chapter 3 experimental setup and...

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

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-

-

-

-

-

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

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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.

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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-

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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


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


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


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.

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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


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


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


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


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.

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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.

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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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


��

��

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


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


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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