Post on 15-Jul-2016
Exp Astron (2005) 20:279–288
DOI 10.1007/s10686-006-9042-5
ORIGINAL ART ICLE
The gamma ray lens – an ESA technology reference study
Craig Brown · Nicola Rando · Alexander Short ·Aleksander Lyngvi · Tone Peacock
Received: 14 October 2005 / Accepted: 10 April 2006C© Springer Science + Business Media B.V. 2006
Abstract The Science Payload and Advanced Concepts Office (SCI-A) of the ESA Science
Directorate conducts a number of Technology Reference Studies (TRS) on hypothetical
scientific missions that are not part of the approved Science programme. Such TRS activities
allow identifying, at an early stage, technology development needs as well as exploring future
mission scenarios.
As part of this effort, the Gamma Ray Lens (GRL) mission, a future generation gamma-
ray observatory, has been the subject of a preliminary internal investigation. The present
paper provides an overview of the science goals assumed for this study, the selection of the
reference mission profile, together with a preliminary description of the spacecraft design. The
reference payload is also described, as well as the list of technology development activities
derived from the study.
1. Introduction
Technology Reference Studies (TRS) are conducted by the Science Payload and Advanced
Concepts Office (SCI-A) of the ESA Science Directorate, with the aim of establishing key
technology development requirements necessary for the realisation of similar, future science
missions [1]. A TRS consists of the investigation of a hypothetical future mission of scientific
worth that is not currently part of the ESA science programme. Each study aims to highlight
areas requiring technology development, as well as to establish mission drivers and areas
of complexity. There are four primary areas for investigation within the TRS programme:
planetary science, fundamental physics, solar physics and astrophysics missions.
Increasing sensitivity of a gamma-ray mission is widely considered as the next important
development in gamma-ray astronomy. The required leap in sensitivity implies the need of
focussing optics that, in this high-energy range, is very difficult to achieve. The Gamma Ray
Lens (GRL) was an ideal candidate for an astrophysics TRS due to its challenging nature,
C. Brown (�) · N. Rando · A. Short · A. Lyngvi · T. PeacockScience Payload and Advanced Concepts Office, European Space Agency, ESTEC, The Netherlandse-mail: craig.brown@esa.int
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280 Exp Astron (2005) 20:279–288
and was studied in order to establish the technology development requirements needed to
realise such a mission in the future.
This paper will introduce the reference mission requirements used in this study, and will
present the preliminary mission design for the Gamma Ray Lens.
2. Reference science requirements
In order to conduct the Technology Reference Study, preliminary science requirements need
to be assumed. There are two primary source types of interest that lead to three main energy
bands for the GRL. Positron-electron annihilation, and the subsequent single, double and
triple Compton backscattering processes, leads to two energy bands: 50–200 keV and 460–
522 keV. The band widths take potential red-shift into account, as well as intrinsic broadening
of the spectral lines. The second source types of interest are sources of explosive nucleosyn-
thesis and, in particular, type Ia Supernovae due to their use as cosmological candles. The
primary energy of interest associated with these sources is the 847 keV 56Co decay line, lead-
ing to an energy band of 825–910 keV. Table 1 shows a summary of the reference science
requirements for the GRL mission. The requirements are compatible with those presented on
behalf of the gamma-ray science community at the 2005 INTEGRAL Workshop at ESTEC
[2].
3. Orbit selection
The GRL mission has the following orbit requirements to best meet the mission goals.� Long, stable observations for formation flying between two spacecraft� Typical observation times of ∼2–3 weeks (80 days for SNe Ia)� Large portion of the sky visible at any one time� Stable thermal environment� Low number of eclipse periods� Minimal radiation damage to the detectors and other systems� Low �v for orbit insertion and maintenance
Table 2 outlines the orbit tradeoff that was performed in order to establish the most appropriate
orbit for the Gamma Ray Lens. Note that a 5-point scale, ranging from –2 to +2, is used to
demonstrate in detail the advantages and disadvantages between the differing orbit types.
Table 1 Summary of preliminary assumed science requirements
Attribute Requirement
Energy band 425–522 keV, 825–910 keV, 50–200 keV
Effective area 10000 cm2 @ 511 keV, 5000 cm2 @ 847 keV
Angular resolution 1 Arcminute
Energy resolution 2 keV @ 600 keV
Line sensitivity ∼5 × 10−7 ph.cm−2s−1
Continuum sensitivity ∼10−8 ph.cm−2s−1
Typical integration time ∼106 sec. @ 511 keV, 2 × 105 sec. for SNeIa
Sun restraint angle 30◦, half cone
Nominal mission lifetime ∼10 years (extendable to 15)
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Exp Astron (2005) 20:279–288 281
Table 2 Outline of the trade-off between orbit types based on the GRL mission requirements
Mission requirement LEO GEO HEO L2 Weighting
Stability and maintenance �v −2 0 −1 2 1
Observation period −2 −1 0 2 1
Visibility of the sky −2 0 1 2 1
Thermal environment stability −1 1 0 2 0.5
Eclipse periods −2 −1 −1 2 0.5
Radiation environment −1 1 −1 1 0.25
Communications and ground operations 0 2 1 0 0.25
Launcher capacity 2 −2 0 −1 1
Total −8 0 −1 10 –
Weighted total −5.75 −2.25 −0.50 7.25 –
Table 3 Laue lens dimensions for the MAX [3], Soyuz Fregat and Ariane 5 configurations
Configuration Focal length [m] Rin Ge [cm] Rout Ge [cm] Rin Cu [cm] Rout Cu [cm]
MAX 133 97 110 87 96
Soyuz fregat 436 318 360 285 314
Ariane 5 504 365 450 328 363
It is clear from this trade-off that an orbit at L2 is the most desirable. For the purpose of
this study it has therefore been assumed that the GRL will utilise a Halo orbit at L2.
4. The gamma ray lens
The energy bands of interest for the GRL mission (see Table 1) suggest two separate optic
technologies: Multilayer Silicon Pore Optics, expected to perform well up to 200 keV, and
Laue crystals which are suitable for the focusing of gamma rays from 200 keV to 2 MeV. Due
to the focusing geometry of the Laue lens, a large (∼500 m) focal length is required. This
implies the need for two separate spacecraft: A focusing optic spacecraft (OSC) housing the
lens and a detector spacecraft (DSC) orbiting at the optic focal spot.
Two mission profiles were investigated as part of the GRL TRS: a medium sized mission
utilising two separate Soyuz Fregat launchers in a dual launch scenario and a single Ariane
5 launcher in a single launch scenario. The smaller Soyuz Fregat launcher capacity results
in a limited payload capability. A silicon pore optic is not included in the Soyuz payload,
meaning that only the two higher energies of interest are considered in this configuration.
The primary advantages of using the larger Ariane 5 configuration are: (1) a larger payload
capacity allowing observation of all energy bands of interest at increased effective areas; (2)
the removal of any problems associated with a dual launch; (3) further potential for payload
expansion.
For comparison, the MAX mission has been considered in this study as a third, smaller
mission concept. MAX is a gamma ray focusing mission based on Laue crystals, proposed
by CESR (Centre d’Etude Spatiale des Rayonnements), Toulouse [3]. The mission is sized
to fit both detector and lens spacecraft into a single Soyuz Fregat launcher. The dimensions
and specifications for MAX were input into the models developed for the Gamma Ray Lens
TRS and are also presented in this paper in order to compare the GRL with MAX.
Table 3 outlines the key dimensions of the Laue lens used for the MAX, Soyuz Fregat and
Ariane 5 profiles.
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282 Exp Astron (2005) 20:279–288
Fig. 1 The modelled effective area for the Ariane 5, Soyuz Fregat and MAX Laue Lens configurations
An effective area and sensitivity model was created in order to analyse and compare
the various Laue lenses proposed for different GRL configurations. Figures 1 and 2 show
the effective area and sensitivity results, respectively, for the Soyuz Fregat and Ariane 5
configurations. The reported dimensions of the MAX lens were also input into the model,
with the results shown here for comparison.
Note that a real measured SPI INTEGRAL background [4], was used in the GRL sensi-
tivity analyses presented in Figure 2. As such we have assumed a SPI INTEGRAL-style Ge
spectrometer with a similar background rejection capability through anticoincidence shield-
ing.
It can be seen from Figures 1 and 2 that the Ariane 5 configuration comfortably meets
both effective area and sensitivity requirements outlined in Table 1. An improvement of ∼100
times better sensitivity than SPI INTEGRAL is achieved by the Ariane 5 configuration.
5. Preliminary spacecraft design
For the purpose of this section, only the Ariane 5 configuration will be discussed. As noted,
two separate spacecraft flying in formation are required for the GRL mission: the OSC
housing the Laue lens and Silicon Pore Optic, and the DSC supporting the SPI INTEGRAL-
style germanium detector. Here, the preliminary spacecraft design is described.
Figure 3 shows a diagram of the deployed OSC. The cylindrical bus was chosen due to the
ring structure of the Laue lens and the ability to more easily stow the large structure during
launch. In order to maintain a clear load path through the spacecraft during launch, the OSC
bus was designed to match the diameter of the 2624 cm Ariane 5 launch adapter. Ensuring a
clear load path will minimise the stress on the stacked configuration during launch.
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Exp Astron (2005) 20:279–288 283
Fig. 2 The modelled sensitivity for the Ariane 5, Soyuz Fregat and MAX Laue Lens configurations, assuminga spectrometer based on an array of cooled Ge detectors
The ring-shaped bus was used due to the necessity of an unobstructed view for the multi-
layer silicon pore optic. Housing the silicon pore optic inside this ring provides a clear line of
sight between the optic and the detector spacecraft. The silicon pore optic will be designed
to have a focal spot that coincides with that of the Laue lens, thus enabling simultaneous
observations in different energy ranges.
The deployment mechanism is designed to fit the lens within the Ariane 5 fairing during
launch and then deploy the lens to the full 9 m diameter after spacecraft separation. It can
be seen that the lens is composed of 30 separate petals. These are sized to simplify the
construction, metrology and testing of the lens. When in stowed configuration, the mass of
the lens is concentrated towards the top of the OSC bus. The height of the bus is therefore
determined by the need to secure the large crystal mass in the stowed configuration, with
stiffening rings used to re-enforce the structure.
The thrusters are positioned to allow for full, three-axis stabilisation and transverse motion.
Star trackers and sun sensors are placed around the craft providing attitude measurements,
while the antennae provide omni-directional communication capability.
Figure 4 shows the DSC spacecraft configuration. The bus consists of two main parts: an
outer octagonal wall and an internal cylindrical wall. The stacked configuration is a primary
driver in the design of the DSC, as this spacecraft has to be able to support the large mass
of the OSC during launch. The cylindrical wall is designed to transfer the load path through
the stacked configuration (Figure 5), and has a 2624 cm diameter to match both the OSC
diameter and the Ariane 5 adapter.
The main detector payload, a cooled germanium spectrometer, is placed in the centre
of the spacecraft. The spectrometer assumed in the GRL study consists of an array of 52
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284 Exp Astron (2005) 20:279–288
Fig
.3(a
)A
3D
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ture
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Exp Astron (2005) 20:279–288 285
Fig
.4(a
)A
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ns
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286 Exp Astron (2005) 20:279–288
Fig. 5 (a) A 3D drawing of theGRL stack inside the Ariane 5short 5660 SPELTRA fairing and(b) the dimensions of the stackedconfiguration
hexagonal germanium detectors, covering an area of 405 cm2. An anticoincidence vetoing
shield surrounds the detector array at the focal plane.
The spacecraft thrusters were positioned on the DSC to allow full three-axis stabilisation
and transverse motion. Star trackers and sun sensors are placed around the craft to provide
attitude measurements, while the antennae on the craft provide omni-directional communi-
cation capability.
The formation-flying package is assumed to be similar to the one baselined for the XEUS
mission [5], modified to account for the large increase in focal length from 50 m to ∼500 m.
The metrology system has three subsystems; (1) coarse radio metrology, (2) fine radio metrol-
ogy and (3) optical metrology. Radio metrology will bring the two spacecraft within 120 m
of each other with an accuracy of a few centimetres. The optical metrology system for XEUS
is very accurate at a focal length of 50 m (∼10’s μm laterally and 100’s μm longitudinally).
The attitude requirements for the GRL are less stringent than that of XEUS, being ± 0.08 m
laterally and ± 1 m longitudinally based on a 1 σ point spread function of 60 cm2. It is
therefore assumed that this system can be scaled for the GRL formation-flying package.
The larger, more active part of the metrology system will be located on the DSC. A laser
will be located on the DSC and is reflected from the OSC in order to perform the optical
metrology measurements and both fine and coarse radio metrology can be performed by the
same set of transmitters.
6. Conclusions
The Gamma Ray Lens Technology Reference Study has highlighted a number of areas
requiring further work if such a mission is to be viable in the future. Table 4 summarises the
key GRL mission drivers and the associated areas of future activities required if the mission
is to be realised.
One of the key outcomes of the study is the fact that increasing the size of the mission
eventually results in a limited sensitivity improvement due to the role of background. In order
to become photon limited rather than background limited, vast improvements in background
rejection should be a priority.
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Exp Astron (2005) 20:279–288 287
Tabl
e4
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mm
ary
of
key
mis
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nd
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dp
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itie
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s•A
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atio
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ol
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tom
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tain
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ack
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s
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ve
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sem
od
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kg
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rd
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op
men
t
Springer
288 Exp Astron (2005) 20:279–288
The mission and spacecraft design presented here are the results of the preliminary science
goals assumed at the start of the study. Nevertheless the suggested technology development
activities would be still applicable if the science requirements would be changed.
Acknowledgements The authors would like to acknowledge the work by Alan Owens, Alex Jeanes, ArvindParmar, Christoph Winkler, Dave Lumb, Hubert Halloin, Peter von Ballmoos, Richard Griffiths and Thijs vander Laan.
References
1. Lyngvi, A. et al.: Technology reference studies, 55th international astronautical congress. Vancouver,Canada (2004)
2. Knodlseder, J. et al: Prospects in space-based gamma-ray astronomy, 39th ESLAB symposium proceedings,Experimental Astronomy 20, DOI: 10.1007/s10686-006-9031-8 (2005)
3. Von Ballmoos, P. et al.: MAX: A gamma-ray lens for nuclear astrophysics, Proc. SPIE 5168, 482–491(2004)
4. INTEGRAL special edition, Astronomy and Astrophysics 411, No. 1 (2003)5. Bavdaz, M., Lumb, D., Peacock, A.: XEUS mission reference design, Proc. SPIE 5488, 530–538 (2004)
Springer