=Paper=
{{Paper
|id=Vol-2507/376-380-paper-69
|storemode=property
|title=Electronics of the Fission Fragments Spectrometer COMETA-F
|pdfUrl=https://ceur-ws.org/Vol-2507/376-380-paper-69.pdf
|volume=Vol-2507
|authors=Oleg Strekalovsky,Dmitriy Kamanin,Yuriy Pyatkov,Alexander Strekalovsky
}}
==Electronics of the Fission Fragments Spectrometer COMETA-F==
https://ceur-ws.org/Vol-2507/376-380-paper-69.pdf
Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30–October 4, 2019
ELECTRONICS OF THE FISSION FRAGMENTS
SPECTROMETER COMETA-F
O.V. Strekalovsky1, D.V. Kamanin1, Yu.V. Pyatkov1,2,
A.O. Strekalovsky1
1
Joint Institute for Nuclear Research, 6 Joliot-Curie, Dubna, 141980, Russia
2
National Nuclear Research University “MEPHI”, Moscow, Russia
E-mail: stroleg1@yandex.ru
The article describes the electronics for the two-arm time-of-flight fission fragment spectrometer
COMETA-F. The time pick-off detector is comprised of a thin electron conversion foil, an
electrostatic mirror, and two microchannel plates supplied by Baspik and mounted in a chevron
configuration. The mosaics of Si PiN diodes is used to measure both energy and time-of-flight. The
waveform of detected signals is digitized by V1742 modules, which sample signals through a
DRS4[1] chip. The DRS4 chip is a switched capacitor array, which can sample the input signal at a
frequency of 5 GHs. The start for registration is provided by a specially designed trigger module. The
V945 discriminator thresholds are individually settable in a range from -1 mV to -255 mV via VME
through an 8-bit DAC.
Keywords: fission fragment spectrometer, SCA ADC, distributed storage
Oleg Strekalovsky, Dmitriy Kamanin, Yuriy Pyatkov, Alexander Strekalovsky
Copyright © 2019 for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30–October 4, 2019
1. Detectors
The study of nuclear fission, both spontaneous and induced, is still of great scientific and
practical interest. The complexity of mathematical calculations of fission processes requires accurate
experimental data to confirm the validity of the models used. In the applied field for fast neutron
reactors, it is important how the additional energy from neutrons is shared between fissile fragments.
Even for such elements as 235U, 238U, and 229Pu, which are important for reactor fuel cells, the transfer
of excitation energy by multiple neutrons affects the fission process differently [2]. Of interest is the
study of neutron multiplicity depending on the masses of fragments, the total kinetic energy, and the
energy of the irradiating neutrons, which makes it possible to determine the total excitation energy of a
compound nucleus. Accurate data on fission of actinides depending on the energy of irradiating
neutrons is required. All this leads to the creation/emergence of new spectrometers for detecting
fission fragments. Of great interest is the study of rare decays of low-excited heavy nuclei in 252Cf (sf)
and 235U(nth, f). We called this decay mode a “collinear cluster tri-partition” (CCT) in view of the
observed features of the effect, that is when the decay partners fly apart almost collinearly and at least
one of them has a magic nucleon composition [3,4].
The Cometa-F spectrometer includes a start-up detector based on microchannel plates (MCP)
with electrostatic mirrors and a matrix of semiconductor silicon detectors with 8 PIN diodes each. The
length of each arm of the spectrometer is 15–25 cm. The spectrometer is placed in a vacuum chamber.
The air inside the chamber is pumped out to a pressure of 10-5 torr by the Pfeiffer HiCube-80 vacuum
station. The principle of operation of the starting detector is based on the use of the secondary
emission of electrons knocked out by an ionizing particle. The detector consists of a conversion foil,
an accelerating grid, an electrostatic mirror, and a MCP chevron assembly. A detected particle (from
protons to heavy ions), when passing through the input foil of the start detector, knocks electrons that
are accelerated in the electric field between the foil and the accelerating grid to an energy of about 3
keV. Then the electrons unfold in the space between two electrostatic mirrors at 90 degrees and after
that they fall on the chevron assembly of the MCP. The start detector was designed so that its output
time signal did not depend on the place of the particle entering the input foil. The foil is a 20-μg/cm2-
thick mylar film with a sprayed layer of gold. The nets are wound from Cu-Be wire 20 μm in diameter
in increments of 1 mm (transparency 98%).The potentials of each element of the detector are set using
a resistive divider located in the vacuum chamber near the MCP. The time resolution of the detector is
100ps ÷ 130ps.
Each PIN diode having a size of 20x20 mm with a thin “dead layer” (less than 100 nm is the
thickness of the implanted p + layer and aluminum coating) is used both to record the energy of the
incident particle and to obtain a time stamp. Signals from PIN detectors are amplified by fast preamps.
The mass of each recorded fragment is restored from the measured values of its flight time and energy
(TOF-E method). Prior to registration fission fragments pass through the source substrate and the start
detector foil where they lose part of their initial energy; therefore, the measured fragment velocities
must be adjusted for energy loss. Energy losses depend on the charge and mass of the particle, its
energy as well as on the substance of the absorber.
2. Preamplifier
To amplify and shape a signal from a PIN detector, two-stage transistor preamplifiers are used.
Since the amplitude of the input signal of the V1742 digitizer should not exceed 1 V and because in
experiments using the same detector it is necessary to register both heavy fragments and light particles
(the energy range differs by 20 times), the gain is limited to the value of 50. The 300-MHz bandwidth
is provided by BFR193 transistors. The consumption current is less than 10 mA, which makes it
possible to place the amplifiers in the close proximity to the detector directly in the vacuum chamber.
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30–October 4, 2019
Figure 1. Preamplifier for PIN detector
3. Data Acquisition System of the COMETA-F spectrometer
Figure 2 shows an electronic block diagram of the COMETA-F setup. Мultichannel modules
from CAEN in the VME standard are used in the spectrometer. After being formed by the
preamplifiers through the passive matching splitters 1–2, the signals from the PIN diodes are fed to the
V895 discriminators [4]. The V895 module has 16 inputs and generates NIM logic pulses when the
input signal exceeds a threshold that can be set in the range from -1 mV to - 255 mV. The duration of
the output pulse of the V895 block is essentially adjustable in the range from 5 to 40 ns, which is not
enough for our experiments, so the range was increased to 200 ns. The discriminator thresholds are
individually configured using the USB / VME VX1718 [5] controller. A signal for starting the data
acquisition system is generated following a signal from the start detector and at least one signal from
PIN diodes.
The TRIGGER GENERATOR unit is used to generate the signal to start the process of
recording events from two digital recorders V1742. The logic for generating the trigger is shown in
Fig. 3. In the presence of at least one signal from the detector (the OR function is implemented for
input signals installed in one of the arms of the spectrometer), a pulse is formed whose duration
exceeds the time of flight of the slowest particle from the target to the PIN detector. This time depends
on the span base of the spectrometer and does not exceed 100 ns at a base of 150 mm. This signal is
applied to the trigger D-input. A delayed signal from the output of the START1 start detector is
applied to the clock input of the same trigger. If the trigger has a resolving potential at the D-input of
the trigger, which means that at least one particle has hit the detector, then a TR0 registration start
signal is generated along the leading edge of the delayed “Delayed St” signal.
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30–October 4, 2019
Figure 2. Data Acquisition System of the Cometa-F spectrometer
Figure 3. Time diagram of control signals of DAQ
To synchronize the operation of the digitizer with other recording subsystems of the
spectrometer, a VETO inhibit signal, a BLK block, and a CLR reset are used. The BLK lock trigger is
set simultaneously with the TR0 signal, and is reset by the external input signal CLR. The external
VETO signal blocks the passage of signals, inhibiting the formation of the TR0 registration trigger.
The mass determination procedure for fission fragments from the measured energy and time of
flight taking into account PHD and PD in PIN detectors is used in our group, which is described in
detail in [6] and is carried out off-line using specially developed algorithms.
Using two 32-channel V1742 recorders, signals are digitized and transmitted via a fiber-optic
cable in accordance with the SONET-2 network standards.
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30–October 4, 2019
The advent of multi-channel digitizers with speeds up to 5 G samples per second opens up
new possibilities, making it possible to register the shape of signals lying in the nanosecond region.
The advantages of recording the shape of the signal under investigation are as follows:
- ability to isolate the situation with the imposition of pulses and to distinguish the interference
and noise from signals, which is relevant when searching for rare events;
- further analysis of digitized and stored data is possible using various digital algorithms;
- reduction in the number of connecting cables, connectors, etc. in a multi-detector installation,
which leads to an increase in reliability, ease of maintenance, and overall lower cost per channel;
- arm length of the time-of-flight system is limited by the working range of the digitizer. The
arm lengths used in our vacuum chamber of tens of mm (the flight time of the recorded fragment lies
in the nanosecond region) are in good agreement with the digitizer range of 200 ns;
- hardly any effect of low-frequency pickups on the determination of the amplitude (energy) of
a short nanosecond detector signal;
- a possibility of obtaining a temporal and energy resolution, which is not worse than those
obtained by the methods of classical analog electronics, using algorithms applied to the digitized
signal.
A typical sequence of operations for processing digitized signals includes filtering, zero line
determination, amplitude determination or calculation of the area of the signal to estimate the energy,
and timing determination. The experience of using DT5742 digitizers from CAEN (Italy) has shown
that accuracy in the choice of filters is required. A simple median filter reduces the amplitude and
worsens the timing. Particular attention is required when working with several digitizers.
When switching to work with high-speed digitizers, you must be prepared for a significant
increase in the volume of accumulated data. Our group uses a terabyte storage Synology of the
DS2415+ type with 12 removable drives organized as RAID-5. The network infrastructure of the
Laboratory of Nuclear Reactions is used for communication.
The software for setting discriminator thresholds, V1742 recorder parameters and
spectrometer data acquisition is implemented in a LabView environment using CAEN libraries.
References
[1] S.Ritt, Nucl. Instr.And Meth.A518 (2004) 470.
[2] F.Bush, W.Pfeffer, B.Khlmeyer, D.Schull, F.Puhlhoffer, Nucl. Instr.And Meth.171(1980) 71.
[3] Yu.V.Pyatkov, D.V.Kamanin, W. von Oertzen, A.A.Alexandrov, et.al. The collinear cluster tri-
partition (CCT) of 252Cf (sf): New aspects from neutron gated data Eur. Phys. J. A: Hadrons and
Nuclei. 2012. V. 48, No. 7. pp. 94–109.
[4] Yu.V.Pyatkov, D.V.Kamanin, A.A.Alexandrov, I.A.Alexandrova, et.al. Examination of evidence
for collinear cluster tri-partition. Physical Review C 96, 2017, art. 064606.
[5] Available at: http://caen.it.
[6] D.V.Kamanin, Yu.V.Pyatkov, A.O.Strekalovsky, V.E.Zhuchko, et.al. New approaches to
determination of the heavy ion's mass in mesurements with PIN
diodes. IzvestiyaRossiiskoiAkademiiNauk. SeriyaFizicheskaya, 2018, V. 82, No. 6, pp. 804-807.
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