Table of Contents

1. The prototype geometry
2. Structure of the geometry description
3. Reference
   1. Constants
   2. Solids
   3. Logical volumes
   4. Physical volumes
      1. Notes on the CopyNumber
   5. User information
   6. Python code with all variables for testing

The prototype geometry

This document describes the current version of the GDML description of the LDMX testbeam prototype. The hadronic calorimeter prototype, shown below, is a sampling calorimeter with steel absorber plates and alternating layers of polystyrene scintillator bars. There are in total 19 layers, divided into two regions with 8 (shown in green below) and 12 (shown in blue below) scintillator bars per layer respectively.

The scintillator bars are currently not represented individually in the geometry description and are treated as a single box of scintillator material.

Structure of the geometry description

The geometry is divided into several files to make working with it easier.

• detector.gdml This is the entrypoint of the geometry. Contains the actual implementation of the prototype geometry using the definitions from hcal_volumes.gdml
• constants.gdml Contains variable definitions used elsewhere in the geometry such as positions and geometrical properties of different volumes.
• materials.gdml Contains definitions of isotopes and materials
• userinfo.gdml Contains auxiliary information (see the GDML manual for details), such as sensitive detector volumes, visualization information, version and author information.
• hcal_solids.gdml Contains definitions of the absorber, scintillator, and parent boxes
• hcal_volumes.gdml Contains the definition of the scintillator and absorber logical volumes, see Logical volumes.
• hcal_physvol.gdml Contains the Hcal physical volumes, see Physical volumes
• trigger_solids.gdml Contains definitions of the trigger scintillator boxes
• trigger_volumes.gdml Contains the definition of the trigger scintillator logical volumes, see Logical volumes.
• trigger_physvol.gdml Contains the trigger scintillator physical volumes, see Physical volumes

Reference

The following is a list of all the variables defined in the protoype geometry description.

Constants

Dimension constants

• world_dim is the size of the sides of the “world_box” box, set to 10 m

• air_thickness is the thickness (Δ z_air) of the layers of air in between the absorber and scintillator bars. The thickness of each region is set to 2 mm, which contributes twice to the thickness of an individual layer.

• absorber_width is the width (Δ x_absorber) of an absorber layer, set to 665 mm

• absorber_height is the height (Δ y_absorber) of an absorber layer, set to 700 mm

• absorber_thickness is the thickness (Δ z_absorber) of an absorber layer, set to 20 mm

• absorber_radius is the radius of the two holes on the absorber plate, set to 6.5 mm

• absorber_hole_distance_from_edge is the distance of each hole's center from the edges of the absorber plate, set to 16. mm

• bar_mounting_plate_thickness is the thickness of the plates upon which the bars are mounted, set to 3. mm

• scint_thickness is the thickness of a scintillator bar layer (Δ z_scintillator). This is equivalent to the thickness of an individual scintillator bar and is set to 20 mm

• scint_bar_width is the width of an individual scintillator bar, it is set to 50 mm.

• scint_bar_length is the length of an individual scintillator bar. In horizontal layers, this is equivalent to the width (Δ x_scintillator) of the layer while in vertical layers it is equivalent to the height (Δ y_scintillator) of the layer. It is set to 2000 mm.

• layer_thickness is the combined thickness of a layer (Δ z_layer). It consists of one layer of absorber, absorber_thickness, one layer of scintillator bar mounting plates, bar_mounting_plate_thickness, one layer of scintillators, scint_thickness, and on layer of air, air_thickness. The resulting layer thickness is thus given by absorber_thickness + bar_mounting_plate_thickness + scint_thickness + air_thickness, 45 mm

• num_bars_front is the number of scintillator bars in the front region. It is set to 8

• num_bars_back is the number of scintillator bars in the back region. It is set to 12

• num_layers_front is the total number of layers in the front region of the prototype and is set to 9

• num_layers_front_vertical and num_layers_front_horizontal are the number of vertical/horizontal layers in the front region. They are set to 4 and 5 respectively

• num_layers_back_vertical and num_layers_back_horizontal are the number of horizontal or vertical layers in the back region. They are both set to 5

• num_layer_back is the total number of layers in the back region of the prototype and is set to 10

• num_layers is the total number of layers. It is set to 19 and corresponds to num_layers_front + num_layers_back

• back_start is the location of the first layer in the back region. It is set to num_layers_front * layer_thickness, i.e. 405 mm

• The length of the sides of the various scintillator layers are defined as

  • Front vertical layers have scint_front_vertical_x which is 400 mm, scint_front_vertical_y which is the bar length, 2000 mm. The 400 mm corresponds to 8 bars each having a width of 50 mm, num_bars_front * scint_bar_width
  • Front horizontal layers have scint_front_horizontal_y which is 400 mm, scint_front_horizontal_x which is the bar length, 2000 mm. The 400 mm corresponds to 8 bars each having a width of 50 mm, num_bars_front * scint_bar_width
  • Back vertical layers have scint_back_vertical_x which is 600 mm, scint_back_vertical_y which is the bar length, 2000 mm. The 600 mm corresponds to 12 bars each having a width of 50 mm, num_bars_back* scint_bar_width
  • Back horizontal layers have scint_back_horizontal_y which is 600 mm, scint_back_horizontal_x which is the bar length, 2000 mm. The 600 mm corresponds to 12 bars each having a width of 50 mm, num_bars_back* scint_bar_width

• The length of the sides of the two scintillator bar mounting plates are defined as

  • Vertical layers have bar_mounting_plate_vertical_x which is 600 mm, bar_mounting_plate_vertical_y which is the bar length, 2000 mm. The 600 mm corresponds to the width of 12 50 mm bars, num_bars_back * scint_bar_width
  • Horizontal layers have bar_mounting_plate_horizontal_y which is 600 mm, bar_mounting_plate_vertical_x which is the bar length, 2000 mm. The 600 mm corresponds to the width of 12 50 mm bars, num_bars_back * scint_bar_width

• dx and dy, the width and height of the prototype respectively are both set to 3000 mm

• dz is the depth of the prototype and is defined as num_layers * layer_thickness which correpsonds to 855 mm

• trigger_bar_gap is the gap between individual trigger scintillator bars. It is 0.3 mm

• trigger_bar_dx (40 mm), trigger_bar_dy (3 mm), and trigger_bar_dz (2 mm) are the dimensions of a trigger scintillator bar.

• There are two trigger scintillator layers, and each layer includes 6 bars, as set in number_of_trigger_bars (=6). This means that the detector has 12 bars in total

Position constants

• center, identity, and hadron_calorimeter_pos are positions that are all defined as (x,y,z) = (0,0,0). hadron_calorimeter_pos is used to place the prototype_volume physical volume within the World volume.

• The location (in z) of the first absorber layer is given by absorber_first_layer_zpos which is defined as -dz/2, corresponding to -427.5 mm

• The location (in z) of the first layer of each scintillator bar mounting plate group are given by

  • bar_mounting_plate_horizontal_first_layer_zpos which is placed in the middle of the plate after one absorber and behind one bar layer absorber_first_layer_zpos + absorber_thickness/2.0 + bar_mounting_plate_thickness/2.0, corresponding to -416 mm
  • bar_mounting_plate_vertical_first_layer_zpos which is placed one layer_thickness after the first horizontal layer at layer_thickness + bar_mounting_plate_horizontal_first_layer_zpos, corresponding to -371 mm

• The location (in z) of the first layer of each scintillator group are given by

  • scint_front_horizontal_first_layer_zpos which is placed in the middle of the bar after one absorber layer and one air layer at absorber_first_layer_zpos + absorber_thickness + air_thickness + scint_thickness/2, corresponding to -404.5 mm
  • scint_front_vertical_first_layer_zpos which is placed one layer_thickness after the first horizontal layer at layer_thickness + scint_front_horizontal_first_layer_zpos, corresponding to -359.5 mm
  • scint_back_vertical_first_layer_zpos which is placed at the start of the back region at a distance back_start from the first scintillator layer at back_start + scint_front_horizontal_layer_zpos, corresponding to 0.5 mm
  • scint_back_vertical_first_layer_zpos which is placed one layer_thickness after the first horizontal layer in the back region at layer_thickness + scint_back_vertical_first_layer_zpos, corresponding to 45.5 mm

• trigger_layer_distance_from_detector defines the distance (the exact length of the empty space) between the trigger scintillator layer and the detector.

Solids

Solids are objects with purely geometrical properties such as a box, a sphere, or a more complicated geometry

• absorber is the solid that represents the absorber layer. It is a plate with a small hole drilled near the top left edge and another small hole drilled near the top right edge. Within Geant4, it is described as a box with two tubes subtracted from it. Its description uses these constants:
  • absorber_width is the overall width of the absorber plate. It is 665 mm.
  • absorber_height is the overall height of the absorber plate. It is 700 mm.
  • absorber_thickness is the thickness of the absorber plate, i.e. the length that the beam travels through the material. It is 20 mm.
  • absorber_hole_radius is the radius of the two holes on the absorber plate, set to 6.5 mm
  • absorber_hole_distance_from_edge is the distance of each hole's center from the edges of the absorber plate, set to 16. mm
• There are two boxes representing the two different types of scintillator bar mounting plates (vertical, horizontal). These are defined using the corresponding width/height from Constants, e.g.
  • horizontal_bar_mounting_plate_box has width bar_mounting_plate_horizontal_x (2000 mm) and height bar_mounting_plate_horizontal_y (600 mm)
  • vertical_bar_mounting_plate_box has width bar_mounting_plate_vertical_x (600 mm) and height bar_mounting_plate_vertical_y (2000 mm)
  • For both, their thickness is defined by bar_mounting_plate_thickness, set to 3. mm.
• There are four boxes representing the four different types of scintillator layers (front vertical, front horizontal, back vertical, back horizontal). These are defined using the corresponding width/height from Constants, e.g.
  • front_vertical_scint_box has width scint_front_vertical_x (400 mm) and height scint_front_vertical_y (2000 mm)
  • front_horizontal_scint_box has width scint_front_horizontal_x (2000 mm) and height scint_front_horizontal_y (400 mm)
  • back_vertical_scint_box has width scint_back_vertical_x (600 mm) and height scint_back_vertical_y (2000 mm)
  • back_horizontal_scint_box has width scint_back_horizontal_x (2000 mm) and height scint_back_horizontal_y (600 mm)
• air_box is the a box representing a single air layer and has width dx (3000 mm), height dy (3000 mm), and depth air_thickness (2 mm)
• prototype_box is the parent volume for the prototype and is defined as a box with width dx (3000 mm), height dy (3000 mm), and depth dz (855 mm)
• world_box is the parent volume for all the other parts of the geometry and is defined as a box with all sides having length world_dim (10 m)
• trigger_bar_box represents an individual trigger scintillator bar. trigger_bar_dx (40 mm), trigger_bar_dy (3 mm), and trigger_bar_dz (2 mm) are the dimensions of a trigger scintillator bar.

Logical volumes

In Geant4, a logical volume can contain all of the information about a volume except for its position. This allows you to use one logical volume to create several distinct daughter physical volumes. The position and rotation of a daughter volume is defined in terms for the mother volume. The logical volumes that we use in this geometry can contain the following tags

• <solidref> is a reference to one of the solids defined in hcal_solids.gdml
• <materialref> is a reference to a material defined in materials.gdml
• <auxiliary> allows us to add any other kind of information that is used by the simulation, such as defining if a volume is supposed to be a sensitive element or how the volume should be visualized by default. Most auxiliary tags will be references to groups of properties defined in User information.
• <physvol> any daughter volumes that are to be placed within the logical volume, see Physical volumes

Furthermore, each logical volume has a name as part of the <volume> tag which can be used to refer to the volume using the <volumeref> tag. At least one logical volume has to be the “World” volume. This volume determines the global coordinate system and has to completly contain all other volumes, sharing surfaces with none of them.

• World is the “World” volume. It is defined in detector.gdml

  • Material: G4_AIR
  • Solid: world_box
  • Daughter volumes:
    • prototype_volume
  • Auxiliary information:
    • “DetElem”: “Top”

• prototype_volume represents the entire prototype and is defined in detector.gdml

  • Material: G4_AIR
  • Solid: prototype_box
  • Daughter volumes:
    • absorber_physvol
    • front_horizontal_scint_physvol
    • front_vertical_scint_physvol
    • back_horizontal_scint_physvol
    • back_vertical_scint_physvol
  • Auxiliary information:
    • “Region”: “CalorimeterRegion”
    • “VisAttributes”: “HcalVis”
    • “DetElem”: “Hcal”

• absorber_volume represents one layer of the steel absorber and is defined in hcal_volumes.gdml

  • Material: Steel
  • Solid: absorber_box
  • Auxiliary information:
    • “Color”: “Red”
    • “VisAttributes”: “HcalVis”

• There are two volumes representing each of the two different types of scintillator bar mounting plate layers called vertical_bar_mounting_plate_volume, and horizontal_bar_mounting_plate_volume, all defined in hcal_volumes.gdml. They differ in name and which corresponding solid they make use of

  • Material: Steel
  • Solid: One of vertical_bar_mounting_plate_box, and horizontal_bar_mounting_plate_box
  • Auxiliary information:
    • “Color”: “Red”
    • “VisAttributes”: “HcalVis”

• There are four volumes representing each of the four different types of scintillator layers called front_vertical_scint_box_volume, front_horizontal_scint_box_volume, back_vertical_scint_box_volume, and back_horizontal_scint_box_volume, all defined in hcal_volumes.gdml. They differ in name and which corresponding solid they make use of

  • Material: “Scintillator”
  • Solid: One of front_vertical_scint_box, front_horizontal_scint_box, back_vertical_scint_box, and back_horizontal_scint_box
  • Auxiliary information:
    • “SensDet”: “HcalSD”
    • “Color”: “Blue”
    • “VisAttributes”: “HcalVis”

• trigger_bar_volume represents a single trigger scintillator bar and is defined in trigger_volumes.gdml

  • Material: Polyvinyltoluene
  • Solid: trigger_bar_box
  • Auxiliary information: “SensDet”: “TriggerPadUpSD” (meaning that the simulation will consider it a sensitive detector region, and will categorize its measurements under “TriggerPadUpSD&rdquo in the output root file.

Physical volumes

A physical volume is a logical volume with a position and, optionally, a name and a so-called CopyNumber. The CopyNumber should be unique for each physical volume. In LDMX-sw, the CopyNumber is used to identify which readout-channels a given physical volume corresponds to so some care must be taken when working on the geometry to ensure that the position of the physical volume and the corresponding CopyNumber aligns. For details see Notes on the CopyNumber.

• The physical volume representing the prototype volume is unnamed
  • Mother volume: World
  • Logical volume: prototype_volume
  • Position: hadron_calorimeter_pos
  • Rotation: identity
• absorber_physvol: There are 19 physical volumes representing the absorber layers
  • Logical volume: absorber_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [1 .. 19]
  • Position of the first layer:
    • x: 0 mm
    • y: 0 mm
    • z: absorber_first_layer_zpos, i.e. -427.5 mm
  • Distance (z) to subsequent layer: layer_thickness, i.e. 45 mm
• horizontal_bar_mounting_plate_physvol: There are 10 physical volumes representing the horizontal scintillator bar mounting plate layers. They all have odd CopyNumbers.
  • Logical volume: front_horizontal_scint_box_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [1,3,5,7,9,11,13,15,17,19]
  • Position of the first layer:
    • x: 0 mm
    • y: 0 mm
    • z: bar_mounting_plate_horizontal_first_layer_zpos, i.e. -416 mm
  • Distance (z) to subsequent layer: 2 * layer_thickness, i.e. 90 mm
• vertical_bar_mounting_plate_physvol: There are 9 physical volumes representing the horizontal scintillator bar mounting plate layers. They all have even CopyNumbers.
  • Logical volume: front_horizontal_scint_box_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [2,4,6,8,10,12,14,16,18]
  • Position of the first layer:
    • x: 0 mm
    • y: 0 mm
    • z: bar_mounting_plate_horizontal_first_layer_zpos, i.e. -371 mm
  • Distance (z) to subsequent layer: 2 * layer_thickness, i.e. 90 mm
• front_horizontal_scint_physvol: There are 5 physical volumes representing the horizontal scintillator layers in the front region. They all have odd CopyNumbers.
  • Logical volume: front_horizontal_scint_box_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [1,3,5,7,9]
  • Position of the first layer:
    • x: 0 mm
    • y: 0 mm
    • z: scint_front_horizontal_first_layer_zpos, i.e. -404.5 mm
  • Distance (z) to subsequent layer: 2 * layer_thickness, i.e. 90 mm
• front_vertical_scint_physvol: There are 4 physical volumes representing the vertical scintillator layers in the front region. They all have even CopyNumbers.
  • Logical volume: front_vertical_scint_box_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [2,4,6,8]
  • Position of the first layer:
    • x: 0 mm
    • y: 0 mm
    • z: scint_front_vertical_first_layer_zpos, i.e. -359.5 mm
  • Distance (z) to subsequent layer: 2 * layer_thickness, i.e. 90 mm
• back_vertical_scint_physvol: There are 5 physical volumes representing the vertical scintillator layers in the back region. They all have even CopyNumbers.
  • Logical volume: back_vertical_scint_box_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [10, 12, 14, 16, 18]
  • Position of the first layer:
    • x: 0 mm
    • y: 0 mm
    • z: scint_back_vertical_first_layer_zpos, i.e. 0.5 mm
  • Distance (z) to subsequent layer: 2 * layer_thickness, i.e. 90 mm
• back_horizontal_scint_physvol: There are 5 physical volumes representing the horizontal scintillator layers in the back region. They all have odd CopyNumbers.
  • Logical volume: back_horizontal_scint_box_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [11, 13, 15, 17, 19]
  • Position of the first layer:
    • x: 0 mm
    • y: 0 mm
    • z: scint_back_horizontal_first_layer_zpos, i.e. 45.5 mm
  • Distance (z) to subsequent layer: 2 * layer_thickness, i.e. 90 mm
• trigger_physvol: There are 12 physical volumes representing the trigger scintillator bars
  • Logical volume: trigger_bar_volume
  • Mother volume: prototype_volume
  • CopyNumbers: [1 .. 12]
  • Position of the first layer:
    • x: 0, i.e. 0 mm
    • y: -trigger_bar_dy/2, i.e. -1.5 mm
    • z: -trigger_bar_dz - trigger_bar_gap -dz/2 -trigger_layer_distance_from_detector, i.e. ** -? mm**
  • Position of the second layer:
    • x: 0, i.e. 0 mm
    • y: 0, i.e. 0 mm
    • z: -dz/2 -trigger_layer_distance_from_detector, i.e. ** -? mm**
  • Distance (y) between bars in the same layer: trigger_bar_dy + trigger_bar_gap, i.e. 3.3 mm
  • Distance (z) between the two layers: trigger_bar_dz + trigger_bar_gap, i.e. 2.3 mm

Notes on the CopyNumber

For the hadronic calorimeter, the CopyNumber encodes the layer number and the section number of the scintillators. The section number refers to the five distinct sections of the full LDMX Hcal (Back, Top, Bottom, Left, Right). The prototype geometry consists entirely of the “Back” section of the Hcal which has section number 0. The section number is derived from the CopyNumber by taking the modulo of the CopyNumber with 1000

The layers of the protoype are numbered from 1 to 19 (note the non-zero based indexing). The layer number is derived from the CopyNumber by dividing the CopyNumber with 1000 and taking the remainder.

Furthermore, the rotation of the layer is determined by the layer number, which in turn depends on the CopyNumber. /This relationship differs between the current (April, 2022) mainline hcal geometry and the prototype geometry/. An odd CopyNumber corresponds to a vertical layer (length of the bars is along the y-axis) while an odd CopyNumber corresponds to a horizontal layer (length of the bars is along the x-axis).

The bars of the trigger scintillator are numbered from 0 to 11 (note the zero based indexing), in strictly increasing order as the y coordinate increases, with the first (the one closer to the origin of the beam) layer having even CopyNumbers and the second layer having odd ones. The reasoning is that this scheme allows the simplest possible grouping of adjacent hits and subsequent tracking in a barID coordinate system, which in turn makes tracking equally effective regardless of whether the bars are in a vertical or horizontal direction.

User information

Python code with all variables for testing

prototype-geometry-reference.py is a short python script containing definitions for all of the variables used to describe the physical volumes that can be used to test or calculate. All units are mm