Alignment Electronics
for the New Small Wheel

Existing System

The ATLAS End-Cap Alignment System Uses roughly:

• 24,000 infra-red LEDs (850 nm)
• 4,400 red lasers (650 nm)
• 4,000 lenses (various diameters and focal lengths)
• 4,000 image sensors (TC255P CCD)
• 2,700 RASNIK mask (various sizes and square pitches)

All powered, controlled, and read out with the Long-Wire Data Acquisition System (LWDAQ).

• 4,500 LWDAQ Devices
• 4,500 Branch Cables (CAT-5, stranded-wire, shielded, up to 10 m)
• 900 LWDAQ Multiplexers
• 900 Root Tables (CAT-5, solid-wire, shielded, up to 130 m)
• 120 LWDAQ Drivers (in USA15 VME crates)

Same system used by: ALICE Geometric Monitoring System, ALICE Space Frame Monitor, LHCb Inner Tracker Alignment, HIE-ISOLDE Alignment, and CMS Technical Coordination.

Overall: The system works well enough.

Obsolete Image Sensor

• Texas Instruments stopped making the TC255P nine years ago.
• We took images with two monochrome sensors and compared them to the TC255.

Importance of Fill Factor

• The "fill factor" is the fraction of each pixel's surface area that detects light.
• A BCAM images are spots of light. We calculate position of the weighted centroid.
• Insensitive regions interact with bright and dark features in the spot.

Figure: BCAM Images At Various Ranges. Source is 650-nm laser diode. Sensor is TC255.

• Spots with interference fringes generate systematic errors with position within a pixel.
• Rightmost image on TC255P, no such errors.
• Leftmost image on TC255P, cyclic error period three pixels, amplitude 0.5 μm.
• The ICX424AL has fill factor close to 100%, as well as uniform sensitivity and dark current.
• We built a prototype ICX424AL BCAM and tested its linearity over the full dynamic range.

Linearity of ICX424AL BCAM

Figure: ICX424AL BCAM Non-Linearity Across Entire Field of View. Pixels 7.4 μm square. Residuals in microns on sensor plotted versus stage position. Image sharply-focused. Two red lasers at 3.1 m on 300-mm stage, camera focal range 3 m, lens focal length 48 mm, camera V0359. Repeat with poorly-focused spot residuals remain 0.2 μm rms.

Changes Required by ICX424

• Existing LWDAQ Driver with VME Interface (A2037A) cannot support ICX424AL.
• Need a larger logic chip on the driver.
• For nSW, will design and build twenty new LWDAQ Drivers with VME Interface.
• Existing TCPIP-VME Interface (A2064F) too slow for full-resolution ICX424AL images.
• For nSW, will design and build three TCPIP-VME Interfaces seven times faster.
• The ICX424AL is a three-phase device, while TC255P was single-phase.
• The ICX424AL permits pixel binning, which requires double-clocking.
• The ICX424AL camera needs four times the logic gates as the TC255P camera.
• We plan to replace VHC-series logic chips with a programmable device.
• The programmable device must be non-volatile, instant-on, and consume less than 5 mA.

Challenges of New Small Wheel

• The nSW requires triple the number of light sources as the oSW.
• We want to use only the existing LWDAQ root cables.
• The nSW radiation dose will be five times higher.

We solve both the radiation resistance and density problems for light sources by switching to fiber-optic light sources rather than laser diode light sources.

• We can inject power into dozens of optical fibers in one LWDAQ device.
• We can place the injector towards the outer rim of the nSW, so less radiation.
• We run the fibers wherever a light source is needed.

We solve the radiation problem for our cameras by study and adaptation.

• Measure the camera tolerance to ATLAS background radiation.
• Eliminate insufficiently tolerant components and replace with more tolerant ones.

Fiber-Optic Light Sources

• We press the polished end of an optical fiber up against a blue LED.
• The far end of the fiber is a zirconia ferrule with a fiber centered to ±5 μm.

Figure: Contact Injection into Optical Fiber. Fiber diameter 125 μm.

• Blue LEDs are more efficient than any other color.
• In the nSW plan to use LuxeonZ LED, no top-side bond wires.
• Must deliver 1-A drive current to inject the required 100 μW.

Buck Regulator for LED Power

• We plant to take the 30-V LWDAQ power and convert to 4-V to provide 1-A LED drive.

• Air-core toroidal inductor will operate in arbitrarily strong local field.
• Bipolar transistors are well-known for their radiation resistance.

Breadboard Buck Regulator

• Efficiency 70% with 20-V input.

Figure: Breadboard Buck Regulator

• Most of power loss is in resistance of the coil.
• Will try coils with lower resistance, and more compact.

Importance of Radiation Spectra

• To measure tolerance of ATLAS background, must model it in the laboratory.
• To model one spectrum with another, must know both spectra, and prove equivalence.

Figure: Energy Absorption Coefficient for Air and Silicon. Data from NIST.

• A 100-keV γ dose that deposits 1 Gy in Pb deposits only 0.01 Gy in air.

Specrum of ATLAS Background

• Charlie Young provided a list of particles generated by 10k simulated 14-TeV p-p interactions.
• We obtain the following spectrum of photons and neutrons at the inner edge of nSW.

Figure: Simulated Spectra of Photons and Neutrons at Inner Edge of nSW. Bin for <=1keV contains 40,000 thermal neutrons.

Spectrum of ATLAS Background Dose Rate

• Assume 72-mbarn for cross section of p-p inelastic collisions.
• Assume 10³⁴ 1/cm²s luminosity and 10⁷s/yr running time.
• Convert photon and neutron frequency into Gy/yr in Si and 1 MeV ea. n/cm²/yr.

Figure: Simulated Dose Rate Spectrum for Photons and Neutrons at Inner Edge of nSW.

Irradiation with X-Rays

We are using x-rays to deliver slow ionizing doses.

• Use 50-keV continuous x-ray source with 3.2-mm Al absorber.
• X-ray spectrum constrained to 14-50 keV.
• These x-rays will penetrate circuit boards and chip packages easily.
• Dose in Si is roughly 7.3 times greater than dose in air.
• Measure dose in air with calibrated ionizing chamber.
• Administer dose over days or weeks, monitor electronics continuously.
• Observe in repeated experiments the same increase in ICX424AL dark current.

We confirmed out dose calibration with a cesium-137 source of 500-keV γ.

• Place ionizing chamber in 500-keV dose and measure dose rate.
• Place ICX424 in chamber and irradiate while monitoring dark current.
• Increase in dark current matches expectation from x-ray experiment.

Performance of Prototype ICX424AL Camera

• ICX424AL dark current increases linearly with ionizing dose.
• Dark current doubles every 8°C increase in temperature.
• With quadruple-pixel readout, can operate at 450 Gy at 20 °C.

Figure: ICX424AL Image after 450 Gy. Circle marks tungston sphere absorber.

• EZ500 blue LED unaffected by 1 kGy.
• DG419DY analog switch fails after 300 Gy.
• LC4064ZC logic chip fully functional after 1 kGy.
• Total logic quiescent current increases from 3 mA to 4 mA after 1 kGy.
• Note: TC255P barely affected by 1 kGy, still functional after 10 kGy.

Expected ATLAS Doses

Figure: Expected Dose Rate versus Radius in nSW. Assume 5×10³⁴ 1/cm²s and 14 TeV.

• We plan to place our sensors at radius ≥2.7 m.
• Ten-year dose at full energy and luminosity is ≤100 Gy.

Neutron Tests

• We plan to perform two fast neutron irradiations this summer.
• TC255P dark current increased linearly with neutron dose.
• Expect ICX424AL dark current to increase with neutron dose also.
• Will determine if neutron and ionizing damage is additive.
• If neutron damage is severe, will double the readout speed with new LWDAQ Drivers.
• Doubling the speed doubles the ICX424 radiation tolerance.
• Expect some drop in LED power with neutron dose.
• Do not expect any problems with logic chips in neutrons.

Single Event Upset Tets

• None of our devices run code or contain RAM cells.
• Power down after data acquisition is standard operating procedure.
• Any disrupted action will be noticed and rectified by repetition.
• Therefore: we are not vulnerable to single event upsets.
• Have no plans to perform single event upset tests.

Conclusion

• New light source: fiber optic cable with contact injection, proven in BEE.
• New image sensor: ICX424AL, accuracy established by prototype BCAMs.
• Will design and build new LWDAQ drivers, multiplexers, and patch panels.
• Believe we understand the ATLAS Background well enough to test radiation tolerance.
• Have been performing ionizing tests.
• Starting neutron tests this summer.
• If ICX424AL is too vulnerable to neutrons, will speed up readout with new drivers.
• Will replace analog switches with discrete mosfet switches.
• New, non-volatile, programmable logic more compact, flexible, tolerates >1 kGy.
• Confident that buck regulator LED power supply will tolerate >1 kGy.

Appendix: Linearity with Quadruple-Pixel Readout

Figure: ICX424 BCAM Non-Linearity Across Entire Field of View with Quadruple-Pixel Readout. Pixels 14.8 μm square. Residuals in microns on the image sensor plotted versus stage position. Image sharply-focused. Two red lasers at 3.1 m on 300-mm stage, lens focal length 48 mm, camera V0359. Repeat with poorly-focused spot residuals remain 0.2 μm rms.

Appendix: Dose Map from Background Group

Figure: Simulated Total Annual Ionizing Dose, Gy/yr. Luminosity 10³⁴ 1/cm²s, energy 14 TeV, 10⁷ s running per year, 2012 detector geometry.