ACD Subsystem

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ACD Subsystem

• Jonathan F. Ormes
• ACD Subsystem Manager
• Rudolph K. Larsen
• ACD Project Manager
• Laboratory for High Energy Astrophysics
• NASA Goddard Space Flight Center
• ormes_at_lheavx.gsfc.nasa.gov

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Anti-Coincidence (ACD) Subsystem
Outline

• Technical overview
• Requirements
• Status
• Organization
• Schedule
• Budget
• Technical issues and mitigations
• Summary

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Large Area Telescope (LAT) Design Overview
Instrument
16 towers ? modularity height/width 0.4 ?
large field-of-view Si-strip detectors 228 mm
pitch, total of 8.8 x 105 ch.
hodoscopic CsI crystal array ?
cosmic-ray rejection ? shower leakage
correction XTkr Cal 10 X0 ? shower max
contained lt 100
GeV segmented plastic scintillator ?
minimize self-veto gt 0.9997 efficiency
redundant readout
Tracker
Calorimeter
Anticoincidence Detector Shield
3000 kg, 650 W (allocation) 1.75 m ? 1.75 m ?
1.0 m 20 MeV 300 GeV
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ACD The First Line of Defense Against Background

• The purpose of the ACD is to detect incident
  cosmic ray charged particles that outnumber
  cosmic gamma rays by more than 5 orders of
  magnitude. Signals from the ACD can be used as a
  trigger veto or can be used later in the data
  analysis.
• Segmented plastic scintillator (Bicron-408) read
  with wave-shifting fibers (BCF- 91MC) PMT
  (Hamamatsu R1635, R4868) readout.
• Each segment (tile) has a separate light tight
  housing.
• Separate tile housings provide resistance to
  accidental puncture by micrometeoroids.
• Wave-shifting fiber readout provides light
  collection uniformity
• Gaps between tiles are deliberately misaligned
  with the gaps between tracker towers and covered
  by scintillating fiber "tapes"

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Design Heritage

Backsplash reduced the EGRET effective area by
50 at 10 GeV compared to 1 GeV. GLAST will
be studying photons to above 300 GeV.
Anticoincidence Detector for GLAST is
subdivided into smaller tiles to avoid the
efficiency degradation at high energy.
0.2-2 MeV "backsplash" photons
GLAST
EGRET
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Design Approach
Lip to "hide" thermal blanket and micro-meteorite
shield
Segmentation for side entry events
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Science Requirement Efficiency

• SRD Background Rejection requirement
  Contamination of the high latitude diffuse gamma
  rays by background in any decade of energy for gt
  100 MeV shall be less than 10. The goal shall
  be 1.
• Charged particle background rejection involves
  the use of pattern recognition in the tracker and
  the calorimeter as well as the ACD.
• For protons, calorimeter and tracker are
  powerful.
• 105 1 at system level
• Electrons are more problematic.
• Electrons create showers in the calorimeter
  identical with photon showers.
• Worst case is 3 x 103 1 at 10 GeV
• The required ACD efficiency for charged particles
  (detector efficiency hermeticity) is 0.9997

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Additional Requirements

• Tile segmentation
• Efficiency gt 80 at 300 GeV relative to that at 1
  GeV
• Effective area at gt60o gt0.1 of on-axis value
• No more than 10 loss of effective area
• 6 in the plastic and structure
• 1 from ACD deadtime
• 1 loss from noise
• lt5 chance of loss of tile in 5 years
  (electronics)
• lt1 per year for loss of tile by micrometeoroid
  puncture

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Physical interface specifications

• Mass 200 kg 60 kg (30 reserve) 260 kg
• Electronics 34 kg 17 kg (50 reserve) 50
  kg
• Power 30 watts 25 watts (80 reserve)
• Volume - see materials presented by Martin Nordby
• Electronic signals
• Fast VETO
• Pre-primitives for Trigger thresholds
• Hi Z trigger for calibration events
• Addresses of hit tiles
• Pulse heights of hit tiles
• Rate date from all tiles
• Housekeeping data
• Command, command verification, and control

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ACD Technical Status

• Detectors
• Scintillator tile light output tests have been
  performed.
• Tile overlapping and detector tapes are planned
  to cover gaps
• Electron background rejection analysis cuts
  being developed
• Side tile segmentation is being reevaluated
• Trade studies are being performed
• tile thickness, segmentation and PMT placement
• Electronics
• ASIC development proceeding
• First prototype submission was in January
• Higher gain Hamamatsu PMTs are being evaluated
• Electronic parts have been submitted for
  acceptance
• High voltage power supply specs, SOW and cost
  estimate created
• Procurement for prototype sent to identified
  vendors
• Mechanical
• Tile support structure has been improved over
  proposal design

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ACD WBS Organization Chart
All ACD team members above are GSFC civil service
employees or GSFC contractors
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Contributing GSFC Lead Organizations

• WBS 4.1.6 ACD Subsystem - Dr. Jonathan Ormes,
  GSFC Code 600, Space Science Directorate
• WBS 4.1.6.1 ACD Management - Rudy Larsen, GSFC
  Code 700.1, Project Formulation
• System Engineering - Tom Riley, GSFC Code
  730.4, Instrument Systems Office
• Science Support- Dr. Alexander Moiseev,
  GSFC/USRA Code 661, Gamma Ray and Cosmic Ray
  Astrophysics Branch
• WBS 4.1.6.2 ACD Reliability and Quality
  Assurance - Patricia Huber, GSFC Code 303,
  Assurance Management Office
• WBS 4.1.6.3 ACD Detectors - Dr. Alexander
  Moiseev, GSFC/USRA Code 661
• WBS 4.1.6.4 ACD Electronics - Dave Sheppard,
  GSFC Code 564, Microelectronics and Signal
  Processing Branch
• WBS 4.1.6.5 ACD Mechanical Components - Tom
  Johnson, GSFC Code 543, Mechanical Engineering
  Branch
• WBS 4.1.6.6 ACD Software - Bob Schaefer,
  GSFC/HSTX Code 664, Data Management and
  Programming Office
• WBS 4.1.6.8 Instrument Subsystem Integration
  Test - John Lindsay, GSFC Code 568, Flight
  Systems Integration and Test Branch
• WBS 4.1.6.9 Mission IT Support - John Lindsay,
  GSFC Code 568
• WBS 4.1.6.A Mission Operations Data Analysis
  - Dr. Dave Thompson, GSFC Code 661
• WBS 4.1.6.B ACD Micrometeoroid Shield/Thermal
  Blanket - Tom Johnson, GSFC Code 543 /
• Louis Fantano, GSFC Code 545, Thermal
  Engineering Branch
• --------------------------------------------------
  --------------------------------------------------
  --------------------------------
• USRA- University Space Research Association
  EITI - Emergent Information Technologies Inc.

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4.1.6 ACD Schedule
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ACD Milestones

• ACD Thermal Blanket Requirements
  Review 03/21/01
• ACD Thermal Blanket PDR 06/27/01
• LAT Instrument PDR 08/06/01
• ACD Thermal Blanket CDR 06/26/02
• LAT Instrument CDR 08/05/02
• ACD Engineering Model (EM) Complete 05/15/03
• ACD Flight Subsystem Assembly Complete
  10/01/03
• Thermal Blanket / Micrometeoroid Shield Ready
  for Integration 02/01/03
• (with thermal model)
• Delivery of Calibration Unit ACD to
  SLAC 05/15/03
• Flight ACD Ready for Integration 01/26/04

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Interim ACD Cost Estimate
(Escalated K)
DOE/NASA funding.
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Technical Issues and Mitigations

• Increase technical margin for light collection
• Add reflective termination to fibers or read out
  at both ends
• Monitor and adjust PMT gain in flight
• Request 25-30 kg additional mass for thicker
  tiles on top
• Required volume for electronics may exceed
  available
• Place PMTs under ACD
• Place some of electronics underneath the grid
• Reduce side segmentation
• Parts acceptance and procurement
• Prototype HVPS procurement is in preparation
• Parts list submitted to Quality Assurance Branch
• Integration highly coupled to design
• ACD IT manager appointed to work with design team

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Summary

• ACD team is in place and progress is quite rapid
• Necessary trade studies underway
• Optimization of side segmentation
• Optimization of redundancy
• Optimization of light collection tile thickness
• Requirements have been established
• Near term schedule advanced
• Received additional funding for FY '01
• Added staff to prepare for PDR
• Draft Level 6 schedule is in hand
• Grass roots costing being scrubbed