1 Stony Brook Update T.K. Hemmick for the Tent Crew

2 Some minor surgery We failed to take pictures inside the glovebox that are any more revealing than those already circulated in (the microscope is difficult to use in glovebox). We also failed to find a scanner that gives revealing images of GEM holes. So, we decided to remove one-by-one the GEMs in stack WS-3 for microscope inspection. The top GEM’s non-linear character was (unfortunately) somehow cured in the removal process. However, examination of this GEM’s underside revealed the most surprising result to date.

3 Damage is Worse Down Below! worseAmazingly, the damage to this GEM is worse underneath than on top!!! The damage is visible to the naked eye with no magnification and no special lighting. This is extremely important. WS3 Top A Cu-colored mesh pattern!!

4 With the microscope… Top of the GEM shows small pock marks. Bottom of the GEM shows full grid pattern. The major question is how the grid pattern gets through.

5 More Bottom Pics… Some holes on bottom are enlarged.

6 Some top pics… Some holes enlarged. Two “flecks” in the gap between strips.

7 Grid pattern on the bottom??? There are two possible ways that the damage on the bottom could retain the grid pattern: –Communication straight through the kapton. –Communication through the holes making an image of the initiating spark point. My intuition tells me that it is highly unlikely that damage propagating through the kapton would be more severe on the bottom than on the top. I propose the wild theory that electrons released during a mesh  GEM spark avalanched though the holes and that the gain of the avalanche made the damage more severe on the bottom of the GEM than on the top.

8 Time for the wild theories… My intuition tells me that when a trip occurs, the transfer gap voltages drop quickly (small capacitance), but that the dV on the GEMs drop more slowly (large capacitance). As the transfer gap voltage drops away, field lines from the GEM-top terminate on the GEM-bottom. When the mesh  GEM spark is initiated, there is still a significant voltage (gain) across the GEM. The electrons freed from the top surface’s spark avalanche through the holes. With the transfer gap voltage small, many of these deposit on the bottom surface of the top GEM. The damage to the bottom surface is larger since it is induced following some gain.

9 Why this process makes an image When the transfer gap has little or no voltage, there is a symmetry principle that applies to the field lines. A field line that starts at one (x,y) location on one side of the GEM, ends at the identical location on the back side of the GEM. In this way, the pattern of the places that release electrons at the top of the GEM surface, matches the pattern at which they land on the bottom. This is my belief for the explanation of how a stronger grid pattern is made on the bottom of the GEM than the top of the GEM.

10 Further wild speculation colorOn the bottom surface, the color of the GEM was changed. Au is inert, meaning that this is either a deposit on the Au or it is a removal of the Au (looks like exposed Cu to me, but not yet sure). If the Au is removed, it will be a trapped contaminant that affects the GEM below it. Top GEM strip 18 showed the non-linear current. On the middle GEM, strips 19&20 were shorted. Note that 19&20 are not “radially” below strip 18, but they are GRAVITATIONALLY lower! My further wild speculation is that the material removed from the bottom side of the top GEM is exactly the material that shorted the GEM below it.

11 The “one source of all troubles” theory The floating grid’s filter cap stored energy that was deposited via a large spark onto the top of the top GEM every time that the LeCroy would trip. The GEM was often still at gain when the mesh sparked and the electrons released by that spark avalanched through the hole and sputtered the Au away from the bottom of the top GEM. The sputtered Au was trapped and became the source of failure for the GEM below it. The light from the spark caused this process to repeat in nearly every module in the HBD for nearly every spark. Magnetic eddy forces on the metal dust generated by these sparks caused the non-linear currents to change characteristics during rapid magnet ramps.

12 “One Source” Theories are Dangerous! If one thing could explain all observed failures, that could make us complacent. If we’re complacent, we could skip finding a second source of trouble. We must continue to both trace down the “back-side-damage” story and also look for other troubles in the detector. I propose a two-pronged plan for making this progress.

13 Continuing the Back-Side Story Sasha should make a spice calculation to verify the scenario that once the mesh  GEM voltage was big enough for a spark, that the GEM dV was still in the avalanche regime. Microscope tests should continue on the GEMs removed from WS3. We should characterize more deeply the damages on both sides of the top GEM. We should search for the trouble on the middle GEM –Visually see the shorts. –Are they at the same Zed as the mesh  GEM sparks? We should find a way to localize where the trouble is on the bottom GEM of the same stack –Has VERY high but not infinite resistance, hard to localize. We should repeat this analysis on three other stacks. One procedural alteration on next three stacks: –The top GEMs from the next three stacks to be dis-assembled should go from the glovebox to the evaporator for a QE test! –After QE tests are made, then we can do microscope tests.

14 “New Stack” Tests I would like to propose to operate a few (4) “new” stacks in CF4 inside the real HBD. These new stacks should be rigged so that mesh  GEM sparks are impossible –I recommend a pair of series zeners to prevent the mesh  GEM voltage from ever exceeding 100 Volts in either direction. These should be able to operate stably and at full gain (measured via 55 Fe) over an extended period. They can be compared to neighboring stacks that have “been through the war” in side-by-side operation. In this way we can see that older stacks still exhibit problems and that (hopefully) newer ones do not. Anything that fails in the new stacks will be a different trouble than the mesh  GEM sparks. Can we get this number of fresh GEMs from WIS??

15 What’s this??? Grid marks on back of gem More than just pock-marks!

16 Some GEM features

17 Some more GEM features

18 Even more GEM features

19 The grid pattern on both… the frontthe back&