Imaging at MeV Energies

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1 Challenges of MeV Instrumentation How work What a Lens Mission could look like Suitable Focal Planes Performance Estimates & Outlook v. Ballmoos et al, 2004

2 Imaging 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)

3 Challenges Materialsmost transparent at MeV energies massive detectors needed complete shielding impossible Imaging traditionally via collimation, coded mask, or single-photon interactions (Compton, Pair) Traditionally, collection area = detection area Photon cross sections in Ge 50 kev-30 MeV Huge and complex instrumental backgrounds Induced by space environment Often instrumental lines underlying astrophysical ones background prediction crucial for instrument development! RHESSI spectrometer background simulations (red) compared with measured background (black). Simulations use the GEANTbased MGGPOD suite (Weidenspointner et al., 2004) kev Wunderer et al., 2004

4 (near) Future Astrophysical Observatories at MeV Energies Advanced Compton Telescope Gamma-Ray Lens (Boggs et al. 2005) Sensitive Survey (explore the unexpected!) Mapping diffuse emission Transient detection Energy range 200 kev 10 MeV Angular resolution ~ 1º (v. Ballmoos et al. 2005)

5 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. Laue Lenses (v. Ballmoos et al., 2004)

6 Principle of Laue Lenses 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

7 Mosaic 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 Crystal Crystallite Mosaic crystals allow bandpasses of several kev per crystal with diffraction efficiencies up to ~ 30% These can be combined to achieve bandpasses of hundreds of kev for a ~ 1 arcmin FoV Detector Lens crystals arranged in rings, crystals at different radii diffract photons of different energies

8 Focal Spot Size Crystal size assuming no mosaicity (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 crystal size dominates bigger crystals are easier to mount) Mosaicity (dominates at long focal lengths) Mosaicity results in photons of one energy being diffracted at slightly mosaicity dominates different angles Focal spot widens with longer focal length Focal length For 30 mosaicity (FWHM gaussian) and 2 cm crystals, equal effects at ~150 m focal length

9 Laue Lenses 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 technologydemonstration flight Halloin et al, 2004 v. Ballmoos et al, 2004

10 A Lens for a ~MIDEX-size Instrument 800 Eff area [cm 2 ] 450 kev 530 kev band 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. Eff area [cm 2 ] 600 Calculated effective Ge-Cu lens diffraction areas for MAX in 2 energy bands, based on CLAIRE experience (N. Barriere, 2005) 800 kev 920 kev band Energy [kev]

11 200 kev 2000 kev Bandpass Combine rings of different crystal materials Make use of 2 nd and 3 rd order diffraction 3 rd order 3.6 m 5 m max 1 st order 2 nd order Continuous energy coverage from 200 kev 2 MeV Lens effective areas of 1000 cm 2 and more (H. Halloin, 2005)

12 Added 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 Beam_0asec cm cm cm Beam_20asec 3 cm Beam_10asec cm Beam_30asec cm 3 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 cm cm (beam images created using SimuLens, H. Halloin 2005)

13 Lens angular response 3 point source lens response (event density on detector plane) on axis, 2 off axis, 4 off axis GRI imaging of 4 point sources model reconstruction GRI 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)

14 Principles of a Laue Mission 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 ~ cm 2 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

15 Gamma-Ray Imager A Gamma-Ray Imaging Observatory is part of ESA s Cosmic Vision for It will probably rely on diffractive optics i.e. use Laue Lenses and shall cover the energy range kev The GRI working group intends to respond to an ESA AO expected within the next few weeks 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: the Gamma-Ray Imager Mission

16 GRI sketch optics spacecraft detector spacecraft edge-on face-on structure / spacecraft detector 3.8 m DSC mask crystal lens m sketch not to scale (Knödlseder et al., GRI consortium)

17 Lens crystals SiGe crystal ingot (courtesy: IKZ) Cu crystal monochromator (courtesy: ILL) CLAIRE Ge crystal lens (courtesy: P. von Ballmoos) Cu crystals with mosaicities of arcsec are available (courtesy: ILL) Lens crystals: Ge crystals used for CLAIRE balloon Cu crystals with required properties available Production & mounting of several is a challenge R&D work underway (Knödlseder et al., GRI consortium)

18 Crystal developments Goal: Improve the efficiency of crystals Silver and gold crystals Better diffraction efficiency for the same mass Gradient crystals Reduce the backreflection Efficiency gain of factor ~2 measured (courtesy: B. Smither) Composite crystals Build a gradient crystals from individual crystal wafers (e.g. Si) (courtesy: N. Barrière) (Knödlseder et al., GRI consortium)

19 Lens performance 60" 30" Lens summary (optimisation studies still ongoing): Ge crystals, Cu crystals Lens effective area: cm kev & 700 cm kev Mosaicity (= angular resolution): 30 (high-energy) - 60 (low-energy) (Knödlseder et al., GRI consortium)

20 Focal 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, 2004 Ge-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

21 Ge Configurations Considered Monolith Segmented SMALL 2x6 Rectangles Compton Compton 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 cm 3 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

22 Performance 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 cosmicray 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 to both datasets to obtain sensitivity estimate

23 Ge-Det Performance Estimates Monolith Segmented SMALL kev sensitivity [ph cm -2 s -1 ] 530 kev sensitivity [ph cm -2 s -1 ] 847 kev sensitivity [ph cm -2 s -1 ] 847 kev sensitivity (3% FWHM) [ph cm -2 s -1 ] Photopeak Efficiency (after all cuts) Total mass active Ge Number of Channels 1.1 kg kg kg kg 2376 (for 1000 cm 2 eff. area lens, 10 6 s, 3σ, realistic lens beam)

24 Ge, CZT, and Si-CZT line sensitivities Using only Compton interactions, 10 6 s effective observation time, optimized event selections Ge Small Ge Medium CZT Small CZT Small w Collimator Si+CZT Large Geometry (not to scale) Andreas Zoglauer Expected performance of CZT and Ge based focal plane detectors for GRI HEAD meeting 2006 San Francisco 511 kev sensitivity [ph cm -2 s -1 ] 847 kev sensitivity [ph cm -2 s -1 ] (ΔE/E=3%) < 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)

25 Continuum sensitivity Crab-like spectrum, 10 6 s effective observation time, ΔE=E/2 Andreas Zoglauer Expected performance of CZT and Ge based focal plane detectors for GRI Continuum sensitivity [ph/cm 2 /s/kev] 1.0E E E Energy [kev] CZT small CZT small with collimator HEAD meeting 2006 San Francisco Si+CZT large Ge medium (Zoglauer et al., IEEE 2006)

26 Preliminary Findings Our preliminary, conservative estimates indicate that sensitivities of 10-6 ph/cm 2 s For a 511 kev narrow line For a 847 kev 3% FWHM broadened line are easily achievable with a Laue lens of ~1000 cm 2 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/cm 2 s 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!

27 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

28 for SNe Ia 3 different SN Ia explosion Models delayed detonation 15 d 25 d 50 d Ch-mass deflagration sub-chandrasekhar-mass Contribute to understanding SN Ia lightcurve variations (correlation of 56 Ni yields and brightness correction?) Discriminate SN Ia explosion models (Milne et al 2004)

29 SN Ia continued discriminations per 5-yr period between delayed-detonation (dd202c) and deflagration (W7) models (Milne 2005)

30 Novae Novae could be significant sources of 7 Li Measurement of 7 Be decay (478 kev, T 1/2 =53 d) from novae would confirm this Flux at maximum expected at ~ ph/cm 2 s (at 1 kpc, Hernanz et al. 2004) A Laue mission with 511 kev sensitivity of ph/cm 2 s could be expected to have 478 kev narrow-line sensitivities of better than ph/cm 2 s. Expect line width ~ 3-7 kev (Hernanz 2004) sensitivity around ph/cm 2 s Should provide 7 Be decay detections for CO novae out to > 1kpc

31 Astronomy without Radioactivities a few examples : 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/cm 2 /s at 511 kev in 10 6 s

32 Gamma-Ray Imaging at the Diffraction Limit the Future? Fresnel phase-shift lenses exploit small deviation of index of refraction from 1 at MeV energies Skinner et al., 2003 Potential lens efficiency near 100% μarcsec resolution Negligible absorption Skinner 2005 Very small field of view Energy bands ~ tens of MeV So far at gamma-ray energies on paper only Focal lengths of Mkm Instrument implementation straightforward once (sub-)μarcsec pointing on Mkm baseline is solved kev Skinner 2005

33 Summary A First Laue-Lens Lens Observatory could: 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 7 Be decay detections for CO novae Measure 44 Ti 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 and focusing will be required to step beyond ACT!

34 Questions 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