IIIIISD9800001

STUDIES OF SOME ALLOYS

USING

X-RAY FLUOft£SC£NCE

A thesis Submitted in Partial fulfilmentof the requirement for the Master

Degree in Physics

ByElamin Musaid Elmahi

Supervised By

Dr. Farouk Idris Habbani

Department of PhysicsFaculty of Science

University of Khartoum

January 1997

2 9 - 30

We regret thatsome of the pagesin this report may

not be up to theproper legibilitystandards, eventhough the best

possible copy wasused for scanning

CONTENTS

PageACKNOWLEDGEMENTS IABSTRACT IICHAPTER ONE IINTRODUCTION 11.1.Gold 31.2 Steel • 41.3 Brass 61.4 Coins 6CHAPTER TWO 7

THE X-RAY FLUORESCENCE METHOD 72.1 Interaction of x-rays with matter 72.1.1 Photo-electric effect 72.1.2 Compton scattering 72.1.3 Coherent scattering 102.2 Fluorescence yield . 1 02.3 Characteristic lines and selection rules 112.4 Relation between intensity and concentration 12CHAPTER THREE 17EXPERIMENTAL MEASUREMENTS 173.1 Experimental setup in XRF 173.1.1 Introduction 173.1.2 The radioactive source and the Si[Li] detector 173.1.3 Electronics 183.2 Experimental measurements 233.2.1 Itfoduction 233.2.2 Analysis of gold (use of x-ray fluorescence) 243.2.2 Analysis of gold using the chemical method (cupellation) 383.3 Measurement of steel 393.3.1 XRF measurements 393.3.2 Determination of carbon in steel using the spark method 403.3.2 Plain carbon content • 423.4 Analysis of brass 433.5 Analysis of coins 43CHAPTER FOUR 46RESULTS AND DISCUSSION 46CHAPTER FIVE 53CONCLUSION 53REFERENCES 54

I would like to express my gratitude and appreciation to my

supervisor Dr. Farouk I dris Habbani for his skillful guidance, keen

supervision and continuous assistance.

My deep gratitude to the director of Precious Metals Assay and Hall

Marking, Omer Ibrahim for his assistance.

My thanks also goes to Ust. Mohamed Younis of the English

servicing unit for proof reading the thesis and Mr. A/Razig, for his

continuous help.

My thanks to the member of the Physics Department who help me

during this work, especially the staff of the applied nuclear science

labrotary.

My thanks extend to Ishraga who print this thesis.

Finally, I am indebted to all my family members especially my

brother Eltigani, for his encouragement during the period of my studies.

In this project an attempt has been made for the study of

alloys commonly used using x-ray fluorescence (XRF)

technique. The alloys selected for the study included gold

jewellery, steels, brasses and coins. The XRF method proved to

be simple, fast, non-destructive and reliable as compared to

chemical methods. The results showed that most of the gold

jewellery used in this country have carat value of 18 and 21.

Also most coins used in different countries are alloys of Cu and

Ni. A simple spark method was used for the determination of C

in steels, since C is not possible to analyze by XRF.

CHAPTER ONE

In this work quantitative XRF analysis was used for studying

concentrations of elements in some alloys. Of the important elements

which enter in the formation of alloys are fij:

Copper:

Iron:

Gold:

is reddish - coloured, takes a bright metallic lustre, malleable,

ductile and a good conductor of heat and electricity ( second only

to sliver in electrical conductivity). The electrical industry is one

of the greatest user of copper. Its alloys are brass and bronze.

copper, tin, zinc, gold and silver were used long before iron.

These metals were also easier to work than iron. In addition, the

removal of impurities from iron ore was difficult. Malleable iron

had limited uses because of the inclusion and impurities in its

structure. ' In the middle of the nineteenth century, Bessemer

process was used to make steel. It was discovered that blowing

air through molten iron burned out the impurities, thus making

the iron workable at room temperature. The pure iron metal is not

often encountered commercially, but is usually alloyed with other

metals. Iron is a main constituent of steel.

is found in nature as a free metal. It is metallic, having a yellow

colour. It is a good conductor of heat and electricity and it is

unaffected by air and most reagents. It is used in coinage and is a

standard for monetary systems and it is used also for jewellery,

decoration, dental^vork and for plating.

Silver:

it is a little harder than gold and is very ductile and malleable.

Pure silver has the highest electrical and thermal conductivity of

all metals and possesses the lowest contact resistance. It is stable

in pure air and water but tarnishes when exposed to ozone,

hydrogen or sulphite.

Zinc:

is a white lustrous metal. It is brittle at ordinary temperatures but

malleable at 100° - 150°C. It is a fair conductor of electricity and

burns in air with high red heat.

Nicke l :

is silvery - white and, takes on a high polish. It is hard, malleable

and ductile. It is somewhat ferromagnetic and a fair conductor of

heat and electricity. Nickel plating is often used to provide a

protective coating for other materials. It is also used in ceramics

and in Edison storage battery.

Carbon:

solid carbon is found in nature in three forms : amorphous,

graphite and diamond. In gas form carbon is found as carbon

dioxide in the atmosphere of the earth.

Some properties of the above - mentioned metals are given in table

(1.1) below:

Element

Cu

FE

Au

Ag

Zn

Ni

C

Atomic -weight

63

55

197

107

65

59

12

Atomicnumber

29

26

79.

47

30

28

6

Meltingpoint(°C)

1063

1535

1063

960

419

1453

355

Boilingpoint (°C)

2595

3000

2966

2212

907

2732

4827

Specificgrav"

increases. Green gold, with gold 75 % and silver 25 % is used in jewellery

also. When the silver is over 70 % the alloy is white. Gold silver alloys are

used to make trial plates or standards of reference with which the fineness

of gold - wares and coins are compared. Naturally occurring metallic gold

usually has variable amounts of silver, copper, platinum, palladium and

other elements. The unit of gold is the carat. Table (1.2) shows the carat

and the corresponding composition of gold \i\.

Carat

24

21

18

16

14

12

Percentage of gold %

100

87.5

75

66.7

58.3

50

Percentage of others %

0

12.5

25

33.3

41.7

50

Table (1.2): The carat and gold composition

1.2. Steel:

SteeUare ferrous alloys, consisting usually of Fe, Ni and Cr. Steels

with chromium over 5 % are called stainless. The important element which

enters in the formation of steel is carbon. Hardness increases as the carbon

content increasesup to about 2.0 %. The tensile strength and yield strengthTo

also increase up'about 0.83 % carbon pj.

A relationship exists between the carbon content of steel and its

usage. This relationship will change with the elements in the steel, the heat

treatment and many other variables. Table (1.3) relates the carbon content

and usage [3].

Carbon content %

0.02-0.10

0.10-0.20

0.20-0.30

0.30 - 0.40

0.40 - 0.50

0.50 - 0.60

0.60 - 0.70

0.70 - 0.80

0.80 - 0.90

0.90- 1.00

1.00-1.10

1.10-1.20

1.20-1.30

1.30-1.40

Usage

Nails, stampings, welding materials, wire rivets.

Free-cutting materials, carbonizing materials, structural

steel, heavy - duty bolts. '

Cams, camshasfts, gears (carbonized), structural steel,

cranks and levers.

Heat-treated bolts, screws, nuts and axles, free -cutting

manganese steel, key stock, cold heading, machine parts.

Heat-treated parts, axles, bolts, camshafts, carbon steel

forgings, studs, gears, adapters.

Oil hardening gears.

Lock washer, forging dies, screw drivers, set and socket

screw, low carbon tool steel.

Wrenches, saws (band), hammers, medium tool steels.

Agricultural, harrow knives, spring steel, punches, cold

chisels, rivet sets, shear blades, rock drills, music wire,

mower blades.

Harrow disks, springs, knives, dies.

Ball bearings, drills, tool bits, cutters, taps.

Cutting tools, essentially some as 1.00 -1.10

Files, cutting tools.

Saws, boring tools, instruments.

Table (1.3): Carbon Content and Usage.

i.%. Brass:

Brass is an, alloy, that consists mainly of copper and zinc. Brass

may be conveniently divided into two groups by the test of malleability,

the dividing line being a composition of 55% copper and 45% zinc. All the

lower copper brasses are unworkable and are known as the white brasses.

They are not of great industrial importance. All the higher copper alloys

are workable. Red brasses contain up to 20% zinc. The amount of copper

determines the colour of the alloy [2]. Various types of brass alloys are

shown in table (1.4).

1

Basic composition

Gilding (210)

Commercial bronze (210

Jewellery bronze (226)

Red brass (230)

Fourdrinier brass (234)

Low brass (240)

Yellow brass (268)

iCartridge brass (260)

Muntz metal

Percentage

ofcopper %

95

90

87.5

85

83

80

70

66

62

Percentage of

zinc %

5

10

12.5

15

17

20

30

34

38

Table (1.4): Various Alloys of Brass [2]

1*4.. Coins:

Gold and sliver were used in the past as coins. Presently coins aremade of alloys of the elements : copper, nickel and zinc in variousamounts. Sliver appears in some coins also in various amountsj.

, CHAFfER TWO

TOE X-RAV FLUORESCENCE METHOD

In this chapter, the following topics are discussed: the interaction

of x-rays with matter, fluorescence yield, characteristic lines and selection

rules and the relation between intensity and elemental concentration.

2x Interaction! of x°rays wi th ma t t e r : -

In x-ray fluorescence (XRF) we are dealing with x-rays of energies

less than 20 Kev. Fig. (2.1) showsx-ray excitation of a sample. When an x-

ray photon interacts with an atom of the material the following processes

occur[4j:

2xi . P h o t o - e l e c t r i c effsect:

In this process we get ionization of the atom, electrons from K or L

shell of the atom are ejected from the atom followed by filling the vacancy

by electrons from outer shells and emission of characteristic x-rays. If

the fluorescent x-ray photon escapes from the atom we get characteristic

x-rays of the element. If it is absorbed within the atom on its way out andi

ionizes the atom in an outer shell by ejecting an electron out, we get Augereffect (4). Fig. (2.2) shows excitation of an atom by the photo-electric

effect.

2x2. C o m p t o n scat ter ing ' :

An incident photon of energy (E=hv) gives part of its energy to an

electron in an atom (considered to be loosely bound to the atom) which

recoils and the photon is scattered with less energy (E=hv). This process is

also called incoherent scattering. Fig. (2.3) shows interaction of x-rays

with matter through the £ompton effect.

x-ray source sample

Si [Li] detector

—fluorescence processin element " i "

emitted x-ray characteristic forthe element "i"

Fig. (2.1): X-ray excitation of a sample.

incident photon

N2

nucleus

emitted photon

r vacancy

ejected electron

Fig. (2.2): Schematic representation for excitation of an atom by thephoto-electric effect.

E=hvJ

incident photon

scatterd photon

fa

recoil electron

Fig. (2.3): The £ompton effect.

2.1.^. C o h e r e n t scattering":

This process occurs as a result of x-ray photon collisions with rigid

or firmly bound electrons in the atom. In this case the photons are

scattered with no loss of energy. The large number of firmly bound

electrons in the heavier elements form the greater contribution of coherent

scattering which increases with increasing atomic number (4).

1.1. F luorescence Yield:

When x-rays interact with matter leading to its ionization

characteristic x-rays are emitted. The intensity of a particular x-ray

emission depends on three probabilities:

1. The incident photons will ionize the atom in a certain level.

2. The created vacancy on the level will be filled by another subshell

electron.

3. The emitted photon in the process will leave the atom without being

absorbed in it.

The first factor is related to the absorption process while the

second one is governed by quantum mechanics and the last factor is called

fluorescence yield. Fluorescence yield is defined as follows:

w = n f /n

where:

n: is the number of primary photons that have induced the ionization in a

given level or the number of secondary photons that are subsequently

emitted.

n f: is the number of secondary photons that effectively leave the atom.

10

Therefore the difference n-nf, is the number of secondary photons

that are absorbed within the atom on their way out (the Auger effect)(4).

Fig. (2.4) shows fluorescence yield'a function of atomic number of the

material.i

l.\, CharacteHstiic Mines and select ion rules :

If as a result of x-ray interaction with matter an electron is ejected,

an electron from higher energy levels in the atom will transfer to the lower

level which is vacated, and characteristic radiation is emitted in the

process. The radiations emitted in such cases will have wave - lengths and

energies related to the difference between the binding energies of the two

levels through which transitions occur. According to this picture a vacancy

in a given shell may be filled by electrons from shells further out from the

nucleus, and each shell-to-shell transition leads to a line in the x-ray

emission. Such transitions follow certain selection rules which are as

follows:

An =1,2,3.

Al = + 1

where

n = principal quantum number.

• ' i

1 = angular quantum number.

j = total quantum number.

Generally if K-shell (n=l) electrons are removed, the electrons

from higher energy states falling into the K-shell produce a series of lines

denoted in the x-ray notation as Ka, Kp ... lines. If L-shell (n=2) electrons

are removed, another series of lines called L-series is produced, and so

l i

forth. Upon closer observation each line of the characteristic x-rays is

found to be composed of a number of closely spaced lines. This splitting in

the lines results from the fine structure of the energy levels. The

interpretation of x-ray spectra in terms of quantum mechanics can lead to a

great deal of information regarding exact values of electronic energy states

within the atom(6). Fig. (2.5) shows some main lines in the K andL

spectra.

2.4. Rela t ion b e t w e e n in tens i ty and concen t ra t ion ;

The x-ray fluorescence is characterised by the non-linear

relationship between the measured intensity of the characteristic x-rays

and the concentration of the respective element in the sample.

The non-linearity is caused mainly by the self absorption of the

excitation and fluorescent radiation in the sample. Namely deeper layers of

the sample are reached by a rather attenuated beam of the excitation

radiation and the excited fluorescent x-rays emitted there are partially1

absorbed before they reach the surface of the sample and escape towards

the detector.

The relation between the measured intensity of an element and its

respective amount in the sample can be presented in the following form [5]:

Ii = Si. Q . Tj. (cic2, c n) . Hi (ci, c2, Cn) (2.1)

where:

Si = element sensitivity

Cj = element concentration

Ti = the absorption correction factor

Hi = the enhancement correction factor

In case of extremely thin sample:

12

Is = Si. (pid) (2.2)

where:

Sifi = element concentration,

d = sample thickness

The above relation becomes linear, since the elemental sensitivity

Si is constant. We have for S\.

Si = G.Ki (2.3)

where ,

Gi = Ao. Qi.Q2- cosec ¥1 (geometrical factor)

Ki = P# (Ei). (1 - 4 - )i Wki.f,ka. 6rd (Ej) ( fundamental

7k

parameter factor).

and

Ao: activity of the excitation source.

Q.], Q.2'. solid angle of the sample from the source and detector

respectively.

8iph (Ei): photo effect cross - section at energy Ei in element

"i"

(1 )i : relative probability for excitation of k-shell of

element "i".

Wika: fluorescent yield for k-shell of element " i"

fika: relative transition probability for k a x-rays of element "i

13

erel (^i) = relative detector efficiency for x-rays of element

Absorption and enhancement correction factors Tj and Hjctively, can be explicitly calculated if the composition of thele is known.

Absorption and enhancement corrections factors Tj and Hj

id on the combined absorption of primary and secondaryion in the sample :

aj = jns (Ej). cosecxl/i+ Jis (Ej)-cosec ^ 2

:̂ j.is(Ej) and )is(Ej) are the respective absorption coefficients for^citation and fluorescent x-rays in the sample, and

cosec^j > c o s e c ^

he average cosecf of the incident primary and take-offascent radiation from the sample.

The absorption correction factor Tj has the following form

Tj = (l-exp(- ajpd))/ aj

the enhancement correction factor Hj is rather complicated:ssion, which can be found in (2.1).

14

\.o

.8

0.6

OM

o.i

atomic number z

Fig (2.4): Fluorescent yield as afunction of atomic number;

15

Fig (2.5): Characteristic lines in the K & L spectra

16

CHAPTER THREE

Experimental setup urn X1RF :

• Introduction:

In this section, a brief description of instruments used in XRF is

n. First, a block diagram of the XRF system is shown, and the

tation radioactive source is briefly described. Second, the Si[Li]

ctor and the electronics associated with the system are.briefly

:ribed, including the preamplifier, the linear amplifier and the multi-

mel analyser.

I. T h e radioactive s o u r c e a n d t h e Si.|Li.| d e t e c t o r :

The excitation source used in this work is £d-109, with 22.6 Kev

: tation energy, which is optimum for the excitation of elements from K

io (KW)p) radiations.

The Si [Li] detector has proved to be a useful and appropriate tool

energy dispersive XRF systems. It has adequate energy resolution to

>lve the K^p-lines from adjacent elements for atomic numbers greater

\ 14 (Si). In fig. (3.1) is shown a block diagram of the Si[Li]

ctrometer and in fig. (3.2) is shown the excitation source and sample

ier.

For the detector to operate a high voltage of- 1500 V is required.

s voltage is provided by the detector high voltage bias supply. To

\imize the electronic noise added to the signal the Si [Li] detector must

mounted in a tight vacuum cryostat and operated at liquid nitrogen

ling temperature (77°k)[6].

17

5«*«?« E lec t ron ic s :

T h e Preamplif ier :

The function of the preamplifier is to collect the charge pulsesfrom the detector, and provide the low driving impedance necessary topass the signal through a co-axial cable to the main amplifier, which isusually located some distance away. The charge pulse from the detector iscollected in the preamplifier by integrating it on a capacitor to produce avoltage pulse. The height of this pulse is proportional to the energy of theincident x-ray[6].

Such a pulse suffers from three problems as it emerges from thepreamplifier output. These are:

1 .The pulse amplitude is extremely small.

2.The pulse duration is too long.

3.There is generally an unacceptable noise level superimposedon the signal by the preamplifier.

To overcome these problems an amplifier is connected with thispreamplifier.

T h e Aimplilf Heir:y

To solve the problems associated with the pulse at thepreamplifier output, the main amplifier is set to serve three purposes :

(i) it amplifies the signal to make it in the 0-to-10 V pulseheight range-

(ii) suitable pulse shaping filters are incorporated to yield ashorter pulse duration so that high counting rates can be handled, withminimal dead time losses.

(iii) the filters are selected to minimize the noise contributionfrom the preamplifier.

18

T h e Multii-chaunurnel Analyser (MCA]):

The purpose of the multichannel pulse height analyser is to

measure the height of each amplifier output pulse, and to represent this

amplitude by an integer number. This is an analog - to - digital conversion

process. The number of times a pulse of each height has been detected is

accumulated in the analyser memory to form a spectrum of the pulse

heights. Subsequently, this information can be displayed as energy

spectrum.

Since most quantitative fluorescence spectrometers include small digital

computer, the computer is generally used to store data, to perform fitting

of spectra and finally to perform quantitative analysis of samples using

appropriate software, such as AXIL.

19

SI [ LI I DETECTOR

MCA

LIQUID NITROGEN

COPPER ROD COMPUTER

Fig (3.1): Block diagram of the Si [Li] spectrometer

-20

sample |

0.86cm

0.3cm

0.1 =Q03 —

cd-109source

1.33cm

•I

Be^windowi\\\y NS\ \W\\N

Detector |

•Ma.

0.5cm

v

(3.2): Excitation source and sample holder.

21

Measuremen t s using" XRF:

The XRF system used was based on Cd-109 excitation source.

The Cd-109 source has average energy of 22.6 Kev and is able to excite

the elements from z = 15 up to z = 92 using both k and L lines. The

spectrometer used was Si [Li] detector spectrometer system. The amplifier

settings were adjusted for optimum conditions of measurements. The

spectra obtained were transferred to IBM compatible computer for

storage and data analysis.

The spectra were first analysed using AXIL program in the

computer. The AXIL software is able to separate overlapping peaks, and

in this way to identify the elements and determine the net area of the

peaks. The net area of a peak will be proportional to the concentration of

the element in the sample.

The accuracy with which elemental concentrations can be

determined by XRF depends upon the conversion of the characteristic x-

ray intensities. The x-ray spectrometer has to be calibrated for the range

of elements expected to be measured by the specific x-ray system.

Calibration is v ; one of the most important steps towards

quantitative analysis. In the AXIL software the calibration of the system

determines the sensitivities for all elements, which can be excited in a

chosen excitation mode, and emit the characteristic x-ray^which are

detected with the particular x-ray spectrometer^].

A software called QAES, prepared by Dr. P., . is also available

for data analysis{5).

There are two types of calibration of the system in QAES, basic

calibration (with pure elements or compounds) and calibration by standard

sample.

22

Basic calibration (with pure elements or compounds):

First the detector characteristics are input. The target material

(usually pure elements like Cu, Zn and Mo) is placed 4nm above the

sample position, and respective K-series x-rays are measured at first

without the sample. Later the measurement is repeated with the sample in

its position. The area for ka and kp for each element is then measured and

intensity determined.

Pure elements like Zn, Fe, Pb, Sn, Ti and Cu are then placed at

the sample position and the peak area for each element is measured using

the AXIL software.

When all the measured standards have been input, the program

starts to evaluate the sensitivities and from them the geometrical constant

for all measured elements. For each element the geometrical constant

must be theoretically the same. Discrepancies are caused by experimental

errors, bad knowledge of the cofripound composition, and partially by the

errors in the tabulated absorption coefficients of pure elements.

When performing the calibration of the XRF system the

experimentally determined geometrical constants of nearly the same value

are adopted. The average geometrical constant is used for the calculation

of sensitivities for those elements, which have not been measured [5].

5.2. EXPERIMENTAL MEASUREMENTS;

5.2.1 I n t r o d u c t i o n :

In this chapter, the experimental techniques and measurements

are presented. These include the analysis of gold, determination of carbon

and other elements in steel and studies of brass and coins. Measurements

and calculations were first done for some standard samples to test the

methods before applying them to all other samples.

23

5*2.2. Analys is of Gold

Use of x-ray

XRF was used for the analysis of various gold samples for the

determination of Au and Cu contents, and in this way to specify the carat

value of the gold. Two types of standard gold samples were taken from the

Precious Metals Assay and Hall Marking for analysis. Their masses and

areas are as shown in table (3.1). The results are as shown in table (3.2).

The intensity of gold in carat per area over the intensity of pure

gold per area as well as gold concentration is shown in table (3.3).

The intensity of copper in carat per area over the intensity of pure

copper per area as well as Cu concentration is shown in table (3.4).

In fig. (3.3) is plotted the intensity ratio of carat An over pure Au

as a function of carat gold concentration for type 1 carat gold.

In fig. (3.4) is plotted the intensity ratio of carat Au over pure Au

as a function of carat gold concentration for type 2 carat gold.

In fig. (3.5) is plotted the intensity ratio of Cu in carat gold over

pure copper as a function of copper concentration for type 1 carat gold.

In fig. (3.6) is plotted the intensity ratio of Cu in carat gold over

pure copper as a function of copper concentration for type 2 carat gold.

In fig. (3.7) to fig. (3.11) are shown the XRF spectra for carat

gold of type 2 samples.

24

Table (3.1) : Weights And Areas of the Two

Types of Gold Standards

Type

First

type

Second

type

Carat value

12

14

18

21

12N

14N

18N

21N

23.5N

Weight(g)

5.1

1.7

2.1

0.7

6.6

3.6

3.4

3.3

4.6

2

Area(cm )

3.6

1.8

2.7

0.9

3.2

2.8

2.9

2.8

2

Table (3.2) : Concentrations ofElements in the Standard Gold Samples.

Type

First

type .

Second

type

Carat

12

14

18

21

12N

14N

18N

21N

23.5N

Concentration •%

Au

61.3±3.1

75.9±3.8

77.2±3.8

88.3±4.4

61.9±3.1

69.1±3.5

79.4±4.0

91.3±4.6-

99.8±5.O

Cu

35.7±1.9

24.1±1.2

22.811.1

11.710.6

27.811.4

18.110.9

14.510.7

8.7±0.4

0.16+0.01

Ag

10.310.5

12.910.6

6.1±0.3

Table (3.3) : Ratio of the Intensity of Gold in Carat per

Area over the Intensity of Pure Gold per Area.

Type

First

type

Second

type

Carat

12

14

18

21

12N

14N

18N

2 IN

Intensity ofgold incarat(c/s)

870.7

675.9

912.0

437.0

766.4

955.3

1191.4

1595.8

intensity ofgold incarat perarea

(c/s.cm J

241.7

375.5

373.8

485.6

234.5

341.2

410.8

480.3

intensity ofpure gold(c/s)

1600

1600

1600

1600

1600

1600

1600

1600

Area of pure

gold (cml

2.9

2.9

2.9

2.9

2.9

2.9

2.9

2.9

Intensity ofpure goldper area

(c/s.cm J

551.7

551.7

551.7

551.7

551.7

551.7

551.7

551.7

I. in caratAu

over I.Au

pure

0.44

0.68

0.61

0.88

0.43

0.62

0.75

0.87

concentration ofAu in carat %

61.3

75.9

77.2

88.3

61.9

69.1

79.5

91.3

Table (3.4) : Intensity of Copper in Carat per Area over the

Intensity of Pure Copper per Area

Type

First

type

Second

type

Carat

1 2 •

14

18

21

12N

14N

18N

21N

Intensity ofcopper incarat(c/s)

971.2

261.7

345.7

68.6

695.3

401.4

338.7

178.7

intensity ofcopper incarat per area

(c/s.cm )

269.8

145.4

128.0

76.2

217.3

142.8

115.2

63.4

intensity ofpure copper(c/s)

4464.9

4464.9

4464.9

4464.9

4464.9

4464.9

4464.9

4464.9

Area of purecopper

(cm2)

4.9

4.9

4.9

4.9

4.9

4.9

4.9

4.9

Intensity ofpure copperper area

(c/s.cm )

911.2

911.2

911.2

911.2

911.2

911.2

911.2

911.2

I in caratover I_

Cu

pure

0.30

0.16

0.14

0.08

0.24

0.16

0.13

0.07

concentrationof Cu in carat%

38.7

24.1

22.8

11.7

27.8

18.1

14.5

8.7

Fig(3.3): IAu in carat per area over Impure per area Versus theconcentration %.

Un in carat ( c/s.gm2)Uu pure ( c/s.cm )

In x-axis lcm s 10 percent

Iny-axis lcmsO.l

&

gold concentration %

So

Fig(3.4): IAU in carat per area over IAu pure per area Versus theconcentration %.

IAu in carat (c/s.cm2)

I pure ( c/s.cm2 )

In x-axis 1 cm s 10 percent

Iny-axis lcm = 0.1

gold concentration %

to 2-0 3

Fig(3.5): Icu in carat per area over Icupure per area Versus theconcentration %.

-„ in carat (c/s.cm2):u pure ( c/s.cm )

In x-axis lcm H 5 percent

Iny-axis lcm s 0.05

o

copper concentration %

31

So

Fig(3.t): ICI in carat per area over Impure per area Versus the concentration %.

n^aj^lXc/sxmpure (c/s.cm )

In x-axis lcm = 5 percent

In y-axis lcm = 0.05

copper concentration %

la ~7z I'U 26

31

3

Oo

"TI

rr

£ cP5

3o7

3-3

,-a

00

c o n(/I

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ooo

c>

Zc

ooo

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Fig. (3.9) : Spectrum .SPE"

ceu11-tc/

k

nnCi.L

j O C i ) •

k-'..-.;. V.,

. •• i •

• » ' • * * »

• ».* , ; .

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£01-

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"^.I.I* Analys is of croldl using* t h e chemica l m e t h o d

(cupel la t ion) :

The method described below is used by the Metal Assay and Hall

Marking Unit, Khartoum, for routine analysis of the gold sample.

The method consists of an oxidizing fusion in a porous vessel

called cupel. The gold sample is wrapped in a lead foil. If the proper

temperature is maintained the lead oxidizes rapidly to form lead oxide and

then the lead oxide oxidizes the basic metals in the sample and all the

oxides are absorbed by the cupel. When this process has been carried to

completion, the gold and silver are left in the cupel in a form of a bead .

The cupel surface may be regarded as a membrane permeable to

molten metal oxides and impermeable to gold and silver.

Piracedlmnre:

Initially a test is performed to estimate roughly the carat value of

the gold sample using a test stone. About 0.1 to 0.15 gram of the gold

sample is then weighed accurately and about 2 to 2.5 times of pure silver

is added to it and wrapped in 4 - 6 gram of lead foil and rolled in a form

of a ball.

The furnace is heated to light red and the empty cupel is then

carefully introduced and allowed to remain in the furnace for at least 10

minutes in order to expel the moisture and organic matter .

When all is ready the balls (samples) are placed carefully in the

cupels and the furnace door is closed for 15 mimrtes.Thcn the temperature

should be allowed to cool until the beads solidify. The beads will consist

of gold and silver only.

Then the beads are flattened and rolled each in a form of a cornet.

Ihey are then placed in a silica tray apparatus for the separation of silver

from gold.

38

The silver is separated from gold by nitric acid. The silica tray

with the cornet is placed in boiling nitric acid of specific gravity 1.2 for 15

minutes and then in another nitric acid of specific gravity 1.3 for 15 min.

Finally the yield cornets are weighed accurately to compare to the

weight of the original gold sample in order to calculate the carat. This

process takes nearly about 2 hours for completion [8].

^.^ Meatsureinnieintt of Steel:

Various steel samples were measured for the determination oftheir composition. Steel is mainly coumposed of Fe and some otherelements are added to it in order to improve its quality and durability.XRF is used to determine most of the elements present in steel. However,XRF is unable to determine the carbon content of steel. The SparkMethod has been used in this work to determine the carbon content insteels.

5.^.1 XRF m e a s u r e m e n t s :

11 samples of steels were measured by the XRF method. A steelstandard from Good Fellow Co. (UK) obtained from the IAEA (Vienna)was used for checking the calibration and performance of the XRF system.The results of the measurements for the steel standard are given in table(3-5).

Table (3.5): Measurement for the Steel Standard.

Elements

Cr

Fe

Ni

Declaredcompisition (%)

19.2±1.0

72.8±3.6

7.5±6.4

Measured compisition(%)

18

72

10

39

5»5*̂ determination of carbon tin steel:

The Spark Test Method;

This method was used , to determine the percentage of carbon insteels. A spark is obtained from the sample when the electric power is

switched on in the apparatus as shown in fig.(3.12). Thedetermination of carbon by this method depends upon three parameters :colour, shape, and band of the signal. A ccording to this method we havethree cases:

l.the percentage of carbon in the sample of 0.15 ; in this case the colourincreases gradually from darker yellow to brighter yellow, and theamount of the band is small.

2. the percentage of carbon in the interval (0.15-1); the colour increasesgradually from brighter yellow to brilliant yellow. The end of the sparkdistrbuted as stars^and the amount of the band increases gradually.

3. the percentage of carbon, in the interval (1-1.7) ; the co{.4 r̂ is brilliant.The end of the spark distrbuted as stars in all directions and the amountof the band increases due to more carbon content.

To determine the percentage of carbon more accurately when thereare more than one sample (xi, x2, , xn) in the interval. These samplesare arranged according to their carbon content from lower to higher. Thisis determined by the above cases. The lower carbon content is assigned tothe minimum limit of interval, the higher carbon content is assigned to themamximum limit and the samples in between are arranged so that thevalue between every two of them is equal. This is done and the error isfound by using the following mathematical method:

lv r t / \ 2

standard deviation ( f) =-\|—!j=-^— —V n-\

where x is the mean

40

Jstandard error (s.e) =

Six samples are arranged in the interval (0.15-1). The values of thesamples are as follows:

0.15,0.32,0.49,0.66,0.83,1

- 0.15 + 0.32 + 0.49 + 0.66 + 0.83 + 1 3.45the mean x = = =0.58

then the following table is made:

Sample

0.15

0.32

0.49

0.66

0.83

1

X j - X

-0.43

-0.26

-0.09

0.08

0.25

0.42

(Xi- X)2

0.185

0.077

0.008

0.006

0.062

0.176

then the following table is made:

Sample

1

1.14

1.28

1.42

1.56

1.7

X j - X

-0.35

-0.21

-0.07

0.07

0.21

0.35

(xi- x)2

0.122

0.044

0.005

0.005

0.044

0.122

I(Xi-X) =0.34

•f = = 0.26

0-26 n 1s.e =0.10

2.5 .

These samples are considered to be standards to compare with.

^1 Plaint Carbon Comteintt:Plain carbon steel contains iron and carbon with small amounts of

other elements. They represent the most important group of engineeringmaterials. fMain carbon steels are classified as follows:

l.low carbon steels; in which the carbon content is below0.20 %.

2. medium carbon steels; in which carbon content is between0.20 and 0.50 %.

3.high carbon steels; in which the carbon content is above0.50 %.

The average physical properties of plain carbon steels depend on thecarbon content as shown in fig (3.13). The tensile strength, yield strengthand hardness increase with i increasing carbon content. Elongation,

42

reduction in area and impact values show a marked decrease withincreasing carbon content.

5«tJ-4 Analys ts of brass:

XRF was used for the analysis of various brass samples for thedetermination of Cu and Zn contents. A standard from Good Fellow Co-(U.K), 2mm thick and composition (63% Cu and 37% Zn) was analysed tocheck on the system. The result is as shown in table (3.6).

Elements

Cu

Zu

Suppliedcomposition

6 3 %

37%

Measured composition

68.7+3.4 %

31.3+1.6%

Table (3.6): Measurement for Elemental Composition of the StandardBrass

The results for the measurements of the brass samples are shownin chapter four.

3.'. c Analys ts of co ins :

In the past coins were made up of pure metals such as gold orsiliver. Nowadays coins are usually made up of alloys of such metals asCu, Ni, Zn, Ag, etc.

XRF was used for the analysis of various coins from differentcountries of the world for the determination of their metallic composition.

The results for such measurements are given in chapter four.

43

grinding wheel

li

spark

Fig (3.12): The Spark Method

44

ELCNGATICN ANO REDUCTION OF TENSILE AND YIELD STRENGTHAREA, PERCENT (THCU. L3. PEH SQ.iN.)

3)On2

3)CDO

p

o

ro

_i

bJGA

T

2) N

O

IN.)

[/j

roO

o

JI 1

ft-

o

mCJo- I

2O

§

o o

I I I ^

i i [5

o01

o

3RINELL HARCNE3S IMPACT VALUE..rT-L3Ol Ol OiO - 2 * C 0 — r o o i ^ mo o o o o c a o o

o i a Nc o o o

!\

JVH

i

m01

•

1

1

I !\

\

/

/

A 0

M*, ̂

!1

Fig (3.13): Average mechanical properties of plain carbon steels versus carbon content.

CHAPTER FOUR

RESULTS AND DISCUSSION

The XRF measurements have been carried out for the analysis of

some alloys: gold jewellery, steels, brasses and coins. The irradiation of

each sample was for 200 seconds and the analysis by the computer takes

about 15 minutes. The AXIL software is used for fitting of spectra and

QAES software for determination of concentrations. We find that total

time for the analysis is about 1100 sec.(18.3 min).

The results of the analysis for 15 samples of gold are shown in

table (4.1). The elements which have the highest values are Au and Cu.

Seven of these sample were assigned to carat 21, four samples were

assigned to carat 18, one sample was assigned to carat 14 and one sample

was found to be carat 24. The gold content of sample number 4 was found

to be 25.9 %. This sample is a kind of mixture alloy. Sample 15 was

found to . contain 0.22% of Au which is assumed to be a process of

guilder at the surface of the sample. Sample 14 contained 2.4% nickel.

This was considered to be due to the presence of impurities in copper or

may be added for hardening.

The results obtained for 11 steel samples are shown in table (4.2).

The elements which have the highest values are Ni, Cr and Fe, whereas

Cu, Mn and Pb have the less values. Samples 3, 10, 11 contain 0.15

percent carbon, and are classified as low carbon steels." Samples 4, 5, 6, 7,i

8, 9 contain carbon in the range (0.66-1.42) percent and are classified as

high carbon steels. Samples 1, 2, 3 contain Cr in the range (15.9-20.0)

percent, and are classified as stainless steels. Samples 5,6 contain nearly

equal concentrations of the elements, with a high value of Fe (> 98%) and

about 0.2% Cr, 1% Mn and 0.66% C. Two of the gun barrels appear to be

identical, with the third one clearly different.

46

Table (4.3) shows 4 samples of brass. Cu and Zn show the

highest values, with Fe, Ni, Au having lesser values. Sample 1 with a

concentration of Zn of 30.2 percent is usually classified as yellow brass[2].

Samples 2, 3 which contain 18.1, 19.3 percent Zn respectively are

classified as red brass. Sample 4, which contains 42 percent Cu is

classified as white brass. This brass also contains 1.8 Au and 15.6 % Ni,

which means it is required for some specific tasks in machinery.

Table (4.4) shows the results for elemental composition of 20

coins from different countries. On looking at the results, one observes that

the elements which have the highest values are nickel, copper and zinc.

Whereas Fe, Mn and Pb have much lesser values. Samples 8,9,10,12,

14, 15, 16, 17 contain Ni in the range (26.0 - 27.1) percent and Cu in the

range (72.6-73.8) percent. Therefore these coins are nearly similar.

Samples 3 and 4 are also similar, with main components 84.6 % Cu and

14.4 %Zn.

The accuracy of the results will depend greatly on the reliability

of the standards used. The standards of gold used in the work were taken

from Precious Metals Assay and Hall Marking Unit, Ministry of

Commerce Khartoum. The standards for steel and brass are from Good

Fellow Co- (UK), supplied by the IAEA (Vienna).

According to the communication by the Director of Precious

Metals Assay and Hall Marking Unit, Ministry of Commerce Khartoum,

the carat assignment to gold is made only if the gold content is greater than

37.5 %. Also if the percentage of gold lies between two carats the sample

is assigned the lower carat value [8].

As can be seen from the measurements made in this work for

various alloys using the XRF technique, XRF proved to be a simple, fast,

non-destructive and reliable method for the analysis of alloys. Chemical

and other methods are time-consuming and are not so reliable. However,

it should be pointed out that XRF has its limitations also and care must be

47

taken in carrying out such measurements. It should be put in mind that

XRF is unable to see deep inside a metallic sample due to absorption.

Also corrections should be made for the enhancement effect between the

elements contained in the alloy. The software used in the analysis of the

samples in this work allowed for enhancement corrections.

48

Table (4.1): Concetrations of Elements in Gold Samples Using XRF.

No

12345678

9101112131415

Sample

RinglRing 2Egyptian PoundRing without stoneBracelet 1 from one sideBracelet 1 from other sideBracelet 1 from the third sideA Small piece of pure gold (fromGood fellow Co.)NecklaceRing 3Ring 4(1821-1923) Mexican dollar _jKing Goerge the fifith dollarRing 5Bracelet 2

Concentration %

Au

90.7±4.569.1±3.588.5+4.425.9±1.390.0+4.589.8±4.589.6+4.5100+5

83.2+4.293.7±4.789.3+4.588.5±4.486.4+4.378.1±3.90.22±0.01

Cu

9.3±0.531.9+1.611.5+0.674.1+3.79.9±0.510.2±0.510.4±0.5

-

16.8±0.76.3+0.310.7+0.511.5±0.613.6±0.719.1±1.098.4+4.9

M

2.4+0.1

Zn

1.3±0.06

Assignedcarat

211418

21212124

182121211818

Table (4.2): Concetrations of Elements in Steel Sampls Using XRFand Spark Method for C.

No

1234

5678910

11

Samples

Standard from Good fellow Co (U.K).Standard frvm Good fellow Co (U.K).Watch Cover (stainless steel).A steel Sample 1 from Mechinical Workshop,Department of Phiysics.A piece of steel from Sudan Railways 1.A piece of steel from Sudan Railways 2.gun Barrel 1.gun Barrel 2.gun Barrel 3.Steel Sample from Sudan University for Science &Technology.A steel Sample 2 from Sudan Mechanical Workshop,Department of Physics. ~

Concetrations %

NI75.3±3.87.5 ±0.4

CR15.9+0.819.2+1.020+1

0.20+0.010.18±0. 013.5±0.20.64+0.033.7±0.20.37±0.02

0.56+0.03

FE8.8+0.472.8+3.680.0+4.099.815.0

98.3±4.998.9±4.995.9±4.998.7±4.995.5±4.899.6+4.9

• 97.9+4.9

MO

0.2410.01

0.0510.01

0.0710.010.0710.010.5410.03

0.5410.03

0.0910.01 -

cu0.1810.01

0.32+0.02

MN

1.410.10.8510.04

0.6610.03

1.1310.06

PB

0.0410.01

c

0.15+0.010.8310.13

0.6610.130.66+0.130.6C.+0.131.4210.1A0.8310. If0.1510.01

0.15+0.01

Table (4.3): Concetrations of Elements in Brss Using XRF.

No

1234

Samples

A piece of brass.Gun cartridge!.Gun cartridge 2.A piece of brass from Mechanical Workshop,Department of Physics.

Concetrations %

Cu69.8±3.481.7+4.180.5±4.042±2

Zn30.2+1.618.1±0.919.3±1.040.4±2.0

Fe

0.16±0.010.19+0.010.17±0.01

Ni

15.6±0,8

Au

1.80±0.09

No

1

2

3

456789101112131415161718

.1920

Table (4.4): Concetrations of Elements in VariousSamples

(5MARK) DDR (GERMAN "DEMOCRATIC REPUBLIC) 1969.(10 PENCE)UK(UNITED KINGDOM)1969.(10CENTS) REPUBLIC OF KENYA1990.((100L) ITALIA 1956.(1 KORONA) SWEDEN 1884.(1 FRANK) FRANCE 1975.(1 GUILDER) NETHERLANDS 1963.(1 GUARTER DOLLAR)USA 1986.(10 SHELING) TANZANIA 1987.(1 SHILING) TANZANIA 1981.(10 FORINT) HUNGARY 1977.(10 DRAHM A) GREECE 1984. -(1 RUBLE) USSR 1970.(1 RUPEE) INDIA 1985.(1 DINAR TUNIS 1976.(10 PIASTERS) EGYPT 1984.(10IASTERS) SUDAN 1983.(1 POUND) SUDAN 1989.(1 DINAR) SUDAN 1994.(2 DINAR) SUDAN 1994.

Coins Using XRF.Concetrations

NI9.4±0.5

26.3±1.3

0.7310.04

0.73±0.0425.1±1.399.7±5.0

26.4±1.327.111.426.7+1.399.815.026.511.312.310.626.711.326.211.326.511.326.011.396.2+4.8

FE1.2010.06

1.6+0.1

0.19+0.01

0.1910.010.2110.010.1410.010.5410.030.1610.010.21+0.010.2010.010.0910.010.3510.020.3010.020.15+0.010.2610.020.2610.020.2010.013.610.20.5110.030.6510.03

CU88.914.4

73.513.7

84.614.2

84.614.274.613.7

99.0+5.073.113.772.613.6-73.013.7

72.9+3.657.212.973.013.773.413.773.213.773.813.7

78.213.999.315.0

MN0.49+0.02

0.09+0.01

0.09+0.010.17+0.01

0.32+0.020.13+0.010.09+0.010.07+0.010.36+0.023.510.20.14+0.010.19+0.01

0.13+0.01

ZN

14.410.7

14.410.7

26.711.3

21.0+1.1

PB

0.41+0.02

0.21+0.01

CHAPTER FIVE

C O N C L U S I O N

The purpose of this project is to study alloys particularly those

related to gold, steels, brasses and coins, by using the x-ray fluorescence

(XRF) method.

The Precious Metals Assay and Flail Marking Unit (PMAHMU)

uses a chemical method for analyzing gold and assignment of the carat

value. This method takes about 2 hours for completion. It is lacking in

precision and accuracy in most cases. The XRF method proved to be

faster (It takes for the analysis about 18.3 minutes) with better precision

and accuracy, as well as being non-destructive.

During this work it was discovered that the gold content of

standard carats 12 and 14 taken from PMAHMU were of higher values

than specified on them. The standard carat 12 was found to be 14 and

standard 14 was found to be 18. In this way the gold from the market

would be assigned less carat than its actual value. The XRF method

proved to be very useful in discovering checking in gold jewellery. It is to

be noted that the carats which were known and accepted in Sudan are 12,

14, 18, 21 and 23.5, with 18 and 21 widely used. With regard to the

analysis of steels no reliable results could be obtained when using the

spark test method for determining carbon content because the method

depends on the eye observation only and is rather subjective.

The XRF method proved to be fast and reliable in thei

composition measurements for brasses and coins. Coins were found to be

predominantly made up of Ni and Cu in most countries.

53

REFERENCES

1. "Hand Book of Chemistry And Physics" By Robert C.Weast,

Forty-fifth Edition, The chemical Rubber Co, (1964).

2. "Chemical And Process Technology Encyclopedia" By Douglas

M. Considine, New York: Me Grow Hill, (1977).

3. "Material Science And Metallurgy, 3rd Ed , By Herman W.Pollack,

Reston Publishing Company, Enc, (1981).

4. "Report On X-Ray Fluorescence" , By Dr. Farouk Habbani,

Department of Physics, University of Khartoum.

5. "Quantitative Analysis of Environmental Samples", By P.Kump,

University of Ljubliana, Ljubliana, Slovenia (1995) (QAES).

6. Elsiddik T.Kafi, M.sc Thesis, Khartoum Unevirsity (1993).

7. "Determination of Copper Impurity In Gold Ornaments Using

Source-Exited X-Ray Fluorescence", By M.Hussain And

F.Hussain (Appl. Radiat. Isot, Volume 39, page 331, (1988).

8.Private Communication, Director of Precious Metals Assay And

Hall Marking Unit Khartoum, Sayed Omer Ibrahim.

54