glast large area telescope: instrument design steven m. ritz goddard space flight center

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13-15, 2001

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GLAST Large Area Telescope:GLAST Large Area Telescope:

Instrument Design

Steven M. RitzGoddard Space Flight CenterLAT Instrument Scientist

[email protected]

Gamma-ray Large Area Gamma-ray Large Area Space TelescopeSpace Telescope


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From Science Requirements to DesignFrom Science Requirements to Design

From LAT proposal Foldout D:


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Simplified FlowSimplified Flow

Science Requirements Document (SRD)

LAT LAT Performance Performance SpecificationsSpecifications

Interface Requirements

Document (IRD)

Design Trade Study Space

Mission System Specification (MSS)

LAT Subsystem Requirements

Mission Assurance Requirements (MAR)


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LAT Performance SpecificationLAT Performance Specification

The LAT PS sets instrument performance requirements on:– energy range and effective area– energy resolution– single photon angular resolution (68% and 95% containment)– field of view– source location determination– point source sensitivity– single-event time accuracy– background rejection– dead time– gamma-ray source transient detection capabilities on board

The LAT PS also includes the physical, operational and communications requirements.


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Experimental TechniqueExperimental Technique• Instrument must measure the direction, energy, and arrival time of high energy photons (from approximately 20 MeV to greater than 300 GeV):

- photon interactions with matter in GLAST energy range dominated by pair conversion: determine photon direction

clear signature for background rejection

e+ e– calorimeter (energy measurement)

particle tracking detectors

conversion foil

anticoincidenceshield

Pair-Conversion Telescope

Energy loss mechanisms:

- limitations on angular resolution (PSF) low E: multiple scattering => many thin layerslow E: multiple scattering => many thin layers high E: hit precision & lever armhigh E: hit precision & lever arm

• must detect -rays with high efficiency and reject the much higher flux (x~104) of background cosmic-rays, etc.;

• energy resolution requires calorimeter of sufficient depth to measure buildup of the EM shower. Segmentation useful for resolution and background rejection.


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Some Constraints (MSS & IRD) on InstrumentSome Constraints (MSS & IRD) on Instrument

• Lateral dimension < 1.8m

Restricts the geometric area.

• Mass < 3000 kg

Primarily restricts the total depth of the CAL.

• Power < 650W

Primarily restricts the # of readout channels in the TKR (strip pitch, # layers), and restricts onboard CPU.

• Telemetry bandwidth < 300 kbps orbit average

Sets the required level of onboard background rejection and data volume per event.

• Center-of-gravity constraint restricts instrument height, but a low aspect ratio is already desirable for science.

• Launch loads and other environmental constraints.


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Overview of LATOverview of LAT

• 4x4 array of identical towers4x4 array of identical towers Advantages of modular design.

• Precision Si-strip Tracker (TKR) Precision Si-strip Tracker (TKR) Detectors and converters arranged in 18 XY tracking planes. Measure the photon direction.

• Hodoscopic CsI Calorimeter(CAL)Hodoscopic CsI Calorimeter(CAL) Segmented array of CsI(Tl) crystals. Measure the photon energy.

• Segmented Anticoincidence Detector Segmented Anticoincidence Detector (ACD)(ACD) First step in reducing the large background of charged cosmic rays. Segmentation removes self-veto effects at high energy.

• Electronics System Electronics System Includes flexible, highly-efficient, multi-level trigger.

Systems work together to identify and measure the flux of cosmic gamma Systems work together to identify and measure the flux of cosmic gamma rays with energy 20 MeV - >300 GeV.rays with energy 20 MeV - >300 GeV.


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Choice of DetectorsChoice of Detectors

• TRACKERsingle-sided silicon strip detectors for hit efficiency, low noise

occupancy, resolution, reliability, readout simplicity.

• CALORIMETER hodoscopic array of CsI(Tl) crystals with photodiode readout

• ANTICOINCIDENCE DETECTORsegmented plastic scintillator tiles with wavelength shifting

fiber/phototube readout for high efficiency and avoidance of ‘backsplash’ self-veto.

for good resolution over large dynamic range; modularity matches TKR; hodoscopic arrangement allows for imaging of showers for leakage corrections and background rejection pattern recognition.


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Pre-Proposal Trade Studies SummaryPre-Proposal Trade Studies Summary


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Benefits of ModularityBenefits of Modularity

• Construction and Test more manageable, reduce costs and schedule risk.

• Early prototyping and performance tests done on detectors that are full-scale relevant to flight.

• Aids pattern recognition.

• Good match for triggering large-area detector with relatively localized event signatures.

Issue: demonstrate that internal dead areas associated with support material and gaps between towers are not a problem.

Resolution: Detailed Monte-Carlo model of instrument, combined with beam-test data of prototype hardware, used to validate design performance.


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Design Performance Validation: Design Performance Validation: LAT Monte-Carlo ModelLAT Monte-Carlo Model

The current LAT design is based on detailed Monte Carlo simulations.

Two years of work was put into this before any significant investment was made in hardware development.

Cosmic-ray rejection of >105:1 with high gamma ray efficiency. Solid predictions for effective area and resolutions (computer models now verified by beam tests). Current reconstruction algorithms are existence proofs -- many further improvements under development. Practical scheme for triggering.Design optimization.

Simulations and analyses are all OO (C++), based on GISMO toolkit.

Zoom in on a corner of the instrument

First TKR module plane

module walls

scintillators

front scintillators

gaps, dead areas included

The instrument naturally distinguishes most cosmics from gammas, but the details are essential. A full analysis is important.

proton

gamma ray


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X Projected Angle3-cm spacing, 4% foils, 100-200 MeV

Data

Monte Carlo

Experimental setup in ESA for tagged photons:

101 102 103 104

Energy (MeV)

0.1

1

10

Con

tain

men

t S

pace

Ang

le (

deg)

68% Containment95% Containment

(errors are 2)

GLAST Data

Monte Carlo

Monte Carlo Modeling Verified inMonte Carlo Modeling Verified inDetailed Beam TestsDetailed Beam Tests

See NIM A446(2000), 444;and SLAC-Pub 8682 (submitted to NIM)


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1999-2000 Beam Test at SLAC1999-2000 Beam Test at SLAC

• data system, trigger• hit multiplicities in front and back tracker sections• calorimeter response with prototype electronics.• time-over-threshold in silicon • upper limit on neutron component of ACD backsplash• hadron tagging and first look at response

Using beams of positrons, tagged photons and hadrons, with a ~flight-size tower, studies of


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Beam Test Engineering ModelBeam Test Engineering Model

Flight-scale tower

constructed

• Beam test at SLAC in December 1999

• Balloon flight in 01

CAL and TKR have prototype custom electronics, with all functionality for flight demonstrated.


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• Evolving understanding of the fluxes, new sources of backgrounds included. Right now:

• Work is ongoing: update background fluxes and selections. L1T rate estimate being revised.

Background RejectionBackground Rejection

Source % rate Avg L1T Rate [Hz]

Chime 36 2019

albedo 4 196

Electron 1 30

Albedo p 59 3224

TOTAL 5470

Analysis done thus far for two main reasons:

(1) Necessary for realistic estimate of effective area.

(2) A proof of principle, demonstration of the power of the instrument design.

Capabilities demonstrated.Capabilities demonstrated. Next:Next:Optimize selections. Some science topics may require less stringent background rejections than others.


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Performance PlotsPerformance Plots

Derived performance parameter: high-latitude point source sensitivity (E>100 Derived performance parameter: high-latitude point source sensitivity (E>100 MeV), 2 year all-sky survey: MeV), 2 year all-sky survey: 1.6x101.6x10-9-9 cm cm-2-2 s s-1-1, a factor > 50 better than , a factor > 50 better than

EGRET’s (~1x10EGRET’s (~1x10-7-7 cm cm-2-2ss-1-1).).

(after all background rejection cuts, being updated)

FOV w/ energy measurement due to favorable aspect ratio

Effects of longitudinal shower profiling


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Instrument Triggering and Onboard Data FlowInstrument Triggering and Onboard Data Flow

Hardware trigger based on special signals from each tower; initiates readout

Function: • “did anything happen?”

• keep as simple as possible

• TKR 3 x•y pair planes in a row**

workhorse workhorse triggertriggerx

xx

Instrument Total L1T Rate: <5 kHz>

Function: • reject background efficiently & quickly with

loose cuts,

• reduce computing load

• remove any noise triggers

xx

x

• tracker hits ~line up

• track does not point to hit ACD tile

L3T: full instrumentFunction: reduce data to fit within downlink

Total L3T Rate: <30 Hz>

• complete event reconstruction

• signal/bkgd tunable, depending on analysis cuts: cosmic-rays~ 1:1

Spacecraft

OR

(average event size: ~7 kbits)

rates are orbit averaged; peak L1T rate isapproximately 10 kHz. L1T rate estimate being revised. **ACD may be used to throttle this rate, if req.

Upon a L1T, all towers are read out within 20s

Level 1 Trigger

• CAL: LO – independent check on TKR trigger. HI – indicates high energy event ground

Level 2 Processing Level 3 Processing

L2 was motivated by earlier DAQ design that had one processor per tower. Using single-tower info only, background rate reduction was typically a factor 5. On-board filtering hierarchy being redesigned.

On-board science analysis:On-board science analysis: transient detection (AGN

flares, bursts)


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Pre-Proposal Formal Peer ReviewsPre-Proposal Formal Peer Reviews

Subsystem designs and critical technology

development decisions are periodically reviewed

by independent panels that include members

outside of the collaboration.


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Flow into Tracker SubsystemFlow into Tracker Subsystem

-ray conversion efficiencyConverter configuration

Other material

Geometric area

Aspect ratio

Charged particle detection efficiency

Spatial measurement resolutionDead area

Ionization measurementSelf-trigger

Trigger efficiency

Trigger noise rate

Data noise occupancy

Trigger saturation recovery time

Dead time

Total mass

Total power

Environmental

Effective Area

Effective Area Knowledge

Single photon angular resolution requirements

Field of view

Bkgd rejection

Dead time

MSS&IRD requirements

Tel. bndwdth


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-ray conversion efficiencyConverter configuration

Other material

Geometric area

Aspect ratio


Spatial measurement resolutionDead area

Ionization measurementSelf-trigger

Trigger efficiency

Trigger noise rate


Trigger saturation recovery time

Dead time

Total mass

Total power

Environmental

Effective Area

Effective Area Knowledge

Single photon angular resolution requirements

Field of view

Bkgd rejection

Dead time


Tel. bndwdth

Flow into Tracker SubsystemFlow into Tracker Subsystem


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Flow into Calorimeter SubsystemFlow into Calorimeter Subsystem

Depth

Geometric area

Passive material

Spatial resolution of energy depositions

Trigger signals


Dynamic range

Total mass

Total power

Environmental

Energy range

Energy resolution

Background rejection

Effective area

Effective area knowledge

Dead time


Tel. bndwdth

Measurement dead time


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Flow into ACD SubsystemFlow into ACD Subsystem


Coverage

Maximum false veto due to backsplash

Noise

CNO signals

Trigger signals

Deadtime

Total mass

Total power

Environmental

Energy range

Energy resolution

Background rejection

Effective area


Dead time


Tel. bndwdth


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Flow into Electronics SubsystemFlow into Electronics Subsystem

Trigger

Dead/live time measurement

Data flow, deadtime

Command & configuration

Transient analysis and notification

Environmental & Housekeeping data flow

Event filtering

Total mass

Total power

Environmental

Time accuracy

Transient detection and

notification

Energy range

Effective area


Dead time


Tel. bndwdth


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Flow into Flight SoftwareFlow into Flight Software

Commanding

Data flow

Monitoring, reporting

Calibration

Event filtering

Transient detection & onboard science analysis

Reprogrammability

Integration and Test

Transient detection and

notification

Energy range

Effective area


Dead time

Tel. bndwdth


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Looking forward to…Looking forward to…

glast large area telescope: instrument design steven m. ritz goddard space flight center

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