PRAMANA
c Indian Academy of Sciences
— journal of
physics
Vol. 85, No. 3
September 2015
pp. 483–495
Detector instrumentation for nuclear fission studies
AKHIL JHINGAN1,2
1 Inter
University Accelerator Centre, P.O. Box 10502, New Delhi 110 067, India
of Physics, Panjab University, Chandigarh 160 014, India
E-mail: [email protected], [email protected]
2 Department
DOI: 10.1007/s12043-015-1067-8; ePublication: 3 September 2015
Abstract. The study of heavy-ion-induced fusion–fission reactions require nuclear instrumentation that include particle detectors such as proportional counters, ionization chambers, silicon
detectors, scintillation detectors, etc., and the front-end electronics for these detectors. Using the
detectors mentioned above, experimental facilities have been developed for carrying out fusion–
fission experiments. This paper reviews the development of detector instrumentation at IUAC.
Keywords. Multiwire proportional counter; ionization chamber; charge-sensitive preamplifier.
PACS Nos 29.40.Cs; 25.85.−w; 84.30
1. Introduction
The experimental nuclear physics programmes in many accelerator laboratories are
focussed at reactions around Coulomb barrier. The experiments that are routinely carried out are fission angular and mass distribution, fusion cross-section measurements,
pre-scission neutron and charged particle multiplicity experiments, etc. To probe these
reactions more deeply, it is required to have detectors with higher detection efficiency,
large solid angle coverage, good energy, position and timing resolutions along with high
count rate handling capability. At the same time, detectors are required to detect different
particles right from protons to heavy evaporation residues, heavy fission fragments, etc.
Using a single detector, it is impossible to carry out all these measurements mentioned
above. In recent years, attempts have been made to carry out multiparametric measurements with multidetector systems to investigate these phenomena. The commonly used
detectors for particle detection are silicon detectors, gas detectors such as ionization
chambers and proportional counters, hybrid telescopes (combination of gas and silicon
detector), inorganic and organic scintillators, etc.
Fission reactions are investigated by detecting both binary fragments and characterizing them. This is done by either measuring the energies of both or one fragment, or by
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measuring the velocities of two fragments. Apart from fragments, there is also emission
of γ -rays as well as light particles such as neutrons, protons, alphas, etc. Detection of
these particles in coincidence with fission fragments can provide complete information
on fission dynamics. Detection of fusion evaporation products gives the fusion crosssection and survival probability of the compound nucleus formed in heavy-ion fusion
reactions. We have developed detector systems for the investigation of fusion–fission
dynamics in the energy domain near the Coulomb barrier using the general purpose scattering chamber (GPSC) [1], national array of neutron detectors (NAND) [2], heavy-ion
reaction analyser (HIRA) [3] and hybrid recoil analyser (HYRA) [4] facilities at the IUAC
(formerly Nuclear Science Centre) [5].
2. Multiwire proportional counter (MWPC)
Multiwire proportional counters (MWPC) are perhaps the most important detectors used
in fission experiments. They provide good timing and position resolutions, are insensitive
to radiation damage, have high count rate handling capability, and can be fabricated with
ease in various sizes and geometry to make it compatible with the given experimental
requirements. They are being extensively used for carrying out mass distribution of fission fragments, and for detection of evaporation residues. An experimental programme
for investigating mass distribution of fission fragments and their total neutron multiplicity
is being carried out at IUAC using GPSC and NAND Facilities. Time-of-flight (TOF)
systems based on MWPC have been developed for carrying out these experiments. An
experimental programme for measuring fusion cross-sections, so as to look for fusion
enhancement or fission hindrance [6], is being carried out at IUAC using the spectrometers HIRA and HYRA. Both the spectrometers detect fusion products at their focal plane
MWPC.
MWPC can be fabricated with a three-electrode geometry [7] having a cathode sandwiched between two position-sensitive anodes (x and y). The wires in x and y are oriented
orthogonal to each other. Cathode is generally made using thin Mylar foil which is
aluminized on both the sides. Cathode is kept at negative potential and provides fast
timing signal for TOF or velocity measurements. Another option is to have the
multistep geometry [8] using four- or five-electrode configuration. All the electrodes are
made using wire frames. The detector has two cathodes (one each at the entrance and
the exit), one anode sandwiched between two position electrodes (x and y). The region
between cathode and position electrodes are made to operate in the drift region, whereas
that between positions and anode in the avalanche region. Drift region acts as a preamplification region, where the charge produced due to ionization drifts into avalanche region
where it subsequently gets multiplied. The last electrode can be removed to provide a
four-electrode configuration. This design provides higher gains as compared to the threeelectrode design. In comparison, a three-electrode MWPC with aluminized Mylar cathode
provides better timing due to more uniform field. In both cases, positions are extracted
using the delay line technique. At IUAC, we use the commercially available Rhombus
delay chips model TZB 12-5. The chip has an impedance of 50 , and each chip has
10 taps with a delay of 2 ns per tap. Wire frames in both cases are fabricated using
gold-plated tungsten wires.
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2.1 Fission fragment TOF spectrometer for NAND
This TOF spectrometer [9] is based on a pair of MWPC fabricated using three-electrode
geometry. A schematic of the detector system along with the read-out electronics is shown
in figure 1. The core of MWPC is made of three electrodes: an anode wire plane (x), a
central cathode foil and strip plane (y). The cathode is symmetrically placed 3.2 mm
apart between the two anodes, and is made of a 2 μm-thick Mylar foil aluminized on
both surfaces. The x-plane is made of 200 wires, 10 μm diameter, stretched on a 2.4 mm
thick PCB with an active area of 125 × 75 mm2 . Interwire spacing is 0.63 mm. The
Y-plane consists of 30 tin-plated copper strips (perpendicular to the wires of x-plane),
2.2 mm wide with a step of 2.54 mm, made on a printed circuit board. In x-plane four
wires are grouped together and connected to one tap of the delay chip. MWPC is isolated
from chamber vacuum (low 10−6 Torr) using 1 μm thick Mylar foil at the entrance of
the detector. The timing signals from positions and cathode are extracted using the inhouse-developed fast timing current-sensitive preamplifiers. Position signals use the noninverting configuration whereas the cathode uses the inverting configuration. More details
about preamplifers are provided in §5. Position signals were subsequently fed to Ortec
CF8000 CFD and cathode to Phillips 715 CFD.
The MWPC has been tested offline with 241 Am α source at 7 mbar isobutane. A bias of
−600 V was applied to the cathode. The cathode signals, after preamplifier, had an amplitude of about 300 mV with rise times close to 3 ns. For position signals the amplitude
varied between 30 and 100 mV with rise times around 5 ns. The detector was operated at
3 mbar isobutane and cathode bias of −450 V was applied for in-beam fission experiments
[10,11]. For these experiments, one detector is generally placed at 40◦ (target-to-detector
distance = 22 cm) and the other at around 120◦ (target-to-detector distance = 17 cm) in
the spherical scattering chamber of 60 cm diameter as shown in figure 1b. The detector
at 40◦ could handle count rates exceeding 100 kHz. Mass information of fission events
and its separation from beam- and target-like particles was performed using TOF from
cathode with respect to RF (pulsed beam from Pelletron + LINAC). Timing signals (from
Figure 1. (a) Schematic of a three-electrode MWPC. (b) Assembled MWPCs
mounted inside the NAND scattering chamber.
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Figure 2. (a) TOF spectrum for 19 F + 208 Pb. (b) x-position spectrum.
CFD) from both cathodes were fed to Ortec CO4010 logic module where OR of these two
signals were AND gated with beam RF. This logic signal provides the master start signal
for TOF and position signals. The TOF of other detectors such as neutron detectors are
also recorded with respect to this signal. These signals were delayed using Ortec octal
gate and delay generator GG8000 and fed to the stop inputs of the TDC (Phillips 7186).
Figure 2a shows the TOF spectra showing clean separation between beam-like particles
and fission events as well as target-like particles. A time resolution of about ∼300 ps
was observed for the position-gated elastic events in 28 Si beam. Figure 2b shows the
x-position spectra. Each peak corresponds to one delay tap of the delay chip.
2.2 MWPC for GPSC
Two large-area multistep position-sensitive MWPCs [12], with five-electrode geometry,
have been developed for experiments involving the study of fission dynamics using GPSC
Facility at IUAC. Both detectors have an active area of 200 × 100 mm2 , and provide
position signals in both horizontal (x) and vertical (y) planes, timing signals (from anode)
for TOF measurements, and energy signals (from cathode) giving differential energy loss
in the active volume.
Figure 3a shows a MWPC with four-electrode configuration and figure 3b shows the
MWPC set-up inside GPSC for mass distribution experiments. The distance between
adjacent wire frames is 3.2 mm. The separation between adjacent wires (20 μm diameter) is 1.27 mm. The x-frame has 160 wires whereas the remaining frames have 80
wires each. In position electrodes, wires are shorted in pair and connected to one tap
of delay chip. The position frames are kept at ground potential by terminating both ends
of delay lines through 150 k resistors. Gas detector is isolated from the vacuum chamber using 1 μm Mylar foil. They have a very poor energy resolution, but is sufficient in
many cases to discriminate between heavy fission fragments and lighter projectiles, and its
combination with the TOF signals gives a very clean separation of fission fragments from
target- and projectile-like background. MWPCs are generally operated with isobutane gas
at pressures ranging from 1 to 4 Torr depending upon the nature of the particle (mass and
energy). The timing and position signals of the detectors are used for fission coincidence
measurements and subsequent extraction of their mass, angular and total kinetic energy
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Detector instrumentation for nuclear fission studies
Figure 3. (a) Schematic of a four-electrode MWPC. (b) Assembled MWPCs mounted
inside the GPSC.
distributions. Signal processing is similar to that described in §2.1. More details on
instrumentation can be found in [12]. The timing signals from the central anode and
position signals are amplified by the in-house-developed non-inverting and inverting fast
current amplifiers (§5) respectively. Both the cathodes are shorted electrically and signal
is processed using a charge-sensitive preamplifier (CSPA) developed in house.
The detector was tested with 241 Am α source for determining its position, timing resolution as well as its detection efficiency. For α-particle detection, the detector was operated
using isobutane gas at a pressure of 4 Torr with an operating voltage of +540 and −180 V
on the anode and the cathode, respectively. Figure 4a shows the masked 2D plot with αs
and figure 4b is the projection of x-axis. The holes of 1 mm diameter are placed 5.08 mm
apart in a rectangular matrix. A position resolution of ∼1.1 mm has been observed for
alphas. The detector system has been used to study the fission mass distribution [13]
and fission-gated neutron multiplicity [14] for various systems. Figure 5a shows the plot
of mass ratio of one fission fragment with the mass of compound nuclei against velocity ratio of the parallel component of fission fragment with that of compound nuclei.
There is clean separation between the transfer channels and the compound nuclei fission for the 18 O + 232 Th system. Figure 5b shows the plot between TOF and energy loss
showing clean separation between the projectile-like fission fragments and the target-like
Figure 4. (a) Masked 2D plot with alphas. (b) x-projection of the masked spectra.
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Figure 5. (a) Mass ratio against the ratio of velocities (parallel to compound nucleus).
(b) Plot of TOF against energy loss.
particles for 19 F + 208 Pb system. To extract TOF information, pulsed beams were used
from IUAC tandem with a bunchwidth of ∼1.1 ns. The detectors in all these experiments
were operated at 1.5 Torr isobutane, and the optimum operating voltages were +400 and
−180 V on the anode and the cathode, respectively. At forward angles the count rates
exceeded 20 kHz. For beam-like elastics, a timing resolution of ∼ 2.5 ns is observed.
For the fission fragments, time resolutions are expected to be better (∼ 1 ns) because of
higher amplitude pulse.
2.3 Focal plane detector systems for HIRA and HYRA
Electromagnetic separator (HIRA) [3] and gas-filled separator (HYRA) [4] are used for
the detection of fusion evaporation products or evaporation residues (ER) by focussing
them at the focal plane detectors and suppressing the primary beam-like particles.
MWPCs are used at the focal plane of both the spectrometers to detect ERs. MWPCs
have an active area of 150 × 50 mm and use mutistep geometry as described earlier. The
HIRA focal plane detector [15] has a five-electrode geometry. All electrodes are wire
frames. The distance between the adjacent wire frames is 3.2 mm. All wire frames are
made from gold-plated tungsten wires (diameter 20 μm) stretched on a 1.6 mm-thick printed circuit board. The adjacent wires are 1.27 mm apart. The x-position frame has 120
wires and the remaining frames have 40 wires each. As shown in figure 6a, the entire
electrode assembly is housed inside a cylindrical housing milled out of a solid aluminum
sheet. For HYRA focal plane MWPC [16], a four-electrode geometry is used by removing the last cathode. Design features are identical to that of HIRA focal plane detector
except that the anode and cathode wires are placed 0.63 mm apart and the anode uses 10
μm gold-plated tungsten wire. Such a design provides higher gains [17]. The geometrical transmission efficiency is 92%. Both MWPCs are operated with isobutane gas at
pressures ranging from 1 to 3 Torr. The gas medium is isolated from the vacuum using a
0.5 μm Mylar entrance window. The foil is transparent to heavy recoils, A ∼ 200 and
above, which are produced in highly asymmetric reactions, and have very low kinetic
energies (0.02–0.1 MeV/A) [18,19]. Front-end electronics are identical to that described
in §2.2.
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Detector instrumentation for nuclear fission studies
Figure 6. (a) Assembled MWPC. (b). Three silicon wafers stacked together at the
HYRA focal plane.
The HYRA MWPC is followed by the position-sensitive large-area silicon detector.
This may be a strip detector or resistive anode detectors. Silicon detector gives the energy
of the implanted ER. The detectors can be stacked together to provide larger detection area
at the focal plane. Figure 6b shows three resistive anode detectors from Eurysis/Canberra,
having read-outs from the four corners, stacked together to give an active area of 15 ×
5 cm2 . The detectors belong to the CATE [20] set-up of RISING campaign at GSI.
2.4 Time-zero MWPC
A transmission-type fast timing MWPC [21] has been developed and used in combination with large area position-sensitive MWPCs [12] to obtain absolute timing of the fission
fragments, and the subsequent extraction of their mass–energy distributions. The detector
can be used as a trigger in multidetector set-ups for measuring neutrons and γ s in coincidence with the fission fragments. The detector has an active area of 40 × 40 mm2 . It uses
a mutistep design with four electrodes made from wire frames that are 1.6 mm apart. The
separation between the adjacent wires is 0.63 mm. Cathode and the first ground at the
entrance operate in the drift region whereas the anode sandwiched between two grounds
operate in avalanche or proportional region. Such a design was preferred to the conventional PPAC using two parallel aluminized Mylars to avoid straggling of low-energy
heavy ions (0.5 MeV/A in mass 100 region). The detector is operated with isobutane
gas at 2–4 mbar pressure. To avoid straggling, the entrance and the exit foils used are of
0.5 μm Mylar. Anode is read using fast timing amplifier (§5) whereas cathode is read by
charge-sensitive preamplifier.
The detector was tested offline with radioactive sources 241 Am and 252 Cf. For α detection, detector was operated at 3 mbar gas (isobutane) pressure. A bias voltage of +450
and −180 V was applied at the anode and the cathode, respectively. As shown in figure 7a,
signal strengths upto 300 mV with 3.5 ns rise times were observed in contrast to 10 ns
rise times and 100 mV strength for MWPC with a wire spacing of 1.27 mm. To evaluate
the timing performance, TOF was set up between time-zero and large-area MWPCs [12].
Both the MWPCs were exposed to fission fragments. The large-area MWPC acts as a
stop detector. The distance between two MWPCs was 15 cm. A neutron detector was
also placed at a distance of 1 m from the source to collect coincident neutrons and γ s.
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Figure 7. (a) Trace showing comparisons between signals from two MWPCs.
(b) TOF with 252 Cf between two MWPCs.
Both the detectors were operated at 2 mbar gas pressure. TOF was also generated between
the start MWPC and neutron detector. Figure 7b shows the TOF spectrum for fission fragments. Figure 8a shows the raw TOF between MWPC and neutron detector depicting the
splitting of γ peaks due to two different groups of fission fragments. On applying the
TOF correction from the two MWPCs, a single γ (figure 8b) peak, having 2.5 ns FWHM
was obtained. Subtracting the contributions (time resolution of large MWPC and neutron
detector), we can estimate the intrinsic resolution of time-zero MWPC to be ∼400 ps.
The detector system was used in one of the experiments to study the mass distribution
for the 6,7 Li + 238 U system at 30–50 MeV energies. The start MWPC was placed at a
distance of 7.5 cm from the target on one of the GPSC arms followed by a large-area
position-sensitive MWPC. The second MWPC was placed on the other arm. The timezero detector provided master trigger (initiates for TDC) for all timing signals such as
TOF for two position-sensitive MWPCs and their position signals. The electronic delay
between the start and the stop detectors was determined using monoenergetic αs from
the 241 Am source. Absolute TOF was obtained for one of the fragments whereas time
Figure 8. (a) Raw TOF spectrum between start MWPC and neutron detector.
(b) Corrected TOF spectrum showing single γ peak.
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difference between one fragment and its complementary fragment was recorded for the
other fragment.
3. Hybrid telescopes for heavy-ion detection
Hybrid telescopes, having a combination of gas and silicon detectors, have been developed [22] for heavy-ion detection and particle identification. The detector telescope has
been used for studying the angular distributions of fission fragments. The detector system
can also be used to identify projectile-like fragments and thus can be used for studying
transfer reactions. As a standard practice, silicon telescopes with very thin (10 μm) silicon
detectors (E) and thick (100–300 μm) (E) are used for particle identification. The
E − E identification technique is threshold-dependent and is governed by the thickness of the E detector. The 10 μm-thick detectors are opaque to low-energy heavy ions
such as the fission fragments. In such cases a gas detector is extremely useful because
its active thickness can be varied by simply adjusting the gas pressure, thus making it
transparent for low-energy heavy ions.
The telescope consists of a gas ionization chamber, operating in the axial field geometry
mode, followed by a silicon detector. The ionization chamber (IC) is composed of three
wire frames of 10 mm active diameter. The wire frames consist of a cathode, a central
anode frame and another cathode wire frame. The distance between adjacent wire frames
is 10 mm. All wire frames are made from gold-plated tungsten wires of 20 μm diameter.
The adjacent wires are 1 mm apart. The two cathodes are grounded whereas the anode
operates in the ionization region at 150 V with a gas pressure of 100 mbar isobutane
(2.5 μm Si equivalent). Entrance foil used is 1 μm Mylar (diameter 10 mm). Anode is
read using CSPA with a gain of 90 mV/MeV (Si equivalent) and the silicon detector has a
CSPA of 20 mV/MeV gain. The preamplifiers are placed next to the detector in vacuum.
Figure 9a shows the detector set-up for investigating the fission anisotropy [23]. Three
hybrid telescopes are placed at a distance of 30 cm from the target. Figure 9b shows the
scatter plot of E against E from the 19 F + 194 Pt system. Fission fragments are well
Figure 9. (a) Detector set-up inside GPSC. (b) E − E plot (FF – fission fragments,
PL – projectile-like).
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separated from projectile-like particles. Various transfer channels of the projectile-like
particles are shown in the inset.
4. CsI detectors for fission experiments
A charged particle detector array for investigating reaction dynamics has been developed
at IUAC [24]. The array consists of CsI(TI) detectors coupled to photodiode and is used
to study fusion–fission dynamics. The array can be used with other detectors such as
MWPC, silicon detectors, gas ionization chambers, neutron detectors, etc. for investigating pre-scission charged particle multiplicity, deep inelastic collisions, etc. Silicon
detector telescopes have been used conventionally. Silicon detectors are highly prone to
radiation damage. CsI(TI) scintillators coupled to photodiodes offer a very flexible and
inexpensive solution for the same. One of the main characteristics of CsI(TI) detectors is
its intrinsic ability to discriminate between different light charged particles such as protons, αs, electrons (γ photons), etc. based on their stopping power. This gives rise to
different decay time constants in the light output (fast component) for different particles.
They also show reasonably good energy resolution. Currently, the array has 16 detectors.
Each scintillator is of 3 mm thickness with a 20 mm × 20 mm active area coupled to a
10 mm × 10 mm photodiode. A thickness of 3 mm is sufficient to stop up to 25 MeV of
protons. The detectors are mounted in groups of four (2 × 2) on a common board with
preamplifiers on its back as shown in figure 10a.
The photodiodes are read by CSPA. As the charge generated in photodiodes is
extremely small, it is desirable for the CSPA to have a high gain, good timing features
(ability to distinguish between different decay times from CsI) and low power consumption so that it can be placed next to photodiode in vacuum to avoid degradation of signals.
A preamp has been developed in-house with the above requirements. It has a low power
consumption of 30 mW, a gain of 2 V/pC (Si equivalent) and exhibits good timing
characteristics for particle identification. Ballistic deficit technique has been used for
particle discrimination. For each detector, two shaping amplifiers are required: one with
Figure 10. (a) Fission-gated charged particle set-up inside the GPSC. (b) Scatter plot
showing separation between different particles.
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shorter shaping time (0.5 μs) and the other with larger shaping time (3 μs – gives total
energy).
Offline tests were performed using radioactive sources. Energy resolutions of 200
keV (5.48 MeV α) and 68 keV (1.33 MeV γ ) were observed. The preamplifier output
was fed to Mesytec STM 16+ amplifier units via a differential driver unit. The output of these units were digitized using 7164H ADC (Phillips). The detector system was
used to study fission gated pre- and post-scission charged particle multiplicity for the
16
O + 194 Pt system at 100 MeV. As shown in figure 10a, four quads of CsI along with
two MWPCs [12] were placed inside the GPSC. The MWPCs detect fission fragments
and CsI scintillators detect light charged particles emitted during the reaction. One quad
is placed behind one of the MWPC to detect post-scission light charged particles whereas
the remaining quads are placed around 90◦ with respect to the MWPC to detect the prescission particles as well as their angular distributions. The detectors are at a distance
of 23 cm from the target. Figure 10b shows the scatter plot between the long and short
shaping times, showing clean separation between different particles.
5. Preamplifiers for signal extraction from the detector
Custom-designed preamplifiers have been developed for extracting signals from the detectors. Two versions have been developed. First one is charge sensitive for extracting
energy information. Second one is fast current sensitive for extracting timing information. Advantages of both the versions are that they have very low power consumption and
thus can be operated in vacuum close to the detector, eliminating the need of long signal
transmission cables which deteriorate the quality of signals from the detector. Another
advantage of developing the preamplifiers in-house is that there is flexibility of varying
the gains and frequency characteristics depending upon the experimental requirements as
well as the detectors with which they are being used.
Charge-sensitive preamplifier (CSPA) have been developed using surface mountable
devices such as chip resistors, capacitors and transistors. It uses a conventional circuit
[25] having a JFET at the input stage followed by a transimpedance amplifier made from
BJT. The CSPA has been realized in the form of a eight-pin SIL hybrid (figure 11a) with
Figure 11. (a) CSPA hybrid. (b) Eight-channel CSPA box.
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Figure 12. (a) Versions of fast timing preamplifiers developed. (b) Trace showing
rise time of 2 ns from the silicon detector.
25 × 25 mm dimension. Two versions have been developed: one with a power consumption of 160 mW showing a higher dynamic range (up to 300 MeV) and the other with
a power consumption of 30 mW. Charge sensitivities vary from 7 to 90 mV/MeV (Si
equivalent). These preamplifiers are being used for extracting signals from proportional
counters, gas ionization chambers, silicon detectors, CsI scintillators coupled to photodiodes. They have been used as a single unit and stacked together in groups of four/eight
on a common mother board (figure 11b) for the read-out of multielement detectors.
The fast current sensitive version [26] utilizes multiple common emitter amplifier
stages with an emitter follower at the output stage. Input impedance is 50 . Two versions are available: inverting and non-inverting (depending upon the polarity of the input
signal to drive the negative signals directly to timing discriminators). Both versions show
a voltage gain of 100. Amplifiers have been developed as single-, four- and eight-channel
NIM modules (figure 12a). Rise times of 2 ns (figure 12b) and a timing resolution of
130 ps (FWHM) have been experimentally observed with a 40 μm thick silicon detector
with respect to RF from superbuncher of LINAC.
Acknowledgements
Author acknowledges the support of the group members P Sugathan, N Madhavan,
T Varughese, S Nath, K S Golda, J Gehlot, N Saneesh, E T Subramaniam, S Venkataramanan
and R Ahuja. The author also acknowledges the encouragement and support from A Roy,
S K Datta, R K Bhowmik and G K Mehta. The author thanks Pelletron and LINAC
group for providing good quality beams. The author thanks the collaborators B R Behera
(Panjab University), S K Mandal (Delhi University), R G Thomas (NPD-BARC),
B K Nayak (NPD-BARC), A Saxena (NPD-BARC). He also thanks H J Wollersheim
(GSI) and G Pascovici (IKP, Koln) for technical support in detector instrumentation. He
thanks and acknowledges the hard work of the students H Singh, P Shidling, S Kalkal,
R Sandal, V Singh, S Appanababu, E Prasad, G Mohanto, S Goyal, M Kaur, G Kaur, R
Mahajan, M Thakur, P Sharma, K Kapoor, R Dubey and T Banerjee.
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