Elsevier

Astroparticle Physics

Volume 27, Issue 5, June 2007, Pages 339-358
Astroparticle Physics

The anti-coincidence detector for the GLAST large area telescope

https://doi.org/10.1016/j.astropartphys.2006.12.003Get rights and content

Abstract

This paper describes the design, fabrication and testing of the Anti-Coincidence Detector (ACD) for the Gamma-ray Large Area Space Telescope (GLAST) Large Area Telescope (LAT). The ACD is LATs first-level defense against the charged cosmic ray background that outnumbers the gamma rays by 3–5 orders of magnitude. The ACD covers the top and four sides of the LAT tracking detector, requiring a total active area of ∼8.3 m2. The ACD detector utilizes plastic scintillator tiles with wavelength shifting fiber readout. In order to suppress self-veto by shower particles at high gamma-ray energies, the ACD is segmented into 89 tiles of different sizes. The overall ACD efficiency for detection of singly charged relativistic particles entering the tracking detector from the top or sides of the LAT exceeds the required 0.9997.

Introduction

The Gamma-ray Large Area Space Telescope (GLAST) is a new gamma-ray observatory scheduled to be launched in 2007. Developed by an international collaboration, including contributions from the US National Aeronautics and Space Administration and Department of Energy, it contains two instruments: the Large Area Telescope (LAT) [1] and the GLAST Burst Monitor [2]. The LAT will detect celestial gamma-rays in the energy range from ∼20 MeV to >300 GeV with angular, energy, and time resolution that are substantially better than in the earlier Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory [3]. The scientific tasks for LAT originate from results obtained by EGRET and a number of other astrophysical space missions as well as results from TeV ground based gamma-ray instruments. LAT goals cover a wide range of topics: understanding of the mechanisms of charged particle acceleration in active galactic nuclei, pulsars, and supernova remnants, determining the nature of the still-unidentified EGRET sources, detailed study of gamma-ray diffuse emission (both Galactic and extragalactic, as well as that produced in molecular clouds), high energy emission from gamma-ray bursts, transient gamma-ray sources, probing dark matter and the early Universe.

The LAT consists of three main detector systems, a silicon strip tracker, a CsI calorimeter, and an Anti-Coincidence Detector (ACD). The conceptual design of the LAT is shown in Fig. 1. The tracker, in which the gamma rays interact by pair production, provides instrument triggering and determines the arrival direction of detected photons. The calorimeter also provides instrument triggering and measures the energy of detected photons. The ACD surrounds the tracker and provides rejection of charged particles. One challenging feature of the LAT is that it does not have a specially designed directional trigger system, as for example the time-of-flight system in EGRET. The LAT hardware trigger is created by the tracker from the coincidence of signals in three consecutive tracker XY layers, or by the calorimeter if the energy deposition there exceeds some pre-selected level. This approach results in very high rate of first-level triggers, up to 10 kHz, mainly caused by primary and Earth albedo cosmic rays (protons, helium and other nuclei, electrons), which over the energy range of interest outnumber gamma-rays by 3–5 orders of magnitude. Most of this charged particle background must be removed on-board, prior to transmission of data to the ground, to make the event rate consistent with the available data downlink rate. This requirement makes the task of charged particle identification and rejection one of the main problems in designing the instrument. This responsibility is primarily assigned to the ACD.

Section snippets

Charged particle detection efficiency

The purpose of the ACD is to provide charged particle background rejection. This purpose dictates its main requirement to have high charged particle detection efficiency. The LAT specification is to have any residual background or “fake photons” at the level of no more than 10% of the diffuse gamma-ray background intensity.

Fig. 2 compares the differential spectra of cosmic ray protons [4] and electrons [5] to that of the extragalactic diffuse gamma-ray background [6] (extrapolated beyond the

ACD design – issues and implementation

In this section we describe the top-level ACD design drivers and solutions, list trade-off issues considered, and discuss in more detail the most important design issues.

ACD performance and operational issues

This section discusses specific aspects of ACD operation that have been investigated during the ACD design.

ACD integration

The ACD integration was a delicate process, requiring careful sequencing of operations. All of the integration steps were tested on prototypes before moving to the flight structure. The final appearance of the ACD (before it was covered by the MMS) is shown in Fig. 17.

  • 1.

    The ACD mechanical structure was assembled and equipped with tile and ribbon mounting flexures, FREE chassis attachment brackets, 44 thermistors and 9 accelerometers.

  • 2.

    The ribbons were installed and fastened to stand-offs in a

Conclusions

As a result of approximately eight years of effort, the LAT Anti-Coincidence Detector has been successfully designed, built, tested, and integrated into the LAT. The ACD array of plastic scintillator tiles with wavelength shifting fiber light collection plus scintillating fiber ribbons and photomultiplier tube readout meets all the science performance and spacecraft reliability requirements for the GLAST LAT. Currently LAT is in environmental testing, after which it will be integrated into the

Acknowledgements

During the years of ACD activity more than 100 people have been involved in the design, fabrication and test processes. We are grateful to Steve Ritz, LAT Project Scientist, for his constant interest, support, and valuable suggestions throughout the ACD development. Early in the ACD program, we benefited greatly from consultations with Don Stillwell, Andy Dantzler, Jay Norris, and John Mitchell of GSFC, and also Pavel DeBarbaro of Fermilab. Bill Atwood of UCSC provided important design and

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