	Document Number:	Release Date:
	LAT-TD-04631-02	March 8, 2005
	Authors: Eduardo do Couto e Silva, Lee Steele	Supersedes: Rev 1

GLAST LAT Technical Document
	Subsystem/Office:
	Integration and Test
Document Title:
GLAST LAT Instrument Data Analysis Primer

Gamma Ray Large Area Space Telescope (GLAST)

Large Area Telescope (LAT)

Integration & Test (I&T)

Instrument Data Analysis Primer

Change History Log

Revision	Effective Date	Description of Changes
1	27 September 2004	Initial release
2	8 March 2005	Revised Fig. 6, Fig. 12, Table 2 and text to reflect change in Recon file plane numbering scheme.

Contents

1. Introduction. 9

1.1. Hardware Overview. 9

1.2. Overview of Data Taking During LAT Integration. 10

2. Geometry and Numbering Scheme. 12

2.1. ACD Geometry and Numbering Scheme. 12

2.2. TKR Geometry and Numbering Scheme. 12

2.2.1. CAL Geometry and Numbering Scheme. 15

3. Readout Sequence. 17

3.1. ACD Readout Sequence. 17

3.2. TKR Readout Sequence. 17

3.2.1. Mapping across TKR Physical and Electronic Space. 17

3.3. CAL Readout Sequence. 19

3.3.1. Mapping across CAL Physical and Electronic Space. 21

4. Global Trigger and Dead Time. 22

4.1. Time Delays. 23

4.2. Dead Time. 26

4.3. Live Time. 26

5. Nominal Register Settings. 27

5.1. ACD Nominal Register Settings. 27

5.1.1. ACD Veto (hit) Threshold Discriminator 27

5.1.2. ACD Zero-Suppression Threshold. 27

5.1.3. ACD Time Delays. 27

5.1.4. ACD Known Features. 27

5.2. TKR Nominal Register Settings. 27

5.2.1. TKR Hit Threshold Discriminator (DAC) 27

5.2.2. Some Relevant TKR GTFE and GTRC Registers. 29

5.2.3. TKR Time Delays. 29

5.2.4. TKR Known Features. 29

5.3. CAL Register Nominal Settings. 30

5.3.1. CAL Time Delays. 31

5.3.2. CAL Known Features. 32

6. Calibrations. 33

6.1. ACD Calibration. 33

6.2. TKR Calibration. 33

6.3. CAL Calibration. 34

7. Event Data. 35

7.1. Event Contributions. 35

7.1.1. AEM.. 35

7.1.2. TEM.. 35

7.1.3. GEM.. 36

7.2. Data Analysis Files. 36

7.2.7. Reports: Configuration and SVAC. 38

7.3. Data Processing Steps. 38

8. TKR Reconstruction (To Be Re-written for new TKR Recon) 40

8.1. Clustering (TrkClusterAlg) 42

8.2. Track Finding (TrkFindAlg) 42

8.3. Track Fitting (TkrFitTrackAlg) 43

8.4. Vertexing (TkrVertexAlg) 44

8.5. Track Hypothesis for Integration and Test 45

9. CAL Reconstruction. 46

10. Monte Carlo Simulations. 47

10.1. Low Energy Photon Source. 47

10.2. Cosmic Ray Source. 47

10.3. Thresholds and Noise. 48

11. Data Analysis. 49

11.1. Identification of Minimum Ionizing Particles. 49

11.2. CALTowerGap. 50

11.3. Timing Analysis. 50

12. Resources. 51

Appendix A - eLog. 53

A.1. Shift Logbook Index 53

A.2. Run Selection Index 54

A.3. List Runs. 57

List of Illustrations

Figure 1: The LAT. 9

Figure 2: Principal LAT Components (Block Diagram) 10

Figure 3: Tower Placement for Cosmic Ray Data Taking. 11

Figure 4: Grid Tower Positions for Monte Carlo Simulations. 11

Figure 5: LAT Tower Numbering and Grid Coordinate System.. 12

Figure 6: TKR Tower Numbering Scheme. 13

Figure 7: TKR Plane Physical Details (X-View) 14

Figure 8: CAL Crystal Layer Numbering and Orientation. 15

Figure 9: CAL Module Cross Section. 15

Figure 10: Zoom of Region between Two Adjacent CAL Modules (3 layers shown) 16

Figure 11: CAL Crystal Dimensions. 16

Figure 12: The Four Sides of the TKR Tower with Cables. “X” or “Y” Means Measured Coordinate. 19

Figure 13: CAL FEE Simplified Schematic Diagram.. 20

Figure 14: CAL Channel Signal Range Energy Overlap. 21

Figure 15: The Four Sides of the CAL Module with Cables. 21

Figure 16: Trigger Studies of Real Triggers vs. MC Simulations. 23

Figure 17: Conceptual Trigger Delay Adjustments Diagram (GEM Inputs) 24

Figure 18: Conceptual Trigger Delay Adjustments Diagram (GEM Outputs) 25

Figure 19: TKR FEE Readout Channel Splitting. 28

Figure 20: Shaper Output Time over Threshold. 29

Figure 21: TDS Input and Output 35

Figure 22: Data Analysis Process Flow. 39

Figure 23: Four TKR Reconstruction Steps (Block Diagram) 40

Figure 24: Four Steps of TKR Reconstruction (Illustrated) 41

Figure 25: Iterative TKR Reconstruction Algorithms (Block Diagram) 41

Figure 26: Combinatoric Pattern Recognition: ComboFindTrack Tool 43

Figure 27: Simulation of VDG Gammas – Simulated Particle Source Generation. 47

Figure 28: Particle Flux vs. Kinetic Energy for surface_muon Source. 48

Figure 29: TrkTrackLength Example of Easily Misinterpreted Data. 49

Figure 30: A Charged Particle’s Path is Parallel to the Z Axis and only Strikes Every Other Crystal 50

Figure 31: GLAST Shift Log Index 53

Figure 32: Logbook Shift Run Info. 54

Figure 33: List Runs Table. 57

List of Tables

Table 1: Detector Readout Channels. 17

Table 2: TKR Mapping between Physical and Electronics Space. Note that “X” or “Y” Means Measured Coordinate, T= Top Side of Tray, B=Bottom Side of Tray. 18

Table 3: CAL FEE Signal Gain Characteristics. 20

Table 4: CAL Mapping between Physical and Electronics Space. 21

Table 5: Number of Timing Delay Registers. 23

Table 6: ACD Delay Register Nominal Settings. 27

Table 7: TKR GTFE and GTRC Registers. 29

Table 8: TKR Delay Registers Nominal Settings. 29

Table 9: TKR Electronics Known Features. 30

Table 10: CAL Registers for Gain, Triggering and Data Volume. 30

Table 11: CAL Thresholds Nominal Settings. 31

Table 12: CAL Delay Registers Nominal Settings. 31

Table 13: CAL Known Features. 32

Table 14: CAL and TKR Trigger Primitive Data. 35

Table 15: GEM Event Contribution. 36

Table 16: Data Analysis Files Locations. 37

Table 17: TKR Reconstruction Clustering Methods. 42

Table 18: TKR Reconstruction Combinatoric Track Finding Methods. 43

Table 19: TKR Reconstruction Vertexing Tools. 44

Table 20: Logbook Shift Run Menu Fields. 54

Table 21: Logbook Shift Run Menu Active Buttons. 56

Table 22: List Runs Fields. 57

1. Introduction

This document is intended to provide the LAT collaborators with sufficient information to perform data analysis during LAT integration. It is intended for users who are familiar with the LAT instrument, however a brief overview is provided.

Most of the information in this document is either copied from the website of Instrument Analysis Workshop presentations, or existing LATDocs documents. A list of references is provided in Resources, section 12.

1.1. Hardware Overview

The Large Array Telescope (LAT) is an integrated instrument consisting of 16 towers set into a 4x4 grid. Each tower consists of a Tracker (TKR), Calorimeter (CAL), and Tower Electronics Module (TEM). The LAT is shown in Figure 1. The 16 towers are surrounded by an Anti-Coincidence Detector (ACD) which is surrounded by a micro-meteorite shield.

Figure 1: The LAT

The following paragraphs provide a brief description of how the major components are used during pre-launch tests and are shown in Figure 2.

The ACD is mostly used to either identify charged particles for cosmic ray calibration runs, or to reject charged particles during Van de Graaff photon calibration runs.

The TKR’s function is to reconstruct the original direction of travel of either incoming photons (from 18 MeV photons from a Van de Graaff generator) or of charged particles (cosmic rays).

The CAL’s function is to measure the energy deposited by incoming charged particles, either from photon pair creation or cosmic rays.

The TEM assembles trigger primitives from the TKR and CAL (or simulated input) to determine whether or not there has been an event in the LAT. If so, it alerts the GLAST Electronics Module (GEM). It also communicates with the Event Builder Module. Trigger parameters are stored in the TEM.

The GEM (also referred to as the GLAST LAT Trigger - GLT) responds to a TEM’s message that an event has been detected and decides whether or not to generate a trigger. The GEM is an important component when performing Dead Time and Trigger analyses.

The Anti-Coincidence Detector Electronics Module (AEM) performs the same function for the ACD as the TEM does for the TKR and CAL detectors.

The Event Builder Module (EBM) communicates with the GEM, TEM and AEM.

The Global-trigger/ACD-module/Signal-distribution Unit (GASU) performs the highest logic level of event decision making, and comprises the AEM, GEM and EBM.

The Power Distribution Unit (PDU) supplies DC current to operate the electronics.

Figure 2: Principal LAT Components (Block Diagram)

NOTE: Real detectors (ACD, TKR, CAL) can be replaced by simulated input that mimics the behavior of the physical detectors beginning at the cables that readout the detectors. Although the Front End Electronics (FEE) are not simulated, a pattern can be created to generate events.

1.2. Overview of Data Taking During LAT Integration

Data taking with cosmic ray muons will occur with 1, 2, 4, 6, 8, 10, 12, 14 and 16 Flight Modules (FMs) installed in the LAT grid. The first position filled is position #0. The second position filled is #4. 15 hours of cosmic ray data taking (plus one hour with zero suppression OFF) occurs every time towers are added to the LAT. 16 hours of data taking with Van de Graaff photons occurs for tower A. Please refer to LAT-MD-00575 for detailed information on data taking during integration. Figure 3 shows the tower positions filled in each data taking configuration. Note that shaded squares indicate a tower installed in the grid.

For each hardware configuration there will be a baseline cosmic ray data-taking run for which the hardware is configured with nominal settings (please refer to the Nominal Register Settings in section 5) for ground analysis for the integrated hardware (towers).

Figure 3: Tower Placement for Cosmic Ray Data Taking

Monte Carlo simulations for Flight Modules will be performed for 1, 2, and 8 towers, and the fully integrated LAT grid, as shown in Figure 4.

Figure 4: Grid Tower Positions for Monte Carlo Simulations

2. Geometry and Numbering Scheme

The global instrument coordinate system for the LAT is consistent with the coordinate system for the observatory. It is a right-handed coordinate system with the Y-axis parallel to the solar panel axis, the Z axis normal to the planes of the TKR, CAL, and Grid (i.e., parallel to the “bore sight”), and the X axis is mutually perpendicular to Y and Z. The positive Z-axis points from the CAL to the TKR. Particles entering the instrument at normal incidence are thus oriented along the -Z direction (please see Figure 5).

The point X=Y=0 is at the center of the Grid. The Z=0 plane is at the top face of the Grid, between the TKR and CAL units on the TKR side of the Grid. The TKR silicon plane closest to Z = 0 is at Z = +33 mm; the crystal plane closest to Z = 0 is at Z = -46 mm.

Figure 5: LAT Tower Numbering and Grid Coordinate System

2.1. ACD Geometry and Numbering Scheme

The active elements of the ACD consist of 89 tiles and 9 ribbons. (A figure will be added later.)

2.2. TKR Geometry and Numbering Scheme

The tracker is made up of 19 trays comprising 36 planes as shown in Figure 6. The 36 planes are mated into 18 layers.

The TKR trays are numbered in increasing order with increasing Z. Each tray has two active planes, except the top half of the top tray (+Z) and bottom half of the bottom tray.

A tray measures in either the X or Y direction, i.e., has an X or Y view. To get X and Y information, planes from two adjacent trays are electronically combined. Mated X and Y planes are about 2 mm apart. This arrangement leaves the top-most and bottom-most planes without a partner and without silicon detectors.

A tray with detector strips physically parallel to the Y axis is an X tray: it measures the X coordinate (has an X view) and is called an X tray. Most planes have an embedded tungsten foil for g conversion: The top 12 X and Y pairs have a thin foil (3% of X₀), the next four have a thick foil (18% of X₀), and the bottom two X and Y pairs have no tungsten.

The active region of each TKR plane is comprised of 16 square Silicon Strip Detectors (SSDs - please see Figure 7). Each SSD has 384 conducting strips. Four SSDs are end-joined to make a ladder with the four SSDs in a given ladder joined mechanically and electrically to make 384 long strips. Four ladders per plane laid side-by-side make up a total of 1536 strips per plane. Each plane is about 360 mmby 360 mm in area.

Figure 6: TKR Tower Numbering Scheme.

Figure 7: TKR Plane Physical Details (X-View)

2.2.1. CAL Geometry and Numbering Scheme

Each CAL module is made up of 96 crystals oriented in a hodoscopic configuration of 8 layers of 12 crystals each. In contrast to a TKR plane, a CAL crystal makes its coordinate measurement along its principal axis: an X crystal has its principal axis along the X direction, as shown in Figure 8.

Each crystal has two PIN diodes at each end for reading out the signal. Each PIN diode (at either end) reads out for either the low or high energy measurement. The low energy PIN has an area four times greater than the high energy PIN.

The CAL layers are numbered from 0 – 7 in increasing order with decreasing Z. The CAL layers closest to the TKR is plane 0. CAL layers 0 has X crystals; CAL layers 7 has Y crystals. Each CAL crystal is read from each end, and each crystal end is either plus or minus: the end with the larger value of the coordinate is the “plus” end and the end with the smaller value of the coordinate is the “minus.”

Figure 8: CAL Crystal Layer Numbering and Orientation

Figure 10 shows an accurate representation of a CAL module.

Figure 9: CAL Module Cross Section

Figure 10 shows a close-up view of the displacement of CAL crystal ends of two adjacent CAL modules. It is important to note that crystal ends that face each other are at a different spacing than the closest crystals in adjacent modules that are parallel to each other.

Figure 10: Zoom of Region between Two Adjacent CAL Modules (3 layers shown)

The CAL crystal profile is shown in Figure 11 along with the dimensions of a CAL crystal, including its carbon fiber enclosure.

Figure 11: CAL Crystal Dimensions

	ACD		TKR	CAL
	Tiles/Channels	Ribbons/Channels	Planes/ Channels	Crystals/ Channels
1 Tower