Presentazione di PowerPoint

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Timing properties of silicon drift detectors for
scintillation detection
C. Fiorini, A. Gola, A. Longoni Politecnico di
Milano, INFN Sezione di Milano, Milano, Italy
F. Perotti IFCTR-C.N.R., Milano, Italy
L. Strüder MPI für Extraterrestrische Physik
Halbleiterlabor, München, Germany
Work supported by Italian INFN
carlo.fiorini_at_polimi.it
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Abstract
In this work we have evaluated the timing
properties offered by silicon drift detectors to
be used as scintillation photodetectors in
systems for medical imaging. The peculiar
drifting mechanism of the charge created inside
the SDD volume is responsible for a rise time of
the signal at the output of the device when this
is irradiated over its whole active area. Despite
this effect, the rise time is in the order of 200
ns for a 5mm2 device, therefore still comparable
with the shaping time used for timing
measurements. In the paper, the effect on the
timing performances of SDDs due to the drifting
mechanism is first theoretically evaluated. We
have then carried out the experimental
characterisation of the timing properties of a
5mm2 SDD coupled to a GSO crystal, in coincidence
with a NaI-PMT detector, using a 22Na source.
Despite the low conversion gain of the system
(1240e-/MeV), due to the low light output of the
crystal and the no-optimized quantum efficiency
of the SDD, a timing resolution of 22 ns was
measured for 511keV photons. This corresponds to
a product resolution times number of collected
electrons of about 13.9 x 103 ns?e-h which is
comparable to the one achieved with APDs of
similar areas. By irradiating the SDD directly
with laser pulses, a resolution better than 1 ns
was achieved with more than 60.000 electrons,
showing no relevant limitations due to possible
jitters of the drift time.
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Motivations of the work
Evaluate the timing performances of Silicon Drift
Detectors used in scintillation detection

• possible application of SDDs in medical imaging
  systems
• (PET, Compton Camera as II detector) to replace
  PDs and APDs
• ? already demonstrated excellent energy
  resolution, position resolution, stability, array
  uniformity,
• however, the drift time of the carriers inside
  the SDD delays
• the complete collection of the scintillation
  signal charge. This could
• prevent, in principle, these devices to achieve
  satisfactory timing
• performances

• up to which degree, SDDs can be used also for
  timing applications with scintillators ?

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The Silicon Drift Detector for scintillation
detection
on-chip JFET operated in Source
Follower configuration (signal charge integrated
on the detector capacitance)
JFET integrated on the detector

• capacitive matching Cgate Cdetector
• minimization of the parasitic
  capacitances

• reduction of the microphonic noise
• simple solution for the connection
  detector-electronics in monolithic arrays
• of several units

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Crossover Timing

• Insensitivity to Amplitude Walk
• Improvement of S/N (e.g. by
• derivation of a semigaussian filter
• Time Jitter depending on Amplitude fluctuation
  thorugh
• pulse derivative at zero-crossing

derivative normalized in time and amplitude
S/N
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Derivative vs. shaping time

• Ideale Case front-end rise time supposed to be
  zero
• the derivative tends to - ?
• by reducing the zero-crossing time
• Real case front-end rise time
• limited by three main effects
• SDD drift time
• rise time of the integrated
• JFET which is operated in a
• Source Follower configuration
• scintillator decay time

Pulse derivative at the Zero Crossing time vs. ZC
time (defined our shaping time)

• the abs. value of the derivative has a maximum
  and then its reduces to zero.
• there is an optimum shaping time for
• the derivative

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Three effects cause the slowering of the
front-end rise time

• The signal can be considered to pass through four
  filtering process
• Current shape at the output of the SDD
• SDD supposed to be uniformly illuminated
• ramp beaviour of the signal current
• initial value not zero (because of the charge
• under the anode region at t0)
• Two exponential time constants
• 1) scintillator, 2) Follower rise time
• Integration on the anode capacitance

the inpulse response h(t) of each process is shown

• SDD output current has
• a triangular shape

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Simulation of the overall rise time of the
front-end

• S. Follower rise time 2.2 ? 180 ns
• scintillator decay time 60ns (GSO)
• S. Follower rise time 2.2 ? 35ns
• Drift time 200ns (estimated from
• device geometry)

note the good agreement between model simulations
and measurements
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Worsening of the derivative because of the SDD
drift time
Tdrift 0
Tdrift 2
Tdrift 4
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Theoretical evaluation of the timing performances
electronics noise
derivative
n filter order
timing resolution
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Rise time reduction by means of the II follower
I follower
II follower
preamplifier
current source
80ns
Intrinsic rise time of the front-end reduced from
400ns to 80ns
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Timing performances with laser pulses
- Good agreement with the 1/X fit - Measured
resolution of 19.7ns for 1000 e- signal (room T)
theoreticaly expected 18.6ns x 1000e- - Best
resolution of 0.8ns with 67.000 e- ? no
noticeable limitations due to jitter effects of
the drift mechanism
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Timing measurements with SDD coupled to GSO
crystal
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Experimental setup

• GSO (Gadolinium Ortosilicate) scintillator
• Fast decay time (60ns)
• Low Light Yield
• Millipore white paper used as
• light diffuser
• maximum convertion gain measured 2.5
  electrons/keV
• Tennelec TC244 bipolar shaping
• with 600ns zero-crossing time
• SDD one unit of a linear array
• Area 5mm2
• ENC 27e- rms at 10C
• (ref. 16.6e- rms unipolar sh.)

10mm
2.5mm
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Experimental results

• 511 keV 1257 electrons
• Temperature 0C
• resolution 13.8 ns FWHM
• ? 17.3 e- for 1000e-
• 511 keV 633 electrons (worse scintillator
  packaging)
• Temperature -10C
• resolution 22.0 ns FWHM
• 13.9 e- for 1000e-
• - comparable with APDs
• - as theoretically expected

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Conclusions

• SDD of small areas (up to few tens of mm2) can
  be used for
• timing applications the drift mechanism does
  not limits significantly
• the performances
• the measured product resolutioncharge of 13.9ns
  1000e- is
• comparable or better than the one achieved with
  PDs and APDs of
• similar areas
• however, the absolute performances (timing
  resolution for 511keV)
• achieved in this work are still poor because of
  the small signal
• available on the SDD (low light output of the
  available scintillator,
• not optimized QE of the SDD, packaging..)
• best performances (lt few ns) will be explored by
  means of better
• scintillator choice (LSO, RGB) and new optimized
  SDDs