New X-Ray Detectors for Exotic Atom Research - MPG HLL

New X-Ray Detectors for Exotic Atom Research
Johann Marton, M. Cargnelli, T. Ishiwatari, P. Kienle, K. Nikolics, E. Widmann, J. Zmeskal, M. Bazzi,
M. Catitti, C. Curceanu (Petrascu), C. Guaraldo, M. Iliescu, P. Levi Sandri, V. Lucherini, S. Okada, D. Pietreanu,
A. Romero Vidal, A. Scordo, D. L. Sirghi, F. Sirghi, O. Vazquez Doce, G. Beer, L. Bombelli, C. Fiorini,
T. Frizzi, A. Longoni, A. M. Bragadireanu, T. Ponta, F. Ghio, B. Girolami, R. Hayano, H. Tatsuno, S. Xe-Hi,
M. Iwasaki, P. Lechner, H. Soltau, and L. Strüder
Abstract—Hadronic atoms being Coulomb-bound systems in
which the electron is substituted by a hadron like a negatively
charged K meson are perfect probes for the study of strong interaction
at lowest energies using x-ray spectroscopy. New large area
x-ray detectors (silicon drift detectors, SDDs) were developed to
provide excellent energy resolution as well as timing capability.
For the first time large area SDDs were employed to measure
the L-lines of kaonic helium at high precision thus succeeding in
clarifying a puzzle persisting for 30 years since the 70’s. An array
of about 200 even more sophisticated SDDs (1 cm SDDs with
monolithically integrated FET) will be used to perform precision
measurements (i.e., at the eV level) of kaonic hydrogen and kaonic
deuterium at the DAFNE factory of LNF/Italy. The new x-ray
detectors and their performance for precision experiments in the
accelerator environment are presented.
Index Terms—kaonic atoms, silicon drift detectors, strong interaction
studies, X-ray spectroscopy.
I. INTRODUCTION
Exotic atoms like mesonic (pionic, kaonic) or baryonic
(antiprotonic) atoms provide a rich research field for
studies of fundamental interactions and symmetries. The study
of the x rays emitted from kaonic atoms formed with hydrogen,
deuterium and helium isotopes are exceptionally interesting
because of the opportunity of investigating the low-energy
antikaon-nucleon interaction in electromagnetically bound
Manuscript received June 30, 2008; revised July 24, 2008. Current version
published June 17, 2009.
J. Marton, M. Cargnelli, T. Ishiwatari, P. Kienle, K. Nikolics, E. Widmann,
and J. Zmeskal are with the Stefan Meyer Institute of the Austrian Academy of
Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
M. Bazzi, M. Catitti, C. Curceanu (Petrascu), C. Guaraldo, M. Iliescu, P. L.
Sandri, V. Lucherini, S. Okada, D. Pietreanu, A. R. Vidal, A. Scordo, D. L.
Sirghi, F. Sirghi, and O. V. Doce are with the INFN, Laboratori Nazionali di
Frascati, Italy.
G. Beer is with the Department of Physics and Astronomy, University of Victoria,
Victoria, BC, Canada.
L. Bombelli, C. Fiorini, T. Frizzi, and A. Longoni are with the Dipartimento
di Elettronica e Informazione, Politecnico di Milano, Milan, Italy.
A. M. Bragadireanu and T. Ponta are with the IFIN-HH, Bucharest, Romania.
F. Ghio and B. Girolami are with the INFN Sez. di Roma I and Instituto
Superiore di Sanita, Rome, Italy.
R. Hayano, H. Tatsuno, and S. Xe-Hi are with the University of Tokyo, Tokyo,
Japan.
M. Iwasaki is with the RIKEN, the Institute of Physical and Chemical Research,
Muon Science Laboratory, Saitama, Japan.
P. Lechner and H. Soltau are with the PNSensors GmbH, Munich, Germany.
L. Strüder is with the MPI for Extraterrestrial Physics, Munich, Germany.
systems. Due to resonances like the elusive resonance—whose
nature is still to be explored in more detail—the
strong interaction antikaon-nucleon is neither simple nor well
understood. Many up-to-now unanswered questions are dating
back to the 60’s and are subject of intense studies in theory
and experiment at present. There are severe challenges: (i) The
kaon intensity is usually very low compared e.g., with pion
beams and at proton accelerators contaminations with pions
are present, (ii) x-ray detectors have to provide high detection
efficiency, excellent energy resolution and good background
rejection capability. In the past experiments lithium-drifted
silicon detectors (Si(Li)) [1], [2] or charge coupled devices
(CCDs) [3]–[5] were employed as x-ray detectors. Some years
ago small size (5–30 mm silicon-drift detectors (SDDs) [6],
[7]) but also large area SDDs for position sensitive charged
particle detection [8] and up to 1 cm large SDDs with external
field effect transistor (FET) for x ray-detection [9] were produced.
These detectors are based on the concept of sideward
depletion developed about 20 years ago by Gatti and Rehak
[10], [11]. Only recently large area SDDs were used in precision
studies of exotic atoms for the first time [12]. For new
exotic atom studies novel large area SDDs with an integrated
FET were developed [13], [14]. This paper describes the application
of this type of large-area SDDs in future high precision
experiments on kaonic hydrogen and kaonic deuterium atoms
[15], [16].
II. X-RAY SPECTROSCOPY OF HADRONIC ATOMS
In the case of kaonic hydrogen the strong interaction effect on
the 1s state (ground state) is detectable whereas the states with
principal quantum number are practically not influenced
(the effect of strong interaction is several orders of magnitude
smaller). The transition energies of the kaonic hydrogen Lyman
series s are in the range of 6–8 keV and the strong
interaction induced 1s state shift is in the order of 200 eV
[3].
Therefore, the observables of hadronic atom studies are the
shift in energy and the width of the x-ray transitions to
states influenced by the strong interaction. The strong interaction
shift in hydrogen is given below, where represents
the pure electromagnetic transition energy (which can be calculated
to a precision of about 1 eV by solving the Klein-Gordon
equation) and
the transition energy affected by the
strong interaction -p.
(1)

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TABLE I
THE PROPERTIES OF KAONIC HYDROGEN AND KAONIC DEUTERIUM ATOMS
ARE SHOWN. THE VALUES AND OF KAONIC DEUTERIUM ACCORDING
TO RECENT THEORETICAL STUDIES ARE GIVEN.
TABLE II
PROPERTIES OF X-RAY DETECTORS USED FOR EXOTIC ATOM RESEARCH:
LITHIUM DRIFTED SILICON SI(LI) DETECTOR, CCDS (TYPE CCD-55 USED IN
KAONIC ATOM RESEARCH BY DEAR) AND LARGE AREA SDDS (THIS WORK).
Fig. 1. The last stages of the electromagnetic transitions in kaonic hydrogen
are governed by x-ray transitions. Since only the 1s state is shifted due to the
strong interaction antikaon-nucleon from the energy of the Lyman transitions
( , ) the strong interaction shift can be determined by the deviation
from the pure electromagnetic transition energies.
Especially interesting systems are kaonic hydrogen and
kaonic deuterium atoms since they are the most simple kaonic
systems well accessible by theory. From the strong interaction
shift and width values of both systems the isospin-dependent
antikaon-nucleon scattering lengths can be extracted in a fairly
model independent way [17].
A. The Scientific Case: Kaonic Hydrogen and Helium Isotopes
In the following Table I the properties of simple kaonic atoms
are given. The required precision of the shift and width measurements
is at the level of percent corresponding to a precision level
of a few eV. According to the experience of the DEAR experiment
[3] even more important is the background rejection.
B. SDDs: New X-ray Detectors With Timing Capability
The electrons produced by an x-ray hit on a SDD are radially
drifting toward the anode in the center of the device. The
drift field is provided by concentric electrodes on one side. The
anode size itself is very small and independent of the detector
area. Therefore, one can produce large area SDDs with still
small detector capacitance and good energy resolution. In addition,
the small anode area makes it possible to reduce the
active layer thickness, while the capacitance is small. A thin
active layer reduces the continuum background caused by the
Compton process. There are 2 types of large area SDDs: (i)
SDDs with external first amplification transistor and (ii) SDDs
with the integration of the first amplification transistor (FET)
on chip. The first type was used in the experiment on kaonic
helium-4 [12] whereas the second type was developed for the
SIDDHARTA (Silicon Drift Detectors for Hadronic Atom Research
by Timing Application) experiment [19] on kaonic hydrogen/deuterium.
1) Design for Exotic Atom Research: Silicon drift detectors
for exotic atom research have to fulfill the following requirements
(i) timing capability with sufficient time resolution
s to suppress the asynchronous background efficiently,
(ii) high efficiency close to 100% up to about 15 keV due to
Fig. 2. Front side of the SDDs developed for the SIDDHARTA experiment.
3 SDDs with an active area (rounded quare) of 100 mm each are arranged on
one chip (A). Picture B shows the center of a SDD with the integrated transistor.
Picture C shows 2 SDD chips (2 3 SDDs) with ceramics and connector cables
mounted.
the sensitive layer thickness
, (iii) energy resolution
in the order of 150 eV at 6 keV (achievable with cooling
to ) to allow the precise extraction of the position and
width of theKlines, (iv) low noise due to the small anode with
low capacitance, (v) short shaping time s for high detection
rate capability, (vi) low internal electromagnetic background
compared with thicker (crystal) detectors, (vii) good energy
separation
between minimum ionizing particles
and photons, (viii) compact design for arrangements in
x-ray detector arrays.
In order to fulfill these requirements new large area SDDs
were developed for the SIDDHARTA experiment. The work
was done within a Joint-Research-Activity (JRA10) of the
Integrated Infrastructure Activity HadronPhysics of the 6th
framework of the European Community. These SDDs were
designed and produced by the Max-Planck Institute for Extraterrestic
Physics together with PNsensor Munich. The
bonding and glueing were performed by the Fraunhofer Institute
Berlin. Finally SMI/Vienna was responsible for the SDD
testing and assembling. In Fig. 2 details of the SIDDHARTA

3 / 5
TABLE I
THE PROPERTIES OF KAONIC HYDROGEN AND KAONIC DEUTERIUM ATOMS
ARE SHOWN. THE VALUES AND OF KAONIC DEUTERIUM ACCORDING
TO RECENT THEORETICAL STUDIES ARE GIVEN.
TABLE II
PROPERTIES OF X-RAY DETECTORS USED FOR EXOTIC ATOM RESEARCH:
LITHIUM DRIFTED SILICON SI(LI) DETECTOR, CCDS (TYPE CCD-55 USED IN
KAONIC ATOM RESEARCH BY DEAR) AND LARGE AREA SDDS (THIS WORK).
Fig. 1. The last stages of the electromagnetic transitions in kaonic hydrogen
are governed by x-ray transitions. Since only the 1s state is shifted due to the
strong interaction antikaon-nucleon from the energy of the Lyman transitions
( , ) the strong interaction shift can be determined by the deviation
from the pure electromagnetic transition energies.
Especially interesting systems are kaonic hydrogen and
kaonic deuterium atoms since they are the most simple kaonic
systems well accessible by theory. From the strong interaction
shift and width values of both systems the isospin-dependent
antikaon-nucleon scattering lengths can be extracted in a fairly
model independent way [17].
A. The Scientific Case: Kaonic Hydrogen and Helium Isotopes
In the following Table I the properties of simple kaonic atoms
are given. The required precision of the shift and width measurements
is at the level of percent corresponding to a precision level
of a few eV. According to the experience of the DEAR experiment
[3] even more important is the background rejection.
B. SDDs: New X-ray Detectors With Timing Capability
The electrons produced by an x-ray hit on a SDD are radially
drifting toward the anode in the center of the device. The
drift field is provided by concentric electrodes on one side. The
anode size itself is very small and independent of the detector
area. Therefore, one can produce large area SDDs with still
small detector capacitance and good energy resolution. In addition,
the small anode area makes it possible to reduce the
active layer thickness, while the capacitance is small. A thin
active layer reduces the continuum background caused by the
Compton process. There are 2 types of large area SDDs: (i)
SDDs with external first amplification transistor and (ii) SDDs
with the integration of the first amplification transistor (FET)
on chip. The first type was used in the experiment on kaonic
helium-4 [12] whereas the second type was developed for the
SIDDHARTA (Silicon Drift Detectors for Hadronic Atom Research
by Timing Application) experiment [19] on kaonic hydrogen/deuterium.
1) Design for Exotic Atom Research: Silicon drift detectors
for exotic atom research have to fulfill the following requirements
(i) timing capability with sufficient time resolution
s to suppress the asynchronous background efficiently,
(ii) high efficiency close to 100% up to about 15 keV due to
Fig. 2. Front side of the SDDs developed for the SIDDHARTA experiment.
3 SDDs with an active area (rounded quare) of 100 mm each are arranged on
one chip (A). Picture B shows the center of a SDD with the integrated transistor.
Picture C shows 2 SDD chips (2 3 SDDs) with ceramics and connector cables
mounted.
the sensitive layer thickness
, (iii) energy resolution
in the order of 150 eV at 6 keV (achievable with cooling
to ) to allow the precise extraction of the position and
width of theKlines, (iv) low noise due to the small anode with
low capacitance, (v) short shaping time s for high detection
rate capability, (vi) low internal electromagnetic background
compared with thicker (crystal) detectors, (vii) good energy
separation
between minimum ionizing particles
and photons, (viii) compact design for arrangements in
x-ray detector arrays.
In order to fulfill these requirements new large area SDDs
were developed for the SIDDHARTA experiment. The work
was done within a Joint-Research-Activity (JRA10) of the
Integrated Infrastructure Activity HadronPhysics of the 6th
framework of the European Community. These SDDs were
designed and produced by the Max-Planck Institute for Extraterrestic
Physics together with PNsensor Munich. The
bonding and glueing were performed by the Fraunhofer Institute
Berlin. Finally SMI/Vienna was responsible for the SDD
testing and assembling. In Fig. 2 details of the SIDDHARTA

4 / 5
Fig. 8. Energy spectrum of the Lyman transitions in kaonic hydrogen (background
components are removed) resulting from the DEAR experiment [3].
Fig. 6. Foto of the Day-1 setup of SIDDHARTA showing the cryogenic gas
target cell and the array of SDDs.
Fig. 9. Monte Carlo simulation of the energy spectrum of kaonic hydrogen to
be measured by the SIDDHARTA experiment. Compared with the DEAR experiment
the SIDDHARTA experiment will improve the signal-to-background
ratio by more than 2 orders of magnitude. This simulation for SIDDHARTA was
performed with an anticipated 5:1 signal-to-background ratio.
Fig. 7. In SIDDHARTA a triple coincidence will be applied for background
suppression (top). The light-weight crygenic gas target cell (bottom left) will be
surrounded by the SDD array (bottom right).
emitted charged kaon pair and the x ray is used for the suppression
of uncorrelated background events (see Fig. 7). Cryogenic
nitrogen is used as target gas filling because of the high x-ray
yield. The kaonic nitrogen x-ray transitions first measured by
the DEAR experiment will be used to study the background suppression
as well as the energy resolution.
In Fig. 7 below the main parts of the SIDDHARTA setup
are displayed. The cryogenic hydrogen gas target consists of
an aluminum grid structure with thin Kapton windows.
The target will be operated at a temperature of 22 K and a
working pressure of 2.5 bar. Up to 216 SDDs ordered in subunits
will surround the cryogenic gas cell. The final setup of SID-
DHARTA will be mounted in 2008. At the highest priority in
the SIDDHARTA experiment are precision measurements of the
x-ray spectra (K transitions) of kaonic hydrogen and kaonic deuterium
[27]–[31]. An integrated luminosity of about 400
is planned to be used in the kaonic hydrogen measurement.
The precision of the DEAR experiment was mainly limited
by the poor signal-to-background ratio (1:100).
This problem will be overcome with the SIDDHARTA setup.
Monte-Carlo simulations show the excellent performance of
SIDDHARTA (see Fig. 9)—in the case of kaonic hydrogen a
signal-to-background ratio of 5:1 is the goal of this experiment.
In the more difficult case of kaonic deuterium (the x-ray yield
is by far lower, see Table I) a signal-to-noise ratio of 1:1 is
anticipated.
From the measured shift and width of the 1s state in kaonic
hydrogen and deuterium the isospin-dependent scattering
lengths can be extracted with unprecedented accuracy. New
experimental information about the kaon-proton interaction
at threshold and the resonance—important for the
research on deeply bound kaonic systems—will be provided.
III. SUMMARY
The potential of new x-ray detectors for exotic atom research
was shown in an experiment at KEK for the first time. The

5 / 5
research on kaonic atoms provides insight in the low-energy antikaon-nucleon
interaction. Long-standing puzzles were solved
but precision experiments are highly requested to provide
high quality data for a clear decision on different theoretical
approaches. New experiments with large area SDDs are in
preparation at LNF and at J-PARC. New x-ray detectors were
developed which are anticipated to reach the goal of a precision
in the eV range—thus a breakthrough in the antikaon physics
at lowest energies is within reach.
ACKNOWLEDGMENT
This work is partly supported by the EU within Joint Research
Activity 10 of I3-HadronPhysics and Transnational Access to
Research Infrastructures Contract No.:RII3-CT-2004-506078.
Gratefully acknowledged is the important contribution to the
electronics by Giovanni Corradi and Diego Tagnani from LNF-
INFN.
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