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**GLAST BURST MONITOR**

**SIMULATION AND DETECTOR RESPONSE SOFTWARE**

**FUNCTIONAL SPECIFICATION DOCUMENT**

Prepared by:

R. Marc Kippen

Los Alamos National Laboratory

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**GLAST BURST MONITOR**

**SIMULATION AND DETECTOR RESPONSE SOFTWARE**

**FUNCTIONAL SPECIFICATION DOCUMENT**

PREPARED BY:

R. Marc Kippen Date

Los Alamos National Laboratory

SIM/DRM Lead

APPROVED BY:

Robert D. Preece Date

University of Alabama in Huntsville/NSSTC

GBM MO&DA Software Lead

APPROVED BY:

Charles Meegan Date

NASA/MSFC SD50

GBM Principal Investigator

APPROVED BY:

Steve Elrod Date

NASA/MSFC SD20

GBM Project Manager

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**TABLE OF CONTENTS**

1\. SCOPE [1](#scope)

2\. APPLICABLE DOCUMENTS [1](#applicable-documents)

2.1 Reference Documents [1](#reference-documents)

3\. SOFTWARE Specifications [2](#software-specifications)

3.1 General Specifications [2](#general-specifications)

3.1.1 Core Simulation Package [2](#core-simulation-package)

3.1.2 Radiation Transport Physics [3](#radiation-transport-physics)

3.1.3 Input Mass Models [4](#input-mass-models)

3.1.4 Input Photon Energy Distributions
[4](#input-photon-energy-distributions)

3.1.5 Input Photon Spatial Distributions
[4](#input-photon-spatial-distributions)

3.1.6 Input Flux [4](#input-flux)

3.1.7 Instrumental Effects [5](#instrumental-effects)

3.1.8 Recorded Data [5](#recorded-data)

3.2 Specifications Specific to GBM Design Phase
[6](#specifications-specific-to-gbm-design-phase)

3.2.1 Design-phase Core Simulation Package
[6](#design-phase-core-simulation-package)

3.2.2 Design-phase Mass Models [6](#design-phase-mass-models)

3.2.3 Design-phase Environment Models
[6](#design-phase-environment-models)

3.2.4 Design-phase Instrumental Effects Models
[6](#design-phase-instrumental-effects-models)

3.3 Specifications Specific to GBM Calibration Phase
[6](#specifications-specific-to-gbm-calibration-phase)

3.3.1 Supported Calibrations [6](#supported-calibrations)

3.3.2 Detector-Level Calibrations [6](#detector-level-calibrations)

3.3.2.1 Detector Mass Models [7](#detector-mass-models)

3.3.2.2 Detector Calibration Environment Model
[7](#detector-calibration-environment-model)

3.3.2.3 Detector Calibration Instrumental Effects Model
[7](#detector-calibration-instrumental-effects-model)

3.3.3 System-Level Calibrations [7](#system-level-calibrations)

3.3.3.1 Detector Mass Models [7](#detector-mass-models-1)

3.3.3.2 System Calibration Environment Model
[7](#system-calibration-environment-model)

3.3.3.3 System Calibration Instrumental Effects Model
[7](#system-calibration-instrumental-effects-model)

3.3.4 On-spacecraft Calibrations [8](#on-spacecraft-calibrations)

3.3.4.1 Detector Mass Models [8](#detector-mass-models-2)

3.3.4.2 Spacecraft Mass Model [8](#spacecraft-mass-model)

3.3.4.3 On-spacecraft Calibration Environment Model
[8](#on-spacecraft-calibration-environment-model)

3.3.4.4 On-spacecraft Calibration Instrumental Effects Model
[8](#on-spacecraft-calibration-instrumental-effects-model)

3.4 Specifications Specific to GBM Operations Phase
[8](#specifications-specific-to-gbm-operations-phase)

3.4.1 Coding Standards [9](#coding-standards)

3.4.2 Maintenance & Configuration Control
[9](#maintenance-configuration-control)

3.4.3 Mass Models [9](#mass-models)

3.4.3.1 Detector Mass Models [9](#detector-mass-models-3)

3.4.3.2 Spacecraft Mass Model [9](#spacecraft-mass-model-1)

3.4.4 Atmospheric Scattering Model [9](#atmospheric-scattering-model)

3.4.5 Instrumental Effects Models [9](#instrumental-effects-models)

3.4.6 Detector Response Matrix Generation
[10](#detector-response-matrix-generation)

3.4.6.1 DRM Definition [10](#drm-definition)

3.4.6.2 Direct Response [10](#direct-response)

3.4.6.3 Atmospheric Scattering [10](#atmospheric-scattering)

3.4.6.4 Source Geometry [10](#source-geometry)

3.4.6.5 Changing Detector Aspect [10](#changing-detector-aspect)

3.4.6.6 Statistical Uncertainty [10](#statistical-uncertainty)

3.4.6.7 DRM Data Format [10](#drm-data-format)

3.4.6.8 DRM Database [10](#drm-database)

3.4.6.9 DRM Database Interpolation [11](#drm-database-interpolation)

3.4.7 Energy Deposit Spectral Data Generation
[11](#energy-deposit-spectral-data-generation)

4\. Notes [11](#notes)

4.1 List of Acronyms [11](#list-of-acronyms)

# SCOPE

This document defines the functional specifications for the GLAST Burst
Monitor (GBM) Simulation (SIM) and Detector Response (DRM) software.
This collection of software will serve several different purposes during
the course of the GBM mission. In the design phase, simulations will aid
in the design and optimization of GBM hardware and flight software
systems. In the test/calibration phase, simulations will be used to aid
in the analysis and interpretation of calibration data. In the
operations phase, simulations will be used to generate detector response
functions (with input from calibrations) crucial to the analysis of
science data. Because of its varied role, the SIM software is an
evolving collection of tools that tracks the development of GBM
hardware, flight software, and data analysis software. The SIM software
is particularly tied to the development of all GLAST mechanical systems,
since detailed mass models are a major source of input that must be
updated when the hardware designs change.

# APPLICABLE DOCUMENTS

The following documents form a part of this specification to the extent
specified herein. Documents referenced in the following specifications,
standards, publications, and procedures are also a part of this
specification and are applicable as described to meet the specifications
of this document.

Document Number Title

GBM-SPEC-1031 GBM Mission Operations and Data Analysis Software
Functional Specification Document

GBM-PLAN-1023 GBM Ground Software Development Plan

GBM-REQ-1007 GLAST Burst Monitor (GBM) Requirements, Verification, and
Compliance (RVC) Document/Database

GBM-MPE-PL-1-1 GLAST Burst Monitor Calibration Plan Document

## Reference Documents

The following documents contain general background information and are
for reference only. As such, they do not constitute part of this
document, but may be of interest to the reader.

Document Number Title

n/a GEANT4 User's Documents. <http://wwwinfo.cern.ch/asd/geant4>

n/a GLAST Burst Monitor: A Proposal to NASA for a Burst Monitor for the
GLAST Mission, Nov. 1999.
<http://gammaray.msfc.nasa.gov/gbm/publications/proposal>

433-SRD-0001 GLAST Project Science Requirements Doc., Draft, July 2000.
<http://glast.gsfc.nasa.gov/project/cm/mcdl/>

# SOFTWARE Specifications

## General Specifications

This section addresses general SIM software specifications that are
independent of individual program elements. The major components are
illustrated in Figure 1.

![](media/image2.pct){width="4.526388888888889in"
height="2.1319444444444446in"}

**Fig. 1.** Major components of SIM software.

### Core Simulation Package

The SIM software shall use a general-purpose radiation transport
simulation package that includes tools for the definition of complex
geometrical mass models, and Monte Carlo simulation of particles through
these models.

A trade study shall be performed to identify the core SIM package. This
study shall consider at least the following parameters:

(1) Ability to meet required functionality (as defined by the
    specifications in this document)

(2) Support and maintenance (especially during the GBM operations &
    analysis phase)

(3) Support by the scientific user community (including gamma-ray
    astronomy)

(4) Support by the Large Area Telescope (LAT) team

(5) Maturity of code design and implementation

(6) Code availability on different computers and operating systems,
    particularly UNIX

(7) Ease of use

(8) Ease of customization

(9) Computing performance

A study was performed considering these factors and the GEANT3 package
from CERN was chosen for initial prototype (design phase) development
because of heritage use and understanding of the code. For all post-
design phase development, the GEANT4 package from CERN shall be used.
This package exceeds all of the functionality requirements described
herein, and will be supported and maintained by its developers (and the
scientific community) for the foreseen duration of the GLAST mission. It
is also the package being used by the LAT simulation team.

### Radiation Transport Physics

In order to simulate the physical response of the GBM detectors, the SIM
software shall have the capability to perform Monte Carlo particle
transport simulations of primary photons and their secondary photons and
electrons. The simulation process shall include at least the following
electromagnetic interaction processes (preferably as part of the core
simulation package):

(1) Photoelectric absorption, including emission of florescence X rays

(2) Incoherent (Compton) scattering of photons, including scattering
    form factors

(3) Coherent (Rayleigh) scattering of photons

(4) e^+^e^-^ pair production by photons

(5) Ionization losses of electrons and positrons

(6) Multiple scattering of electrons and positrons

(7) Bremsstrahlung radiation from electrons and positrons

(8) Positron annihilation

The total cross-sections used in the simulations for these processes
shall be demonstrated to be accurate to 10% over the energy range 1 keV
to 1 GeV.

### Input Mass Models

The SIM software shall accept as input descriptions of mass model
geometry and composition stored in external data sets. Mass model data
sets shall contain all information needed to construct the mass model,
such that the SIM software shall be independent of specific mass models.

### Input Photon Energy Distributions

The SIM software shall be capable of generating random incident photon
energies from at least the following spectral distributions:

(1) Mono energetic

(2) Combination of up to 10 discrete energies with user-specified
    intensity weighting

(3) An arbitrary tabulated differential spectrum read from a file within
    a user-specified energy range

(4) Power law with user specified parameters and energy range

(5) Broken power law with user-specified parameters and energy range

(6) Band gamma-ray burst function with user specified parameters and
    energy range

### Input Photon Spatial Distributions

The SIM software shall be capable of generating random incident photon
directions from at least the following angular distributions:

(1) A plane-parallel beam from a user-specified direction

(2) An isotropic distribution with user-specified angular limits

(3) A divergent beam emanating from a user-specified location, with
    user-specified angular limits

(4) An arbitrary angular distribution specified by a tabulated map of
    flux as a function of direction read from a file in FITS format.

The starting positions shall be randomly selected from a simple,
user-specified, surface such as a sphere or cylinder that envelops the
region of interest.

### Input Flux

The SIM software shall accept as input user-specified incident flux
defined as either the total incident flux at a particular energy (in
physical units such as photons/cm^2^/keV), the total number of incident
photons over some energy range, or the total number of detected photons.

### Instrumental Effects

The SIM software shall include the capability to simulate instrumental
effects unique to individual GBM detectors, such as energy resolution
and energy thresholds. Parameters defining such effects shall be read
from input data sets. The portion of software that applies instrumental
effects shall be made separate from the radiation transport code such
that instrumental effects may be applied with different parameters
without repeating the radiation transport.

### Recorded Data

The SIM software shall include the capability to accumulate and record
the energy deposited by incident particles and their secondaries in
sensitive components of the mass model. Sensitive components will be
defined by the GBM science team as the SIM system is developed. Recorded
data shall also include flags to indicate the type of the first
interaction (i.e., photoelectric absorption, Compton scatter, etc.) and
the mass model component where the interaction took place

![](media/image3.png){width="5.5375in" height="4.539583333333334in"}

**Fig. 2.** Development flow of SIM input models.

## Specifications Specific to GBM Design Phase

This section addresses SIM software specifications that are specific to
the GBM design phase. The major goal in this phase is to assist in the
development of the instrument design.

### Design-phase Core Simulation Package

The core radiation transport simulation package used for the
design-phase system is not required to be the same as that adopted for
later phases of SIM software.

### Design-phase Mass Models

The SIM software shall include mass models of the individual GBM
detector modules in prototype configuration. These models shall at least
include major detector components such as crystals, housings, and PMT
assemblies, but not minor components. Dimensions of components shall be
accurate to less than 1 mm, and material compositions shall include all
species with 10% or greater contribution to the component mass.

### Design-phase Environment Models

The SIM environment model during the GBM design phase shall be limited
to the individual detectors placed in a uniform surrounding medium, such
as vacuum or air.

### Design-phase Instrumental Effects Models

For the design phase, SIM software shall include instrumental effects
limited to a user-specified detector energy threshold, and a model for
the detector energy resolution as a function of energy deposit. In this
phase it is not required for the instrumental effects software to be
separate from the radiation transport code.

## Specifications Specific to GBM Calibration Phase

The following specifications address SIM support of GBM calibrations.
The primary SIM function in this phase is to verify the simulation model
through comparison with experimental calibration data (and adjust the
model as needed).

### Supported Calibrations

The SIM software shall support calibrations of the GBM flight
instrument, as defined in the GBM Calibration Plan Document.
Specifications for the support of additional calibrations (such as
calibrations at different facilities) are limited to those covered here.

### Detector-Level Calibrations

In this document, detector-level calibrations refer to those
calibrations where detectors are tested independently, with separate
data processing systems.

#### Detector Mass Models

The SIM software shall include high-fidelity mass models of the
individual GBM flight detector modules in their detector-level
calibration configuration. The detector models shall include the mass of
all detector components as documented during their fabrication. The
elemental mass composition and geometry shall be based on
design/fabrication documents. Dimensions of components shall be accurate
to less than 0.1 mm, and material compositions shall include all species
with 1% or greater contribution to the component mass.

#### Detector Calibration Environment Model

The SIM software shall include models of the detector-level calibration
environment sufficient to account for major sources of scattered
radiation, such as source holders and mounting fixtures. Where possible,
the environment models shall be based on measured (not estimated)
properties of the experimental setup.

#### Detector Calibration Instrumental Effects Model

The SIM software shall include instrumental effects models appropriate
for conditions during detector-level calibration, including at least
energy resolution and thresholds for individual GBM detectors.

### System-Level Calibrations

In this document, system-level calibration refers to calibrations where
detectors are tested together with a common data processing unit.

#### Detector Mass Models

The SIM software shall include high-fidelity mass models of the
individual GBM flight detector modules in their system-level calibration
configuration. These models shall be adapted from those used to simulate
detector-level calibrations. The elemental mass composition and geometry
shall be based on design/fabrication documents. Dimensions of components
shall be accurate to less than 0.1 mm, and material compositions shall
include all species with 1% or greater contribution to the component
mass.

#### System Calibration Environment Model

The SIM software shall include models of the system-level calibration
environment sufficient to account for major sources of scattered
radiation, such as source holders and mounting fixtures. Where possible,
environment models shall be based on measured (not estimated) properties
of the experimental setup.

#### System Calibration Instrumental Effects Model

The SIM software shall include instrumental effects models appropriate
for conditions during system-level calibration.

### On-spacecraft Calibrations

In this document, on-spacecraft calibrations are those calibrations of
the GBM system performed after the GBM detectors and LAT hardware are
mounted on the GLAST spacecraft.

#### Detector Mass Models

The SIM software shall include high-fidelity mass models of the
individual GBM flight detector modules in their on-spacecraft
calibration configuration. These models shall be adapted from those used
to simulate detector or system-level calibrations. The elemental mass
composition and geometry shall be based on design/fabrication documents.
Dimensions of components shall be accurate to less than 0.1 mm, and
material compositions shall include all species with 1% or greater
contribution to the component mass.

#### Spacecraft Mass Model

The SIM software shall include a mass model of the GLAST spacecraft in
its configuration during GBM on-spacecraft calibration (with the
integrated LAT). The spacecraft/LAT mass model shall at least include
all major assemblies, as well as minor assemblies located near the GBM
detectors. This model shall be derived from data provided by the
spacecraft contractor and/or LAT team. Models of the GBM detectors shall
be incorporated into the spacecraft model, including the appropriate
mounting hardware.

#### On-spacecraft Calibration Environment Model

The SIM software shall include models of the on-spacecraft-level
calibration environment sufficient to account for major sources of
scattered radiation, such as source holders and mounting fixtures. Where
possible, environment models shall be based on measured (not estimated)
properties of the experimental setup.

#### On-spacecraft Calibration Instrumental Effects Model

The SIM software shall include instrumental effects models appropriate
for conditions during on-spacecraft calibration.

## Specifications Specific to GBM Operations Phase

In this document, operations phase refers to the operation of the GBM
system after successful launch and turn-on when it is part of the GLAST
observatory. The SIM software for the mission operations phase is
considered part of the GBM Mission Operations and Data Analysis
Software. The primary function of the SIM software in this phase is the
generation of Detector Response Matrices (DRM) used for analysis of
flight science data. Software for this purpose is called "SIM-DRM"
software.

### Coding Standards

In general, the SIM-DRM software shall abide by the coding standards
defined in the GBM Mission Operations and Data Analysis Software
Functional Specification Document. However, the programming language
used for SIM software is not required to be the same as that used for
other data analysis programs.

### Maintenance & Configuration Control

The SIM-DRM software and its key input datasets, such as mass models and
instrument performance parameters, shall be maintained and configuration
controlled in the manner defined in the GBM Mission Operations and Data
Analysis Software Functional Specification Document.

### Mass Models

#### Detector Mass Models

The SIM-DRM software shall include high-fidelity mass models of the
individual GBM flight detector modules in their flight configuration.
These models shall be adapted from those used to simulate on-spacecraft
calibrations. The elemental mass composition and geometry shall be based
on design/fabrication documents. Dimensions of components shall be
accurate to less than 0.1 mm, and material compositions shall include
all species with 1% or greater contribution to the component mass.

#### Spacecraft Mass Model

The SIM-DRM software shall include a mass model of the GLAST spacecraft
in its operational flight configuration (including the LAT). The
spacecraft/LAT mass model shall at least include all major assemblies,
as well as minor assemblies located near the GBM detectors. This model
shall be derived from those used to simulate on-spacecraft calibrations.
Models of the GBM detectors shall be incorporated into the spacecraft
model, including the appropriate mounting hardware.

### Atmospheric Scattering Model

The SIM software shall include a model of the mass distribution of
Earth's atmosphere for use in simulating the radiation from cosmic
sources that scatters from the earth into the GBM detectors.

### Instrumental Effects Models

The SIM software shall include instrumental effects models appropriate
for conditions during in-flight operation. These models shall be
implemented to allow changes in the instrument performance parameters
during the course of the mission.

### Detector Response Matrix Generation

#### DRM Definition

The SIM-DRM software shall generate detector response matrices that
tabulate the effective detection area of individual GBM detectors as a
function of incident and measured photon energy. The definition of
energy grid shall be an input parameter.

#### Direct Response

The SIM-DRM software shall compute the response of individual GBM
detectors to the flux from cosmic sources at arbitrary directions,
including the response from photons and their secondary particles that
scatter from the spacecraft into the detectors.

#### Atmospheric Scattering

The SIM-DRM software shall compute the response of individual GBM
detectors to the flux from astrophysical sources at arbitrary directions
that scatters from Earth's atmosphere into the detectors. The response
due to atmospheric scattering shall be computed independently from the
direct response.

#### Source Geometry

The SIM-DRM software shall generate detector response matrices for
arbitrary Earth--Source--Spacecraft geometry.

#### Changing Detector Aspect

The SIM-DRM software shall have the capability to generate averaged DRMs
appropriate for time intervals when the spacecraft/detector aspect or
spacecraft--earth--source angle changes by more than 1 degree.

#### Statistical Uncertainty

The SIM-DRM software shall generate DRMs with user-specified statistical
uncertainty.

#### DRM Data Format

The SIM-DRM software shall record DRM data files that adhere to the
high-level data products standards defined in the GBM Mission Operations
and Data Analysis Software Functional Specification Document.

#### DRM Database

The SIM-DRM software shall be used to generate a database of DRMs that
cover the full range source directions and Earth-source geometry with a
sampling resolution of at least 10 degrees. The response due to
atmospheric scattering shall be stored in the database independently
from the direct response. This database shall be stored using the CALDB
architecture as specified in the GBM Mission Operations and Data
Analysis Software Functional Specification Document.

#### DRM Database Interpolation

The SIM-DRM software shall include the capability to generate DRMs for
an arbitrary source direction and arbitrary Earth-source geometry by
interpolating an existing DRM database. This software shall have the
ability to generate DRMs with, or without the response due to
atmospheric scattering. The combination of DRM database and
interpolation algorithm shall be capable of generating DRMs to a
resolution of 0.5 deg with a precision of less than 5% compared to
non-interpolated DRMs.

### Energy Deposit Spectral Data Generation

The SIM-DRM software shall have the capability to generate, for any GBM
detector, energy deposit spectra for cosmic sources at arbitrary
(user-specified) directions and Earth-source geometries. The user shall
have the option to include direct and/or Earth-scattered response. These
spectra shall be recorded in data files that adhere to the high-level
data products standards defined in the GBM Mission Operations and Data
Analysis Software Functional Specification Document.

# Notes

## List of Acronyms

BGO Bismuth Germanate

Dec. Declination

deg degree

DRM Detector Response Matrix

GBM GLAST Burst Monitor

GLAST Gamma-Ray Large Area Space Telescope

GRB Gamma-Ray Burst

keV kilo electron Volts

LAT Large Area Telescope

MB Megabyte

MeV mega electron Volts

MSFC Marshall Space Flight Center

NaI Sodium Iodide

NASA National Aeronautics and Space Administration

PMT Photomultiplier Tube

R.A. Right Ascension

SC Spacecraft

SIM Simulation and detector response

TBR To Be Resolved

TBD To Be Defined

UTC Universal Time (Coordinated)