Transcript GroupMeeting_sccho_20050125_TPC

Group Meeting
TPC
的基本原理
S.C.CHO.
TPC的介紹
Field Cage
MWPC
eHV Plane Drift Volume
示意圖
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with Gas
Charged
Particle
ionization electrons drift wire chambers  avalanches
position dependence of the induced signal the point of arrival
of the electrons
drift time for each of the pad signals fully 3-D
wire signals know specific energy loss, dE/dx
FADC measure the pulse height
E//B
電場與磁場的作用
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電場
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Ionization electrons drift toward the the MWPC
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磁場
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Measure the particle momenta from track curvatures
Improves the spatial resolution
Decreasing the transverse diffusion of electrons during the drift
process.
How to measure information
E
B
Particle Trajectory
T1
T2
Sense
Wire
T3
Avalanche
Trajectory
Projected
Onto Pad Plane
Pad
Row
M.W.P.C.圖解
Drift Volume
M.
Gating Grid
Cathode Grid
(Potential Grid)
Sense/Field Wire
Grid
Cathode Plane
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W.
P.
C.
The electrons arriving there are multiplied by gas amplification
and the avalanche process generates electrical signals both on
the wire itself and by induction on the segmented cathode plate
underneath.
M.W.P.C.內層結構
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Sense grid
Act as the anode wires of the proportional cell and
are at positive high voltage with respect to the
surroundings.
The electrons arrive from the drift volume towards
these wires and initiate there an avalanche process
due to gas amplification as in any multiwire
proportional chamber.
Each of sense wire is connected to a preamplifier
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Field wire
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serve to electrically decouple the sense wire
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Null potential (cathode) grid
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serve to close the proportional cell and to separate it
from the drift volume. This grid is held at the same
potential as the cathode plane.
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Potential distribution
calculate the electric field configuration for such a wire chamber.
V1(x,y;x0,y0,s)=-λ＊ˇ（x,y;x0,y0,s
s: wire pitch
Vω(x,y)= - sum<i=1,n>ˇ（x,y;x0i,y0i,s）* λi
n grid
If y>>y0,y>>s V constant
VHV=4πσ* y
V(x,y)=4πσ* y - Vω(x,y)
Space Charge的產生
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Positive ions
1.最初的游離過程: electron—drift HV plane neglected
2.avalanche process +ions (gas gain 10^3~10^4)cannot be
neglected
大部分被neutralized
1.cathode plan 2.field wire 3.cathode grid
一部分到達drift volume they travel only slowly towards the
HV plane, they can build up a considerable space charge in the
drift volume, thereby destroying the uniformity of the electric
field.
How to reduce the space charge
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Gating grid
Layer
MWPC
Of
Drift
Volume
Open Gate
Wire
Nearly not disturbing the original field
Gating Grid
Closed Gate
A potential difference between the two semi-grids is applied
such that a dipole field is created which can serve twofold
purpose
1.it stops the ions created in the avalanche process from
traveling back into the drift volume; instead they are neutralized
at the gating grid.
2.it can also be used to stop the electrons arriving from the drift
volume before they enter the proportional region
Asynchronous Mode&
Synchronous Mode
event
event
event
open
closed
Protection against ageing
Lost track information
Positive
Negative
Negative
Trigger
Trigger
Trigger
event
event
event
open
closed
非同步模型
同步模型
No protection against ageing
No track information is lost
Positive
Negative
Negative
Trigger
Trigger
Trigger
Some design considerations
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Avalanche process:
High electric field
Diameter of the sense wire small
Tolerance on the variation of the diameter very
tight
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Potential strips:
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讓邊緣線圈的電場較為均勻Guard Strips
Guard Strips
▼
▼
Normalised pulse height
▼
■
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■
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■●▲
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Potential on guard strips
▼
no guards
▲+750v/+750V
■ 0V/0V
■
●
▲
●
Anode: +1550V
Field wires: 0V
■
●
2
▲
4
Cathodes: 0V
6
8
▼-180V/-180V
-180V/0V
10 12
……………
Distance from the frame
24