Pulsed Proton Beam as a Diagnostic Tool for the
Characterization of Semiconductor Detectors at High
Charge Densities
L. Carraresi, A. Castoldi, IEEE Member, N. Grassi, C. Guazzoni, IEEE Member, R. Hartmann, D. Mezza and
F. Taccetti
Abstract– We exploited the possibility of using a pulsed monoenergetic proton beam – coming from the DEFEL beam-line of
the Tandetron accelerator at LaBEC (Laboratorio di Tecniche
Nucleari per i Beni Culturali) in Sesto Fiorentino, Italy – as a
diagnostic tool for the characterization of the response of
semiconductor detectors at high charge densities. In fact
accelerated protons owing to their limited range in silicon can
deliver a large and precisely calibrated amount of charge along a
track well matched to the typical silicon wafer thickness. As a
case study we considered the characterization at high level of
charge injection of a Multi-Linear Silicon Drift Detector
prototype for position-sensing applications. The focus is on the
potentiality of the experimental technique and on the first results
of the experimental characterization of the detector.
I. INTRODUCTION
the field of semiconductor detector development it is of
Iinterest
to probe the energy and the timing response of the
N
detection system as a function of the charge injection level.
Moreover it is of interest to experimentally evaluate the
impact of electron-hole plasma on the silicon detector
properties. For example, heavy ion collisions at Fermi
energies (15 AMeV < E/A < 100 AMeV) or central collisions
in pre-relativistic energy domain (up to few GeV/A), of
interest in intermediate energies nuclear physics, produce
reaction products that span from protons to very heavy ions
(gold or uranium) in a range of energies varying from few
MeV to several GeV. Another example is the qualification of
the imaging detectors being developed for the novel FELbased X-ray sources that push the limits of brilliance farther
than any light source today. The required dynamic range in
fact can range from single photon counting to 104 times
12 keV photons per pixel per pulse. To this aim table-top
pulsed laser systems at different wavelengths are a useful test
stand. However, despite their ease-of-use, the stability in the
Manuscript received November 18, 2010. This work was supported by
Istituto Nazionale di Fisica Nucleare (INFN) under RIXFEL experiment.
L. Carraresi is with Universita’ degli Studi di Firenze, Dip. Fisica e
Astronomia and INFN, Sezione di Firenze, Italy (e-mail: [email protected]).
A. Castoldi, C.Guazzoni and D. Mezza are with Politecnico di Milano,
Dip. Elettronica e Informazione and INFN, Sezione di Milano, Italy (e-mail:
[email protected],
[email protected],
[email protected]).
N. Grassi was with INFN, Sezione di Firenze, Italy. She is presently with
Dip. di Fisica Sperimentale, Universita’ degli Studi di Torino, Torino, Italy (email: [email protected])
R. Hartmann is with PNSensor GmbH, Munchen, Germany (e.mail:
[email protected]).
F. Taccetti is with INFN, Sezione di Firenze, Italy (e-mail:
[email protected]).
generated charge level and the maximum level of charge
injection can be sometimes a limiting factor in detector
characterization.
In this work we exploited the possibility of using a pulsed
mono-energetic proton beam as a diagnostic tool for the
characterization of semiconductor detectors at high charge
densities. By using a pulsed beam, single mono-energetic
protons can be generated which can be used to precisely probe
the detector-electronic system response with high spatial and
temporal resolution. Moreover by tuning the proton energy or
by increasing the number of protons per bunch it is possible to
probe different levels of charge density and/or different
ionization profiles across the detector depth.
As a case study we carried out the experimental
characterization of a Multi-Linear Silicon Drift Detector
(MLSDD) [1], a recent evolution of silicon drift detectors for
position-sensing in which the signal electrons generated by the
radiation interaction are confined within parallel drifting
columns (channels) and transported towards point-like anodes
by the electrostatic field. Using single protons with energies in
the range 1-5 MeV we investigated the drift time as a function
of drift coordinate, charge sharing between adjacent channels
and longitudinal broadening of the charge cloud. A dedicated
deconvolution method was also developed to extract the true
shape of the anode current pulses that allow further insight and
analysis of detector performance.
The paper is organized as follows. Section II describes the
pulsed beam-line DEFEL used during the beam time at
LABeC. Section III describes the experimental setup and the
MLSDD prototype used for the case study. Section IV is
divided in three different subsections showing the main
measurements results and their analysis. Conclusions and
future perspectives are summarized in Section V.
II. THE PULSED BEAM-LINE DEFEL
The proton beam comes from the DEFEL beam-line of the
Tandetron accelerator of LABeC (Laboratorio di Tecniche
Nucleari per i Beni Culturali), located in Sesto Fiorentino (FI),
Italy, that is equipped with a fast electrostatic chopper that
allows creating a pulsed proton beam. A sketch of the DEFEL
beam-line [2] is shown in Fig. 1.
The operating principle of the DEFEL beam-line consists of
deflecting the continuous beam coming from the accelerator
transversally across a slit allowing a bunch of protons (or ions)
to proceed downstream through an aperture for a short time
deep p implants HE phosphorous
implant
+
deep n implants
p field strips
p+ back electrode
HV
Fig. 1. Sketch of the pulsed proton beamline DEFEL (not to scale): S1, S2, S3
are motorized slits acting as collimators, RPs are rotating platforms, A1, A2,
A3 are tantalum apertures, V1,V2,V3 are high vacuum pumping station.
lapse (1 ns). The adjustment of the size of the aperture with a
motorized slit, and of the intensity of the continuous beam, is
the key feature to create a pulsed beam with a variable and
finely controllable number of particles in each pulse (down to
an average value much below 1 proton per trigger and with a
position of interaction in a defined area of down to
100 μm × 100 μm) with energies tunable in the range 1-6 MeV
in the case of protons, 1-8 MeV in the case of α particles and
1-14 MeV in the case of carbon ions. In this energy interval,
the proton range is perfectly matched with the typical wafer
thickness, namely proton range in silicon is 16 μm at 1 MeV
and 295 μm at 6 MeV, as shown in Fig. 2. The inset of Fig. 2
shows the corresponding ionization profiles [3]. The repetition
rate is selectable from a manual single shot up to hundreds of
kHz (limited by power dissipation effects on the drivers). The
end-station of the beam-line is equipped with a vacuum
chamber. It houses the detector under test mounted on X-Y
stages that allows the selection of the impact point of the ions
in the two directions orthogonal to the beam-line.
III. CASE STUDY AND MEASUREMENT METHOD
A. MLSDD for 2D Position Sensing
MLSDDs are special silicon drift detectors in which signal
electrons are confined within parallel drifting columns (or
channels) by means of a suitable combination of deep p- and
n-implants [2]. Fig. 3 shows a sketch of the architecture of a
typical MLSDD prototype.
The measurement of the drift time, i.e. the time interval
Fig. 2. Proton range in silicon for 1 up to 6 MeV and ionization profiles. The
error bars shows the longitudinal straggling. The inset shows the
corresponding ionization profiles.
protons
LV
y
E1
outn+1
z
outn
x
outn-1
anodes
reset FET
first FET
Fig. 3. Sketch of the architecture of a typical MLSDD prototype. Three
adjacent drift channels are shown together with some interaction points along
y.
between radiation interaction (trigger signal needed) and the
arrival of the signal charge at the collecting electrode,
provides one coordinate of interaction, while the orthogonal
coordinate is provided by the anodes segmentation.
The starting material of the prototype used in the present
tests is a detector-grade high-resistivity n-doped silicon
substrate (450 μm thick). An array of narrow p+ strips is
implanted on the front side (anode side) and is reverse biased
by means of an integrated resistive voltage divider in order to
fully deplete the detector volume. An array of deep p-implants
(channel stops) defines the border between adjacent drifting
channels (width of drift channels 200 μm). An array of deep nimplants (channel guides) located in the center of each drifting
channel has the two-fold effect of increasing the potential
barriers along the lateral direction i.e. perpendicular to the
drift (therefore of improving charge confinement) and of
enhancing electron drift velocity [4]. The presence of a highenergy uniform n-type implant allows setting the potential
minimum for electrons at a given depth from the surface.
Signal electrons, generated at a given detector depth
(depending on the energy of incident protons), are focused by
the component of the electric field, directed along the detector
depth (z), henceforth called depletion field, toward the
detector front surface, until they reach one of the drifting
channels. While being focused into the center of the drift
channel the signal electrons drift toward the anode owing to
the field component along the y direction (maximum drift
length 1.1 cm), henceforth called the drift field. Signal holes
are immediately collected by the p+ strip closest to the
interaction point. At a first level of approximation, the electron
cloud spread along x and y is approximately isotropic during
the initial motion across the thickness toward the drift
channels near the surface. This initial broadening process lasts
from few ns to tens of ns – according to the depth where the
interaction takes place and the depletion field.
Once the signal electrons reach the drift channels the
focusing process is terminated and the charge distribution is
then “frozen” into the drift channels. Electrons are transported
to the readout anodes without further broadening in the lateral
(x) direction, irrespective of the drifted distance, due to the
lateral potential barriers. Only longitudinal broadening (i.e.
along the y direction) continues to increase while electrons
move within the drift channel. At low levels of charge
injection (when electrostatic repulsion is negligible) the charge
cloud expansion, during the focusing process, is dominated by
thermal diffusion. At higher levels of charge injection, a
further and (in this case) dominant contribution to charge
broadening arises from electrostatic repulsion.
The MLSDD prototype used as case study features different
topologies of anodes, some with integrated JFETs (as shown
in Fig. 3), one (first FET) in source-follower configuration and
the other (reset FET) to restore the anode potential, and some
conventional collecting anodes connected to an external
charge preamplifier. The latter were used in these
measurements.
B. Experimental Setup
A photograph of the detector prototype mounted in the
DEFEL vacuum chamber is shown in Fig. 4. The detector
board is housed on a mechanical frame coupled to 2D motion
stages that can be controlled remotely. The mounting frame
houses also a diode detector for beam intensity adjustment and
a quartz glass for precise positioning and alignment of the
proton beam. An optical camera allows alignment of the
proton beam to reference markers on the quartz glass and on
the detector chip. Three anodes corresponding to three
adjacent drifting channels were instrumented by charge
preamplifiers. The beam was aligned to the middle of the
central instrumented channel, on the front side of the detector.
The central channel was irradiated with the proton beam at
different positions along the drift coordinate (as shown in
Fig. 3). Tested proton energies were 1, 3 and 5 MeV.
The waveforms (104 points) at the three preamplifier
outputs are digitized with a remotely controlled digital
oscilloscope (Tektronix DPO4104, 8 bit, 1 GHz analog
bandwidth, 2.5 Gs/s). The LOGIC trigger option was used to
set different thresholds for each channel and therefore acquire
only on occurrence of the desired logic pattern. The trigger
signal comes from the chopper of the beam-line which allows
precise synchronization of the acquired waveforms and,
therefore, the correct evaluation of the drift time of the charge
cloud.
IV. EXPERIMENTAL MEASUREMENTS
The proton beam was focused to a spot of approximately
100 μm × 100 μm on the detector surface and the detector
response was acquired at different drift coordinates (y), as
shown in Fig. 3. In the next subsections the results of three
types of measurements are discussed.
A. Drift Time as a function of Drift Coordinate
Figures 5a and 5b show the measured drift times at six
different incident coordinates of the proton beam along y at,
respectively, 3 and 5 MeV proton energy. The drift field is set
to 400 V/cm. The drift time for every interaction point is
obtained averaging over 142 preamplifier output waveforms,
and the corresponding standard deviation is computed. The
average electron drift velocity, derived from the linear fit on
all measured coordinates, is 0.475 cm/μs for 3 MeV protons
and 0.478 cm/μs for 5 MeV protons which shows the good
accuracy of the system. The small difference in the measured
values can be attributed to small variations in the detector
operating temperature that was monitored but not stabilized.
(a)
Fig. 4. Photograph of the experimental setup in the DEFEL vacuum
chamber.
(b)
Fig. 5. Average drift time (over 142 preamplifier’s output waveforms) as a
function of drift coordinate for 3 MeV protons (a) and 5 MeV protons (b).
The error bars show the standard deviation.
Fig. 6 shows the statistics of the measured drift times for
5 MeV protons at the each of the six drift coordinates.
B. Distribution of the amplitudes of the output waveforms
For the same proton energies used in the previous
subsection, at a fixed drift coordinate y = 5000 μm we
computed the distribution of the amplitudes of the collected
waveform in order to analyze the entity of charge sharing
between neighbor channels.
As an example, Fig. 7a shows the distribution of the
amplitudes collected at the central channel for 3 MeV protons.
Fig. 7b shows the same distribution for 5 MeV protons. The
low energy shoulder, well visible in both spectra, is due to
charge sharing between the central channel and its neighbor
channels due to the finite aperture of the beam profiling slits
(on the order of 100 μm) and to the broadening of the charge
cloud, which is larger at higher level of charge generation.
C. Longitudinal Broadening of the Charge Cloud
In order to evaluate the broadening of the charge cloud
along the longitudinal direction we developed a dedicated
algorithm able to extract the induced anode pulses from the
preamplifier outputs. The energy of the protons was set to
1 MeV and 10 different equally spaced longitudinal points
(along y) were acquired. The induced anode pulses must be
de-convolved from preamplifier output waveforms, Vout(t),
since
V out ( t ) =
+∞
∫I
in
(τ ) h ( t − τ ) d τ
problem that tends to be unstable in presence of noise which is
responsible for a large number of small eigenvalues for the
operator  . In other words we can say that the problem is
ill-conditioned. A way to solve this problem is to perform the
de-convolution in the time domain by using a conjugate
gradient based algorithm [5]. We start constructing the
functional F(Iin(t)) given by
(4)
F(I (t)) = R, R = Aˆ I (t) −V (t), Aˆ I (t) −V (t)
in
in
out
in
out
where R is called remainder and measures the norm between
the real output Vout(t) and the output obtained by reconvolution between the pulse response h(t) and the input
current Iin(t) obtained at a given iteration number (R is a
function of the iteration number). At each iteration the
conjugate gradient method searches within a functional space
the input waveform that minimizes the remainder R by
following the directions given by the major of the remaining
eigenvalues. It is therefore possible – provided an ad hoc
termination criterion is implemented – to stop iteration before
the directions given by the smaller eigenvalues (noise
eigenvalues) are explored. When the obtained input waveform
is re-convolved with the system impulse response, the
resulting curve will fit the measured output waveform ideally
with negligible noise. In other words the ill-conditioning of
(1)
0
where Iin(t) is the unknown anode current and h(t) is the pulse
response of the system. We can re-write Eq. (1) as an operator
equation
(2)
Aˆ I ( t ) = V ( t ) .
in
out
The numerical computation of Iin(t) is a typical inverse
(a)
Fig. 6. Time statistics for six different drift coordinates and proton energy
set to 5 MeV.
(b)
Fig. 7. Amplitude histogram for protons incident on central channel at
y = 5000 μm at 3 MeV (a) and 5 MeV (b).
the problem still remains but it does not affect the obtained
solution.
In order to solve Eq. (2) it is necessary to accurately
measure the pulse response, h(t), of the detector-electronic
system. This is not trivial, if done with the proton beam,
because charge broadening due to diffusion and Coulomb
repulsion and the related delays are already significant at this
generation level. In order to minimize collection time and to
approach the zero-charge pulse response, the proton beam was
aligned on the anode (y = 0) and the proton energy (1 MeV)
was further moderated by interposing a 12.5 μm thick
UPILEX layer in front of the detector. The reduction of the
deposited energy is however at the expense of a slightly worse
spatial resolution of the proton beam. Fig. 8 shows a set of deconvolved anode pulses at ten different injection points along
the drift channel. The shape of the anode pulses is strictly
connected with the longitudinal profile of the charge could
which can be studied in detail. We want to stress that no a
priori assumption about the shape of anode current is made.
Fig. 9 shows the rms spread of the charge cloud as a
function of pulse centroid (i.e. time of arrival), obtained by
least-square fitting of the anode pulses with a gaussian
function. In this preliminary study every point is averaged
over 10 measurements at the same incident coordinate. The
results show that for 1 MeV proton the charge cloud spread is
significantly larger than thermal diffusion; as expected,
Coulomb repulsion is the dominant effect at this level of
charge injection (277,778 electrons). Further analysis on a
larger set of data is currently under way.
Fig. 9. Longitudinal broadening of the charge cloud as a function of the
drift time (blue symbols) and thermal diffusion broadening (continuous red
line).
response with intrinsically calibrated charge levels. Moreover
the proton energy, finely controllable in the range 1-6 MeV
allows generating ionization profiles in silicon that range from
few μm (at 1 MeV) to about 300 μm (at 6 MeV). The output
waveforms of the detector channels are digitized in shape at
high speed and their time relationship is precisely preserved
thanks to the precise trigger signal coming from the
electrostatic chopper of the beam-line. A deconvolution
method to extract the induced current pulses at the anodes has
been developed to accurately study the dynamics of the mobile
carriers. In perspective, 2D mapping of the detector-electronic
response with micrometer resolution, high resolution in time
and energy over a wide range will be possible.
V. CONCLUSIONS AND PERSPECTIVES
The pulsed proton beam at the DEFEL beam-line of the
Tandetron accelerator in Sesto Fiorentino (Firenze, Italy) has
been evaluated as a tool to map the detector and electronic
system response at high charge levels. The possibility of
having single or multiple protons allows probing the detector
ACKNOWLEDGMENT
The authors wish to thank Marco Manetti of the LABeC
mechanical workshop for his effective support during the
beam time.
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Fig. 8. Deconvolved anode pulses for ten equally spaced drift coordinates
(spaced by 1080 µm).The proton energy is set to 1 MeV. The blue pulse on
the left refers to proton injection at y = 0 (anode) while the dark cyan pulse on
the right to proton injection at y = 9720 µm.
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