKEL METEORITE

The Mungindi Iron Nickel Meteorite was investigated by neutron diffraction. All layers of the γ-phase (separated from each other by α-phase bars) form, from the crystallographic point of view, the same, big single crystal. The perfection of this single crystal is very high. In the kamacite phase each individual “single crystal” is broken into a group of crystals.     

Laue diffraction lenses for astrophysics: Theoretical concepts

Beyond the present technologies, Laue diffraction lenses are very promising tools in the field of gamma-ray astrophysics. The theoretical concepts of this kind of instruments are based on the Laue diffraction in crystals, discovered almost 100 years ago. Though they are commonly used in crystallography, their application to γ-ray focusing in astrophysics requires some specific developments, e.g. in terms of energy and imaging responses. The present article describes the physics of X-ray diffraction in crystals. In the context of the Darwin model of mosaic crystals, some peculiar aspects, relevant to the astrophysical observation, are discussed. The evaluation and optimization of diffraction efficiency are discussed, especially with rigards to the crystal’s mosaicity and thickness, its spatial extent and deviations to the “ideally imperfect” Darwin model. PACS 95.55.Ka, 61.50.Ah, 61.10.−i, 41.50.+h     

The Ferrara hard X-ray facility for testing/calibrating hard X-ray focusing telescopes

We will report on the current configuration of the X-ray facility of the University of Ferrara recently used to perform reflectivity tests of mosaic crystals and to calibrate the experiment JEM–X aboard Integral. The facility is now located in the technological campus of the University of Ferrara in a new building (named LARIX laboratory= ̳LARge ̳Italian ̳X-ray facility) that includes a tunnel 100 m long with, on the sides, two large experimental rooms. The facility is being improved for determining the optical axis of mosaic crystals in Laue configuration, for calibrating Laue lenses and hard X-ray mirror prototypes.     

A hard X-ray telescope and its observation of the Crab pulsar

The hard X-ray phoswich telescope, HAPI-1, is described. It was designed for making observations on board high-flying balloons. The telescope comprises a 145 cm² primary crystal of 5 cm thick Cs I(T1), effectively shielded over the lower 2π steradians by a 5 cm thick NaI(T1) crystal and has a field of view of approximately 4° HWHM determined by a graded shield and a collimator. The scintillation pulses originating in CsI and NaI crystals are distinguished by pulse shape discrimination. All sub-systems of the telescope are described. The Crab pulsar was obsserved by HAPI-1 in a balloon flight on 1984 May 23 in the energy range 20 to 200 keV and the observed result is presented.     

Effects of capillary reflection in the performance of the collimator of the Large Area Detector on board LOFT

The Large Observatory For X-ray Timing (LOFT) is one of the candidate missions selected by the European Space Agency for an initial assessment phase in the Cosmic Vision programme. It is proposed for the M3 launch slot and has broad scientific goals related to fast timing of astrophysical X-ray sources. LOFT will carry the Large Area Detector (LAD), as one of the two core science instruments, necessary to achieve the challenging objectives of the project. LAD is a collimated detector working in the energy range 2-50 keV with an effective area of approximately 10 m ²at 8 keV. The instrument comprises an array of modules located on deployable panels. Lead-glass microchannel plate (MCP) collimators are located in front of the large-area Silicon Drift Detectors (SDD) to reduce the background contamination from off-axis resolved point sources and from the diffuse X-ray background. The inner walls of the microchannel plate pores reflect grazing incidence X-ray photons with a probability that depends on energy. In this paper, we present a study performed with an ad-hoc simulator of the effects of this capillary reflectivity on the overall instrument performance. The reflectivity is derived from a limited set of laboratory measurements, used to constrain the model. The measurements were taken using a prototype collimator whose thickness is similar to that adopted in the current baseline design proposed for the LAD. We find that the experimentally measured level of reflectivity of the pore inner walls enhances the off-axis transmission at low energies, producing an almost flat-top response. The resulting background increase due to the diffuse cosmic X-ray emission and sources within the field of view does not degrade the instrument sensitivity.     

Deep Hα imagery of the Eridanus shells

A deep Hα image of interlocking filamentary arcs of nebulosity has been obtained with a wide-field (≈30° diameter) narrow-band filter camera combined with a charge-coupled device as a detector. The resultant mosaic of images, extending to a galactic latitude of −65°, has been corrected for field distortions and had galactic coordinates superimposed on it to permit accurate correlations with the most recent H  i (21 cm), X-ray (0.75 keV) and FIR ( IRAS 100 μm) maps.
Furthermore, an upper limit of 0.13 arcsec yr⁻¹ to the expansion proper motion of the primary 25° long nebulous arc has been obtained by comparing a recent Hα image obtained with the San Pedro Martir telescope of its filamentary edge with that on a Palomar Observatory Sky Survey E plate obtained in 1951.
It is concluded that these filamentary arcs are the superimposed images of separate shells (driven by supernova explosions and/or stellar winds) rather than the edges of a single 'superbubble' stretching from Barnard's Arc (and the Orion Nebula) to these high galactic latitudes. The proper motion measurement argues against the primary Hα-emitting arc being associated with the giant radio loop (Loop 2) except in extraordinary circumstances.     

Monte Carlo study of detector concepts for the MAX Laue lens gamma-ray telescope

MAX is a proposed Laue lens gamma-ray telescope taking advantage of Bragg diffraction in crystals to concentrate incident photons onto a distant detector. The Laue lens and the detector are carried by two separate satellites flying in formation. Significant effort is being devoted to studying different types of crystals that may be suitable for focusing gamma rays in two 100 keV wide energy bands centered on two lines which constitute the prime astrophysical interest of the MAX mission: the 511 keV positron annihilation line, and the broadened 847 keV line from the decay of ⁵⁶Co copiously produced in Type Ia supernovae. However, to optimize the performance of MAX, it is also necessary to optimize the detector used to collect the source photons concentrated by the lens. We address this need by applying proven Monte Carlo and event reconstruction packages to predict the performance of MAX for three different Ge detector concepts: a standard coaxial detector, a stack of segmented detectors, and a Compton camera consisting of a stack of strip detectors. Each of these exhibits distinct advantages and disadvantages regarding fundamental instrumental characteristics such as detection efficiency or background rejection, which ultimately determine achievable sensitivities. We conclude that the Compton camera is the most promising detector for MAX in particular, and for Laue lens gamma-ray telecopes in general.     

Microflares and hot component in solar active regions

Open-shutter RHESSI observations of 3–15 keV X-rays are found to exhibit active-region transient brightenings and microflares at a rate of a least 10 per hour occurring even during the periods of lowest solar activity so far in the mission. A thermal component fitted by temperatures of 6–14 MK dominates from 3 keV to about 9 keV, but can be traced up to 14 keV in some cases, and has an average duration of 131(±103) s at 7–8 keV. The duration increases with decreasing photon energy. The peak count rate defined by cross-correlation is delayed at low energies. The temperature peaks early in the event and then decreases, whereas the emission measure increases throughout the event. The properties are consistent with thermal conduction dominating the evolution. In some of the bigger events, a second component was found in the 11–14 keV range extending down to 8 keV in some cases. The duration is typically 3 times shorter and ends near the peak time of the thermal component consistent with the Neupert effect of regular flares. Therefore the second component is suggested to be of non-thermal origin, presumably causing the beam-driven evaporation of the first component. The two components can be separated and analyzed in detail for the first time. Low-keV measurements allow a reliable estimate of the energy input by microflares necessary to assess their relevance for coronal heating. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022496515506     

Progress Toward Light Weight High Angular Resolution Multilayer Coated Optics

We have been working on 3 separate projects that together will give us the ability to make 1 arc second, light weightWolter I optics that work above 40 keV. The three separate tasks are: (a) plasma spraying of metal-coated micro-balloons; (b) coating of the inside of Wolter I mirrors, (c) actuator designs for improving figure quality.We give a progress report on our work on all three areas.     

MARSIS surface reflectivity of the south residual cap of Mars

The south residual cap of Mars is commonly described as a thin and bright layer of CO₂-ice. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is a low-frequency radar on board Mars Express operating at the wavelength between 55 and 230 m in vacuum. The reflection of the radar wave on a stratified medium like the residual cap can generate interferences, causing weaker surface reflections compared to reflections from a pure water ice surface. In order to understand this anomalous low reflectivity, we propose a stratified medium model, which allows us to estimate both the thickness and the dielectric constant of the optically thin slab. First, we consider the residual cap as single unit and show that the decrease in the reflected echo strength is well explained by a mean thickness of 11 m and a mean dielectric constant of 2.2. This value of dielectric constant is close to the experimental value 2.12 for pure CO₂-ice. Second, we study the spatial variability of the radar surface reflectivity. We observe that the reflectivity is not homogeneous over the residual cap. This heterogeneity can be modeled either by variable thickness or variable dielectric constant. The surface reflectivity shows that two different units comprise the residual cap, one central unit with high reflectivity and surrounding, less reflective units.     

Chang’E-1 gamma ray spectrometer and preliminary radioactive results on the lunar surface

The Chang’E-1(CE-1) spacecraft took a gamma-ray spectrometer (hereafter, CGRS) to detect the element distributions on the lunar surface in a circular, 200 km altitude, polar orbit with approximately 2 h periodicity. CGRS consists of two large CsI(Tl) crystals as the main and anticoincidence detectors. The large CsI crystal of CGRS has a higher detector effective area than other lunar gamma ray spectrometers. For its 1-year mission, gamma ray spectra including many peaks of major elements and trace elements on the lunar surface have been measured by CGRS. Global measurement within 0.55-0.75 MeV is given here to describe the distribution of radioactive composition (e.g., uranium and thorium) on the lunar surface. Although CGRS has a lower energy resolution that cannot separate the uranium peak from others in this energy region, 609 keV uranium gamma ray line dominates the shape of the spectrum in this energy region. Therefore, the radioactive map can indirectly describe the uranium distribution on the lunar surface. The radioactive map shows that the higher radiation is concentrated in the Procellarum KREEP Terrene (PKT) on the nearside with an oval shape. The secondary high-radiation is located in South Pole-Aitken (SPA) basin. Lunar highlands have lower concentration. The relationship between radiation and topography displays different linear correlations for lunar highlands and SPA basin, which imply the different processes for these two regions.