Stopping Power of 500keV - 1500keV Protons in Mylar Foil

UNIVERS ITE IT •STELLENBOSCH •UNIVERS ITY

j ou kenn i s v ennoo t • you r know ledge pa r tne r

Stopping Power of 500 keV - 1500 keV Protonsin Mylar Foil

by

Tinyiko Simon Maluleke

14706075

Nuclear Physics Project 754

Department of Physics,University of Stellenbosch,

Private X1, Matieland 7602.

Supervisors:

Dr. P. Papka (US & iThemba LABS)Dr. C. Pineda-Vargas (iThemba LABS)Mr. M. Msimanga (iThemba LABS)

November 2009

Declaration

I, the undersigned, hereby declare that the work contained in this report ismy own work and that work from other authors has been properly

referenced and aknowledged.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Tinyiko Simon Maluleke Date

Copyright ©2009 Stellenbosch UniversityAll rights reserved

Acknowledgments

I like to thank my supervisors, Dr. Carlos Pineda-Vargas and Mr. MandlaMsimanga, for their support and guidance throughout the project. Theirknowledge, dedication and patience made it possible for me to finish thisproject. I also thank the staff at the Material Research Department, whoassisted me during the experiment. I extend my thanks to Dr. Paul Papkaand Mr. J.J. van Zyl for organizing such interesting project for me. Itempowered me and provided me with a way forward for my future studies.

Contents

Contents i

Abstract iii

List of Figures iv

List of Tables v

Nomenclature vi

1 Introduction 1

1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Theory And Techniques 4

2.1 Rutherford Backscattering . . . . . . . . . . . . . . . . . . . . 52.2 Energy Loss, Stopping Power and Stopping cross section . . . 62.3 Energy straggling . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Experimental Setup And Procedure 10

3.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . 103.1.1 Primary Target (Pt/C) . . . . . . . . . . . . . . . . . . 103.1.2 Mylar Foil . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Irradiation And Measurement . . . . . . . . . . . . . . . . . . 12

i

CONTENTS ii

3.2.1 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.2 Analysis Software . . . . . . . . . . . . . . . . . . . . . 153.2.3 Evaluation of Data . . . . . . . . . . . . . . . . . . . . 16

4 Results And Discussion 18

5 Conclusion 24

A The Van De Graaff Accelerator 26

Bibliography 28

Abstract

Stopping power of mylar foil (1.68 µm thick) have been measured for pro-tons in the energy range from 500 keV to 1500 keV, at the Van de GraaffAccelerator at Material Research Department (MRD), iThemba LABS, withan experimental uncertainty of 5.4%. The experimental setup was based onthe Rutherford Backscattering spectroscopy (RBS) technique currently usedat MRD, using the split-foil technique. The stopping power was obtained ataverage scattered proton energies. Since the energy loss in the 1.68 µm mylarfoil was found to be less than 20% of incident proton energy, the proton aver-age path-length was approximated to be equal to the thickness of mylar foiland the stopping power was obtained by taking the ratio of energy loss ∆E

to the thickness ∆x of mylar. The current results have been compared withcalculations done on SRIM2008 and PSTAR computer codes, and with pre-vious experimental results from Damache et al. [1] and Shiomi-Tsuda et al.[2]. The results are a little higher than the calculated results and previousresults, but still within acceptable experimental error limit. In addition, thecurrent results showed a kink between 1200 keV and 1500 keV, previouslyobserved by Ammi et al. [3].

iii

List of Figures

1.1 Stopping power of protons in mylar [3]. . . . . . . . . . . . . . . 2

2.1 Ion beam interaction with target material. . . . . . . . . . . . . 42.2 Scattering kinematics. . . . . . . . . . . . . . . . . . . . . . . . 7

3.1 Schematic diagram of evaporation process. . . . . . . . . . . . . 113.2 Front area of the Si barrier detector. . . . . . . . . . . . . . . . 123.3 The Van de Graaff accelerator with experiment beam. . . . . . . 133.4 The electronics setup for the experiment. . . . . . . . . . . . . . 143.5 Electronics. (1) - Scattering chamber, (2) - Si surface barrier

detector, (3) - Preamplifiers, (4) - Beam line, (5) - Collimator(2mm diameter) and (6) - Target holder. . . . . . . . . . . . . . 15

3.6 Experimental setup. . . . . . . . . . . . . . . . . . . . . . . . . . 153.7 Determination of energy loss through mylar foil using Pt peak

position on the energy spectrum. . . . . . . . . . . . . . . . . . 16

4.1 Comparison of energy loss for incident proton energies E0 = 700keV and E0 = 1500 keV. . . . . . . . . . . . . . . . . . . . . . . 19

4.2 Energy calibration E1 vs. C1. . . . . . . . . . . . . . . . . . . . 204.3 Deviation in stopping power from SRIM2008 calculations due

to thickness of foil. . . . . . . . . . . . . . . . . . . . . . . . . . 224.4 Current stopping power results compared with previous results

and calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . 23

A.1 Generalized schematic diagram of a Van de Graaff accelerator. . 26

iv

List of Tables

3.1 Beam experimental parameters and beam characteristics. . . . . 14

4.1 Experimental results where the energies are in keV and stoppingpowers are in keV/µm. . . . . . . . . . . . . . . . . . . . . . . . 21

v

Nomenclature

Variables

K Kinematic Factor . . . . . . . . . . . . . . . . . . . . . . . [ Unitless ]

E0 Incident proton energy . . . . . . . . . . . . . . . . . . . . [ MeV ]

E1 Backscattered ion energy (not through myalar foil) . . [ MeV ]

E2 Backscattered ion energy (through myalar foil) . . . . . [ MeV ]

E Average backscattered ion energy . . . . . . . . . . . . . [ MeV ]

∆E Energy loss . . . . . . . . . . . . . . . . . . . . . . . . . . . [ MeV ]

θ Scattering angle . . . . . . . . . . . . . . . . . . . . . . . . [ o ]

M1 Mass of incident ion . . . . . . . . . . . . . . . . . . . . . . [ u ]

M2 Mass of target atom . . . . . . . . . . . . . . . . . . . . . . [ u ]

S total stopping power . . . . . . . . . . . . . . . . . . . . . [ MeVµm

, keVmm

, etc. ]

Se electronic stopping power . . . . . . . . . . . . . . . . . . [ MeVµm

, keVmm

, etc. ]

Sn nuclear stopping power . . . . . . . . . . . . . . . . . . . . [ MeVµm

, keVmm

, etc. ]

ε Stopping cross section . . . . . . . . . . . . . . . . . . . . [ keVmg.cm2 , etc. ]

∆x proton path length or thickness of foil . . . . . . . . . . [µm, mm, etc ]

vi

Chapter 1

Introduction

1.1 Overview

Understanding the process of interaction of charged particles with matteris important for applications in atomic and nuclear physics. This includesstudying the energy loss characteristics of charged particles when they tra-verse a target material to extract the stopping power of that particular mate-rial for charged particles. The study of energy loss of light charged particlesin thin polymer films, such as mylar and kapton, has seen an increased in-terest in the recent past. Therefore databases of stopping powers for lightcharged particles have to be constantly updated. Accurate measurementsand calculation of stopping powers can then be achieved if there is enoughdata available to compare with. The availability of data also help to improvetheoretical calculations and simulations performed by computer codes suchas SRIM2008 [4]. Ion beam analysis (IBA) techniques are generally used forstopping power measurements.

1.2 Literature Review

Stopping power for protons in thin polymer films have been measured experi-mentally for proton energy in the range from about 200 keV to several MeV’s.

1

CHAPTER 1. INTRODUCTION 2

Ammi et al. [3] measured the stopping power of protons in 6.30 µm mylarusing Rutherford Backscattering Spectroscopy technique. The experimen-tal results showed about 10% deviation from calculated results of TRIM92,predecessor of latest SRIM and TRIM codes (see Figure 1.1).

Figure 1.1: Stopping power of protons in mylar [3].

Shiomi-Tsuda et al. [2], provide experimental results for stopping powerof protons in mylar for energy range between 400 keV to 3.25 MeV. Howeverthis time, scattered protons were detected at forward scattering angle. Theresults were again compared with theoretical calculations and previous re-sults. The stopping power were calculated at average incident ion energies.Amongst the latest results are the results by Damache et al. [1], which pro-vide the stopping power of protons in mylar for proton energy 236 keV to3.019 MeV. Experimental setup used was similar to that of Ammi et al. [3].The experimental stopping power was compared with results obtained fromtheoretical calculations.

CHAPTER 1. INTRODUCTION 3

Various methods and computer programs to calculate stopping powers forions in matter have been developed based on collected experimental dataand proposed theoretical models (see [4; 5; 6; 7; 8]). However there is less ex-perimental data available on stopping power for protons in the lower energyregion, therefore the accuracy of these models and programs is still in ques-tion, especially in the lower energy region. Overall less experimental datais available for stopping power of protons in the lower energy region (below1.50 MeV) for mylar foil. However the available experimental results, somementioned above, showed agreement with previous results and theoreticalcalculations.

1.3 Scope

Based on the literature survey and current trends on ion solid interaction inrelation to the experimental confirmation of theoretical models for stoppingpower in thin polymer foils, the objective of the present research project isto report on the experimental measurement of stopping power of protons inmylar (C10H8O4) at energies between 500 keV and 1500 keV. The B-Line ofthe Van de Graaff accelerator at the Material Research Department (MRG),iThemba LABS, was used for this purpose. The technique of RutherfordBackscattering Spectroscopy (RBS) was used to measure the stopping powerof protons on a thin mylar foil.

The first chapter gives an overview of a interaction process of the prob-lem. Chapter two presents a description of theory and the techniques usedin charged particle interaction with matter, particularly those related toRutherford backscattering, energy loss, stopping power and energy strag-gling. Chapter three gives the description of the experimental setup andexperimental procedures, including irradiation and measurement, electronicsand the software used for the evaluation of the experimental data. Chapterfour presents the results and discusion of reluts, and chapter five give theconclusion to the overall report.

Chapter 2

Theory And Techniques

To describe the interaction of charged particles with matter, consider a beamof charged particles with initial energy E0, mass M1 and charge Z1 incidenton a target material of thickness x, mass M2 and charge Z2, as shown inFigure 2.1.

Figure 2.1: Ion beam interaction with target material.

These charged particles are produced in a charged particle accelerator

4

CHAPTER 2. THEORY AND TECHNIQUES 5

such as the Van de Graaff accelerator. The charged particles can be trans-mitted, absorbed or backscattered elastically or inelastically. If a chargedparticle is transmitted, it appears with energy E0−∆E where ∆E is the en-ergy lost in the target material. If the incident ion is absorbed, it can exciteor ionize the target atoms if it carries enough energy. Photons are emittedupon de-excitation. Inelastic scattering results in knockout reaction wherean electron is emitted. Elastic scattering results in ions being deflected bytarget nuclei due to Coulomb interation.This is the process of interest whenmeasuring the stopping power of ions in matter.

2.1 Rutherford Backscattering

Consider a beam of charged particles incident on the target material as shownin Figure 2.2. An incident ion will interact with target nuclei and then itis scattered at an angle θ. The scattered ion appears with energy E1 whichdepends on incident energy, ion mass M1, ion charge Z1, target mass M2,target charge Z2 and scattering angle θ [5].

E1 = KE0 (2.1)

where K, the kinematic factor, is defined as

K =

(M2

2 −M21 sin2 θ

) 12 + M1 cos θ

M1 + M2

2

(2.2)

For backscattering spectroscopy, we are interested in scattering angles closerto 180o, where ions retain most of their energy. Backscattering yield can bedetermined from differential Rutherford cross-section dσ

dΩ. Yacobi et al.

[9] gives the Rutherford differential cross section as


dσ

dΩ=

(Z1Z2e

2

4E

)24

sin4 θ

(

1−(

M1

M2sin θ

)2) 1

2

+ cos θ(1−

(M1

M2sin θ

)2) 1

2

2

(2.3)

The number of backscattered ions detected is related to the cross-section by

A =dσ

dΩΩQNt (2.4)

where Q is the total number of incident ions, N is the particle volume densityin atoms/cm3, Ω is detector solid angle and t is the thickness of the target.

2.2 Energy Loss, Stopping Power and

Stopping cross section

As mentioned before, incident ions lose energy when they traverse a targetmaterial. Consider a backscattering configuration in which an ion with initialenergy E0 is incident on a target material of thickness x, as shown in Figure2.2.

The energy at depth ∆x is

E∆x = E0 −∆xdE

dx(2.5)

After scattering at depth ∆x, the ion appears with energy

E2 = KE∆x −∆x

cosθ

dE

dx(2.6)

Therefore the energy lost by the ion in the material is

∆E = KE0 − E2 (2.7)


Figure 2.2: Scattering kinematics.

The stopping power S is defined as the ratio of differential energy loss todifferential path length traversed by the ion.

S =dE

dx(2.8)

Stopping power is also reffered to as the specific energy loss. Units forstopping power is MeV/mm, keV/µm, keV/mg.cm−2, etc. Tesmer and Nas-tasi [5] gives Bethe-Bloch formula for stopping power as

dE

dx= NZ2

(Z1e

2)2

f

(E

M1

)(2.9)

Now combining (2.5) and (2.6), (2.7) becomes


∆E = ∆x

(K +

1

cosθ

)dE

dx(2.10)

or∆E = ∆xSeff (2.11)

where Seff , known as the effective stopping power, is given by

Seff =

(K +

1

cosθ

)dE

dx(2.12)

Interaction processes between incident ions and target atoms depend onion velocity, and ion and target masses. At low velocities nuclear energy lossdue to interation with target nuclei dominates. As velocity increases nuclearenergy loss diminishes and electronic energy losses due to interation withtarget electron dominates. The total stopping power S is then the sum ofelectronic stopping power Se, due to electronic interations, and nuclear

stopping power Sn, due to nuclear interactions.

S = Se + Sn (2.13)

At high energies, S ' Se. Ziegler et al. [6] provide equations for calculatingelectronic and nuclear stopping power. Stopping cross section ε is definedas follows

ε =1

N

dE

dx=

1

NS (2.14)

In case the target of interest is a compound or a mixture Bragg’s rule canbe applied to compute the stopping powers of ions in compounds [9; 5]. LetAmBn be a coumpound or a mixture, according to Bragg’s rule, the stoppingcross section is

εAmBn = mεA + nεB (2.15)


Then the stopping power is calculated as follows

SAmBn = NεAmBn (2.16)

2.3 Energy straggling

When individual ions traverse a target material, they experience differentinteractions with target atom. Some travel longer distances than others.Therefore there is a fluctuation in the energy of the scattered ions. Thisis called energy straggling. Straggling increases with path length traversedby ions in the target material. Other sources of energy straggling includedetector resolution, beam energy spread, geometry of scattering spectroscopyand kinematics effects.

Chapter 3

Experimental Setup And

Procedure

The experimental setup was based on Rutherford Backscattering Spec-

troscopy (RBS technique. The experimental setup comprised of a chargedparticle accelerator (Van de Graaff accelerator), experimental chamber anddata acquisition system. The scattering or experimental chamber was setupas shown in Figure 3.4, similar to the setup of Damache et al. [1] and Ammiet al. [3].

3.1 Sample Preparation

3.1.1 Primary Target (Pt/C)

A primary target of Pt/C was prepared at the Material Science Group labo-ratory at iThemba LABS, by evaporating platinum onto a carbon substrate.Figure 3.1 shows the mechanism used for evaporating a solid. A substrate(in this experiment, carbon C) was fixed on a holder, just above the source(in this case platinum Pt) inside the evapopration chamber. The evaporationchamber was then vacuumed and maintained at a pressure of less than 10−5

mb for the duration of the evaporation process, to avoid contamination oftarget.

10

CHAPTER 3. EXPERIMENTAL SETUP AND PROCEDURE 11

Figure 3.1: Schematic diagram of evaporation process.

High voltage was applied to the thermionic filament, which in turn emit-ted electrons. The electrons were accelerated by the accelerating electrodesand deflecting magnets were used to bend the electron beam and focus it ontothe source (Pt). The electron beam then melted the Pt which evaporatedand deposited on the carbon substrate. The thickness of the Pt evaporantwas monitored, and the process was stopped once the required thickness ofabout 200 angstroms was obtained.

3.1.2 Mylar Foil

The mylar foil of approximately 1.50 µm was used as a stopper foil for themeasurement of stopping power. Molecular formular for mylar is C10H8O4

with density of 1.40 g/cm3. A split foil technique was used to mount the foilin front of the Si barrier detector. In split foil technique, the foil is mountedsuch that it covers half the detector’s front, active surface or window, asshown in Figure 3.2.


Figure 3.2: Front area of the Si barrier detector.

3.2 Irradiation And Measurement

3.2.1 Electronics

The Van de Graaff accelerator at MRG, iThemba Labs, provides five beamlines for various experiments(see Appendix A for general operation of Vande Graaff accelerator). The experiment was performed at the B-Line (seeFigure 3.3) using proton beam with energy from 500 keV to 1500 keV.

The data collection and acquisition system comprised of the Si surfacebarrier detector, preamplifier, amplifier, Analog-to-Digital Converter (ADC),CAMAC interface, and the data acquisition system (XSYS) running on VAX4000/VLC computer (see figure 3.4).

A cylindrical Si surface barrier detector (with FWHM 15 keV) wasused to measure the energy of the backscattered protons. It was biasedto 100 V to keep the depletion region as wide as possible. A preamplifier,connected to the output of the detector, was placed within short distancefrom the detector, outside the scattering chamber. The distance betweenthe detector and preamplifier was kept as short as possible to minimise noisedue to length of cable. The output of the preamplifier was connected to theamplifier for further signal amplification. The amplified signal was fed to theADC, which is used to convert analog signal to digital signal, and then to theCAMAC interface which does the sorting of data using a special computer


Figure 3.3: The Van de Graaff accelerator with experiment beam.

code written in DCL language. The output of the CAMAC was sent to thedata aquisition system, XSYS. The pressure in the scattering chamber wasmaintained at 10−5 mb for the duration of the experiment.

Inside the B-Line scattering chamber (shown in Figure 3.5 and Figure3.6), a ladder was mounted on the target holder at the center. The targetholder could move up and down to bring the target of interest onto the beamaxis, and also rotate about its vertical axis to allow variation of target tiltangle if necessary. The lower the tilt angle, the smaller the path length trans-versed by protons in the primary target Pt/C, and therefore the the lowerthe energy loss in the target material. Together with the proton number Z2

of Pt and incident energy E0, the tilt angle has an effect on the backscatter-ing yield and energy of backscattered protons. The target tilt angle was setto 0o with respect to the incident beam direction. A cyllindrical Si barrierdetector was placed at a scattering angle of θ = 165o.


Figure 3.4: The electronics setup for the experiment.

The primary target was irradiated with a beam of 1500 keV protons, 2 mmin diameter (see Table 3.1 for more beam characteristics and experimentalparameters). First spectrum was recorded for E0 = 1500 MeV. The incidentproton energy was reduce at 100 keV steps and the procedure was repeated forbeam energies down to 500 keV. For each measurement, the charge collectionwas set to 333.33 s at a beam current of about 60 nA.

Parameter Value

Total Collected Charge 20000nC

Scattering Angle 165o

Tilt Angle (target) 0o

Beam Energy (keV) 500 - 1500

Gain 4 keV/Channel

Beam Current 60 nA

Beam Diameter 2 mm

Table 3.1: Beam experimental parameters and beam characteristics.


Figure 3.5: Electronics. (1) - Scattering chamber, (2) - Si surface barrier detector,

(3) - Preamplifiers, (4) - Beam line, (5) - Collimator (2mm diameter) and (6) -

Target holder.

Figure 3.6: Experimental setup.

3.2.2 Analysis Software

The XSYS program running on a VAX4000/VLC computer was used fordata collection and to set the experimental parameters. SIMNRA, a com-puter program developed by Mayer [8], was used for the calculations of the


scattering kinematics. SIMNRA is mainly used for the analysis of RBS,ERDA and PIXE spectra to extract information such as depth profiling andelemental contents of materials. SRIM2008 and PSTAR were used to calcu-late the stopping power for comparison with current results [4; 7]. These twoprograms were developed based on theoretical models of ion beam interactionwith matter and the available experimental data on stopping power. Origin8 Pro, a plotting and data analysis program, was used to plot RBS spectrum[10].

3.2.3 Evaluation of Data

The XSYS program enabled the recording of the output spectrum as a binaryfile, named TINY000*.dat, for each proton beam incident energy E0.

Figure 3.7: Determination of energy loss through mylar foil using Pt peak position

on the energy spectrum.


Each RBS spectrum file contains the channel number and counts perchannel, which was used to plot a spectrum similar to Figure 3.7 using Origin8 Pro. The centroids correspond to backscattered proton energies, E1 (MeV)and E2 (MeV), for a given incident proton energy E0 (MeV). E1 correspondsto protons that backscattered on Pt and do not pass through the mylar foil,while E2 corresponds to protons that pass through the mylar foil. Then theenergy loss ∆E was computed as the difference between E1 and E2. Thisenergy loss is directly proportional to the stopping power of protons in mylar.

Chapter 4

Results And Discussion

The recorded RBS spectra, for different incident ion energies, were analysed.A typical RBS spectrum is shown in Figure 3.7. The spectrum depicts twoprominent peaks, one peak corresponds to protons that backscattered on Ptand do not pass through the mylar foil, while the other peak corresponds toprotons that pass through the mylar foil.

The peaks broadened due to energy straggling of protons as they inter-act with the primary target, Pt/C. Otherwise sharp peaks were going to beobserved if all backscattered protons were having the same types of inter-actions and therefore same energy. Comparing the peak separation for highincident (1500 keV) and low incident energy (700 keV)(see Figure 4.1), its isclear that peak separation at high energy is much smaller than at low energy.This shows that energy loss is a function of incident ion energy or velocityand increases with decrease in incident proton energy.

Energy calibration was performed to convert channel numbers to energyvalues. Let C1 and C2 be peak centroids corresponding to backscattered pro-ton energies, E1 (keV) and E2 (keV) respectively, for a given incident protonenergy E0 (keV). There centroids of each the two peaks were determined byGaussian fit. Now, E1 was computed from first principles using equations(2.1) and (2.2), for M1 = 1.007825 u, M2 = 194.964774 u, θ = 165o andincident energies, 500 keV ≤ E0 ≤ 1500 keV . Then a graph of E1 versus

18

CHAPTER 4. RESULTS AND DISCUSSION 19

Figure 4.1: Comparison of energy loss for incident proton energies E0 = 700 keV

and E0 = 1500 keV.

C1 was plotted, as shown in Figure 4.2. The relationship between protonenergy and channel number was found to be linear, and expressed as a linearequation

E1 = 0.00311C1 + 0.005155 (4.1)

To convert C2 to E2, the energy of backscattered protons that passthrough mylar, the equation become

E2 = 0.00311C2 + 0.005155 (4.2)

After obtaining E1 and E2, the energy loss ∆E of protons in mylar wascomputed as the difference between E1 and E2


Figure 4.2: Energy calibration E1 vs. C1.

∆E = E1 − E2

= (0.00311C1 + 0.005155)− (0.00311C2 + 0.005155)

= 0.00311 (C1 − C2)

(4.3)

The energy loss ∆E was found to be less than 20% of the backscatteredenergy E1. Damache et al. [1], Ammi et al. [3] and Shiomi-Tsuda et al. [2]showed that if ∆E

E1≤ 20%, the stopping power can be computated accurately

as

S (E) =∆E

∆x(4.4)

where ∆x is the thickness of the mylar foil, assuming that the average path


E0 E1 E2 ∆E E S1.50µm S1.68µm SSRIM2008

500 489.94 389.48 100.46 439.71 66.98 59.80 53.53

600 587.93 500.97 86.96 544.45 57.97 51.76 47.15

700 685.92 606.93 78.99 646.42 52.66 47.02 41.97

800 783.90 711.34 72.56 747.62 48.37 43.19 38.48

900 881.89 814.19 67.70 848.04 45.13 40.30 35.61

1000 979.88 916.20 63.68 948.04 42.45 37.91 33.19

1100 1077.87 1017.48 60.39 1047.68 40.26 35.94 31.52

1200 1175.86 1121.21 54.64 1148.53 36.43 32.53 30.11

1300 1273.84 1217.26 56.58 1245.55 37.72 33.68 28.71

1400 1371.83 1318.89 52.95 1345.36 35.30 31.51 27.36

1500 1469.82 1421.06 48.76 1445.44 32.51 29.02 26.15

Table 4.1: Experimental results where the energies are in keV and stopping powers

are in keV/µm.

length of protons in mylar equals the thickness of mylar. Stopping power iscalculated at average energy

E =E1 + E2

2(4.5)

The experimental stopping power is approximately equal to electronicstopping power for this energy range (500 keV ≤ E0 ≤ 1500 keV ). The cur-rent experimental stopping powers were initially calculated mylar of thick-ness 1.50µm, as provided by the manufacturer, and plotted against the av-erage energy E. The results show a large deviation from calculations doneby SRIM2008 (see Figure 4.3). Therefore due to these deviation the mylarfoil thickness was measured through the energy loss of 5.48 MeV α-particlesfrom a 241Am source, and found to be 1.68 µm. The results at this thicknessshow a better agreement with SRIM2008 calculations, as shown in Figure4.3. However, since the SRIM2008 stopping power of 241Am α-particles wasused in calculating the foil thickness, the stopping power values reported hereare relative values.


Figure 4.3: Deviation in stopping power from SRIM2008 calculations due to

thickness of foil.

Energy resolution of the Si surface barrier detector used is 15 keV. Thenuncertainty in the energy loss measurements was calculated to be 3.1%. Theuncertainty in foil thickness measurement was calculated to be 4.4%. Thecombine to give a uncertainty in the measured stopping power

∆S

S=

√(0.031)2 + (0.044)2

= ±5.4%

(4.6)

The stopping power is seen to decrease with energy increase over the whole500 keV to 1500 keV energy range. Current data is, within experimentallimits, in agreement with data from Damache et al. [1] and Shiomi-Tsudaet al. [2] (see Figure 4.4). SRIM2008 predictions underestimate stopping


power, especially at lower energies, close to the stopping power maximum.The step observed between 1200 keV and 1500keV and cannot be explainedat this stage. However results by Ammi et al. [3] show a similar kink at aboutthe same energy (see Figure 1.1)

Figure 4.4: Current stopping power results compared with previous results and

calculations.

Chapter 5

Conclusion

Very few data is currently available on stopping power of protons for mylarin the energy range from 500 keV to 1500 keV. In this experiment, we suc-cessfully measured the stopping power of protons in mylar foil, in this energyrange. The results show a slight deviation from the semi-empirical calcu-lations and previous experimental results. This deviation is largely due touncertainty in measurement of the thickness of mylar foil. However, Paul andSchinner [11] showed that the uncertainty in stopping power data obtainedfrom computer codes such as SRIM is about 7-8% accurate. Therefore currentresults, with a 5.4% experimental uncertainty, show a good agreement withwith SRIM2008 and PSTAR predictions and previous experimental results,and therefore it can be taken as reliable.

Among other observations of stopping power in this results is the suddenincrease in stopping power in the 1200 keV - 1500 keV region. This is also beenobserved in the results of Ammi et al. [3]. The presence of this kink couldnot be explained at the moment, and must be investigated more extensivelyin future experiments.

The determination of stopping power for thin polymer foils requires aspecial, dedicated experimental setup and techniques. Since new type oftechnical materials are being produced in the nanotechnology, the knowledgeand availability of extensive and accurate database is required for charac-

24

CHAPTER 5. CONCLUSION 25

terisation of these materials. This can be done using, in particular, IonBeam Analysis techniques, especially RBS and ERDA techniques with lightnuclides (protons and α-particles) and heavier nuclides (Z ≥ 3).

Appendix A

The Van De Graaff Accelerator

Charged particle beams or ion beams are produced from a suitable ion sourceand accelerated using particle accelerator, such as the Van de Graaff accel-erator. A Van de Graaff accelerator, named after R.J. Van de Graaff, can

Figure A.1: Generalized schematic diagram of a Van de Graaff accelerator.

26

APPENDIX A. THE VAN DE GRAAFF ACCELERATOR 27

provide 1H+, 2H+, 3He+ and 4He+ beams. The accelerator can be of ver-tical or horizontal alignment (most tandem accelerators). Figure A.1 showsthe generalised schematic diagram of a vertically aligned Van de Graaff ac-celerator. The Van de Graaff accelerator used at iThemba Labs, is verticallyaligned and produces 1H+, 2H+, and 4He+ ion beams with energies up toabout 3.50MeV (see Figure 3.3). The ion source is located inside the con-ducting sphere which is maintained at a high voltage using the high-voltagesupply. The ions produced through ionization of the ion source are accel-erated through the high potential difference towards the target. Bendingmagnet is used to bend the ion beam 90o towards the target , and to alignthe beam. Collimators are used to shape the beam. The beam current canbe determined and varied.

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