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Forum on Tracking Detector Mechanics 2013 University of Oxford June 19 th -21 th. Construction/position alignment requirements for large scale tracking detectors. Steve McMahon RAL/STFC. What is a Forum anyway ? What does OED say?.
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Forum on Tracking Detector Mechanics 2013University of OxfordJune 19th-21th Construction/position alignment requirements for large scale tracking detectors Steve McMahon RAL/STFC
The Problemto go from the performance spacialaccuracy that can be realized by a module …
Outline • I would like to concentrate the discussion around: the mechanical support, positioning and alignment with special reference to the tracker for ATLAS. • While one would like to think that the arguments are, on the whole, general it has particular importance for large scale multi-mechanical component detectors. • What is your experience in your detector? • Does it reinforce what I present? • Do you have experience to add to/contradict this picture? • Is there anything new that your experience before Phase II
Requirements • The support structures (local and global) should be designed so that errors on the knowledge of the position of the individual detector elements do not degrade the tracking performance significantly beyond the limits imposed by the intrinsic detector (module) resolution, segmentation and multiple scattering. • This positioning and stability of the individual detector elements is key to optimal performance and is achieved by a combination of the design, the engineering of the support structures, surveys during and after the assembly, during operation (hardware alignment) and very importantly the offline alignment corrections by software. • The difficulty (or optimization) comes in trying to resolve often conflicting requirements towards one requirement without giving up too much on another. For example: maximum rigidity against minimum mass.
Our Benchmark is “old ATLAS” • In ATLAS the offline track-based alignment uses a global chi-squared technique that minimizes the residuals to samples of fitted tracks. This technique exploits the so called “strong-modes” and has proven its ability to locate the positions of the individual detector elements (modules) with high accuracy. The performance achieved with this technique will serve as the benchmark for the future tracker. • In current ATLAS tracker, momentum systematic errors are at the per-mill level, impact parameter systematics are at the 1mm level, residual detector misalignments are at the few micron level at the largest radius. At small radii sensitivities to biases due to clustering are important. • At the same time there are deformations which are difficult to reconstruct with this technique, these are the so called “weak modes”. Additional constraints, like cosmic rays or the reconstruction of the invariant masses of resonances, are required. The mechanical design of the future tracker aspires to support the track-based alignment in this task.
Some Definitions : A repeat • Strong modes: Deformations which cause a change in the χ2 of a track fit. These can typically be identified and corrected with a limited amount of data (currently around 1 day) in track-based alignment. Typically, these are uncorrelated deformations in the r-φ direction causing a kink in the track. • Weak modes: Deformations which do not result in a significantly increased χ2of a track fit, but do affect other physics-relevant measurement parameters (e.g. momentum measurement, vertex position or the invariant masses of resonances). Typically these are coherent deformations of larger scale structures of the tracker. In track alignment some of these movements can be constrained from module overlaps (in particular in r due to the closed loop constraint), or from higher-order reconstruction.
How is it done in practice ? • The offline tracker alignment procedure is iterative in nature and works at different levels of detector granularity. The procedure starts with the so-called level-1alignment. • Here the largest macro structures including individual sub-detectors: Barrels, End-Caps etc are aligned with respect to one another. • At the level-2 alignment the structures inside sub detectors (individual cylinders, Disks) are aligned and at level-3 the positions of individual modules are optimized. The number of degrees of freedom in the current ATLAS detector at the different level in the Inner Detector increases from 24 at level-1 to 1878 at level-2 to 23,328 at level-3. Note at Level 3 the NDOF depends on the number of modules.
What is important ? • Module Build Accuracy • These are typically small objects and the internal build accuracy is usually very good ( a few micron level in X-Y) • Placement accuracy • This is the accuracy with which detector elements are placed on the macro-structures during the assembly. Build placement accuracy does include deflections from the design position under standard static loads (gravity, standard operational thermal loads, and standard operational moisture conditions). Placement accuracy is required to maintain gaps between detector elements and structures (clearances, HV separation etc.), and overlaps. Both, clearances and overlaps, depend on the exact location and relation of the components and cannot be specified generally. • Assembly survey • This describes the accuracy with which the positions of detector elements are known when they have been measured during the assembly. Typically global assembly survey accuracy is the result of a hierarchy of surveys, which will result in a build-up of errors, which is difficult to control. • Some dimensions/directions are more important than others. The dimensions along the track direction are the most important (radial in the barrel and Z in the end-caps)
What is really important • Mechanical stability in this context implies the dimensional stability of the active detector elements under a variety of both static and dynamic loads. These loads include: vibration, thermal (temperature), changes in atmospheric pressure, moisture-induced etc. We are particularly interested in the local displacement over time from a known position in a global coordinate system caused by these loads of the detection elements. The absolute scale of the displacement will depend on the time-scale over which the stability requirements need to be achieved. • To perform track based alignment for the current ATLAS Inner Detector (ID) it is required that the time to collect the data is short compared to the time over which the detector elements might move. The current selection of high-momentum tracks used to do a level-1 alignment are such that enough data to be collected in a 24 hour period. It is worth noting that we often collect enough data to align the entire detector at module level but we have chosen to not to perform the alignment as it may introduce weak modes. Further improvements in the alignment may allow for module level alignment to be performed on a daily basis. Corrections to the weak mode deformations require significantly more data and as such it is required that the detector is stable over periods of time greater than a week. At phase II with much higher pile up it will be possible to reduce the data required for a level-1 alignment to a few hours. The time to collect enough data to perform the weak mode analysis will also be dramatically reduced. 100k Tracks for L1 is sufficient 2-5M is needed for L3
What are the Timescales • Short • Time to collect enough data to do a L1 alignment (strong modes) • Currently 24hr at time of Phase II will be < 12hrs • Scales with number of tracks (CoM energy, Evt rate to disk, Luminosity, <m>) • 100k Tracks is sufficient for L1. • Medium • Time to collect enough information to do a weak mode analysis • Use invariant mass of resonances, cosmics, overlaps • With the current trigger menu 1 fb-1 is enough data to remove bulk of the weak modes (>0.3M Z→mmdecays). During these periods (relatively infrequent) external perturbations, or ‘seismic’ events (e.g. magnet ramps, cooling system cycles, power and HV cycles), can occur on top of environmental changes. It currently takes about 1 month to collect these data. • Scales with integrated luminosity and COM energy. • Long • Long term variations deal with stability against relaxation caused by creep, possibly accelerated by irradiation, under realistic operating conditions. The timescale is months to years. For the stability on this timescale we require that the detector positions satisfy the same criteria as in the original placement requirements.
Some comments on “load” stability • It is very important for the tracker design to be able to accommodate changes in module position due to changes in load... • d-Power ~ 10% • 10% changes in FE power consumption is on the high side. • In ATLAS the L1 trigger rate is “levelled”. • d-T • For local supports , during periods of “full” power (like-like: on-on & on-off). • Ref we should use ± 1K over short durations of data-taking cf(ON-ON): • As comment it seems fair to say 0.2K is possible but 0.5K easy • We should use ± 3K over longer durationsoutside data taking cf(ON-OFF) • Specify/Qualify materials against a lowest working point of -45oC • Also need to define a maximum working dT/dt • d-Vibration • See separate slide • d-Pressure • Control the pressure difference between inside and outside volumes.
Vibrations • In choosing materials it is important to have small static displacement and high resonant frequencies within the limits set by the material budget and construction cost. • However, the actual resonant frequencies (which are easy to get out of the FEA’s) are not the critical parameter, rather it is the amplitudes of these frequencies (harder to extract with confidence). We do know that A~1/f2 and it can also be a strong function of the driving force(s) and their power spectrum. It is important to measure this as an input to the models. The risk in not doing this is that one is “driven” to designs with much higher resonant frequencies to improve margin against unknown forces. • We do have some general knowledge of the external driving frequencies. Ground motion tends to be in the range 3-8 Hz and so the structure must have resonant frequencies above 12-15 Hz. • Introduction to ground motion around LHC (&lots of references) see by L. Vos • http://wwwslap.cern.ch/collective/bb-workshop99/wswg/vos.pdf • Unknowns in our application are the other environmental sources in the tracker environment. These include pumps, motors, in fact anything, that can vibrate, hence the need to measure these as part of LS1 (A. Catinacio & M. Guinchard). • We have set on a resonant frequency to be greater than 30.
Humidity Bounds • What are reasonable bounds to work against? • Working lower limit of -45oC as coolest place in the detector • Assuming a safe Td of -70oC , this sets an upper limit of RH of 7.3% of [2.6ppm (v) or 1.6ppm(w) at 1000 mbar]. • If we demand Td = Td of -80oC this requires an RH of <1.5% [0.5ppm(v) or 0.33ppm(w) at 1000 mbar] • Proposal: specify Td of -80oC and check sensitivity to Td of -70oC • This means we check sensitivity from RH=1.5% to 7.5% at -50oC. • This is fairly easy in a nitrogen environment without leaks
Weekly changes in atmospheric pressure Atmospheric Pressure in mbar A reasonable bound would be 980mbar to 1035 mbar : Should also thing about dP/dt
On line surveys : The Past • FSI (SCT frequency scanning interferometer giving Grid Line Array with 842 lines distributed between Barrel and EndCap) • Data recorded at the end of barrel (r-f) which has been shown to be stable at the (sub)micron level. • The FSI was a victim of the success of the excellent design and construction of the SCT. Where good enough data was available over extended periods it showed that the mechanical structure was VERY stable in time.
Putting it all together • The placement accuracy, together with the results from the assembly and online surveys provide the best knowledge of the position of the active detector elements before the corrections from track-based alignment can be determined. It is the gaol of the combination of the assembly and survey to constrain the weak modes to be less than predefined values. For the long term stability we need to understand if an alignment system would be needed to track these modes and where it might be placed. The experience from ATLAS so far is that an alignment scheme based on tracks is sufficient and no additional hardware is required.
How are the stability requirement numbers qualified ahead of construction and installation. • Level 1 (macro-macro) • Cannot be done?? At least, I don’t think so. • Essential to ensure good engineering practice & models • Level 2 (inside detector cylinder-to-cylinder) • Prototype measurements? • Not 100 % sure here either as the comes rather late. • Essential to ensure good engineering practice & qualified models • Level 3 (stave/module) • FEA and prototype measurements are very important and it is important to ensure that the structures meet the requirements for all of the known dynamic stimuli (temp, RH, vibration, power variations …) • See Neal’s talk from this morning.
Requirements.. • Not all directions are equal • With track based alignment it is harder to reconstruct changes in the coordinates along the track than perpendicular to it. This should be reflected in the requirements. This means the r coordinate in the barrel and Z in the barrel are special. • Placement • On local-supports it should be possible to place active detector elements to 100-200mm. • In a global-reference frame these should be sufficient to control weak modes • Also important to understand/respect construction clearances, electrical stand-off etc… • Survey • This is of the same order as the detector resolutions in those directions. • Stability • At phase II It should be possible to track the movements of the macro-structures at the level of a few microns a few times per day. We have already seen that this is possible. • It should be possible to track the weak modes over the periods of a couple of days. It should be said that, as far as ATLAS is concerned, there is no strong evidence for the time evolution of the weak modes!
Conclusion • We start with sensors with an intrinsic resolution that is given by the sensor technology and the geometry of the individual silicon diodes. • We build modules/staves with an intrinsic accuracy which is driven by the design and construction techniques. We then mount/support them in/on global support structures in an imperfect but managed & structured way. • We need to know where the sensors are to start with and need, by construction, to be able to “locate” them with an ultimate accurately that is limited by the resolution, segmentation and amount material. • We need to be able to guarantee that the active surfaces do not move more than a certain amount in a given time. We also need to minimize the available movements in a particular set of directions where it is hard to follow them. • In ATLAS comparisons between, what we measure in the data, and the Monte Carlo seems to indicate that we are close to the intrinsic limits imposed by the detector construction. In ATLAS this is possible with alignments from the tracks alone. • So far we have been able to do all of this with tracks alone.
