Timepix and TOT mode

Report
TIMEPIX AND
TOT MODE
1
Taken from Llopart’s 2007 thesis: Paper X and
J. Jakubek, Nucl. Instr. and Meth. A (2010),
doi:10.1016/j.nima.2010.06.183
Ryan Badman, July 15th
OUTLINE
Introduction to Timepix
 Calibration with charge (test pulse)
 Calibration with energy

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IMAGE OF TIMEPIX
Fig. 1
3
TIMEPIX LAYOUT

Analog part has one band-gap
circuit to internally generate a
stable reference voltage to be
used by the 13 on-chip global
DACs
Reference voltage has sensitivity
of -0.22 mV/ deg C and a power
supply of less than 1mV/V
 Has eight 8-bit current DACs and
four 8-bit voltage DACs, and a
single 14-bit voltage DAC


Digital part has all input/output
control logic, the IO wire-bonding
pads, and a 24-bit fused blown
registry for unique chip
identification

36 million transistors total in the
entire chip, and a read-out time of
less than 300us
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CHARGE COLLECTION IN A SINGLE
CHANNEL




Electronics for each individual pixel
Any charge collected by the pixel anode is integrated and compared to the global
threshold.
If the preamplifier voltage crosses the threshold then the output of the
discriminator generates a pulse whose width corresponds to the length of the
time the preamplifier output voltage remains over threshold
Minimum detectable charge is expected to be 650 e- and the noise is 100 e-rms
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TIMEPIX ABILITIES



Timepix uses an external clock
with frequency up to 100MHz
Each pixel has its own
preamplifier, discminator with
hysteresis, and a 14-bit counter
Each pixel has 4 modes:
Masked mode: pixel is off
 Medipix mode: incoming
particles are counted
 TOT mode: Counter is
incremented continuously as long
as signal is over TH, used to
measure particle energy. (TOT =
time over threshold)
 Timepix mode: counter is
continuously incremented from
the time the fist hit arrives until
the end of the shutter

Fig. 11 shows a
full matrix
response to 10
test pulses of
about 2.3 ke-
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TOT CALIBRATION
TOT vs. Qin (test pulse)
 In TOT mode the energy resolution is better than 5%
if the input charge is greater than 1 keV above
threshold

Fig. 6, TOT measurements
on the right, and the
measured energy resolution
with 1% and 5% markers on
the right. Fig 1 (above,
another Timepix schematic.
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TIME-WALK




Defined as the difference between
the time measured from an input
charge that is 1ke- over threshold
and an infinite input charge.
The faster the preamp peaking
time, the better the time-walk value
One method of compensating the
time walk is to arrange the pixel
matrix in a chess pattern where
pixels alternate between TOT mode
and arrival mode, and then use
knowledge of the input charge in the
neighboring pixels.
Measured time-walk per pixel is
less than 50ns.
Time-walk of 1 pixel at
the matrix center for five
different thresholds. The
thicker lines represent
Preamp 255 (1.8uA) and
the thin with Preamp 127
(900nA). Y axis is time of
arrival.
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TIMEPIX EXAMPLE RESULTS
Fig. 12 shows TOT measurements of a single cosmic
background particle on the left after interacting in
the gas of a GEM detector, and on the right the
arrival time measurements of the cosmic particle
obtained with a Micromegas gas gain grid coupled to
the Timepix chip.
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TOT MODE ENERGY CALIBRATION



Each pixel needs
to be calibrated for
proper energy
measurement.
This calibration is
nonlinear in lower
energy ranges,
and linear in
higher.
The calibration
can be described
by a surrogate
function
containing four
parameters.
Fig. 2 (a is gain, b the y-intercept, and
t relates to threshold)
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HOW THE FIT PARAMETERS VARY PER
PIXEL (MY RESULTS, TH400)
Note: These our for test
pulse, not for Jakubek’s
energy calibration
Results for (col 0-40) x 256 pixel
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COMPARE TO RICHARD’S (SAME BINNING)
TH300, Col (0-40) x
256
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PROBLEM WITH ENERGY CALIBRATION

Calibration requires measurement of at least 4
spectral lines and at minimum 5 least-squares fits
per pixel. However, fitting spectral peaks with
Gaussians in the non-linear portion close to
threshold gives incorrectly shifted results.
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IMPROVEMENTS TO ENERGY CALIBRATION

The solution to the
bad peak shape is
to combine a
Gaussian with the
inverse of the
surrogate function
for the nonlinear
region (called
model M); and just
a Gaussian in the
linear region.
Fig. 3. The TOT spectrum of 55Fe
(the same spectrum as shown in
Fig. 2 on the previous slide) fitted
with the model M.
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IRRADIATION WITH MULTI-ENERGY
CALIBRATION SOURCE.

Jakubek irradiated the Timepix chip to obtain TOT spectra for each
pixel in single pixel clusters.
 The threshold was just above the noise edge, sensor bias voltage
was 100V, the clock frequency was 10 MHz, and Ikrum was 1.
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CALIBRATED GLOBAL SPECTRUM

The calibration
quality was tested
without the Fe layer
and the following
global spectrum was
observed.
Fig. 6. Calibrated spectra of dual energy source
(241Am+In) for different cluster sizes (the cluster
size is denoted by label). Peaks for different cluster
sizes are well aligned. Peaks are fitted with
Gaussians. The energy resolution (sigma of
Gaussian) is 2.3keV for both peaks.
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DISTORTION CORRECTION


Since the cluster
volume spectra is
distorted (its supposed
to be independent of
bias voltage), the
calibration function is
not accurate for high
ionization charges.
The difference
between estimated
and measured cluster
heights is a linear
function so the
calibration function
can be corrected above
0.9MeV (up to
1.2MeV).
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Fig. 12. Cluster volume spectra of 5.5MeV alpha particles. The
distorted response of pixels was corrected above 0.9MeV and the
spectrum shape restored (compare with Fig. 10).
CLUSTER HEIGHT VS BIAS VOLTAGE
Fig. 8. Maximum signal
seen by a pixel in the
cluster (cluster height) as
a function of the bias
voltage for 5.5MeV alphas.
Fig. 9. Dependence of the
cluster height on the bias
voltage for 5.5MeV alpha
particles. When the pixel
response reaches about 0.9MeV
the cluster height starts to
grow unexpectedly.
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EXAMPLE OF DISTORTION

Fig. 10. Cluster
volume spectra
for sensor bias
voltage of 39V
(top) and 72V
(bottom)
showing
distortion
caused by the
deviation of
pixel response
from the
calibration
function f
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SUMMARY
Timepix offers more control over individual pixels
than its predecessor Medipix2.
 The small periphery will cause no loss in
coverage if the chip is chosen for the VELO
upgrade.
 The power consumption, radiation tolerance,
pixelated format, small pitch, and convenient
programmability are also favorable for the VELO
upgrade.
 Jakubek’s correction successfully corrects the
cluster height measurements for higher pixel
responses up to 1.2 MeV.
 We will come up with a better energy calibration?

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