Fast timing - Ettore Majorana Foundation and Centre … 2013 williams.pdfdictates equal gain in all...

Erice School for Sub Nuclear Physics June 2013

Fast timing

C. Williams A. Zichichi and K. Doroud

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Friday, June 28, 13

A brief review of the past

Precise timing has been with us for decades

Important tool for particle IDAlso for trigger selection

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3 2

1

1965 : search for anti-deuterium at CERN PS

three timing counterscarefully timed

together to select velocity of anti-

deuterium

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BELLE experiment at KEK (2002 publication)

TOF scintillators: barrel of 128 counters read out at both ends

time resolution (σ) 100 ps4

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So what happened between 1965 and 2002?

• 3 channels increase to 256 channels• much bigger scintillators• better precision TDCs• BUT no improvement in time resolution

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So for the TOF: time has stood still: maybe the reason for this is the

technology in use.

Both the 1965 and the 2002 experiments used scintillators coupled to phototubes

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Large - difficult to segment - problems in magnetic field - limited timing performance

The Photomultiplier

Text

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In my opinion, there are three items:

n Multigap resistive plate chamber

n Concept of differential readout and the NINO asic

n Silicon photomultiplier

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However during the last decade the field of fast timing has been

invigorated

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The Multigap Resistive Plate Chamber

Invented in 1996

Solves all the problems of the RPC

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Typical Pb-Pb event in ALICENeed highly segmented TOF detector

Clearly a scintillator-phototube approach will

not work here

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So for the ALICE experiment in 2000: everyone knew that the experiment needed a Time-of-Flight array: but

there was no obvious device that could be used

Enter: the Multigap Resistive Plate Chamber

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Idea for multigap was born at an RPC conference

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12

3

4 1 2 3 4

wire in a tube

signal dominated by arrival of closest cluster

of electrons

0 V

+ kV

high electric field

RPC

avalanches start immediately - induced signal is all

avalanches acting in parallel(Santonico 1996 RPC conference)

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Great idea - but unfortunately not correct

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cathode

anode

Only avalanches that traverse full gas gap will produce detectable signals

So only a few ionisation clusters (those created closest to cathode) take part in signal production - (2 mm gap RPC only ionising clusters within 0.5 mm of cathode can grow avalanches big enough to generate signal - if increase E field so that all clusters can grow big enough - we will have sparks - etc)

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but what a nice idea - get many avalanches to act together simultaneously

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Question: Can we increase gas gain such that avalanche produces detectable signal immediately?

(a) Need very high gas gain (immediate production of signal)

(b) Need way of stopping growth of avalanches (otherwise streamers/sparks will occur)

Friday, June 28, 13

but what a nice idea - get many avalanches to act together simultaneously

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Question: Can we increase gas gain such that avalanche produces detectable signal immediately?

(a) Need very high gas gain (immediate production of signal)

(b) Need way of stopping growth of avalanches (otherwise streamers/sparks will occur)

Answer: add boundaries that stop avalanche development. These boundaries must be transparent to the fast induced signal - induced signal on external pickup electrodes due to movement of charge in any of the avalanches

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Multigap Resistive Plate Chamber

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Cathode -10 kV

Anode 0 V

(-2 kV)

(-4 kV)

(-6 kV)

(-8 kV)

Signal electrode

Signal electrode

Stack of equally-spaced resistive plates with voltage applied to external surfaces

Pickup electrodes on external surfaces (resistive plates transparent to fast signal)

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Multigap Resistive Plate Chamber

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Cathode -10 kV

Anode 0 V

(-2 kV)

(-4 kV)

(-6 kV)

(-8 kV)

Signal electrode

Signal electrode

Stack of equally-spaced resistive plates with voltage applied to external surfaces

Pickup electrodes on external surfaces (resistive plates transparent to fast signal)

magic #1Internal plates take correct

voltage - initially due to electrostatics but kept at correct voltage by flow of

electrons and positive ions - feedback principle that

dictates equal gain in all gas gaps

Friday, June 28, 13


ALICE-TOF has 10 gas gaps (two stacks of 5 gas gaps) each gap is 250 micron wide

Built in the form of strips, each with an active area of 120 x 7.2 cm2, readout by 96 pads

Cathode pickup

electrodes

electrically floating

electrically floating

+ve HV

-ve HV

-ve HV

+ve HV

Anode pickup

electrode

Differential signal to

front-end electronics

Schematic view of the ALICE TOF MRPC

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Erice School for Sub Nuclear Physics June 2013 17

Friday, June 28, 13

Erice School for Sub Nuclear Physics June 2013 18

glass: 280 micron thickfishing line: 200 micron thick

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Time does not allow me to discuss the details of the Multigap, but some key points are:

n Easy segmented (ALICE has 160,000 channels c.f. BELLE has 256 channels)

n Excellent timing (~ 75 ps) (n.b. differential readout)

n Low dark count rate (can be used easily as triggering device)

STAR followed our lead and have now built a barrel TOF very similar to ALICE.

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our initial thought was to build large planar chambers

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however we quickly realised that we could only access time resolutions below 150 ps if we moved to differential readout

input impedance

input impedance

input impedance

input impedance

Channel 2

Channel 1

Channel 3

Channel 4

input impedance

detector ground

ground offront-end electronics

input impedance

input impedance

input impedance

Channel 2

Channel 1

Channel 3

Channel 4

DIFFERENTIAL SINGLE-ENDED

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ALICE TOF - detector was designed as strips in order to have access to the anode and cathode signal

DIFFERENTIALSINGLE-ENDED

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Finally a new Ultra-Fast discriminator “THE NINO” was designed

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There are many interesting physics principles governing the operation of the multigap:

SPACE CHARGE LIMITATIONRECOMBINATION

of positive and negative ions

and also some exceptional operational characteristics:

ultra low dark current 10 nA/m2 (ALICE TOF)low dark count rate 0.2 Hz/cm2 (ALICE TOF)

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dE/dx

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The top two panels show the dE/dx in units of multiples of σdE/dx, nσdE/dx , of negatively charged particles (first panel) and positively charged particles (second panel) as a function of mass measured by the TOF system. The masses of 3He (3He) and 4He(4He) are indicated by the vertical lines at 2.81 GeV/c2 and 3.73 GeV/c2 , respectively. The horizontal line marks the position of zero deviation from the expected value of dE/dx (nσdE/dx = 0) for 4He (4He). The rectangular boxes highlight areas for 4He (4He) selections. The bottom panel shows a projection of entries in the upper two panels onto the mass axis for particles in the window of −2 < σdE/dx < 3. The combined measurements of energy loss and the time of flight allow a clean identification to be made.

Combining TOF with dE/dx

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THE FUTURE?

• Exceptional timing performance

• High rate operation

• device with high segmentation

R&D on the following

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ANODE PICKUP ELECTRODE

CATHODE PICKUP ELECTRODE




NINONINO






NINONINO

24 gaps of 160 micron

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T10 test beam

10

0

20

30

40

50

60

-200 -100 0 100 200 300 400 500-500 -400 -300

σ = 35.3 ps

time resolution = 35.3/√2 = 24.9 ps

events / 10 ps

Time difference between MRPC1 and MRPC2 [ps]

Cosmic rays12.5 kV

Cosmic ray

0

10

20

2020

2525

1010

55

1515

0

30

40

50

60

70

80

90

100

10000 11000 12000 13000 14000

Efficiency [%]

efficiency mrpc1 [%]efficiency mrpc2 [%]

Time resolution of mrpc

Timeresolution [ps]

0

20

40

60

80

100

Entries / psTime difference between MRPC1 and MRPC2

-300 -200 -100 0 100 200 300TimeMRPC1 - TimeMRPC2 [ps]

! = 22.35 ps

time resolution is 22.35/"2 = 15.8 ps

12.5 KV T10 test beam

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Rate capability: the problem is that it is a RESISTIVE plate chamber: Current has to flow through these plates and at high rates (i.e. high current) there is a voltage drop across these plates and thus a reduced field in the gas gaps

SOLUTION: use resistive plates of lower resistivity ALICE TOF array constructed using “soda lime” float glass (resistivity: ρ ~ 1012 Ω.cm) and has a rate capability of 1 kHz/cm2

A group in China produced glass with ρ ~ 1010 Ω.cm and the rate capability has been measured to be ~ 50 kHz/cm2

• High rate operation

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• device with high segmentation

Devices with strips on a 2.5 mm pitch have been built and tested at CERN:

strip multiplicity ~ 3 time resolution ~ 50 ps

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• TOF is a technique that performs particle id in the range 1 - 4 GeV/c

• needs: highly segmented detector that works in a magnetic field excellent time resolution.

• MRPC is the only viable technology.

• there are some magic aspects of the mrpc

(a) floating intermediate resistive plates

(b) space charge limits avalanche growth to be below streamer transition

(c) recombination cuts ionic charge residual in the gas gap

• differential readout really cuts noise and is essential to obtain good time resolution.

• time resolutions close to 10 ps are possible - needs very good TDC.

summary

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Friday, June 28, 13


OUTLOOKTOF arrays in operation using MRPCs:

ALICE, STAR, PHENIX, FOPI + others(all work at ~ 80 ps time resolution)

TOF arrays planned using MRPCs:BES III upgrade

CBM and PANDA at FAIR(20 ps is the dream)

MRPC Trigger chambers planned for forward region of CMS with 100 ps timing at trigger level

STAR will build a MRPC wall just outside the iron return yoke and use TOF to separate muons from pion punch through

EEE project: equip Italian high schools with muon telescope

Clement-Ferrand: building ‘EEE-type’ MRPC for muon tomography of volcano

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Friday, June 28, 13

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Fast timing - Ettore Majorana Foundation and Centre … 2013 williams.pdfdictates equal gain in all...

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