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

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.