BES III Main Drift Chamber MDC group Yuanbo Chen June 5,2002, Beijing

General consideration The main functions are: 1. Precise momentum measurement accuracy, with particular emphasis on minimizing the effects of multiple Coulomb scattering; 2. Adequate dE/ dx resolution for particle identification; Short tracks from interaction point can be reconstructed; 3. Charged particle trigger can be realized; 4. Maximum-possible solid angle subtended in the barrel region ( ～ 90%4 π Str.)

General consideration The drift chamber must be : 1. In the limited space, arrange more readout layers,and less materials inside; 2. To have a possible big solid angle, the end-plates of the drift chamber should be adopted a special shape; 3. Have enough accuracy for wire location; 4. Obtain good hit efficiency, a uniform time-to-distance relation and a uniform dE/dx collection region. 5. For the trigger requirement, the drift cell number in one layer should have good symmetry alignment, and super-layer arrangement will be adopted.

General consideration The inner diameter of the drift chamber is limited to 118 mm by the beam pipe and the assembly procedure for the interaction quadrupole. The physical outer diameter is 1600 mm to satisfy the physical requirements. The length is 2400 mm for the outer polar angle at. The drift chamber is a most inner sub-detector of BESIII detector. It is designed in two parts, inner chamber and outer chamber. The consideration is that the inner chamber can be replaced if needed.

MDC structure design.

The drift cell Small cell construction ( square cell ). Average cell half height is 6 mm in the inner chamber; 7mm for the outer chamber. Width / height of the cell is nearly equal. Two kinds of wires are selected for the drift chamber, the sense wire is 30μm Gold-plated tungsten and the field wire is 110μm gold-plated aluminum wire.

The drift cell The position resolution/drift distance(CLEOIII)

The layer arrangement There are altogether 47 sense wire layers in the drift chamber, 8 layers in the inner chamber and 39 layers in the outer chamber. The stereo wire layers are necessary to provide longitudinal measurements in the drift chamber. For measuring the small incident angle particles, 8 layers are stereo layers in the inner chamber, and 20 stereo layers in the outer chamber. There are 19 axial layers in the outer chamber for trigger and reconstruction. The axial layers and stereo layers are arranged alternately according the super- layers. All cells in the inner chamber will be symmetrical in 45 °. All cells in the outer chamber will be symmetrical in ° except stepped range. All cells in the stepped range are symmetrical in 22.5 °(30°). The axial layer will have to start at a small angle with the X-axis.

The layer arrangement

Low Z working gas The helium gas is only low Z working gas. It has a radiation length about 50 times longer (≈ 5300 m) than Argon gas (110m) ; We chose 60%He-40% propane gas mixture, same as CLEOIII.

Low Z field wire 110 μm gold-plated Aluminum wire. California Fine Wire, Al5056, 0.75 μm Au, Ni flash, “ultra-finish”

The feedthrough design The design of the feedthrough is referenced the feedthrough used in CLEO III drift chamber. Three parts: 1. The outer side is insulating bush for high voltage insulation; 2. The middle part is copper tube for inner tube location and a connector of high voltage; 3. The inner part is small tube for wire fixing, copper for sense wire and aluminum for field wire. The cramping method will be used to fix wire in the inner tube. The material of the insulating bush is Vectra A130, a kind of liquid crystal copolyester.

The feedthrough design

The mechanical design (Inner chamber ) To reduce materials,the inner chamber is designed as an open cylinder, no outer skin. All wire tension in the inner chamber will be transferred to the inner skin. It will be a whole chamber after the inner chamber is connected to the outer chamber. However, the deformation caused by wire tension in the inner chamber and the outer chamber will not be the same. It has to be considered in the design, and attention should also be paid to the helium gas leakage. The optimum connection design will be done based on the ANALYSIS simulation.

Inner chamber

The mechanical design (Outer chamber ) The inner radius of the outer chamber is to 198 mm, the outside radius is 810 mm. The length of the inner skin of the outer chamber is 1180 mm, the length of the outer skin of the outer chamber is 2400 mm. The polar angle is COSθ= 0.93, and a large acceptance of COSθ=0.83 in last sense wire layer. An inner skin of the outer chamber is necessary to conquer the deformation in the inner radius. In order to reduce materials in the working range, the thickness should be as thinner as possible. The thickness of the inner skin will be 0.5 mm carbon fiber according to the calculation. The thickness of the outer skin, made of aluminum (Al 5056), is 10 mm (the TOF detector will be banded on the surface of the outer skin).

Outer chamber

The multi-step endplate The endplate of the outer chamber is separated to two parts, one is the multi-steps, and an inclined plane. The design of the multi-step part has to guarantee the polar angle in COSθ= 0.93, to accommodate the interaction quadrupole, and to have space for cables. The stepped endplate is a set 4 aluminum rings interconnected with nonmagnetic steel bands via radial screws. Every ring is for holding 2 sense wire layers.

The multi-step endplate

The inclined plane endplate The another part of the endplate is design to inclined plane, by reason of reducing deformation caused by wire tension. The inclined plane is processed from a whole aluminum plate (Al-5056). It will be processed to some mini -steps to guarantee all feedthroughs are located in parallel with Z axis. The thickness of the endplate is 25 mm.

The inclined plane endplate

Structural analysis of the drift chamber axial stability analysis of inner cylinder (shell) stability safety factor is ： So the inner cylinder is steady enough.

Analysis of outer chamber The max deformation of outside of endplate is 2.44mm is excessive than 1mm. What we should do for reducing the deformation is to add the inner cylinder support to endplate

Analysis of outer chamber Inner skin is 0.5mm Maximal deformation in axial direction is 0.985mm 。 Inner skin is 0.8mm Maximal deformation in axial direction is 0.919mm 。

The drift chamber wiring inner chamber: It is simple to stretch wires in the inner chamber, because there is no outer skin. During the wiring procedure, the inner chamber will be installed on a rolling support. There will have 40,000 wires for the inner chamber and the outer chamber. The total wire tension is nearly 8.5 ton. To guarantee a uniform wire tension in the chamber, pre- stress method will be used in the wiring.

The drift chamber wiring The outer chamber will be set up in vertical for wiring. There is a special support to hold the outer chamber. The chamber can be rotated as needed during wiring. The support is showed in the figure.

The outer chamber wiring

High voltage system A positive high voltage will be connected to the sense wire to form a drift field in the drift cell. An RC low pass filter is used for each supply at the drift chamber end plate before distribution to the chamber wires.

High voltage power supply system Principle diagram of the high voltage power supply system

The electronics readout The BES III drift chamber has total of 9096 sense wires, time ( T ) and electric charge ( Q) information of each sense wire will be read out. Since the single wire spatial resolution is designed to be, the time measurement error from electronics readout <1 ns is desirable, assuming a drift velocity of 3.8cm/μs. The charge deposition could be measured by integrating the signal from sense wire with the accuracy better than the intrinsic resolution of about 6%. Therefore a charge measurement with a precision of 2% is sufficient to match the chamber resolution.

Expected performances Solid angle coverage The solid angle coverage in the layer 17 (sense wire) is and in the last sense wire layer is.

Single wire spatial resolution

The single wire Z-direction resolution According to the experience formula,, L is the half length of the wire, D is the movement of the stereo wire along the circle. There are 28 layers of stereo wires in the chamber, of each layer is listed in the table. Z resolution is decided by stereo wire layers. From TRACKERR the Z resolution of the chamber ≤3mm.

Momentum resolution

The TRACKERR program was developed by Barba drift chamber group to quickly simulate the performances of the chamber. The momentum resolution of BES III drift chamber was simulated by the TRACKERR,, at 1Gev/c,

The dE/ dX resolution From the calculation, is 4.7% for the 1.2 cm long sample, and is 4.5% for the 1.4 cm long sample in He/C3H8 (60/40) gas mixture. In the 1.2 cm long sample, 3 π / K and π / p separation momentum is 0.80 GeV/ c with 1.35 GeV/c respectively. In the 1.4 cm long sample, 3 π / K and π / p separation momentum is 0.81 GeV/ c with 1.40 GeV/ c respectively. The dE/ dX resolution is expected to be 6-7%.

The dE/ dX resolution ( a) Calculated probable energy loss as a function of P, in 1.2 cm sample length. (b) The particle identification capability. (a) Calculated probable energy loss as a function of P,in 1.4 cm sample length. (b) The particle identification capability.

The R & D programs Geometry of the drift chamber. According to the measurement of the MIcro-β, with references to the endplate of CLEO III drift chamber, BES III drift chamber structural design will be done,and all factors will be more detail studied. Cell and layer organization. The optimum ratio of field wires to sense wires, cell size, type of layers -axial and stereo and the arrangement will be evaluated before a final design is chosen. Track reconstruction and full simulation are needed. Cooperation with CLEO III and BELLE experiments. We will make the widest possible use of successful technology to design our drift chamber. This is important to reduce the R&D time and to perfect drift chamber structural design and manufacture. Prototypes & beam test.

Progress plan

The end Thanks a lot!

Radiation Length 0.5 mm carbon fiber ( =0.222m) ~0.225% 800 mm (radius) ( =184.5 m) ~ 0.43% 10 mm outer cylinder( =0.089 m) ~ 11.23%

The parameters of wires

Low Z working gas

Electrons drift line (from field wires) Electrons drift line(from the cell) Electrical fieldX-T relation

Low Z field wire Fig. shows the creeping result from the Al wire using in the CLEO III drift chamber.

Low Z field wire Several kinds of light material wires have been proposed and tested. Aluminum wire has a relatively long radiation length and is a good field wire candidate. CLEO II reported that a long-term creeping effect of partial tempered 5056-Al field wire was observed (less than 15% tension reduction). Several other experiments like BELLE, BABAR, and KLOE have also reported their measurements on the creeping of this kind of wires.

The thickness of the endplate is 25 mm, and the material is aluminum Al The joint space is limited by half width of the cell (7 mm), so the thickness of steel bands will be limited to 1.5 mm. The multi-step endplate is a challenge to the mechanical manufacture.

The drift chamber assembling The outer chamber endplate installation The assembling accuracy of the multiple steps will directly affect the wire position accuracy in the chamber. The tolerance of wire holes in every ring is 25μm. For the demand of the single wire position, all wire hole position tolerance should be below 60μm. A method has to be studied to guarantee the assembling accuracy of multiple steps.

The drift chamber assembling The inner chamber endplate installation The endplate of the inner chamber is processed from a whole aluminum plate, and all wire holes are finished in a digital mechanical center, the hole tolerance could be controlled at 25μm. Because there is no outer skin for the inner chamber, two endplates are only jointed with the inner skin. It is a key point to guarantee two endplates will be paralleled in 250μm and the concentricity will be less than 100μm in a limited connection area.