Transcript Development of GEM at CNS

Development of GEM at CNS
Hideki Hamagaki
Center for Nuclear Study
University of Tokyo
Contents of This Presentation
• What is GEM
• How to make GEM
– Plasma etching and its improvement
– Test of gain variation
• GEM application
– GEM-TPC
– HBD
– X-ray detector with Xe gas
• Summary and outlook
– Improve GEM performance and stability
– Applications
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What is GEM
• Gas Electron Multiplier
– Developed at CERN by Sauli et al.
• Foil of Kapton with Copper coated
at both sides
– typical: Kapton 50mm, Copper 5mm
with holes:
– typical: pitch 140mm, and 70mmf
• Electron multiplication in the
strong field inside the holes
– typical voltage between the two
Copper foils = 300 ~ 550V
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Characteristics of GEM
• simple structure with lost cost material
• low mass
• multiple stacking  high gain
– less risk of severe sparks/breakdown
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S.Bachmann et al,
NIM A438 (1999) 376
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How GEM is or will be used
• COMPASS experiment@CERN
– tracking chambers near beam-pipe
• high rate ~150 kHz
• low mass & high resolution
• Possible applications
– Time Projection Chamber
• PHENIX, STAR, TESLA, JLC, …
– UV photon detector with CsI photocathode
• Gas Cherenkov counter: e.g. PHENIX HBD
– X-ray detection
• Cosmology  T. Tamagawa’s talk
• Biology/Medical applications
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• step1
How to make GEM
– put proper masks to the copper foils
• step2
– wet etching of masked copper foils
• step3
– make holes to the Kapton foil
• wet etching
--- CERN GEM
• laser etching
--- persued by T. Tamagawa
• plasma etching
--- persued by us (CNS)
• hybrid of the above
--- persued by us (CNS)
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Plasma Etching Method
• A method different from CERN
– Fuchigami Micro has expertise on this
• inherent gain variation in CERN GEM
– our suspicion is on the hole shape
CERN-GEM
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CNS-GEM
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Characterstics of CNS-GEM
• Low resistance or
sparks at low HV in the
beginning  aging
– surface roughness
– small residues
• Similar gain to CERNGEM
• Lower breakdown point
than CERN-GEM
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Improvement of CNS-GEM
• Earlier breakdown
– cleaning process
– surface roughness
– due to over-hung of Copper edges
CERN-GEM
CNS-GEM
• Succeed to reduce over-hung
• Still breakdown at lower HV
– This is only crucial for high HV
operation, needed for CF4 gas
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Test of Gain Variation
• gain measurement with Fe55
source
• gain of CNS-GEM seems to
stabilize in shorter time
• After the first try, gain seems
to stabilize much sooner, as
long as GEM is kept inside the
chamber
Blue : CERN-GEM（Gas : flow）
Black: CNS-GEM（Gas : noflow）
Red: CNS-GEM(Gas: flow)
• Need further work
– condition; temperature, oxygen,
moisture, …
– simultaneous comparison
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GEM-TPC; motivation
• Need of 3D-tracking device durable under
high particle density and high rate at RHIC
– position resolution, two-track separation, energy
measurement
• Expected advantage of GEM
– small ion feedback; no need of gating grid
– simple structure
– flexible readout configuration
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GEM-TPC prototype
Tested at KEK-PS
• two types of readout pads
– rectangular & chevron type
– 1.09 mm x 12 mm
• charge-sensitive pre-amp
– 1 ms time-constant
• readout with 100 MHz FADC
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Performance of GEM-TPC (I)
• Position resolution
P10
– z direction
– x direction
– resolution gets worse with
increase of drift length
Ar+C2H6(30%)
CF4
• diffusion effect
• magnitude depends on gas species
Electric
field
(V/cm)
Drift
velocity
(cm/ms)
Diffusion
(T)@1cm
(mm)
Diffusion
(L)@1cm
(mm)
Ar(90%)+CH4(10%)
130
5.5
570
360
Ar(70%)+C2H6(30%)
390
5.0
320
190
CF4
570
8.9
110
80
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R : P10 chevron
B : P10 rect.
Y : Ar+C2H6 rect.
G : CF4 chevron
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Performance of GEM-TPC (II)
Energy loss measurement
36 mm of P10 gas
drift length = 85mm
– P10: s(55Fe;5.9 keV) = 11 %
• Ne(primary) ~ 222 for 5.9keV X-ray
in P10  ~1.7 times larger than
statistical estimate
– obtained energy loss is as
expected for various particles
with different momentum
• Beam rate effect
– no change up to 5000 cps/cm2
– good enough for HI applications
– further studies may be needed
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Z direction
R : P10 chevron
B : P10 rectangular
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Hadron Blind Detector
• UV photon detector
– with CsI cathode
– CF4 gas radiator
– Ne(Cherenkov) > Ne(ionization)
• development at Weizmann
Institute
• Use to measure low mass
electron pairs at RHIC
– Rejection of Dalitz pairs and
external conversion pairs
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Summary and Outlook
• How to improve GEM performance and
stability
– improve etching procedure
– cleaning procedure after etching
• use wet etching (cleaning) to smoothen the surface
• washing out small residues
– choice of material
• base: insulator  resistive plastics
– prevent gain variation due to charge up
• Copper  Aluminum
– low mass, long radiation length
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Summary and Outlook (cont.)
• X-ray detector with Xe gas
– for medical/biological use; two versions
– 2D radiation dose monitor
• taking advantage of simple structure, and
flexible configuration
– precise 2D tomography
• started development of a custom ADC chip with
Tanaka’s group at NIAS
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