INAF - Osservatorio Astrofisico Catania

1
INAF - Osservatorio Astrofisico Catania
II PRIN 2006 Meeting NEWS FROM SINGLE
PHOTONS Sergio Billotta
2
Summary

• Single Photon Avalanche Diode (SPAD)?
• What it is
• How it works
• Dark
• After pulse
• Photon Detection Efficiency (PDE)?
• Silicon PhotoMultiplier (SiPM)?
• What it is
• Dark
• After pulse
• Linearity
• Charge spectrum
• Time jitter
• Photon Detection Efficiency (PDE)?
• Conclusions

3
(No Transcript)
4
(No Transcript)
5
SPADdark
STM SPAD array It is manufactured by the
integration of 25 pixels with a square geometry
of 5 x 5. For these devices, STMicroelectronics
has designed three different pixel diameters 20,
40 and 60 ?m. Separation distances between
adjacent pixels are in the range of 160 and 240
?m according to different diameters.
Anode contacts are in common for each row, while
each cathode is separately contacted and
available from outside by different pads. The
typical breakdown voltage is about 30 V.
We have measured the dark counts rate of each
pixel of several array of SPADs, and we have
found a fairly good uniformity of it.
Median room temperature dark count rate at 4V
overbias as a function of SensL SPAD device
area. Applications of Silicon Photon Counting
Detectors, Stewart et al., JMO in press
6
(No Transcript)
7
(No Transcript)
8
SPADphoton detection efficiency (2)?
PDE QE x Pt x Ae QE Quantum
Efficiency Pt Avalanche Probability Ae Geometr
ical Efficiency
There is a finite probability for a carrier to
initiate an avalanche when passing through a
high-field region. In case of a photogeneration
event, 2 carriers are created travelling in
opposite directions
Pt Pe Ph - PePh
Electron and hole breakdown initiation
probabilities
In case of photogeneration on the right side, the
situation is symmetrical and only electrons
contribute to the triggering probability, thus,
Pt PeM. In the central region, both carriers
contribute to a different extent as a function of
the interaction position and the Pt value is
between PeM and PhM When a pair is generated in
the left side of the high-field region, the
electron is directly collected at the n
terminal thus, it does not contribute to the
triggering probability. The hole is forced to
cross the whole high-field region and so its
triggering probability is maximized and Pt PhM.
9
(No Transcript)
10
(No Transcript)
11

• Single Photon Avalanche Diode (SPAD)?
• What it is
• How it works
• Dark
• After pulse
• Photon Detection Efficiency (PDE)?
• Silicon PhotoMultiplier (SiPM)?
• What it is
• Dark
• After pulse
• Linearity
• Charge spectrum
• Time jitter
• Photon Detection Efficiency (PDE)?
• Conclusions

12
(No Transcript)
13
SiPMdark

• Dark count pulses
• s gt single pixel pulse
• d gt two simultaneously pixels pulse
• a gt
• Characteristics of the single-pixel dark pulse
  (equal of single photon pulse)?
• rise time
• hundreds of ps
• recovery time
• t Rquenching Cmicro-cell 20-30 ns

afterpulses
Applications of Silicon Photon Counting
Detectors, Stewart et al., JMO in press
Dark rate as a function of overbias for a SensL
SiPM at room temperature and at -20C
Measured noise rate as a function of V - Vbd
14
SiPMafter pulse

• For uncorrelated events (SiPM without cross-talk
  and afterpulses)
• The random noise follows the Poisson law
• The distribution of the arrival time between two
  events is exponential

Sketch of the electronics for the self-correlated
timing, employed for afterpulse measurements.
We measured the distribution of time intervals
between two consecutive dark pulses at 20C for
several bias voltages, and built the
corresponding histograms. The lower time
threshold was around 15-20ns, therefore
preventing us from attaining a direct measurement
of cross-talk. The afterpulse effect shows up in
such a distribution as a pronounced deviation
from the perfect exponential distribution of the
uncorrelated dark noise, namely a prominent peak
around 200ns.
15
SiPMdynamic range - linearity
If Nphoton x PDE ltlt Ntotal Output signal a
Nfired When 50 of the cells fire the deviation
from linearity is 20 Best working condition gt
Nphotons lt Ncells

• If Dt gt tR the SiPM dynamic range is larger.
• Dt duration of the light signal
• tR single pixel recovery time

16
SiPM charge spectrum
By making use of a dedicated data acquisition
system one can characterize the SiPM on an
event-by-event basis. Using this method we built
the charge distribution histogram under several
different light intensity values. We employed a
red laser diode (650nm) pulsed at 1kHz, whose
light was conveyed onto the sensor by means of an
optical fiber.
Sketch of the electronics for the charge and time
measurements, employed for SiPM response
characterization.
A typical charge spectrum under very low light
level for a 10x10 device biased at 6 OV. The
multipeak structure reflects the detection of
1-18 photons per event. For this sensor we
measured a 3s resolving power around 20 and a 2s
resolving power around 45.
17
SiPMtime jitter
The width (FWHM) of the statistical distribution
of the delay between the true arrival time of
the photon at the sensor and the measured time
marked by the output pulse current leading edge.

• STM SiPM
• SiPM area 0,5 x 0,5 mm2
• Pixel active area 32 x 32 mm2
• Fill factor 36
• N pixels 10 x 10
• 6 overvoltage
• Apparatus
• Pulsed laser diode (l 650 nm, 1kHz, pulse
  40ps)?
• Optical fiber

Sketch of the electronics for the charge and time
measurements, employed for SiPM response
characterization.
A typical timing spectrum under very low light
level for a 10x10 device biased at 6 OV. The
average number of detected photons was around 6.
The time calibration of the TDC was 50ps/channel,
therefore the time resolution (sigma) is 135ps.
18
SiPMphoton detection efficiency
PDE QE x Pt x Ae QE Quantum
Efficiency Pt Avalanche Probability Ae Geometr
ical Efficiency
Aactive / Atotal

• dead region
• determined by the guard ring
• structure preventing optical cross-talk
• space between the cells for the individual
  resistors
• Considering that the area of a cell can be very
  small (in the order of 30x30 mm2) even few
  microns of dead region around the cell have a
  very detrimental effect on the geometrical
  efficiency.
• Best filling can be achieved with a small number
  of big cells gt SATURATION !!!

19
(No Transcript)
20
Conclusions

• SiPM
• Advantages
• robust and compact
• sensitivity to extremely low photon fluxes
  providing proportional information with excellent
  resolutionand high photon detection efficiency
• extremely fast response with low fluctuation
  (sub-ns rise time and lt100ps jitter)?
• low bias voltage (lt100V)?
• low power consumption (lt50µW/mm2)?
• long term stability
• insensitive to magnetic fields (up to 15T) and EM
  pickup
• low cost (in the future! now 140/mm2) low
  peripheral costs
• Disadvantages
• silicon quality (dark rate, after-pulse)?
• effective area of the cells (gain, fill factor,
  dynamic range, recovery time
• optical cell insulation (optical cross-talk)?
• quenching resistor (recovery time, dynamic range)?

• SPAD
• Advantages
• solid state technology robust, compact,
  mechanically rugged and less expensive
• Geiger mode
• high internal gain of 105 - 106
• faint sources
• high quantum efficiency
• large standardized output signal
• no Read Out Noise
• high sensitivity for single photons
• excellent timing event for single photo electrons
  (ltlt 1ns)?
• good temperature stability
• devices operate in general lt 100V
• no nuclear counter effect (due to the
  standardized output)?
• Disadvantages
• BINARY DEVICE one knows there was at least one
  electron/hole initiating the breakdown but not
  how many of them !!!!!
• Max diameter 100 mm.

21
INAF - Osservatorio Astrofisico Catania
II Meeting PRIN 2006 NEWS FROM SINGLE
PHOTONS Sergio Billotta Grazie
22
(No Transcript)
23
SPADphoton detection efficiency
PDE Detected photons / Incident photons

• Xenon Lamp
• Pre-filtering system
• Monochromator
• Integrating Sphere
• Reference photodiode
• SPAD ( Camera)?
• One ammeter
• (AQC Counter)?
• One PC

• Camera conceived to be anchored to the
  integrating sphere
• Distance (SPAD sphere centre) Distance
  (photodiode sphere centre)?
• BK7 window with MgF2 anti reflection coating
• AQC (OACt)?

vacuum
Peltier Stabilized TSPAD down to -20C

• SPAD biasing
• SPAD ( 10 ns) quenching
• Hold off time variation (400 ns 6 ms)?
• TTL output

24
SiPMwhat it is
The cell is square shaped, with a 50µm/30µm side
over active area ratio, and a resulting 36 fill
factor. We produced a SiPM made of a 10x10 array
with common anode. Each cell has a breakdown
voltage around 29.5V at room temperature, with a
variation coefficient of 35mV/C. For our tests
the SiPM was biased at voltages between 31.5V and
33V.
25
SiPMOV
SiPM breakdown voltage as a function of the
temperature. The breakdown voltage increases
linearly with a temperature coefficient of 35.5
mV/C.
400 channels SiPM output dark noise pulses. The
device was biased at 10 OV three types of
signals can be seen (i) single pulses coming
from the activation of a single pixel (ii)
double pulses corresponding to events affected by
cross-talk effects and (iii) small amplitude
pulses due to the afterpulsing
26
SiPMafter pulses
Probability distribution of the time interval
between two consecutive signals (solid line). The
open circles represent the contribution due to
the dark counts, the open squares the afterpulses.
Example of measured noise rate as a function of
the discriminator threshold. The threshold is
normalized to the 1-photon signal amplitude.
Operation at 0.5 photon threshold is safely in
the plateau region
27
(No Transcript)
28
SiPMphoton detection efficiency (4)?
The PDE at 700nm, measured at 32.5V bias voltage,
as reconstructed with our method using logic
signal durations of 50ns as a function of the
number of photons impinging on the SiPM. Apart
from the first point, measured at very low flux,
the behaviour is constant as expected.
29
SiPMapplications
Positron Emission Tomography (PET)?
Positron emission tomography (PET) is a new
nuclear medicine imaging technique which produces
three-dimensional images or maps of functional
processes in the body. It is heavily used in
clinical oncology (medical imaging of tumors and
search for metastases) and for clinical diagnosis
of certain diffuse brain diseases. In order to
conduct the scan, a short-lived radioactive
tracer isotope which has been chemically
incorporated into a metabolically active
molecule, is injected into the living subject.
There is a waiting period while the metabolically
active molecule becomes concentrated in tissues
of interest then the research subject or patient
is placed in the imaging scanner. The
radioisotope emits a positron which travels few
millimeters before it combines with an electron.
The two 511 keV photons form the annihilation
radiation are absorbed by a scintillator material
in the scanning device, creating a burst of light
which is detected by a photodetector (usually PMT
or SiPM). This technique depends on simultaneous
or coincident detection of the pair of photons
in fact photons which do not arrive in pairs
(i.e., within a few nanoseconds) are ignored.
Hence it is possible to localize the source along
a straight line of coincidence. A statistics from
tens-of-thousands of coincidence events is then
collected. The scanner records these signals and
transforms them into images. The resulting map
shows the tissues in which the molecular probe
has become concentrated, and can be interpreted
by a nuclear medicine physician or radiologist
for the patient's diagnosis and treatment plan.
30
COLD laboratory

• Detectors characterization
• Traditional detectors CCDs
• Innovative ones
• Diamond detectors
• SiC detectors
• Single Photon Avalanche Diode (SPAD)?
• Silicon Photon Multiplier (SiPM)
• Designing and testing of control electronics,
  cryogenics and mechanical equipments for
  detectors
• Software development for management of
  astronomical instruments
• Collaboration in spatial and terrestrial
  telescope instrumentation

31
(No Transcript)