1. Measuring distortions in a TPC with photoelectrons D. Karlen University of Victoria & TRIUMF June 2, 2007

2. Measuring distortions in a TPC with photoelectrons 2 Introduction A controlled and reproducible ionization pattern can be very useful to understand and monitor electron transport in a TPC: A controlled and reproducible ionization pattern can be very useful to understand and monitor electron transport in a TPC: drift velocitydrift velocity diffusiondiffusion gaingain distortions arising fromdistortions arising from electric field, magnetic field non-uniformities electric field, magnetic field non-uniformities positive ions positive ions

3. June 2, 2007 Measuring distortions in a TPC with photoelectrons 3 Using photoelectrons To produce photoelectrons for this purpose, place on the central cathode a pattern of metal with a lower workfunction To produce photoelectrons for this purpose, place on the central cathode a pattern of metal with a lower workfunction Flashing the central cathode with a light pulse of the right wavelength will produce photoelectrons only from the metal patternFlashing the central cathode with a light pulse of the right wavelength will produce photoelectrons only from the metal pattern precise survey of metal pattern allows for absolute measurements of distortionsprecise survey of metal pattern allows for absolute measurements of distortions This technique is used in the STAR TPC and will be used in the T2K TPCs This technique is used in the STAR TPC and will be used in the T2K TPCs

4. June 2, 2007 Measuring distortions in a TPC with photoelectrons 4 Correcting distortions For the test pattern to be useful to help empirically correct distortions (arising because of imprecise knowledge of the fields, or from positive ions) then it is important to consider the ambiguities in solving the inverse problem: For the test pattern to be useful to help empirically correct distortions (arising because of imprecise knowledge of the fields, or from positive ions) then it is important to consider the ambiguities in solving the inverse problem: given the known positions of the test-pattern, and images from the device, determine the displacementsgiven the known positions of the test-pattern, and images from the device, determine the displacements

5. June 2, 2007 Measuring distortions in a TPC with photoelectrons 5 Ambiguities Issue 1: diffusion constant must be known to calculate spatial coordinates from the observed charge distribution Issue 1: diffusion constant must be known to calculate spatial coordinates from the observed charge distribution use line patterns (not aligned with pad boundaries) to determine diffusion constantuse line patterns (not aligned with pad boundaries) to determine diffusion constant Issue 2: lines alone are not enough: Issue 2: lines alone are not enough: readout module test-pattern image possible solutions

6. June 2, 2007 Measuring distortions in a TPC with photoelectrons 6 Having lines in other directions resolves ambiguity, but only near crossing point Having lines in other directions resolves ambiguity, but only near crossing point crossing points more difficult to analyzecrossing points more difficult to analyze Ambiguities

7. June 2, 2007 Measuring distortions in a TPC with photoelectrons 7 T2K TPC test pattern A set of circular dots will be used, along with a few line patterns for diffusion A set of circular dots will be used, along with a few line patterns for diffusion One MM module: 36 x 34 cm 2 1728 pads 10 x 7 mm 2 4 mm wide strips 4 mm radius dots

8. June 2, 2007 Measuring distortions in a TPC with photoelectrons 8 Prototype test for T2K A test system was installed in the T2K- Canada prototype field cage during its initial construction in 2005 A test system was installed in the T2K- Canada prototype field cage during its initial construction in 2005 The pattern consists of lines only The pattern consists of lines only thin aluminum tape applied manuallythin aluminum tape applied manually 2-4 mm wide 2-4 mm wide two contrast materials for central cathode:two contrast materials for central cathode: bare copper bare copper carbon loaded kapton (STAR design) carbon loaded kapton (STAR design) The UV light is provided by a 266 nm laser The UV light is provided by a 266 nm laser acquired just a few months ago (same model as used in our DESY ILC TPC tests in 2004)acquired just a few months ago (same model as used in our DESY ILC TPC tests in 2004)

9. June 2, 2007 Measuring distortions in a TPC with photoelectrons 9 System design ….. Test with CW UV Test with red laser in field cage

10. June 2, 2007 Measuring distortions in a TPC with photoelectrons 10 First events ……. on strip off strip – current through protection resistors change GEM potential Ar CO 2 (90:10) Typical pulses: 100 ns time bins electronics off

11. June 2, 2007 Measuring distortions in a TPC with photoelectrons 11 First events ……. Observed images appear by eye to match the strip pattern Observed images appear by eye to match the strip pattern

12. June 2, 2007 Measuring distortions in a TPC with photoelectrons 12 Movie of 10 sequential events

13. June 2, 2007 Measuring distortions in a TPC with photoelectrons 13 Analysis of laser tracks Segments of pads are analyzed separately, using standard jtpc methods Segments of pads are analyzed separately, using standard jtpc methods Run 732 – 4000 pulses in ~ 1 hour Run 732 – 4000 pulses in ~ 1 hour

14. June 2, 2007 Measuring distortions in a TPC with photoelectrons 14 Track parameters For one strip… For one strip…

15. June 2, 2007 Measuring distortions in a TPC with photoelectrons 15 Drift velocity calibration Simple, precise Simple, precise first run – drift velocity is changing by 0.2% over a period of 1 hourfirst run – drift velocity is changing by 0.2% over a period of 1 hour arrival time (100 ns time bins) time (minutes)

16. June 2, 2007 Measuring distortions in a TPC with photoelectrons 16 Other monitoring for the same run Diffusion and amplitude vs time Diffusion and amplitude vs time

17. June 2, 2007 Measuring distortions in a TPC with photoelectrons 17 Absolute gain.. The variation in collected charge in a row from 1 strip in repeated pulses is Poisson The variation in collected charge in a row from 1 strip in repeated pulses is Poisson variance  amplitudevariance  amplitude gives absolute gain estimategives absolute gain estimate 10 pe 70 pe

18. June 2, 2007 Measuring distortions in a TPC with photoelectrons 18 Measuring distortions Fit tracks to upper and lower modules separately, except for 2 short tracks Fit tracks to upper and lower modules separately, except for 2 short tracks gives 14 measurementsgives 14 measurements compare to specified strip locationscompare to specified strip locations note: no survey done note: no survey done x0 coordinate measured on this centre line

19. June 2, 2007 Measuring distortions in a TPC with photoelectrons 19 Track x0 – specified strip x0 Displacements are small! Displacements are small! apart from one, all are within 0.5 mmapart from one, all are within 0.5 mm No parameters adjusted!

20. June 2, 2007 Measuring distortions in a TPC with photoelectrons 20 Track angle – strip angle upper mod: tracks within 2 mrad of spec. upper mod: tracks within 2 mrad of spec. lower mod: rotation seen – to be studied lower mod: rotation seen – to be studied

21. June 2, 2007 Measuring distortions in a TPC with photoelectrons 21 Intentional field distortions The TPC was operated with intentionally incorrect field cage voltages to verify sensitivity to field distortions: The TPC was operated with intentionally incorrect field cage voltages to verify sensitivity to field distortions: voltage on last strip of field cage (field low) was changed by 10% (about 250V)

22. June 2, 2007 Measuring distortions in a TPC with photoelectrons 22 Nominal field low

23. June 2, 2007 Measuring distortions in a TPC with photoelectrons 23 Field low +10%

24. June 2, 2007 Measuring distortions in a TPC with photoelectrons 24 Nominal field low

25. June 2, 2007 Measuring distortions in a TPC with photoelectrons 25 Field low –10%

26. June 2, 2007 Measuring distortions in a TPC with photoelectrons 26 Considerations for the ILC TPC A combination of dots and nearly radial lines on the central cathode (azimuthal displacements most critical) A combination of dots and nearly radial lines on the central cathode (azimuthal displacements most critical) dots of diameter 2 mm would allow displacements to be measured to a precision of 50 m from a single laser pulsedots of diameter 2 mm would allow displacements to be measured to a precision of 50 m from a single laser pulse laser ports will require some gaps in readout laser ports will require some gaps in readout

27. June 2, 2007 Measuring distortions in a TPC with photoelectrons 27 Uniformity of light, electronics gain The light intensity needs to be relatively uniform across each dot – to not cause a significant shift in the charge centroid The light intensity needs to be relatively uniform across each dot – to not cause a significant shift in the charge centroid similarly the relative gain in neighbouring pads sampling charge from a dot needs to be knownsimilarly the relative gain in neighbouring pads sampling charge from a dot needs to be known example: example: dot diameter: 2 mm (1D  = 0.5 mm)dot diameter: 2 mm (1D  = 0.5 mm) light intensity (or relative gain) differs on two sides of dot by 3%light intensity (or relative gain) differs on two sides of dot by 3% shift is:shift is:

28. June 2, 2007 Measuring distortions in a TPC with photoelectrons 28 Possible locations for installing on LP1 Two fibre ports Two fibre ports separate pulsing checks that light variation across dots not affecting measurementsseparate pulsing checks that light variation across dots not affecting measurements light directed towards centre points on cathode light directed towards centre points on cathode location of port on endplate

29. June 2, 2007 Measuring distortions in a TPC with photoelectrons 29 Summary Reproducible patterns of photoelectrons can be conveniently produced by pulsing UV on the central cathode Reproducible patterns of photoelectrons can be conveniently produced by pulsing UV on the central cathode Proof of principle test completed Proof of principle test completed very good contrast between aluminum and coppervery good contrast between aluminum and copper no need for carbon loaded kapton on central cathode no need for carbon loaded kapton on central cathode laser data easy to use to monitor drift velocity, diffusion, and possibly gainlaser data easy to use to monitor drift velocity, diffusion, and possibly gain system gives essential information to understand distortionssystem gives essential information to understand distortions Would be useful to install in LP1 Would be useful to install in LP1

30. June 2, 2007 Measuring distortions in a TPC with photoelectrons 30 Displacements of tracks 10 mm displacements seen at outer edges, no effect at centre, roughly linear 10 mm displacements seen at outer edges, no effect at centre, roughly linear