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

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Exp Astron (2005) 20:279–288 285

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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

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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