Expanding the detection efficiency of silicon drift detectors - MPI HLL

Expanding the detection efficiency of silicon drift detectors
D.M. Schlosser a,
, P. Lechner a , G. Lutz a , A. Niculae b , H. Soltau a , L. Strüder c , R. Eckhardt a , K. Hermenau a ,
G. Schaller c , F. Schopper c , O. Jaritschin b , A. Liebel b , A. Simsek b , C. Fiorini d , A. Longoni d
a PNSensor GmbH, Römerstr. 28, 80803 München, Germany
b PNDetector GmbH, Otto-Hahn-Ring 6, 81739 München, Germany
c MPI Halbleiterlabor, Otto-Hahn-Ring 6, 81739 München, Germany
d Politecnico di Milano and INFN Sezione di Milano, Milano, Italy
article info
Available online 20 April 2010
Keywords:
Silicon drift detector (SDD)
X-ray detector
Low X-ray energy
Light elements
Gamma ray detection
Quantum efficiency
Scintillator
Hard X-ray
1. Introduction
abstract
Silicon Drift Detectors (SDDs), introduced in 1984 [1], are
nowadays being used in a rising number of different applications.
As a result of the continuous improvements in the detector
technology, the SDDs with integrated FET [2] fabricated by the
Semiconductor Laboratory of the Max-Planck-Institute together
with the company PNSensor have established themselves as stateof-the-art
detectors for Energy Dispersive X-ray (EDX) spectroscopy.
The working principle of a SDD is based on sideward depletion
[3]. The SDD geometry enables a low anode capacitance and the
monolitical integration of the first amplification step, a junction
gate field effect transistor (JFET), in the silicon. This geometry
reduces stray capacitances and avoids pick-up noise or microphony.
The reduction of the detector capacitance leads to a low
electronic noise, hence to an improved energy resolution, even at
high count rates.
The detector efficiency is an important feature of an EDX
system. It influences on the one hand the ultimate quality of the
measurement process and on the other hand the overall
measurement time which is of great importance in industrial
applications. There are three factors determining the detection
efficiency: detector key performance (energy resolution, P/B
ratio), detector quantum efficiency, detector area and geometry.
The influence of all three factors will be discussed in the
following.
Corresponding author.
E-mail address: dieter.schlosser@pnsensor.de (D.M. Schlosser).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2010.04.038
Nuclear Instruments and Methods in Physics Research A 624 (2010) 270–276
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in
Physics Research A
journal homepage: www.elsevier.com/locate/nima
To expand the detection efficiency Silicon Drift Detectors (SDDs) with various customized radiation
entrance windows, optimized detector areas and geometries have been developed. Optimum values for
energy resolution, peak to background ratio (P/B) and high count rate capability support the
development. Detailed results on sensors optimized for light element detection down to Boron or
even lower will be reported. New developments for detecting medium and high X-ray energies by
increasing the effective detector thickness will be presented. Gamma-ray detectors consisting of a SDD
coupled to scintillators like CsI(Tl) and LaBr3(Ce) have been examined. Results of the energy resolution
for the 137 Cs 662 keV line and the light yield (LY) of such detector systems will be reported.
& 2010 Elsevier B.V. All rights reserved.
Detector systems consisting of SDDs in combination with a
scintillator have also been developed and investigated as gamma
ray detectors with superior spatial and energy resolution for
gamma energies above 100 keV in comparison to photodiodes
and photomultipliers (PMTs) [4–7]. The advantages of SDDs as
photodetectors are the high quantum efficiency (QE), the low
electronic noise at moderate cooling temperatures ( 20 1C) and
their compactness.
In the next chapters the detector noise and the P/B ratio, the
light element, medium and hard X-ray detection capability and
the detection of gamma-rays will be discussed.
2. Improvement in detector noise and P/B ratio
For energy dispersive solid state detectors the main contributions
to the energy resolution, expressed as full width at half
maximum (FWHM) (Eq. (1)), is the noise due to the statistical
fluctuation in the number of electron hole (Neh) pairs created by
an incident X-ray, the so-called Fano noise (Eq. (2)), and the
electronics of detector and first amplification stage (Eq. (3)):
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
FWHMðEÞ¼2:35 e ENCFanoðEÞ 2 þENC 2
q
: ð1Þ
The Fano noise (Eq. (2)) depends on detector material properties
including F¼Fano factor and e ¼ mean energy to create an
electron hole pair and the E¼X-ray energy. It sets the lowest limit
for the detector energy resolution:
ENC 2
Fano
F
ðEÞ¼ E: ð2Þ
e
el

The three terms of the electronic noise (Eq. (3)) are describing
the contributions of the serial white noise, the ‘‘1/f’’ noise of the
integrated JFET and the shot noise associated to the leakage
current Il of the detector [8]. Additional noise sources from other
electronic components are neglected in the formula, respectively,
hidden in the factors A 1 A 3.
ENC 2
el
¼ 4kT
3gm
D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 270–276 271
C 2 totA1 1
t þ2paf C 2 totA2 þqIlA3t ð3Þ
with k is the Bolzmann constant, T the temperature, gm the
transconductance of the JFET, a f the constant parameterizing the
JFET ‘‘1/f’’ noise, q the elementary charge, t ¼ shaping time,
and A 1, A 2, A 3 the constants depending on the filter functions of
the shaper.
The serial white and 1/f noise strongly depend on the total
capacitance Ctot seen by the detector anode, which is a sum of the
anode and the gate-drain capacitance of the integrated JFET and
the depletion capacitances between the anode and the neighboring
regions [9] and other parasitic contributions if existent. The
parallel noise is mainly determined by the value of the leakage
current Il.
Fig. 1 shows the FWHM, calculated according to Eq. (1), in
dependence of the incident X-ray energy for two values of the
electronic noise, given in equivalent noise charge (ENC). The lower
limit of the FWHM is determined by the Fano noise (Eq. (2)), if
ENC el¼0 electrons. An increase of ENC el to 4, 10 electrons leads to a
higher relative increase of the FWHM especially at lower X-ray
energies. In the low energy regime the benefit of a low electronic
noise value on the energy resolution is the greatest.
Possibilities to improve the energy resolution are the reduction
of the total detector capacitance and the leakage current.
A reduction of C tot from 150 to 80 fF is achieved by moving
the anode and the integrated JFET from the center (standard SDD)
to the border of the SDD (droplet SDD¼SD 3 ) [10,11]. The JFET
located at the SDD border has a second positive effect. Irradiation
of the integrated JFET can be avoided by mounting an appropriate
collimator. Undesired background events can be reduced. This
circumstance leads to a higher P/B ratio [10]. Furthermore the
leakage current could be decreased, through a new fabrication
technology, poly-silicon, to a stable level down to 200 pA/cm 2 at
room temperature, reducing the energy resolution further.
The dependence of the energy resolution of a 10 mm 2 SD 3 on
the count rate is illustrated in Fig. 2. Energy resolution values
down to a full width at half maximum (FWHM) of 123 eV at
the Mn2Ka line have been currently measured at moderate
temperatures of T¼ 20 1C. The charge sensitive amplifier (CSA)
Fig. 1. Energy resolution against X-ray energy for different contributions of the
electronic noise.
Fig. 2. Count rate dependent energy resolution of the Mn Ka line measured with
a10mm 2 SD 3 . The energy resolution is about 123 eV at count rates of some kcps
and changes to about 125 eV for count rates up to 130 kcps.
Fig. 3. Superposition of spectra of Boron, Carbon and Oxygen measured with a
10 mm 2 SD 3 , with pn-Window at T¼ 20 1C. The FWHM of the B2Ka, C2Ka and
O2Ka lines are down to 38, 42 and 48 eV.
readout configuration in combination with the pulsed reset
operation mode ensures a nearly constant energy resolution up
to a few hundred kcps [9].
3. Optimizing the detector for light element performance
Apart from the detector noise, the detector entrance window is
important at low X-ray energies. A modified new detector
entrance window, pn-Window, has been developed for optimum
light element detection. The pn-Window reduces the loss of
generated electrons in the SDD p+ layer of the entrance window
after an interaction of a X-ray photon in this region, so that a
lower number of events with partial charge collection are
detected. This leads to an improved energy resolution measured
at lower X-ray energies. The spectra of boron and carbon
measured with a 10 mm 2 SD 3 detector are plotted in Fig. 3.
Energy resolution values of 38 eV for Boron line (138 eV) or 42 for
Carbon line (277 eV) have been determined.

272
The P/B and P/V (peak to valley) ratio of the SDDs with
pn-Window is improved by the same physical mechanism also for
higher X-ray energies e.g. Mn–Ka. This effect is shown in Fig. 4.
4. Optimization of area and geometry to increase the
detection efficiency for the fluorescent X-rays
D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 270–276
Fig. 4. Spectra of two 10 mm 2 SD 3 with a standard entrance window (EW) (gray)
and a optimized EW (black), while irradiating with a 55 Fe source. The optimization
of the P/B and P/V ratios by the pn-Window is obvious.
Fig. 5. Spectrum of a 20 mm 2 SD 3 with optimized pn-Window, while irradiating
with a 55 Fe source. The FWHM of the Mn2Ka line is 125 eV and a P/B ratio of up to
16 000 has been measured.
The optimization of the geometry of the detection area is
important for applications requiring maximum collection
efficiency of the incoming photons. This can be achieved by
maximizing the detection area, by matching the detector
geometry to the geometry of the system, as well as by maximizing
the detection solid angle for the incident radiation.
Fig. 5 shows the spectrum of 55 Fe measured with a droplet
shaped, SD 3 , detector, of which the detection area has been
increased to 20 mm 2 . The energy resolution at Mn2Ka line is
125 eV at T¼ 20 1C with a P/B ratio of 16 000. Prototypes of
30 mm 2 SD 3 have also been developed. They are still under
investigation.
Fig. 6 shows the FWHM at Mn2Ka of a 30 mm 2 circular SDD
against shaping time at two different temperatures. In Fig. 7 the
Fig. 6. FWHM at Mn Ka of a circular 30 mm 2 SDD vs. shaping time at 20 and
30 1C. At optimal shaping times of 2 or 3 ms FWHM of 129 eV have been
measured.
Fig. 7. FWHM at Mn2Ka of a square 100 mm 2 SDD vs. shaping time at 20, 25,
30 and 351C.
Fig. 8. Detector consisting of 6 100 mm 2 cells with an on chip collimator.
FWHM at Mn2Ka of a 100 mm 2 circular SDD for four
temperatures is plotted against the shaping time.
Increasing the detector area furthermore to several cm 2 by
maintaining the energy resolution and avoiding dead area is
possible with monolitical SDD arrays. Detectors with large areas
have been produced in that way [11]. The one presented here has
an area of 6 100 mm 2 , consisting of six cells. The energy
resolution of this detector is down to 140 eV at the Mn2Ka line
and moderate temperatures of 201 for each channel. Operation
is possible for input count rates of up to 3 Mcps. By means of an
adapted radiation entrance window, these detectors can also be
used in combination with scintillators for gamma ray detection
(Figs. 8 and 9).

D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 270–276 273
Fig. 10. Rococo 2 consists of four SDDs, with an area of 15 mm 2 each, a central
hole and a on chip collimator.
Fig. 11. Fluorescent X-rays from the sample detected by Rococo 2. The sample is
irradiated by an X-ray beam through the central hole of the SDD. A maximum solid
angle covered by Rococo 2 can be achieved by placing the sample as near as
possible to the detector.
Fig. 9. 55 Fe spectra of each channel of the six element SDD.
Fig. 12. SDD without a housing, electrically coupled by a flex lead, as a detector to
be integrated arbitrarily into analysis instruments e.g. electron microscopes,
where the space for the detector is limited.
cooling plate
ceramic
peltier element
SDD chip
Fig. 13. Example of a construction for the flexlead SDD shown in Fig. 12, which is
cooled in this case by a cooling plate and a peltier element.
Specific devices with a central hole for close arrangement of the
detector to the sample to ensure high collection efficiency of X-ray
fluorescence photons have been developed and tested (Fig. 10)
[12]. In such a geometry the detector covers a bigger solid angle
from the excitation point of the fluorescent X-rays on the sample,
so that the detected fraction of fluorescent X-rays is increased
(Fig. 11). Such detector consists of four SDDs, with an active area of
15 mm 2 each, on a single chip and is suitable in various systems
with X-rays and electrons as exciting beam (Fig. 11). Energy
resolution of the Mn Ka line down to 129 eV has been measured.
Besides the design of the sensors themselves the SDD
mounting is very flexible and accounts for the small amount of
space available in an electron microscope. The fact that for SDDs
with integrated FET an external bulky cooling mechanism is not
necessary to reach optimum performance becomes very important.
As demonstrated in Fig. 12 the detector can be built up in a
very slim version connected electrically by a small flex lead and
cooled by a peltier element, which is connected to the ceramic by
a cooling plate (Fig. 13). This detector architecture allows various
integration possibilities into electron microscopes.

274
5. Optimizing the detection efficiency for medium
and hard X-rays
To improve the detection efficiency for medium and high X-ray
energies the thickness of SDDs ð450 mmÞ has to be increased. This
can be done by using a thicker silicon substrate. A disadvantage
of using SDDs with thicker substrates is besides the difficulty
in starting a new silicon production line the increase of leakage
current. To reach the same electronic noise level and energy
variance the operation temperature has to be decreased.
A new detector arrangement is avoiding these difficulties and
offering new opportunities: It consists of a stack of two SDD
detector chips (Fig. 14) leading to an overall thickness of 900 mm.
The thermal coupling of the SDD chips to the peltier cooler is
good, giving rise to a negligible temperature difference between
the two chips and a typical operation temperature of 20 1C only.
The spectra of 109 Cd + 55 Fetakenwiththe2-layerSDDareshown
in Fig. 15. Opportunities of the 2-layer SDD are obvious: on the one
side the good performance for X-ray energies Eo10 keV remains
unchanged compared with 450 mm thick SDDs, on the other side
trace contaminations of heavy elements can be easily distinguished.
The calculated QE of the 2-layer SDD compared to the SDD with the
standard thickness is plotted in Fig. 16. For energies beyond the
Ag K a line it is increased by almost a factor of two. The ratio of
the theoretical QEs of a standard SDD and a 2-layer SDD is
QE ð450 mmÞ
¼ 0:56 ðFig:16Þ: ð4Þ
QE ð900 mmÞ
ceramic 1
ceramic 2
D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 270–276
Peltier
SDD chip 1
SDD chip 2
Fig. 14. Drawing of the two layer SDD, with a total thickness of 900 mm.
Fig. 15. 109 Cd + 55 Fe spectra of the two channels of the 2-layer SDD shown
in Fig. 14.
Fig. 16. Calculated quantum efficiency (QE) of a 450 and 900 mm thick SDD.
The ratio of the measured counts with the first SDD layer to the total
measured counts at the Ag K a line is
counts layer 1
¼ 0:57 ð5Þ
counts layer 1þcounts layer 2
which is in good agreement with the theoretical value.
6. Extension of the energy range of SDDs into the gamma
range
Silicon of thicknesses up to several millimeters is transparent
for g-rays with energies E4100 keV. Gamma rays of such
energies can be detected with a detector consisting of a SDD
coupled to a scintillator, where the SDD is used as a photodetector
for the scintillation light (Fig. 17).
Fig. 17. CsI(Tl) wrapped into a reflector and coupled to a SDD.
The formula of the relative energy resolution RðEgÞ
ðFWHMðEgÞ=EgÞ of such a detector system is given in Eq. (6). ENCel
is the equivalent noise charge of the SDD and the electronics, Neh
are the generated electron hole pairs in the SDD, Rintr is the
intrinsic resolution of the gamma detector and Eg the energy of
the g-photons.
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ENC
RðEgÞ¼2:35
2
el
þ
ðEgÞ 1
NehðEgÞ þR2 intrðEgÞ s
: ð6Þ
N 2
eh
For a good energy resolution as many of the following criteria as
possible have to be fulfilled. The SDD entrance window should be
optimized to maximize scintillation photon detection, thus
maximizing the number of generated e–h pairs. The electronic
noise, ENCel, should be as low as possible. The scintillation decay
time (Table 1) should be faster than the optimal shaping time for
the SDD and the drift time of the electrons in the SDD to the anode
to ensure the use of the optimum shaping time for minimum
ENCel and a high count rate capability. A high light yield (LY) of

D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 270–276 275
the scintillator (Table 1) is desirable and a low intrinsic resolution,
Rintr, of the gamma detector improves the energy resolution in
addition. For compact scintillators, crystals with high densities are
needed to minimize the volume by keeping the stopping power
for gamma rays.
Two scintillators with high LYs have been investigated, CsI(Tl)
and LaBr3(Ce), manufactured by Scionix Netherlands and Saint
Gobain Crystals. The LY of CsI(Tl) is about 54 photons/keV and
of LaBr3(Ce) 63 photons/keV with a maximum of the scintillation
light spectrum at 550 nm for CsI(Tl) and 380 nm for LaBr 3(Ce).
The scintillation decay times of CsI(Tl) are 600 and 3400 ns and of
LaBr 3(Ce) 16 ns. In Table 1 the values of the refraction index and
density of the scintillators are also listed.
Fig. 18 shows the calculated quantum efficiencies (QEs) of two
SDD entrance windows for photons with a propagation direction
perpendicular to the entrance window. One entrance window has
been optimized for photons with wavelengths around 400 nm and
the other for photons around 550 nm to fit the scintillation photon
spectra.
In Fig. 19 the N eh pairs per keV, which are generated in the
SDD, are plotted against the shaping time. The LY has been
determined from the pulse hight of the 662 keV 137 Cs line.
Because of the slow scintillation decay times of CsI(Tl) (Table 1),
long shaping times of some ms ðt shaping 410 msÞ are needed to
minimize ballistic deficit, which leads to a higher contribution of
the leakage current to the electronic noise. In contrast the optimal
shaping time of a SDD with minimal noise is around 0.5 and 1 ms.
This corresponds better to LaBr 3(Ce). Fig. 19 shows that already at
1 ms all charges are collected. The lower number of eh-pairs when
measuring with LaBr 3(Ce) compared to CsI(Tl) is mainly caused by
the loss of photons on their way out of the vacuum sealed
LaBr 3(Ce) housing into the SDD. This loss of photons can be
Table 1
Properties of CsI(Tl) and LaBr 3(Ce) given by Scionix Netherlands and S. Gobain
Crystals: LY scintillator light yield, tdecay decay time of scintillation light, lmax
maximum of scintillation spectrum, n refraction index at lmax, r density.
Scintillator LY (photons/keV) t decay (ns) lmax (nm) n r (g/cm 3 )
CsI(Tl) 54 600; 3400 550 1.79 4.51
LaBr 3(Ce) 63 16 380 1.9 5.08
Fig. 18. The QEs of two SDD EWs for photons, which propagate perpendicular
towards the EW coming from the vacuum, are plotted against the photon
wavelength (continuous lines). The EWs have been optimized for photons with
wavelengths around 400 or 550 nm. These wavelength ranges fit to the
scintillation spectra of CsI(Tl) and LaBr 3(Ce) (dashed lines).
Fig. 19. Dependence of the LY of the gamma detector, SDD + LaBr 3(Ce) or CsI(Tl),
on the shaping time.
Fig. 20. Spectra of gamma detectors consisting of SDD coupled to LaBr 3(Ce) or to
CsI(Tl) scintillators, while irradiating with a 137 Cs source.
reduced by coupling the scintillator LaBr 3(Ce) directly onto the
SDD. Commercially available LaBr3(Ce) scintillators are normally
packed into vacuum sealed housings, because LaBr 3(Ce) is
hygroscopic. Unfortunately it has been not possible for us to
receive an unpacked LaBr 3(Ce) scintillator yet.
The spectra in Fig. 20 have been measured with a detector
consisting of a SDD, coupled to a cylindrical shaped CsI(Tl) (gray
curve) or LaBr3(Ce) (black curve) scintillator, while irradiating
with a 137 Cs source. Both scintillators are wrapped into reflectors.
The energy resolution of the 662 keV 137 Cs line down to 2.8% has
been measured with LaBr 3(Ce) or 4.3% with CsI(Tl). In spite of a
lower number of optical photons entering the SDD, if the SDD is
coupled to LaBr 3(Ce) (Fig. 19) the energy resolution of the 662 keV
137 Cs line is better compared to a detector consisting of SDD +
CsI(Tl), because of a better intrinsic resolution of LaBr 3(Ce) at
Eg ¼ 662 keV and a lower electronic noise contribution, ENCel, due
to a shorter shaping time of 0:5 ms (LaBr 3(Ce)) compared to 10 ms
(CsI(Tl)).
The energy resolution strongly depends on the scintillator
quality. For a gamma detector consisting of LaBr3(Ce) coupled to a
photomultiplier (PMT) or a large area avalanche photo diode
(LAAPD) energy resolution values of 2.7% or 3.1% have been

276
measured for the 662 keV 137 Cs line [13]. The readout of the light
pulses generated in LaBr3(Ce) with SDDs lead to a better energy
resolution above gamma energies of 100 keV compared to the
readout of the same LaBr3(Ce) scintillator with PMTs, LAAPDs or
photo diodes (PDs) [6]. In case that CsI(Tl) is coupled to a PMT
values of 5.2% have been measured for the 662 keV 137 Cs line [14].
Gamma detectors consisting of CsI(Tl) or LaBr3(Ce) coupled to
SDDs with an adapted entrance window for scintillation photons
achieve similar or superior energy resolutions for the 662 keV
137
Cs line compared to gamma detectors consisting of CsI(Tl) or
LaBr3(Ce) coupled to PMTs, LAAPDs or PDs.
7. Summary and conclusions
D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 270–276
Various possibilities for the enhancement of the detection
efficiency of SDDs have been presented. SDDs with optimum
energy resolution and high count rate capability build up the
baseline.
Improvement of the radiation entrance window leads to a
reduction of partial events, resulting in excellent light element
performance and shifting the detection efficiency to low energies.
Large SDDs with active areas up to 600 mm 2 improve the
detection efficiency for specific applications as TXRF or synchrotron
needs. New fascinating SDD geometries as the SDD with the
hole in the middle for the exciting beam or as the distributed
element systems arranging SDDs via flex lead architecture almost
arbitrarily into the analytical instruments allow much higher
detection efficiency as in the past and will have a strong impact
on the analysis methods especially in micro analysis.
A double stage 1 mm thick SDD shows enhanced detection
efficiency for medium X-rays as Ag with undisturbed energy
resolution.
Detectors consisting of a SDD, with optimized entrance
window for scintillation light, coupled to scintillators, LaBr3(Ce)
and CsI(Tl), for gamma ray detection have been investigated for
gamma spectroscopy and results of the energy resolution for the
137
Cs line, which belong to the best resolution measured yet, have
been tabled.
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