Gamma-ray Spectroscopy with LaBr3:Ce Scintillator ... - MPG HLL

Gamma-ray Spectroscopy with LaBr 3 :Ce Scintillator
Readout by a Silicon Drift Detector
C. Fiorini, member, IEEE, A. Gola, M. Zanchi, A. Longoni, P. Lechner, H. Soltau, L. Strüder
Abstract– In this work we propose a gamma-ray spectrometer
based on a LaBr 3 :Ce scintillator coupled to a Silicon Drift
Detector (SDD). The SDD is a photodetector characterized by a
very low noise thanks to the low value of output capacitance
independent from the active area. With respect to a PMT, the
SDD offers a higher quantum efficiency which reduces the
spread associated to the statistic of photoelectrons generation.
Also with respect to an APD, the SDD offers a lower
photoelectrons statistic contribution, which, in the APD, is
worsened by the excess noise factor with respect to pure Poisson
statistics. Moreover, the SDD has a stable behaviour, less
sensitive to temperature and bias drift. In the past years, good
energy resolutions were measured using a SDD coupled to a
CsI:Tl crystal. However, the long shaping time, to be used with
this scintillator to prevent ballistic deficit, was too far to exploit
the best noise performances achievable with a SDD obtained at
shaping times in the order of 1us. On the contrary, this optimum
shaping time is fully compatible with the short decay time of the
LaBr 3 :Ce crystal (about 25ns). The results of the experimental
characterization of the LaBr 3 :Ce-SDD gamma-ray spectrometer
are presented in this work and are compared with the
performances achieved with the same crystal coupled to a PMT
and to a CsI(Tl) crystal coupled to the same SDD. The SDD has
an active area of 30mm 2 . Antireflective coatings have been
implemented. Good energy resolutions were measured at room
temperature, thanks to the low leakage current of the detector:
2.7% at the 137 Cs 661.7 KeV line and 6.1% at the 57 Co 122 KeV
line. A resolution of 5.7% at 122 KeV line was measured at 0°C.
T
I. INTRODUCTION
he LaBr 3 :Ce scintillator has been recently introduced as a
very valuable alternative to the most conventional
scintillators [1-4]. This crystal, in fact, is characterized by a
5.3 g/cm 3 density, a very high light output (> 60000
photons/MeV), of the same order of the CsI(Tl), and at the
same time by a very short decay time (< 25ns), at the same
level of LSO and much better than NaI(Tl). LaBr3:Ce crystals
have emission peaks at 360 nm and 380 nm, very close to the
wavelength of peak detection efficiency of PMTs and still in
the range of detection efficiency of most solid state
photodetectors. The light output is essentially constant in a
wide range of temperatures. Saint Gobain has recently
measured a very little change of light yield (less than 1%) with
its LaBr3:Ce samples in the range of -10°C to 50°C All these
features make this scintillator very attractive for both space
Manuscript received November 11, 2005.
C. Fiorini, A. Gola, M. Zanchi and A. Longoni are with the Politecnico di
Milano – Dipartimento di Elettronica e Informazione, Milano, Italy and with
the INFN, Sezione di Milano, Milano, Italy.
P. Lechner and H. Soltau are with PNSensor GmbH, Munich, Germany.
L. Strüder is with MPI Semiconductor Laboratory, Munich, Germany.
applications (potentially also as an alternative to CZT
detectors) [5] and for medical imaging [6].
To fully exploit the best performances of a good crystal as
the LaBr3:Ce, also the choice of the best photodetector for the
scintillation light is of relevant importance. For this reason let
us consider the following well-known equation, which gives
the expression of the energy resolution, for a generic γ-ray
spectroscopy system composed by a scintillator and a
photodetector:
ΔE
E
= 2.35
2
ENC
2
M
( ηN
)
ph
2
α
+
ηN
ph
⎛ ΔE
⎞
+ ⎜ ⎟
⎝ E ⎠
2
np , inh
where ENC is the equivalent noise charge of the detector, η
is its quantum efficiency, M the internal gain (if present), α is
a worsening factor due to the statistics of the multiplication
mechanism, N ph is the number of photons generated in the
scintillator. Unitary light collection efficiency has been
considered. The first member is the electronic noise
contribution (R el ), the second is the statistical contribution
(R st ) and the last is the intrinsic contribution due to the
scintillator and the assembly non-idealities (R intr ). Typical
values for the parameters of (1) for the different
photodetectors that are considered here are given in Table 1.
Photomultiplier tubes (PMTs) are still the most used
devices for scintillator readout. The high gain of the PMT
makes the contribution of the electronics noise R el practically
negligible, however the statistical contribution R st plays an
important role. In fact the disadvantage of the PMT is
represented by its low quantum efficiency η (in the order of
25%) which results in a photoelectrons generation spread R st
not optimal with respect to the intrinsic capabilities of the
scintillator. Moreover, the mentioned contribution is worsened
from pure Poisson statistics by the statistics of the
multiplication itself (α ~1.25).
A possible alternative to the PMTs is represented by the
Avalanche Photodiode (APD) [7]. It combines the high η of a
Silicon photodiode, which affects both the electronic and
statistical contributions, with the benefits of avalanche
multiplication which helps to keep R el low. However, also in
the case of APDs, the statistical component to the resolution
R st is affected by the statistics of the multiplication itself: α =
F , where F is the excess noise factor (>2). Moreover, as a
practical drawback, APDs show a high sensitivity of the gain
and η to temperature and bias variations as well as the need to
be operated at high voltages.
Another approach is to use the Silicon Drift Detector
(SDD), which has a low electronic noise, thanks to the low
(1)

value of output capacitance. In this case R el can be
significantly low, without using an internal multiplication
mechanism. The statistical contribution is kept close to the
pure Poisson limit thanks to both the high quantum efficiency
of the photodetector, and by the absence of the multiplication,
so that α = 1 (see Table 1).
If we plot the energy resolution vs. γ-ray energy, as shown
in Fig.1, there is a low-energy region where the electronic
contribution is expected to be dominant, if present, like in
APDs and SDDs; then we find a region dominated by the
statistical contribution, and in this case the SDD is expected to
have better energy resolution performances with respect to the
other photodetectors. Finally there is a region where the
intrinsic contribution of the scintillator is dominant. In this
case, the choice of the photodetector has no significant effect.
II. THE SDD DETECTOR
The Silicon Drift Detector is a silicon detector,
characterized by a particular charge collection mechanism,
such as the electrons generated in the active area of the device
drift towards the anode, which has a small size and a very
small capacitance, independent from the total active area of
the device. Equation 2 expresses the ENC of the detector, as a
function of the shaping time used for signal processing.
2 2
⎛ A
⎞
1
ENC = C
T
⎜ a + A2
2 πa
f
+ A3τb
τ
⎟
(2)
⎝ s
⎠
In this equation C T is the value of the total anode
capacitance (detector + preamplifier + parasitic), τ s is the
shaping time, a is the white series noise, a f is the 1/f noise
coefficient and b is the white parallel noise due to the leakage
current, which can be reduced by cooling. From (2) it can be
seen that a small value of C T reduces the electronic noise
without the need of a multiplication mechanism. Moreover, as
the ENC is dependent on the shaping time, there is an
optimum value for this parameter, which is usually of the
order of 1 μs for the SDDs (this value is dependent on the
operating temperature).
SDDs used for scintillation readout have already
demonstrated to achieve state-of-the-art energy resolution in
γ-ray spectroscopy using a CsI:Tl scintillator [8] as well as
sub-millimetre position resolution in γ-ray imaging [9].
However, the use of a slow scintillator like CsI:Tl (decay time
~1 μs) did not allow to fully exploit the best noise
performances offered by the SDD, which are obtained using
shaping times of the order of 1 μs. Longer shaping times were
needed with CsI:Tl for not being excessively affected by
ballistic deficit. In this case, the electronic noise of the SDD
was dominated by the contribution of the leakage current
(white parallel noise, see (2)) which had to be reduced by
cooling.
The recently introduced LaBr 3: Ce scintillator has a much
shorter decay time, of the order of 25 ns, which does not
generate ballistic deficit, even when used at shaping times of 1
μs or less. This benefit has however to be considered together
with the collection mechanism of the scintillation-generated
charge inside the SDD. In fact, the drift time necessary for the
charge to reach the anode of the photodetector prevents the
use of a too short shaping time. For the device presented in the
next section, the drift time from the outermost region of the
active area is estimated to be about 0.7μs.
III. DETECTOR CHARACTERIZATION
The detector we have used for the measurements is a
circular SDD, with an active area of 30mm 2 , produced at the
Semiconductor Laboratory of Max Planck Institut in Munich.
The light enters from the bottom side of the detector, which is
made by an homogeneous entrance window. The input JFET
of the readout electronics has been integrated in the detector
itself, in order to further reduce the total anode capacitance,
and consequently the electronic noise at short shaping times.
Recent technological improvements in the fabrication process
of the SDD include a reduced leakage current, of about
200pA/cm 2 at 25°C, which makes the device very attractive
for measurements near room temperature. Another feature is
the presence of the bonding pads outside the active area; a
layout of the chip is shown in Fig. 2.
The principle schematic of the readout electronics used in
the measurements with the SDD is shown in Fig.3. The JFET
integrated on the detector is operated in a source follower
configuration with an external current source. The voltage
signal is obtained by the conversion of the charge signal into
voltage on the total capacitance C d connected at the detector
output. The signal is amplified by means of a voltage
amplifier. This is realized by means of a coupling capacitor
followed by a charge preamplifer. The voltage gain on the
signal step is given by C K /C F while the decay time of the
preamplifier waveform is equals to R F C F . The signal is further
processed by a Tennelec TC244 shaping amplifier.
The electronic noise of the device have been characterized
by directly irradiating the device with an X-ray source of 55 Fe
(5.9 KeV Mn Kα), at different temperatures, ranging from -
10°C to 23°C. The optimum energy resolution achieved was
145 eV FWHM (9.8 e - rms), obtained at -10°C and with a
shaping time of 1 μs. At room temperature we obtained 237
eV FWHM at 0.25 μs. The corresponding spectra are shown
in Fig. 4.
If we plot the measured electronics noise with respect to
the shaping time we find a behavior, in accordance to (2),
which is shown in Fig. 5. It is also worth mentioning that the
shaping times compatible with LaBr 3: Ce correspond roughly
to the minimum of the ENC curve, while the CsI:Tl lays in a
region with higher noise, especially at room temperature.
The entrance window of the detector was also provided
with custom anti-reflective coatings (ARCs). The presence of
these coatings is important, because they improve the
Quantum Efficiency η at the wavelengths of interest,
increasing the collected signal charge, and, from (1), this has
positive effects both on the electronic contribution R el and on
the statistical contribution R st . A plot of the measured η vs.

incident wavelength for these ARCs and for standard coatings
is shown in Fig. 6. The ARC coatings were originally
designed to have a maximum efficiency at λ = 565 nm
(CsI:Tl), where we have measured η = 90%, but still have η =
80% at λ = 400nm (the lower end of our QE measurement
sensitivity).
IV. γ-RAY SPECTROSCOPY SET-UP AND MEASUREMENTS
For γ-ray measurements the SDD was optically coupled to a
LaBr 3 :Ce Brillance 380 crystal module, produced by Saint
Gobain. The crystal has a cylindrical shape with 5mm
diameter and 5mm thickness. No specifications were available
on the Ce doping of the crystal and on the light output of the
assembled module. The crystal was already mounted with a
light reflector and sealed in an aluminium housing. A picture
of the scintillator coupled to the SDD photodetector is shown
in Fig. 7.
The back side of the detector was biased at -86 V, while the
last ring, providing the charge drift mechanism, was biased at
about -110V. The first measurement was done to evaluate the
conversion gain between the energy of the γ-photon and the
number of photoelectrons. Using the data from the X-ray
measurements, we obtained a conversion gain of about 28 e -
/KeV. Assuming, from the data sheets, a light yield of the
scintillator of 60 ph/KeV and extrapolating from the curve
shown in Fig. 6 a value for η of about 70% at λ = 370 nm (the
central wavelength of the emission spectrum of LaBr 3 :Ce), we
can evaluate a light collection efficiency of about 70%. This
means that there is still room for improvements, both in the
assembly and in η, which should be tuned to match λ = 370
nm.
With the system described, we carried out a set of
spectroscopic measurements at room temperature, with
different gamma sources. Fig. 8 shows the spectrum obtained
irradiating the scintillator with a 137 Cs source. The energy
resolution is 2.7% and the 662 keV escape peak, as shown in
the expanded region plot on the right side, is clearly
distinguishable. As far as the electronic noise contribution is
already known from the X-ray spectroscopic measurements,
and the statistical contribution can be obtained from the
conversion gain, it is possible to evaluate from (1) the intrinsic
contribution, due to the crystal and to the assembly, which is
2% in this case. This value is quite high, if compared to the
one measured by Shah and co-workers [7], who estimated the
same contribution to be of 0.9%, using a LaBr 3 :Ce scintillator
by RMD, coupled to a 8mm 2 APD cooled at 250K.
It is worth mentioning that the same scintillator was used
with a PMT by the manufacturer, and an energy resolution of
3% have been measured. This is actually not among the best
values measured with a LaBr 3 :Ce scintillator. The same
manufacturer reports resolutions as good as 2.8% with other
samples. We could estimate the potential improvement in
resolution using the SDD photodetector with a LaBr 3 :Ce
scintillator characterized by a better intrinsic contribution. For
instance, considering the intrinsic resolution of 0.9% quoted
by RMD and using the R el and R st contributions for the SDD
quoted in the first column of Table II, a resolution of 2.0% at
662keV can be estimated.
In Fig. 9, the spectrum of a 57 Co source measured at room
temperature is shown. The energy resolution (FWHM) at the
122keV peak is of 6.1%. In this spectrum, the peak at 14 keV
is well distinguished from the noise threshold. The same
measurement, carried out with the PMT by the manufacturer,
has given a value of 6.5%.
In this case the electronic noise is no longer negligible; to
improve the results we repeated the measurements with a
moderate cooling, operating the system at 0°C. The
corresponding spectrum, reported in Fig. 10, shows an energy
resolution of 5.7%.
The energy resolutions measured with the SDD and PMT
are listed in Table 2, together with the estimated contributions.
Combining these measurements with others, made with a
241 Am source, it is possible to draw a plot of the energy
resolution vs. energy, shown in Fig. 11, which fits the
experimental data.
It is also interesting to compare the results obtained at 122
keV with LaBr 3 :Ce with the ones obtained with a CsI:Tl
crystal, using the same SDD as a photodetector. The
comparison is shown in Fig. 12. The main difference between
the two scintillators is that CsI:Tl has a much longer decay
time which forces to use the SDD at longer shaping times and
with a higher electronic noise, as anticipated in section II. The
energy resolution of the LaBr 3 :Ce crystal readout by the PMT
is also reported.
V. CONCLUSIONS
In conclusion the measurements have shown that the SDD
coupled to a LaBr 3 :Ce scintillator is a good photodetector for
γ-ray spectroscopy, and can obtain better results than a PMT.
The performances achieved are due to the low electronic noise
of the SDD and to an improved photodetection efficiency, but
there is still room for improvements, especially in the
optimization of the detection module. Applications in medical
imaging and γ-ray spectrometry are foreseen in the near
future.
VI. ACKNOWLEDGMENTS
The authors would like to thank Mike Mayhugh (Saint
Gobain) for his support.
REFERENCES
[1] E.V.D.van Loef, P.Dorenbos, C.W.E. van Eijk, K.Krämer, H.U.Güdel,
Appl.Phys.Lett., vol.79, p. 1573, 2001.
[2] P. Dorenbos, Nucl. Instr. Meth., vol. A 486, pp. 208-213, 2002.
[3] E. V. D. van Loef, et al., Nucl. Instr. Meth., vol. A 486, pp. 254-258,
2002.
[4] P. Dorenbos, J. T. M. de Haas, C. W. E. Van Eijk, IEEE Trans. Nucl.
Sci., vol.51, n.3, p.1289, 2004.
[5] M.L.McConnel, et al., SPIE 5488, 2004.
[6] W.Moses, K.S.Shah, Nucl. Instr. Meth., vol. A 537, pp. 317-320, 2005.
[7] K.S.Shah, et al., IEEE Trans. Nucl. Sci., vol.51, n.5, p.2395, 2004.

[8] C. Fiorini, et al., IEEE Trans. Nucl. Sci., vol. 44, no. 6, pp. 2553-2560,
1997.
[9] C.Fiorini, F.Perotti, Review of Scientific Instruments, 76, 044303,
2005.

TABLE I
PHOTODETECTOR TYPICAL PARAMETERS
Parameter PMT APD SDD
η ~25% >80% >80%
α 1+υ (~1.25) F (>2) 1
M ~ 10 6 ~ 10 3 1
TABLE II
DIFFERENT CONTRIBUTIONS TO THE ENERGY RESOLUTION
SDD
137 Cs
T amb
PMT
137 Cs
T amb
SDD
57 Co
T amb
PMT
57 Co
T amb
SDD
57 Co
T = 0°C
R el 0.5% ~0% 2.9% ~0% 1.9%
R st 1.7% 2.2% 4.0% 5.4% 4.0%
R intr 2% 2% 3.6% 3.6% 3.6%
R tot 2.7% 3% 6.1% 6.5% 5.7%
Energy resolution (%)
10
electronics
noise
total
statistics
scintillator
non idealities
1
10 100 1000
Energy (keV)
Fig. 1. Plot of the energy resolution vs. incoming γ-ray energy, for a system composed of a scintillator crystal coupled to a photodetector. It is possible to
distinguish three regions, each one dominated by a particular contribution to the resolution. In the region on the right side of the graph, above few hundreds
of keV, scintillator non idealities dominates with respect to the other contributions.
Fig. 2. Layout of the Silicon Drift Detector used for the γ-ray measurement. On the left side of the figure one can see the bonding pads, which are external to
the active area and are connected to the central region of the device.

Fig. 3. Principle schematic of the readout electronics used in the measurements.
4000
5.9 keV
5000
55 Fe spectrum
5.9 keV
Counts
3000
2000
55 Fe spectrum 4000 5000 6000 7000
237eV
FWHM
Counts
4000
3000
2000
145eV
FWHM
1000
1000
4000 5000 6000 7000
Energy [eV]
Energy [eV]
Fig. 4. Energy spectra of a 55 Fe source, measured at room temperature (left) with 0.25μs shaping time and at -10°C (right) with 1μs shaping time. The longer
shaping time used at -10°C is used thanks to the reduced leakage current (see also Fig. 4).

Fig. 5. ENC curve of the 30 mm 2 SDD measured at different temperatures. In this figure the shaping time range to be used with two different scintillators, due
to their decay times, are also shown.
Fig. 6. Graph of the Quantum Efficiency η measured with the 30 mm 2 SDD detector, with custom and standard anti-reflection coatings.
Fig. 7. Picture of the circular SDD coupled to the LaBr 3:Ce module. The detector is placed on a ceramic substrate for biasing and signal extraction.
2x10 6 2x10 4
137
Cs spectrum
661.7 keV
6x10 4
661.7 keV
1x10 6
4x10 4
2.7%
FWHM
escape
5x10 5
32 keV
Ba X-rays
0 200 400 600
E [k V]
600 650 700
Energy [keV]

Fig. 8. Spectrum of a 137 Cs source, measured at room temperature (23°C). In the left side of the figure, the same spectrum is shown in two different vertical
scales to better show both 32keV X-ray lines and 662keV line. The escape peak of the 661.7 KeV line (shown in the expanded region on the right side of the
figure) is clearly distinguishable and the noise threshold is well below the 32 KeV X-ray line.
2000
57
Co spectrum
122 keV
Counts
1500
1000
6.1%
FWHM
500
0
14 keV
Escape
136.4 keV
0 40 80 120 160
Energy [keV]
Fig. 9. Spectrum of a 57 Co source, measured at room temperature (23°C). The 14 KeV line is clearly distinguished from noise. Laescape
peak is visible in figure.
2000
57
Co spectrum
122 keV
Counts
1500
1000
5.7%
FWHM
500
0
14 keV
Escape
136.4 keV
0 40 80 120 160
Energy [keV]
Fig. 10. Same measurements of Fig. 8, but made at a lower temperature, in order to reduce the electronic noise contribution. The energy
resolution has improved to 5.7%.

Fig. 11. Plot of the energy resolution vs. energy for the 30 mm 2 SDD used for γ-ray measurements.
Fig. 12. Comparison between the energy resolution obtained at different shaping times for LaBr 3 :Ce and CsI:Tl. Also the measurement with the
LaBr 3:Ce crystal using a PMT is reported.