Simulations and Resistive Films Results and status report By: Stephen Turnbull

A quick overview  Simulated work; PRF analysis :  What does a good PRF look like anyway?  Calculated first order ILC/T2K Resistive film PRFs  Two capacitance dependencies to further consider  Theoretical work: Si-prot event size problem  What we expect to see verses what we see  Why?!?!?! (solve this, win a big prize)‏  The Real work: Reality is not as pretty as simulation  Measuring Resistivity / square  Capasitence / cm^2  Next Steps, Watching Decay in a resistive film

Simulated work Before I could find the best Resistivity/capacitance for the ILC and T2K I needed to define what a good PRF looks like... I decided that the Ideal Properties of a PRF are: 1. Since the signal is, to the first order, symmetric a PRF should have a normalized magnitude of ½, at one half pad width displacement. 2. The Slope of the PRF in the [0.7,1.3] region should be maximized (approximate bounders), to compensate for the flatness in the [0,0.3] pad width region. And 3. The tails [1.4,infinity] should be as close to 0 as possible.

A Quick look at Possible PRF's

Changing models The Current TPC model assumes a restive film resting on a isolating layer of flowbond with essentially no bulk conductivity. Future resistive anodes might benefit from a bulk conductivity, which would have slightly stronger, but possibly slower, signals and no, or diminished, PRF tails. Future resistive anodes might benefit from a bulk conductivity, which would have slightly stronger, but possibly slower, signals and no, or diminished, PRF tails. For such a system a new, or adapted model would need to be implemented.

Second order concerns Currently the transverse size of the image of a charge cluster is capacitance independent... Whereas in physics the image charge is always more spread out then the source charge by a factor which is capacitance dependent. consider the image of charge on a resister in deep space, verses a conducting wire in a conducting cylinder. A last issue is the capacitance coupling between the resistive film and the ion clusters is not a constant. (It is the decay of this capacitance coupling which we ‘see’ as the ‘Ion Signal’)

A rather bad drawing to illuminate the issue The top and bottom lines represent pads/resist and Micromagas Grid respectively. The oval is the bulk of ions from the avalanche (the funnel part of the avalanche giving us the ‘electron signal’ by the same mechanism) And time progresses from left to right Only once the ions arrive at the conductive mesh the roles of ‘charge’ and ‘image charge’ are reversed and capacitance effect of mesh/ions becomes a constant. This effect is not taken into account in the current simmulation

Theoretical work: Time Pix TPC

“Then I would have been sorry for the dear Lord as my theory is correct ” – Albert Einstein With a Transverse diffusion of approximately 400 or so microns per root centimeter... And a maximum drift distance of 6 centimeters... We expect to see Iron 55 source events with a max size of approximately mm. (1/7 th the width of the Time pix chip).... What we see... Is a LOT larger...

What is going on? How to test it? Paper anybody?  Suggested possible causes:  Some form of surface effect? (Surface tension induced Super low friction electron ‘beads’ similar to the last drops of a cup of coffee) –Paul: test:??  Non uniform – divergent – electric field in the gas chamber: test: collimated/pencil ionizing beam? Drift length dependency?  Some form of charge build up induced divergent surface fields above the resistive pad? Test: Rate dependency?  Add one of your own, Guess right, possibly win a nifty paper. (Room temperature short lifetime superconductivity anybody??)

Real work: I actually did some of this.  Using the large roll of Resistive film, rated at 1.6 Mega Ohm / Square, Arnaud and I build a series of small Resistive films glued onto PCB boards with conductive surfaces.  The first set of 4 PCB films were: 10 mm by 100 mm resistive foil, on two 11 mm by 130 mm flowbond insulation layers, all on a 15 mm by 140 mm PCB board.  The second set had larger PCB’s and Flowbond surface areas (25-30 mms by 140mm, and 20 by 130 mm respectively)  Also a 90mm by 90 mm square of un-bonded film was tested to see if the heating was changing any properties.  Arnaud will discuss a further two films which were bonded to full readout boards and tested for holding voltage

Resistivity measurements of first 8 film strips Contacts between the film and multimeter were mediated by thin bars of conductive tape, or silver paint and conductive tape. (All measurements are +/- 2 mega ohms, or 0.2 Mohms/square) The foil 1A had a ruler drop upon it before being measured, demonstrating dramatically how easily damaged our resistive foil is. From these tests we conclude that resistivity of the foil is on the order of 1.5 Mega ohms to 2.0 Mega ohms per square, but VERY easily damaged leading to highly increased resistivity. The Large 90 mm by 90 mm un-bonded resistive film had a resistivity of 2.03 Mega ohms, but also did not have a silver paint mediating contact between the film and conductive tape.

Next step in Observing/measuring real PCB’s  The next operation will be to attempt to attempt to ‘see’ a injected charge cluster decay.  By injecting a charge pulse into a resistive film with an applied transverse field we can observe the pulse as it passes under each of the readout wires.  By measuring the Halfwidth half maximum of the distribution as it passes under each of the readout wires we can calculate the spatial halfwidth half maximum of the charge cluster as it expands with time. (The transverse drift applied by the electric field in the resistive film does not significantly change the radial decay behavior)