The expected performance of LISA relies on two main technical challenges: the ability for the spacecrafts to precisely follow the free-flying masses and the outstanding precision of the phase shift measurement. This latter constraint requires frequency stabilized lasers and efficient numerical algorithms to account for the redundant, delayed noise propagation, thus cancelling laser phase noise by many orders of magnitude (TDI methods). Recently involved in the technical developments for LISA, the goal of our team at APC (France) is to contribute on these two subjects: frequency reference for laser stabilization and benchtop simulation of the interferometer. In the present design of LISA, two stages of laser stabilization are used (not accounting for the "post-processed" TDI algorithm): laser pre-stabilization on a frequency reference and lock on the ultra stable distance between spacecrafts (arm-locking). While the foreseen (and deeply studied) laser reference consists of a Fabry-Perot cavity, other techniques may be suitable for LISA or future metrology missions. In particular, locking to a molecular reference (namely iodine in the case of the LISA Nd:YAG laser) is an interesting alternative. It offers the required performance with very good long-term stability (absolute frequency reference) though the reference can be slightly tuned to account for arm-locking. This technique is currently being investigated by our team and optimized for LISA (compactness, vacuum compatibility, ease of use and initialization, etc.). A collaboration with a French laboratory (the SYRTE) had been started aiming to study a second improved technique consisting in inserting the iodine cell in a Fabry-Perot cavity. Ongoing results and prospects to increase the performance of the system are presented in the present article.
LISA, the first space project for detecting gravitational waves, relies on two main technical challenges: the free falling masses and an outstanding precision on phase shift measurements (a few pm on 5 Mkm in the LISA band). The technology of the free falling masses, i.e. their isolation to forces other than gravity and the capability for the spacecraft to precisely follow the test masses, will soon be tested with the technological LISA Pathfinder mission. The performance of the phase measurement will be achieved by at least two stabilization stages: a pre-stabilisation of the laser frequency at a level of 10-13 (relative frequency stability) will be further improved by using numerical algorithms, such as Time Delay Interferometry, which have been theoretically and numerically demonstrated to reach the required performance level (10-21).
Nevertheless, these algorithms, though already tested with numerical model of LISA, require experimental validation, including ‘realistic’ hardware elements. Such an experiment would allow to evaluate the expected noise level and the possible interactions between subsystems. To this end, the APC is currently developing an optical benchtop experiment, called LISA On Table (LOT), which is representative of the three LISA spacecraft.
A first module of the LOT experiment has been mounted and is being characterized. After completion this facility may be used by the LISA community to test hardware (photodiodes, phasemeters) or software (reconstruction algorithms) components.
The concept of a gamma-ray telescope based on a Laue lens offers the possibility to increase the sensitivity by more
than an order of magnitude with respect to existing instruments. Laue lenses have been developed by our
collaboration for several years : the main achievement of this R&D program was the CLAIRE lens prototype, which
has successfully demonstrated the feasibility of the concept in astrophysical conditions. Since then, the endeavour
has been oriented towards the development of efficient diffracting elements (crystal slabs) in order to increase both
the effective area and the width of the energy bandpass focused, the aim being to step from a technological Laue lens
to a scientifically exploitable lens. The latest mission concept featuring a gamma-ray lens is the European Gamma-
Ray Imager (GRI) which intends to make use of the Laue lens to cover energies from 200 keV to 1300 keV.
Investigations of two promising materials, low mosaicity copper and gradient concentration silicongermanium
are presented in this paper. The measurements have been performed during three runs: 6 + 4 days at the
European Synchrotron Radiation Facility (Grenoble, France), on beamline ID15A, using a 500 keV monochromatic
beam, and 14 days on the GAMS 4 instrument of the Institute Laue Langevin (Grenoble, France) featuring a highly
monochromatic beam of 517 keV. Despite it was not perfectly homogeneous, the presented copper crystal has
exhibited peak reflectivity of 25 % in accordance with theoretical predictions, and a mosaicity around 26 arcsec, the
ideal range for the realization of a Laue lens such as GRI. Silicon-germanium featuring a constant gradient have
been measured for the very first time at 500 keV. Two samples showed a quite homogeneous reflectivity reaching
26%, which is far from the 48 % already observed in experimental crystals but a very encouraging beginning. The
measured results have been used to estimate the performance of the GRI Laue lens design.
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 Si1-x-Gex 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.
With focusing of gamma rays in the nuclear-line energy regime establishing itself as a feasible and very promising approach for high-sensitivity gamma-ray studies of individual sources, optimizing the focal plane instrumentation for gamma-lens telescopes is a prime objective. The detector of choice for a focusing nuclear-line spectroscopy mission would be the one with the best energy resolution available over the energy range of interest: Germanium. Using a Compton detector focal plane has three advantages over monolithic detectors: additional knowledge about (Compton) events enhances background rejection capabilities, the inherently finely pixellated detector naturally allows the selection of events according to the focal spot size and position and could enable source imaging, and Compton detectors are inherently sensitive to gamma-ray polarization. Suitable Ge-strip detectors that could be assembled into a sensitive high-resolution focal plane for a gamma-ray lens are available today. They have been extensively tested in the laboratory and flown on the Nuclear Compton Telescope balloon from Ft. Sumner in 2005. In the course of the ACT vision mission study, an extensive simulation and analysis package for Compton telescopes has been assembled. We leverage off this work to explore achievable sensitivities for different Ge Compton focal plane configurations - and compare them to sensitivities achievable with less complex detectors - as a step towards determining an optimum configuration.
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 cm3. 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 1h12' 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.
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.
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 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.
Fresnel lenses can focus gamma-rays by using a combination of diffraction and refraction. Such lenses (and variations on them in which the chromatic aberration is much reduced) have the potential for revolutionizing gamma-ray astronomy. Diffraction-limited lenses of several meters in size are feasible and do not require high technology for their manufacture. Focal lengths are long - up to a million kilometers - but developments in formation flying of spacecraft make possible a mission in which the lens and detector are on two separate spacecraft separated by this distance. A telescope based on these principles can have angular resolution better than a micro second of arc - sufficient to resolve the event horizon of black holes in the nucleii of AGNs. At the same time, the sensitivity can be three orders of magnitude better than that of current instrumentation.