gamma-ray lenses – taking a deeper look at sites of...

GammaGamma--Ray LensesRay Lenses

v. Ballmoos et al, 2004

Challenges of MeV InstrumentationChallenges of MeV InstrumentationHow GammaHow Gamma--Ray Lenses workRay Lenses workWhat a Lens Mission could look likeWhat a Lens Mission could look likeSuitable Focal PlanesSuitable Focal PlanesPerformance Estimates & OutlookPerformance Estimates & Outlook

Gamma-Ray Lenses Cornelia Wunderer, SSL, UC BerkeleyKITP, Santa Barbara, Feb 27, 2007

Imaging at MeV EnergiesImaging at MeV Energies

• Conventional lenses cannot be used

• Collimators, earth occultation• Coded aperture imaging (SIGMA, INTEGRAL, SWIFT)

• Imaging through reconstruction of individual photons’ interactions– Compton-scatter (COMPTEL on CGRO, ACT)

– Pair events (COS-B, EGRET on CGRO, GLAST)

• Multilayer grazing-incidence Wolter optics (up to a few 100 keV)

• Focusing via Laue - Diffraction (CLAIRE)

• Focusing with Fresnel Phase-shift Lenses (G. Skinner, 2000)


Challenges …Challenges …• Materials most transparent at MeV energies

massive detectors neededcomplete shielding impossible

• Imaging traditionally via collimation, coded mask, or single-photon interactions (Compton, Pair)

• Traditionally, collection area = detection area

• Huge and complex instrumental backgrounds– Induced by space environment– Often instrumental lines underlying

astrophysical onesbackground prediction crucial for instrument development!

50 keV-30 MeV

Photon cross sections in Ge

RHESSI spectrometer background simulations (red) compared with measured background (black). Simulations use the GEANT-based MGGPOD suite (Weidenspointner et al., 2004) 102 103 104 keV

Wunderer et al., 2004


(near) Future Astrophysical (near) Future Astrophysical Observatories at MeV EnergiesObservatories at MeV Energies

Gamma-Ray LensAdvanced Compton Telescope

(Boggs et al. 2005)(v. Ballmoos et al. 2005)• Sensitive Survey (explore the unexpected!)

• Mapping diffuse emission• Transient detection• Energy range 200 keV – 10 MeV• Angular resolution ~ 1º


Why a Lens?Why a Lens?• The Advanced Compton Telescope (ACT) will push the

envelope of achievable sensitivity for a survey-type imaging spectroscopy γ-ray mission to its limits

• decoupling of instrument effective area and detector volume is the only way to further increase point-source sensitivity in the heavily background-dominated MeV energy regime

• This IS possible even at MeV energies, using e.g. LaueLenses (v. Ballmoos et al., 2004)


Principle of Principle of LaueLaue LensesLenses

• Photon diffraction on crystal planes in the volume of a crystal(follows Bragg’s law)

• Diffraction angle determined by crystal plane spacing and photon energy

• Diffraction angles of ~ 1° feasible at 1 MeV


Mosaic CrystalsMosaic Crystals• A perfect monocrystal’s bandpass is a few eV

at most – too narrow to be very useful for astrophysical observations even of single lines

• Mosaic crystals allow bandpasses of several keV per crystal with diffraction efficiencies up to ~ 30%

• These can be combined to achieve bandpassesof hundreds of keV for a ~ 1 arcmin FoV

• Lens crystals arranged in rings, crystals at different radii diffract photons of different energies

De

tec

tor

Crystal

Crystallite


Focal Spot SizeFocal Spot Size• Crystal size

(dominates at short focal lengths)– Assuming uniform crystals, photons

impinging on an individual crystal are diffracted the same way

– Thus the minimum focal spot size is the size of an individual crystal (and bigger crystals are easier to mount)

• Mosaicity(dominates at long focal lengths)– Mosaicity results in photons of one

energy being diffracted at slightly different angles

– Focal spot widens with longer focal length

• Focal length

assuming no mosaicity

crystal size dominates

mosaicity dominates

For 30” mosaicity (FWHM gaussian) and 2 cm crystals, equal effects at ~150 m focal length


LaueLaue Lenses WorkLenses Work• Ge mosaic crystal lens tested on a balloon at

CESR, France (Halloin, 2003)• 576 crystals concentrated 170 keV photons onto

monolithic Ge detector array at focal length of 2.77 m with average efficiency of ~ 9%

• Crab detection in a short technology-demonstration flight

v. Ballmoos et al, 2004Halloin et al, 2004


A Lens for a ~MIDEXA Lens for a ~MIDEX--size Instrumentsize Instrument

Calculated effective Ge-Cu lens diffraction areas for MAX in 2 energy bands, based on CLAIRE experience (N. Barriere, 2005)

450 keV – 530 keV band

Effa

rea

[cm

2 ]

800

Energy [keV]

800 keV – 920 keV band

Effa

rea

[cm

2 ]

600

Energy [keV]

Cu and Ge mosaic crystals at different orientations, lens radii, and tilt angles combine to cover a large energy band.Crystals weigh 85kg and 75kg for the two segments, respectively.


200 keV 200 keV –– 2000 keV 2000 keV BandpassBandpass• Combine rings of

different crystal materials

• Make use of 2nd and 3rd order diffraction

Continuous energy coverage from 200 keV – 2 MeVLens effective areas of 1000 cm2 and more

1st order 2nd order

3rd order

3.6 m 5 m max

(H. Halloin, 2005)


Added Benefit: some ImagingAdded Benefit: some Imaging• Field of View: few arcmin,

Resolution: ~ arcmin– Unambiguously identify sources– Resolve AGN jets or SNRs– Downside: can’t cover e.g. close

SNRs in one observation• Point-source location to

somewhat higher accuracy

• Plots show beam spot simulations for 0”, 15”, 30”, and 45” off-axis– Assuming 30” crystal mosaicity– Assuming MAX lens

• Deconvolution should enable imaging of strong sources in a few-arcmin field (beam images created using

SimuLens, H. Halloin 2005)

cm-3 -2 -1 0 1 2 3

cm

-3

-2

-1

0

1

2

3

Beam_0asec

cm-3 -2 -1 0 1 2 3

cm

-3

-2

-1

0

1

2

3

Beam_10asec

cm-3 -2 -1 0 1 2 3

cm

-3

-2

-1

0

1

2

3

Beam_20asec

cm-3 -2 -1 0 1 2 3

cm

-3

-2

-1

0

1

2

3

Beam_30asec

Lens angular responseLens angular response3 point source lens response

(event density on detector plane)on axis, 2’ off axis, 4’ off axis

GRI imaging of 4 point sources

model

reconstructionGRI imaging:• FOV defined by detector area (20 x 20 cm ⇒ 10 arcmin diameter)• Dithering allows imaging with ~ arcmin resolution (precision set by mosaicity)

(Knödlseder et al., GRI consortium)


Principles of a Principles of a LaueLaue MissionMission• Laue lens of Ge, Cu, or other crystals at ~ 100 m from the focal plane

– Several wide energy bands, or continuous coverage out to ~ 2 MeV– Lens effective area ~ 500 – 2000 cm2

– Lens radius of 1 m – few m • Small, compact focal plane instrumentation

• ~ 100 m focal length starts to require formation flying, HOWEVER:– Alignment and station keeping requirements modest compared with other

formation-flying missions envisioned(roughly distance ± 10 cm; lateral offset ± 1 cm, knowledge ± 1 mm; lens/det. tilt ± 1°; lens pointing ≤ 10”)

• Thus such a mission also provides a convenient stepping stone for mission concepts requiring more advanced formation-flying


GammaGamma--Ray ImagerRay Imager• A “Gamma-Ray Imaging Observatory” is part of ESA’s Cosmic Vision

for 2015-2025• It will “probably rely on diffractive optics” – i.e. use Laue Lenses – and

“shall cover the energy range 50 – 2000 keV”

http://gri.rm.iasf.cnr.it/

GRI: the Gamma-Ray Imager Mission• The GRI working group intends to respond to an ESA AO expected within the next “few weeks”

– 50 – 200 keV: Multilayer or Coded Mask

– 150 keV – ~1 MeV (goal 1.3 MeV): Laue Lens

– multinational collaboration, US currently represented by SSL/Berkeley and Argonne NL

GRI sketchGRI sketch

60 - 80 m

detector spacecraft

DSC

3.8

moptics spacecraft

face-onedge-on

crystal lens

structure / spacecraft

mask

detector

sketch not to scale


Lens crystalsLens crystals

Cu crystals with mosaicities of 30-60 arcsec are available (courtesy: ILL)

Lens crystals:• Ge crystals used for CLAIRE balloon • Cu crystals with required properties available• Production & mounting of several 10000 is a challenge• R&D work underway

SiGe crystal ingot(courtesy: IKZ)

CLAIRE Ge crystal lens (courtesy: P. von Ballmoos)

Cu crystal monochromator(courtesy: ILL)


Crystal developmentsCrystal developments

Silver and gold crystalsBetter diffraction efficiency for the same mass

Composite crystalsBuild a “gradient” crystals from individual crystal wafers (e.g. Si)(courtesy: N. Barrière)

Gradient crystalsReduce the backreflectionEfficiency gain of factor ~2 measured(courtesy: B. Smither)

Goal: Improve the efficiency of crystals


Lens performanceLens performance

Lens summary (optimisation studies still ongoing):• 13772 Ge crystals, 20697 Cu crystals• Lens effective area:

500 - 900 cm2 @ 160 - 520 keV & 700 cm2 @ 800 - 900 keV • Mosaicity (= angular resolution): 30” (high-energy) - 60” (low-energy)

60"30"



Focal Plane Detector OptionsFocal Plane Detector Options• Ge offers best energy resolution available at ~ 2-3 keV FWHM,

⇒ detectors of choice for MeV spectroscopy

• Monolithic Ge detector would be simplest• Simple segmentation will allow some

background rejection (distinguish lens focal spot) without adding much complexity

• Compton-Detector focal plane enables – Better capability for background rejection– Inherently finely pixellated detector naturally

allows selection of events according to focal spot size and position – and enables imaging

– Inherently sensitive to γ-ray polarization

Boggs et al, 2004Ge-strip Compton detectors have been

tested in the lab and on the NCT prototype balloon in Spring 2005

at the cost of complexity (number of channels, active cooling, …) and compact design


GeGe Configurations ConsideredConfigurations Considered

• Leveraging off ACT Concept Study tools (coll. with G. Weidenspointner and A. Zoglauer)

• Mass models identical to TGRS / built starting from existing NCT (balloon !) detectors; LBL has made 13 × 7 × 1.75 cm3 rectangular Ge detectors before

• “segmented” and “small Compton” detectors have identical detector geometry, only electrode configuration is assumed to differ ⇒ direct comparison

• Spacecraft based on CNES study of MAX mission• Tower concept and BGO shield reduce SC-originating background

Monolith Segmented SMALL 2x6 RectanglesCompton Compton


Performance PredictionsPerformance Predictions• Need to predict interactions in detectors

– From source– From background

• Background predictions from MGGPOD– Geant 3 – based package – ab-initio background simulations

(good estimates for TGRS, INTEGRAL/SPI, RHESSI)– Cosmic photons, prompt and delayed background components from cosmic-

ray proton interactions in detector and spacecraft (L2 orbit)• Must include realistic passive material and assume realistic energy

resolution and thresholds

• Segmented and Compton detectors:must apply same event (reconstruction and) selection criteria toboth datasets to obtain sensitivity estimate

http://sigma-2.cesr.fr/spi/MGGPOD/


GeGe--DetDet Performance EstimatesPerformance EstimatesMonolith Segmented SMALL

3.3-5.5·10-6 1.6·10-6

9.3·10-7

8.6·10-7

2.3·10-6

Photopeak Efficiency(after all cuts)

38% @ 51124% @ 847

~25% @ 511~19% @ 847

6% @ 5116% @ 847

14% @ 51113% @ 847

2.5 kg

380

1.7-2.5·10-6

6.6-9.9·10-6

2.5 kg

10

2 × 6511 keV sensitivity[ph cm-2 s-1]

5.0-7.1·10-6 8.5·10-7

530 keV sensitivity[ph cm-2 s-1]

5.4·10-7

847 keV sensitivity[ph cm-2 s-1]

3.2-4.6·10-6 5.4·10-7

Total mass active Ge 1.1 kg 10.2 kg

Number of Channels 1 2376

847 keV sensitivity(3% FWHM) [ph cm-2 s-1]

1.2-1.6·10-5 1.4·10-6

(for 1000 cm2 eff. area lens, 106s, 3σ, realistic lens beam)

Andreas Zoglauer

Expected perfor-

mance of CZT and Ge based

focal plane

detectors for GRI

HEAD meeting2006 San Francisco

Ge, CZT, and Si-CZT line sensitivities

Ge Small Ge Medium CZT Small CZT Small w Collimator

Si+CZTLarge

Geometry (not to scale)

2.8·10-6

2.4·10-6

< 1.7·10-6

1.6·10-6

1.6·10-6

847 keV sensitivity[ph cm-2 s-1] (ΔE/E=3%)

2.3·10-6 1.4·10-6 2.5·10-6

8.5·10-7511 keV sensitivity[ph cm-2 s-1]

2.2·10-6

Using only Compton interactions, 106 s effective observation time, optimized event selections

Trends: • Larger geometries better sensitivity• Ge better narrow line sensitivity due to better energy resolution• Collimator only helps at lower energies (activation dominates at

higher energies) (Zoglauer et al., IEEE 2006)

Andreas Zoglauer

Expected perfor-

mance of CZT and Ge based

focal plane

detectors for GRI

HEAD meeting2006 San Francisco

Continuum sensitivity

Crab-like spectrum, 106 s effective observation time, ΔE=E/2

1.0E-09

1.0E-08

1.0E-07

0 200 400 600 800 1000

Energy [keV]

Con

tinuu

m s

ensi

tivity

[ph/

cm2 /s

/keV

]

CZT small CZT small with collimatorSi+CZT large Ge medium

(Zoglauer et al., IEEE 2006)


Preliminary FindingsPreliminary Findings• Our preliminary, conservative estimates indicate that sensitivities

of 10-6 ph/cm2s – For a 511 keV narrow line– For a 847 keV 3% FWHM broadened line

are easily achievable with a Laue lens of ~1000 cm2 effective area at 511 keV and 847 keV, respectively, with a Compton focal plane

• Sensitivities for narrow lines other than 511 keV significantly better

• Compton focal plane survey sensitivities, relying on the “usual” Compton technique, around several 10-5 ph/cm2s for 511 keV, again significantly better for other narrow astrophysical lines

• These estimates are based on first guesses at detector configurations, rather than final, optimized designs!


So …So …

• Gamma-ray spectroscopy covering ~ 100 keV to above 1 MeV (or broad bands in this regime)

• A pointed instrument with FoV ~ arcmin and some imaging / point-source localization capabilities

• Secondary capabilities for all-sky monitoring in the unshielded Compton focal plane versions


…… for for SNeSNe IaIa

• Contribute to understanding SN Ialightcurve variations(correlation of 56Ni yields and brightness correction?)

• Discriminate SN Iaexplosion models

sub-Chandra-sekhar-mass

delayeddetonation

Ch-massdeflagration

15 d

50 d

25 d

(Milne et al 2004)

3 different SN Ia explosion Models


…… SN SN IaIa continuedcontinued

discriminations per 5-yr period between delayed-detonation (dd202c) and deflagration (W7) models

(Milne 2005)


NovaeNovae• Novae could be significant sources of 7Li• Measurement of 7Be decay (478 keV, T1/2=53 d) from novae

would confirm this• Flux at maximum expected at ~ 1-2·10-6 ph/cm2s

(at 1 kpc, Hernanz et al. 2004)

• A Laue mission with 511 keV sensitivity of 1·10-6 ph/cm2s could be expected to have 478 keV narrow-line sensitivities of better than 5·10-7 ph/cm2s.

• Expect line width ~ 3-7 keV (Hernanz 2004) ⇒ sensitivity around 8·10-7 ph/cm2s

Should provide 7Be decay detections for CO novae out to > 1kpc


Astronomy without RadioactivitiesAstronomy without Radioactivities

• Environs of compact objects– Spectroscopy and mapping of 511 keV signatures from positron

annihilation e.g. in microquasar jets• Polarization signatures

– γ-ray polarization can carry detailed information about source emission processes and geometry

– With a Compton detector focal plane, a Laue lens mission would provide sensitive polarization measurements

• An unshielded Compton detector focal plane would also have some capability for survey science and transient detection … … sensitivities at least several 10-5 ph/cm2/s at 511 keV in 106 s

… a few examples … :


GammaGamma--Ray Imaging at the Ray Imaging at the Diffraction Limit Diffraction Limit –– the Future ?the Future ?

• Fresnel phase-shift lenses exploit small deviation of index of refraction from 1 at MeV energies

Skinner et al., 2003

Skinner 2005

keV Skinner 2005

• Potential lens efficiency near 100%• μarcsec resolution• Negligible absorption

• Very small field of view• Energy bands ~ tens of keV @ MeV• So far at gamma-ray energies on paper only• Focal lengths of Mkm

• Instrument implementation straightforward once (sub-)μarcsec pointing on Mkmbaseline is solved


SummarySummary

• Perform detailed γ-ray line spectroscopy in nuclear-line energy band(s)• Provide order-of-magnitude sensitivity and angular resolution improvements

⇒ Distinguish between SN Ia models for tens of SN Ia in mission life⇒ 7Be decay detections for CO novae⇒ Measure 44Ti from individual SNRs …⇒ map positron emission in jets⇒ measure γ polarization, …

• Laue lenses and Ge-Strip Compton detectors have been demonstrated on balloons

• Formation flying at the modest level required is feasible today• ESA’s “Cosmic Vision” includes a (Laue) Gamma-Ray Imaging Observatory• NASA has funded Laue-lens technology development

A First A First LaueLaue--Lens Observatory could:Lens Observatory could:

…… and focusing will be required to step beyond ACT!and focusing will be required to step beyond ACT!


Questions to YOUQuestions to YOU

• What would you like to be able to observe?

• What gamma-ray (line) signature(s) do you expect?

• Should we use different benchmarks?• … ?

… I’m here through Thursday, room 2112

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