Download Sample HTPD article for RSI - Narodowe Centrum Badań Jądrowych

Performance of the prototype LaBr3 spectrometer developed for the JET
Gamma-ray Camera Upgradea)
D. Rigamonti,1,2,b) A. Muraro,2 M. Nocente,1,2 V. Perseo,1 G. Boltruczyk,3 A.
Fernandes,4 J. Figueiredo,4,5 L. Giacomelli,2 G. Gorini,1,2 M. Gosk,3 V. Kiptily,5 S.
Korolczuk,3 S. Mianowski,3 A. Murari,5 R. C. Pereira,4 E. P. Cippo,2 I. Zychor,3 M.
Tardocchi,2 and JET. Contributors*
Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Milano, Italy
Istituto di Fisica del Plasma “P. Caldirola”, CNR, Milano, Italy
3
Narodowe Centrum Badań Jądrowych (NCBJ), 05-400 Otwock-Swierk, Poland
4
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal
5
Culham Centre for Fusion Energy, Culham, United Kingdom
*
See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014,
Saint Petersburg, Russia
1
2
(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX)
(Dates appearing here are provided by the Editorial Office)
In this work we describe the solution developed to improve the spectroscopic properties of the existing γ-ray
camera of JET, enabling gamma-ray spectroscopy in JET deuterium-tritium plasmas. A dedicated pilot
spectrometer based on a LaBr3 crystal coupled to a Silicon Photo-Multiplier has been developed. A proper
pole zero cancellation network able to shorten the output signal to a length of 120 ns has been implemented
allowing spectroscopy at MHz count rates. The system has been characterized in the laboratory showing an
energy resolution of 5% at Eγ=0.662 MeV, which extrapolates favorably in the energy range of interest for
the observation of 4.44 MeV gamma-rays.
I. INTRODUCTION
Gamma-ray spectroscopy is a plasma diagnostic technique
investigating the behaviour of fast ions in high temperature
fusion plasmas, as demonstrated at JET1-4. In particular, it plays a
key role in the study of alpha particle confinement, which is
crucial for plasma self heating in a high power discharge.
Gamma-ray emission in thermonuclear plasmas is mainly due to
reactions between fast particles and fuel ions or impurities. Of
particular relevance is the detection of 4.44 MeV gamma-rays
from the 9Be(α,nγ)12C reaction, as it gives information on alpha
particles in deuterium-tritium plasmas. At JET, a horizontal and a
vertical neutron/γ camera provide information on the radial
profile of the neutron/γ emission source by collimated
measurements along 19 channels. The Gamma-ray Camera
Upgrade project aims to improve the spectroscopic properties of
the existing γ-ray camera of JET in terms of energy resolution
and high counting rate capability in order to operate in the
deuterium-tritium (DT) campaign. The required target values
from the project are: energy resolution of 5% (En.Res. =
FWHM/En) at 1.1 MeV and count rate capability exceeding 500
kHz. An important constraint is the limited available space.
Silicon Photo-Multipliers (SiPMs) can represent a good
alternative to photo-multiplier tubes, given their high photon
detection efficiency, high internal gain, insensitivity to magnetic
field and an extremely compact size. SiPMs, which have
experienced great improvements in the last years, still show
voltage-temperature dependence and a limited linearity but both
can be corrected. In particular, gain shifts due to temperature
changes can be compensated by a voltage adjustment while the
non-linearity, due to the limited available number of APD pixels,
can be corrected off-line. In this work we describe the solution
developed to meet the requirements. Our choice fell on a LaBr3
scintillator cystal (25.4 x 16.9 mm2) coupled to a Silicon PhotoMultiplier (12x12 mm2). the final design with the electronic
readout circuit of the pilot spectrometer will be presented
together with the laboratory measurements at low count rate with
radiation sources and at high count rate with a LED pulse.
II.
SILICON
PHOTO-MULTIPLIER
ELECTRONIC READ-OUT CIRCUIT
AND
Silicon Photo-Multiplier detectors, also known as MPPC,
are a relatively novel solid state photo-sensors. they are made up
multiple Avalanche Photo-Diode (APD) pixels connected in
parallel and operating in Geiger mode which allow an internal
gain depending to the operational condition. For each APD cell,
the Geiger mode is activated when a reversed bias above the
electrical breakdown voltage (Vbd) is applied. The chosen SiPMs
are 12x12 mm2 in size, made by 16 channels with 3464 pixels
each and they are manufactured by Hamamatsu. In Fig. 1 the
characteristic I-V curve of the device shows a sharply increase of
the current in correspondence of the breakdown voltage which is
roughly 65 V. In order to achieve high counting rate capability, a
short output signal is necessary. For this purpose, a dedicated
read-out circuit for the MPPC has been developed in Milano by
implementing a CR differential circuit able to shorten the signal
length up to 120 ns. In Fig. 2, an example of the output signal
achieved are shown.
FIG. 1. Current vs. Voltage curve measured for the
12x12mm2 SiPM.
FIG. 3. Pulse Height Spectrum recorded with 137Cs and
60Co sources.
IV. LABORATORY TESTS AT HIGH COUNTING
RATE WITH A LED PULSE
FIG. 2. Output signal from the LaBr3 crystal coupled to the
SiPM with the pole zero cancellation network.
III. LABORATORY TESTS WITH 137Cs AND 60Co
SOURCES
Some standard tests have been performed in order to characterize
the MPPC response with the dedicated electronic readout circuit.
The MPPC was coupled with a LaBr3 crystal (25.4 x 16.9 mm2)
and it was powered by a TTi EX752M voltage supply. Finally,
the output signal was fed into a CAEN digitizer DT5730. This is
a 14 bit and 500 MHz sampling rate digitizer equipped with
CAEN software able to perform on-line measurements of the
pulse area. In Fig. 3 the pulse height spectrum with a 137Cs and
60Co sources is shown. In order to characterize our pilot
spectrometer, several measurements with a 137Cs source (0.661
MeV) have been performed with different applied voltage and
each measurement lasted 120 seconds to limit the temperature
changes. The best energy resolution (En.Res. ≈ 5 %) was
obtained at 67.5 V (roughly 2.5V over V bd) as shown in Fig. 6.
As exhibited in Fig. 4, the trend of the energy resolution is well
fitted by the curve 1/√(E) and this extrapolates favorably in the
energy range of interest (<2.5% in the range 3 – 5 MeV) for the
observation of 4.44 MeV gamma-rays from the 9Be(α,nγ)12C
reaction.
In order to simulate the high rate environment at JET, high
count rate measurements have been performed in laboratory with
a blue LED connected to a pulse generator (Keysight 81150A).
With this kind of generator, it is possible to have signal with
arbitrary amplitude and frequency. All the measurements have
been done with the MPPC coupled to a LaBr3 crystal of 25.4 mm
diameter and 16.9 mm height, covered with teflon. The MPPC
has been placed in a lightproof box with a small 137Cs source
(used as reference) and an optical fiber. Basically, two types of
measurements have been performed: 1) by using the 137Cs as
reference source and the LED pulse as perturbation source (with
amplitude and frequency variables). These measurements lasted
120 seconds. 2) By using two LED pulses, one as reference
(LED2: amplitude and frequency fixed) and the other one as
perturbation source (LED1). In these tests it was possible to run
short measurements (2 seconds), in order to be immune from
temperature effects.
The measurements allowed investigating and reducing the
deviation of the peak position in the pulse height spectrum. The
main reason of the shift is due to the voltage drop across the
resistor R1 (see Fig.) which is bigger when the current flowing
through the MPPC is higher, therefore, when the counting rate is
higher. Another factor which affects the deviation of the peak
position is the supply voltage of the MPPC and the amplitude of
the perturbation source. In fact, by increasing the input voltage,
the deviation increases. Therefore, by choosing a suitable resistor
R1 (10Ω) and suitable supply voltage (65.6V; ≈ 0.6V over the
breakdown voltage) we obtained very good results in terms of
peak shift reduction. As shown in the pulse height spectra in Fig.,
which represent two measurements of a 137Cs source with and
without the use of the LED pulse, a deviation of the cesium peak
position less than 5% occurs when the LED is on at 500 kHz. All
these measurements lasted 120 seconds and this can introduce
temperature changes which increase the peak shift. It is worth
noticing that, in this test, the amplitude of the LED pulse induces
a signal which is roughly three times bigger than the cesium one,
therefore, corresponding to a gamma energy higher than 2 MeV
(considering the MPPC non-linearity).
V. JET-LIKE MEASUREMENTS
By using again the two LEDs, it was possible to do some
the eff ect can be reduced by decreasing the input voltage at
65.8V (≈0.5 V over the breakdown voltage), reaching about 5.5%
of maximum shift at 1MHz in the JET-like measurements (1
minute of measurement with only 10 seconds at high rate).
Moreover, it is clear that the MPPC warms up if the high rate
measurement lasts more than a few seconds.
VI. ACKNOWLEDGMENTS
This work has been carried out within the framework of the
EUROfusion Consortium and has received funding from the
Euratom research and training programme 2014-2018 under grant
agreement No 633053. The views and opinions expressed herein
do not necessarily reflect those of the European Commission.
1
FIG. 4. MPPC energy resolution versus energy. The black
points are obtained by analyzing the full energy peaks.
measurements lasting one minute but with only a window of 10
seconds at high counting rate, in order to mimick the
experimental conditions of a JET shot. Again, LED1 was the
perturbation and LED2 was the reference. LED2 was chosen to
have an amplitude five times bigger than 137 Cs amplitude and a
frequency fixed at 10 kHz. LED1 was chosen to have the same
amplitude of 137Cs and a variable frequency: in the first 20
seconds it was 10 kHz, in the next 10 seconds it was 500 kHz or
1MHz and from 30 seconds to 60 seconds it was again 10 kHz.
The measurement was repeated at two different input voltage,
65.8 V and 66.2 V. An example of the obtained spectrum is
shown in figure 12. From figure 13, it can be seen that when the
perturbation at 500 kHz or 1 MHz is on, the peak is shifting on
the left and, after the end of the perturbation, it does not go back
in its exact previous place because of a change in the MPPC
temperature.
FIG. 4. Pulse Height Spectra of a 137Cs source with and
without the LED pulse
V. CONCLUSIONS
After these tests, we can conclude that the pilot spectrometer
is working properly showing a very good energy resolution, i.e.
between 5% and 6% at 662 keV. The measurements with LEDs
have demonstrated that the MPPC has a gain loss at high rate,
producing a peak shift on the left in the spectra. This shift can
however be minimized by substituting the resistor after the
voltage supply (R1). The phenomenon is certainly related to the
current that flows in the circuit but probably it is not the only
reason, as temperature effects may also play a role. In any case,
M. Tardocchi et al., Phys. Rev. Lett. 107, 205002 (2011).
M. Nocente et al., Nucl. Fusion 52, 063009 (2012).
3
M. Nocente et al., Nucl. Fusion 52, 094021 (2012).
4
M. Tardocchi, M. Nocente, and G. Gorini, Plasma Phys. Controlled
Fusion 55, 074014 (2013).
5
M. Nocente et al., Review of Scientific Instruments 85, 11E108 (2014);
doi: 10.1063/1.4886755.
2