Group Meeting

1. Group Meeting TPC 的基本原理 S.C.CHO.

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

3. 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.

4. E B How to measure information Particle Trajectory T1 T2 Sense Wire T3 Avalanche Pad Row Trajectory Projected Onto Pad Plane

5. Drift Volume Gating Grid Cathode Grid (Potential Grid) Sense/Field Wire Grid Cathode Plane M.W.P.C.圖解 • The electrons arriving there are multiplied by gas amplification and the avalanche processgenerates electrical signals both on the wire itself and by induction on the segmented cathode plate underneath. M. W. P. C.

6. 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 processdue to gas amplification as in any multiwire proportional chamber. • Each of sense wire is connected to a preamplifier

7. 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.

8. 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)

9. 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 wire3.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 electricfield.

10. Layer Of Wire MWPC Drift Volume Gating Grid 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 electronsarriving from the drift volume before they enter the proportional region

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

12. 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

13. Normalised pulse height ▼ ■ ● ▲ ▼ ■ ▲ ▼ ■ ● ▲ ● ▼ Potential on guard strips ●no guards ■ ▲ ▲+750v/+750V ● ■ 0V/0V Anode: +1550V Field wires: 0V Cathodes: 0V ▼-180V/-180V -180V/0V ■ ● ▲ Distance from the frame Guard Strips 2 4 6 8 10 12 …………… 24