Bernard Hamelin

MAX: a gamma-ray lens for nuclear astrophysics

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+e- 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.

High crystalline quality of large single crystals of subglacial ice above Lake Vostok (Antarctica) revealed by hard X-ray diffraction

X-ray diffraction experiments were carried out on large ice crystals from accreted ice above Lake Vostok, a subglacial lake lying beneath the East Antarctic ice sheet. Results indicate a surprisingly very low lattice distortion. This crystalline quality does not seem to be affected by impurities. Abnormal grain growth should occur and could explain both the large grain size and the low lattice distortion. Accreted ice is therefore supposed to be non-plastically deforming. These results should be taken into account for further studies of the permeability of accreted ice to drilling fluid present in the borehole.

Strain gradients and geometrically necessary dislocations in deformed ice single crystals

Abstract Hard X-ray diffraction experiments were performed on ice single crystals deformed in torsion. This work shows the relationship between the density of geometrically necessary dislocations and strain gradients. The torsion strain appears to be totally accommodated by geometrically necessary basal screw dislocations.

Lattice distortion in ice crystals from the Vostok core (Antarctica) revealed by hard X-ray diffraction; implication in the deformation of ice at low stresses

Hard X-ray diffraction experiments have been carried out on ice monocrystals taken from the 3623 m long Vostok core (Antarctica). Strain gradients associated with the storage of geometrically necessary dislocations appear to be a general feature of the deformation microstructure of ice. The observed lattice distortion is related to the bending of the basal plane and the torsion of the lattice around the c-axis. The lattice distortion is shown to be compatible with the basal dislocations generally observed in ice crystals, supporting the assumption of deformation modes governed by basal slip and accommodated by recrystallization processes. The dependence of the ice viscosity on grain size in ice sheets appears to be compatible with these accommodation modes.

Performance of CLAIRE, the first balloon-borne γ-ray lens telescope

CLAIRE is a balloon-borne telescope dedicated to validating the concept of a crystal diffraction lens for nuclear astrophysics. For the first time, focusing γ-rays enters into the domain of the high energy astrophysics. This represents a breakthrough in γ-ray instrumentation, and will allow unprecedent sensitivities. CLAIREs first flights occurred on June 15 2000 and on June 14 2001. Here we present its performance during the two flights in terms of pointing accuracy, background noise and estimated efficiency of the lens.

Development status of a Laue lens for high-energy x rays (>60 keV)

A Laue lens for focusing X-ray photons with energies above 60 keV for astrophysical applications is being developed. The lens is based on mosaic crystals of Cu (111) produced at the Institute Laue-Langevin. A feasibility study has allowed to establish lens geometry and crystal properties required. The test of the crystals has provided very satisfactory results. We are now developing a Demonstration Model (DM) of the lens in order to establish the best assembling technique of the crystals. We will discuss the status of the project and its prospects.

Design and flight performance of a crystal diffraction telescope

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. CLAIREs lens consists of Ge-Si mosaic crystals, focusing gamma-ray photons from its 505 cm2 area onto a small solid state detector, with only 7.2 cm3 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.

A new hard x-ray diffractometer (100-400 keV) for bulk crystalline analysis: applications for nondestructive investigation

The characterization in the bulk of crystalline thick materials (thickness: several cm) can be performed up to now by using high energy X-Ray sources (gamma ray diffractometers or high energy beamlines of synchrotron facilities) or with neutron beams. The Institut Laue-Langevin has developed and built in collaboration with the Laboratoire de Spectrométrie Physique, a new instrument using the continuous high energy X-ray spectrum (100 - 400 keV) delivered by a high voltage, fine focus X-ray generator, previously used for industrial radiography. This article describes the principle of this new diffractometer and presents an overview of the main applications in the field of non destructive crystalline characterization, for both physic researches and industrial applications.

A new method based on hard x-ray diffraction for the investigation of archaeological artefacts

In order to characterize the microstructure of crystalline and polycrystalline thick materials, the Bragg diffraction pattern associated with the diffracted energy analysis allows texture determination, strain analysis, crystalline phase identification and grain size evaluation. This work describes an innovative application of a hard x-ray diffractometer, employed for the first time to obtain, in a non-destructive way, interesting microstructural information related to the production processes of archaeological finds. Two ancient bronze artefacts of different historical periods were selected to demonstrate an application of the method, which could be implemented at a museum. In fact drastically different structural characteristics were observed.

Feasibility study of a Laue lens for hard x rays for space astronomy

We report on the feasibility study of a Laue lens for hard X-rays (> 60 keV) based on mosaic crystals, for astrophysical applications. In particular we discuss the scientific motivations, its functioning principle, the procedure followed to select the suitable crystal materials, the criteria adopted to establish crystal dimensions and their distribution on the lens in order to obtain the best lens focusing capabilities, and the criteria for optimizing the lens effective area in a given passband. We also discuss the effects of misalignments of the crystal tiles due to unavoidable mechanical errors in assembling the lens. A software was developed to face all these topics and to evaluate the expected lens performance.