Prototype double-sided silicon strip detectors (DSSD\(s\)) of  300 \(\mu \)m thickness produced at PTI St. Petersburg (Russia) were tested  for the use as position sensitive, \(\Delta E\) and E detectors for tracking  and particle identification in the EXL (EXotic nuclei studied in Light-ion  induced reactions at the NESR storage ring) setup at the FAIR (Facility for  Antiproton and Ion Research) project at GSI. We describe the characteristics  of detectors with \(16 \times 16,\;64\times 64\) and \(64\times 16\) strips, respectively. The response  of these detectors for \(^{241}\)Am \(\alpha\) particles injected either from  the p or n side was examined. The test measurements were performed partially  at GSI and the University of Edinburgh. A first in-beam test with a proton  beam of 50 MeV with the latter two DSSDs and two 6.5 mm thick Si(Li)  detectors was also done at KVI Groningen, the Netherlands. The results  reveal good spectroscopic properties of these detectors

Borger, R.

Chung, Le Xuan

Davinson, T.

Egelhof, P.

Eremin, V.

Ilieva, S.

Kalantar-Nayestanaki, N.

Khai, Nguyen Tuan

Mutterer, M.

Najafi, A.

Rigollet, C.

Streicher, B.

Von Schmid, M.

Woods, P. J.

English

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 22, No. 3 (2012), pp. 263-273
PROTOTYPING OF DSSDS FOR PARTICLE TRACKING AND
SPECTROSCOPY WITHIN THE EXL PROJECT AT FAIR
L. X. CHUNG
Institute for Nuclear Science and Technology
and
GSI, Helmholtzzentrum fu¨r Schwerionenforschung GmbH, Darmstadt, Germany
N. T. KHAI
Institute for Nuclear Science and Technology
P. EGELHOF, S. ILIEVA, AND B. STREICHER
GSI, Helmholtzzentrum fu¨r Schwerionenforschung GmbH, Darmstadt, Germany
R. BORGER, C. RIGOLLET, N. KALANTAR-NAYESTANAKI, AND A. NAJAFI
KVI, University of Groningen, the Netherlands
T. DAVINSON AND P. J. WOODS
University of Edinburgh, United Kingdom
V. EREMIN
PTI, Ioffe Physical Technical Institute of Russian Academy of Sciences,
St. Petersburg, Russia
M. MUTTERER
GSI, Helmholtzzentrum fu¨r Schwerionenforschung GmbH, Darmstadt, Germany
and
Institut fu¨r Kernphysik, Technische Universita¨t Darmstadt, Darmstadt, Germany
M. VON SCHMID
Institut fu¨r Kernphysik, Technische Universita¨t Darmstadt, Darmstadt, Germany
Abstract. Prototype double-sided silicon strip detectors (DSSDs) of 300 µm thickness produced
at PTI St. Petersburg (Russia) were tested for the use as position sensitive, ∆E and E detectors
for tracking and particle identification in the EXL (EXotic nuclei studied in Light-ion induced
reactions at the NESR storage ring) setup at the FAIR (Facility for Antiproton and Ion Research)
project at GSI. We describe the characteristics of detectors with 16×16, 64×64 and 64×16 strips,
respectively. The response of these detectors for 241Am α particles injected either from the p or
n side was examined. The test measurements were performed partially at GSI and the University
of Edinburgh. A first in-beam test with a proton beam of 50 MeV with the latter two DSSDs and
two 6.5 mm thick Si(Li) detectors was also done at KVI Groningen, the Netherlands. The results
reveal good spectroscopic properties of these detectors.
264 PROTOTYPING OF DSSDs FOR PARTICLE TRACKING AND SPECTROSCOPY ...
I. INTRODUCTION
The objective of the EXL-project, which is part of the NUSTAR (Nuclear STructure,
Astrophysics and Reactions) program at FAIR [1], is to capitalize on light-ion induced
direct reactions in inverse kinematics using novel storage ring techniques. For the key
physics issues addressed in [2], a complex detection system must satisfy the demand of a
highly efficient, high-resolution universal system and to be applicable to a wide class of
reactions.
Double-sided silicon detectors are a major part of the EXL silicon particle array
(ESPA) [2] which, surrounding a gas-jet or hydrogen droplet target, will be used as a po-
sition sensitive integrated ∆E-E detector array. It is dedicated to light recoil particle (p,
alpha) detection in the wide energy range from about 100 keV to nearly 200 MeV. These
detectors should have low detection threshold, high energy resolution and detection effi-
ciency and, possibly, pulse shape discrimination (PSD) capability for particle identification
at low energies.
This paper presents first results on the development of spectroscopic DSSDs for the
EXL experiment which have proper design: moderate pitch size, large strip width, small
interstrip gap and a thin entrance p-side window in order to reach the above requirements.
The detectors were produced in PTI St. Petersburg (Russia) under INTAS-GSI project
#06-1000012-8844.
II. DETECTORS
The prototype DSSDs studied have a thickness of 300 µm and 16×16, 64×64 and
64×16 strips. The smallest sensor has 16 strips at each side and is 7.1×7.1 mm2 in area.
On the p-side, the widths of strips and interstrip gaps are 285 µm and 15 µm, respectively,
while these values are 235 µm and 65 µm on the n-side. Thus, on both sides a 300 µm
wide pitch is formed. There are 64 strips at each side of the bigger 64×64 DSSD at an area
of 21.2×21.2 mm2. The sensor strips have the same configuration as in the small detector.
The second big detector tested is the 64×16 DSSD which has the same outer dimension
as the 64×64 DSSD. The only difference between them is that at the n-side there are 16
strips with a pitch size of 1200 µm formed by 1135 µm wide strips and 65 µm interstrip
gaps. The characteristic features of the three DSSDs are summarized in table 1.
Fig. 1 is a schematic view of the structures of both DSSD faces processed on to
a 300 µm thick n-bulk wafer. On the front side (p-side) of all DSSDs, the strips are
separated by a gap passivated by SiO2 layer and, besides an approximately 400 nm thick
p+ implantation layer windowless. The electric contact is realized through a 800 nm
thick and 17.5 µm wide aluminum frame. The strips at the rear side (n-side) are entirely
covered with aluminum. The isolation between them is established by a 450 nm thick p+
implantation. There are 5 guard rings surrounding the DSSDs. The bias voltage is applied
to the innermost ring serving as bias ring (punch-through biasing).
III. MEASUREMENT AND DATA ANALYSIS
In the EXL experiment particles will always impact perpendicularly on the surface
of the DSSDs and be injected from the p-side. As mentioned above, the DSSDs will be
L. X. CHUNG et al. 265
Table 1. DSSD characteristic features (in µm)
DSSD
p-side n-side
Strip Gap Pitch Strip Gap Pitch
16×16 285 15 300 235 65 300
64×64 285 15 300 235 65 300
64×16 285 15 300 1135 65 1200
Fig. 1. P-side and n-side cross section of 16×16 DSSD (n-side strips are rotated
by 90o for illustration)
dedicated to light particle detection with energy range spreading from about 100 keV to
nearly 200 MeV. Therefore, in the test experiment, an alpha source and a 50 MeV proton
beam were used.
The 241Am-α particles with 5.5 MeV energy are stopped in the detector at a pen-
etration depth of 27 µm. This low-energy particles allowed to explore the DSSD wafer’s
structure and the influence in charge collection towards strips both, with p-side and n-side
injection. Because of the current limitation in read-out electronics to 32 channels only
the 16×16 DSSDs and the 64×16 DSSDs with 4 adjacent strips coupled together at the
64-strip side were tested at GSI. The 64×64 DSSD test was performed at the University
of Edinburgh where electronics equipped with 128 read-out channels was available.
Fig. 2 displays the electronic scheme of the test experiments at GSI. The total
depletion voltage was established at -50 V, but an overbias of up to -200 V was applied to
the p-side with AC- coupled preamps. The ground potential was connected to the n-side
through DC-coupled preamps. The signals of the strips on the front side were connected
to channel 1 to 16, and the rest (17-32 channels) was used for the strip signals from the
rear side. The DAQ recorded events only when the energy signal was coincident with the
gate produced as OR of all timing signals from front side. This trigger was kept in both
p- and n-side injection measurement discussed in the next section.
A second in-beam test stage was done with a 50 MeV proton beam at KVI Gronin-
gen. The 64×64 and 64×16 DSSDs were chosen. The experimental setup is shown in
266 PROTOTYPING OF DSSDs FOR PARTICLE TRACKING AND SPECTROSCOPY ...
Fig. 2. The electronic scheme of GSI test experiments
Fig. 3. KVI experimental set up, two DSSDs and two Si(Li)s were placed in
vacuum chamber.
Fig. 3. The protons punched through both DSSDs and were stopped in the second Si(Li)
detector [3].
IV. RESULTS AND DISCUSSION
IV.1. Tests with alpha source
Fig. 4 shows the peaks of the alpha source. With p-side injection, the FWHMs
for 5.5 MeV α particles were between 14 keV and 26 keV for both, p- and n-side energy
spectra of the small detector (panel a and b), dependent on electronic settings. The five
energies of 241Am are separated. In contrast, these energies are no longer resolved in the
case of n-side injection (panel c and d). The energy resolution is about 50 keV due to the
aluminum metallic layer at the entrance of the n-side strips (Fig. 1).
L. X. CHUNG et al. 267
Fig. 4. Energy spectrum of 16×16 DSSD. a) and b): p-side injection, c) and d):
n-side injection
A characteristic energy spectrum is displayed in Fig. 5 which is taken from strip
#5 at the p-side. The spectrum consists of 3 regions: main peaks (of 241Am source),
shifted peaks (energy lost by metallic frame, by the partial overlap of bonding pads or by
inducing positive signal in adjacent strips) and the continuous background region due to
interstrip events. Each strip energy spectrum consists of the contributions from all event
types discussed below.
The performance of the 64×64 DSSD measured at the University of Edinburgh with
different electronics was very similar to that of the 16×16 DSSD. Measured energy reso-
lutions were 29 and 32 keV for energy spectra of the strips on p- and n-side, respectively.
High efficiency is an important requirement for DSSDs in the EXL setup. To deter-
mine this information, we plotted a correlation between the energy signals recorded on p-
and n-side strips, respectively. Then, the scatter plot (Fig. 6) was divided into 4 regions
corresponding to 4 event types: (1) strip-strip, (2) strip-interstrip, (3) interstrip-strip and
(4) interstrip-interstrip. Where, ‘strip’ means the signal was created by the incident parti-
cle only on one strip, while ‘interstrip’ means the signal was shared between 2 neighboring
strips. The DSSDs’ efficiency is tabulated in table 2. This gives the relative efficiency of
each event type in the total detected event.
268 PROTOTYPING OF DSSDs FOR PARTICLE TRACKING AND SPECTROSCOPY ...
Mainpeaks
Background
(interstrip events)
Shifted peaks
p-side strip # 5
Fig. 5. Characteristic energy spectrum of 16×16 DSSD, p-side injection
(2)
(3)
(1)
(4)
(4)
Fig. 6. 2-D energy signal correlation between p- and n-side, p-side in jection
One possible explanation for the bending in region (3) in Fig. 6 is a certain re-
combination and trapping probability of electrons in the interstrip region due to positive
charges created at the Si-SiO2 interface. Consequently, the amount of collected charges
toward the n-side is reduced. In addition, these positive charges would create locally a
field inversion which would modify charge collection in the interstrip zone and make the
situation dependent on particle penetration depth [4].
Table 2. DSSD Efficiency (%), 5.5 MeV alpha particles
Event type 16×16 *64×16
(1) 76.8 93.9
(2) 18.2 6.1
(3) 4.1 ∼ 0.0
(4) 0.9 ∼ 0.0
L. X. CHUNG et al. 269
Fig. 7. P- and n- side energy spectra of 4 event types, detail is explained in text.
270 PROTOTYPING OF DSSDs FOR PARTICLE TRACKING AND SPECTROSCOPY ...
The front and rear side energy spectra of all event types are shown in Fig. 7. Panels
a, b, e and g are the energy spectra of events induced signal in only one strip in one DSSD
side. They contribute to the strip spectrum (Fig. 5) as the main and shifted peak regions.
Except panel g, panels a, b and e show good energy resolution (FWHM=14 ÷26 keV).
The main peaks of the α-source are separated. The rest panels in Fig. 7 present the
energy spectra of events which gave energy sharing between 2 adjacent strips. In the strip
spectrum, these appear as the continuous background region. The correlation of sharing
energy E1 and E2, where E1 and E2 are the energy signals of 2 neighboring strips at one
side of the DSSD, is plotted in Fig. 8. Comparing with Fig. 11 in [4], because our ADCs
were not able to measure signals with both polarities, we couldn’t see (in panel a) the parts
of the signals with a reduction of total energy and of inverse signals in adjacent strips. In
addition, since our DSSD’s interstrip gap is small (15 µm) the energy correlation of the
detector entrance face strips does possibly not consist of the parts related with particles
impinging on the region of field inversion. Panel b shows a very similar pattern of the
energy correlation of exit face strips to Fig. 13 in [4]. The reconstructed energy spectra
(E1+E2) in p- and n-side are displayed in Fig. 9.
Fig. 8. E1, E2 correlation, p-side injection
The original energy of the incident particles when signals are induced at 2 adjacent
strips is equivalent to the sum of E1 and E2. Except the outermost ones, each strip has
two neighbors. The percentages of interstrip events were calculated by half the ratio of
interstrip and strip count rates. The results were approximately 5 % and 17 % for strips
at the p- and n-side, respectively (Fig. 10.a), consistent with the geometry of the 16×16
DSSD (Table 1). For n-side injection, the results were about 21 % and 0.3 % (Fig. 10.b).
The response of the DSSDs with rear side injection is not fully understood and still needs
more investigation.
IV.2. Test with proton beam
The second test phase of the DSSDs was done in an EXL demonstrator experiment
at KVI Groningen with a 50 MeV proton beam [5]. The experimental setup is shown in
Fig. 3. It was the aim to study the DSSD response for punching through high-energy
particles and to check the quality of total energy reconstruction of a telescope setup.
L. X. CHUNG et al. 271
FWHM=116keVFWHM=274 keV
Fig. 9. E1+E2 spectra, p-side injection
Fig. 10. Percentage of interstrip events of the 16×16 DSSD
The response of the DSSDs to punching through proton is shown in Fig. 11. The
peaks are located at about 700 keV with ∼ 200 keV FWHM for strip energy spectra of
64×64 and 64×16 DSSDs.
The same 2-D energy signal correlation method as shown in Fig. 6 was used to
determine the DSSD efficiency (Fig. 12). The results for the efficiencies are given in table
3. The reconstructed energy of the beam protons is summed up from the energy losses in
the 2 DSSDs and the first Si(Li) and the residual energy in the second Si(Li). The FWHM
for about 45.2 MeV total energy was determined to be 396 keV (Fig. 13).
Table 3. DSSD Efficiency (%), 50 MeV proton beam
Event type 64×64 64×16
(1) 91.3 82.6
(2) 3.1 15.0
(3) 3.3 2.4
(4) 2.3 ∼ 0.0
272 PROTOTYPING OF DSSDs FOR PARTICLE TRACKING AND SPECTROSCOPY ...
Fig. 11. Energy spectra of the DSSDs. 64×64 DSSD p-side spectrum - top left,
n-side spectrum - top right; 64×16 DSSD p-side spectrum – bottom left; n-side
pectrum - bottom right.
(1): strip-strip
Fig. 12. 2-D energy signal correlation be-
tween p- and n-side of 64×64 DSSD
Fig. 13. Energy reconstruction for single pro-
ton events
V. SUMMARY AND OUTLOOK
Within an R&D program using prototype detectors for the EXL project, several
DSSDs were tested with α particle injected from both sides. With p-side injection, the
result shows good spectroscopic properties of the detectors. The best resolution is 14 keV
FWHM for the p-side strip response of 5.5 MeV alpha particle energy. The efficiency
L. X. CHUNG et al. 273
of the DSSDs is good with the main contribution from strip-strip events. The interstrip
behavior conforms with the detector geometry. The DSSD performance with n-side in-
jection becomes worse and this case still needs more investigation. For a demonstrator
detector telescope consisting of DSSDs and Si(Li)s bombarded with 50 MeV protons the
total energy was reconstructed with 396 keV FWHM resolution which is already close to
the designed value. The possibility of pulse shape discrimination with the DSSDs has been
investigated [6]. Also, the larger 64×64 DSSD chips will be tested for their capability of
operation in the ultra-high vacuum environment of a storage ring.
ACKNOWLEDGMENTS
The authors L. X. Chung and N. T. Khai would like to thank Vietnam National
Foundation for Science and Technology Development (NAFOSTED) for the support under
grant number 103.01-2011.17.
REFERENCES
[1] Facility for Antiproton and Ion Research, http://www.fair-center.com.
[2] http://www-linux.gsi.de/∼wwwnusta/tech report/05-exl.pdf.
[3] http://www.fair-center.de/Publikationen.171.0.html?&0=&L=1.
[4] http://www.gsi.de/informationen/wti/library/scientificreport2006/PAPERS/FAIR-
EXPERIMENTS-31.pdf
[5] Y. Blumenfeld et al., Nucl. Instr. and Meth. in Phys. Res. A421 (1999) 471.
[6] B. Streicher et al., GSI Scientific Report 2009, p 36.
[7] M. von Schmid et al., Nucl. Instr. and Meth. in Phys. Res. A629 (2011) 197-201
Received 14 March 2012.


Prototyping of DSSDs for Particle Tracking and Spectroscopy within the EXL Project at Fair

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