Micromegas Gain and Stability in Negative Ion Drift Chamber

1
Micromegas Gain and Stability in Negative Ion
Drift Chamber

• Michael Dion, C.J. Martoff, M. Hosack
• Temple University

2
Outline

• Mechanical Overview of 3M/Purdue Micromegas
• Experimental set-up
• Why Negative Ion Gas?
• Calibration data, and simulations
• Results in Negative Ion Gas(NI-gas)
• Conclusions

3
Mechanical Overview

• Purdue/3M produced
• Base material 50µm Kapton layer with 15µm copper
• Overall area 7x8cm Active area 6x6cm.
• Copper chemically etched in active area to
  achieve 35µm holes, 70µm spacing
• Kapton pillars 50µm height, 1mm spacing
• Ribbed and ribbless design
• Electron multiplication(avalanche) takes place
  between Micromegas and anode

4
Experimental set-up

• Micromegas is kept at ground, the anode positive
  potential and the cathode negative potential.

5
Experimental Set-up

• Drift gap 1.4 cm, drift field 714V/cm
• Anode voltage 300-500 volts, gt gain field
  8x104V/cm
• Pulses read from anode, using Tennelec 174
  pre-amp, Ortec 571 shaping amp, Ortec MCA.

6
Set-up cont.
Anode must be clean and optically flat for
uniform, stable response.
Sandbags added to reduce microphonic noise.
7
Why Negative Ions?

• Diffusion during drift
• 
  Ldrift distance, Edrift
  field, ekavg. (thermal) energy
• High drift fields reduce the diffusion.?
• BUT electron-atom mass mismatch gt ek
  increases at high field. ?
• Ion-atom mass matching gt ek thermal to very
  high drift field. ? ? ? ?

Negative Ion drift reduces diffusion to thermal
limit (in all 3 directions!) at very high drift
field, without magnetic field. (see Martoff et
al, Nucl. Inst. Meth. A440, 355 (2000) )
8
Why Negative Ions?

• By drifting negative ions, drift diffusion in the
  gas is at the thermal(lower) limit up to high
  E-fields. Without the use of a magnetic field.
  This gives an extreme advantage in resolution.

9
Calibration, Data Simulation

• Argon/Isobutane (90/10, 80/20) used for
  calibration
• NI gas was CS2 at 40 Torr
• Collimated Fe-55 source
• Energy deposits simulated with GEANT3

10
Calibration, Data Simulation cont.
Our spectra are very comparable to Purdue
group Notice visible escape peak
Fe-55 80/20 Ar/Iso 425v/-1000v
11
Calibration, Data Simulation cont.

• Gas gains from electronic gains and primary
  number of electrons.
• Result are comparable to others previous work.

12
Results in NI-gas(CS2)

• Micromegas performed poorly in NI-gas low gain,
  high sparking rate gt low signal to noise ratio
• Deposits developed on Micromegas
• Sparking limited the available statistics

13
Results in low pressure Ar/Iso

• Tried low pressure Ar/Iso mix at same density of
  40 Torr CS2 without better results. Escape peak
  is not present in the noise.
• Still high sparking rate and poor stability and
  S/N

14
CS2 gain and GEM Comparison

• In 40 Torr CS2 Micromegas gain and stability are
  markedly inferior to GEM (NIM A 526, 409 (2004)).

15
Breakdown
e-

• What caused the Micromegas to breakdown?
• We suspect positive ion feedback can be causing
  breakdown.
• 1 Colas, P et al Electron drift velocity
  measurements at high electric fields. Available
  here
• www-dapnia.cea.fr/Phocea/file.php?classstd
  fileDoc/Publications/Archives/dapnia-01-09.pdf
  

16
Conclusions

• Micromegas is not a good candidate for TPC
  readout in low pressure NI-gases
• In electron-drift gas near 1 bar, performance is
  good
• Anode prep is crucial
• Acknowledgements to Dr. Martoff and Dr. Hosack