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

Abstract

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. SCIENTIFIC MOTIVATIONSThe role of hard X-ray astronomy (> 10 keV) is now widely recognised. The numerous results obtained with the most recent satellite missions (BeppoSAX , Rossi-XTE , INTEGRAL) on many classes of X-ray celestial sources have demonstrated the importance of the high (> 60) energy band in order to derive an unbiased picture of the radiation emission mechanisms of the celestial sources, their nature, their evolution with time, their contribution to the Cosmic X-ray Background (CXB). In spite of the excellent results obtained thus far, the statistical quality of the measured spectra with the best instrumentation of the current generation becomes poor at these energies. Thus only the strongest sources can be accurately studied. The development of focusing optics at these energies and, more generally, in the entire passband covered by the BSAX/PDS and beyond, is of key importance to study the high energy spectra of the celestial sources with the same detail which is achieved at lower energies (< 10 keV), where focusing optics are available.X-ray mirrors based on Bragg diffraction from multilayers of bi-strates made of high and low Z materials, 1 can overcome the sensitivity problem up to about 70 keV. At higher energies the Bragg diffraction from mosaic crystals in transmission configuration (Laue geometry) appears the most efficent technique for focusing photons. A Laue lens is a set of properly oriented crystals that focus X-ray photons via Bragg diffraction in Laue geometry. X-ray diffraction can occur in crystals because the lattice structure is periodic and its period is comparable with the wavelength λ of the X-rays: crystals are a natural diffraction grating for X-rays. However, standard (almost perfect) crystals are not suitable for building lenses for astrophysical investigations. Indeed they allow a high reflection efficiency in a very narrow energy band (in the keV fraction range) and a very narrow angular range of photon acceptance (in the arcsecond range), while for studying properties of celestial sources, a good reflection efficiency is required in a relatively broad energy band (tens to hundreds of keV, depending on science goal), along with a relatively large field of view (in the arcminute range). For these types of lens mosaic crystals appear the best solution.

Gamma-ray lens development status for a European gamma-ray imager

“…3-5 They concern a theoretical feasibility study devoted to establish the best design of a Laue lens telescope along with its sensitivity expectations, 3,4 Monte Carlo simulations of the expected optical properties of Laue lenses, reflectivity measurements of mosaic crystal samples of Cu(111). 5 Here we summarize the most relevant results.…”

Development status of a Laue lens project for gamma-ray astronomy

“…5. In short, the activity has mainly concerned a theoretical design study to establish the best design of a Laue lens telescope, 7,8 Monte Carlo simulations of the expected optical properties of Laue lenses, 9 reflectivity measurements of mosaic crystal samples of Cu [111]. 10 We have investigated the geometry of the lens, the crystal material and lattice configuration that optimize the crystal reflectivity and energy passband.…”

Laue Gamma-Ray Lenses for Space Astrophysics: Status and Prospects

“…It can be seen [11,[29][30][31] that, even if a higher mosaicity gives a larger lens effective area (see Figure 9), a higher spread also produces a larger defocusing of the reflected photons in the focal plane and thus a lower lens sensitivity.…”