R&D Activities with GEM Trackers for Nuclear Physics and Medical Imaging at BNL

1.  R&D Activities with GEM Trackers for Nuclear Physics and Medical Imaging at BNL  B. Azmoun BNL RD 51 Collaboration Meeting Stony Brook, NY Oct. 4 2012

2. New Applications for GEM Tracking Detectors at BNL • sPHENIX • PHENIX → sPHENIX (major upgrade) • Augment silicon tracking in central region • Large area tracking in forward direction • eRHIC • Central TPC • Planar GEMs in forward direction • Need to be low mass for measuring scattered electron • Medical Imaging • Tracking positrons from PET isotopes → tomography • Useful in plant biology → biofuels, environmental science • Initial R&D Effort • Reconstructing tracks from a beta source • Cosmic rays • SRS/APV, and first look at the VMM1 chip • GEM based PET

3. From PHENIX to Central Detector GEM Tracker • sPHENIX • Smaller, more compact, but with • larger acceptance (|h|<1.1, Df =2p) • Central solenoid magnet with high precision silicon tracking with • additional GEM tracking • Forward spectrometer with large • area GEM trackers Forward Spectrometer GEM Trackers GEM Trackers

4. Central Detector EIC Detector – Conceptual Design Forward/Backward Detectors GEM TPC GEM Tracker Planar GEM Trackers • Large acceptance: -5 < h < 5 • Asymmetric • Nearly 4p tracking and EMCAL coverage • HCAL coverage in central region and hadron direction • Good PID • Vertex resolution (< 5 mm) • Electron is scatted over large range of angles (up to 165˚) • Low Q2 → low momentum (few GeV) • Requires low mass, high precision tracking

5. Mini-Drift GEM Det. + SRS Readout • Std. 10x10cm CERN 3-GEM Det. • ArCO2 (70/30) • Gain ~ 6500 • ~17mm Drift Gap • Drift Time ~600ns • SRS /512 channels APV 25 • 30 x 25ns Time Samples • Martin Purschke’s RCDAQ • affords high flexibility • COMPASS style Readout: • 256 x 256 X-Y Strips • ~10cm x 400um pitch Mesh Primary Charge Fluctuation 17mm Drift Gap GEM 1 Transfer 1 1.5mm GEM 2 Transfer 2 1.5mm GEM 3 2mm Induction X-Y Strips Pitch: 400um Preamp/Shaper

6. Data Processing Raw Data: Waveforms in Time Vector Signature: “Charge square” Vector Recon. Z-residual < 0.5mm • Linear Fit to determine arrival time = x-int. • 30 samples x 25ns = 750ns window • Vector Recon: • X -coord. = middle of pad • Y-coord. = drift time * Drift Vel. • Fit (x,z) points to line Propagated Errors: Angle: ~+/-18mrad Charge arrival time: ~+/-1.8ns

7. Some Limitations on Track Recon. MC Results on Track Reconstruction Errors Fluctuations in Primary Ionization T. Cao • For tracks near zero degrees, less pads fire and the track reconstruction gets more ambiguous, leading to larger errors. Here it is better to rely on the centroid for giving the position of the track, where high gas diffusion is preferable. • For larger angled tracks, gas diffusion and charge sharing between pads is the major source of error, since the true arrival time of the column of charge above a given pad is distorted. • Charge fluctuations on the primary ionization lead to small charge clusters, which can be difficult to measure. This can put a limit on the arrival time calculations at each strip.

8. Brass Source Holder Measuring Low Energy Collimated Beta Source using External Trigger Sr-90 ~50mm 1.00mm hole Tungsten Collimator Sr90 b -decay spectrum Endpoint ~2.2MeV Plastic Veto Scintillator (5mm) For example, even observe occasional scattering in gas Plastic Trigger Scintillator (0.5mm) Light guide (5mm) • External Trigger allows for precise timing of hits, with no dependence on the detector’s • ability to measure first pad hit, but… • Low momentum electrons suffer greatly from multiple Coulomb scattering by any scintillator used to produce the external trigger

9. Brass Source Holder Measuring Betas with Self-Triggered System Sr-90 ~50mm • Several Advantages to having a Self Triggered System: • Ease of use, independent readout, and can be used for applications where an external trigger is not readily available • GEM trigger doesn’t provide precise timing so rely on ability to measure the first pad fired as a measure of t_ZERO • Detector requirements: • High Gain • Low Noise • Wide Drift Gap • Low Diffusion Gas (CF4?) Beam Angle = 590 mrad Spread due to beam div./scattering 1.00mm hole Tungsten Collimator 1nF GEM TRIGGER Preamp/Shaper Capacitivelycouped to bottom GEM electrode Beam cross section @17mm = 2mm

10. Tracking Cosmics X-Axis Y-Vector Top Scintillation Counter Detector Bottom Scintillation Counter Y-Axis Y-Axis (mm)

11. GEM Detector + VMM1 Readout VMM1 FEC USBPC VMM1 Labview Control panel • Peak sensing ASIC that provides charge amplitude, and peak-time with minimal time walk • Programmable electronic gain, memory depth (we use 1usec) • Records only pads with charge above threshold • Labview interface allows for Plug n’ Play • Despite only spending a day’s worth of time with the chip, we were able to take some reasonable data • Preliminary Results (64 ch.): • Measured Fe55 spectrum • Measured Sr90 vectors at ~35o

12. Medical Imaging: using mini-drift to do PET Emax (b+ )= 640 keV e+ Positron Annihilation Positron Escape (50%) γ e- ~0.2 mm Thin plant tissue (eg, Leaf) e+ γ • Plant tissue absorbs radioactive tracer • b+ decay , followed by positron annihilation • Traditionally back to back gammas are measured to reconstruct image • New Concept: Use mini-drift detector to measure escaping positrons directly Fig. C.2.2-5 Escaped positron fraction vs. thickness of [18F]-FDG solution as determined by microPET imaging of our positron escape phantom.

13. Preliminary Results with FDG (proof of principle) FDG is a radioactive tracer and analog of glucose, commonly used in PET scans Vial of liquid FDG ~1cm mylar window Sigma of Reconstructed position ~5.6mm

14. Summary • Used Mini-drift GEM detector to reconstruct vectors from ionization trails using the SRS system with a relatively slow sampling rate ADC(40MHz). • Successfully read out a GEM based detector with the VMM1 chip. • Successfully measured tracks produced by b+ particles and have provided a proof of principle that the mini-drift GEM detector may be applicable for doing PET. • Outlook: Will produce high precision, silicon based cosmic ray telescope to study the performance of the detect0r further. Also, we have a beam test at CERN planned later this October for studying the detector under very controlled conditions.