19 September 2006SuperNEMO tracker, Manchester status The SuperNEMO Tracker Manchester status Steve Snow Ray Thompson Stefan Soldner-Rembold Irina Nasteva

19 September 2006 SuperNEMO tracker, Manchester status

The SuperNEMO TrackerManchester status

Steve Snow

Ray Thompson

Stefan Soldner-Rembold

Irina Nasteva

James Mylroie-Smith

Nasim Fatemi-Ghomi

19 September 2006 SuperNEMO tracker, Manchester status 2

Outline

Electrostatic simulations of Geiger cells:

• Comparison between Garfield and FlexPDE

• Results for 9-cell prototype layouts

• Results for different layouts of the SuperNEMO tracker

• To do…

Construction of 9-cell prototype:

• Status of first 9-cell prototype

• Status of second 9-cell prototype

• Single Geiger cell


Electrostatic simulations

of Geiger cells


Simulations of 3x3 cells

The 9-cell prototype is simulated with:

• X pitch = 30 mm

• Y pitch = 30 mm

• Gap = 10 mm

• Cathode diameter 50 m

• Anode diameters 50 and 30 m

Possible layouts:

• Basic octagonal cells

• Octagonal cells with 4 extra wires around mid cell

• Octagonal cells with 4 extra wires around all cells

ground plane

extra cathodes


Garfield and FlexPDE

Garfield:

• electrostatic simulations of wire chambers in 2D

• makes use of symmetries

• can simulate gases with Megaboltz

• used and tested in many gas detector simulations (NEMO3)

FlexPDE:

• finite element analysis

• user supplies differential equations to be solved (programme knows nothing about the physics)

• can do simulations in 3D

• easy to use


Applied and effective voltagesIn a wire chamber we have -

An arrangement of wires with voltages applied to them.

A resulting field distribution that can be calculated with Garfield or FlexPDE.

Very near the wires, the field always has the form E=A/r. Equivalently, the potential contours are circles centred on the wire.

It is the strong E field within 1.5 mm of the anode wire that determines the avalanche gain, which in turn drives the Geiger plasma propagation.

It is the strong E field at the surface of the cathode wire that can drive electron emission processes, leading to self-sustained discharge.

So the electrostatics of a wire chamber is characterised by the A values near each of the wires.

Instead of quoting A directly, we usually convert it to the effective voltage: the voltage necessary to produce the same value of A when the wire is in the centre of a 30mm tube:

Veff = ∫ A/r dr = A ln( rtube /rwire )


Gain versus Voltage and Anode radius

14.6

14.8

15

15.2

15.4

15.6

15.8

16

16.2

16.4

1620 1640 1660 1680 1700 1720 1740 1760

Voltage

ln(gain)

50 micron30 micron-

1654 V 1700 V

Nemo 3 gas

To compare layouts with different anode diameters we need to know how the Townsend coefficient varies with E.

We used predictions from Magboltz for the NEMO-3 gas mixture.

The avalanche gain is given by integration of (E) in the high field region:

Gain = exp( ∫ (A/r).dr )

The result of the integration for 30 and 50 micron wire diameters, at a range of effective voltages, is shown in this plot.

This shows that a 50 micron wire with Veff = 1700 V will give the same gain as a 30 micron wire with Veff = 1654 V.

-500.000

0.000

500.000

1000.000

1500.000

2000.000

2500.000

3000.000

0.00 50.00 100.00 150.00 200.00

Field (kV/cm)

Rate ( /cm )

Alpha

Attachment

Alpha-Att


Basic octagonal 3x3 cells - results

50 micron anodes: 30 micron anodes:

On the anodes we show the applied voltage, necessary to produce a gain equivalent to 1700 V on a 50 micron wire in a 30 mm tube.

On the cathodes we show (-1x) the effective voltage.


Octagonal+4 (mid cell) - resultsOn the anodes we show the applied voltage, necessary to produce a gain equivalent to 1700 V on a 50 micron wire in a 30 mm tube.


50 micron anodes: 30 micron anodes:


Summary of 3x3 cells results

• Garfield and FlexPDE agree to within 0.4%

• We can go on to use FlexPDE for 3D simulations (wire ends)

• Adding extra cathodes around mid cell reduces Veff on cathodes …

• Decreasing anode diameter to 30 mm gives a higher gain at a given voltage


SuperNEMO module assumptions

We assume that:

• There will be a continuous block of Geiger cells filling nearly all the space between the source foil and the calorimeter.

• All cells have the same layout except for possible minor variations on the surface layers.

• The space between foil and scintillator must be >30 cm for TOF to work. But total module thickness should be kept down.

• The structure will be 9 cells deep in the X direction and very large in the Y direction. So the unit cell for electrostatics is the pink area.

• X pitch and Y pitch need not be identical.


Octagonal layouts

Simulated with

• X pitch = 30 mm

• Y pitch = 30 mm

• Gap = 10 mm




Hexagonal layouts

Simulated with

• X pitch = 30 mm

• Y pitch = 30 mm

• Gap = 10 mm




Edge effects are small in this cell, important in the last cell

30 micron anodes

50 micron anodes

These three should be equal to 1700/4 = 425 V. Difference is due to limited simulation accuracy.

Basic octagonal layout - resultsOn the anodes we show the applied voltage, necessary to produce a gain equivalent to 1700 V on a 50 micron wire in a 30 mm tube.



30 micron anodes

Edge effects are negligible except for the last cell

50 micron anodes

Field lines are no longer shared equally between these four cathodes. We could benefit by increasing the separation of the closest pair.

Octagonal+2 layout - resultsOn the anodes we show the applied voltage, necessary to produce a gain equivalent to 1700 V on a 50 micron wire in a 30 mm tube.



30 micron anodes

Edge effects are negligible except for the last cell

50 micron anodes

Field lines are now shared equally between these six cathodes.

Octagonal+4 layout - resultsOn the anodes we show the applied voltage, necessary to produce a gain equivalent to 1700 V on a 50 micron wire in a 30 mm tube.



Hexagonal+4 layout - resultsOn the anodes we show the applied voltage, necessary to produce a gain equivalent to 1700 V on a 50 micron wire in a 30 mm tube.


50 micron anodes

30 micron anodes


Hexagonal+6 layout - resultsOn the anodes we show the applied voltage, necessary to produce a gain equivalent to 1700 V on a 50 micron wire in a 30 mm tube.


50 micron anodes

30 micron anodes


Figures of merit

We want: wires/cm to be small for transparency,

cathode Veff to be small for stability.

The dominant parameter is cathodes/cell, followed by anode diameter, then Hex/Oct.

We choose the octagonal as our baseline design.

Anode Cathodes X pitch Wires Anodes Anode voltage Cathode(micron) per cell (mm) per cm per cm Max Range V_eff

Octagonal 50 4 30 15.67 3.00 2014 78 429 too highOct+2 50 5 30 18.33 3.00 1944 42 359 just OKOct+4 50 6 30 21.33 3.00 1891 32 290 safeHexagonal 50 2 30 10.58 3.27Hex+2 50 3 30 13.66 3.27Hex+4 50 4 30 16.74 3.27 2010 78 430 too highHex+6 50 5 30 20.01 3.27 1927 40 350 just OK

Octagonal 30 4 30 14.47 3.00 1937 72 386 too highOct+2 30 5 30 17.13 3.00 1874 38 323 safeOct+4 30 6 30 20.13 3.00 1877 30 269 very safeHexagonal 30 2 30 9.28 3.27Hex+2 30 3 30 12.36 3.27Hex+4 30 4 30 15.43 3.27 1931 68 385 too highHex+6 30 5 30 18.71 3.27 1859 36 315 safe

Layout


To do …

Geiger cells:

• Simulate the NEMO3 cell to give an estimate of tolerable cathode Veff.

• Check a handful of points on the gain versus voltage and anode diameter plots by operating a test cell in proportional mode.

• Check whether Geiger propagation depends only on gain, as assumed, or whether it has some extra dependence on wire diameter.

• Find out experimentally the highest tolerable cathode surface field a) on a fresh wire, b) after some ageing.

Physics simulation:

• Are 40mm cells are acceptable for two-track resolution?

• Which of the following have most influence on acceptance of 0v events?energy loss or multiple scattering, in the gas or in wires,wire length, source foil area, foil-to-scintillator distance, …

• This was partially studied by Darren Price, could be a new student project


Construction of 9-cell

tracker prototype


First 9-cell prototype

• 3x3 cells (as in simulation)

• X,Y pitch = 30 mm

• Length = 2 m


• Anode diameter 50 m

• Wires from NEMO3

• Gas system, He-Ar, ethanol cooler

• Trigger system for cosmics: 2 scintillators in coincidence


Status of the first 9-cell prototype

Prototype was wired:


Status of the first 9-cell prototype

… and closed in the vacuum vessel


Second 9-cell prototype

Based on Forget concept of separate stackable cells:

Rail glides

Pick up points


Some alterations to allowprototype to be fabricated by CNC machining rather than molding.

Wireclamps screwed rather than ultrasonic welding.


Single Geiger cell

A single Geiger cell was constructed to study plasma propagation:

• Single anode inside a tube

• Diameter 26 mm

• Length = 3 m


Total time of pulses agrees with plasma propagation at 2.5 cm/ 3- s along a m wire

0

5

10

15

20

25

90 100 110 120 130 140 150 Times

Events

Single cell tests

• He-Ar gas mixture, no alcohol yet

• Trigger on 2 scintillators

• We have seen the first signals


Status Summary

Single long tube - Pulses.(simulation reference)

First 9-cell prototype - Wired, in clean vacuum vessel.conventional crimp design awaiting cleaning of gas piping.

ready to switch on after Dubna meeting.

Second 9-cell prototype Most of endcap components CNCForget concepts machined.

Awaiting side closure pieces.2nd vacuum vessel ready.Need to build wired cell carrier.

Readout Currently using a scope and LabView.Need a multichannel ASIC readout card (LAL).We have bid for H1 ADC boards after decommissioning (2007).

Documents

19 September 2006SuperNEMO tracker, Manchester status The SuperNEMO Tracker Manchester status Steve Snow Ray Thompson Stefan Soldner-Rembold Irina Nasteva