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Design of active-target TPC

ContentsI. Physics requirements

II. Basic structure

III. Gas property

IV. Electric field• Distortion by ground• Distortion of electric filed by ions created by beam

V. Pad shape

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Physics requirements Study for nuclear property of unstable nuclei

→ Use of inverse kinematics is needed.

Measurement of forward scattering → Measuring the recoil light nuclei can lead to precise measurement.→ But the energy of the recoil nuclei for forward scattering is very small.→ Gaseous target (or thin foil) is needed.

GasTarget : → He gasTarget : d → d gas (or Cd4)

To separate the objective reaction from other reaction,• Angler resolution : 7.45mrad(RMS)• Position resolution of vertex point : 1mm(RMS)• Energy resolution : 10%(RMS)

is needed for .

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Basic structure Mask the beam track area

•TPC can be operate in high rate beam condition.(Only the rate of recoil nuclei has to be taken into account.)

Use of GEMElectron multiplication can be done at high rate.

Pad shape(rectangular triangle)•To lessen the number of pads, rectangular triangle is used for the pad shape.•Position is derived by the charge ratio of the neighboring two pads.

Beam

Field cage

Pad

GEM

Recoil nuclei25cm

10cm 10cm☓Thickness : 100m

Total volume :565mm 668mm 520mm☓ ☓

Schematic view

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GEM

GEM

Beam

Recoil nuclei

Field cage

GEM

Pad

Recoil nuclei

25cm

Beam

Field cageWire(pitch: 2.5mm)

4cm

Schematic view 2

Mesh

NaI (CsI)

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Gas property (simulated by Garfield)• He(90%) + CO2(10%) (760 Torr, 300 K)

Drift velocity Townsend coefficient

Longitudinal diffusion Transverse diffusion

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• D2(100%) (760 Torr, 300 K)

Drift velocity Townsend coefficient

Longitudinal diffusion Transverse diffusion

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Distortion by ground The effect of ground is checked for 3 configurations.

•pitch : 2.5mm; double wire• pitch : 5mm; double wire• pitch : 2.5mm; single wire

Put electrons at • x : every 5mm from x=0.5cm to x=13.0cm(active area of GEM : 2.5<x<12.5)• y : 24.0cm

Drift electrons to the end of field cage, and subtract the point where electron is put from end point.

→ Position difference < 0.745mm.

The effect of diffusion is not considered.

Field cage Field cage

y

0x

y=24cm

13cm

Result

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2.5mm pitch; single 5mm pitch; double

Active area of GEMActive area of GEM

2.5mm pitch; double

Active area of GEM

Wire of field cage has to be double.

Active area of GEM is 2.5cm < x < 12.5cm

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Distortion of electric field by ions

If the beam rate is very high, beam comes before the ions created by previous beam go away.→ Ions (electrons) created by beam are piled up, and distorts the electric field.

After a few seconds, charge distribution will be stationary. ← This Charge distribution is simulated by Monte-Carlo simulation.

Distortion of the electric field is simulated by Garfield,← Put stationary charge distribution into Garfield’s configuration, substitute wire for electric charge.

and simulation of the position gap during electron drift has done.

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Condition for simulation Gas property

• Gas: He(90%) + CO2(10%) • Electric field : 1kV/cm • Pressure : 760 Torr • Temperature : 300 K

Drift velocity : 3 [cm/s] Ion mobility : 2.5 10☓ 3[cm2·Torr·V-1·s-1]

Beam•Beam rate : 107 Hz• Energy loss : 4 MeV/cm = 105 ions(electrons)/cm

← corresponds to Sn with 100MeV/u• Beam spread :

5cm (RMS) for drift direction1cm (RMS) for the other direction

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Charge distribution

• Move each bin data to the next bin.← corresponds to time change, the move of ions(electrons).

• Generate random number (Gaussian; mean: 12.5cm, RMS: 5cm). ← corresponds to beam hit position, where ions(electrons) are created.

• Add to the histogram.

Field cage

y y

GEMPad

count count

Recoil nuclei

25cm

Beam

Field cage

y

repeat

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Distribution of ion

stationary

Take the average of these histogram

Ion distribution for each 1[ms]

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Field cage Field cage

y

0x

Put electrons at • x : every 5mm from x=2.5cm to x=13.0cm• y : 24.0cm

Drift electrons to the end of field cage, and subtract the point where electron is put from end point.

→ Position difference < 0.745mm.

The effect of diffusion is not considered

Simulate position difference in 3 different shield wire configuration.

• Without shield wire• 5mm pitch• 2.5mm pitch

y=24cm

Position difference

2cm 13cm

Shield wire

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Result

Without shield wire Shield wire : 5mm pitch

Active area of GEM

Active area of GEM is 2.5cm < x < 12.5cm

Active area of GEM

• Without shield wire : Maximum position difference is over 1mm• Shield wire : 5mm pitch : Maximum position difference : ~ 0.745mm• Shield wire pitch : 2.5mm : Maximum position difference is 0.3mm < 0.745mm → Change of track angle is less within 3mrad.(flight length: 10cm)

Shield wire : 2.5mm pitch

Active area of GEM

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Pad shape

16mm

16mm

• Pad shape : rectangular triangle (16mm☓16mm)

• Position is derived by the charge ratio of the neighboring two pads.

• Angler resolution < 7.45mrad(RMS)

• Hit position is fitted by line using the least squares method.

z

xQ1

Q2

Recoil nuclei

z = Q2 / (Q1 + Q2) ☓ 16mmx = Q2 / (Q1 + Q2) ☓ 16mm

xQ1

Q2 Recoil nuclei

z

z

x

Q2

Recoil nuclei

Q1 Derive wrong position!→ Thinking about the algorism to derive correct position in such case

Derivation of position 1

z = Q1 / (Q1 + Q2) ☓ 16mmx = Q1 / (Q1 + Q2) ☓ 16mm

For these cases, position is derived by the same way.

Derivation of position 2

z

xQ1

Q2

Recoil nuclei

z

xQ1

Q2

Recoil nuclei

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Condition

Energy loss of recoil nuclei : 500 electrons/cmproton 30MeV @Ar(70%)+CO2(30%) : 700 electrons/cm with 15MeV/u @He(90%)+CO2(10%) : 500 electrons/cm

Transverse diffusion （ RMS ）Transverse diffusion coefficient of He(90%)+CO2(10%) : 200m for 1cm(RMS)

• 200m• 400m• 600m• 1000m

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Simulation Arrival position of electronx : z :

Ru : uniform random number between -1 and flight length+1Rdx : Gaussian random number, which corresponds to diffusion length for x directionRdz : Gaussian random number, which corresponds to diffusion length for y direction : incident anglez0 : incident position

Number of generated random number : n±√n n=500[electrons/cm]×(flight length+2)

Number of events : 10000

dzdxu RRR sin0cos zRRR dzdxu

16mm

16mm

z

x

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Diffusion

: 200m: 400m: 600m: 1000m

Diffusion

• Other than the border of 2 pads → Almost same• Border of 2 pads → Tracking algorism is not good.

0z

z : injection position for z

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Pad size

: 8mm×50mm: 16mm×50mm: 16mm×25mm: 16mm×16mm: 20mm×20mm

Pad size

z : injection position for z

16mm×16mm : best angular resolution

Diffusion : 1000m

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Inclined incidence16mm

16mm

z

x

In the case of inclined incidence, angular resolution and position resolution of vertex point are simulated.

: -30°, -15°, 0°, 15°, 30°

Pad size : 16mm × 16mm

Number of generated random number : n±√n n=500[electrons/cm]×(flight length+2)

Number of events : 10000

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Result (angular resolution)

= 30° = 15° = 0°

= -15° = -30°

Diffusion : 1000m

Better angular resolution can be achieved in the case of <0 than in the case of >0.

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Result (vertex resolution)

= 30° = 15° = 0°

= -15° = -30°

Diffusion : 1000m

Vertex resolution is less than 1mm.

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Summary & Outlook Final design is

• Wire pitch : 2.5mm (double)• Pad size : 16mm × 16mm

→ Performance• Angular resolution : < 4.5mrad• Position resolution of vertex point : 0.5mm• Position difference : < 0.3mm

Outlook• Tracking algorism• Include the effect of straggling

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Backup

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Field cage Field cage

y

0 x

Substitution wire for electric charge (x=1)

To consider the beam spread for x-axis, wires are put at x=1.

The voltage which supplied to substitution wire(V) is

V0 : electrical potential made by field wiresq : electric charge at unit lengthlground : distance from wire to groundlwire : diameter of wire

)ln(2 0

0wire

ground

llqVV

Substitution wire

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• Ar(70%) + CO2(30%) (760 Torr, 300 K)

Drift velocity Townsend coefficient

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• Ar(70%) + CO2(30%) (760 Torr, 300 K)

Longitudinal diffusion Transverse diffusion

Electric field(distorted)

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Without shield wire Shield wire : 5mm pitch

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Electric field(distorted)

Shield wire : 2.5mm pitch

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Position difference between mesh to pad

-15.4 -12.7 -2.3 2.3 12.7 15.4

1.3-1.30

0.19 ~ 0.200.25

0.6

x

0.14

GEM Pad Frame of GEM

Put electrons at • x : every 1mm (-13.5cm<x<-11.5cm & -3.5cm<x<-1.5cm)• y : Between mesh and GEM

→ 0.59cm(0.01cm below Mesh)Between GEM(or frame) and pad

→ 0.18cm(-12.6cm<x<-11.5cm or -3.5cm<x<-2.4cm; 0.01cm below GEM) or 0.13cm(other area; 0.01cm below frame)

Drift electrons from mesh to GEM & from GEM(frame) to pad, and derive the position difference.

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Active area ofGEM

-3.5 < x < -1.5

-13.5 < x < -11.5Result (Mesh-GEM)

Overlap with GEM & frame : 1mm

Overlap with GEM & frame : 0.5mm

Overlap with GEM & frame : 0mm

Frame width : 10mm

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Active area ofGEM

-3.5 < x < -1.5

-13.5 < x < -11.5Result (GEM-Pad)

Overlap with GEM & frame : 1mm

Overlap with GEM & Frame : 0.5mm

Overlap with GEM & Frame : 0mm

Frame width : 10mm

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Active area ofGEM

-3.5 < x < -1.5

-13.5 < x < -11.5Result (Mesh-GEM)

Frame width : 5mm Frame width : 10mm Frame width : 15mm

Overlap with GEM & frame : 0mm

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Active area ofGEM

-3.5 < x < -1.5

-13.5 < x < -11.5Result (GEM-Pad)

Frame width : 5mm Frame width : 10mm Frame width : 15mm

Overlap with GEM & frame : 0mm