1. Development of a Digital Hadron Calorimeter (DHCAL)
Lei Xia
Argonne National Laboratory
HEP Division

2. RPC DHCAL Collaboration
Argonne
Burak Bilki
Carol Adams
Mike Anthony
Tim Cundiff
Eddie Davis
Pat De Lurgio
Gary Drake
Kurt Francis
Robert Furst
Vic Guarino
Bill Haberichter
Andrew Kreps
Zeljko Matijas
José Repond
Jim Schlereth
Frank Skrzecz
(Jacob Smith)
(Daniel Trojand)
Dave Underwood
Ken Wood
Lei Xia
Allen Zhao
Boston University
John Butler
Eric Hazen
Shouxiang Wu
Fermilab
Alan Baumbaugh
Lou Dal Monte
Jim Hoff
Scott Holm
Ray Yarema
IHEP Beijing
Qingmin Zhang
University of Iowa
Burak Bilki
Edwin Norbeck
David Northacker
Yasar Onel
McGill University
François Corriveau
Daniel Trojand
UTA
Jacob Smith
Jaehoon Yu
IIT
Guang Yang
Daniel Kaplan
RED = Electronics Contributions
GREEN = Mechanical Contributions
Yellow = Students
BLACK = Physicist

3. Motivation: physics at the next lepton collider
– Physics Benchmarks for the ILC Detectors
Required: excellent Jet energy/mass resolution Solution: Particle Flow Algorithm (PFA)

4. Particle Flow Algorithms
Need new approach
Particle Flow Algorithms
ECAL
HCAL γ π+ KL
The idea…
Charged particles Tracker
measured with the
Neutral particles Calorimeter
Particles in jets
Fraction of energy
Measured with
Resolution [σ2]
Charged
65 %
Tracker
Negligible
Photons
25 %
ECAL with 15%/√E
0.072 Ejet
Neutral Hadrons
10 %
ECAL + HCAL with 50%/√E
0.162 Ejet
Confusion
≤ Ejet
18%/√E
Required for 30%/√E
Requirements for detector system
→ Need excellent tracker and high B – field
→ Large RI of calorimeter
→ Calorimeter inside coil
→ Calorimeter as dense as possible (short X0, λI)
→ Calorimeter with extremely fine segmentation
→ Single particle energy resolution not critical
thin active medium
Opens up new
possibilities

5. Digital Hadron Calorimeter (DHCAL) and Choice of active medium
PFA hadron calorimeter  ‘Digital’ readout  implication for active medium
Extremely fine segmentation (~1x1cm2 readout cell)  simple hit counting provide sufficient energy resolution  1bit readout/Digital Hadron Calorimeter (DHCAL)
DHCAL  counting charged particles in hadronic showers (instead of measuring dE/dx in the active medium)  the active medium can have poor, or even no dE/dx resolution
Thin active medium, embedded readout
Cost …
What RPC can offer
Good position resolution  great for fine segmentation
Excellent efficiency for charged particles  great for counting charged particles
Large charge fluctuation for single MIP  no dE/dx resolution, poor particle counting on single pad  but this is exactly NOT needed for a DHCAL
Can be made very thin
Low cost
RPC is a perfect choice for a Digital Hadron Calorimeter!

6. DHCAL R&D started with small prototypes
Small size RPC prototype
Studied several RPC designs and narrowed down to 1-gap design
Measured RPC parameters: efficiency, multiplicity, noise, etc.
Invented 1-glass RPC
Small size DHCAL readout prototype
Prototyped the whole DHCAL readout chain
4-asic, 256ch, 16x16cm2 active area FEB/pad board
Thoroughly debugged
Small DHCAL prototype – the slice test stack
Up to 9 layers
Tested with muon, positron and hadron beams
Analysis done: 6 papers published on NIM, JINST
Gave us valuable experience and confidence in building larger prototype

7. 1 m3 – Digital Hadron Calorimeter Physics Prototype
Description
Readout of 1 x 1 cm2 pads with one threshold (1-bit) → Digital Calorimeter
38 layers in DHCAL and 14 in Tail Catcher, each ~ 1 x 1 m2
Absorber: 16mm Fe + (2mm Fe + 2mm Cu [cassette]) or 10mm W + (2mm Fe + 2mm Cu [cassette]),
thicker Fe plates in Tail Catcher
Each layer with 3 RPCs, each 32 x 96 cm2
~500,000 readout channels
Purpose
Validate DHCAL concept
Gain experience running large RPC systems
Measure hadronic showers in great detail
Validate hadronic shower models (Geant4)
Status
Started construction in 2008
Completed in January 2011
Test beam runs with Fe absorbers started in Oct at Fermilab
Finished Fermilab test beam by the end of 2011
Test beam runs with W absorbers at CERN in 2012
Critical for PFA validation 7 7

8. RPC Construction RPC design RPC production Resistive paint spraying
Glass
Pad board
Frame
RPC design
2 – glass RPCs (chosen for construction)
1 – glass RPCs (developed at Argonne)
Gas gap size 1.1mm
Well arranged gas flow (by fishing line + sleeve spacer)
Total RPC thickness < 3.4mm
Dead area ~5% (frame ~3%, spacer ~2%)
RPC production
~114 for DHCAL + 42 for TCMT + spares  at the end, produced ~ 205 RPC’s
Resistive paint spraying
Uses two-component artist paint
Built dedicated spraying booth
Accept plates with value 1 – 5 MΩ/□
Achieved a yield of ~60% overall
Gap assembly
Developed precision cutting/gluing fixtures
Production rate 1RPC/day/tech 8 8

9. Quality Assurance Pressure tests Gap size measurement HV tests
All RPC’s are tested with 0.3 inch of water pressure
Gap size measurement
Thickness of 1st batch RPC’s measured along the edges
(Gap size away from the edges is assured by spacers)
Gap sizes along edges vary within 0.1 mm
Corners typically thicker (up to 0.3mm, but limited effect)
HV tests
ALL RPC’s tested up to 7.0 kV before placing readout
board on top (nominal operating voltage is 6.3 kV)
Cosmic ray test
9 of the 1st batch RPC’s were tested on a cosmic ray test
stand for performance (efficiency, multiplicity, uniformity)
All other RPC’s were later tested in vertical position
with cosmic rays, after assembled into cassettes
Overall yield: ~95% 9

10. Design consideration for readout system
System is built around a custom ASIC (DCAL chip)
Designed to handle huge number of channels + low channel density
(1cm2 pad, 1m2 planes, 54 planes  497,664K channels)
64ch, handling 8x8 RPC active area
System tailored for test beam / prototype tests, NOT for real colliding beam detector
Facilitates all possible tests (test beam, cosmic ray test, noise runs, system diagnostics, etc)
Avoided cutting-edge technology and fancy functionality
Didn’t optimize for minimum power consumption, minimum data links, minimum thickness, etc.
Compromises aimed at proving detector concept  key in getting system up quickly and running reliably
Two basic running mode
External triggering mode  primary method for beam events, also used for cosmic ray test
20-stage pipeline  2μs 100ns clock cycle
Deadtimeless readout (within rate limit)
Self triggering mode primary method for noise runs, also used for cosmic ray, beam runs
Powerful running mode for monitoring RPC condition
Equally powerful for cosmic ray running and some beam running as well

11. Readout system overview
VME Interface
Data Collectors – Need 10
Timing Module
Double Width
- 16 Outputs
Ext. Trig In
master IN
6U VME Crate
To PC
Data
Concentrator
Front End Board
with DCAL Chips
& Integrated DCON
Chambers – 3 per plane
Data Collectors – Need 10
VME Interface
master IN
Ext. Trig In
To PC
Timing Module
Double Width
- 16 Outputs
Communication Link
- 1 per Front-End Bd
6U VME Crate
Square Meter Plane

12. FrontEnd/DCON board + Pad board
The DCAL Chip
Developed by
FNAL and Argonne
Input
64 channels
High gain (GEMs, micromegas…) with minimum threshold ~ 5 fC
Low gain (RPCs) with minimum thrshold ~ 30 fC
Readout mode
Triggerless (noise measurements)
Triggered (cosmic, test beam)
Versions
DCAL I: initial round (analog circuitry not optimized)
DCAL II: some minor problems (used in vertical slice test)
DCAL III: no identified problems (final production verision)
produced 11 wafers, 10,300 chips for DHCAL physics prototype
Hits/100 tries
Threshold (DAC counts)
FrontEnd/DCON board + Pad board
Build FE and pad boards separately to avoid blind and
buried vias (cost and feasibility issue)
Each board contains 1536 channels and 24 ASICs
The data concentrator is implemented into the same board
Glue the two boards together with conductive epoxy 12

13. Tests in the Fermilab Test Beam
Fermilab Test Beam Facility
120 GeV primary proton beam
1 – 60 GeV/c secondary beam (pions, positrons, muons…)
Broadband muon beam
3.5 second long spills every minute
Muon data taking
+32 GeV/c secondary and 3 m Fe absorber
DAQ rates 500 – 2000/spill
DAQ triggered by a pair of 1 x 1 m2 Scintillator paddles
Secondary beam data taking
DAQ rates 50 – 500/spill
DAQ triggered by a pair of 19 x 19 cm2 Scintillator paddles
1 x 1 m2 Scintillator Paddle A
1 x 1 m2 Scintillator Paddle B
DHCAL
Tail
Catcher
Trigger
DHCAL
Tail
Catcher
Trigger
Cerenkov
19 x 19 cm2 Scintillator A
19 x 19 cm2 Scintillator B

14. The DHCAL in the Test Beam
Fermilab test beam
run dates
DHCAL layers
RPC_TCMT layers
SC_TCMT layers
Total RPC layers
Total layers
Readout channels
10/14/2010 – 11/3/2010 38 16 54
350,
1/7/2011 – 1/10/2011 8 46
350,
1/11/2011 – 1/20/2011 4 42 50
387,
1/21/2011 – 2/4/2011 9 6 47 53
433,
2/5/2011 – 2/7/2011 13 51
470,016+0
4/6/2011 – 5/11/2011 14 52
479,232+0
5/26/2011 – 6/28/2011
11/2/2011 – 12/6/2011
460800
Run I
Run II
Run III
Run IV
Run V
~ 480K readout channels ~ 35M events
DHCAL
Tail Catcher (TCMT)

15. Some event display One muon 60 GeV pion 32 GeV positron
Occasionally, neutral hadron

16. Energy reconstruction in the DHCAL
Data
Consists of hit patterns of pads with signal above 1 threshold and
their time-stamps with 100 ns resolution
Incident particle energy reconstruction
To first order
E ∝ N N = ∑layer Ni … total number of hits
Correction for contribution from noise
E ∝ N - Nnoise Nnoise … accidental hits
Correction for variation in chamber inefficiency
E ∝ ∑layer Ni ·(ε0 /εi) – Nnoise ε0 … average DHCAL efficiency
εi … efficiency of layer i
Correction for variation in pad multiplicity
E ∝ ∑layer Ni ·(ε0 /εi) ·(μ0 /μi) – Nnoise μ0 … average pad multiplicity
` μi … average pad multiplicity of layer i
Second order corrections
Compensate for e/h ≠ 1
Saturation (more than 1 particle/pad) …

17. General DHCAL Analysis Strategy
Noise measurement
- Determine noise rate (correlated and not-correlated)
- Identify (and possibly mask) noisy channels
- Provide random trigger events for overlay with MC events (if necessary)
Measurements with muons
- Geometrically align layers in x and y
- Determine efficiency and multiplicity in ‘clean’ areas
- Simulate response with GEANT4 + RPCSIM (requires tuning 3-6 parameters)
- Determine efficiency and multiplicity over the whole 1 x 1 m2
- Compare to simulation of tuned MC
- Perform additional measurements, such as scan over pads, etc…
Measurement with positrons
- Determine response
- Compare to MC and tune 4th (dcut) parameter of RPCSIM
- Perform additional studies, e.g. software compensation…
Measurement with pions
- Compare to MC (no more tuning) with different hadronic shower models
- Perform additional studies, e.g. software compensation, leakage correction…

18. Estimation of contributions from noise
Data collection (in general)
Trigger-less (all hits) mode for noise, cosmics
Triggered (record hits in 7 time bins of 100 ns each) for noise, cosmics, testbeam
→ Only hits in 2 time bins used for physics analysis
Noise measurement
These results from trigger-less mode
In quantitative agreement with measurements with random trigger
Results
Noise rate measured to be 0.1 – 1.0 Hz/cm2
Rate strongly dependent on the temperature of the stack
(FE-electronics embedded into stack and generates heat)
Tail catcher in 4/2011 run
DHCAL in 4/2011 run
DHCAL in 10/2010 run
Noise rate [Hz/cm2]
0.1
0.5
1.0
Nnoise/event in DHCAL + Tail Catcher (2 time bins)
0.009
0.05
0.09
Nnoise/event in DHCAL + Tail Catcher (7 time bins)
0.033
0.165
0.33
1 hit corresponds to ~ 60 MeV!
Contribution from noise negligible for most analysis

19. Tracking Clustering of hits Loop over layers 1 cluster 2 clusters
Performed in each layer individually
Use closest neighbor clustering (one common side)
Determine unweighted average of all hits in a given cluster (xcluster ,ycluster)
Loop over layers
for layer i request that all other layers have Njcluster ≤ 1
request that number of hits in tracking clusters Njhit ≤ 4
request at least 10/38(52) layers with tracking clusters
fit straight line to (xcluster,z) and (ycluster,z) of all tracking clusters j
calculate χ2 of track
request that χ2/Ntrack < 1.0
inter/extrapolate track to layer i
search for matching clusters in layer i within
record number of hits in matching cluster

20. Calibration constants, etc…
Calibration factors = mean of multiplicity distribution/(average over detector) = ε·μ/ ε0·μ0

21. Simulation Strategy Comparison Parameters GEANT4
Experimental set-up
Beam (E,particle,x,y,x’,y’)
Measured signal Q distribution
Points (E depositions in
gas gap: x,y,z)
GEANT4
RPC response simulation
Hits
Parameters
Distance cut dcut
(within which only 1 avalanche)
Charge adjustment Q0
(if needed)
Exponential slopes a1, a2
(of signal spread in pad plane)
Ratio R
(between 2 exponentials)
Threshold T
(of discriminator)
Hits
Comparison
DATA
With muons – tune a1, a2, R, T, (dcut), and Q0
With positrons – tune dcut
Pions – no additional tuning 21

22. x = Mod(xtrack + 0.5,1.) for 0.25 < y < 0.75
y = Mod(ytrack – 0.03,1.) for 0.25 < x < 0.75
Scan across one 1 x 1 cm2 pad
Note These features not implemented explicitly into simulation
Simulation distributes charge onto plane of pads…

23. Efficiencies, multiplicities
Select ‘clean’ regions away from
- Dead ASICs (cut out 8 x 8 cm2 + a rim of 1 cm)
- Edges in x (2 rims of 0.5 cm)
- Edges in y (6 rims of 0.5 cm)
- Fishing lines (12 rectangles of ±1 cm)
- Layer 27 (with exceptionally high multiplicity)
Measured average response
Add in dead
regions
Note: Simulation of RPC response tuned to this data
Tail towards higher multiplicity reproduced with 2nd exponential

24. Analysis of Secondary Beam Data: Topological Particle ID
Muons: All active layers have aligned clusters with no more than two consecutive layers with nonisolated clusters.
Pions: At least one track segment in the interaction region that spans at least four layers and is not compatible with the beam direction. If such a track segment is not found, at least one pair of track segments that span three layers with at least 20o angle in between. Or remaining events with rrms > 5 cm.
Positrons: rrms < 5 cm.
Where rrms is defined as,

25. Unidentified μ's, punch through
Results - October 2010 Data
CALICE Preliminary
Gaussian fits over the full response curve
Unidentified μ's, punch through 25

26. Pion Response N=aE As expected, linear up to > 25 GeV
CALICE Preliminary
(response not calibrated)
N=aE
Standard pion selection
+ No hits in last two layers
(longitudinal containment
16 (off), 32 GeV/c (calibration off)
data points are not included in the fit.
As expected, linear up to > 25 GeV 26

27. Energy Resolution for Pions (response not yet calibrated)
CALICE Preliminary
(response not yet calibrated)
B. Bilki et.al. JINST 4 P10008, 2009
MC predictions for a large-size DHCAL based on results from small scale prototype.
32 GeV data point is not included in the fit.
Standard pion selection
+ No hits in last two layers (longitudinal containment)
Measurements confirm prediction 27

28. (response not yet calibrated)
B. Bilki et.al. JINST 4 P04006, 2009
Positron Response
CALICE Preliminary
(response not yet calibrated)
N=a+bEm
Non-linearity
Due to saturation (more than 1 hit/pad)
Can be improved with software compensation
Not detrimental: this is a hadron calorimeter!
Data (points) and MC (red line) from small prototype test and the MC predictions for a large-size DHCAL (green, dashed line).
Measurements confirm prediction 28

29. Positron energy resolution
CALICE Preliminary
(response not yet calibrated)
Un-corrected for non-linearity
Corrected for non-linearity

30. The DHCAL at CERN Some repairs done before CERN test beam
Some RPC’s are taken out to fix gas leakage – they are repaired, retested
and insert back to the DHCAL
A small number of RPC’s are replaced due to low efficiency
2 more DHCAL layers were produced
They are made out of the spare parts
54 layers (96 x 96 cm2) of RPCs with 1 x 1 cm2 readout pads
Up to 497,664 readout channels (world record for calorimetry and RPC systems)
Transport to CERN
Built spring-damped transport fixture
All RPCs survived intact
Installation at CERN
39 layers into Tungsten (1 cm ~ 3 X0 plates) absorber structure
15 layers into Steel (2 cm and 10 cm plates) tail catcher

31. Beams at CERN PS Trigger recorded so far (million events) SPS Testbeam
Covers 1 – 10 GeV/c
Mixture of pions, electrons, protons, (Kaons)
Two Cerenkov counters for particle ID
SPS
Covers 12 – 300 GeV/c
Mostly set-up to either have electrons or pions (18 Pb foil)
Trigger recorded so far (million events)
Testbeam
Configuration
Muons
Secondary beam
Total
CERN2
DHCAL + TCMT
5.6
23.4
29.1

32. 300 GeV pion showers

33. A few event variables The interaction layer Longitudinal barycenter
100 GeV π simulation
The interaction layer
Algorithm tuned with Monte Carlo events
Defined as first of two consecutive layers with more than 3 hits
Longitudinal barycenter
with Ni ... hits in layer I
Hit density R
with sgn(Ni ) = 1 for Ni > 0, = 0 for Ni = 0
80 GeV ‘pion’ beam
Muons Pions

34. Event selection
General cut: 1 cluster in layer 0 with less than 12 hits
Particle selection:
Particle
Cerenkov BC R IL N0 μ
>20
<3.0 -
>0
>10 e±
C1·C2=1
<8
>4.0 for
E>12 GeV
>4 for π-
C1+C2=0
>2.0 – 5.0
>2 for
E>3 GeV π+
C1=0 and C2=1
(p ≤ 10 GeV/c)
(p > 10 GeV/c) p
BC … Longitudinal barycenter
R … Average number of hits per active layer
IL … Interaction layer
N0 … Hits in layer 0

35. Response at the PS (1 – 10 GeV)
Fluctuations in muon peak
Data not yet calibrated
Response non-linear
Data fit empirically with αEβ
β= (hadrons), 0.78 (electrons)
W-DHCAL with 1 x 1 cm2
Highly over-compensating
(smaller pads would increase the
electron response more than the
hadron response)
Remember: W-AHCAL is compensating!

36. Resolution at the PS (1 – 10 GeV)
Resolutions corrected for
non-linear response
Data fit with quadratic sum of
constant and stochastic term
Particle α c
Pions
(68.0±0.4%
(5.4±0.7)%
Electrons
(29.4±0.3)%
16.6±0.3)%
(No systematics yet)

37. Comparison with Simulation – SPS energies
Data
Uncalibrated
Tails toward lower Nhit
Simulation
Tuned with Fermilab data
Rescaled to match peaks
Shape surprisingly well reproduced

38. Response at the SPS (12 – 300 GeV)
Fluctuations in muon peak
Data not yet calibrated
Response non-linear
Data fit empirically with αEβ
β= (hadrons), 0.70 (electrons)
W-DHCAL with 1 x 1 cm2
Highly over-compensating
(smaller pads would increase the
electron response more than the
hadron response)

39. Further R&D Large single glass RPC High rate RPC
ANL invention: thinner, can go to smaller pad sizes
Built and tested 2 large single glass RPCs (32x48cm2)
Showed good efficiency and uniformity
High rate RPC
Several approaches are going in parallel: low resistivity material, new RPC design
Making progress on every approach: new glass sample, new Bakelite board, new RPC
R&D towards a realistic DHCAL
Need: low power, better timing resolution (~1ns), better mechanical design, gas recycling, HV distribution
Have some ideas already
In discussion with new collaborators

40. Summary
Digital Hadron Calorimeter (DHCAL) concept has been proven by a large DHCAL physics prototype
Test beam at Fermilab with Fe absorber is completed, data analysis made a lot of progress already
Test beam at CERN with W absorber is ongoing, and will finish next week. First look at the data suggests that the data quality is very good
Further R&D has started

41. Additional slides

42. Cassette Assembly Assembly Cassette Testing
- FEB’s are placed onto RPC’s directly (no gluing), positioning and
contact is assured by pressure asserted from cassette
- Cassette is compressed horizontally with a set of 4 (Badminton) strings
- Strings are tensioned to ~20 lbs each, very few broken strings
Cassette Testing
- Cassettes were tested with CR
before shipping to test beam
38+14 cassettes assembled 42

43. Calorimeters/configurations during Fermilab test beam
DHCAL
38 layers, each 1 x 1 m2
1.2 X0 Fe (+ Cu) absorber plates
350,000 readout channels
Tail Catcher
14 layers, each 1 x 1 m2
8 1.7 X0 Fe (+Cu) X0 Fe (+Cu) absorber plates
129,000 readout channels
Silicon – Tungsten Electromagnetic Calorimeter
30 layers, each 18 x 18 cm2
Variable thickness Tungsten absorber
10,000 readout channels
Minimal Absorber Structure
50 layers, each 1 x 1 m2
0.27 X0 Fe (+Cu) absorber plates
461,000 readout channels

44. Events taken with Minimal Absorber
Stack
50 layers
Cassette contain 2 mm Cu and 2 mm Fe
0.27 X0/layer → 13.4 X0 total
0.038 λI/layer → 1.9 λI total
Beam
1,2,3,4,6,8,10 GeV/c secondaries
8 GeV e+
16 GeV/c π+
8 GeV/c π+