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Custom IC Design and Data Acquisition

Robert P. JohnsonSanta Cruz Institute for Particle PhysicsUniversity of California Santa Cruz

Proton Computed Tomography using Silicon-Strip Detectors5/22/20141Proton Computed Tomography

Collaborators in Building the pCT Scanner5/22/2014Proton Computed Tomography2

U.C. Santa Cruz:Robert JohnsonHartmut SadrozinskiJoel DeWittAndriy ZatserklyaniyTia PlautzLLUMC:Reinhard Schulte, M.D.Vladimir BashkirovFord HurleyNick VenceValentina Giacometti

Front-end and DAQ electronicsOutline5/22/2014Proton Computed Tomography3Introduction to pCTPre-clinical scanner prototypeTracking SystemEnergy DetectorData AcquisitionTracker Front-End ASICTracker PerformanceNoise occupancy Hit efficiencyBad channel ratesDAQ System PerformanceProton Radiation Therapy5/22/2014Proton Computed Tomography4

The proton beam is tuned such that the protons stop in the tumor, depositing most of their energy there.

Compared with photon radiation (X-ray or -ray), a higher does can be delivered to the tumor while minimizing exposure to surrounding tissue.Energy deposition vs. depth for several beam energies.Total proton dose vs. depth (sum of blue curves)-ray dose vs. depthBragg Peak placed in tumor regionZero dose behind the tumor!Why the Interest in Proton CT?5/22/2014Proton Computed Tomography5

Alderson Head PhantomRange Uncertainties(measured with PTR)> 5 mm > 10 mm > 15 mm

Schneider U. & Pedroni E. (1995), Proton radiography as a tool for quality control in proton therapy, Med Phys. 22, 353.X-ray CT use in proton cancer therapy can lead to significant uncertainties in range determination, which limits its use in the case of some tumors located close to critical healthy tissue.Proton CT can measure directly the density distribution needed for range calculation, is less affected by intervening dense structures, and is unaffected by beam hardening.

Fermi-LAT Tracker/Converter Heritage5/22/2014Proton Computed Tomography6

73 m2 of Si (>10,000 SSDs)880,000 channels160 WattsIn orbit since June 2008W. Atwood et al., Design and Initial Tests of the Tracker-Converter of the Gamma-ray Large Area Space Telescope, Astroparticle Physics 28, 422-434, 2007.Our first prototype pCT scanner used the Fermi Tracker electronics and SSDs, but a much faster, specialized ASIC is needed for clinically realistic rates.pCT Instrument Concept5/22/2014Proton Computed Tomography7

Object being imaged (a calibration device is shown)Proton range detectorPair of silicon-strip based trackers.Incoming 200 MeV KE protonTracking detectors based on silicon-strip sensors measure track vectors entering and exiting the object being imaged.From this the protons path through the object is estimated.A range detector stops the proton and measures the residual range.From this the Water Equivalent Path Length through the object is inferred. TUVH.F.-W. Sadrozinski et al, Development of a Head Scanner for Proton CT, Nucl. Instr. Meth A 699 (2013) 205.The Actual Instrument5/22/2014Proton Computed Tomography8

Tracker ModulesBeamRotating StageRange DetectorRange Detector ElectronicsEvent BuilderTVSilicon-Strip Sensor Orientations5/22/2014Proton Computed Tomography9Measures V CoordinatesMeasures T Coordinates384 stripsASICsSingle-sided, AC coupled, 400m n-on-p HPK sensors, left-over from the Fermi-LAT.We sawed off the sensor edges to minimize the gaps!

After cutInside of one Tracker Module5/22/2014Proton Computed Tomography10

VASICSpartan-6 FPGATwo V boards and two T boardsOne Tracker Cassette, T side5/22/2014Proton Computed Tomography11

Vertical stripsSpartan-6 FPGAsASICsDVI connectorPower connectorThe T side has twice and many amplifier channels as the V side, and two FPGAs, each with a dedicated data line to the event builder.TRange Detector Digitizer Board5/22/2014Proton Computed Tomography12

FPGA14-bit 65 MHz ADC (AD9244)Amplifier and differential ADC driver (AD8138)PMT signalFast inverting amplifierThreshold DACTrigger comparator with LVDS outputEach of the 5 channels has a 14-bit 65 MHz pipeline ADC plus a separate amplifier and discriminator for triggering.

Typical digitized PMT signal.Data Acquisition Concept5/22/2014Proton Computed Tomography13ASICSSD ( V or T)ASICFPGAVirtex-6EventBuilderFPGAFPGAFPGAFPGAFPGAFPGALVDS (Printed Circuits)LVDS (DVI Cables)DAQComputerEthernet32 SSD total144 ASIC total4 V layers4 T layersV layers have 12 ASICsT layers have 24 ASICsSSD ( V or T)9 MHz clock sync. from accelerator1 Spartan-6 FPGA per V board; 2 per T board100 Mbps per LVDS linkFPGAFPGAFPGAFPGAFPGAFPGAADCsFPGAADCsFive-Stage Scintillator800 MbpsDuring acquisition of image data, the system is triggered by the first scintillator stage.Triggered readout, not data driven.Designed to read out more than a million proton events per second.5/22/2014Proton Computed Tomography14

pCT Tracker ASIC Layout100 MHz digital clock and 100 Mbps LVDS data output.Single-threshold digitization (binary output).64 amplifiers are always active and supply a 64-wide-OR trigger as well as data.Output data are formatted as a list of clusters (first strip and number of strips).

TSMC 0.25 micron CMOSASIC Features5/22/2014Proton Computed Tomography15

Internal calibration system to pulse any set of channels with a programmable charge.RC/CR pulse shape with 200 ns peaking time.Two gain options.Also a 400 ns peaking time option.Extra optional inverter for p-on-n SSDs.Threshold programmed by an 8-bit DAC common to all channels.Digital 1-shot on each channel to define a hit window comparable to the accelerator beam RF period (~110 ns).64-wide OR for self triggering.Two 64-bit masks to isolate noisy channels from the trigger and/or data stream.32 deep FIFO to hold the hits during trigger latency.4 independent event buffers, each with a processor to form the cluster lists for output.Digital commands to configure the system, load registers, and verify registers.e.g. set sample clock frequency.R. Johnson et al., Tracker Readout ASIC for Proton Computed Tomography Data Acquisition, IEEE Trans. Nucl. Sci. 60, 3262-3269, 2013ASIC Noise Performance5/22/2014Proton Computed Tomography16

Example threshold scans for eight adjacent channels.

18-cm long SSD strips (V-board strips).

From the erfc fits, the noise sigma is about 1100 e.The relatively large total bias current on the edge-cut SSDs does not impact the strip noise performance.

ENC = 280 + 35 CpF electrons

The expected signal from a 200MeV proton is about 60,000 electrons, so there is excessive noise margin!It would be advantageous to go to 200 m SSDs to reduce multiple scattering.Gain variation:2% rms within a chip5% rms chip to chipThreshold DAC Setting

Efficiency60120Analog power: 1.2 mW/channelDigital power: 2.3 mW/channel (100 MHz)Hold the charge-injection level constant and measure the detection efficiency versus discriminator threshold.Noise Occupancy (Random Triggers)5/22/2014Proton Computed Tomography17

Note: fC scale presently has an uncertainty of up to 30%.The signal from our highest momentum protons is over 9 fCThus the amplifier noise is nearly negligible. The few noisy strips that are present in the system are due to defective SSD strips.Tracker Hit Efficiency5/22/2014Proton Computed Tomography18To minimize confusion from overlapping protons, the time window to capture a hit must be less than about100 ns.Hence the threshold must be set low enough relative to the expected signal that time jitter will be low.And the efficiency of getting all 8 SSD hits must be high.We have no redundant detector planes, except that in the front telescope the beam spot can substitute for one measurement.If a hit is missing and the track projects close to a known dead strip or a gap, that location can be assumed to be hit.This emphasizes the great importance of having high signal/noise.5/22/2014Proton Computed Tomography19

Bad stripsGaps between sensorsTracking resolution near the edge of the acceptanceThe measured hit efficiency generally lies between 98% and 99%, including gaps between detectors and bad strips.Note: the SSDs were run under-depleted at 75V bias (full depletion requires about 100V).Bad Channels5/22/2014Proton Computed Tomography20The ASICs were not tested prior to assembly, and so far the yield has been 100% out of about 200 chips.Noisy and dead channels typically are due to bad detector strips, not the ICs.We found only 24 such channels in the system, out of 9216 channels, or about 0.3%.These are leftover Hamamatsu-Photonics sensors from fabrication of the tracker of the Fermi orbiting gamma-ray telescope.Residuals from Straight-Line Fits5/22/2014Proton Computed Tomography21

(Before making alignment corrections.)

Data Acquisition Rate5/22/2014Proton Computed Tomography22

Trigger RateAcquired-Event RateThe accelerator spill intensity needs to be improved in uniformity, but the instrument succeeds in delivering our goal of a million events per second.Conclusions5/22/2014Proton Computed Tomography23Our pCT scanner is now fully operational and able to take data at an event rate above 1 MHz.The Tracker ASIC met all of our requirements in the first submission and had a very high yield.The custom data acquisition system based on FPGAs meets our goal of >1 million protons per second.

Future goals: Increase the proton rate up to 5 to 10MHz by designing and building a segmented range detector (to measure simultaneous protons).This could get the scan time down to 1 or 2 minutes.Transfer the technology to industry (e.g. Varian).Extra Slides5/22/2014Proton Computed Tomography245/22/2014Proton Computed Tomography25

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Residuals from Straight-Line Fits5/22/2014Proton Computed Tomography28