Chapter 3
Experimental setup and details
This chapter provides the details of the experimental setup, measurements, detectors
and electronic setup used in the present study. In the present study we measured the
ER cross sections, fission fragment angular distribution and fragment mass distributions
for two reactions forming the same compound system 210Rn. The accelerator facility,
target fabrication techniques, different kinds of detectors used, electronics, and other
experimental facilities used in the present study are briefly discussed in this chapter.
3.1 15 UD Pelletron accelerator at IUAC
The 15 UD Pelletron accelerator at Inter University Accelerator Centre (IUAC), New
Delhi, is an electrostatic tandem accelerator [1, 2] capable of accelerating any ions,
except inert gases, from proton to uranium upto an energy of about 200 MeV depending
upon the ions. The accelerator is installed in vertical configuration in an insulating tank
of length 26.5 m and diameter 5.5 m, filled with SF6 gas. The schematic diagram of
the accelerator is shown in Fig. 3.1.
The ion source in the top produces negative ions. There are three different types of
ions sources available, which are, (i) Alphatros (ii) MCSNICS and (iii) Duoplasmatron.
Among these MCSNICS (Multi Cathode Source of Negative Ions by Cesium Sputtering)
is commonly used for the ion production. These negative ions are pre-accelerated to ∼200 KeV by the deck potential and then focussed and are mass analyzed using the 90o
injector magnet before injecting them into the low energy accelerating tubes. Injector
magnet bends the ions by 90o in vertical direction down in the accelerating column.
Inside the vertical accelerating tank a high voltage terminal is located at the center.
This terminal can be charged to very high potential varying from 4 MV to 15 MV.
A potential gradient is maintained through the tube from the top of the tank to the
terminal and from terminal to the bottom. There are thirty 1 MV modules 15 on
59
Experimental setup and details
-
-
-
-
-
Ion source
Injector deck
Injector magnet
Ion accelerating tube
Accelerator tank
High voltage terminal Charge stripper
Equipotential
rings
Sulphur hexafluoride
Pellet chains
Analyser magnet
To swithching
magnet
Negative ion
Positive ion
+
+
+
Figure 3.1: Schematic representation of IUAC Pelletron accelerator.
either side of the terminal. The portion above the terminal is called low energy section
and portion below the terminal is called high energy section. The injected ions get
accelerated down towards the high voltage terminal at the middle. At the terminal
the accelerated negative ions pass through a stripper, which can be a very thin carbon
foil or a small volume of gas. During this passage through this stripper, the negatively
charged ions lose electrons and thus result in a distribution of positive charges. This
distribution depends upon the velocity of the ions. These positively charged ions now
get repelled down towards the ground potential through the high energy accelerating
tube. A second stripper assembly is located in the high energy dead section. This will
help in further stripping and yield higher charge state and hence higher energy. The
60
Experimental setup and details
energy gained by the ions after emerging out of the accelerator is given by
Efinal = E0 + V (q + 1) (3.1)
if only single stripper is used, where E0 is the energy gained from the ion source deck,
V is the terminal potential and q is the charge state of the ion. When both strippers
are used during acceleration, final energy is given by,
Efinal = E0 + V (1 + 0.4 × q1 + 0.6 × q2) (3.2)
where q1 and q2 are the ion charge states. These ions after exiting from the tank are
bent by 90o using the analyzer magnet. This magnet also helps in selecting the energy
of the ions. These beams are then switched to any of the beam line using the switching
magnet.
IUAC pelletron accelerator gives both dc and pulsed beam at the target depending
upon the experimental requirements. The components of the pulsing system are based
on the principles of electrostatic or magentostatic deflection of ions and the velocity
modulation. Beam pulsing system is placed in the pre-acceleration stage and consists
of (i) Chopper (ii) Buncher and (iii) Travelling Wave Deflector (TWD). The chopper
chops the dc beam and produces the ion pulses which are compressed by buncher to
still narrower pulses. The function of the TWD is to change the repetition rate of the
ion beam pulses.
3.1.1 Chopper
Here the ion beam is swept across a slit by applying a RF field across a pair of plates
through which it passes. Chopper is located after the injector magnet and the slit used
is the image slit of the magnet. The ion pulse width at the exit of the slit depends
on several factors such as beam energy, RF voltage, applied frequency of the RF field,
distance between the slit and the pair of plates, slit width and the gap between the
plates. Chopper introduces an energy spread in the beam, which should be kept low to
have a very good performance of the buncher. This energy spread can be reduced by
having a dc bias volatage at the plates. IUAC chopper consists of two pairs of plates
provided with a dc bias. One pair of plates has 4 MHz RF field, while the other pair of
plates has 8 MHz RF field applied to it. Chopper produces ion pulses of width varying
from 10 ns to 60 ns.
61
Experimental setup and details
3.1.2 Buncher
The pulsing system consists of two bunchers. (i) Light Ion Buncher (LIB) and (ii)
Heavy Ion Buncher (HIB). The buncher works on the principle of velocity modulation.
RF is applied to a cylindrical tube through which the beam passes. Two other tubes at
ground potential are kept on either side of this tube. The phase of the applied RF field
is so adjusted that the particles in the leading edge are deccelerated, while particles
which are coming later are accelerated. This way the chopped beam gets compressed
and results in a time focus at a certain distance from the buncher. The compression of
the ion beams mostly occurs at the drift tubes and first few accelerating tubes. The
length of the bunching tube should be such that the time of transit in the tube is an
odd multiple of T/2, where T is the time period of RF. LIB consists of five bunching
tubes housed in a vacuum chamber and can bunch ions upto mass 80. HIB consists
of three bunching tubes housed in vacuum chamber and can be used to bunch ions of
mass ranging from 80 to 220.
3.1.3 Travelling Wave Deflector (TWD)
The TWD is located immediately after the chopper. It consists of twelve pairs of plates,
each of length ∼ 3 cm and provided with a dc bias of ± 200 V. The voltage pulses from
the amplifiers are applied to these plates at the same frequency as the repetition rate
of the pulsed beam. There is appropriate delay in these pulses to match the velocity
of the incoming ions. The over all effect of the dc bias and the voltage pulses at the
plates is that the plates remain at zero potential allowing the desired ion bursts to pass
through and rejecting the unwanted ion beam pulses.
3.2 Preparation of isotopic targets
Preparation of thin, isotopically enriched target is an important and challenging task
in any nuclear reaction experiments. To study the heavy ion induced reactions, in par-
ticular fission fragment angular and mass distribution measurements, fusion excitation
measurements at near barrier energies etc., require very thin targets. The reason is
that the incident beam and the reaction products loose energy as they pass through the
target material. This energy loss will alter the energy definition of the incident beam
as well as the resolution of the energy spectra of the reaction products. If the target is
sufficiently thick the low energy reaction products like evaporation residues and even
high energy fission fragments, will be stopped inside the target itself. Hence, in ideal
case we prefer self supporting targets or targets with very thin backing material. Iso-
62
Experimental setup and details
topically enriched targets of 194Pt and 186W used in the present study were prepared at
IUAC target laboratory using vacuum evaporation technique.
3.2.1 Preparation of isotopic 194Pt target on carbon backing
Natural and isotopically enriched thin Pt targets were prepared using the vacuum evap-
oration technique. The material available for evaporation was only 100 mg and the
target thickeness required for ER and fission measurement was 200-300 µg/cm2. Since
it was very difficult to prepare self supporting Pt targets, very thin carbon foil (20-30
µg/cm2) was chosen as the backing material. Since the available isotopic material was
very less, natural platinum was used for perfecting the method of fabrication and then
followed this method for the preparation of isotopic targets.
First step in target preparation was to prepare very thin carbon foil. For this,
cleaned glass slides were used as the substarte and barium chloride (BaCl2) was used
as the parting agent. Carbon evaporation was achieved using the resistive heating
method. Glass slides were kept at 15 cm away from the resistive heating arrangement.
Though commonly used parting agent teepol was used in the initial trials, BaCl2 was
found to be more suitable because of its high melting point. The evaporation was
carried out in the high vaccum evaporator. About 100 µg/cm2 BaCl2 was deposited
on the glass slides at a pressure of 2 × 10−6 Torr. The deposition rate was 0.2 nm/sec.
The chamber was allowed to cool to room temperature. After the successful deposition
of the parting agent film, carbon was deposited on the slides using electron gun (2
kW) bombardement technique, without disturbing the vacuum inside the chamber.
The pictorial representation of high vacuum chamber is shown in Fig. 3.2. 100 to 180
mA current was used during the evaporation. The thickness of the film deposited
was monitored online using the quartz crystal monitor setup. After the deposition
of required carbon film, the chamber was allowed to cool for 6 hours and then the
slides were carefully taken out and transported to ultra high vacuum evaporator. The
thickness of the carbon foils deposited were around 15 - 20 µg/cm2.
Isotopically enriched (96.5%) 194Pt deposition was done using the ultra high
vacuum evaporation setup. The carbon deposited glass slides were mounted at a
distance of 10 cm above the crucible. During the trial runs, it was observed that
platinum forms alloys with commonly used W, Ta and Mo boats. Hence, specially
designed carbon crucibles were made for the evaporation of platinum pellets. However,
due to the fact that the boiling point of platinum and the sublimation temperature
of carbon are not very different, it was very important to ensure that electron beam
63
Experimental setup and details
Figure 3.2: The high vacuum chamber used for the evaporation of barium chlorideand carbon.
was not falling on carbon crucible and care was taken to avoid any sharp edge in the
crucible. The evaporation was performed using the 6 kW electron gun setup. Vacuum
of around 2×10−8 Torr was maintained inside the chamber during the evaporation. At
the beginning, a low current of about 30 - 40 mA was maintained for about 15 minutes
so that uniform heat is produced inside the chamber. Later, the current was increased
to 90 mA and the deposition was started at a rate of 0.1 Ao/sec. The evaporation was
continued till targets of thickness 250 - 300 µg/cm2 were obtained. Exact thickness of
these targets were measured by the energy loss of the alpha particles in these foils from241Am source. However the films were not stable due to internel stress developed. In
order to make them stress free, these slides were annealed [3] to 425oC for two hours
in dry nitrogen gas and then cooled to room temperature. The targets were seperated
from the glass slides by floating them in warm distilled water and taken into respective
target frames. Fig. 3.3 shows the ultra high vacuum chamber used for the evaporation
of enriched targets.
3.2.2 Preparation of isotopic 186W targets
Preparation of tungsten targets were more difficult because of its very high melting
point. As in the case of platinum targets, the isotopic material available for the film
preparation was very less. Due to the difficulty in preparing self supporting tungsten
64
Experimental setup and details
targets, carbon foil of about 50 - 100 µg/cm2 was chosen as the backing material.
Figure 3.3: The ultra high vacuum chamber used for the evaporation of 194Pt and186W.
Carbon backing was prepared in the same way as described in section 3.2.1. using
the high vacuum chamber. It was found that the direct deposition of tungsten on the
carbon deposited slides would not work as the foils were unstable and were breaking
while floating in distilled water. Hence the deposited carbon films on the glass slides
were separated by floating them in warm distilled water. These films were not very
stable due to internal stress developed and were not strong enough to bear the heat
developed during the tungsten evapoartion. In order to make them stress free, these
slides were annealed in the tubular furnace at a temperature of 325oC for a period of
an hour. Dry nitrogen gas was continuously circulated through the furnace tube during
the annealing process. These films were later separated from the slides and mounted
on the target frames. In this way carbon foils of very high thickness and good quality
could be easily made. Fig. 3.4 shows the target frame and frame holders used in the
preparation of tungsten targets.
Tungsten deposition was carried out in the ultra high vacuum chamber using the
6 kW electron gun. Before depositing enriched 186W, trial runs were performed using
natural tungsten. Since availabe tungsten material was in powder form, special punch
65
Experimental setup and details
Figure 3.4: The target frames and target holder used in the preparation of isotopictargets.
and die were fabricated for making tungsten pellets of 3 mm diameter and 3 mm
length. During the deposition of tungsten, it was observed that the carbon foils were
curling due to excess heat generated on the films. This curling may later lead to the
damage of the films. In order to minimize the excess heat in the carbon foils, silver
paste was used to mount the foil in the target frame in subsequent trials. Silver paste
helps in conducting the heat from the carbon foil to the metalic frame holder and
thereby protects the foils from curling and damage. The target frames were mounted
on the target frame holder, made of stainless steel. These frames were also made heavy
to enhance the dissipation of heat which results in a minimized heat accumulation on
the carbon foils. After arranging the carbon foils (kept at a distance of 8 cm above the
pellet) inside the ultra high vacuum chamber, the chamber was evacuated to 2×10−8
Torr. A small current of about 40 mA was given for about 15 minutes and later
increased to 120 mA and tungsten started evapoartion at this stage. The evaporation
was kept low (0.1 Ao) to have better quality tungsten film. Target thickness of about
250 µg/cm2 was achieved in three hours. After the evaporation process the chamber
was allowed to cool for 6 hours and later vented using dry nitrogen gas. Target
thickness was measured using the alpha energy loss method.
66
Experimental setup and details
3.3 Charged particle and radiation detection
Typical heavy ion collision results a variety of charged and uncharged products, like
elastic, inelastic, fusion and fission products. For example, in fusion reactions the
formed compound system undergoes decay via the emission of various particles and
radiation (protons, neutrons, alpha particles, gamma quanta etc.) and also via fission.
The CN may also survive against fission and can result in the formation of ER. Hence,
to understand the reaction mechanism, it is highly essential to detect these reaction
products and analyze them systematically. Depending upon the interaction of these
charged particles and radiation with matter different kinds of detectors are used in
nuclear reaction experiments.
3.3.1 Charged particle detection
Charged particles are detected by their interaction with matter [4]. When a charged
particle enters a material it interacts with the matter primarily through the Coulomb
forces between their positive charge and negative charge of the orbital electrons within
the absorber atoms. Interactions with nuclei are also possible. However, such interac-
tions occur very rarely and are not very significant in the response of radiation detectors.
Upon entering any absorbing medium, the charged particles immediately interact simul-
taneously with many electrons. In any one such encounter, the electron feels an impulse
from the attractive Coulomb force, as the particle passes its vicinity. Depending upon
the proximity of the encounter this impulse may be sufficient either to raise the electron
to a higher lying shell within the absorber atom (excitation) or to remove completely
the electron from the atom (ionization). The energy that is transferred to the electron
must come at the expense of the charged particle and its velocity thus decreases as a
result of these encounters. In this way, the effect is to decrease its velocity continuously
until the particle is stopped inside the medium. The specific energy loss of a charged
particle inside a material is given by the Bethe formula,
−dEdx
=4πe4z2
m0v2N [ln
2m0v2
I− ln(1 − v2
c2) − v2
c2] (3.3)
where v and ze are the velocity and charge of the primary particle, N and Z are the
number density and atomic number of the absorber atom, m0 is the rest mass of elec-
tron, e is the electronic charge and I represent the average excitation energy. From
equation 3.3, it can be seen that the specific energy loss is proportional to the square of
67
Experimental setup and details
the charge of the incident particle and inversely proportional to the energy of the inci-
dent particle. For light charged particles (protons and alpha particles) specific energy
loss is maximum towards the end due to decreasing particle velocity. However for heavy
charged particles like fission fargments, specific energy loss decreases along the track.
Since fission fragments enter the medium with very large positive charge, the electron
pick up starts from the very begining and the decrease in dEdx
due to reduction in charge
is more dominant than increase due to the reduction in their velocity. Hence, when
charged particles enter inside a detector volume, they ionize the medium and result in
the production of electric charge pairs. In a gas detector electron-ion pairs are created,
while in a solid state detector electron-hole pairs are created. These charge pairs are
collected by applying suitable electric field and generate electric signals. These signals
from the detector contain information on the properties of the incident particles like
energy, timing etc. Detectors have been developed with liquid, solid and gas media.
Solid state detectors and gas detectors are widely used in nuclear reaction experiments.
Semiconductors are extensively used for detector fabrication. The average energy re-
quired to create an electron-hole pair in a semiconductor material is much samller
than that required for ionizing the gas. Hence the amount of ionization produced in
a semiconductor for a given energy is an order of magnitude larger than that in a gas
detector, which results in a better energy resolution in solid state detectors. Silicon
and germanium are widely used semicondutor materials for detector fabrication. Si,
which can be operated at room temperature, is extensively used for producing surface
barrier detectors and are used for charged particle detection. Detectors are also made
to provide position information of the particles incident on it. Position sensitive multi
wire proportional counters used for fission fragment measurements is discussed in detail
in section 3.4.1
3.3.2 Interaction of gamma rays with matter
When a nucleus is in its excited state, it may decay to its lower excited state or possibly
the ground state by the emission of a photon (gamma ray) of energy equal to the
difference in energy betwen the nuclear states (less a usually negligible correction energy
for the ”recoil energy” of the emitting nucleus). Gamma ray emission is observed in
all nuclei that have excited bound states. They are chargeless, massless and have the
velocity equal to that of light. They have more penetrating power than charged particles
and when they pass through a thickness of matter, their energy do not get degraded,
instead, the intensity gets attenuated. Gamma rays interact with matter mainly by
three different processes namely (i) Photo electric absorption, (ii) Compton scattering,
68
Experimental setup and details
and (iii) Pair production.
In photo electric absorption, the photon energy is fully absorbed and an energetic
electron (photo electron) is emitted from one of the bound shells of the atom. The
energy of the outgoing electron is given by
E = hν −BE (3.4)
where BE is the binding energy of the electron. Since a free electron cannot absorb
a photon and also conserve momentum, photo electric effect always occur on bound
electrons with the nucleus absorbing the recoil momentum. The cross section for photo
electric absorption depends upon the atomic number Z of the material. In MeV energy
range, this dependence goes as Z5. Hence high Z materials are the most favoured
materials for photo electric absorption. This is an important factor while choosing the
material for γ detectors.
In Compton scattering, the photon transfers a part of its energy to the electron.
The photon gets deflected through an angle φ from its original direction during this
process. Since scattering is possible in all angles, the energy transferred can also vary
from zero to a large fraction of the initial photon energy. The scattered photon energy
is given by,
hν ′ =hν
1 + hνm0c2
(1 − cosφ)(3.5)
where, m0 is the mass of the electron. In pair production, an energetic photon with
energy greater than 1.022 MeV get transformed into an electron-positron pair. This
process require a third body to conserve momentum, usually a nucleus. In all the above
interaction mechanisms the photon energy is transferred to electrons (and to positrons in
the case of pair production). These particles loose their energy in the detector material
and produce ionized atoms and electrons. Basic detection hence depends upon the
collection of these secondary paricles.
General criteria for a good γ-ray spectrometer device are that they should have
(i) excellent energy resolution, (ii) good photo peak efficiency and (iii) good timing
properties [4]. Germanium detectors (Z of Ge is higher than that of Si) are widely
used for γ-ray detection. Although sodium iodide detectors have better efficiency than
germanium, the excellent energy resolution of Ge makes it the gamma detector of choice
for high resolution studies. However, there are some problems with Ge detectors which
are (i) Most probable interaction of most of the gamma rays is Compton scattering and
69
Experimental setup and details
upon entering the detector material, they will scatter out before the full energy has
been absorbed by the material, which results in a large Compton background and (ii)
Ge detectors must be kept at liquid nitrogen temperature for very good resolution. In
scintillator detectors as the radiation passes through the scintillator, it excites the atoms
and molecules and make them to emit light. This light is transmitted to the optically
connected photomultiplier, where it is converted into a weak current of photoelectrons,
which is further amplified by an electron multiplier system. Crystals of NaI, Bi4Ge3O12
(Bismuth Gemanate Oxide, BGO) etc., are used as detector material in this case.
3.4 Fission fragment mass distribution measure-
ments
Heavy ion induced fusion-fission has been a topic of great interest because of the
anomalous behaviour of fragment angular and mass distributions at near barrier
energies. Fusion-fission dynamics is very much dependent on the entrance channel
parameters at near barrier energies. At moderate beam energies, the total cross section
consists of elastic, inelastic, fusion and fusion-like (non-compound nucleus processes
like quasi fission, fast fission and pre-equilibrium fission) processes. In fission fragment
mass and angular distribution measurements, it is very essential to separate out the
fission events from the CN process, from elastic, quasi-elastic and other non-compound
nucleus fission channels. Experimentally this separation can be achieved by keeping
the detectors at proper folding angles for the complimentary fragments. The concept
of folding angle is discussed in detail in chapter 4. Folding angle distribution is the
experimental signature of the linear momentum trasferred in the reaction process.
Conventionally silicon detectors are used for fission fragment detection in light heavy
ion (A < 20) induced fission reactions. However, these detectors are not very efficient to
completely separate fission fragments from the contaminants like elastic, quasi-elastic
and other non-compound nucleus fission channels, due to very large energy straggling
of the fission fragments. Other disadvantages of silicon detectors include their limited
count rate handling capability, sensitivity to radiation damage, high cost and small
area that reduces the overall detection efficiency. Position sensitive, large area gas
detectors like multi wire proportional counters (MWPCs) with small radiation length
are transparent to elastic and quasi-elastic particles in heavy ion (A < 40) induced
reactions. Also, they are helpful in differentiating the fragments from CN fission and
non-compound nucleus fission by accurate determination of the folding angle of the
fission fragments. These detectors are inexpensive and can be made with ease in
70
Experimental setup and details
various sizes. They are insensitive to radiation damage, and have high count rate
handling capabilities. Most importantly, these detectors provide very good position
and timing resolution.
Fission fragment mass ratio and mass angle distribution measurements for the two
reactions 16O+194Pt and 24Mg+186W, both forming the same CN 210Rn, were performed
using the General Purpose Scattering Chamber (GPSC) [5] at the Inter University Ac-
celerator Centre, using the 15 UD pelletron beams. The schematic of the experimental
setup used for the fragment measurements is shown in Fig. 3.5. Two reactions were
studied in two separate runs. Pulsed beam of 16O and dc beam of 24Mg were used in
these experiments, to bombard the isotopically enriched 194Pt (96.5% enriched) target of
thickness 300 µg/cm2 on 20 µg/cm2 thick carbon foil and 186W target (99.5% enriched)
of thickness 110 µg/cm2 on 20 µg/cm2 carbon backing, respectively. The detectors
used and the electronic set up were slightly different in the two runs. Two multi wire
proportional counters were mounted on the moving arms of the scattering chamber,
which were placed at exact folding angles, were used for the fragment detection.
ML
Beam
MWPC1
MWPC2
FF1
FF2
MR
Figure 3.5: The shematic representation of the experimental setup used for fragmentmass distribution measurements. ML and MR are monitor detectors used at ±10o
with respect to the beam. FF1 and FF2 are the complimentary fission fragments.MWPC1 and MWPC2 are the two gas detectors kept at folding angles for fragmentdetection.
71
Experimental setup and details
3.4.1 Muti wire proportional counters
In fission fragment mass and angular distribution measurements, fission fragments have
to be isolated from the large background of unwanted events like elastic, quasi-elastic
and other non-compound reaction channels. At near and below barrier energies this
task becomes increasingly difficult and position sensitive MWPCs are excellent in sep-
arating the fragments from the contaminants at all energies. The operating parameters
such as gas pressure, voltages on electrode etc., can be adjusted to make the detector
transparent to the unwanted light particles and make it sensitive to heavier particles
such as fission fragments. The energy resolution of these detectors are not very good.
However they provide very high gain, fast rise time, good position resolution and very
high ( > 90%) detection efficiency. The design geometry of these detectors are similar to
that of Breskin [6] detector. The schematic diagram of the detector is shown in Fig. 3.6.
These are transmission type detectors. The detector used in 24Mg+186W reaction [7]
consisted of 5 wire frames, namely a cathode, a position wire frame to measure the
horizontal (X) position, central anode, another position wire frame to measure vertical
(Y) position and and again a cathode frame. Ative area of the detector is 20×10 cm2.
All wire frames are made of gold plated tungsten wires of 20 µm diameter, at a sep-
aration of 1.27 mm. The separation between adjacent wire farmes is 3.2 mm. X-wire
frame consists of 160 wires while all other wire frames consist of 80 wires. Position in-
formation of the particles hitting the detectors were obtained from the delay-line chips.
Each chip has 10 taps with a delay of 2 ns/tap. End to end delay in X and Y positions
are hence 160 and 80 ns, respecively. 1 µm thick mylar foil was used to isolate the gas
detector from the vacuum chamber. Isobutane was used as the operating gas at very
low pressure (< 3 mbar) during the fragment measurements. In the second measure-
ment (16O+194Pt reaction) slightly bigger detectors of active area 24×10 cm2 [8] were
used for the fragment detection. In this case, anode wire plane consisted of 12.5 µm
diameter gold plated tungsten wires soldered at 1 mm apart. X and Y sense wires are
of 50 µm diameter gold plated tungsten wires at 2 mm separation while cathode wires
are also of 50 µm diameter gold plated tungsten wires placed 1 mm apart. X sense wire
plane consisted of 120 wires at a pitch of 2 mm and Y sense wire consisted of 50 wires
at 2 mm pitch. The delay between successive X-wires were 2 ns while that between
successive Y-wires were 5 ns. Stretched polypropylene film of thickness 50-100 µg/cm2
was used in the entrance window. Typical bias voltage applied to anode and cathode
were +400 V and -180 V, respectively, during the experiments.
72
Experimental setup and details
Figure 3.6: The shematic representation of the MWPC used in mass distributionmeasurements.
3.4.2 MWPC electronics
The block diagram of the electronics used for a single MWPC is shown in Fig. 3.7.
The fast timing signals from the central anode are amplified by ortec VT120A fast
pre-amplifier. Position signals (XL, XR, YU and YD signals) were amplified by fast
current pre-amplifiers VT120B. The primary function of the pre-amplifier is to extract
the signal from the detector without degrading the signal to noise ratio. There are
different kinds of pre-amplifiers such as current sensitive, charge sensitive and parasitic
capacitance pre-amplifiers in use. Charge sensitive pre-amplifiers are normally used in
energy spectroscopy applications and the pre-amplifier output is proportional to the
quantity of charge inside the current pulse. For timing applications with rise-time <
1 ns, model VT120 is the ideal choice. The output from a pre-amplifier has a small
rise-time and long fall time. Pre-amplifiers are adjusted in this way to ensure maximum
charge collection. For further amplification of the signal and shaping, these signals are
processed through amplifiers. Shaping is achieved through differentiation and several
integration stages using RC circuits, which results in a Gaussian shaped pulse, whose
amplitude is proportional to the charge collected and hence the energy of the incident
particle. The cathode signal from the MWPC detector is processed in this way. The
pulse height of the signal is then digitized using the anlog to digital converter (ADC)
and the spectra will be stored in a computer using the CAMAC dataway. If the timing
signals are not very strong, they are amplified using timing filter amplifiers (TFA) before
giving to the discriminators. However in fission fragment measurements, these delay-
line signals are not very weak and timing amplifiers are not required. Timing signals are
given to the time to digital converter (TDC) for digitization after proper delay. Timing
pulse from the anode is processed through the constant fraction discriminator (Ortec
CFD 935). This signal can be used as the start of the TDC and position signals will
73
Experimental setup and details
be used as individual stops after giving proper delay with respect to the start signal
(anode signal here). The stretched anode signal (5-6 µs width) itself can be used as the
master strobe for ADC.
A
Anode
XL
XR
YU
YD
Cathode
CFD 935VT 120
FPA
FPA
FPA
FPA
PA AMP
OCT
CF
DISC
CF8000
GDG
GDG
GDG
GDG
GDG
T
D
C
A
D
C
M
A
C
M
W
P
C
Master
Start
StopStopStopStop
C
Figure 3.7: Block diagram of electronic set up required for a single MWPC.
3.4.3 Fragment mass distribution measurements
The mass distribution measurements for the reactions 16O + 194Pt and 24Mg + 186W
were perfromed in two separate runs. The experimental setup for the former reaction
is discussed first.
Pulsed beam of 16O with a pulse separation of 250 ns and pulse width of ∼ 1 ns, in
the energy range 75 to 102 MeV, was used in the experiment. Two large area, position
sensitive MWPCs [8] of active area 24 cm × 10 cm, were used for fission fragment
measurement, by forming a time of flight (TOF) setup. These detectors were mounted
on the two arms of the scattering chamber, the forward detector centered at polar
angle θ= 45◦ (azimuthal angle φ = 90◦) and backward detector centered at θ = 115◦
(azimuthal angle φ= 270◦). The nearest distance to the forward detector from the
target was 56 cm and that to the backward detector was 30 cm. The forward detector
has an angular coverage of about 24o. Backward detector had an angular coverage of
about 43o in this geometry. The basic reason for keeping the detectors at different
distances was to make sure that we were not missing any complimentary fragment
corresponding to the fragment that is being detected at the backward detector. The
target was kept at 45◦ with respect to beam direction, which minimized the energy
loss of the fragments and also avoided the shadowing of the detectors by the target
ladder. The gas detectors were operated with isobutane gas at low pressure (∼ 3.5
Torr). The fission fragments were well seperated from the elastic and quasi elastic
74
Experimental setup and details
channels, both in time and energy loss spectra. Two solid state detectors mounted at
±10◦ with respect to the beam axis, were used to monitor and position the beam at
the center of the target throughout the experiment. One of this monitors was used to
get the time structure of the beam by generating a TAC signal with RF signal. The
position information of the fragments entering the detectors was obtained from the
delay-line read out of the wire planes. The fast timing signals from anode of both,
MWPC1 and MWPC2 were used to obtain the TOF of the fragments with respect to
the beam pulse. These anode signals were processed through constant fraction discrim-
inators. The position signals (XL, XR, YU and YD) were also processed through the
discriminator (CF8000) and further delayed using gate and delay generator (GDG),
with respect to the start signal and given to the time to digital converter (TDC)
as stop signals. A fast coincidence between the logically ’OR-ed’ output signal of
two monitors and two MWPC anode signals and RF pulse (using the multi purpose
coincident unit MPCU) was used as the master trigger for the data acquisition system.
Energy signals from the two monitors and two MWPCs were given to the 16 chan-
nel analog to digital converter (ADC-Philips 7164 model) through 571 Ortec amplifiers.
In the case of 24Mg + 186W system, the detectors used were slightly smaller with an
active area of 20 cm × 10 cm [7]. The forward detector was centered at polar angle θ =
38◦ (azimuthal angle φ = 90◦) and backward detector centered at θ = 113◦ (azimuthal
angle φ = 270◦). The nearest distance to the forward detector from the target was
55.5 cm and that to the backward detector was 40 cm. As the beam current was very
low for 24Mg in the required energy range, dc beam was used in the measurements (in
the energy range 111 to 125 MeV in laboratory frame) and the time difference method
was used for obtaining the mass ratio distributions of the complimentary fragments.
Any of the signals of two MWPCs and two monitor detectors formed the master strobe
for the data acquisition system in this measurement. Individual TDCs were used for
individual MWPCs with anode as the start and four position signals as individual stop.
A TAC signal was formed by taking start from the anode signal from the back detector
and stop from the delayed anode signal from the front detector. The block diagram for
the electronic setup used for the two experiments are shown in Fig. 3.8 and Fig. 3.9.
The data were collected using the linux based IUAC online data acquisition program
FREEDOM [9]. The NIM and CAMAC standards are followed in implementing this
data acquisition system. The system is capable of running on a single computer or
on a network, under the operating system linux. Experimental signal conversion is
performed by various NIM and CAMAC modules and data is read through the CAMAC
dataway. The Offline data analysis was performed using IUAC’s advanced data analysis
75
Experimental setup and details
package CANDLE [10] and also using the software ROOT [11]. The detailed calibration
procedure of MWPC detectors, analysis and results of mass distribution measurements
are discussed in chapter 4
M
W
P
C
1
XL
XR
YU
YD
A
CAAMP
VT
2
C
P
M
W
C
YD
YU
XR
XL
A
AMP
VT
CFD 935
GDG
GDG
GDG
GDG
GDG
GDG
GDG
GDG
GDG
GDG
CFD 935
MPCU
T
D
C
A
M
A
C
C
A
D
C
PA
PA
’OR’ ’ AND ’
ML PA AMP
TSCA
RF
CFD 935
GDG
MR PA AMP TSCA
FPA
FPA
FPA
FPA
FPA
FPA
FPA
FPA
TACStartStopRF
Start
MasterStrobe
OCT
CF
DISC
OCT
CF
DISC
Figure 3.8: The block diagram of electronics used in the time of flight setup toobtain the mass distribution of the fission fragments.
3.5 Evaporation residue measurements
Evaporation residues (ERs) are important probes, which provide a lot of information in
heavy-ion induced fusion reactions. ER excitation function can provide valuable infor-
mation on the onset of nuclear dissipation as well as non-compound nucleus processes.
Dissipation results in an enhancement of ER cross section. On the contrary onset of
non-compound nucleus processes reduce the ER cross section over the statistical model
predictions. In the present study we measured the ER cross section for 16O+194Pt reac-
tion, in the laboratory energy range 75.4 MeV to 103.1 MeV. The measurements were
76
Experimental setup and details
2
C
P
M
W
YD
YU
XL
FPA
FPA
FPA
FPA
FPA
VT CFD 935
VT
PA
CFD 935
AMP
GDG
GDG
GDG
GDG
GDG
ML PA AMP
OCT.
CF
DISC
OCT
CF
DISC
Start
T
D
C
1
GDG
GDG
GDG
GDG
GDGStart
MR PA
TSCA
TSCAGDG
LOGIC
FIFO
FPA
FPA
FPA
PA
AMP
MasterStrobe
A
D
C
T
D
C
2
XR
M
W
P
C
AMP
Bi polar
Biplora
C
A
M
A
C
C
C
A
XL
XR
YU
YD
A
1
Figure 3.9: The block diagram of electronics used in the fragment mass distributionmeasurement using time difference method.
carried out using the HYbrid Recoil mass Analyzer (HYRA) at IUAC. The gas filled
mode of the separator was used in the measurements.
3.5.1 HYbrid Recoil mass Analyzer(HYRA)
HYRA [12, 13] is a dual mode, dual stage spectrometer/separator with its first stage
capable of operating in gas filled mode in normal kinematics and both stages in vacuum
mode, in inverse kinematics. In fusion reactions, the ERs produced are kinematically
forward focussed in a narrow cone. Measuring these low intensity, low energy reac-
tion products from the intense beam background is a challenging task. Recoil Mass
Separators/spectrometers are used (i) to separate the low intensity reaction products
from high intensity beam background, (ii) to analyze the mass of the reaction products
and (iii) to carry the reaction products to a background free area (focal plane) and
focus them on the focal plane detector assembly. Though both vaccum mode and gas
filled mode separators are in use, separators using vaccum mode are limited by their
77
Experimental setup and details
poor transmission efficiency for very asymmetric reactions. This is a severe concern
in experiments involving very low cross sections. Gas filled separators offer very high
transmission efficiency in comparison to vaccum mode separators, due to their inherent
velocity and charge state focusing.
3.5.2 Operational principle
When a charged particle (ion) moves through a magnetic field, the Lorentz force ~F
acting on it is given by,
~F = q[~v × ~B] (3.6)
where q is the charge and ~v is the velocity of the ion. ~B is the magnetic flux density.
The resulting magnetic rigidity of the ion of mass m is given by
Bρ =mv
q(3.7)
ρ is the curvature of the radius. If the medium is filled with dilute gas, the moving
ions undergo multiple collisions with the atoms of the gas, which change their energy,
direction and charge state. After a statistically large number of collisions with the atoms
of the medium, the charge distribution approaches equilibrium and the ions follow some
mean trajectory determined by the mean charge state (q̄) value. This mean charge state
is approximately represented by,
q̄ ≈ v
v0
Z1/3 (3.8)
where v 0 is the Bohr velocity and Z is the ion atomic number. Substituting Eqn. 3.8
in Eqn. 3.7,
Bρ = 0.02267A
Z1/3(3.9)
Hence it is clear that the curvature radius of the ion in a magnetic field region filled
with dilute gas is determined mainly by mass, to a lesser degree by Z and is independent
of charge and velocity. Ions with different masses hence will follow different trajectories
and can be effectively separated.
.
78
Experimental setup and details
Figure 3.10: First stage of HYRA used in gas-filled mode.
3.5.3 Electromagnetic configuration and features
First stage of the HYRA was used for the ER excitation function measurements in the
present study. First stage can be operated in momentum dispersive mode in gas-filled
mode or as a momentum achromat in vacuum mode. The gas-filled mode was used
for the ER measurement studies for 16O+194Pt reaction. In the gas-filled mode, the
mass resolution is poorer due to multiple scattering effects in the gas medium. The
electromagnetic configuration of first stage of HYRA is QQ-MD-Q-MD-QQ, where
QQ stands for quadrupole doublet, MD stands for magnetic dipole and Q stands for
quadrupole singlet. The overall length from target chamber to focal plane chamber is
∼ 7.6 m in first stage. The first stage of HYRA is shown in Fig. 3.10. The second stage
of HYRA will be operated only in vacuum mode. This stage has the configuration
QQ-ED-MD-QQ, where ED stands for electrostatic dipole. The elements in the first
stage (Q1Q2-MD1-Q3-MD2-Q4Q5) are designed for 2.25 T-m magnetic rigidity (with
radii of 1.5 m and and maximum magnetic field of 1.5 T) to be able to handle very
heavy residues in gas-filled mode. In vacuum mode the momentum dispersion of
HYRA is 8 mm/% at Q3 centre and zero at the focal plane (after Q5). The target to
Q1 distance is kept minimum (40 cm) and Q1 and Q2 are used in strong focusing mode
to increase the acceptance of the spectrometer. Even though the original design is to
use superconducting quadrupoles for Q1 and Q2, in the present study we used room
temperature quadrupoles (Q1 and Q2) for initial focusing. The first magnetic dipole
79
Experimental setup and details
Figure 3.11: The shematic of focal plane detector system used in the gas filled modeof HYRA. Window foil separating the isobutane gas region from helium gas region,MWPC detector, removable shutter and 2-dimensional silicon detector are shown.
(MD1) is provided with 50 mm tall tantalum linings with water cooled copper at the
back. The primary beam will strike the tantalum linings with larger radius in vacuum
mode and smaller radius in gas-filled mode.
The choice of the filling gas depends on the application. In fusion studies, helium
gas is used as the standard gas. Different laboratories use different gases. At Dubna
H2 is being used. Separation of transfer products is better with H2 gas. In atomic mass
spectroscopy (AMS) applications, where high resolution is required, N2 gas at higher
pressure is used. In our measurements, we used helium gas in the separator at very
low pressure (0.15 Torr). The helium was introduced into the target chamber through
a solenoid based valve controlled by a MKS gas pressure controller which compares the
set pressure value with that measured using a baratron gauge. Helium was pumped
out through MD1 roughing pump system. A continuous flow of helium was maintained
for dynamic control of the gas pressure. This gas filled region was separated from the
high vaccum of the accelerator beam line using stationary window foil. We used a Ni
foil (1.5 mg/cm2 thick) and a carbon foil (300 µg/cm2 thick) as the window foil in our
two separate runs.
3.5.4 Charged particle detector setup
In very asymmetric fusion reaction, detecting the low energy ERs at the focal plane
is very challenging. In the present measurements we used two monitor detectors at
80
Experimental setup and details
the target chamber, which were implanted silicon detectors. These detectors were
mounted at ±22o with respect to the beam direction. These detectors were of 300 micron
thickness with an active area of 100 mm2. They were used for beam flux normalisation
and also for beam monitoring. Monitor counts were used for normalisation of absolute
cross section, discussed in detail in chapter 5.
The Focal plane detector system consisted of a position sensitive MWPC [14]
followed by a two-dimensional position sensitive silicon detector. Fig. 3.11 shows
the focal plane detector system used in the gas filled mode operation. The MWPC
is of Breskin type with very good position and timing resolution. Active area of
this detector is 57 × 57 mm2 with five sets of wire frames (cathode, followed by
a vertical wire plane which gives Y position, anode, another horizontal wire plane
which will give X position and a second cathode). However, while mounting the
detector at the focal plane of HYRA, we removed the last cathode wire-plane and
hence had only four wire planes. The cathode wire plane was connected to a 142IH
charge sensitive pre-amplifier. The position signals were taken from the two ends
of the X and Y frames through delay-line chips. Timing signals were taken from
the anode wire. This fast anode signal was used as the common start for the TDC
with position signals as individual stops. The MWPC was operated with isobutane
gas of about 2 mbar pressure, filled in the focal plane chamber. The chamber filled
with isobutane gas was separated from the helium gas filled region of HYRA using a
large area (5 inch × 2 inch) polypropylene foil of 0.5 micron thickness. The detec-
tors were operated with bias voltages of +380 V (anode bias) and -180V (cathode bias).
MWPC detector was followed by a two-dimensional position sensitive silicon detec-
tor with resistive layer at the front. A removable shutter was placed in between these
two detectors. This shutter was inserted during the field optimization and tuning to
protect the silicon detector from any radiation damage. This detector has an active
area of 50 × 50 mm2 and was suppiled by EURISYS. There are five outputs, four posi-
tion signals and one energy signal. These four position signals were taken from the four
corners of the front side of the detector. Energy signal was taken from the back side.
The detector was used with a bias voltage of +100V and leakage current was about
0.35 µA. The four position signals and the energy signals were processed through Ortec
571 amplifiers. The X and Y position signals were obtained from the detector outputs,
using the following relations
X =C +D −A−B
E(3.10)
81
Experimental setup and details
Y =A+ C −B −D
E(3.11)
where A, B, C and D are the position signals and E is the energy signal as shown in
Fig. 3.12.
Figure 3.12: The shematic of two-dimensional position sensitive silicon detectorused in HYRA focal plane.
Apart from these detectors we used a high resolution High Purity Germanium
(HPGe) detector of 23% relative efficiency, mounted at the bottom side of the tar-
get chamber, to measure the transmission efficiency of the separator using the gamma
method. In this method, the gamma rays are recorded in the HPGe in singles and in
coincidence with the ERs. The ratio of counts of a specific gamma line in the coinci-
dent spectrum to that in the singles spectrum gives the absolute transmission efficiency.
However, this method did not work in the present measurements, because of very large
singles gamma background arising from fission, and also from the reaction products of
beam with the Ni foil. A new method, using a well studied calibration reaction and
simulation code, was employed in determining the transmission efficiency, discussed in
detail in chapter 5.
3.5.5 Experimental details
The experiment was performed using the first stage of HYRA. Chopped 16O beam
with a pulse separation of 4 µs was used in the experiment to bombard isotopically
enriched 194Pt (96.5% enrichement) target of thickness 300 µg/cm2 on 20 µg/cm2 thick
carbon foil. Two silicon detectors were mounted at ±22o (at a distance of 108 mm
from the target) with respect to the beam direction and was used for monitoring the
82
Experimental setup and details
beam incidence on the target. Monitor counts were also used to normalize the absolute
ER cross sections. The target ladder was mounted 51 mm upstream to the center of
the target chamber. The diameter of the target chamber was 200 mm. Apart from194Pt target, 184W (225 µg/cm2 on 110 µg/cm2 carbon backing), 27Al (220 µg/cm2 self
supporting) targets and quartz were mounted on the target ladder. The 16O+184W
reaction [15], which was well studied using the vacuum mode spectrometer HIRA [16]
was used as the calibration system for the magnetic field as well as gas pressure
optimization. The same reaction was used to obtain the transmission efficiency of
the spectrometer. ER excitation functions were measured at eight laboratory beam
energies, (at the center of the target, after correcting the energy loss in the nickel foil,
carbon backing and half thickness of the target film) 75.4, 79.5, 83.7, 87.8, 91.9, 96.0,
101.1 and 103.1 MeV. At the highest energy, (i.e., at 103.1 MeV), dc beam was used to
compensate for the low beam current. At 101.1 MeV, pulsed beam with 4 µs repetition
was also used. Thus we confirmed that the contaminations at the focal plane arising
from the scattered particles were less than 1.5 %. At energies above 101.1 MeV, the
possible contamination at the focal plane was expected to be even less. This was due
to the very low scattering cross sections at higher energies, especially at energies well
above the Coulomb barrier.
In the gas-filled mode, the particles undergo multiple collisions with the gas atoms.
These collisions change the charge state as well as the energy of the particles. Par-
ticles experience a continuous reduction in their energy during their transit through
the gaseous medium. At optimum gas pressure and field values, the charge state and
velocity focusing occur and the particles follow a mean trajectory decided by the mean
charge state, as discussed in section 3.5.2. The mean charge state was calculated using
a simulation code developed in-house [17] using the empirical formula [18, 19] available
in literature. The energy loss of the particles in the gas medium, in multiple steps,
was also incorporated in this simulation. The magnetic field values of each dipole were
calculated using the average charge state and the energy of the particle at the center of
the dipole. The beam was first tuned on the quartz mounted on the target ladder. After
succesfully tuning the beam on quartz, 184W was introduced in the beam position. The
calculated field values were used to set the dipole fields. The gas pressure was first set
for 0.5 torr and later varied to different values. Data were collected for the maximum
transmission at the focal plane detector. The field values of the magnets Q1 and Q2
were scaled by MD1 scaling factor. Similarly Q4 and Q5 field values were scaled using
MD2 scaling factor and Q3 fields were scaled using the mean of MD1 and MD2 scaling
factors. The field values were varied ±10% in steps of 2% at the set-gas pressure by
83
Experimental setup and details
maximizing the ER yield at the focal plane and HYRA was set for the optimum field
settings. After the field optimization, the gas pressure was varied to different values
(0.25 Torr, 0.15 Torr etc) and ERs were collected at the focal plane. It was observed
that helium gas pressure of 0.15 Torr gives the best transmission. The background was
also found to be the least in this setting.
A self supporting Ni foil of thickness 1.5 mg/cm2 was used as the window foil to
separate the beam line vacuum from the gas filled region of HYRA in this experiment.
The reaction 16O+194Pt being very asymmetric, the ERs produced were of very low
energy and detecting these low energy ERs at the focal plane was very challenging.
As mentioned earlier the focal plane detector system consisted of an MWPC followed
by a two dimensional position sensitive silicon detector. At the lowest bombarding
energy (75.4 MeV), after losing energy in the target foil, helium gas, polypropylene
foil and in isobuatane gas, final energy of the ER at the focal plane was around 1 -
1.5 MeV. In fusion cross section measurements, the forward focused recoils have to
be separated from the background arising from the scattered beam-like as well as
target-like particles. Beam-like contamination can be handled easily, as they differ
very much in their energy and mass from the heavy ERs. However, target-like recoils
are a major concern in gas-filled separators. At higher energies, particularly at energies
well above the Coulomb barrier, the contamination from target-like particles are very
less owing to the negligibly small cross sections. However, at near and below barrier
energies target-like particles contribute maximum to the contamination at the focal
plane. The slowly moving ERs, produced at the target chamber took about 3.5 to 4
µs to reach the focal plane, where they were detected. A time-of-flight (TOF) setup
was formed by taking the start signal from the MWPC anode and stop signal from
the RF. This TOF setup helped us to have a very clean separation of ERs from the
beam-like and target-like contaminations. Fig. 3.13 shows the two-dimensional plot of
energy of the particles deposited in silicon detector vesus TOF at 101.1 MeV beam
energy. Fig. 3.14 shows the two-dimensional plot of energy loss (∆E) of the particles
in MWPC detector versus TOF at 101.1 MeV.
Major ERs (205Rn and 206Rn) detected at the foal plane produced in 16O+194Pt
reaction, decayed via alpha decay (with half lives greater than few seconds). These de-
cayed alpha particles were also detected in the two-dimensional silicon detector. These
alpha particles further confirmed that the particles reaching the focal plane were true
ERs. It may be also mentioned that in the case of 16O+184W reaction, no decay alphas
were seen at the focal plane. Fig. 3.15 shows the alpha decay at the focal plane detector.
84
Experimental setup and details
Figure 3.13: Two-dimensional plot of energy versus TOF for the reaction 16O+194Pt at 101.1 MeV beam energy. ERs are seen at the center, while scatteredparticles (which are very less in number) are seen on the top.
Figure 3.14: Two-dimensional plot of energy loss (∆E) in MWPC versus TOF at101.1 MeV. ERs are seen at the center, while scattered particles (which are very lessin number) are seen on the top and beam-like particles at the bottom
85
Experimental setup and details
1000 2000Energy (arb. unit)
0
2000
4000
Co
un
ts
16O +
194Pt
16O+
184W
ER
α-particles
ER
Figure 3.15: The particles detected in the two-dimensional silicon detector at thefocal plane in 16O+194Pt and 16O+184W reactions. ERs and decayed alpha particleswere detected in 16O+194Pt reaction, while no decay alphas were detected in the caseof 16O+184W reaction.
27Al target mounted on the target ladder was used for checking the beam-like
contamination at the focal plane, during the experiment. Magnetic fields were set for
the ERs from 16O+194Pt reaction and data were collected for some fixed duration.
Later 27Al was put in the beam position and without changing the fields originally set
for 16O+194Pt reaction, data were collected for the same duration of time. Fig. 3.16
shows the two-dimensional plot of energy deposited in the silicon detector versus
energy loss in the MWPC for (A) 16O+194Pt reaction and (B) 16O+27Al reaction,
respectively. This confirmed the excellent beam rejection capability of the separator.
The block diagram of the electronics used in the experiment is shown in Fig. 3.17.
Energy signals from the two-dimensional silicon detector (A, B, C, D and E), monitor
detectors (ML and MR) and cathode signal from MWPC were processed through charge
sensitive pre-amplifiers (Ortec 142IH), amplifiers (Ortec 571) and were given to the ADC
(Philips 7164, 16 channel-4K-10V ADC). MWPC anode signal was processed through
the fast timing pre-amplifier (VT120) and CFD 935. MWPC-RF TAC output was also
86
Experimental setup and details
Figure 3.16: Two-dimensional plot of energy deposited in the silicon detector ver-sus energy loss in the MWPC (both in arbitrary units) for (A) 16O+194Pt reactionand (B) 16O+27Al reaction, respectively. Data collected for the same duartion forboth the reactions. Magnetic fields were set for the ERs from 16O+194Pt during themeasurements.
given to the ADC. Four position signals (XL, XR, YU and YD) from the MWPC were
processed through fast pre-amplifiers and octal-CFD (CF8000) and were given to TDC
as individual stops. Timing signals from the two monitor detectors, MWPC anode
and the two-dimensional silicon detector were logically ’OR’-ed using logic fan in - fan
out (LF4000) unit. This signal was used as the master strobe for the data acquisition
system. Data were collected and later analyzed using the IUAC software CANDLE.
The detailed analysis and results are discussed in chapter 5.
87
Experimental setup and details
ML
MR
M
W
P
C
S
S
B
D
PA
PA
PA
PA
PA
AMP
AMP
AMP
AMP
AMP
PA
PA
AMP
AMP
A
D
C
7164
AMP
CFD935
OCT.
CF
DISC.
GDG
GDG
GDG
GDG
LF
4000GDG
MasterStrobe
Master
TWD CFD GDG
TACStart
Stop
C
A
M
A
CT
D
C
A
B
C
D
E
VT120Anode
XL
XR
YU
YD
Cathode
FPA
FPA
FPA
FPA
PA
Figure 3.17: The shematic block diagram of the electronics used in the ER mea-surements of 16O+194Pt reaction.
3.6 Fission fragment angular distribution measure-
ments
Though nuclear fission was discovered more than 65 years back, it continues to be a
hot topic even today. Considerable effort [20, 21] has been made over the years to
understand the dynamics of fusion-fission reactions, both experimentally and theoret-
ically. The angular distribution of the fission fragments is an effective probe to study
the dynamics of fusion-fission process. Through this study, it has been possible to have
insight into the evolution of the composite system after the capture, as it relaxes in
various degrees of freedom such as energy, mass, angular momentum and shape degrees
of freedom. Fragment angular distribution studies are also important as they appears
to be sensitive not only to the entrance channel of the interacting particles, but also to
the statistical aspects of the intermediate system. The observed anomalous behaviour
of fragment angular distributions and their dependence on various entrance channel
parameters such as mass asymmetry, deformataion etc, are not fully understood till
date. In this section, we discuss the experimental setup used for the fission fragment
88
Experimental setup and details
angular distribution measurements in 16O + 194Pt reaction (α = 0.847) populating the
compound system 210Rn (αBG = 0.857, N = 124). The entrance channel mass asymme-
try of this reaction is lying very close to αBG value. Hence the angular anisotropies are
expected to be normal. As the target 194Pt is less deformed, the effects of deformation
in fission dynamics were expected to be small. The measurements have been carried
out in the range of Ec.m./VB = 0.95 - 1.09, where Ec.m. is the centre-of-mass energy of
the system and VB is the corresponding Coulomb barrier (Bass model) height.
Figure 3.18: The experimental setup used for fragment angular distribution mea-surements in 16O+194Pt reaction.
3.6.1 Experimenatl setup and details
The experiment was performed using the BARC-TIFR 14UD Pelletron accelerator
facility at Mumbai [22]. Negative ions were extracted at the top from a SNICS source
and were mass analyzed before injecting into the accelerating tube, in the accelerator.
The terminal voltage in this acceleartor can be raised as high as 15 MV. The basic
principle and acceleration mechanism are the same as discussed section 1.1. Accel-
erated beams are given to five beam lines here. The fission fragment measurements
were performed using the twelve sided general purpose scattering chamber with an
equivalent diameter of one meter, located at the 0o beam line. 16O beam (dc) in the
energy range 79 - 90 MeV was used to bombard 194Pt target of thickness 300 µg/cm2
89
Experimental setup and details
��������������� �����
�����������������
Figure 3.19: Typical two-dimensional spectrum of Eres versus ∆E showing thefission and quasi-elastic events. Fission counts were taken from the Y-projection ofthis spectrum during the analysis.
on 20 µg/cm2 thick carbon foil. The beam energies were corrected for the energy loss
in the half thickness of the target. The fission fragments were collected using three ∆E
- E silicon detector telescopes consisting of 15 - 20 µm thick ∆E detectors and 300 -
500 µm thick E detectors with a collimator of diameter 5.0 mm. These telescopes were
placed at 20o apart, at a distance of 13.6 cm from the target, on the same movable
arm of the scattering chamber. Two silicon surface barrier detectors were placed
at a distance of 42.0, cm at an angle of ±20◦ with respect to the beam direction.
These detectors were used to monitor the beam incidence on the target. Another such
monitor detector was mounted at 40◦ with respect to the beam, at a distance of 42 cm
from target. This was used for the normalization of fission yields and estimation of the
absolute fission cross sections.
In ∆E - E telescopes, two detectors are placed one after the other. When the
particle passes through the detector, it deposits a fraction of its energy in the first
90
Experimental setup and details
Figure 3.20: The two-dimensional spectrum of Eres versus ∆E projected onto Y-axis.
detector which is normally very thin (hence called ∆E detector, with typically 10
- 15 micron thickness) and the remaining energy in the E-detector. It is obvious
from Bethe formula that the energy loss in the ∆E detector is proportional to z2/v2
and the total energy loss is proportional to mv2 which yield mz2 on multiplication.
Hence a two-dimensional plot of ∆E versus ∆E+E yield different hyperbolas grouping
particles having same mz2 values. As the fission fragments are very heavy and loose
energy very fast, they get stopped in the first detector itself. However the scattered
particles deposit very little energy in the first detector. In this way, the fission frag-
ments can be easily separated from the scattered particles using the detector telescopes.
The angular distribution of the fission fragments were measured at 10◦ intervals from
80◦ to 170◦ in the laboratory frame. The trigger of the data acquisition was derived from
the timing signals from the ∆E detectors. The relative solid angle of the telescopes were
taken care by measuring the data at overlapping angles. The ∆E detector thickness
was so chosen that most of the fragments were stopped at the thin ∆E detector itself
and fragments reaching the E detector were well seperated in energy from elastic,
quasielastic and other channels. Fig. 3.18 shows the experimental setup used for the
fragment measurements. The monitor and telescope signals were processed through
pre-amplifiers (142 IH) and amplifiers (Ortec 571) and were given to the ADC. The
timing signals from the three ∆E detectors and three monitor detectors were processed
91
Experimental setup and details
through timing filter amplifier (TFA) and constant fraction discriminator (CFD, Ortec-
935 Model) and were ’OR’-ed using the multi purpose coincident unit (MPCU). This
’OR’-ed output signal was used as the trigger for the data acquisition system, as shown
in Fig. 3.21. The online data were collected, eventwise for sufficient time period required
for reasonable statistical accuracy, and later analyzed using the BARC-TIFR software
LAMPS [23]. LAMPS again is a list-mode data acquisition system running on linux
(like CANLDE and FREEDOM) and uses CAMAC hardware for data acquisition. One
dimensional spectra for monitor detectors, E detectors and ∆E detectors were created
online during the experiment. Two-dimesional spectrum of ∆E versus energy of the
particles reaching the E-detector (Eres) was used to separate the fission fragments from
quasi-elastic particles. Fig. 3.19 shows the two-dimensional spectra of Eres versus ∆E
showing the fission and quasi-elastic events. The fission counts were taken from this
spectrum after projecting it on Y- axis. Fig. 3.20 shows the Y-projected spectrum of
Fig. 3.19. The anlysis methods, results and conclusions are discussed in chapter 6
PA TFA CFD
E2
E3
Mon1
Mon2
M
P
C
U
G
D
G
Mon3
Det: E1
E1
E2
PA AMPDet: E1
E3E1
E2E3
Mon2
Mon3
Mon1
A
D
CA
C
M
A
C
MasterStrobe
Figure 3.21: The shematic block diagram of the electronics used in fission fragmentangular distribution measurements in 16O+194Pt reaction.
92
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