Investigation of LaBr3 Detector Timing Resolution

1
Investigation of LaBr3 Detector Timing Resolution

• Kuhn1, S. Surti1, K.S. Shah2, and J.S. Karp1
• 1Department of Radiology, University of
  Pennsylvania, Philadelphia, PA
• 2Radiation Monitoring Devices, Watertown, MA

2
Abstract
Lanthanum bromide (LaBr3) scintillation detectors
are currently being developed for use in
time-of-flight (TOF) PET. In recent years,
studies have been aimed at the parameterization
of the LaBr3 scintillation properties. We have
utilized the findings of these studies in the
development of simulation tools to investigate
and predict the performance of TOF PET detectors
of realistic geometries. Here, we present a
model to simulate the combined scintillator and
photomultiplier tube (PMT) response to incident
photons. This model allows us to study the
effects of crystal response, geometry, and
surface finish, PMT response, transit time
spread, and noise, as well as discrimination
techniques on the coincidence resolving time
achievable in various detector configurations.
Results from the simulations are benchmarked
against several experimental measurements with
two different PMTs and LaBr3 crystals of varying
cerium concentration and geometry. A comparison
is also made to the time resolution achievable
with LYSO. Good agreement between measurement
and simulation has been achieved with detectors
consisting of 4x4x30 mm3 crystals suitable for
use in a TOF PET scanner. Ultimately, this
guides the improvement of TOF detectors by
identifying the individual contribution of each
detector component on the time resolution that
can be achieved.
3
Properties of LaBr3

• Fast Rise and Decay Times
• Reduction in random coincidences
• Excellent coincidence time resolution
• Excellent Energy Resolution
• Reduction in scattered events and random
  coincidences
• Very High Light Output
• Good crystal discrimination with long narrow
  crystals (i.e., 4x4x30 mm3)
• Low Melting Point (783 C)
• Easier crystal growth, reduction in material costs

Scintillator t (ns) m (cm -1) DE/E () At 662 keV Relative Light Output ()
NaI(Tl) 230 0.35 6.6 100
BGO 300 0.95 10.2 15
CsF 3 0.39 18.0 5
BaF2 2 0.45 11.4 5
GSO 60 0.70 8.5 25
LSO/LYSO 40 0.86 10.0 75
LaBr3 25 0.47 2.9 160
Values obtained from reference 5-11
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Model Introduction (I)

• Photon Transport (MonteCrystal)
• Gamma-ray trajectory
• Tracks gamma interactions (Compton
  Photoelectric)
• Defined detector materials geometry
• Crystal type (LaBr3 and LYSO)
• Crystal Size (varied crystal length
• with 4x4 mm2 cross-section)
• Single crystal/PMT and Anger-logic
• detector geometries
• Scintillation photons generated at each
  interaction point
• Crystal scintillation response parameterized 3
• Path of scintillation photons traced
• Modeled crystal surfaces, boundaries and
  reflector material

5
Model Introduction (II)

• Modeled PMT Parameters
• Transit time spread (jitter)
• Quantum efficiency
• Response of PMT (single photoelectron)
• Signal noise from dark current
• Discriminator Time Pick-off
• Leading edge model

Two PMTs Modeled
The XP20D0 represents good timing performance in
a 2 inch diameter PMT and is being used in our
prototype LaBr3 scanner, the HM R4998 was chosen
because of its extremely fast response and low
TTS.
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Model - Block Diagram
Montecrystal
Crystal Surface And Reflector Properties
Detector Geometry
Crystal Response
Interactions in Crystal (Compton Photoelectric)
Generation of Scintillation Photons
Gamma ray Transport
Track Scintillation Photons
PMT Signal Model
PMT Transit Time Spread
Threshold Setting
Noise
Convolve PMT Response
Anode Signal
Discriminator
Event Time
7
Simulation of Pulse Shapes
5.0 Ce LaBr3 Response
Photoelectrons created at PMT cathode
Taken from reference 3
Response at photocathode is convolved with the
measured single photo-electron PMT response
Measured single photoelectron response for XP20D0
Simulated Pulse Shape
5.0 Ce LaBr3
Measured Noise Histogram XP20D0
Dark current noise (Gaussian fit to measured
noise histogram) is added to the simulated PMT
pulse shape
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Single Crystal on XP20D0 PMT
Simulation
Measurement
LYSO
LYSO
All Crystals are 4x4x30 mm3 Measured pulse
shapes include oscilloscope response

• Rise time of 30 Ce LaBr3 (3.5 ns) is faster
  than 5.0 Ce LaBr3 (5 ns)
• Simulated pulse shapes have slightly faster rise
  and decay compared to those measured due to the
  finite response of the oscilloscope used to
  record the pulses
• LYSO pulses have 20 signal amplitude compared
  to LaBr3

9
Single Crystal on HM R4998 PMT
Simulation
Measurement
LYSO
LYSO
All Crystals are 4x4x30 mm3 Measured pulse
shapes include oscilloscope response

• Response of R4998 is faster than XP20D0
• Reduced rise time of 30 Ce LaBr3 (2 ns) and
  5.0 Ce LaBr3 (3 ns), thus improving the ability
  to accurately determine the start time of the
  pulses

10
Relative Light Output Crystal Surface Finish

• Comparison of light collection for various
  crystal surface finishes
• Large light loss for a crystal with all diffuse
  surfaces
• Previously tested crystal samples indicate that
  the light output behavior is comparable to the
  simulation of a crystal with both specular and
  diffuse surfaces (i.e., 1 diffuse and 4 specular
  surfaces) for crystal lengths up to 30 mm (i.e.,
  30 reduction in light collection compared to
  very small samples)

Simulated Light Collection
Crystal cross-section is 4x4 mm2
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Coincidence Time ResolutionLaBr3 5.0 Ce
Coupled Directly to PMT
Measured Coincidence Time Resolution Two 5.0Ce
LaBr3 (4x4x30 mm3)
Simulated Coincidence Time Resolution
XP20D0
XP20D0
FWHM 280 ps
Simulation
HM R4998
Simulation
HM R4998
(Crystal cross-section is 4x4 mm2)
FWHM 240 ps
- Measured resolution with XP20D0
- Measured resolution with HM R4998
12
Coincidence Time Resolution LaBr3 30 Ce
Crystal Coupled Directly to PMT
Measured Coincidence Time Resolution Two 30Ce
LaBr3 (4x4x5 mm3)
Simulated Coincidence Time Resolution
Simulation
XP20D0
XP20D0
FWHM 190 ps
Simulation
HM R4998
HM R4998
(Crystal cross-section is 4x4 mm2)
FWHM 145 ps
- Measured resolution on XP20D0
- Measured resolution on HM R4998
13
Coincidence Time Resolution LYSO Crystal
Coupled Directly to PMT
Measured Coincidence Time Resolution Two LYSO
crystals (4x4x20 mm3)
Simulated Coincidence Time Resolution
XP20D0
XP20D0
Simulation
FWHM 380 ps
HM R4998
Simulation
HM R4998
(Crystal cross-section is 4x4 mm2)
FWHM 310 ps
- Measured resolution on XP20D0
- Measured resolution on HM R4998
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Anger-logic DetectorCoincidence Time Resolution

• Detector Geometry
• 7 PMTs coupled to a light guide
• and 4x4x30 mm3 crystal array
• PMT transit times varied by 200 ps
• Simulation indicates a significant improvement in
  time resolution can be achieved by utilizing a
  PMT with faster response

7 XP20D0s Coincidence Time Resolution
7 HM R4998s Coincidence Time Resolution
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Conclusions

• Simulated time resolution is in good agreement
  with the measured data points for LaBr3 and LYSO
  crystals coupled directly to PMTs as well as in
  an Anger-logic design
• The faster response and lower transit time spread
  of the HM R4998 PMT leads to a significant
  improvement in the coincidence time resolution
  achieved
• Simulation and experimental measurements with 30
  Ce LaBr3 indicate an improvement in coincidence
  time resolution over the 5.0 Ce LaBr3 on the HM
  R4998 PMT due to the faster response
• Utilizing a PMT with the properties of the HM
  R4998 in an Anger-logic detector design can
  potentially yield a coincidence time resolution
  of 200 ps with LaBr3 and 400 ps with LYSO

16
Acknowledgments
This work was supported by NIH R33EB001684 and a
research agreement with Saint-Gobain. We would
like to thank the research members at
Saint-Gobain and Radiation Monitoring Devices for
their continued support.
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