Honeycomb Chambers for

1
Honeycomb Chambers for Photon Multiplicity
Measurements
G. S. N. Murthy, VECC
2
Photon Multiplicity Measurements..
Task To measure multiplicity and spatial
distribution of photons at forward rapidity on an
event by event basis in high multiplicity
environment
Where _at_ RHIC in STAR experiment- 5.4m from
IP _at_ LHC in ALICE experiment - 3.6m from IP

• Multiplicity numbers
• STAR PMD - dn/d 600
• ALICE PMD - dn/d 4000

3
Basic Electromagnetic Interactions
g
e / e-
s
Photoelectric
dE/dx
E
Ionisation
E
s
Compton
Bremsstrahlung
dE/dx
E
E
s
Pair Production
E
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Basic Electromagnetic Interactions

• Energy loss by Bremsstrahlung is proportional
  to E/m2
• effect plays role only for e/- ultra
  relativistic muons
• gt1000GeV
• 10GeV/c electron looses only 0.4MeV by dE/dx
  and 100Mev by bremsstrahlung.
• Photoelectric cross section is proportional to
  Z5
• At high energies pair production cross section
  is independent of energy lpair 9/7 Xo where
  Xo Radiation length (gm/cm2)

5
Electromagnetic Shower development..
e ? Critical energy
g
e, g
e
E
Shower maximum tt(E,e)and there must be a
difference between e and g
for e
for g
The lateral spread of the shower is mainly
governed by the multiple scattering of the
electrons. Define the Molière Unit RM
Characteristic Energy Radiation Length /
Critical Energy 21 MeV X0 / e ? A/Z (g
cm-2) 95 of the shower is contained inside a
cone with radius 2RM
Radiation Length X0 The average length in a
specific material in which a relativistic charged
particle will lose 67 percent of its energy by
bremsstrahlung.
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Photon Detectors..

7
Pre-Shower detector..
Photons
Pre-Shower Detector
(Shallow detector to avoid shower blowup)
Veto Detector
Converter
(High Density, Low Molier radius)
8
Converter thickness..
9
Detector design considerations..

• Explore suitable detector technology options
  (good cost / performance ratio)
• Run simulations based on distance from IP,
  Multiplicity and limits on multi-hit probability
  to obtain optimum granularity required
• Study feasibility of implementing the required
  granularity in the chosen detector medium

10
Detector design considerations..

• Silicon Good but very expensive to cover
  large areas
• Scintillator
• Readout costs
• Difficult to implement for
  small pads (low light
• collection)
• Neutron response
• Gas Low cost , easy to implement at
  high
• granularities
• No Neutron response

11
Gas detector basics recap

• Noble gases like Argon are used as main
  component in gas detectors due to
  absence of energy loss mechanisms other than
  ionization excitation.
• In thin layers total specific ionization
  (primarysecondary) follows Landau distribution.
• Quenching component of the gas mixture absorbs
  photon emission from the main component. Also
  electron drift velocities are enhanced due to
  addition of quenchers.
• Positive ions drift a longer distance and hence
  have a major contribution towards signal
  formation.
• Positive ions during their drift towards the
  cathode, transfer their charge to other
  additive(quencher)molecules of lower ionization
  potential. At the end only the components of the
  gas with lowest ionization potential reach
  cathode.
• Gas detectors have two regions of electric field
  -drift and multiplicative.

12
Limitations of MWPCs..
We need one hit - one pad (cell) response
13
Detector design considerations..
14
Tests with single cell prototype..

15
Concept of Honeycomb detector..
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Cell Wall Optimisation..

17
Cell Simulation..
18
Cell Simulation..

19
Detector Elements of PMD..
G. S. N. Murthy, VECC
20
Components of a Unit Module
Top PCB
FPC Connector
Edge Frame
Honeycomb
Bottom PCB
21
STAR PMD Unit Module..
Gassiplex boards plugged onto the connector

• Each unit module contains
• 24x24 array of cells (576)
• Size of unit module
• Rhombus of side 255 mm
• Weight of unit module 700 gm

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Photon Multiplicity Detector
Charge
PHOTON
PHOTON
particle
Charge particle
Ch particle
Ch particle
Veto detector
Veto detector Plane
SS support
SS support
Pb
converter
Pb converter
Pre
-
shower
Pre-shower Detector Plane
-
detector
g
honeycomb
honeycomb
cells
planes
Isolated Ch
Isolated Charge
particle hit
particle hit
-
cluster
-
g
g
cluster
Honeycomb array
Honeycomb array
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Layout ALICE PMD
24
PMD in ALICE
25
Single cell to Honeycomb (STAR PMD)..
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Unit Module..
Unit Honeycomb (ALICE PMD)
Single Cell
Unit Honeycomb from factory 48 x 12 Cell array

• 8 unit honeycombs to be joined by conductive
  epoxy to form an 48 x 96 cell array for
  unit module

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Unit Module..
Bonding cells to form Honeycombs (ALICE PMD)
28
Honeycomb sizes ..
419
242.5
482.5
Type B
Type A
210
29
Unit Module types..

• Why two types?

To keep the alignment of the FPC connectors
vertical on the entire PMD plane for streamlined
air cooling
30
Cooling FEE

New ½ plane scale cooling model is under study
Warm air out
Ambient air in
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Top PCB of the Unit Module (ALICE PMD)..
Solder pads for FPC connector
Solder pads for FPC connector
Solder pads for Anode Wire
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Unit Module Assembly..
Edge Frame
HV Cable
Top PCB
Fasteners
Bottom PCB
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Unit Module..
Wiring jigs
Unit Module
Counter Weight
Height adjust slider
Lever arm
Slot for holding Unit Module
Anode Wire
Teflon pully
Support columns
Base Plate
Top View
Anode wire
Teflon Pully Cross-section
Base Weight
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Unit module..
Wiring..
Hypodermic needle (0.25mm dia)
Tungsten wire (20 micron)
Top PCB
Solder
Honeycomb Cathode
Bottom PCB
STAR PMD Unit module wiring
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Wire cutting near FPC connector pads..

FPC Connector
Solder pads for FPC connector
Sequence Wiring gt connector soldering gt wire
cutting
36
Gas Flow inside the Module..
37
Unit Module Edge Frame..
38
How the modules are positioned on the planes..
39
Test Beam Prototype..
40
G. S. N. Murthy, VECC
41
Prototype performance..
42
Performance..
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ready to move in soon