Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 1

LHCb Outer Tracker Cross-talk measurements with Fe-55

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 2

Cross-talk, past & present

• A particle traversing a straw tube causes not only current in anode wire but also produces electrical signal induction in other electrodes, such as neighbouring straws. This is cross-talk.

• CT was investigated on a prototype module in 2000 by Gromov & Sluijk. For a Kapton XC + aluminium shielding the analogue CT was measured to ~0.5% (adjacent).

• This study concerns CT between adjacent wires in same plane. Measurements done on module #2.

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 3

The choice of source

• A Fe-55 source was used to induce ionization in the wires.– Primarily emits 5.89 keV photons from EC.

– A photon induces a cloud of ~226 electrons in one straw tube.

– If a signal is recorded simultaneously in an adjacent wire it is highly probable that it is cross-talk

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 4

Some words on the gain

• To read out signals from module the latest VV-50 preamp board from Heidelberg was used.

• Calibrated with pulse generator

• Saturates just around 1800V

• Gain calculated using conversion factor 0.42 V/pC (from calibration)

• Gain increases rapidly in region 1500-1800 V• It is expected that cross-talk increases with higher voltage →

measurements done at 1800V.

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 5

Experimental setup

• Wires are connected to HD-preamp board which also supplies HV.

• Output from preamp on two separate channels to scope card in computer. – Active channel is illuminated by source, used as a

trigger for the– Channel of interest

• adjacent wire (to measure cross-talk)• distant wire (for reference)

• LabView is used to set trigger levels, sampling rate etc. Transfers digitized signals to txt-file.

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 6

Cross-talk measurements

• CT between straws , 1800V– Adjacent wires (32/33)– Distant wires (32/48)

• Electronic CT on preamp, disconnected from module, using pulse generator– Adjacent channels (32/33)– Distant channels (32/48)

• To eliminate electronic noise the signals are averaged

+ + →

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 7

Pulse characteristics

• Pulse generator was set to give an output signal looking like a Fe-55 signal in amplitude and time duration.

x-axis in ns, y-axis in V

Fe-55(averaged)

1800 V

Pulse gen.

Flatness due to range settings in read-out

1800 V

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 8

Electronics cross-talk

Adjacent channel 32/33 Distant channel 32/48 ch33-ch48

• Dotted line=active channel, solid line=ch. of interest*100. • Electronics cross-talk is present. Ratio between amplitudes

gives a measure of the Elec-CT: ~ 2‰• CT is bigger on adjacent channels, right plot shows

difference between ch33 and ch48. Differences are of the order of 0.5 mV.

- =

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 9

Cross-talk between wires

• Since elec. CT is present it’s expected to make up one component in the total CT measurement.

Adjacent straw 32/33 Distant straw 32/48 str33-str48

- =

• Similar patterns as in the electronics CT measurement can be seen, the elec. CT component is present. The question is of course…

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 10

…can we disentangle straw CT?

• Due to differences in time duration of pulses…– Fe-55 pulse ~ 175 ns– Generated pulse ~ 110 ns

… the two “difference plots” cannot easily be subtracted from each other in order to remove the electronics cross-talk component.

• There are still some observations that can be done though at this stage:

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 11

“First peak” observations

str33-str48 ch33-ch48

Due to differences between active signals the red line is drawn to indicate comparable regions in time

0.70 mV0.50 mV

Pulse gen. signal amplitude is 20% higher than Fe-55 but the first peak in Fe-55 difference plot is 40% bigger than pulse gen. difference peak…

“First peak”

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 12

A straw cross-talk estimate

• If one assumes that the amplitude of electronics CT grows linearly with the pulse amplitude on the active channel then– The Fe-55 first peak should have a elec. CT-

component of about ~ 0.4 mV… … but the Fe-55 first peak has an amplitude of 0.70 mV!

• So, from this (too?) simple reasoning one can draw the conclusion that the straw CT component is ~ 0.30 mV

• Analogue CT between adjacent straws ~ RCT= 0.30 mV / 500 mV = 0.6 ‰

Aras Papadelis,OTR meeting NIKHEF, June 17th 2004 13

Conclusions and outlook

• Hard to resolve inter-straw cross-talk due to electronics cross-talk on preamp board. – In any case, the analogue CT is less than 2‰– Could be as small as ~ 0.6‰

• The straw tubes are well-isolated so a possible scenario is that the main source of CT will come from the read-out electronics.

• For more accurate measurement: – Re-measure with separate preamps (not possible with current

VV-50 preamp board). – The other preamps available now (old HD-preamp board and

Gras’ preamp) are not silent enough for this.