LEM Seminar
George glanced at the time readout hanging in midair at the top of his field of view. It read “05/31/04, 11:32 CDT, LEM Auditorium, SSC, Texas.” He mentally translated the time as 9:32 AM, Seattle time. He was bored and becoming slightly irritated. Larry’s rambling “10 minute” talk on the status of the data compression software had now been going on for almost half an hour and showed no sign of concluding. George heard his remote’s motor whirr softly as he turned to look at Jake, who sat in his special chair against the wall at the front of the auditorium. Jake’s eyes were hooded in thought, and his ivory hands were in his lap with the fingertips together, making a horizontal steeple.
He turned toward Wolfgang, who sat in an aisle seat in the LEM auditorium, next to the spot where George’s remote was parked. “Jake is a ticking bomb,” George whispered to him. “Watch. He’ll go off any second now.”
As if responding to the cue from George, the spokesman and absolute monarch of LEM, Harvard University Professor Jian-Kwok “Jake” Wang, stood up. “Enough,” he said and paused, his head tilted slightly backward, staring at Larry. George was familiar with that Stare. All too familiar.
Larry looked toward Jake. “B-but, I wasn’t quite finished,” he said.
“Time is short,” said Jake. “LEM is running, and data are coming in at this very moment. Many of us have shifts that will start soon. If you have more to tell us, Larry, you may continue at another group meeting.” Jake consulted the list in his hand. “Next,” he said, “George Griffin, who was at CERN last week and is now back in Seattle, will tell us about radiation damage and vertex detectors.”
Larry stood bewildered at the overhead projector for a moment, then attempted to collect his numerous scattered transparencies. About half of them slipped from the pile and skittered across the floor. A few in the audience of about 300 people gathered in the LEM Auditorium laughed nervously, but most sat quietly in sympathetic silence.
George’s remote whirred up to the podium and with a robotic hand he expertly scooped up the loose transparencies in his path. The robot arm telescoped outward, offering them to Larry, who snatched them and retreated to the back of the auditorium in disarray, arms crossed over the transparencies and notes clutched against his chest.
George’s remote turned off the overhead projector and activated the projection monitor system. Through electronically tracked camera eyes, George looked out at the large group. “First,” he said, “I want to introduce a visitor who is newly arrived from CERN, Dr. Wolfgang Spiegelmann, a distinguished physicist, a micro-electronics specialist, and a valued member of the ATLAS collaboration.” At George’s urging, Wolfgang stood, turned, and nodded to the audience. “Wolfgang will be working with our group for the next month,” George continued. “His stay with us is relevant to the present discussion because he’s an expert on radiation damage in pixel detector microchips.”
George looked at a sensitive spot in his field of view and blinked, and his first figure appeared on the screen. It showed a cylindrical device with flat black surfaces on its sides. “I’ll keep this short,” he said, “but I want to start with a bit of orientation for the new students. This is the vertex detector for the LEM experiment. It’s placed as close as possible to the point where the two proton beams collide, and the emerging particles reach it first. It’s made of five concentric layers of slabs of silicon containing several million tiny pixel detector elements, along with some of their associated electronics.” He blinked again and a second figure appeared. “A pixel detector is a square of silicon 20 microns on a side that gives a current pulse when a charged particle passes through it.” His pink wireframe hand appeared in the projection and pointed at the dark square area on the diagram. “These pixel detectors and their electronics are made directly on a slab of silicon, using the same fabrications processes used to make integrated circuits for commercial electronics.”
George blinked, and the picture changed to a display of curving colored tracks. “The vertex detector provides some valuable information for LEM. Using all of the pixels together, we get a close-up image of the charged particles from the collision. We’re able to track some of the secondary products of short-lived particles from the collision and establish the location where their breakup occurred. This helps in following the production of strange and charmed quarks in the collision. The secondary vertex particles, as we say, have strangeness and charm.”
Another blink, and the picture changed again, displaying the vertex detector now mottled with damage points shown in purple. “Now the problem is that we’ve been running LEM for less than a year, not even at full luminosity yet, and we’re already losing pixels from the vertex detector due to radiation damage. We were expecting to be in our present state of radiation damage after perhaps two or three years of operation at full luminosity, not after six months at low luminosity. So we’ve got a problem.”
“We all know we have a problem, George,” Jake interrupted. “Do we have a solution?” He stared across the room, seeming to focus his gaze several yards behind George’s remote.
George noticed that Jake’s famous Stare was less intimidating through a telepresence remote. The remote’s motor whirred as he turned. “Patience, Jake, patience,” he said with a grin. “First the problem, then the solution. It’s important to do these things in the proper order.” A few in the audience laughed appreciatively. Fewer than would have liked to laugh, George suspected. As a full professor with tenure, George had little to fear from Jake, but others here were not so fortunate.
He blinked his next figure to the screen, a diagram of an electronic circuit. “This shows one pixel detector and its associated on-chip electronics. We made some tests with our probe station of a few of the radiation damaged chips that were replaced last month. The good news is that it is not the pixel detectors themselves that are failing. Even in the failed units, the detector leakage currents are staying low.” He gestured and two areas of the circuit became circled in red. “The bad news is that there are two key components that are failing: the first FET of the input stage and the power-drive junction transistor of the output stage. What we’ve learned so far, Jake, is that with no input and no output you don’t get much data.” Again there was scattered laughter from the group.
“We did a lot of radiation damage testing of vertex detector prototypes several years ago, and we never saw such problems. So the question is, why are these components failing now? The answer is, we don’t know, yet. It could be an undetected beam halo from the SSC. It could be some subtle impurity in the kind of silicon we’re using. It could be the on-chip layout of the integrated circuit components, which is slightly different from the layout we tested. It could be the design of the circuit itself, also slightly different. It could be some glitch in the details of the fabrication process. It could be a lot of other things.
“Fortunately, we have a way of finding out which it is. Our European competitors at the LHC were very accommodating during my visit last week in sharing their experience. In their ATLAS detector at the LHC they have a similar vertex detector with very similar on-chip electronics. Moreover, because of the LHC runs at a lower energy, the radiation damage at full LHC luminosity is about the same as ours to ours at reduced luminosity. And, as Wolfgang will tell you in a seminar scheduled for next week, their vertex detector is performing above their expectations. It has not had the radiation damage problems that ours is suffering. The question is, why not?”
George’s remote held up a small plastic box with its right robot arm. Something wrapped in pink polyethylene was visible inside. “This is one of the spare silicon chips from the ATLAS vertex detector that Wolfgang brought us. He brought a dozen more of them, and also some electronics and components. We’re going to test their chips along with ours and find out why theirs live and ours die. That’s about all for now. Thanks for your attention.”
He stopped, turned to look at Jake, and nodded, indicating that he was finished. There was scattered applause.
Jake stood and leveled the Stare at George’s remote for what must have been a full minute. “This is not satisfactory,” he said finally. “We should not humiliate ourselves before our competitors by admitting our failings and begging their help. We should go to CERN to discuss our victories, not our defeats.”
George shook his head, producing a whirr. “First we have to have victories, Jake,” he said with a twinkle, drawing more laughter from the group and discharging the tension that had been building a moment before. “Our resources in this experiment, as you know better than anyone else here, are very limited. If we can shortcut many man-months of work by getting assistance from CERN and directly comparing a chip that works with one that doesn’t, then the effort saved can go elsewhere. I’m just goddamned glad that the ATLAS people were willing to provide us with spare chips and expertise. I think we would have done the same, if the circumstances were reversed. At least, I hope we would have.” He stared back at Jake. He was certain that Jake would never have diverted any LEM resources to help ATLAS with their problems.
Jake made a dismissive motion with his hands, a gesture that one might use to send away a servant. George rolled the remote back up the aisle and positioned it next to Wolfgang’s seat. Jake could be such a jerk sometimes, he thought.
Jake peered at his notes. “The next speaker,” he announced, “is Pierre Barbotin, who is in Nantes at the moment. Pierre will tell us about the performance of the straw tubes.”
Wolfgang looked at George’s remote inquiringly.
“Welcome to the LEM collaboration,” George whispered.