Bahadur Singh's Seminar on Multifregmentation

8/9/2019 Bahadur Singh's Seminar on Multifregmentation

http://slidepdf.com/reader/full/bahadur-singhs-seminar-on-multifregmentation 1/36

1

CHAPTER 1:

HEAVY ION REACTION:

1.1 Introduction:

Multifragmentation is one of the active branch of nuclear physics to study the

nuclear matter by heavy ion collision under the energy range between 10 A MeV and 2

A GeV to explore the properties of nuclear matter like nuclear interactions, fusion ±

fission, cluster radio activity, formation of super heavy nuclei [1-2] etc.many attempts

have been made to study this domain of nuclear physics theoretically and

experimentally by various scientists at various places around the globe. In the following

we will discuss some outlines of multifragmentation from various aspects.

The interest to study the low energy nuclear physics or heavy ion collision is to

look for the low density phenomenon. The reaction cross section at low energy

composed of three parts i.e the fusion, quasi elastic and deep inelastic scattering and

these processes depends upon the projectile-target combination, the bombarding

energy of projectile and on the angular momentum. In fig1.1 two nuclei collide and make

compound nucleus and then fragmented in all possible direction. One can explore these

aspects of low energy physics by studying variety of experiments which involves

symmetric (e.g16O+16O, 40Ca+40Ca,197 Au+197 Au) or very asymmetric nuclei (e.g.

16O+118Sn, 16O+114Nd etc. )[3-4]

Fig 1.1 two nuclei collide to form compound nucleus and the fragmented.



2

With the development of technologies one was able to accelerate heavy ions with

bombarding energy comparable to its rest mass. This opened up a new dimension of

intermediate and relativistic energy heavy ion physics. The compound nucleus formed

at intermediate and relativistic energies gives information to study the nuclear matter

under extreme conditions. The properties of this hot and dense nuclear matter depend

upon the pressure, density and temperature. This hot and dense nuclear matter is

composed of hadronic matter which may have rich structure in this energy domain. As

indicated in fig(1.2).

Fig 1.2. Phase diagram of nuclear matter at various temperature and densites.

Figure.1.2 shows the four various phases of nuclear matter at different

temperature and densities. At lower densities and lower temperature it behaves like

liquid, at low density and higher temperature it becomes hadron gas, at higher density

and lower temperature the matter is in the state of condensed phase and fourth

possibility is at higher density and higher temperature this state of nuclear matter turned

in quark-gluon plasma. The above described facts are not only important to nuclear

physicists but have great importance for cosmologists to study astrophysical

happenings like supernova explosions, which are good candidates for studying the



3

nuclear matter at higher temperature and higher densities. But these happening are far

away and rare so nothing can be extracted from these happenings. On the other hand

the giant resonances generate the densities which are close to the normal nuclear

densities. The only remaining candidate is the heavy ion collision at intermediate and

relativistic energies. In this domain we can compress the nuclear matter (neutron and

proton) within nucleus by 2-3 times more than the normal nuclear matter and

temperature of 100 MeV can be reached [5]. At this stage we can extract the

information about how it respond and what role the strange particle play in hot and

dense nuclear matter? These questions are of fundamental importance for

understanding strong interaction and astrophysical research.

It is well known that when two nuclei collide they break into several pieces and also

lot if nucleon are emitted. This branch is termed as multi-fragmentation. Many attempts

were made theoretically and experimentally to explore the question why do nuclei break

into several fragments? How and when they formed? What is the mechanism behind

the multi-fragmentation? Why does nuclei shatter into several fragments if it hit by

projectile? Is this a statistical process? Making new micro-canonical phase space

models the proper tool for its description or is this dynamical process? Can we relate

this process to astronomical happening? etc etc.

Theoretically several models have been developed which make the simulation

more complicated. The key point to remember is that the heavy ion collisions involve

very complicated non- equilibrium physics. Due to lack of free space at low incident

energies about 98% of the attempted collisions are blocked. The whole dynamics at low

energies is governed by the mean field, at relativistic energy (2A GeV) Pauli principle¶s

role quite small (roughly 4%collisions are blocked) and hence the dynamics of reactionis governed by Cascade picture. On the other hand both cascade and mean field picture

appear at intermediate energies.



4

1.2 Basics of Cyclotron:

First we have to understand two basic points about electric and magnetic fields and

their effects on charged particles.[6]

1. When a charged particle is in a electric field it feels a force that accelerates it in

the direction of the field (or in the direction opposite to that direction if it is a

negatively charged particle). If this force is in the direction that the particle is

already traveling then clearly this acceleration speeds up its motion and thus

adds energy (and this is what we want our accelerator to do).

2. When a charged particle is moving through a magnetic field region it feels a force

that is perpendicular to its direction of motion (and also perpendicular to the

magnetic field). Such a force makes the particle change direction but does not

change its speed. This means that in a large enough region of magnetic field the

particle will travel in a circle. The size of the circle depends on the speed of the

particle and the strength of the magnetic field.

Now how can we use these two facts to design an accelerator --a cyclotron. Fig.(1.3)

Fig1.3. Cyclotron when viewed form above.



5

We make the region of magnetic field by having a pair of large flat magnets, one

above the other, with opposite poles facing, so there is magnetic field pointing down

from the upper magnet towards the lower one. We arrange two such regions, each one

D-shaped (when looked at from above) with the straight sides of the two D's facing one

another (i.e. one D is backwards). Now we have a place where a moving electric charge

(or rather a bunch of such charges) goes around half a circle in one D, then goes

straight ahead till it reaches the other D, and makes another semicircle in that one, and

so on.

So now what we have to do is arrange to have an electric field turn on in the right

direction (and at the right time) to give the charges a bit of a push each time they cross

the gap between the two D's. We are applying square radio frequency to the D¶s so thatthe electric field has to reverse its direction while the charge is going around the

semicircle inside the D, so that when the charges cross the gap again in the opposite

direction they are again accelerated a little.

We also need to build a chamber that can evacuate to very low air pressure in the

entire region where our charged particles are traveling -- between the two pairs of D-

shaped magnets and in the gap between them. This is because we want no collision of

accelerated particle with air molecules so we want ultra high vacuum inside our

accelerator.

Because the particle is speeding up each time it crosses from one D to the other it

travels in a spiral path with increasing radius. So the limit on what energy we can get

with such a machine is given by the size of the D-shaped magnets, and the vacuum-

chamber between them.



6

Chapter 2:

Experimental detail of 4 Array:

2.1 Introduction of K1200 cyclotron:

In Michigan state university scientist use (MSU) 4 to collect data for heavy ion

collision. Development at national superconducting cyclotron laboratory (NSCL). They

use K1200 cyclotron[7] for heavy-ion beam acceleration. K1200 cyclotron can

accelerate beam upto 115 MeV/nucleon. Prior to this experiment, the MSU 4 array

was updated with high rate array (HRA). This MSU 4 array has been recently used in

various experiments as diverse in purpose as sub threshold pion production, proton-

proton correlation experiments and fusion-fission studies. Other accomplishments with

the MSU 4 array include the first measurement of the balance energy, the mass

dependence of the systematic event shape analysis. During the experiment information

from each collision which satisfied a minimum bias hardware. Trigger was digitized on

an event-by-event basis, written to magnetic tape and later analyzed off line.

The K1200 cyclotron is the larger of the NSCL's two superconducting cyclotrons

and provides a higher-energy beam. Like the K500, it accelerates ions of any element in

the periodic table²through uranium²to more than half the speed of light. The K1200was used for nuclear science research at the NSCL from its completion in 1989 until

1999, the beginning of the construction to couple the two cyclotrons. The K1200 is now

the high-energy half of the two coupled cyclotrons; acceleration begins in the K500, and

the beam from the K500 is injected into the K1200 for further acceleration.

The K1200 is 14 feet, 7 inches in diameter, and the magnet is 9 feet, 7 inches tall.

Its beam energy can be selected within the range from 20 MeV per nucleon to 200 MeV

per nucleon (about 38,000 miles per second to 105,000 miles per second). The K1200

can run independently, but it is usually coupled to the K500 cyclotron. Coupling the two

cyclotrons allows them to accelerate more intense beams of all the elements, and it also

allows them to accelerate higher-energy beams of the heavier elements. For example,



7

uranium can be accelerated to 90 MeV/nucleon, while the limit with the K1200 alone is

about 25 MeV/nucleon.

In the coupled system, the K500 cyclotron accelerates ions of low charge state (the

ion source can provide high intensities of low-charge ions) to an energy suitable for

injection into the K1200 (less than 20 MeV/nucleon). When injected into the K1200, the

ions pass through a thin foil and lose many electrons, typically emerging with a charge

2.5 times larger. The accelerating high voltage, acting on this higher charge, can boost

the ions to a higher energy. The coupling of the two cyclotrons greatly increases the

beam intensity and makes it possible to explore very rare exotic isotopes.

In stand-alone operation, ions from the ion source are injected directly into the K1200

for normal acceleration.

A target of 1.0 mg/cm2 natural Sc(Scandium) was bombarded with 40 Ar(Argon)

projectiles ranging in energy in energy between 35 and 115 MeV/nucleon in 10

MeV/nucleon steps. For symmetric systems the experimental identification of specific

collective effects is less ambiguous, because then it can assumed that the projectile and

target contribute equal to the participation (overlap) region. This produce a source that

moves with the well defined centre of mass velocity regardless of impact parameter.Experiment was successfully run with the nearly symmetric entrance channel 40 Ar +

45Sc , but now improved acceptance would allow better impact parameter selection.

Scandium was also the closest naturally pure stable isotope for a mass forty target

available.



8

Firure.2.1 The K1200 cyclotron at Michigan state university.

Experiment was run at bean energies of 65,75,85,95,105,115 MeV/nucleon and balance

energy comes out to be 8712 Mev/nucleon for central 40 Ar + 45Sc collisions. At the

time the experiment was run 115Mev/nucleon was the highest 40 Ar 16+ energy the K1200

(shown in fig. 2.1) could produce.

Beam intensities were approximately 100 electrical pA. with RF beam bursts

approximately every 50 ns , these currents corresponded by only one or two 40 Ar ions in

each burst, thus the possibility of multiple events in a single burst is quite small. In order

to reduce tuning time for beams, a primary 40 Ar beam degraded in the A1200 to produce

a series of several lower energy secondary 40 Ar beams without a significant loss of

intensity in the following section we will describe in detail the MSU 4 array its various

components and their acceptance and the methods used to calibrate them.

2.2 DETECTORS:

The geometry of the MSU4 array is a 32 faced truncated. Of which twenty faces

are hexagonal and twelve are pentagons. This geometric configuration allow close-



9

packing nearly 4(sr) coverage in solid angle. Two of the pentagonal faces

serve as the entrance and exit for the beam.

All the remaining sites are filled by detector modules. Each hexagonal (pentagonal)

module contain a sub-array of six(five) close packed fast/slow plastic detectors resulting

in a total of 170 phoswiches in the main ball.

The detectors of the main ball covers laboratory polar angels 180lab<1620. The

individual phoswich detectors in the main ball are truncated triangular pyramids which

sub divide either hexagons (600,600,600) or pentagons (720,540,540). The solid angle

subtended by each of the ball phoswiches is listed in table(2.1).

Table 2.1 The solid angle subtended by phoswiches.

Module type Ideal (msr) True

Hexagon (6x) 75.2 66.0

Pentagon (5x) 59.0 49.9

Figure.2.2. The MSU 4 Array is constructed as a main ball augmented with a forward

array. The main ball contains 170 fast/slow scintillator for protons up through carbon

and 55 gas Bragg counters for lithium through argon fragments covering angles from 20

to 160 degrees. The forward array consists of 45 fast/slow scintilallator detectors

covering angles from 5 to 20 degrees.



10

Fig 2.2. Basic geometry of the MSU 4 Array

True solid angels are smaller than those predicted by ideal geometry because there

is a space between the detectors (dead space) which reduce the angles subtended by

phoswiches.



11

2.3 HRA (High rate array):

Prior to this experiment the MSU 4 array was upgraded to include HRA. The HRA

is a close packed pentagonal configuration of 45 phoswich detectors spanning polar

angle 30lab<180. This array has acceptable granularity, minimum dead area and high

rate capability(HRA count rate 25000 events/s). The HRA consist of 10, 15 and 20

fast/slow plastic counters as shown in fig(2.3)

Fig.2.3. High Rate Array (HRA)

2.4 MFA (Maryland forward array):

The major constrain in the design of the HRA was that it subtended the solid angle

between the Maryland Forward Array and the main ball of MSU 4 array. In this



12

experiment the MFA was a close packed annular configuration of 16 phoswich detectors

spanning polar angle 1.50lab<30. A schematic view is shown in fig(2.4).

Fig.2.4 The Maryland Forward Array (MFA)

The whole setup is three step setup first the main ball module include angle

180 lab<1620 secondly the HRA subtend the angle 30lab<180 and finally the MFA

include angel 1.50lab<30. The HRA subtends all solid angle between MFA and Main

ball module resulting in over 90% geometric efficiency for the entire MSU4 array.



13

Fig.2.5 schematic view of the HRA mounts in the MSU 4 Array between the

MFA and the modules in the main ball.

Simulated events were run through a software replica of the HRA to determine the

position and sizes of the 45 HRA elements that provide the optimal granularity for these

generated events and minimize the probability for double hits in each detector. For HRA

three designed were proposed (1) 20-15-10 (2) 15-15-15 (3) 10-15-20 (counting from

the ring closest to the beam axis), at different incident energies events were generated

randomly with specific multiplicities in projectile like frame and centre of mass frame in

each event particles were distributed isotropically in each frame with thermal (T=

10MeV) kinetic energy distribution and the graphs we obtained for these two frames.



14

Fig.2.6 Double hit probabilities for design(1),(2) and (3).

Fig.2.6 simulation result for double hits probabilities in various proposed designs

for the HRA. At each incident energy the upper curves are for projectile source

emission, and the lower curves are emission at midrapidity from 40 Ar+45Sc reaction.

Crosses are for DESIGN(1); diamonds are for DESIGN(2); and squares are for

DESIGN(3) as described in the text. In graph the vertical axis is the probability that two

or more particle hit the same HRA element.

And the result indicates that the design (3) (10-15-20) is the most acceptable andconsidered the most suitable for experiment.



15

Figure.2.7 The number scheme for the phoswiches in the HRA.

The number scheme for the detectors in HRA is Shown in Fig(2.7). The array is

composed of five wedges of nine detectors. The solid angle subtended by the HRA

detector is given in table.(2.2).

HRA detector no. Solid angle

12,34,5,6,7,8,9,10 5.1

12,15,1,8,21,24 6.27

11,13,14,16,17,19,20,22,23,25 6.18

27,28,31,32,35,36,39,40,43,44 6.88

26,29,30,33,34,37,38,41,42,45 6.65

Table 2.2 Shows the solid angle subtended by HRA.



16

The HRA is positioned as close to the target as would allow a 5cm diameter

photomultiplier tube to optically coupled onto the back of each detector.

2.5 Specification:

Fig (2.8) Showing the two dimensional layout of the components of a hexagonal

module of the MSU 4 Array. The phoswiches consist of a thin layer of fast plastic

scintillator, followed by a thick block of slow plastic scintillator, which is optically coupled

to a PMT. These detectors will stop all but the most energetic light ions. Mounted in

front of each phoswich subarray is a gas ionization chamber known as a Bragg curve

counter(BCC), which primarily measures intermediate mass fragments(IMFs), i.e.,

particles with charge 3 Z 20. The hexagonal anodes of the five most forward BCCs

are segmented, resulting in a total of 55 separate detectors if this type. Mounted in front

each BCC is another gas detector called a low pressure multi-wire proportional counter

(MWPC) for detection of heavy slow fragments, e.g., fission fragments.

Figure.2.8 schematic diagram showing the components of a hexagonal MSU 4

module.



17

When a charged particle impinges on the scintillator elements of the phoswiches,

light is produced, which is collected by the PMT and turned into a current pulse? The

fast and slow plastics have different decay times as shown in table.(2.3)So that their

individual contribution to this current pulse can be electronically separated. This is

called the E-E method because the fast component of this signal is a measure of the

energy loss in transmission through the thin fast plastic, while the slow component is a

measure of the total energy deposited in the thick slow plastic.

Table2.3 phoswich scintillator specifications.

Element Plastic Thickness(mm) Rise(ns) Decay

time(ns)

Ball Fast E

Ball slow E

BC 412

BC444

3.2

250

1.0

2.0

3.3

180

HRA Fast E

HRA Slow E

NE110

NE115

1.7

194

1.1

8.0

3.3

320

The current pulse from a phoswich detector, and the E and E gates to separate the

fast and slow components of this signal are schematically shown in Figure.(2.9).

Figure.2.9 Diagram of the phoswich signal and gates.



18

The phoswich detectors were painted with an epoxy based paint pigmented with

TiO2. Only this opaque epoxy layer and thin layer and a thin reflective foil( to insure no

crosstalk) separate adjacent detectors, effectively minimizing the dead area. A 4.5 kA0-

thick (0.12 mg/cm2) aluminum layer was evaporated onto the front face of the HRA

detector to minimize light leaks without compromising the kinetic energy thresholds. The

25 kA0-thick Al layer on the front face of the main ball phoswich subarray serves as the

anode for the BCC.

The Z resolution of HRA is up to the charge of the 40 Ar projectile, and mass

resolution for the hydrogen isotopes. Table(2.4) shows the energies in MeV for various

particle types entering the slow plastic of an HRA phoswich.

Table 2.4 The particles and their threshold energies.

Particle

type

Punch in

Energy(MeV)

Particle

type

Punch in

Energy(MeV)

Particle

type

Punch in

Energy(MeV)

P

D

T

He

Li

Be

B

13

17

20

50

99

152

212

C

N

O

F

Ne

Na

Mg

269

341

419

515

591

687

767

Al

Si

P

S

Cl

Ar

877

962

1079

1170

1294

1455

Figure (2.10) Displays a schematic cross-sectional view showing the components of a

BCC. For this experiment, the BCC were operated in ion chamber mode with a pressure

of 125 Torr of C2F6 gas (cathode voltage = -500 V and anode voltage = +150 v).



19

Fig.2.10 A schematic cross section view of a MSU 4 Bragg curve counter (BCC).

The BCCs were used to measure the energy loss of charged particles that stopped

in the fast plastic scintillator of the main ball. This is similar to the method used in the

phoswiches, but with significantly lower kinetic energy thresholds (especially for heavier

mass fragments). In Table(2.5) shows the energies in MeV for various particle types

entering the fast plastic of a main ball phoswich in this experiment.

Table2.5 shows the low energy threshold for the ball telescopes.

Particle

type

Punch in

Energy(MeV)

Particle

type

Punch in

Energy(MeV)

Particle

type

Punch in

Energy(MeV)

HeLi

Be

B

C

1223

34

46

59

NO

F

Ne

Na

7491

108

123

146

Mg Al

Si

P

S

163184

202

224

242



20

2.6 Electronics:

The PMT base and voltage divider card for each phoswich detector are contained

within the vacuum chamber of the MSU 4 Array. All amplification of the light produced

by the fast/slow scintillator plastic takes place in the PMT. Typical voltages on the

PMT¶s range between +1300 and +1900. Figure (2.11) is a schematic diagram of the

electronics layout for one of the phoswich detector in the MSU 4 Array. Each detector

receives its bias and transmits its signal over a single SHV cable. These cables are

connected to splitter box modules in the electronics racks where the phoswich signal is

passively separated from the high voltage into its fast, slow and timing signals.There are

twelve banks of signals for the main ball, three for the HRA and one for MFA.

The triggering system allows on-line selection of events on the basis of particle

multiplicity for storage to magnetic tape. The trigger condition can be on the number of

hits in the main ball, the HRA, the MFA or the entire detector system. The trigger stream

for the MSU 4 Array starts with the analog timing signals out of the splitter boxes.

These signals are sent to leading edge discriminators with thresholds set to fire above

noise. The twelve sum signals for the main ball and the three sum signals for the HRA

are then separately combined in an analog summer box. The total sum the main ball,

HRA and MFA is further combined to provide a sum signal proportional to the total hit

multiplicity in the entire detector array.



21

Figure.2.11 A schematic diagram of the electronics layout for the MSU 4 Array.



22

The four corresponding sum signals are passed into a constant fraction

discriminator (CFD) which can be programmed to select a multiplicity greater than or

equal to a given value for any of the four inputs (Ball, HRA, MFA or total system). Theoutput of this octal CFD becomes the master trigger.

For events that satisfied trigger, the availability of the 4 tranputer based data

acquisition (DAQ) system is checked. A master-Live signal is created which commands

the DAQ system to initiate an event. Gates are generated via a second octal CFD which

indicates at least one signal present in the main ball, HRA or MFA. Coincidence

between these ³singles´ and Master-Live, to create Ball-live or MFA-live insures gate

generation only for acceptable events.

In general, a HRA or MFA multiplicity trigger will enrich the data sample with

peripherial events due to the forward focusing of particle in fixed target experiments,

while a main ball trigger will select more central events in which particles are emitted at

larger polar angles with respect to the beam axis.

Chapter: 3

The ALADIN SPECTROMETER (A LARGE ACCEPTANCE DIPOLE MEGNET):

3.1 Introdution:

The experiments performed at GSI with ALADIN forward spectrometer were done

with Au as projectile with energy range from 100 MeV to 1000 MeV with different fixed

targets of C, Al, Ca, Pb and Au. ALADIN gave the data for multifragmentation.

The ALADIN spectrometer consist of many detectors namely Si-CsI(TI)

hodoscope, TP-MUSIC,LAND,TOF and central plastic detector collectively covers

almost 100% of fragments and particles. A schematic layout of the experimental set up

is shown in fig.3.1For each beam particle, its arrival time and it position in the plane

perpendicular to the beam direction were measured upstream of the target with thin



23

plastic scintillator. Their effective thicknesses were 110 and 50 m. The geometric

acceptance of the spectrometer of lab 9.20 horizontally and 4.30 vertically and its

matched with the dimensions of the multiple sampling ionization chamber TP- MUSIC III

and by the extended time of flight (TOF) wall. These detector systems permitted the

detection of clos to 100% of all projectile fragments with atomic number Z 2. At lower

bombarding energies, the angular distribution of some lighter fragments extended

beyond the acceptance of the spectrometer but stayed within the acceptance of Si-

CsI(TI) hodoscope array that surrounded the entrance to field gap of the magnet.

3.2 Detectors:

Next we will discuss all the detectors in series as they are in set up first is the Si-

CsI (TI) hodoscope. This hodoscope placed in the entrance of the beam in the field of

ALADIN magnet. It is positioned 60 cm downstream from the target and covered the

solid angle surrounding the spectrometer acceptance up to angles lab 160. Its 84

telescope modules were mounted in close geometry. Each detector consisted of a

300 m Si detector followed by a 6 cm long CsI(TI) detector which was viewed by a 1

cm2 photodiode from the end face. The active area of each detector was 30 × 30 mm2 ;

the solid-angle coverage rectangular opening, amount to 85%. Light fragments with

approximately beam velocity were not stopped by the telescope. For these particles,theidentification had to be based on the two fold measurement of their energy loss. The

peaks caused by fast hydrogen, helium and lithium ions are clearly visible in the

identification spectrum obtained from weighted sum of the two E measurement with



24

Fig.3.1 cross sectional view of the ALADIN experimental set up. The beam enters from

the left and is monitored by two beam detectors before reaching the target. Projectile

fragments entering into the acceptance of the magnet are tracked and identification in

the TP-MUSIC III detector and in the time-of-flight (TOF) wall. The central plastic

detector covers the hole in the TOF wall at the exit for the beam. Fragments and particle

emitted in forward direction outside the magnet acceptance and up to

lab = 16

0

aredetected in the Si-CsI array. Neutrons emitted in direction close to 00 are detected with

the large area neutron detector (LAND). The dashed line indicates the direction of the

incident beam. The dash-dotted line represents the trajectory of beam particles after

they were deflected by an angle of 7.30 .



25

the hodoscope detector (fig.2.2). For the actual analysis, gates were set in the two

dimensional E1 versus E2 spectra which permitted the identification of stopped

fragments and a more efficient suppression of background.

Figure.3.2 The Si-CsI(TI) hodoscope contained 84 telescope modules.



26

Figure.3.3. Z identification spectrum obtained from a weighted sum of the energy-loss

measurements with the Si-CsI(TI) telescope of the hodoscope for reaction

197

Au on197 Au.

The yield of fragments with Z 3, selected in this way, is given by dark shaded

distribution shown in fig.3.3. The inset shows the distribution of these fragments, mostly

lithium, across the solid angle subtended by the hodoscopes. Each square represent a

detector element, with its area being proportional to the number were considered as

belonging to the spectator source at this incident energy.



27

Figure.3.4. The layout of TP-MUSIC III detector

The TP_MUSIC III detector (figure.3.4.) capable of identifying the individual

elements for Z 8 with resolution between Z 0.8 (Z 20) and Z 0.4 (Z 60),

served to calibrate the charge response of the TOF wall. The main purpose of the TP-

MUSIC, in these experiments, was to provide the tracking information for other analysis

that involved the fragment momenta. The dynamic range for tracking has been

extended down to Z = 2 by using gas amplification over part of the length of the detector

which permits the measure of isotopic yield ratios of light fragments [8]. The high charge

resolution of TP-MUSIC was essential for the analysis of fission decay in experiment

with the uranium beams [9-10]. The Z resolution achieved with the two detector systems

off-line calibration is demonstrated in fig.3.5.



28

Figure.3.5. The Z identification spectra measured with the TOF wall (top) and TP_MUSIC

(bottom) for the reaction 197 Au on 197 Au at E/A = 600 MeV.

The atomic number Z and the velocities of nuclear fragments were determined

with the TOF wall (figure 3.6) located at the end of the ALADIN spectrometer and

extended over 2.4m in horizontal and 1.0 m in vertical direction. It consisted of two

layers of vertically mounted scintillator strips of 2.5 cm width 1.0 cm thickness, viewed

by photomultiplier tubes at both ends[11]. The two layers were offset by half a width with

respect to each other, the primary beam was directed through a hole of 4.8 × 6.0 cm2

cut into the middle section of the central slats. The upper and lower halves of these

central slats were optically connected by a hollow light guide built from aluminized mylar

foil. The discriminator threshold was set to be Z = 2 particles. For the time-of-flight



29

calibration, primary beams of reduced intensity were swept across the wall with Aladin

magnet. By inducing fragmentation reactions a thick aluminum target positioned

immediately in front of the wall the response to fragments of different Z was determined.

A resolution of about 400-ps and 180-ps for fragments of Z= 2 and Z Zp, respectively,

was achieved (Zp denotes the atomic number of the projectile). These values include

the systematic uncertainty of the calibration. The intrinsic time-of-flight resolution

measured for beam particle was 120 ps.

Fig.3.6 The time-of-flight wall (TOF).

The algorithm used for fragment identification in TOF-wall analysis took into

account that fragments may pass into the narrow gap between adjacent slats in one of

the two layer of the wall and that heavy fragments are accompanied by a considerable

number -rays. The identification spectrum of the TOF wall detectors was obtained by

projecting along the ridges of constant Z in two-dimensional maps of the measured



30

pulse height versus time-of-flight. Elements with Z 15 were resolved individually. For

heavier fragments the resolution assumed values of up to Z 1.5

A central plastic detector with light fiber readout was installed behind the central

hole of TOF wall. It served for the detection of the primary beam particles and of

projectile fragments emitted direction close to the beam and for their detection

according to the measured energy loss signals.

The large area neutron detector LAND (figure 3.7) was positioned close to zero

degree with respect to the incident beam direction and operated in calorimetric mode.

After collision when fragmented beam entered in the field gap then the charged particles

get deflected by the magnetic field of the Aladin magnet but rest residual fragments or

neutron having no charge undergoes without deflection along the original trajectory of

the entering beam and these neutral fragments or neutron are then detected by LAND

placed exactly in front of the entering beam. Other charged fragments get deflected by

an angle of 7.30 with respect to the original beam direction by the magnetic field of the

Aladin magnet and pushed towards MUSIC and TOF wall where they are detected by

their threshold values in respective manner.



31

Fig.3.7 A large area neutron detector.(LAND)

The on-line trigger condition consisted of the logical product of the requirement of a

beam particle in the start detector(figure 3.8), no fragment with Z close to that of the

beam in the central plastic detector or in the central part of the TOF wall, and the

detection of at least one particle with Si-CSI(TI) hodoscope. It had the effect of

supperessing the most peripheral interactions except those leading in the binary fission.



32

Figure 3.8 The start detector.

For the present study of multi-fragment production, the peripheral fission events

which may have large cross section, in particular for the case of 238

U projectile[18], were

suppressed off-line. In the experiment with the 197 Au beam of 400 MeV per nucleon, a

beam particle in the start detector and a minimum of one particle detected with the Si-

CsI(TI) hodoscope were required. Scaled down events with less restrictive trigger

conditions, including beam events triggered only by the beam detectors, were recorded

for normalization.

Absolute cross section were determined by normalizing the measured event rate

with respect to the thickness of the target and the rate of incoming particle. The error of

the normalization, dominated by the uncertainty of the aerial density of the targets, is

between 1% and 5%.



33

Fig.3.9 The faculty of ALADIN colaboration.

Figure 3.9 shows the experimental set up of ALADIN spectrometer and also the

scientists and research scholars involved in ALADIN colaboration.



34

Summary:

Heavy ion experiments performed with ALADIN and 4 spectrometer has been

explained in this report. With 4 experiment the information about the asymmetric

collision of Ar + Sc at energies 65, 75, 85, 95, 105, 115 MeV/A have been studied. In

ALADIN experiment Au beam with varying energy with 100 to 1000 MeV/A collided with

fixed targets of C, Al, Cu, Pb and Au. ALADIN provided the data about

multifragmentation. The extracted data gives us lot of information about the balance

energy and equation of state. The mean dependence and impact parameter of the

different reactions studied here throw light on different aspects of the reaction dynamics.



35

Bibliography

[1] L. C. Vaz, J. M. Alexander and G. R. Satchler, Phys. Rep. 69, 373 1981; M.

Beckerman, Rep. Prog. Phys. 51, 1047 (1988).

[2] See e.g. Heavy Elements and Related New Phenomena Vol. I and II EDS, w.

Greiner and R. K. Gupta, World Scientific, Singapore (1999).

[3] R. K. Puri, Ph. D. Thesis, P.U. Chandigarh, (1990).

[4] M. K. Sharma, Ph. D. Thesis, P. U. Chandigarh, (1998)

[5] H. Stocker and W. Greiner, Phys. Rep. 137, 277 (1986).

[6] http://www2.slac.stanford.edu/vvc/faqs/faq3.html

[7] http://www.nscl.msu.edu/tech/accelerators/k1200

[8] J. Pochodzalla, T. M6hlenkamp, T. Rubehn, A. Schiittanf, A. WOmer, E. Zude, M.

Begemann-Blaich,

Th. Blaich, H. Emling, A. Ferrero, C. Grog, G. Imm6, I. Iori, G.J. Kunde, W.D. Kunze, V.

Lindenstruth,

U. Lynen, A. Moroni, W.EJ. MUller, B. Ocker, G. Raciti, H. Sann, C. Schwarz, W.

Seidel, V. Serfting,

J. Stroth, W. Trautmann, A. Trzcinski, A. Tucholski, G. Verde and B. Zwieglinski, Phys.

Rev. Lett. 75

(1995) 1040.

[9] Th. Rubehn, W.EJ. Miiller, R. Bassini, M. Begemann-Blaich, Th. Blaich, A. Ferrero,

C. Grog, G. Imme,

I. Iori, G.J. Kunde, W.D. Kunze, V. Lindensrtuth, U. Lynen, T. M6hlenkamp, L.G.

Moretto, B. Ocker,

J. PochodzaHa, G. Raciti, S. Reito, H. Sann, A. Schiittauf, W. Seidel, V. Serfling, W.

Trautmann,

A. Trzcinski, G. Verde, A. W6mer, E. Zude and B. Zwieglinski, Z. Phys. A 353 (1995)

197.



[10] Th. Rubehn, R. Bassini, M. Begemann-Blaich, Th. Blaich, A. Ferrero, C. Grol3, G.

Imm6, I. Iori,

G.J. Kunde, W.D. Knnze, V. Lindenstruth, U. Lynch, T. M6hlenkamp, L.G. Moretto,

W.F.J. Miiller,

B. Ocker, J. Pochodzalla, G. Raciti, S. Reito, H. Sann, A. Schiittauf, W. Seidel, V.

Serfling, W. Trautmann,

A. Trzeinski, G. Verde, A. W6mer, E. Zude and B. Zwieglinski, Phys. Rev. C 53 (1996)

3143.

[11] J. Hubele, P. Kreutz, J.C. Adloff, M. Begemann-Blaich, P. Bouissou, G. Imme, I.

Iori, G.J. Kunde,

S. Leray, V. Lindenstruth, Z. Liu, U. Lynen, R.J. Meijer, U. Milkau, A. Moroni, W.EJ.

Miiller, C. Ng6,

C.A. Ogilvie, J. Pochodzalla, G. Raciti, G. Rudolf, H. Sann, A. Schiittauf, W. Seidel, L.

Stuttge,

W. Trautmann and A. Tucholski, Z. Phys. A 340 (1991) 263.

Documents

Bahadur Singh's Seminar on Multifregmentation