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

Abstract

We report the status of the HAXTEL project, devoted to perform a design study and the development of a Laue lens prototype. After a summary of the major results of the design study, the approach adopted to develop a Demonstration Model of a Laue lens is discussed, the set up described, and some results presented. Comment: 11 pages, 11 figures, 2007 SPIE Conference on Optics for EUV, X-Ray, and Gamma-Ray Astronomy III

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arXiv:0712.1132v1 [astro-ph] 7 Dec 2007
Development status of a Laue lens project for gamma-ray
astronomy
F. Fronteraa,d, G. Loffredoa, A. Pisaa, L. Milania, F. Nobilia, N. Auricchioa, V. Carassitib, F.
Evangelistib, L. Landia, S. Squerzantib, K.H. Andersenc, P. Courtoisc, L. Amatid, E. Carolid,
G. Landinid, S. Silvestrid, J.B. Stephend, J. M. Poulsene, B. Negrif, G. Pareschig
aUniversity of Ferrara, Physics Department, Via Saragat 1, 44100 Ferrara, Italy;
bIstituto Nazionale Fisica Nucleare, Sezione di Ferrara, Via Saragat 1, 44100 Ferrara, Italy;
cInstitute Laue–Langevin, 6 Rue Jules Horowitz, 38042 Grenoble, France
dINAF, IASF Bologna, Via Gobetti 101, 40129 Bologna, Italy
eThales Alenia Italia SpA – Laben, S.S Padana Superiore, 290, 20090 Vimodrone, Italy
fAgenzia Spaziale Italiana, Viale Liegi, 26, 00198 Roma, Italy
gINAF, Osservatorio Astronomico di Brera, 23807 Merate, Italy
ABSTRACT
We report the status of the HAXTEL project, devoted to perform a design study and the development of a Laue
lens prototype. After a summary of the major results of the design study, the approach adopted to develop a
Demonstration Model of a Laue lens is discussed, the set up described, and some results presented.
Keywords: Laue lenses, gamma-ray instrumentation, focusing telescopes, gamma-ray observations
1. INTRODUCTION
The hard X–/gamma–ray astronomy is moving toward a new generation of telescopes: from direct sky-viewing
telescopes to focusing telescopes. Nowadays it is happening something similar to what happened in the late
’70s to the soft X–ray astronomy (<2 keV) and in the ’90s to the 2–10 keV X–ray astronomy, in the first case
with the Einstein satellite, in the second case with the launch of the ASCA and BeppoSAX satellites, when the
first X–ray focusing telescopes were flown. With the advent of focusing telescopes in the hard X–/gamma–ray
astronomy, it is expected a big leap in sensitivity, by a factor 10–100 with respect to one of the most sensitive
instruments of the current generation, and a significant increase in angular resolution (from ∼ 10 arcmin of the
mask telescopes like the INTEGRAL IBIS to less than 1 arcmin).
Both the hard X-ray (<100keV) and the gamma–ray (> 100 keV) focusing telescope generation make use
of the Bragg diffraction technique, in the first case, from multilayer coatings (ML) in reflection configuration
(supermirrors), in the second case, from mosaic crystals in transmission configuration (Laue lenses).
A mission proposal that makes use of both supermirrors and Laue lenses, named Gamma Ray Imager (GRI),
has been recently submitted to ESA in response to the first AO of the ’Cosmic Vision 2015–2025’ plan.1It covers
with unprecedented sensitivity the energy band from 10 keV to 1 MeV. While below 100 keV other missions
are now under study, like Simbol-X2and NeXT,3above 100 keV GRI is unprecedented. For the astrophysical
importance of the >100 keV band see, e.g., Refs. 4–6.
Here we report on the current status of our project HAXTEL (= HArd X-ray TELescope) devoted to develop
the technology for building broad energy passband Laue lenses, mainly for the study of the continuum emission
of celestial sources above 100 keV.
Further author information: (Send correspondence to F.F.)
F.F: E-mail: frontera@fe.infn.it, Telephone: +39 0532 974 254

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2. SUMMARY OF THE LENS DESIGN STUDY RESULTS
Results of the previous activity have been reported and discussed in Ref. 5. In short, the activity has mainly
concerned a theoretical design study to establish the best design of a Laue lens telescope,7,8Monte Carlo
simulations of the expected optical properties of Laue lenses,9reflectivity 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. Given that we have to cover with good reflection
efficiency a relatively broad energy band (several hundreds of keV), special crystals, with properly controlled
lattice deformations, appear to be more useful. Crystals of this kind include mosaic crystals, bent crystals and
crystals with non constant lattice spacing d induced by doping materials or thermal gradients. For our project
we have assumed mosaic crystals, made of crystallites misaligned each with other with controlled angular spread
β (FWHM of the Gaussian-like angular distribution of the crystallite misalignments). The growing technique of
mosaic crystals with the desired spread is now being consolidated (e.g., Courtois et al.11).
Crystal tiles of thickness t are assumed to have their mean crystalline plane normal to the tile main facets,
which are assumed to be square of side l.
To correctly focus photons, the direction of the vector perpendicular to the mean lattice plane of each crystal
has to intersect the lens axis, while its inclination with respect to the focal plane has to be equal to the Bragg
angle θB(see figure in Ref. 10). The angle θBdepends on the distance r of the tile center from the lens axis and
on the focal length f. For a correct focusing, it is needed that θB= 1/2arctan(r/f). Once the crystal material
is established, the Bragg angle increases with r/f, while the energy of the focused photons decreases with r/f.
More generally, once the focal length is established, the outer and inner lens radii, rmaxand rmin, depend on
the nominal energy passband of the lens (Emin, Emax) and on the crystal lattice spacing: higher dhkl implies
lower radii.5For given crystal material, outer lens radius and the focal length, the minimum energy that can be
focused is established.
Thus, for a fixed inner and outer radius, the lens passband can be established by the use of a combination of
different crystal materials. Among the candidate materials for their high reflectivity and for which the mosaic
technology has been developed (see, e.g., Ref. 11), Cu[111] appears very promising for the hard X-/gamma–ray
range.5However, for a long focal length (100 m), like in the case of the GRI mission, given the low lattice spacing
value d111= 2.087˚ A of Cu[111], the minimum photon energy that can be focused is 320 keV for an outer lens
radius of 185 cm (that of GRI lens). The extension of the Laue lens passband below 320 keV can be obtained by
the use of other crystal materials, e.g., Ge[111] with mosaic structure (d111= 3.266˚ A) or Si1−xGex[111] (Silicon
d111= 3.135˚ A) with a composition–gradient.12
Mosaic spread and, for a fixed material, crystal thickness are the most crucial parameters for an optimization
of the lens performance. A single crystal thickness is not the best solution for optimizing the lens effective area in
its entire passband. However the optimization of the lens effective area at the highest energies could imply large
thicknesses, that could be incompatible with lens weight constraints. We have investigated this issue,9finding
that a good compromize between crystal thickness and lens weight can be found.
Also the mosaic spread β issue has been investigated. A higher spread gives a larger effective area, but also
produces a larger defocusing of the reflected photons in the focal plane. By introducing a focusing factor G
G = fphAeff
Ad
(1)
in which Aeff is the effective area of the lens and Adis the area of the focal spot which contains a fraction fph
of photons reflected by the lens, it is found that, for long focal lengths like in the case of GRI (100 m), for its
maximization, a very low spread (∼30 arcsec) is requested. However a lower spread requires a higher accuracy
in the positioning of the crystals in the lens. A compromize has to be found.
Another issue we have investigated is the disposition of the mosaic crystal tiles in the lens for a uniform
effective area of the lens passband. The best crystal tile disposition is an Archimedes’ spiral that provides
a smooth behavior of the lens effective area Aeff with energy. However the Archimedes’ spiral becomes less
important for long focal lengths (> 30 m) and other approaches are needed.

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Table 1. Main features of the simulated Laue lens.
Parameter
Focal length (m)
Nominal passband (keV)
Inner radius (cm)
Outer radius (cm)
Crystal material
Mosaic spread (arcmin)
Tile cross section (mm2)
Tile thickness (mm)
Number of crystal rings
No. of tiles
Crystal weight (kg)
Effective area (cm2) @ 200 keV
Effective area (cm2) @ 400 keV
Effective area (cm2) @ 511 keV
Half power radius(mm)
100
200–530
88
185
Ge[111], Cu[111], Cu[200]
0.5
15 × 15
optimized
61
17661 (Ge[111]), 3254 (Cu[200]), 3386 (Cu[111])
155
500
530
430
12
Figure 1. Properties of the Laue lens described in Table 1 in the 200–530 keV band for an on-axis source(see text). Left
panel: On-axis effective area. Right panel: the corresponding 3σ sensitivity for an observation time of 106s and energy
channel width ∆E = E/2.
Also the required accuracy of the crystal tile positioning in the lens has been investigated.7It depends not
only on the mosaic spread but also on the focal length. Higher focal lengths require higher positioning accuracies,
which at the current stage of development is one of the major problems to be faced for the realization of a Laue
lens.
Results from a Monte Carlo (MC) code, that has been developed to derive the properties of different lens
configurations, like their Point Spread Functions (PSF), for either on–axis and off–axis incident photons, have
been already reported.5,8,9They confirm the results obtained from the theoretical investigation and extend
them.
We show in Fig. 1 the expected on–axis effective area and 3σ sensitivity of the lens described in Table 1.
This lens configuration nicely fits also the GRI requirements.
The expected angular resolution of the lens is better than 1 arcmin.
Results of reflectivity measurements of Cu[111], discussed elsewhere,10,13confirm our expectations.

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Figure 2. Drawing of the lens Demonstration Model.
3. DEMONSTRATION MODEL DEVELOPMENT
A lens Demonstration Model (DM) is being developed. Unlike the Laue lens in Ref. 14, the lens under devel-
opment will be made of crystal tiles rigidly fixed to the lens frame, without mechanisms for adjustment of their
orientation. Thus their positioning in the lens has to be correctly performed during the lens assembling. The
goal of the DM development is just to establish the best crystal assembling technique of the lens. Figure 2 shows
the drawing of the DM under development.
It is composed of a ring of 20 mosaic crystals with diameter of 36 cm. The tiles are made of Cu[111] with
∼ 3 arcmin spread, 15×15 mm2front surface and 2 mm thickness.
3.1 Lens assembly technique
The adopted lens assembling technique is based on the use of a counter-mask (see Fig. 3). The counter-mask
is provided with holes, two for each crystal, with their axes directed toward the center of curvature of the lens.
Actually, in the case of the DM under development, the hole axis is parallel to that of the lens axis. This
simplification has been adopted in this phase in order to focuse gamma–rays coming from a source at finite
distance d from the lens. In our case d ∼ 6 m.
Each crystal tile is positioned on the counter-mask by means of two pins (see Fig. 4) that are glued to each
crystal with their axis parallel to the direction of the average crystalline plane.
Once each crystal is positioned on the counter-mask, the lens frame is glued to all the crystals. The lens frame
is made of carbon fiber and is obtained starting from a mould. The lens is then separated from the counter-mask
by means of a chemical treatment that dissolves an aluminum cup that covers the pin base closest to the crystal.
Thus the assembling of the DM lens comprises the following main activities:
• Proper alignment of each crystal on the X–ray optical bench;
• Mounting of two support pins and bonding of pins to each crystal on the bench (see Figs. 4 and 5);
• Positioning of all crystals (with support pins) on the counter-mask;
• Bonding of all crystals to the lens frame;

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Figure 3. DM counter-mask. The counter-mask is made of stainless steal. The holes, 2 per each crystal tile, are apparent.
Figure 4. Configuration of a crystal provided with 2 pins.

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Figure 5. A view of the set up for the crystalline plane determination and control and for the pin positioner.
• Chemical etching of the support pins, and removal of the counter-mask.
3.2 Test facility
The set up for the crystalline plane determination and for the alignment of the two pins with the crystalline
plane direction, along with the gluing system of the pins to the crystal tiles, is shown in Fig. 10.
It is located in the X–ray facility LARIX (LARrge Italian X–ray facility) of the University of Ferrara (for a
description see Ref. 15). The gluing is performed when the crystal correctly reflects the X–ray beam toward the
lens focus.
A view of the experimental apparatus for assembling and testing the lens DM is shown in Fig. 6. The
apparatus includes an X–ray generator tube (see Fig. 7) with a Tungsten target, a fine focus of 0.4 mm radius, a
maximum voltage of 150 kV and a maximum power of ∼ 200 W. The X–ray tube, mounted on (X,Z) translation
stage, is located in a lead box in which a hole is made in correspondence of the X–ray. The X–ray photons coming
out from the hole are collimated by a pyramidal collimator, at the end of which it is mounted a Tantalum slit
with selectable aperture along the vertical and the horizontal directions, both perpendicular to the pyramid axis.
The slit can be also translated perpendicularly to the pyramid axis (see Fig. 8). The slit defines the cross section
of the X–ray beam that irradiates the crystal main surface.
The radiation coming out from the slit collimator is used to both determine the mean lattice crystalline plane
of the crystal tile and to align the pins to the X–ray beam. The image and spectrum of the X–ray beam, either
the direct one or that transmitted through the crystal tile or that diffracted, can be detected. The available
detectors include an X–ray imager, a cooled HPGe detector and a position sensitive scintillator detector (see
Fig. 9). All of them can be rotated along the horizontal axis, and can be translated along the vertical and along
the horizontal. The position resolution of the X–ray imager is 0.3 mm.
3.3 Determination of the average crystalline plane
In order to determine the direction of the average lattice plane of the mosaic crystal tiles using the set up shown
in Fig. 5, the crystal tile is located on a crystal positioner. This can be rotated around a vertical axis and tilted
along two orthogonal axes until the diffracted beam from two symmetrical orientations of the crystal tile give
symmetrical images and coincident spectra (see Fig. 10). An accuracy better than 10 arcsec in the determination
of the average crystalline plane is achieved.
3.4 Alignment of the pin axis to the average crystalline planes
The 2 pins needed for each crystal are mounted on the pin positioner (see Fig. 5). This can be rotated along a
circle with its center located in the vertical axis of the crystal positioner, and can be tilted along two orthogonal
directions like the crystal positioner. The pin axis is requested to be parallel to the beam axis, previously made
parallel to the average crystalline plane. This alignment is obtained by using X–ray shadow projected by two
Tungsten crosses located along the pin positioner, with their axes parallel to the pin axes (see Fig. 11).

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Figure 6. A view of the apparatus for assembling the lens DM. The apparatus is located in the LARIX facility of the
University of Ferrara.
Figure 7. A view of the X–ray generator tube, located in a lead box.

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Figure 8. A view of the pyramidal collimator during its assembling phase. In front of the collimator a movable Tantalum
slit is visible.
Figure 9. A view of the detector set. The largest detector is the X–ray imager.

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Figure 10. Left panel: Superimposed images of the transmitted and diffracted X–ray beam, once the crystalline plane is
found. Right panel: Diffracted spectra once the crystalline plane is found.
Figure 11. Image of the X–ray shadow projected by the two crosses when their axes are almost aligned to the X–ray beam.

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4. DM ASSEMBLING STATUS
A preliminar DM model with few mosaic crystal tiles of Cu[111] with ∼ 3.5 arcmin spread is expected to be
assembled and tested in a short time to evaluate the cumulative error budget of the assembling technique. Soon
after, the DM model made of a ring of Cu[111] mosaic crystal tiles will be assembled. Results of the first rigid
Laue lens are expected in a few month time.
ACKNOWLEDGMENTS
We acknowledge the financial support by the Italian Space Agency ASI and a minor contribution by the Italian
Institute of Astrophysics (INAF). The design study was also possible thanks to the received Descartes Prize 2002
of the European Committee.
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