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

BNL 

B. AzmounBNL

RD 51 Collaboration MeetingStony Brook, NY

Oct. 4 2012

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

B. Azmoun, BNL 2

From PHENIX to Central Detector

Forward SpectrometersPHENIX • Smaller, more compact, but

with larger acceptance (|h|<1.1, Df =2 )p• Central solenoid magnet

with high precision silicon tracking with

additional GEM tracking• Forward spectrometer with

large area GEM trackers

GEM Tracke

rs

GEM Tracke

rs

GEM Tracker

B. Azmoun, BNL 3

EIC Detector – Conceptual Design Central Detector

Forward/Backward Detectors

• 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

GEM TPC

Planar GEM Trackers

GEM Tracker

B. Azmoun, BNL 4

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

Drift Gap

Transfer 1

InductionTransfer 2

GEM 1GEM 2GEM 3

Mesh

X-Y StripsPitch: 400um

17mm

1.5mm

2mm1.5mm

Preamp/Shaper

Primary Charge Fluctuation

B. Azmoun, BNL 5

Data Processing

Propagated Errors:Angle: ~+/-18mradCharge arrival time: ~+/-1.8ns

• Linear Fit to determine arrival time = x-int.

• 30 samples x 25ns = 750ns window

Raw Data: Waveforms in Time Vector Signature: “Charge square”

Vector Recon:• X -coord. = middle of pad• Y-coord. = drift time *

Drift Vel.• Fit (x,z) points to line

Vector Recon. Z-residual < 0.5mm

B. Azmoun, BNL 6

Some Limitations on Track Recon.

• 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.

MC Results on Track Reconstruction ErrorsFluctuations in Primary Ionization

T. Cao

B. Azmoun, BNL 7

Measuring Low Energy Collimated Beta Source using External Trigger

Plastic Veto Scintillator(5mm)

Plastic Trigger Scintillator(0.5mm)

~50mm

Sr-

90Bra

ss

Source

Holder

Tungsten

Collim

ator

1.00m

m

hole

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

Sr90 b -decay spectrum Endpoint ~2.2MeV

For example, even observe occasional scattering in gas

B. Azmoun, BNL 8

Measuring Betas with Self-Triggered SystemSeveral 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 cross section @17mm = 2mm

Beam Angle = 590 mrad

Spread due to beam div./scattering

Preamp/ShaperCapacitively couped to bottom GEM electrode

~50

mm

Sr-90

Brass Source

Holder

Tung

sten

Col

lim

ator1.00m

m

hole

GEM TRIGGER

1nF

B. Azmoun, BNL 9

Tracking Cosmics

X-Axis

Y-Axis

Top Scintillation Counter

Bottom Scintillation Counter

Detector

Y-Vector

Y-Axis (mm)

B. Azmoun, BNL 10

GEM Detector + VMM1 Readout

• 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

VMM1 FEC USBPC VMM1 Labview Control panel

Preliminary Results (64 ch.):• Measured Fe55 spectrum• Measured Sr90 vectors at ~35o

B. Azmoun, BNL 11

Medical Imaging: using mini-drift to do PET

e-

e+

γ

γ

e+

~0.2 mm

• 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

Thin plant tissue(eg, Leaf)

Positron Escape (50%)Positron Annihilation

Fig. C.2.2-5 Escaped positron fraction vs. thickness of [18F]-FDG solution as determined by microPET imaging of our positron escape phantom.

B. Azmoun, BNL 12

Emax (b+ )= 640 keV

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

B. Azmoun, BNL 13

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.

B. Azmoun, BNL 14