Status of the technology of the current space missions including the Fermi gamma-ray space telescope

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Abstract

We review the status of X-rays and γ-rays space missions operational in 2009. We provide highlights of the current semiconductor technology instrumentation and briefly discuss challenges to be addressed by future missions.

Introduction

We were presented with the task to summarize the status of the technology of all space missions. We selected the field of high-energy astrophysics and classified missions according to the astrophysical messengers they can detect, i.e. cosmic rays and neutrinos, gravitational waves and photons from the entire electromagnetic spectrum. We soon realized that the number of telescopes/instruments was daunting for the time and the space allotted. Therefore, we concentrate on missions that probe the high-energy part of the electromagnetic spectrum, i.e. X-rays and γ-rays.

Fig. 1 further motivates our choice to focus on X-rays and γ-rays. It shows operating missions in X-rays and γ-rays (in 2009), an incomplete list of current missions at other wavelengths and some approved missions (up to year 2014). For the next 5 years there is a clear absence of new approved missions at higher energies. On-going efforts in the scientific community will fill this gap in the next decade, since simultaneous observations from radio to γ-rays, will be essential to optimize the scientific return of these telescopes.

Since X-rays and γ-rays are absorbed by the atmosphere,1 telescopes are deployed into orbits around the Earth at distances from 500 to 150 000 km. By detecting X-rays and γ-rays space telescopes provide knowledge of thermal (order keV) and of non-thermal emissions (>100keV) in the Universe. They probe extreme processes in Nature dominated by strong general relativistic fields and large magnetic fields (e.g. 1012G) Among the detailed objectives one finds

  • to study hot plasmas and their corresponding dynamics,

  • to understand the origin of X-ray and γ-ray backgrounds and cosmic-rays,

  • to explain how elements form in astronomical sources and their surroundings (nucleosynthesis) and how galaxies and clusters form and evolve,

  • to study the mechanism behind cosmic accelerators, how energetic relativistic outflows produced in astronomical sources propagate and accelerate, and what their particle and radiation contents are, and

  • to search for physics beyond the standard model of particles and interactions.

To accomplish these scientific goals, the ideal detector should have excellent spatial, energy and timing resolution, high quantum efficiency and sensitivity, large effective area (and field of view) and small deadtime. The science goals will determine the choice between pointing and all-sky survey modes. To simplify operations in space one should minimize dissipated power, mass, and avoid combustibles, complex cooling schemes and non-radiation-hard materials. These missions are typically approved for a period of two to five years. When successful, it is not uncommon for them to operate continuously for as long as 10 years.

In this paper, we provide insights on the instrumentation used in this field and illustrate, with an example, the importance of a technological breakthrough in silicon microstrip detectors used for γ-ray telescopes.

The choice of detector technology depends on their operating energy range, i.e. whether X-ray and γ-ray interactions with matter are dominated by photoelectric, Compton scattering or pair production processes. The choice of low-Z materials is more appropriate for detectors that measure Compton scattering, while high-Z materials are used when the other two processes dominate.

Table 1 summarizes the properties of main semiconductor materials currently in use or considered for approved space missions. Silicon is the semiconductor of choice for energies below 10 keV, and most of the instruments rely on Charge Coupled Devices (CCDs) or Silicon PIN diodes (for a nice review of CCDs see Ref. [2]). For energies greater than 50 keV, the Compton cross-section in silicon is larger than that of photoelectric emission. Therefore, silicon is used for hard X-rays/soft γ-rays as a low-Z absorber in Compton cameras. At higher energies, where the pair conversion cross-section dominates, silicon microstrip detectors are considered the best option for tracking devices (preceded by a high-Z converter).

The hard X-ray range of few 100s of keV is dominated by CdTe and CdZnTe, which are used either as high-Z scatterer in Compton cameras or as detector arrays in non-focusing telescopes (see a definition below). Improvements in the last 10 years have indicated that CdTe and CdZnTe are strong contenders for the future missions (for a nice review see Ref. [3]). They can operate with passive cooling or at room temperatures, because leakage currents are not large due to the relatively wide band-gap energy.

Germanium becomes the material of choice for MeV energies (for a nice review see Ref. [4]). Although Ge provides the best energy resolution, it requires cryogenic temperatures; not straightforward for operations in space.

Table 2, extracted from Ref. [5], shows some of the properties of inorganic scintillators used either as calorimeters or as anti-coincidence shields in operational X-ray and γ-ray missions or considered for approved missions. The light yield is measured relative to that of NaI(Tl), the mean free path (λ) is for 511 keV photons, Tsci corresponds to the scintillating time and ρ is the density. As argued in Ref. [5] Bismuth Germanate (BGO) is a great choice for high energy astrophysics applications due to its large stopping power and availability of large volumes.

In the next sections we do not describe all instruments in the current missions, but instead highlight those relevant to this review. A telescope that collect photons with detectors located at the focal plane is referred to as focusing telescope.

Section snippets

X-ray and γ-ray missions

Table 3 summarizes the five X-ray missions operational in 2009. The third column shows the number of instruments aboard each mission. The energy range displayed in the fourth column corresponds to the total range covered by the mission and not necessarily that of each instrument. To understand details in gaps of energy coverage the reader should consult the references for each instrument or mission at the end of this paper.

We can briefly overview the operational X-ray missions in the following

Non-focusing telescopes

Non-focusing telescopes typically employ collimators and coded masks to determine the direction of incident photons and the corresponding source positions. Fig. 2 illustrates how the spatial resolution is determined for both methods. Collimators are followed by layers of detectors, and the segmentation is fixed by the width of the collimator walls. Coded masks patterns cast shadows on detectors, and the segmentation depends on the detector layout.

If one uses different scintillators (see Table 2

Focusing telescopes

With the exception of RXTE, all current X-ray missions employ focusing telescopes. To cope with the X-ray background of a few 1000 counts per second and faint signals from astronomical X-ray sources (10−15−16 ergs/cm2/s), the detection and collection areas are decoupled. Large collecting areas are realized by nesting concentric shells of X-ray mirrors around a mast that can be as long as 7.5 m (e.g. XMM). The effective area is limited by the size of the outer-shell mirrors that are typically of

Summary

Table 6 summarizes the largest sensitive areas for semiconductor materials used in X-ray and γ-ray missions operational in 2009. The Fermi LAT instrument clearly demonstrates that single-sided silicon microstrip technology is robust for space applications. Large area detectors (m2) using other semiconductor materials may happen in the next decade. Note that the MAXI CCDs are not focal plane detectors. CCD arrays placed in focal planes are typically smaller than that.

The soft X-ray missions

Acknowledgments

I am grateful to Junji Haba and the organizers of the First International Conference on Technology and Instrumentation in Particle Physics. I was honored by their invitation and impressed with the vibrant program and excellent discussions during the conference. I should acknowledge the encouragement and key discussions with Tadayuki Takahashi (ISAS/JAXA). Many thanks to G. Kanbach and J. Greiner (MPE), N. Gehrels (GSFC/NASA), H. Tajima, T. Tanaka, S. Kahn, T. Kamae (SLAC National Accelerator

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