Ray Thompson /Steve Snow Supernemo meeting MSSL 4-6 April 06

Stefan Soldner-RemboldSteve SnowRay ThompsonNasim Fatemi-GhomiIrina Nasteva - new RA

Manchester Status

Plane and modular geometry: ~ 5 kg of enriched isotope per module

Top view Side view5 m

1 m

4 m

20 modules: 100 kg of enriched isotope ~ 60 000 channels for drift chamber ~ 20 000 PMT if scint. block ~ 2 000 PMT if scint. bars

1 module: source (40 mg/cm2) 4 x 3 m2

Tracking volume: drift wire chamber in Geiger mode, ~ 3000 cells Calorimeter: scintillators + PMTs

1 m

SuperNEMO preliminary designSuperNEMO preliminary design

Basic Nemo3 Geiger Cell

9 cell prototype aims -

Technology Transfer - understand Nemo 3 cell parameter space Gas composition He + 1%Ar + 4% Ethanol

1. Extension of cell to 4m Efficency of Geiger propagation Plateau width?

2. Ageing - high gain – small fraction continuous afterpulsing 3. Reduce scattering - Wire diameter - looks unlikely - Cell size optimisation why 3cm ? 2 track resolution - channel

count

4. Test ideas for modular cell construction/wiring ultrasonic welding to avoid crimps ??

3cm 2.7m

Possible end cap concepts

Classic large machined end plates - individual crimps - well understood - difficult to wire - large - don’t really need the mechanical accuracy - hard to test

Modular cells - cell units wired individually – - stored on simple frames - tested in small units - shipped to final assembly site then mounted on big frame

Plastic moulded endcap- One continuous cathode wire wrapped round lugs- split cathode ring allowing access to anode wire- Ultrasonic welding of plastic to fix - wires ?-Worry about fields on insulating surfaces?

Endcap molding front

ENDCAP MOLDING BACK

Cell

Cell stacking

Mechanics• Gathering tools

• New 150m2 clean room suite now ready – can start wiring

• Kit of parts to construct test cells• Clean gas tanks 2 x 2m x 200mm

f Or 1 x 4m• Up to 5x 5 cells

• Endplates to suit cell designs • Clean gas system – revamped gas microstrip system

+ ethanol cooler

Software tools

MAGBOLTZ (Steve Biagi NIM A 421 1999 234-240) is a programme that calculates properties of electron drift in most wire chamber gasses and their mixtures. I have got a copy and tried it on the Nemo 2 and 3 gasses.

FlexPDE is a programme that solves systems of differential equations using the finite element method. I am already familiar with it, so I have tried it on the 2D electrostatics of the Nemo 3 cell. It also works in 3D but I have not tried this yet.

GARFIELD is well known wire chamber simulation software, already used for Nemo 3. It includes interfaces to MAGBOLTZ and other useful tools. It solves 2D electrostatics of wires and planes, but no 3D.

MAGBOLTZ

Calculates drift velocity, diffusion and Townsend , starting from electron-molecule collision cross sections and the Boltzmann transport equations.

Examples

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Field (kV/cm)

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ft v

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cit

y (

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Nemo 3

Nemo 2Nemo 3 gasHe(94.85):Elthanol(4):Ar(1):water(0.15)

Nemo 2 gasHe(97),Ethanol(3)

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Alpha-Att

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Actual velocity plateau measured in Nemo is about 20% lower than MAGBOLTZ prediction

FlexPDE

Used to simulate this wire layout

Minimal cell to represent 3-layer plane.

Symmetry boundary conditions on dashed line. Value boundary conditions in coloured areas.

FlexPDE output example.

Potential map, contours from 60 to 400 V Anode at 1840 V, all wires 50 micron.

Interpretation of potential mapsThe thing that matters is the E field strength near to ( < 1mm ) the anode wire.

In this region the field is extremely close to E = A/r, where A is a constant to be determined by the simulation.

Instead of plotting A directly, I interpret it as the equivalent voltage on a 50 micron wire at the centre of a 30 mm tube:

No contours: field is very close to 1/r. Effective voltage is 1560 V

Some contours: field is slightly stronger on one side of the wire than the other. Effective voltage is 1605 V.

Conclusion

middle plane needs to be run 45 V higher to give the same gain as edge planes.

Becomes 20 V when extra ground wires are added.

Nemo 3 test cell equipotentials

Comparison with other experiments

There are plenty of other helium based drift chambers out there. None use Geiger mode so comparisons with Nemo may be unfair but ... Wire diameter (microns) Quencher

Experiment Anode Cathode Gas Proportion X0 (m)Finuda 25 50 isobutane 30% 524Babar 20 120 isobutane 20% 748Belle 30 125 ethane 50% 630Hades 20 80 isobutane 40% 402Kloe 25 80 isobutane 10% 1310Nemo 3 50 50 ethanol+Ar 4% + 1% 2179Nemo 2 100 100 ethanol 3% 3060

Other experiments find it necessary to have Øcathode = few × Øanode , otherwise high field on cathode surface causes continuous discharge. Nemo has unusually thick anode and equal cathode. Survives because of low rate ? Improved stability or transparency if anode could be reduced ?

Nemo has unusually low proportion of quencher good X0. But a further compromise of 50% on X0 would allow use of Kloe gas and proportional mode. Worth investigating for Supernemo ?

Something I don't understandThe very tight tolerance on the wire diameter, 50 microns ± 1%, "is necessary to ensure proper plasma propagation."

There should be a simple relation between wire diameter and operating voltage that give equivalent gain. Use Magboltz value of vs E and basic electrostatics to give E vs radius for wire in centre of Ø30 mm tube. Integrate dr to get total gain:

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Wire potential (V)

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ain

)

Wire diameter 50 microns

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35 40 45 50 55 60 65

Wire diameter (microns)

ln(g

ain

)

Wire at 1600 V

1% on wire diameter corresponds to 1.3 V on wire potential

15 % on wire diameter corresponds to 20 V on wire potential

Another thing ...

In Nemo 3 the sagitta of tracks bending in the magnetic field is only a factor ~2 greater than the sagitta error caused by multiple scattering.

Naive calculation using 50 cm electrons to 25 G field

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In SuperNemo, could we make better use of the good intrinsic resolution of the tracker ( ~0.4 mm ) by using a higher magnetic field and shorter path length ?

+ Better rejection of wrong-sign tracks.

+ Independent measurement of electron energy.

– Compromises rejection of crossing tracks by timing.

+ Reduces thickness of module.

Summary

Various tools / components in placeNew RA – Irina 100%

Can now start serious work…..

- Set up test cells- Test endcap designs---

- Simulations - Electrostatics- - Physics optimisation, multiple scattering 2 track resolution

Timescale

Geiger mode

trying to -

• Understand the self-limited Geiger mode in general,

• Collect the software tools that will be necessary to understand our test cells,

• Understand how Nemo 3 has been optimised,

• Think about optimising SuperNemo.

Steve Snow

3/4/06

Self limited Geiger mode

I waded through the series of papers by D.H.Wilkinson.

As far as I understand, he says there are just two key parameters that control the plasma propagation:

1. Gain excluding feedback: the average number of secondary electrons produced by one electron drifting in to the anode from 'far' away.

2. Photon feedback probability: the probability that a UV photon is produced in the avalanche and then travels far away from the anode and liberates a photoelectron in the gas. Assumed proportional to the number of secondary electrons.

Neither parameter is directly measurable, but

Gain can be measured at lower voltage and extrapolated upwards, or else simulated with existing tools (later slide).

The onset of Geiger mode is when gain x feedback > 1. We can measure this and infer the feedback probability per secondary electron.