Upload
elwin-byron-burns
View
218
Download
3
Tags:
Embed Size (px)
Citation preview
Spark-protected high-rate... P.Fonte CERN 1998a1
Spark-protected high-rate parallel geometry gas chambers
P.Fonte
Laboratório de Instrumentação e Física Experimental de Partículas (LIP)
e
Instituto Superior de Engenharia de Coimbra (ISEC)
Coimbra, Portugal
CERN 1998
Spark-protected high-rate... P.Fonte CERN 1998a2
Summary
• Brief survey of the basic parallel plate detector types, physics and operation modes.
• Hybrid wire mesh/resistive plate detector, made with a medium resistivity plate for improved rate capability and spark protection.
• Rate-gain limitations on parallel geometry chambers and other gaseous detectors.
• Thin gap parallel mesh chambers.
Spark-protected high-rate... P.Fonte CERN 1998a3
Basic parallel plate detector types
metal plate
amplification gap
metal plateParallel Plate Chamber (PPC)
(J.S.Townsend, 1900)
resistive plate
amplification gap
resistive plateResistive Plate Chamber (RPC)
(Pestov, 1978)
wire mesh
amplification gap
wire meshParallel Plate Avalanche Chamber
(PPAC)(G.Charpak and F.Sauli, 1978)
Hybrid types
amplification gap
wire mesh cathode
resistive plate anode
A subject of this talk
amplification gap
wire mesh cathode
metal plate anode
Another subject of this talk
metal plate
amplification gap
resistive plate
Not very popular
etc.....
Spark-protected high-rate... P.Fonte CERN 1998a4
PPC operation modes
anode
cathode
ionizing particle
high-voltagepulsespark Spark Chamber
(optical or electric read out)
anode
cathode
ionizing particle
narrow high-voltage
pulse
streamersStreamer Chamber(optical read out)
anode
cathode
ionizing particle
constantvoltage
Proportional mode(electric readout)
avalanche*
* eventually there will be also a few sparks
Spark-protected high-rate... P.Fonte CERN 1998a5
RPC operation modes
ionizing particle
streamer
Streamer mode
(development of a sparkis avoided by the current
limitation provided by theresistive electrodes)
higher constant voltage
resistive anode
resistive cathode
resistive anode
resistive cathode
ionizing particle
Limited proportional mode(sub-exponential gain due
space-charge effect in single avalanches)
avalanche constantvoltage
PPAC operation modes
Proportional mode only*, but a large number of configurations canbe formed.
A drift gap providesgood energy resolutionand efficiency for MIPs
drift gap
pre-amplification gap
ionizing particlePhoton
transfer gap
amplification gap
transfer gap
read out electrode
gate electrode
Example of a multistep PPAC
* plus a few occasional sparks
Spark-protected high-rate... P.Fonte CERN 1998a6
Basic physics
Electron (fast) signal Ion (slow) signal
Charge in slow signalNi = Nt (1-1/ln(G))
in practice about 90% of Nt
Charge in slow signalNi = Nt (1-1/ln(G))
in practice about 90% of Nt
Charge in fast signalNe = Nt/ln(G)
in practice 7% to 14% of Nt
50ns 10 to 30 s
Gain: G(x)=exp(x); = First Townsend coefficient
Total collected charge: Nt = N0·G(x)
cathode
anode
Primary electron createdat distance x from the anode
Avalanche multiplication
xamplifying
gap
Primary electron cloud
x
Ionizationtrack
Ionizing particle
N0 = number of primary electrons
Photon
Total collected charge: Nt = N0 /d·G(d)
d
The electrons closer to the cathode generate most of the final charge localization of the entry point
If N0=50, then the equivalent numberof concentrated primary electrons will be only 500.07 = 3.5
Spark-protected high-rate... P.Fonte CERN 1998a7
From avalanche to streamer
Slow breakdown mode Fast breakdown mode
Slow breakdown depends on photon feedback to the cathode: “generations” mechanism.
Fast breakdown is a local process:“streamer” mechanism.
When the amount of hydrocarbons (quencher) is increased thereis always a transition from the slow to the fast breakdown mode.
No matter the gain or the gas composition there is a hard limit on the total avalanchecharge of a few times 108 electrons.
Mixtures with TEA
Mixtures with CH4 and C2H6
Spark-protected high-rate... P.Fonte CERN 1998a8
Streamer theory
( M e e k , L o e b , R a e t h e r , L o z a n s k i i )
P h o t o n - m e d i a t e d l o c a l f e e d b a c k i n a s t r o n g s p a c e - c h a r g e f i e l d
C o m p l e x p h y s i c a l p r o c e s s , i n v o l v i n g : e l e c t r o n t r a n s p o r t i n v a r i a b l e f i e l d s e l e c t r o n m u l t i p l i c a t i o n i n h i g h f i e l d s s p a c e - c h a r g e d i s t o r t e d e l e c t r i c f i e l d e m i s s i o n o f p h o t o n s a b l e t o p h o t o i o n i s e t h e g a s a t a
c e r t a i n d i s t a n c e ( g a s s e l f - p h o t o i o n i z a t i o n )
O n l y q u a l i t a t i v e a n a l y t i c a l s o l u t i o n s a r e p o s s i b l e , b u tn u m e r i c a l s o l u t i o n s h a v e b e e n s u c c e s s f u l l y a p p l i e d t o s e v e r a ld i s c h a r g e s i t u a t i o n s .
The streamer process can be separated in three stages- proportional avalanche stage- avalanche-streamer transition stage- streamer development stage- spark
No SQS mode
Spark-protected high-rate... P.Fonte CERN 1998a9
Proportional avalanche stage
Avalanche-streamer transition stage
Spark-protected high-rate... P.Fonte CERN 1998a10
Streamer development stage
The "precursor": Corresponds to the initial avalanche The current reduction between the precursor and the
discharge is due to the absorption of the majority of theelectrons by the anode
The electrons absorption causes important electric fielddistortions, so the precursor is a necessary feature ofthe breakdown process at low gains.
Spark-protected high-rate... P.Fonte CERN 1998a11
From streamer to sparkA
vala
nche
/str
eam
er
Glo
w f
orm
atio
n
Dif
fuse
glo
w
Fila
men
tary
glo
w
Spa
rk
(S.C.Haydon, 8th Int. Conf. on Phen. in Ion. Gases, Vienna, 1967)
Spark-protected high-rate... P.Fonte CERN 1998a12
Motivation for an hybrid PPAC/RPC detector
PPAC
•High counting rate (up to 105/mm2)•Violent sparks•Versatile (drift, multistep, etc..)•No dark noise•Proportional mode only•Almost not used
PPAC +RPC
•Fast signal (tens of ns)•Large areas•Breakdown via streamers at a few times 108 electrons/avalanche•Good position resolution (100 m) in 2 dimensions (prop.mode). •Good timing (less than 1 ns ).
RPC
•Low counting rate (up to a few times 10/mm2)•Mild sparks / “indestructible”•Less versatile (MIPs only)•Dark noise•Streamer mode or proportional mode•Widely used
Hi-rateRPC
•High counting rate•Mild sparks / “indestructible”•Versatile (MIPs, X-rays)•No dark noise•Proportional mode•Good timing•Widely used
(wish list)
PPAC RPC
18 orders of magnitude in electrode resistivity
Explore the electrode resistivity parameter
High-rate PPAC/RPC
Spark-protected high-rate... P.Fonte CERN 1998a13
How fast can a PPAC count and at what gain?
Pulse-height doesn’t depend on rate
Maximum counting rate is determined by the
appearance of sparks
For low gains the maximum counting rate Rmax and the gain G=Q/(eN0) are related by RmaxQ/D=C were C is
a constant with a value around 100 nA/mm or 1012 electrons/(s mm).
There is a linear dependence on the beam diameter and
not on the beam cross-section!
High-rate PPAC/RPC
Principle of measurement
1,0E+02
1,0E+03
1,0E+04
1,0E+05
1,0E+06
1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 1,0E+07
Rate/mm2 (from scaler and beam diameter)
Ga
in (
fro
m P
H)
Allowed region
Forbidden region
Spark-protected high-rate... P.Fonte CERN 1998a14
A new material is needed
if = 1011 cm 10 Hz/mm2
then = 107 cm 105 Hz/mm2
Our solution: epoxy+ink• black soft rubber (not staining)• = 1012 to 2107 cm• dielectric strength > 16 kV/mm
1,0E+06
1,0E+07
1,0E+08
1,0E+09
1,0E+10
1,0E+11
1,0E+12
1,0E+13
0% 10% 20% 30% 40% 50% 60% 70%
Ink concentration (per weight)
cm
97-06-30
97-07-15
97-07-31
• Controllable resistivity.
• Ohmic behavior
• Mechanically inconvenient. Too soft and the surface resistivity is strongly affected by dryness.
• A much more convenient material, based on ABS plastic, is now being investigated in collaboration with a specialized group.
Bulk resistivity
1.1E+07
1.3E+07
1.5E+07
1.7E+07
1.9E+07
100 400 700 1000
Applied voltage (V)
cm
60% Ink
High-rate PPAC/RPC
Spark-protected high-rate... P.Fonte CERN 1998a15
Setup
15 mm
3.5 mm
Drift
Amplification
3
300 pF
To scopeResistive plate
over a metal base
Wire meshes
Sharp focusX-ray gen.
40 mm
Collimator
To current amp
Gas: Ar + 10 to 20% C2H6 + 30 % of methanol V.P.
Since Cgap « Cplate « Creadout
the voltage change across Cgap
is mainly determined by the signal charge stored in Cplate
3 Cgap
Cplate
Creadout
Signalcurrent
Rplate
Equivalent electrical circuit
Mechanical arrangement
High-rate PPAC/RPC
Spark-protected high-rate... P.Fonte CERN 1998a16
Low-rate behavior
0
50
100
150
200
1 8
15
22
29
36
43
50
57
64
71
Channel
Co
un
ts
55Fe = 3x108 cmFWHM = 20%
0
200
400
600
800
1000
1200
1.00E+04 1.00E+05 1.00E+06 1.00E+07
Gain
Cou
nt
rate
(H
z)
55Fe
Dark
Sparks
Dark sparks
= 3108 cmL = 1.5 mm
High-rate PPAC/RPC
Spark-protected high-rate... P.Fonte CERN 1998a17
Counting rate capabilities
Ohmic model
fittedmeasured V01 V02
4.0E+07 5.8E+07 3.8E+07 3.8E+08 3.5E+08 1.8E+08
4.1E+11 8.7E+11 6.1E+11
Reasonable agreement,considering that the model doesn’t take into account beam-edge effects, materialnon-linearities, etc...
High-rate PPAC/RPC
1,0E+04
1,0E+05
1,0E+06
1,0E-01 1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06
Counting rate (Hz/mm2)
Effe
ctiv
e ga
in
N0 = 200 e-
"metallic" limit (PPC)=4x107 cm
=3x108 cm=4x1011 cm
open symbols: 5 mm diam. beamsolid symbols: 2 mm diam. beam
It is known that RPCs can reach higher gains than
PPCs
Spark-protected high-rate... P.Fonte CERN 1998a18
Streamer charge
50 mV/div 250 ns/div
20 mV/div 10 s/div
50 mV/div 250 ns/div
50 mV/div 250 ns/div
50 mV/div 10 s/div
50 mV/div 10 s/div
Streamer current on 3 (not all pictures on same gas)
= 41011 cmL = 0.25 mm (melamine)
Qmeas. 33 nCQstored 40 nC/cm2
= 3108 cmL = 1 mm
Qmeas. 28 nCQstored 7 nC/cm2
= 4107 cmL = 1.5 mm
Qmeas. 21 nCQstored 11 nC/cm2
There is some contribution from conduction current across the plate, but it is comparable to the discharge of the plate-equivalent capacitor.
High-rate PPAC/RPC
Spark-protected high-rate... P.Fonte CERN 1998a19
Summary of high-rate PPAC/RPC
• The detector can operate in proportional mode up to the intrinsic counting rate limits of metallic PPACs (about 105 Hz/mm2 with gain above 104).
• Materials with suitable mechanical properties are needed with resistivity about 107 cm.
• The streamer has a charge typically of the order of 10 nC (relatively independent of the substrate resistivity) and poses no threat to the integrity of the detector.
• Beam area seems to have only a minor effect on rate capability.
High-rate PPAC/RPC
Spark-protected high-rate... P.Fonte CERN 1998a20
0.5 s
Detector current (on 1 M)
Slow currentincrease justbeforebreakdown
Cyclic breakdown, with frequency dependenton detector current.At higher gain there is continuous sparking.
This kind of continuous sparking is totally absent in low-rate sparking.
1 s
The breakdown pulse is precededby many individual spurious avalanchesat a growing rate and amplitude
We call this the “cathode excitation” effect (cannot be photon feedback). May be improved by choice of cathode materials, geometry or gases.
Why there is rate-induced breakdown in PPACs?
Rate-induced breakdown
Aftercurrent
Spark-protected high-rate... P.Fonte CERN 1998a21
What about other detectors?
Rate-induced breakdown
Data presented by V.Peskov at the Wien WCC98
It seems that there is a general tendency for a reduction in themaximum gain as the counting rate increases.
Unknown physical origin.
Spark-protected high-rate... P.Fonte CERN 1998a22
How can we improve gain x rate (at least in PPACs)?
Rate-induced breakdown
If we succeed this will clarify the physical nature of the rate-induced breakdown process.
The result may be useful also for other detectors.
We measured the maximum current in a 3 mm gap PPAC for the following combinations of parameters
Anode or cathode copper plate or mesh
150 m or 50 m mesh wires
Check if the effect depends on the ion density over the cathode surface
Oxidized or clean copper
High (ethane) or low (methanol) ionization potential ions in gas
Presence or absence of a drift gap
The current ranges from 100 to 200 nA (3 mm2 beam)independently of any of these parameters
but… it jumps to 1 A if the gap width is reduced to 0.6 mm!
Spark-protected high-rate... P.Fonte CERN 1998a23
SetupSingle gap
10 mm
0.6 mm
Drift
Thin amp. gap
300 pF
Slow current amp
Metal plate
Wire meshes
Sharp focusX-ray gen.
100 mm
Collimator (2 mm diam.=3.1 mm2)
Fast current amp
5 M
-HV
Pre-amplified thin gap
10 mm
0.6 mm
Drift
Thin amp. gap
300 pF
Slow current amp
Metal plate
Wire meshes
Sharp focusX-ray gen.
100 mm
Collimator (2 mm diam.=3.1 mm2)
Fast current amp
5 M
-HV
2 mm Transfer
2 mm Thick preamp. gap
Electronics
Slow current amp. (DC coupled): sensitivity 1 or 10 V/A averaging time 5 ms
Fast current amp. (AC coupled): sensitivity 0.1 V/A averaging time 200 ns
Thin gap PPAC
Spark-protected high-rate... P.Fonte CERN 1998a24
Gain calibration
Example of gain calibration curve
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
0 500 1000 1500 2000
Applied voltage (V)
Ga
in
Ionization region (current)
High gain region (to be fitted)
Counting mode points (check)
Fitted exponential
Ion pulse shape
The gap current was measured by the slow current amp., calibrated against a Keithley 414s picoammeter.
200 nA
1 sQion 1.6 pC
Qelectrons Qion/(ln(G)-1) Qion/10 160 fC
velectrons 5 cm/s Pulse widthelectrons gap/velectrons 12 ns
Pulse heigthelectrons (Q/ Pw)electrons 13 A
G Qion / (e N0)
Gap field30 kV/cm
Thin gap PPAC
Spark-protected high-rate... P.Fonte CERN 1998a25
Rate-gain capabilities
Mesh pitch = 500 m
Gap thickness = 600 m
Thin gap PPAC
1,0E+02
1,0E+03
1,0E+04
1,0E+05
1,0E+06
1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08
Rate/mm2
Ga
in
ABCD (pre-amp gain=30)Thick gap (3 mm)MSGC (NIM 1996-1998)Micromegas (1997 data)GEM+MSGC*
0
* V.Peskov, Wien WCC, 1998
Spark-protected high-rate... P.Fonte CERN 1998a26
10
100
1000
10000
1,00E+02 1,00E+03 1,00E+04 1,00E+05 1,00E+06
Gain
Imax
(n
A)
3 mm gap
0,6 mm gap
From the point of view of current
Thin gap PPAC
Spark-protected high-rate... P.Fonte CERN 1998a27
Discharges
1 A
20 ms
Qdischarge 20 nC
but in the readout capacitor there were
QC 1.5 kV 300 pF = 450 nC
For some (yet unknown) reason in thin gaps full sparks don’t develop and the discharge is self-limited!
0.5 A
500 ms
Rate-induced spark in a thick gap chamber using the same amplifier as the previous one
Thin gap PPAC
Spark-protected high-rate... P.Fonte CERN 1998a28
0
20
40
60
80
100
120
1
15 29 43 57 71 85 99
113
127
141
155
169
183
197
PH(arb. units)
Cou
nts
050
100150200250300350400
1
11
21
31
41
51
61
71
81
91
10
1
11
1
12
1
PH(arb. units)
Co
un
ts
0
20
40
60
80
100
120
1
17
33
49
65
81
97
11
3
12
9
14
5
16
1
17
7
19
3
PH(arb. units)
Co
un
ts
Detector “B”
Detector “C”
Detector “D” (preamp)
Energy resolution
•The mesh pitch is comparable to the gap width causing the field in the gap to be non uniform•May be improved by a finer mesh
5.9 keVX-rays
5.9 keVX-rays
5.9 keVX-rays
Thin gap PPAC
Spark-protected high-rate... P.Fonte CERN 1998a29
Position resolution
From the literature on thick-gap PPACs and RPCs: about 100 m.
Quadratic contributions to the distributionbeam width:100 mbeam divergence: 20 melectronics noise: 22 mdetector: 48 m
Our own measurements usinga 9 mm gap RPC and a collimatedX-ray beam
Timing accuracy
From the literature on PPCs and RPCs: better than 1 ns .
(Arefiev et al.)
Thin gap PPAC
Spark-protected high-rate... P.Fonte CERN 1998a30
Thin gap parallel mesh chamber summary
Negative points
Mesh pitch comparable with the gap width
Bad energy resolution (50%) when comparedwith the best resolution of PPACs (14%).
Intrinsic granularity
May be solved (or not) by using a finer mesh
Probably affects negatively the timing accuracy
Detector physics not fully understood (but quite a lot is know about parallel geometry chambers)
Discharges are not totally avoided.
Thin gap PPAC
Spark-protected high-rate... P.Fonte CERN 1998a31
Thin gap PPAC summary
Positive points
Large current capability (essentially gainrate)
Made with standard stainless steel wire mesh
Mechanically robust
No melting
Can be strongly stretchedto achieve good parallelism
Free of defects(spikes, etc..)
Large maximum(low-rate) gain
107 Hz/mm2 @ gain 1000
105 Hz/mm2 @ gain 104
Very mild discharges + strong electrodes virtually indestructible
“Macro”-technology cheap and easy to build
Free of dielectrics nocharging-up effects
Good timing and position resolution expected (from experience with thick gap PPCs and RPCs)
Thin gap PPAC