Space based γ-ray detectors

Space based γ-ray detectors

Sara Rydbeck

Experimental techniques in particle astrophysics

1 June 2007

Outline

• What are γ-rays? historical highlights & physical processes

• Science goals: γ-ray bursts & indirect dark matter search

• Basic principles of detection: spectrometers & imagers + dealing with background & orbital environment

• Experiments: CGRO, GLAST and AGILE

• Οutlook

• tens of keV & up - “soft γ-rays” overlap with “hard X-rays”

• Very high energy γ-rays ~ 100 GeV-10 TeV

• Ultra high energy γ-rays ~10 TeV & up

• Characteristic scale - wavelength λ

What are γ-rays?

characteristic scale of atom ~Å characteristic

scale of atomic nucleus ~fm

Atom is mostly empty space to γ-ray!

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History of γ-ray - highlights

• 1900: P. U. Villard studied uranium - saw rays not bent by magnetic fields

• 1914: E. Rutherford & E. Andrade showed it to be EM radiation

Villard named it γ-radiation (came after Rutherford’s α- and β-radiation)

Cosmic γ-rays? theoretically predicted but γ-ray astronomy had to wait for balloons & spacecraft

• 1961: 1st γ-ray telescope on Explorer-XI satellite - saw γ-ray background due to CRs in the ISM

• late 60’s/early 70’s: US Vela military satellites discovered γ-ray bursts!! when searching for secret USSR nuclear tests (astronomers got the news in -73)

• 1977: NASA plans Compton Gamma Ray Observatory (CGRO) - launched in 1991 & terminated in

2000

Physical processes giving rise to γ

• Thermal γ-rays from violent explosions, Wien’s law λ

*T=0.3: SNe

• Synchroton or cyclotron radiation from near strong magnetic fields: neutron stars, pulsars

• Brehmsstrahlung: cosmic electrons

• De-excitation of atomic nuclei: cosmic rays hit interstellar gas

• Decay of pion: cosmic proton accelerators

• Particle, anti-particle annihilation

• Inverse Compton scattering: Galaxy plane, compact objects

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γ ray bursts

• rapid flares of γ-rays

• Vela saw strong isotropy

• cosmological distances proved by BeppoSAX 1997

• model:

• relativistic speed of matter outflow -> short time-difference between flashes

• duration of burst increases with cosmological redshift

afterglow, as shown by Molinari et al. (16). The lack of evidence for strong polarization just after the afterglow onset could suggest that large-scale magnetic fields are not likely playing an important role in driving gamma-ray burst outflows. However, if the magnetic fields are carried by the outflow ejected from the central source, as is commonly hypothesized, the prediction of the polarization during the afterglow onset depends on poorly known details of the magnetic energy transfer from the outflow to the shocked medium around the burst (6, 17).

More observations tracing the early time evolution of the polarized flux, and an adequate modeling of these first phases of evolution of the outflow interaction with the matter surrounding the progenitor, will definitively settle the question. In addition, the possibility of carrying out polarimetric observations at high energies, in particular in the x-ray region, is an exciting future possibility. It could open the way for measurements during the prompt event, much closer to the progenitor, in a region where the possible effects of large-scale magnetic fields should be stronger.

References and Notes

1. C. G. Mundell et al., Science 315, 1822 (2007). Published online 15 March 2006 (10.1126/science.1140172).

2. T. Piran, Phys. Rep.314, 575 (1999).

3. P. Meszaros, Rep. Prog. Phys. 69, 2259 (2006).

4. B. Zhang, Chin. J. Astron. Astrophys.7, 1 (2007).

5. M. Lyutikov, V. I Pariev, R. D. Blandford, Astrophys. J. 597, 998 (2003).

6. B. Zhang, S. Kobayashi, Astrophys. J.628, 315 (2005).

7. M. Lyutikov, N. J. Phys. 8, 119 (2006).

8. A. Gruzinov, E. Waxman, Astrophys. J. 511, 852 (1999).

9. G. Ghisellini, D. Lazzati, Mon. Not. R. Astron. Soc.309, L7 (1999).

10. R. Sari, Astrophys. J. 524, L43 (1999).

11. E. Rossi, D. Lazzati, J.D. Salmonson, G. Ghisellini, Mon. Not. R.

Astron. Soc.354, 86 (2004).

12. J. Granot, A. Königl, Astrophys. J. 594, L83 (2003).

13. D. Lazzati et al., Astron. Astrophys. 410, 823 (2004).

14. A. Sagiv, E. Waxman, A. Loeb, Astrophys. J. 615, 366 (2004).

15. N. Gehrels et al., Astrophys. J. 611, 1005 (2004).

16. E. Molinari et al., www.arxiv.org/astro-ph/0612607.

17. Y. Fan, T. Piran, Mon. Not. R. Astron. Soc.369, 197 (2006) 18. I thank G. Ghisellini, D. Lazzati, and D. Malesani for numerous fruitful discussions.

Controlling the flow. Gamma-ray bursts originate either from (top) collapsing stars or (middle) mergers of binary stars. The resulting high-energy event (bottom) creates ultrarelativistic outflows and very bright bursts of gamma- radiation.

Dark matter annihilation

• need matter density Ω

h^2~0.1 (Ω ^B h^2 ≤ 0.02)

• For massive particle non-relativistic at freeze-out

• weak scale a few 100 GeV, SUSY neutralino 100 GeV - 10 TeV

• WIMP candidates give right relic density

• γ-ray from χχ (neutralino its own anti-particle) or χZ annihilation tells us the

WIMP mass!

Basic principles of detection

• γ-ray interaction with matter:

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γ-ray detectors

imagers spectrometers

scintillators, solid state pair production, Compton

scattering, coded-mask

eV- eV-

eV-

Crystal scintillators

• Scintillator: material that emits low-energy photons when hit by high-energy charged particle

• γ-ray enters scintillator crystal, produce charged particles

• low-energy photons collected in PMTs

• scintillator materials: NaI(Tl) CsI(Na)

• used in CGRO, GLAST...

inorganic crystal

impurity,

“activator”

Solid state detectors

• semiconductors:

• better energy resolution, less noise, better spatial resolution

• expensive, need cooling (Ge)

• similar to scintillator detector but photoelectric absorption gives rise to electron/hole instead of electron/ion

• materials: Ge CdZnTe

Compton scatter telescopes

• two-level

Compton scatter law incident angle of γ-ray

scintillator

scintillator

γ-ray

compton scatters -> scintillation

light scattered photon

complete absorption

Coded mask imaging

• partially mask opening of telescope

• detector records shadow of mask

• need of complex software tools to reproduce image

• used on INTEGRAL, Comptel...

Pair telescopes

• converter

• & tracker

layers intervened

• another tracker: silicon strip detectors

• better spatial resolution

Energy determined by

• analysis of scattering pair

• absorption of pair by scintillator detector or calorimeter

heavy metal - where the e-/e+ pair is created

• spark chamber: gas filled & criss-crossed with wires

• gas ionized by e-/e+ pair

• triggering -> electrified wires attract the free e-’s

• signal: trail of sparks

silicon strips in x-direction

silicon strips in y-direction

Background suppression

• Anti-coincidence shields: covers entire telescope - if sees charged particle, the event is not triggered

• Time-of-flight system: determines if the e-/e+ comes from the right direction

• higher energy of γ <-> lower flux;

area more important!

• EGRET: peak effective area 1500 cm^2

• groundbased air cherenkov ~ 10^8 cm^2

CGRO (Compton Gamma Ray Observatory)

30 keV-30 GeV

Burst & Transient Source experiment. 8 detectors facing outward from each corner - all sky. NaI, 20 keV - >1 MeV, γ- ray bursts.

Oriented Scintillator Spectrometer

Experiment. 4 NaI detectors, 50 keV-10 MeV. Can be pointed ->

enables background subtraction. Focusing of sources found by BATSE.

Imaging Compton Telescope. 2 layers: liquid scintillator & NaI, 1-30 MeV. Time, location, energy -> image & energy spectrum.

Energetic Gamma Ray Experiment Telescope. High- voltage gas-filled spark

chambers, energy recorded by

NaI crystals.

GLAST (Gamma-ray Large Area Space Telescope)

• 20 MeV-300 GeV, peak effective area >8000 cm^2

• LAT (Large Area Telescope)

• silicon tracker

• CsI(Tl) calorimeter

• segmented anti-coincidence detector

• GBM (Gamma-ray Burst Monitor)

- large field-of-view, will localize bursts & alert ground-based detectors

4x4 array of towers,

40x40cm^2 each

Tracker

• 19 trays x SSD & converter plate & SSD

• each tray rotated 90° with respect to next, giving 18 x-y pairs

• 12 thick converter layer, 4 thin, then 2 layers without converter foil -> good tracking

• 55000 strips/tower - many channels! ->

challenges like noise and power consumption

Calorimeter

• electromagnetic, homogenous calorimeter

• energy measurement

+ cosmic-ray background rejection

• 1500 kg & 60% of satellite mass!

• each module: 8 layers of 12 CsI(Tl) crystals

• 90° rotation between each -> x-y array - important if γ not converted in tracker

• segmentation -> distinction between EM & hadronic showers

• read-out with PIN photodiodes (not PMTs)

semiconductors: p-type, intrinsic, n-type

Anti-coincidence detector & trigger

• cosmic rays 10^4 as many as the γ, also Earth albedo particles

• ACD of plastic scintillator tiles covers top & sides of tracker, not calorimeter

• segmentation -> no confusion with backward scattered particles from inside detector

• 3-level trigger system

1. 3 succeeding x-y silicon pairs? event! check ACD 2. check alignment of track

3. event reconstruction using data from entire instrument

AGILE (Astro-rivelatore Gamma a Immagini LEggero)

• γ-ray imager (30 MeV-50 GeV) + hard X-ray imager (15-45 keV)

• GRID: Si-W tracker, CsI calorimeter, ACD

• Super-AGILE: coded-mask system of Si detector plane & thin W mask

• was launched in April

Data so far & prospects of GLAST

• Dark matter annihilations in center of Galaxy?

• dark matter profile or smoking-gun γ-line

• HESS: power-law spectrum not consistent with annihilating dark matter

• Still hope for GLAST!

• Understand background?

• More fruitful look away from galactic center?

Data so far & prospects of GLAST

• EGRET saw excess diffuse galactic flux at GeV energies

• De Boer 2005: dark matter rings

• Stecker 2007: new all-sky analysis -> isotropy

• just wrong sensitivity calibration above 1 GeV?

10^-5 10^-4

10^-1 1 10 10²

E [GeV]

E2 * flux [GeV cm-2 s-1 sr-1 ]

EGRET background signal

m_WIMP=50-70 GeV Dark Matter

Pion decay Inverse Compton Bremsstrahlung

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Model Flux [10

ph cm

s

sr

] 0.0

0.2 0.4 0.6 0.8 1.0 1.2

Observed Flux [10

ph cm

s

sr

] (b)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Observed Flux [10

ph cm

s

sr

] (a)

γ-ray lense?

• focusing -> lower background & higher sensitivity

• possible for X-rays - γ?

• Multilayer coating & Silicon Pore Optics -> max energy of 300 keV

• Laue diffraction through crystal (Bragg’s law) - large focal length

~500 m focal length - need two spacecraft, detector & optic

Many challenges! Formation flying, crystal development, multilayer & Si

optics development, etc....

References

• Agile Home Page @ Roma2, http://people.roma2.infn.it/~agile

• S. Bergenius Gavler “Counting Calories - Studies of Energy Loss in Segmented Calorimeter, Doctoral Thesis, Stockholm, 2006

• L. Bergström & A. Goobar “Cosmology and Particle Astrophysics” 2nd edition 2004

• De Boer et al “EGRET excess of diffuse galactic gamma-rays as tracer of dark matter” 2005, astro-ph/0508617

• R. Diehl “The Physics of Gamma-Ray Sources”, 2001

• ESA Science & Technology Home Page, http://science.esa.int

• GLAST home page, http://www-glast.stanford.edu/index.html

• NASA Goddard Space Flight Center, http://www.gsfc.nasa.gov

• Stecker et al “The Likely Cause of the EGRET GeV Anomaly and its Implications, astro-ph/07054311

• M. Tavani et al “The AGILE Mission and its Scientific Instruments”, 2006

• G. Zaharijas & D. Hooper “Challenges in Detecting Gamma-Rays From Dark Matter Annihilations in the Galactic Center”,arXiv:astro-ph/0603540