# Presentation: 078075

## GLAST Large Area Telescope:

**AntiCoincidence Detector (ACD) Overview**
**WBS 4.1.6**
**David J. Thompson**
**NASA Goddard Space Flight Center**
**ACD Subsystem Manager**
**djt@egret.gsfc.nasa.gov**

## Outline - ACD

**Overview**
**Level III Requirements Summary **
**Technical Heritage**
**Organization**
**Flight Experience **
**Status of January Recommendations**
**Summary Schedule**
**Summary Cost Plan**

## Anticoincidence Detector Overview

**Anticoincidence Detector Overview**

**TILE SHELL ASSEMBLY**
**89 Plastic scintillator tiles**
**Waveshifting fiber light collection (with clear fiber light guides for long runs)**
**Two sets of fibers for each tile**
**Tiles overlap in one dimension**
**8 scintillating fiber ribbons cover gaps in other dimension (not shown)**
**Supported on self-standing composite shell**
**Covered by thermal blanket + micrometeoroid shield (not shown)**
**BASE ELECTRONICS ASSEMBLY**
**194 photomultiplier tube sensors (2/tile)**
**12 electronics boards (two sets of 6), each handling up to 18 phototubes.  High voltage power supply on each board.**

**Base Electronics Assembly (BEA)**

**Tile Shell Assembly (TSA)**

**Prototype ACD tile read out with Wavelength Shifting Fiber**

## Level III Key Requirements Summary

**Reference:  LAT-SS-00016**

## ACD Technical Heritage

**Plastic Scintillator**** - used in all previous gamma-ray telescopes OSO-3, SAS-2, COS-B, CGRO (all 4 instruments), plus many cosmic ray experiments.**
**Waveshifting fibers**** - used in GLAST LAT Balloon Flight Engineering Model (BFEM).  Waveshifting bars used by HEXTE on RXTE (same material in a different geometry)**
**Photomultiplier tubes**** - used in all previous gamma-ray telescopes. HEXTE/RXTE used a commercial version of the same tube we are using (Hamamatsu 4443), and GOLF on SOHO used the same tube as the ACD except for the cathode material (Hamamatsu 4444)**
**High Voltage Bias Supplies**** - used in all previous gamma-ray telescopes, plus many cosmic ray experiments.**
**Electronics**** - similar ASIC’s (same designer) used on the BFEM.  Discriminators, PHA and logic signals similar to many flight instruments.  **
**Micrometeoroid Shield**** - Improved version (more layers, stronger materials) of shield that protected EGRET successfully for nine years. **

## ACD Organization Chart

**ACD Systems Engineering**
**4.1.6.1.2**
**George Shiblie **
**Mike Amato**

**Tile Shell Assembly**
**4.1.6.3**
**Ken Segal, Lead**

**Base Electronics **
**Assembly**
**4.1.6.4**
**Glenn Unger, Lead**

**Micrometeoroid Shield /**
**Thermal Blanket**
**4.1.6.5**
**Ken Segal, Lead**
**Carlton Peters, Thermal**
**                       ****Lead**

**ACD Design and Science Support**
**4.1.6.1.3**
**Alexander Moiseev,Lead**

**Tile Detector **
**Assemblies**
**4.1.6.3.2**
**A. Moiseev, Lead**

**ACD Reliability and Quality Assurance**
**4.1.6.2**
**Patricia Huber, Lead**

**Hardware**
**Integration & Test**
**4.1.6.7**
**Jim La, Lead**

**Mission Integration**
** ****& Test Support**
**4.1.6.9**
**Bob Hartman, Lead**

**Ground Support **
**Facilities & Equipment**
**4.1.6.B**
**Jim La**
**Ken Segal**
**Glenn Unger**

**LAT Instrument **
**Integration & Test**
**Support **
**4.1.6.8**
**Jim La, Lead**

## ACD Team Space Flight Experience

**Science**
**Dave Thompson - SAS-2, EGRET **
**Bob Hartman - SAS-2, EGRET **
**Alex Moiseev - GAMMA-1**
**Engineering**
**Tom Johnson - BBXRT, COBE, EUVE, SAMPEX, TRMM, HST **
**George Shiblie - FUSE, MAP**
**Mike Amato - Spartan 201, STIS (HST), Stereo COR1**
**Ken Segal - TRMM, HST, POES, EOS**
**Glenn Unger - MOLA, XTE, MAP**
**Dave Sheppard - BBXRT, XTE, TGRS, POEMS, GRS, Swift**
**Satpal Singh - EPACT and TGRS on WIND, Swift**
**Art Ruitberg - EGRET, COBE, POLAR, WIND, CASSINI, Triana**
**Bob Baker - HEAO, SMM, EGRET, BBXRT, XRS, XTE, Swift**
**Jim La - TDRS, POES, VCL/MBLA, Spartan, ROMPS, SLA, SEM**
**Carlton Peters - VCL, CATSAT, MAP, Triana**

## Status of January Review Recommendations

**Finalize tile and fiber layout, build mock-up**
**In progress using completed mock-up**
**Test light yield of full optical system **
**Delivery of prototype flight-like tile assembly occurred last week**
**Demonstrate electronic noise is low **
**Use first engineering board - Sept. **
**Thermal cycle tile assemblies**
**In progress (no problems after 140 cycles)**
**Plan for calibration of ACD system**
**Basic approach verified, draft completed**
**Improve ASIC schedule margin**
**New approved screening process helps**
**Complete streamlined WBS in Primavera**
**Done**
**Analyze critical path and contingency**
**Done**

- Full-scale mock-up of ACD being used for tile placement and fiber routing

## ACD Work Flow Overview

- Shell Subassembly

- ASIC Development

- High Voltage Power Supply

- Photomultiplier tubes

- Front End Electronics Card Assembly 07/03

- Base Frame Subassembly

- Base Electronics Assembly 09/03

- ACD Integration 10/03

- Tile Shell Assembly 10/03

- ACD Performance and Environmental Test  03/04

- Thermal Blanket Mircometeoroid Shield

- Completion Dates Shown

**ACD Work Flow Overview**

- Ship 04/04

## Top-Level Schedule

**Top-Level Schedule**

## ACD Critical Path - Digital ASIC

## Cost/Manpower Overview by Fiscal Year

## Cost/Manpower Overview by Task

- Civil Service personnel salaries are paid by Goddard, not the LAT.
- Taxes: Goddard overhead, charged on the basis of on-site FTE and total cost.

## Manpower Skill Mix by Fiscal Year

## ACD - Largest Procurements

## ACD Cost & Commitments

**ACD Cost & Commitments**

## ACD Cost Type

## Some ACD Risks - Not Likely, But Possible

## ACD Issues/Concerns

**The schedule is tight.****  The ACD critical path (digital ASIC) has little room for delay.  We are drawing on SLAC ASIC experience to minimize risk. **
**There is no ACD Engineering Model.****  Although testing will be done on components and subassemblies, the first full-scale test will be on the flight unit. **
**The ACD is not completely redundant.**** Loss of a tile, a phototube, a high voltage supply, or an electronics channel would reduce the ACD efficiency, although risk assessment shows not enough loss to have a significant impact on science. **
**The ACD is dependent on Goddard Civil Service manpower and test facilities.****   We have a commitment from Goddard management and fairly high visibility, but crises in other programs have been known to pull manpower away and tie up facilities.**

## Backup material

## What Happened in January?

**In late Summer, 2001, the ACD was descoped (fewer tiles, fewer HV supplies, some electronics shifted to LAT) for cost saving.**
**At the same time, the ACD team had personnel changes:**
**Subsystem Manager:   Ormes  ********  Thompson**
**Instrument Manager:    Larson ********   Johnson**
**Also new mechanical, electrical, I&T, systems engineers**
**Rebudgeting and development of the new WBS schedule were complicated by the mismatch between Goddard and PMCS accounting methods.**
**By the January review, we had not completely incorporated the new cost/schedule into the PMCS.**
**The PDR/Baseline Review team correctly concluded that the ACD did not have an adequate baseline plan.**
**We spent January-March streamlining and documenting the cost and schedule within the PMCS.  We now have a coherent,  practical plan for building the ACD.**
**The ACD team is now stable and ready to move on.**

## Level 3 Key Milestones

## Selected Level IV Milestones

## Meeting the Level III Key Requirements

**Meeting the Level III Key Requirements**

- Detection Efficiency 0.9997

- Black line: measured efficiency
- Green line: efficiency with 15% loss
- Blue line: efficiency with 40% loss

- Backsplash Loss <20% at 300 GeV

- Measurements at SLAC and CERN