CASTERA Concept for a

Black Hole Finder Probe

Mark McConnellUniversity of New Hamsphire

The CASTER CastMark McConnell, Peter Bloser,

John Macri, Jim RyanUniversity of New Hampshire

Gary Case, Mike Cherry, T. Greg Guzik, Brad Schaefer, J. Greg Stacy, John Wefel

Louisiana State University

R. Marc Kippen, W. Tom VestrandLos Alamos National Laboratory

Bill Paciesas, Rich MillerUniversity of Alabama – Huntsville

Jim CravensSouthwest Research Institute

Beyond Einstein Roadmap

Three Einstein ProbesExpected launch date : 2012-2020

Black Hole Finder ProbeAll-sky black hole census

Total energy range : 10 – 600 keV

Sensitivity goal ≈0.02 mCrab in 20–100 keV

≈1000x more sensitive than HEAO A-4

1–20x more sensitive than Swift

≈20x more sensitive than BATSE for GRBs

Angular resolution of 3-5 arcmin will be

CASTER

One of two mission concepts proposed for the Black Hole Finder Probe.

Coded aperture imaging (10-600 keV).

Detectors based on new scintillator technologies.

Coded Aperture Survey Telescope for Energetic Radiation

Motivation for CASTER

New scintillator and readout technologies.

New scintillator with high light output :Improved energy resolutionImproved spatial resolution

Traditional technology simplifies implementation.

Potential for low cost detector technology.

Emphasize the importance (uniqueness) of observations at higher energies (up to ≈600 keV).

The CASTER PayloadFour High Energy Telescope (HET) modules

50-600 keVdetector area (total) ≈ 6.0 m2 ∆θ ≈ 10’

Two Low Energy Telescope (LET) modules50-600 keVdetector area (total) ≈ 3.0 m2

∆θ ≈ 10’The FoV for each array will be 60° x 120°

Zenith-pointed mode will scan much of the sky every orbit.

Detector Requirements

Energy range ≈ 10-600 keV

Good stopping power for energies up to ≈600 keV

Spatial resolution ≈ 1-2 mm in x, y, and z

Availability in large areas and at low cost

Energy resolution << NaI

Environmental tolerance

Good timing resolution

Scintillator Materials

LaBr3 LaCl3 LuI2 NaI(Tl) CsI(Na) BGO

Density 5.29 3.86 5.6 3.67 4.51 7.13

Light Outputphotons/MeV 63,000 49,000 >50,000 39,000 39,000 9,000

∆E/E @ 662 keV <3% 4% 10% 7% 7.5% >10%

Peak λ(nm) 358-385 330-352 475 415 420 480

Fast Decay (ns) 25 25 23 230 630 300

New Scintillator Technology

High Z material (comparable to NaI)

High density (higher than that of NaI)

Higher light output (60% more than NaI)

Significantly improved linearity (E vs. light output)

Significantly better energy resolution (<3% vs. 7%)

Significantly faster decay (35 ns vs. 230 ns)

Lanthanum Bromide (LaBr3)

Stopping Power

Comparable to CZT.

Thick scintillators are easier to fabricate.

At higher energies, scintillators may offer a significant advantage.

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102 3 4 5 6

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CZT 5mm thick

LaBr3 5mm thick

LaBr3 20mm thick

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CZT 5mm thick

LaBr3 5mm thick

LaBr3 20mm thick

Energy Resolution

≈ 2.7% @ 662 keV≈ 3.8% @ 511 keV≈ 6.8% @ 122 keV

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Comparable to CZT

Comparable to Swift (Hullinger et al. 2004).

Lanthanum Bromide (LaBr3)

Scintillator Imaging

Goal is to achieve spatial resolution of ≈1-2 mm in all three dimensions (x, y, z).

Performance will depend on several parameters :light output of scintillatorthickness of scintillatorenergy

Imaging configurations :

Traditional Anger Camera ImagingDepth-Encoding Anger CameraCross-Fiber Readout

Depth of Interaction

Depth measurement comes from light cone projected onto sensor array.

The number of triggered sensors provides a measure of depth.

Anger Camera Imaging

MCP-PMT (Burle) Flat-Panel PMT(Hamamatsu)

An array of sensors can be used to determinethe x-y interaction location.

Studies at UAH and UNH will be providing results for LaBr3 and LaCl3 using latest technologies.

Simulation tools are also being developed.

Depth-Encoding Anger Camera

WLS fiber ribbons used to determine depth.

X-Y location and total energy provided by an array of PMT anodes.

Matthews et al. 2003

Crossed Fiber Readout

Use of orthogonal WLS fibers for light readout can be used to determine x-y.

Tests have been made (UNH) using plastic scintillator.

Same approach could also be used for crystals.

Bravar (2005)paper 5901-20

Crossed Fiber Readout

LSU is developing a coded aperture imager for homeland security.

HISGRI - High Sensitivity Gamma Ray Imager.

Initial tests with LaBr3 and LaCl3 are promising.

Case et al. (2005) - paper 5898-21

Pixellated Scintillator ArraysPixellated arrays may be needed to concentrate light at lower energies.

At lower energies, depth measurement is not as important, but we need to get X-Y.

Cherry et al. 2004, Case et al. (2005) - paper 5898-21

Outstanding Issues

Availability in large volumes

Intrinsic background

Radiation hardness

Activation

Background at balloon altitudes

Background at orbital altitudes

Lanthanum Bromide (LaBr3)

Availability of Detector Material

Both LaBr3 and LaCl3 still under development (St. Gobain, RMD)

LaCl3 - BrilLanCe™ 350 (2’’)

LaBr3 - BrilLanCe™ 380 (1.5’’)

No fundamental barriers to larger volumes 3” x 3” LaCl3

(St. Gobain)∆E/E = 4.1%

Ongoing ActivitiesRadiation Beam Tests (UNH) -To evaluate the sensitivity (radiation hardness and activation) of LaBr3 and LaCl3 to various forms of radiation.

Balloon Background (UAH, LSU) - Sample crystals will be flown on a balloon flight this fall (2005).

Orbital Background Modeling (LANL, UNH) -Estimates of the orbital background will be derived from MMGPOD suite of simulation tools (Weidenspointner et al. 2005).

Summary

Coded aperture imaging is an attractive way of doing a hard X-ray survey (10-600 keV).

Alternative detector technologies are worth considering.

The goal of the CASTER mission concept study will be to consider some of these alternative technologies and their implications for mission design.

Other Challenges

Spatial resolution of ≈ 1-mm in x, y and z

Handling of multiple interaction sites?

Ability to do polarimetry?

Implications for Mission Design

Thicker material ==> greater weight, background?

Thicker mask ==> greater weight, background?

Thicker detector/mask ==> restricted FoV?

Separate low-energy and high-energy imagers?

Daily sky coverage?

CASTER Mission Concept Study

Continued Development of LaBr3

Detector Design Studies (various scintillators)

Imager Design Studies

Background Studies (beam tests, MGGPOD)

Sensor Ruggedization

Data Handling

Spacecraft Design

Mission Design

Cost of Detector Material

LaX fabrication geometries are expected to be like those of other inorganic scintillators.

LaX costs are expected to be comparable to that of other inorganic scintillators.

Cost < $30 / cm3.

Background Issues

Can be problematic for coded-aperture telescope.

Fast response of LaX scintillator may make shielding more effective.

Depth information can also be used to reject some level of background.

Thicker detectors do not necessarily imply a larger background.

Environmental Tolerance

LaX is hygroscopic (like NaI)

Response to large doses of radiation is unknown

induced background (activation)

radiation damage

Beam tests are required (and planned)

Scintillator Comparison

Segmented CsI array from St. Gobain. Individual cells are 2 mm x 2 mm. Overall size is 5 cm x 5 cm x 0.6 cm thick. Energy resolution comparable to monolithic CsI (19% FWHM @ 60 keV)

Pixellated Scintillator Arrays