Group Meeting TPC 的基本原理 S.C.CHO.

TPC 的介紹 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 MWPC Charged Particle Drift Volume with Gas e- Field Cage HV Plane - E//B 示意圖

電場與磁場的作用 電場 Ionization electrons drift toward the the MWPC 磁場 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 Particle Trajectory Sense Wire Pad Row Avalanche T1 T2 T3 Trajectory Projected Onto Pad Plane EB

M.W.P.C. 圖解 Gating Grid Cathode Grid (Potential Grid) Sense/Field Wire Grid Cathode Plane Drift Volume M. 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 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

Field wire serve to electrically decouple the sense wire Null potential (cathode) grid 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.

Potential distribution calculate the electric field configuration for such a wire chamber. V1(x,y;x 0,y 0,s)=-λ ＊ ˇ （ x,y;x 0,y 0,s s: wire pitch V ω (x,y)= - sum ˇ （ x,y;x 0 i,y 0 i,s ） * λi n grid If y>>y 0,y>>s V  constant V HV =4πσ* y V(x,y)=4πσ* y - V ω (x,y)

Space Charge 的產生 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 Gating grid Open Gate Nearly not disturbing the original field 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 MWPC Layer Of Wire Drift Volume Gating Grid

Asynchronous Mode& Synchronous Mode event open closed Positive Trigger Positive Trigger Negative Trigger Negative Trigger Negative Trigger Negative Trigger 非同步模型 Protection against ageing Lost track information 同步模型 No protection against ageing No track information is lost

Some design considerations Avalanche process: High electric field Diameter of the sense wire  small Tolerance on the variation of the diameter  very tight Potential strips: 讓邊緣線圈的電場較為均勻  Guard Strips

Guard Strips Potential on guard strips ● no guards ▲ +750v/+750V ■ 0V/0V ▼ ▲■● ▼ ▼ ▼ ■ ■ ■ ▲ ▲ ▲ ● ● ● ■●▲ ▼ -180V/-180V -180V/0V Anode: +1550V Field wires: 0V Cathodes: 0V Distance from the frame Normalised pulse height …………… 24