D. Gonzalez-Diaz, KEK, 19-01-2017

I. A contemporary recap
II. Historical introduction
III.Technological pillars / few
things not to forget
IV. Faster
V. Bigger
VI. Better

I. parallel plate geometries

II. Cherenkov light + solid converters
V0

Limitedbyfluctuations in themultiplication andprimary ionization
statistics

III. Multi-sampling

Limited by primary statistics andelectron diffusion

Limited by ionization mean
free path and drift velocity

D. Gonzalez-Diaz et al. arXiv:1606.08172v2
R. de Oliveira arXiv:1503.05330v1

History of timing with gaseous detectors(I)E. K. Zavoisky and G. E. Smolkin, At. Energ. (USSR), 4(1956)46[?]

1956

track

σ=100 psreadout
triggertails!
chambers• Long dead-time (time to reload the chamber capacitance after a spark was ~1-10ms) .
• Read-out was performed by optical means .

History of timing with gaseous detectors(II)
1970

1 mm

• Time resolution limited due to a relatively big gas gap .
• Discharge takes energy only from the locally affected area, limited by surface resistivity .

History of timing with gaseous detectors(III)19780.1 mm• High efficiency enforces operation at around 12bar .
• Time resolution consistently below 100ps .

History of timing with gaseous detectors(IV)1981

1.5 mm• Time resolution limited due to a relatively big gas gap .
• Avoids the necessity of using soviet technology (Bakelite plates used instead) .
• In subsequent works, the authors introduced C2H2F4 and SF6 and a new operation mode(limited proportionality / saturated avalanche mode).

History of timing with gaseous detectors(V)
1996

3 mm

• Time resolution limited due to a relatively big gas gap .
• Time resolution and efficiency can be improved with addition of more gaps. Each new gas
gap behaves like a detector replica (i.e., a parallel current generator) improving avalanche
statistics, and resistive plates can be simply left floating .

History of timing with gaseous detectors (VI)2000

0.3 mm• Time resolution good due to small gap .
• Efficiency good due to large number of gaps .
• Standard materials, standard gases, standard pressure, standard 1GHz electronics .

History of timing with gaseous detectors (VIII)
J. Wang et al. NIM A, 621(2010)151 [Tsinghua University]
charged particle
readout pads/strips-V+V•••

Small improvements since 2000 in order to stablish reliable production techniques .
50-90ps achieved on large 2m-scale areas .
Technology stabilized.

>2004

I.

The characteristics of the induced signal are only mildly affected by theresistive material (through its dielectric constant and thickness).

II. The transition avalanche-streamer-filamentary discharge-spark is quenched down tothe energy available in a small local area, provided the flow of current through the resistive
electrode is limited. In practice big avalanches end as streamers, and stable operation of theamplification electronics is possible.
III. The use of multiple gas gaps (acting in practice as parallel current generators) allows tokeep the high efficiency characteristic of large gaps and the high time resolution
characteristic of narrow ones. The associated reduction in the induced charge
fluctuations improves both.
IV. The use of electronegative gases helps at stabilizing le operation up to very high gains,even in the presence of strong Space-Charge effects.
V. The maximum operating rate is limited by charge build-up and the characteristic time forthe released charge to abandon the system, by conduction through the resistive electrodes.
VI. Scalability is relatively easy due to use of common materials.

log( s / A)

1GeV
•

10GeV

100GeV

FOPI
technology introduced (100ps)
HADES
2000

•

simulations demonstrate operation
under deep space-charge conditions
NIM A 517(2004)54
ALICE module (50ps)
NIMA 533 (2004)932005

warm RPCs
NIM A 527(2004)471
NIM A 555(2005)72•24-gap module (20ps)
NIM A 594(2008) 39•

Chinese glass RPC
NIM A 621(2010)151•Electrostatic compensation
NIM A 648 (2011) 52

HARP

CBM
R&D

MPD
R&D2010BGO
LEPS22015•

Ceramics RPC
NIM A818 (2016) 45–50

E
STAR (barrel, MTD)

year

•

10000GeVALICE

NIM A 443(2000)201•1000GeVPHENIXEEESHOWERdetectormagnethigh angle
TOF

inner MDCs (I-II)

RPC wallhadron blind RICH

outer MDCs (III-IV)

8 m2

 0.27 mm × 4 gaps
 minimum for good efficiency
 Aluminum and glass, 2mm-thick electrodes minimize amount of glass for maximum
rate capability
 try to keep good mechanics
 Heat-tolerant materials

fully shielded

Aluminium

Glass

spring-loadedpressure plate

HV &readout atthe center

wall
1116 tRPC individual detectors
6 sectors x2layers
3columnsgas
box ofxone
sectorx 31 cells

variable cell overlap for providing full angular coveragecolumn (1-3)
variable cell width for matching occupancy

layer(1-2)

row (1-31)

detailed info in D.Gonzalez-Diaz 2006 JINST TH 003 and D.Belver et al. NIM A, 602(2009)687

overall cross-talk 0.4%

13cm length

55cm length
overall resolution 77ps
S1

σT [ps]S2S3
S4S5S6A. Blanco et al., NIM A, doi.10.1016/2010.08.068

deterioration of resolution for acoincident track (around 100ps)

distance (in rows) between primary and secondary hits

G. Kornakov et al., 2014 JINST 9 C11015

tNormal
V

Rgap
ECgap

g

t=0

t=∞

E?

E?

0E=Vg

E=Vg

ResistiveVE0σ, εrg

Rgap

Cgap

RplateCplateE?

E?

d
E=C plate

VC plate +C gap g

E=Vg

transient! (in practice, few sec)

V=VoCglass

V=VodEgasgEglass ~0Eglass

Eglass

Egas

Egas+++++++++-

Egas

EglassEglassEgas = Ctot/Cgas Vo/g

Egas = Vo/3g+++++++++-

CglassEglassEglass

V=0
V=0
1/Ctot =1/Cglass+1/Cgas+...

(‘DC limit’)
V=Vo

V=Vo

Eglass = Ctot/Cglass Vo/d+++++++++-

Cgas

glass capacitance un-loaded,
gap capacitance fully loaded

RPC capacitance loaded+++++++++-(‘AC limit’)Vo
ΔVo=1/3Vo
2/3Vo

RglassRgas
Rglass

V=0
V=0
Rtot =Rglass+Rgas+...

V=Vo

----

V=Vo

V=VoVo++++2/3Vo+
1/3Vo------VoVo++2/3Vo+ΔV1+1/3Vo-ΔV2--<V>=2/3Vo...
<V>=1/3Vo++++++0
V=000
V=0

V=0E. Cerron Zeballos et al., NIM A, 374(1996)132*Indeed, it has not been possible to quantify the effect of the fluctuations on the plates’ potentials. But

no practical difference has been observed between leaving them floating or fixing the potential…

Avalanche field (Ez=100 kV/cm, 0.3 mm gap)!!

C. Lippmann, W. Riegler, NIM A 517(2004)54

Semi-quantitative derivation of the maximum attainable gain before streamers appear:

xr = 4Dv

αx2qe eEav ( x) ≅4πε o r 2q e eα c x c≅ EoEav ( xc ) ≅24πε o r

α c xc ≈ 16 + ln(ε k [eV]) + ln( xc [cm])α g ≈ 16 + ln(ε k [eV] g[cm])Experimentally observed limit for ~ cm gap PPCs:

α g ≈ 20

The Raether limit, 1964

αg=25- 33 in tRPCs!

D=λvth3qe λv≅
Eomvthcharacteristic energy:

qe Eo Dεk == mvth2v
in equilibrium

σT =σT =σ v2,noise + σ v2,avalanche

1.28(α − η )vddV(V = Vth )dt
Exact solution for a single
electron avalanche!

W. Riegler et al., Nucl. Inst. Meth. A 500(2003)144

MC simulationconsider only 1 gap
VΔVglass/R∆VDC‘cellmodel’I(t)=qδ(t)long transient behavior,'equilibration time'
simple circuit calculation

average voltage drop in
stationary conditions
voltage fluctuations

D. Gonzalez-Diaz et al., Nucl. Phys. B (Proc. Suppl) 158(2006)111
D. Gonzalez-Diaz et al., Nucl. Instr. Meth. A 602(2009)713

fluctuations aroundthe average field
Campbell theorem
for shot noise

average field
the DC limit must satisfy=1(Vo − φρdq )g rmsq2 1 +2
rms E gapq=
Eo − E gap
2NDC modelrmsEgap/(Eo-Egap)

V(Vo − Vth )E gap = th +g g (1 + aφρd )teq ~τln(1 + aφρd )aφρdno dependence on the area A
influenced (cell) by the shot!

N = Aτφif we further assume thatq ~ a (Vgap − Vth )simple analytical estimate

MCteq/ τ1E gap = (Vo − I R )gtransient time

Campbell theorem

Φ [Hz/cm2]
for the typical values A>1mm2
the importance is small.

Φ [Hz/cm2]
forfloatglass
theequilibration time typicallyobserved is ~2-3s.

D. Gonzalez-Diaz et al., Nucl. Phys. B (Proc. Suppl) 158(2006)111

electrical ageing (material)

avalanche ageing (material+gas)(dry)

Next generation
experiments

From M. Morales, C. Pecharromán, PoS(RPC2012)024

300 Hz/cm2
2pC

Deposits observed for ~100mC/cm2 but
no degradation of performance!

5 years

Q/A = q x ϕ x Δt = 100 mC/cm2

S. Gramacho, L. Lopes et al.,
NIM A 602(2009)775

2dd2
I ( y, t ) = Cˆ Lˆ 2 I ( y, t )2dtdyzyI(t)?

2d2dV ( y, t ) = Lˆ Cˆ 2 V ( y, t )2dydtxCm
Lo+V-V

Lm
Cg

D~1m

D~1m

D. Gonzalez-Diaz, H. Chen, Y. Wang,
Nucl. Instr. Meth. A 648, 1(2011)

standard RPC

centerend‘compensated’ RPC

centerend

intrinsic time resolution

average ohmic drop at the insulator

time resolution at highparticle fluxD. Gonzalez-Diaz et al arXiv:1606.08172v2

The problem is to find a 'bad insulator' developed by industry!
not enoughstreamerquenchingconductors

semi-conductors

bad insulators

good insulators
too low
rate capability
From C. Pecharroman (CSIC-Madrid), "Understanding the ageing process in RPC’s from an ion
conductivity approach'. Talk at the X Workshop on Resistive Plate Chambers and related detectors.

float glass
...

x100

low resistivity
chinese glass~1C/cm2
new insulators
are needed !
Ra_d
Rq_d
Ry_d
Ra_c
Rq_c
Ry_c

Roughness(nm)10010J. Wang et al. NIM A 621(2010)151

good surface uniformity
10.10123456

Measured point

78

continuous line:
doped glass
dashed line:910
commonglassVth(Vo − Vth )+g g (1 + aφρd )σT[ps]

E gap =

phenomenological:

ε ( E gap ) =1+ e

ε0−ϑ ( E gap − Eref )σ T (φ )Φ[kHz/cm2]

ε (φ )

for small particle fluxes:

σ T = σ 0 + KT q φρdε = ε 0 − K ε q φρdapproximate
linear behavior

yields the following scaling
I. Deppner et al.,doi:10.1016/j.nima.2010.09.165

rate capability [kHz/cm2]

K
σ T ( E gap ) =S ( E gap )εfrom theoretical considerations:ρ0d0/ρdTypical sizes of interconnects, cross-talk and signal attenuation inside the counter limit
the performances for counters at the 1-2m scale unless proper precautions are taken.

Virtually all next-generation RPCs are based, to be based, or considering the multi-strip
technology (EEE, STAR-MTD/RHIC, CBM/SIS300, MPD/NICA, BGO, LEPS2)

arXiv:1606.08172v2

BGO-EGG counteruse shielding vias

fancier:electrostatic compensation!

Cm Lm
=CoLo

x10!
D. Gonzalez-Diaz, H. Chen, Y. Wang,
Nucl. Instr. Meth. A 648, 1(2011)

in time-domain

in frequency-domaincompensatedunder-compensatedover-compensateddielectriclosses
What makes the trick?
In this particularconfiguration... increasing the coupling!

within ±0.2mm cross-talk increases a factor 2-3!

charge threshold ~ 30fC
jitter@100fC~ 15psgain~ 100BW~ 2GHz
Q-Width algorithm built-in

FEE-MBOs (power regulationand distribution, sensing, signaldistribution)

D. Belver et al. IEEE TNS 57(2010)2848

t1

low-noisecustomized LV
system basedon switching
power supplies

t2

A. Gil et al. IEEE TNS 56(2009)382

FEE-DBOs (amplification, discrimination, Q-ToT algorithm)

TDC resolution at the level of 25 LSB (HPTDC) or ~10ps (FPGA-based)Increase number of gaps and field σ T ~as much as possible
No streamers.

σ T best = σ T ( Emax ) ~1
N o (α − η )vdrift

αg ≤C(~30 for 0.3mm gaps
and electronegative gas)gK ( No )∝N o (α ( Emax ) − η ( Emax ))vdrift ( Emax ) vdrift ( Emax )So... make very thin capacitors!

problems:•Accuracy on the gap definition better than ~0.05gap x sqrt(Ngap) ~10μm–30μm, since
the field variations are directly depending on it.
•Low efficiency, due to the low ionization probability (use multi-gap!).
•Theoretical arguments apart, obtaining resolutions below 50ps is technically difficult(FEE-BW, noise, interconnects, transmission characteristics, TDC resolution...).
•Detector stability?. Respecting the analogous to the Raether criteria for thin gaps
does not guarantee stability, due to unavoidable fluctuations in avalanche
multiplication and primary ionization that can trigger streamers.

S. An et al., Nucl. Instr. Meth. A, 594(2008)39160um gas gap

N. Tomida, C.-Y. Hsieh, et al JINST 7 (2012) P12005

I(t)?
RR+V-VNe-

e--I+

D~1m

D~1m

1/ α

and summing overall gaps!

Imain-

currents produced by
the 'mirror charges'

Ineigh-+V-V

Ne-,v

D~1m

D~1mz
y
Imain+

Ineigh+xI(t)?

CmLo+V-V

LmCg

D~1m

D~1mzyxI(t)?
TDCADCD. Gonzalez-Diaz
doi:10.1016/j.nima.2010.09.067

σT~1nsσT~100ps

computing time <50ms
per impinging particle

E=100kV/cm

6cm

CBM-prototype injection
at center(multi-pad)

pad readout
Zc = 21Ω (simulation)

Jingbo Wang et al., NIM A
621, 1-3(2010)151

sim (BW~0.5GHz)

15-55cm

HADES(shielded)

injection
at center

20mV

100ns

15cm-long, 2.2cm-wide (short cell,high granularity region)
Zc = 9.0Ω (measurement)
Zc = 10.5Ω (simulation)
D. Belver et al., NIM A
602, 3(2009)687

sim (BW=2GHz)

40cm

Fast neutron
detector-I

Zc=9.3Ω

D. Yakorev et al. doi:10.1016/j.nima.2011.05.031

injection
at centersimsim(BW=1.25GHz)(BW=1.5GHz)

200cmFast neutron
detector-II
Zc = 13.0Ω

injection
at centersimsim(BW=1.25GHz)(BW=1.5GHz)

injection
Fast neutron
at center
detector-II (optimized)

200cm

Zc = 14.1Ω
very slightly modified(additional material required ->less than 10%)sim
sim(BW=1.25GHz)(BW=1.5GHz)

• Seventeen successful years of multi-gap timing RPCs development. This is still by far
the best technology for sub-100ps timing with gaseous detectors, although new
approaches are being tried.
• Large module sizes (~1m x 1m, 2m x 2m) achieved.
• Large system sizes implemented (~100m2).
• Time resolution at the scale of 50-100ps for minimum ionizing particles (and 95-100%efficiency) in any modern detector system.
• Demonstrated technological limit: σt=20ps, 10ps possibly achievable.
• Rate capability from ~0.5-1kHz/cm2 (standard float glass), to 5-10kHz/cm2 (warm glass),
30-50 kHz/cm2 (chinese glass), 100 kHz/cm2 (ceramics).
• Demonstrated maximum detector size keeping a 1-2GHz bandwidth: 2m.
• Good performance demonstrated also for relativistic heavy ions, neutrons andannihilation photons (with accuracies around 100ps in either case).