Banha University Faculty of Science Chemistry Department
Radiochemical Study on the Medically and Technologically Radionuclides of Some Lanthanides
Presented by
Hany Abd ElEl----HamidHamid Abd ElEl----AzizAziz Aglan Egyption Atomic Energy Authority Nuclear Research Center Cyclotron Project
For The degree of Master of Science in Chemistry (Physical chemistry)
Supervised by
Prof. Dr. Mahmoud Ahmed Mousa Prof. Dr. Zeinab Abdou Saleh Professor of Physical Chemistry Professor of Nuclear Physics Faculty of Science Nuclear Research Center Benha University Atomic Energy Authority
Dr. Hassan Ali Hanafi Lecturer of Physical and Applied Chemistry Cyclotron Project Nuclear Research Center Atomic Energy Authority
2010 Approval Sheet
Title : Radiochemical Study on the Medically and Technologically Radionuclides of Some Lanthanides
Name: Hany Abd ElEl----HamidHamid Abd ElEl----AzizAziz Aglan
Supervisors:
Name Position Signature Prof. Dr. Mahmoud Ahmed Professor of Physical Mousa Chemistry - Benha University Prof. Dr. Zeinab Abdou Professor of Nuclear Physics Saleh Atomic Energy Authority Dr. Hassan Ali Hanafi Lecturer of Physical and Applied Chemistry Atomic Energy Authority
Head of Chemistry Vice – Dean Dean of Faculty Department for Graduate Studies and Research
Prof. Dr. S. G. Donia Prof. Dr. M. A. El Fakharany REFEREES DECISION
Title: Radiochemical Study on the Medically and Technologically Radionuclides of Some Lanthanides Name: Hany Abd El El----HamidHamid Abd ElEl----AzizAziz Aglan Referees: Name Position Signature
Date of Discussion: Degree of Discussion: Referees Signatures:
Name Signature
Head of Chemistry Vice – Dean Dean of Faculty Department for Graduate Studies and Research
Prof. Dr. S. G. Donia Prof. Dr. M. A. El Fakharany
آ ا م ا ء
درا آ ا ا ا ات ذات ا ه ا و ا
ﺑﺤﺚ ﻣﻘﺪﻡ ﻣﻦ ه ا ا ن ﻛﺠﺰﺀ ﻣﻦ ﻣﺘﻄﻠﺒﺎﺕ ﺍﻟﺤﺼﻮﻝ ﻋﻠﻰ ﺩﺭﺟﺔ ﺍﻟﻤﺎﺟﺴﺘﻴﺮ ﻓﻰ ﺍﻟﻌﻠﻮﻡ - ﻛﻴﻤﻴﺎﺀ ﻛﻴﻤﻴﺎﺀ ﻓﻴﺰﻳﺎﺋﻴﺔ - ﺟﺎﻣ ﻌﺔ ﺑﻨﻬﺎ
ﺗﺤﺖ ﺇﺷﺮﺍﻑ
ا ذ ا آ ر / د أ ا ذة ا آ رة / ز أ ذ ا ء ا أ ذة ا ء ا و آ ا م – آ ا ث ا و – ه ا ا ر
ا آ ر / رس ا ء ا وا وع ا ون آ ا ث ا و – ه ا ا ر
2010
آ ا م ا ء
ﻋﻨﻭﺍﻥ ﺍﻝﺭﺴﺎﻝﺔ : ﺩﺭﺍﺴﺔ ﻜﻴﻤﻴﺎﺌﻴﺔ ﺍﺸﻌﺎﻋﻴﺔ ﻋﻠﻰ ﺍﻨﻭﻴﺔ ﻤﺸﻌﺔ ﻝﺒﻌﺽ ﺍﻝﻼﻨﺜﻨﻴﺩﺍﺕ ﺫﺍﺕ ﺍﻷ ﻫﻤﻴﺔ ﺍﻝ ﻁﺒﻴﺔ ﻭ ﺍﻝ ﺘﻜﻨﻭﻝﻭﺠﻴﺔ
ﺍﺴﻡ ﺍﻝﺒﺎﺤﺙ : ه ا ا ن
ﺍﻝﻤﺸﺭﻓﻭﻥ:
ﻡ ﺍﻻﺴﻡ ﺍﻝﻭﻅﻴﻔﺔ ﺍﻝﺘﻭﻗﻴﻊ ١ ﺍﻷﺴﺘﺎﺫ ﺍﻝﺩﻜﺘﻭﺭ / ﻤﺤﻤﻭﺩ ﺃﺴﺘﺎﺫ ﺍﻝﻜﻴﻤﻴﺎﺀ ﺍﻝﻔﻴﺯﻴﺎﺌﻴﺔ ﺃﺤﻤﺩ ﻤﻭﺴﻰ ﺠﺎﻤﻌﺔ ﺒﻨﻬﺎ ٢ ﺍﻷﺴﺘﺎﺫﺓ ﺍﻝﺩﻜﺘﻭﺭﺓ / ﺯﻴﻨﺏ ﺃﺴﺘﺎﺫ ﺍﻝﻔﻴﺯﻴﺎﺀ ﺍﻝﻨﻭﻭﻴﺔ ﻋﺒﺩﻩ ﺼﺎﻝﺢ ﻫﻴﺌﺔ ﺍﻝﻁﺎﻗﺔ ﺍﻝﺫﺭﻴﺔ ٣ ﺍﻝﺩﻜﺘﻭﺭ / ﺤﺴﻥ ﻋﻠﻰ ﺤﻨﻔﻰ ﻤﺩﺭﺱ ﺍﻝﻜﻴﻤﻴﺎﺀ ﺍﻝﻔﻴﺯﻴﺎﺌﻴﺔ ﻭﺍﻝﺘﻁﺒﻴﻘﻴﺔ - ﻫﻴﺌﺔ ﺍﻝﻁﺎﻗﺔ ﺍﻝﺫﺭﻴﺔ
ﺭﺌﻴﺱ ﻤﺠﻠﺱ ﻗﺴﻡ ﺍﻝﻜﻴﻤﻴﺎﺀ ﻭﻜﻴل ﺍﻝﻜﻠﻴﺔ ﻝﺸﺌﻭﻥ ﺍﻝﺩﺭﺍﺴﺎﺕ ﻋﻤﻴﺩ ﺍﻝﻜﻠﻴﺔ ﺍﻝﻌﻠﻴﺎ ﻭﺍﻝﺒﺤﻭﺙ
ﺃ ﺩ. / ﺸﺎﻓﻌﻰ ﺠﻼل ﺩﻨﻴﺎ ﺃ ﺩ. / ﻤﺤﻤﺩ ﻋﺒﺩ ﺍﷲ ﺍﻝﻔﺨﺭﺍﻨﻰ
آ ا م ا ء ﻗﺭﺍﺭ ﻝﺠﻨﺔ ﺍﻝﺘﺤﻜﻴﻡ
ﻋﻨﻭﺍﻥ ﺍﻝﺭﺴﺎﻝﺔ : ﺩﺭﺍﺴﺔ ﻜ ﻴﻤﻴﺎﺌﻴﺔ ﺍﺸﻌﺎﻋﻴﺔ ﻋﻠﻰ ﺍﻨﻭﻴﺔ ﻤﺸﻌﺔ ﻝﺒﻌﺽ ﺍﻝﻼﻨﺜﻨﻴﺩﺍﺕ ﺫﺍﺕ ﺍﻷﻫﻤﻴﺔ ﺍﻝﻁﺒﻴﺔ ﻭﺍﻝﺘﻜﻨﻭﻝﻭﺠﻴﺔ ﺍﺴﻡ ﺍﻝﺒﺎﺤﺙ : ه ا ا ن ﻝﺠﻨﺔ ﺍﻝﺘﺤﻜﻴﻡ: ﻡ ﺍﻻﺴﻡ ﺍﻝﻭﻅﻴﻔﺔ
ﺘﺎﺭﻴﺦ ﺍﻝﻤﻨﺎﻗﺸﺔ -: -: ﺘﻘﺩﻴﺭ ﺍﻝﺭﺴﺎﻝﺔ-: ﺘﻭﻗﻴﻌﺎﺕ ﻝﺠﻨﺔ ﺍﻝﺘﺤﻜﻴﻡ: ﻡ ﺍﻻﺴﻡ ﺍﻝﺘﻭﻗﻴﻊ
ﺭﺌﻴﺱ ﻤﺠﻠﺱ ﻗﺴﻡ ﺍﻝﻜﻴﻤﻴﺎﺀ ﻭﻜﻴل ﺍﻝﻜﻠﻴﺔ ﻝﺸﺌﻭﻥ ﺍﻝﺩﺭﺍﺴﺎﺕ ﻋﻤﻴﺩ ﺍﻝﻜﻠﻴﺔ ﺍﻝﻌﻠﻴﺎ ﻭﺍﻝﺒﺤﻭﺙ
ﺃ ﺩ. / ﺸﺎﻓﻌﻰ ﺠﻼل ﺩﻨﻴﺎ ﺃ ﺩ. / ﻤﺤﻤﺩ ﻋﺒﺩ ﺍﷲ ﺍﻝﻔﺨﺭﺍﻨﻰ
ACKNOWLEDGMENT
At the beginning, I would like to knee to ALLAH , the most Beneficent, the most Merciful, for helping me to do this research work and I am asking Him to guide me for a more successful scientific career.
I would like to articulate my deepest appreciation and gratitude to Prof. Dr. Mahmoud Ahmed Mousa, Professor of Physical Chemistry, Faculty of Science, Benha University , for his kind guidance, excellent supervision subjective and precise criticism, as well as his continuous valuable encouragement and support all over each stage of this work.
No words can express my gratitude and appreciation to Prof. Dr. Zeinab Abdou Saleh ; Professor of Nuclear Physics-Nuclear Research Center-Atomic Energy Authority ; for the suggestion of the research point, instructive supervision, subjective criticism and the continuous guidance and efforts that she kindly and generously offered; throughout each stage of this study. I would like to express my gratitude to Dr. Hassan Ali Hanafi , Lecturer of Physical and Applied Chemistry , Cyclotron Project , Nuclear Research Center , Atomic Energy Authority for his supervision, constructive criticism and deep concern in this work.
I really appreciate the efforts of Dr. Shaban Abd Allah Kandil Lecturer of Inorganic and Analytical Chemistry , Cyclotron Project , Nuclear Research Center , Atomic Energy Authority for his constant help in the experimental work, guidance and the countless hours of attention he devoted throughout the course of this work.
My personal gratitude is also extended to all my colleagues in Cyclotron project, for their continuous and sincere help and support.
Finally, I would like to thank my family members for their support and encouragement which enabled this work to be completed.
Hany Abd ElEl----HamidHamid Abd ElEl----AzizAziz Aglan
i CONTENTS
ACKNOWLEDGMENT i LIST OF FIGURES vi LIST OF TABLES viii LIST OF ABBREVIATIONS ix ABSTRACT x CHAPTER 1 INTRODUCTION 1.1 Production of Radionuclides Using Cyclotrons...... 1 1.1.1 Types of Cyclotrons...... 1 1.1.2 Medical Radioisotopes...... 2 1.1.3 Optimum Conditions for Production...... 4 1.2 Production of Cerium and Praseodymium Radioisotopes...... 5 1.2.1 Importance of Ce Radioisotopes...... 5 1.2.2 Importance of Pr Radioisotopes...... 5 1.2.3 Production of Cerium 139...... 6 1.2.4 Production of Praseodymium 142...... 7 1.3 General Properties of Elements Concerned in This Study...... 8 1.3.1 Chemistry of Lanthanum...... 8 1.3.2 Oxides of the Lanthanides...... 9 1.3.3 General Properties of Cerium...... 11
1.3.4 Cerium Dioxide, CeO 2 ...... 13 1.3.5 General Properties of Praseodymium...... 14 1.4 Review on Separation Chemistry of Cerium and Praseodymium...... 16 1.4.1 Solvent Extraction...... 16
ii 1.4.2 Ion Exchange...... 18 1.5 Aim of the Work...... 22 CHAPTER 2 EXPERIMENTAL 2.1 Chemicals and Reagents...... 24 2.2 Instrumentation...... 25 2.3 Chemical Separation Processes...... 29 2.3.1 Separation of Cerium from Lanthanum...... 29 2.3.1.1 Solvent Extraction Separation...... 29 2.3.1.2 Ion Exchange Chromatography...... 30 (A) Determination of the Distribution Coefficients.... 30 (B) Cation Exchange Column Chromatography...... 31 a) 0.1 M Citrate Buffer...... 31 b) 0.1 M EDTA...... 32 c) α HIBA...... 32 2.3.2 Separation of Radioactive Cerium from Lanthanum Oxide Cyclotron Target...... 33 2.3.2.1 Target and Irradiation...... 33 2.3.2.2 Separation of 139 Ce from Irradiated Lanthanum Target Using Solvent Extraction Technique...... 35 139 2.3.2.3 Separation of Ce from Irradiated La 2O3 with Cation Exchange Column Chromatography...... 36 a) Purification of the Resulting Radionuclide...... 36 2.3.2.4 Determination of Radionuclidic Purity, Chemical Purity and Yield Measurement...... 36 2.3.3 Separation of Praseodymium from Lanthanum...... 37 2.3.3.1 Solvent Extraction Separation...... 37 2.3.3.2 Ion Exchange Separation...... 38 (A) Determination of The Distribution Coefficients.... 38
iii (B) Cation Exchange Column Chromatography...... 38
CHAPTER 3 RESULTS AND DISCUSSION Part I: Separation of Cerium from Lanthanum I.1 Solvent Extraction Separation...... 40 139 I.1.1 Separation of no Carrier added Ce from Irradiated La 2O3...... 40 A) Equilibrium Time...... 41 B) Extraction Behavior of no Carrier added 139 Ce Using DEE..... 42 C) Extraction Behavior of no Carrier added 139 Ce Using TBP.... 44 D) Extraction Behavior of no Carrier added 139 Ce Using TPPO... 45 E) Comparison of Investigated Methods for the Separation of Radiocerium Using Solvent Extraction Technique...... 47 F) Optimum Conditions for Separation of 139 Ce...... 48 G) Production Yields...... 51 I.2 Ion Exchange Separation...... 52 I.2.1 Adsorption Behaviors of Ce and La on Cation Exchanger...... 52 I.2.2 Cation Exchange Column Chromatography...... 55 A) 0.1M Citrate Buffer...... 55 B) 0.1 M ED...... 57 C) 0.2 M α HIBA...... 58 I.2.3 Optimum Conditions for Separation of no Carrier added 139 Ce
from Irradiated La 2O3 Using Cation Exchanger Technique...... 59 A) Primary Chromatography Column for Separation of 139 Ce
from Irradiated La 2O3...... 59 B) Purification of the Resulting Radionuclide...... 59 I.2.4 Production Yields...... 60 I.3 Comparison of the Investigated Separation Methods of Radiocerium...... 61
iv
Part II Separation of Praseodymium from Lanthanum via Solvent-Extraction and Ion-Exchange Techniques II.1 Solvent Extraction Separation...... 63 II.2 Ion Exchange Separation...... 66 II.2.1 Adsorption Behaviors of Pr and La on Cation Exchanger...... 66 II.2.2 Cation Exchange Column Chromatography...... 69 CONCLUSION ...... 71
SUMMARY ...... 72
REFERENCES ...... 75
v LIST OF FIGURES
Figure Description Page 2.1 The Egyptian cyclotron facility at Inshas...... 26 2.2 Schematic layout of the cyclotron central line...... 27 2.3 Inductive coupled plasma (ICP OES)...... 28 2.4 Pressing device used for sample preparation 34 nat 3.1 Gamma ray spectrum of La 2O3 target irradiated with 14.5 MeV protons...... 41 3.2 % Uptake of radiocerium in the first extraction as a function of contact time with the three extractants...... 42 139 3.3 Distribution coefficients of Ce and La versus HNO 3 concentration over the range of 2 11 M, using DEE, shaking time 3 minutes...... 43 139 3.4 Distribution coefficients of Ce and La versus HNO 3 concentration over the range of 2 11 M, using TBP, shaking time 3 minutes...... 45 139 3.5 Distribution coefficients of Ce and La versus HNO 3 concentration over the range of 2 11 M, using 3 % TPPO in chloroform, shaking time 3 minutes...... 46 139 3.6 Flow sheet of separation of Ce from La 2O3 target...... 50
3.7 Distribution coefficients of La and Ce as a function of HNO 3 concentration on Dowex 50W X8 resin, shaking time 2 hours...... 53 3.8 Distribution coefficients of La and Ce as a function of pH of 0.1 M acetate buffers on Dowex 50W X8 resin, shaking time 2 hours...... 54 3.9 Distribution coefficients of La and Ce as a function of pH of 0.1 M citrate buffers on Dowex 50W X8 resin, shaking time 2 hours...... 55 3.10 Elution profile of La and Ce using 0.1 M citrate buffer of pH 5.5 as an eluant from a column packed with Dowex 50W X8. Fraction volume = 25 ml...... 56 3.11 Elution profile of La and Ce using 0.1 M EDTA as an eluant from a column packed with Dowex 50W X8. Fraction volume = 25 ml...... 57
vi 3.12 Elution profile of Ce by 0.2 M α HIBA and of La by 6 M HNO 3 in sequence from a column packed with Dowex 50 W X8 (H form). Fraction volume = 15 ml...... 58 3.13 Elution profile of 139 Ce using 6 M HCl as an eluant from a column packed with Dowex 50W X8. Fraction volume = 15 ml...... 61
3.14 Distribution coefficients of Pr and La versus HNO 3 concentration over the range of 2 11 M, using DEE, shaking time 3 minutes...... 63
3.15 Distribution coefficients of Pr and La versus HNO 3 concentration over the range of 2 11 M, using 3 % TPPO in chloroform, shaking time 3 minutes...... 64
3.16 Distribution coefficients of Pr and La versus HNO 3 concentration over the range of 2 11 M, using 30 %TBP in chloroform, shaking time 3 minutes...... 65
3.17 Distribution coefficients of La and Pr versus HNO 3 concentration over the range of 1 12 M, using Dowex 50W X8, shaking time 2 hours...... 66 3.18 Distribution coefficients of La and Pr versus acetate buffer over the range of (3 – 5.5 PH) with Dowex 50W X8, shaking time 2 hours 67 3.19 Distribution coefficients of La and Pr versus citrate buffer over the range of (3 – 5.5 PH) with Dowex 50W X8, shaking time 2 hours...... 68
3.20 Elution profile of Pr by 0.2 M α HIBA and of La by 6 M HNO 3 in sequence from a column packed with Dowex 50 W X8 (H form). Fraction volume = 15 ml...... 70
vii LIST OF TABLES
Table Description Page 1.1 Classification of cyclotrons used for radionuclide production...... 2 1.2 Classification of radionuclides for in vivo studies...... 3 1.3 Properties of lanthanum...... 9
1.4 Properties of rare earth oxides Ln mOn...... 10 1.5 Properties of cerium...... 13 1.6 Properties of praseodymium...... 15
2.1 Characteristics of radionuclide formed in irradiation of La 2O3 target...... 35 3.1 Comparison of separation methods for no carrier added cerium
from La 2O3 target irradiated with protons...... 48 3.2 Effect of washing time for organic phase on the efficiency of 139 separation of no carrier added Ce from La 2O3 target irradiated with protons...... 48 3.3 Thick target yield of 139 Ce...... 49 3.4 A summary of the results on the four investigated methods for the separation of no carrier added radiocerium from proton irradiated
La 2O3 target...... 62
viii LIST OF ABBREVIATIONS
Single Photon Emission Computed Tomography SPECT Positron Emission Tomography PET Lanthanide Element Ln Inductively Coupled Plasma Optical Emission Spectrometry ICP OES Tri Phenyl Phosphine Oxide TPPO Tri Butyl PhosPhate TBP Di Ethyl Ether DEE Ethylene Di Amine Tetra Acetic Acid EDTA Alpha Hydroxy Iso Butyric Acid α HIBA
Energy of proton beam EP End Of Bombardment EOB
ix ABSTRACT In this work, trials for the production of the medically and technologically interesting 139 Ce and 142 Pr radionuclides through cyclotron irradiations using protons and alpha particles were studied. The radiochemical separation of no carrier added cerium from proton irradiated lanthanum was studied by solvent extraction using DEE, TBP and TPPO, the latter reagent being employed for the first time for separation of radiocerium from bulk of lanthanum. Distribution coefficients of cerium and lanthanum were investigated as a function of equilibrium time and HNO 3 concentration. A mixture of 0.05 M K 2Cr 2O7 and 0.1 M H 2SO 4 was used as an oxidizing agent to improve the separation efficiency of cerium. A comparative study of the three extractants released that DEE is the best for separation of cerium from bulk of lanthanum oxide. The target was prepared by pressing. The production of 139 Ce of high radionuclidic and chemical purity via irradiation of lanthanum oxide target at MGC 20 cyclotron with protons of energy 14.5 is described. The experimental yield was found to be 153 kBq/ A h . The adsorption behaviour of La/Ce system on Dowex 50W X8 in different media, namely, nitric acid, acetate buffer and citrate buffer was studied as a function of the concentration of nitric acid and buffer pH. In addition, in cation exchange column chromatography experiments, three different eluants, namely, citrate buffer of pH 5.5, 0.1 M EDTA and 0.2 M α HIBA, were employed for separation of Ce (III) from La (III). The optimum conditions for improvement of radiochemical separation of no carrier added 139 Ce from proton irradiated lanthanum were applied using the most suitable chelating agent 0.2 M α HIBA. The purification of
x 139 Ce from macro amount of La (III) was done using two columns in a sequence. The experimental yield was found to be 200 kBq/ A h. The adsorption behaviour of La/Pr system on Dowex 50W X8 in different media, namely, nitric acid, acetate buffer and citrate buffer was studied as a function of the concentration of nitric acid and buffer pH. In addition, in cation exchange column chromatography experiment, 0.2 M α HIBA was employed for separation of Pr (III) from La (III). Also the extraction behaviour of La/Pr system in nitric acid of different concentrations was studied by solvent extraction using DEE, TBP and TPPO.
xi
CHAPTER 1
IIINTRODUCTIONINTRODUCTION
I- INTRODUCTION CHAPTER 1
1.1. Production of Radionuclides Using Cyclotrons The cyclotron is a device to accelerate charged particles. Today a large number of cyclotrons are used worldwide for medical radioisotope production. The common terminology for these cyclotrons is "medical cyclotrons" or "compact cyclotrons". Many of them have been installed in hospital environments and are employed extensively for preparation of short lived radionuclides with very high specific activities for direct use on site. Irradiation of the target material for routine radionuclide production can be performed using the internal or external beam of the cyclotron. The internal beam current of the cyclotron is relatively high and it decreases to less than 60% by extraction process. While using internal irradiation, the target is located at a certain radius of the particle trajectory.
1.1.1 Types of Cyclotrons Cyclotrons can be classified according to their maximum energy of acceleration, the type of accelerated particles, and consequently, the type of radionuclides that can be produced. Some terminological classification was given to the commercial cyclotrons as medical cyclotrons, small cyclotrons, baby cyclotrons, low energy cyclotrons, etc. Other classification referred to the type of the accelerated charge as negative ion or positive ion machines. Cyclotron is widely used in the production of radioisotopes, especially for medical purposes. There are two types of radioisotopes commonly produced at cyclotron, positron and photon emitters. Table 1.1 gives the categories of cyclotrons used for radionuclide production and their maximum energy of acceleration (1 3).
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I- INTRODUCTION CHAPTER 1
Table 1.1 Classification of cyclotrons used for radionuclide production (1). Major Energy Classification Characteristics radionuclides [MeV] produced
Level I Single particle < 4 15 O (d)
Level II Single particle ≤ 11 11 C, 13 N, 15 O, 18 F (p)
Single or two 11 C, 13 N, 15 O, 18 F Level III ≤ 20 particle 123 I, 67 Ga, 111 In (p, d )
38 73 75,77 K, Se, Br, 123 I, 81 Rb, 81 Kr, Level IV ≤ 40 Single or multiple 67 Ga, 111 In, 201 Tl, particle 22 Na, 57 Co (p, d ,3He, 4He)
28 Mg, 72 Se (72 As), Level V Single or multiple ≤ 100 82 Sr, 82 Rb,117 mSn, particle 123 I (p, d ,3He, 4He) 26 32 44 Al, Si, Ti, 67 Cu, 68 Ge ,68 Ga, Level VI Single particle ≥ 200 82 Sr ,82 Rb, 109 Cd, (p) 95 mTc, etc.
1.1.2 Medical Radioisotopes The choice of a suitable radionuclide for certain medical application depends on several nuclear and chemical aspects. The nuclear decay data of radionuclides help to decide whether they can be used for therapeutic or diagnostic purposes. The medical requirements impose many restrictions on half life, energy of the emitted gamma ray, energy of the emitted positrons, etc. On the other hand, the chemical characteristics of the radioisotope are very important to be used in their
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I- INTRODUCTION CHAPTER 1
proper function. The major medical applications are in vivo imaging using Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT). Both techniques are used for imaging the tumors and following up metabolisms in human organs. The PET camera is based on the detection of the two gamma rays resulting from positron annihilation of the injected radionuclides in the target organ. The SPECT camera detects gamma photons emitted from the injected radionuclide of a suitable gamma ray energy. For a description of the physical and technical basis of PET and SPECT see ref. [4]. From the chemical point of view, the radionuclides used for in vivo applications are classified(5) into organic, halogens, alkali and alkali like metals and inorganic. Table 1.2 gives a list of the classified in vivo radionuclides according to their chemical behavior. In this work we are paying attention to the inorganic radioisotopes of cerium and praseodymium. Further discussion about their nuclear and chemical properties is given in this chapter. Table 1.2 Classification of radionuclides for in vivo studies(5)
Group Radionuclides
Organic short lived β + emitters 11 C, 13 N, 15 O, 18 F, 30 P, etc.
Halogens and rare gases 34mCl, 75,77 Br, 123 I, 77,79,81mKr, 125 Xe, etc.
Generator isotopes 68 Ge 68 Ga, 81 Rb 81mKr, 99 Mo 99mTc, etc.
Alkali and alkali like metals 38,43 K, 81 Rb, 128,129 Cs, 201 Tl, etc.
Inorganic radionuclides 28 Mg, 45 Ti, 48 Cr, 73 Se, 97 Ru, etc.
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I- INTRODUCTION CHAPTER 1
1.1.3 Optimum Conditions for Production Before starting the production of a radionuclide several experimental tests have to be performed to find out under which conditions high product yield and low impurities level can be achieved. The test includes irradiation and separation experiments. The good irradiation conditions avoid damage to the target material and its holder (6 8). The optimum conditions for irradiation include the beam energy, the required target thickness, time of irradiation and the suitable beam current. The optimum beam energy is chosen such that the maximum reaction cross section lies within this energy and that there are no interfering reactions. The irradiation time is chosen relative to the half life of the produced radionuclide. In most cases the irradiation time amounts to one or two half lives of the produced radionuclide. Finally, the estimation of the optimum irradiation current needs several experimental investigations on the used target material. It is known that there is a limit (9) to the increase of the product activity by increasing the beam current. At high beam currents there is a loss of the produced radionuclide due to the thermal effect of the beam. Therefore, the current activity relationship should be estimated experimentally over a wide range of the beam current values. From this relation one can choose the optimum beam current to produce the maximum activity.
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I- INTRODUCTION CHAPTER 1
1.2 Production of Cerium and Praseodymium Radioisotopes 1.2.1 Importance of Ce Radioisotopes Radioactive isotopes of cerium are very frequently used in technology. There are more than thirty isotopes for cerium with mass numbers beginning from 119 to 155. The relatively long lived 139 (10) radionuclide Ce (T 1/2 = 137.6 d ) is useful as a standard for the calibration of γ ray detectors. This radioisotope can be produced through 139 La(p,n) 139 Ce reaction (Q value 1.06037 MeV) and has only one strong γ ray line of energy 165.857 keV (10) with 80 % intensity, which is within the optimum energy range for detection with a gamma camera. Image degradation during single photon emission tomography (SPECT) due to attenuation and Compton scattering of photons can cause clinical image artifacts. Duraan et al.(11) has shown that a 139 Ce line source can be used to determine attenuation maps for SPECT. The interest in this radionuclide is therefore increasing. 141 The neutron rich radionuclide, Ce, (T1/2 = 32.5 d) decays through β emission to stable 141 Pr (12). The shorter half life isotope, 143 Ce 143 (T 1/2 = 1.4 d) decays also by β to the rather long lived Pr (T 1/2 = 13.6 d) which can be used in radiotherapy (12).
1.2.2 Importance of Pr Radioisotopes Several radionuclides of praseodymium may be useful for nuclear medicine. There are more than thirty isotopes for praseodymium their mass numbers beginning from 121 to 155. As an example, 139 Pr has a half life of 4.5 hours and decays by EC and β+ emission which makes it feasible for diagnostic applications with positron emission tomography (13) 142 (PET) .The neutron rich isotope Pr(T 1/2 = 19.13 h) has 59 protons
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and 83 neutrons, therefore, lies in the region of the table of isotopes that should be well described by the shell model and has good physical characteristics for internal radiotherapy. The wide use of 142 Pr almost relates to the β emission of maximum energy 2.16 MeV and an average energy of 0.83 MeV, as well as rather adequate intensity (93 %). These particles penetrate approximately 3 mm of soft tissue (14). 142 Pr has another advantage, It emits one gamma photon at 1.58 MeV with an intensity of 3.7% which does not interfere with internal medical applications due to its relatively low intensity and low specific γ dose (15,16) 143 constant . The longer lived Pr (T 1/2 = 13.15 d) also decays by the emission of β particle (100%)(15,17), which are highly attractive features for possible application in radionuclide therapy. It can be produced using accelerators via 146 Nd(p,α)143 Pr or 142 Ce(d,n) 143Pr (13) reactions.
1.2.3 Production of Cerium 139 139 Ce can be produced in cyclotron by a number of possible nuclear reactions from natural lanthanum target. 139 Ce can be produced through the 139 La(d,2n)139 Ce reaction which can be carried out using a cyclotron deuteron beam, and also can be produced by irradiation of lanthanum target with proton through the nuclear reaction 139 La(p,n) 139 Ce. Radionuclide 139 Ce was produced previously by irradiation of high purity La 2O3 (Johnson Matthey) target with 13 MeV deutrons. The target thickness was 160 mg/cm². The 139 La(d,2n)139 Ce reaction took place for lanthanum of natural abundance ( 139 La 99.9% , 138 La 0.09%). Beside the main product, Some quantity of 140 La formed via the reaction 139 La(n,γ) 140 La. Under the irradiation conditions the yield of the reaction leading to 139 Ce was 159 kBq/ Ah. The product 139 Ce was
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separated by extraction with diethyl ether (DEE) in the system DEE (18) 139 (19) HNO 3 . Production of Ce was also carried out by Neirinckx et al in which, targets of natural lanthanum foil (99.911% 139 La) were bombarded in the internal deuteron beam of the cyclotron at the CSIR (Pretoria). The targets were cooled to allow for the decay of l40 La. The extraction was done using di (2 ethyl hexyl) phosphate in n heptane, from a 10 M HNO 3 + KBrO 3 solution, which allows the production of a much purer 139 Ce cyclotron product. Also 139 Ce was produced by a proton – induced nuclear reaction on praseodymium (20) . A target of praseodymium disc was encapsulated in aluminum canister. The target was intermittently bombarded with a 66 MeV proton beam. 139 Ce was separated from praseodymium by anion exchange chromatography on a AG MP – 1 resin in a nitric acid – bromic acid mixture. Mayer et al (21) , separated 139 Ce from lanthanum targets by anion exchange chromatography in bromic acid – nitric acid.
1.2.4 Production of Praseodymium 142 The beta emitter 142 Pr can be produced by a number of nuclear reactions either in cyclotrons or reactors. it can be produced in reactor via the following nuclear reaction: 141 Pr (n,γ) 142 Pr which leads to two 142m 142 (13) states of Pr, Pr (T 1/2 =14.1 m), and Pr(T 1/2 = 19.13 h) . In cyclotron 142 Pr can be produced via the following nuclear reactions: 142 Ce(p,n) 142 Pr, 139 La(α,n) 142 Pr and 142 Ce (d,2n)142 Pr.
The excitation functions for the reaction 139 La(α,n) 142 Pr have been measured previously (22). The (α,n) excitation functions rise to a
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maximum of about 130 mb at 15 MeV and falls to about 10 mb at 38 MeV exitation energy.
1.3 General Properties of Elements Concerned in This Study 1.3.1 Chemistry of Lanthanum Lanthanum is the first member of the lanthanide series and is recovered from the minerals bastnasite and monazite. It is the second most abundant lanthanide after cerium and was first isolated in 1839 by C. G. Mosander from cerium. Its name is derived from the Greek word “lanthanein” which means “to lie hidden”. In broad terms the chemical properties of the lanthanides fall between those of the alkaline earths and that of iron/aluminum. La(III) is the only easily accessible valence state for lanthanum and the chemistry of La(III) species is the prototype for the behavior of all the trivalent lanthanides (Ln 3+)(23). Lanthanum is the least acidic of the lanthanides. Table 1.3 gives some of lanthanum properties.
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Table 1.3 Properties of lanthanum
The Element Lanthanum
Chemical Symbol La Atomic Number 57 Atomic Weight 138.91 Electronic Configuration (Xe)4f05d16s2 Valency 3 Ionic Radius, 8 coordination 117.2 pm Magnetic Moment 0 B
Lanthanum Metal
Crystal Structure dhcp Melting Point 918 oC Boiling Point 3464 oC Density 6.15 g/cm 3 Metallic Radius 188 pm Thermal Conductivity 0.134 w/cm k Specific Heat 0.165 j/g K Heat of fusion 6.20 k j/mole Thermal Expansion 4.5 K 1 Electrical Conductance 1
1.3.2 Oxides of the Lanthanides
The Ln 2O3 sesqui oxides are the stable well defined solids usually obtained as the final product of the calcination in air of most Ln metals and Ln salts such as oxalates, carbonates and nitrates. This is a consequence of the high thermodynamic affinity of the lanthanide elements for oxygen and the stability of the Ln(III) valence state. The lanthanide oxides have the greatest, most negative, standard free energies of formation for any oxides, accounting for their exceptional thermodynamic stability (24). The basic physical and chemical data for the rare earth oxides under investigations are summarized in Table 1.4.
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Table 1.4 Properties of rare earth oxides Ln mOn under investigations
Ln stable formula element color structure density m.pt oxide wt % type g.cm 3 0C
La La 2O3 325.8 85.3 white A 6.6 2305
Ce CeO 2 172.1 81.4 off white fluorite 7.3 2600
Pr Pr 6O11 1021.5 82.8 black fluorite 6.9 2200
The Ln(III) ion’s size, ranging for 8 coordination from 116 pm for La to 98 pm for Lu, is not so different from that of the O 2− anion. This, along with the imposed Ln 2O3 formula, means that the crystal structures of the Ln 2O3’s are not easy to visualize, unlike oxides of smaller elements where the cations fit into holes within arrays of close packed 2− O ions. The A and B type Ln 2O3 structures can be described as based n+ on layers of polymeric [LnO] n cations separated by discrete layers of 2− n+ O anions. (Similar [LnO] n layers are seen in many Ln oxo compounds). The arrangement around the Ln ion is 7 coordinate with several differing Ln O distances (25).
The C type Ln 2O3 structure is related to the fluorite, CaF 2, system by the ordered removal of one quarter of the anions. (The Ln atom is coordinated to 6 oxygens but there are two vacant sites at cube corners). The oxides of the lanthanides are among the most thermally stable materials known, melting in the range 2200 2500 ◦C. In general, the metal cation structure is rigid up to the melting point whereas there is a high mobility in the oxide anion lattice, starting above ~ 300 ◦C. The oxides dissolve in aqueous acids to produce the corresponding salt, e.g. nitrates and chlorides from HNO 3 and HCl respectively. The reactivity with acid is noticeably slower for C type
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structures, i.e. the heavy Ln oxides, than for the A and B types, the light Ln oxides. To avoid hydrolysis of the hydrated lanthanide ion, the solutions should be kept acidic (pH < 4). Furthermore, oxides with Ln(IV) ions are even less reactive and a trace of reducing agent, e.g. H 2O2, may be required to take the oxide into solution. All the oxides will absorb water and/or carbon dioxide onto their surfaces, forming a layer of hydrate, carbonate or hydroxy carbonate (26). The A and B types do this more readily with lanthanum oxide being the most hygroscopic of the series.
1.3.3 General Properties of Cerium Cerium is the most abundant member of the lanthanides. Chemically, cerium can be characterized as having two valence states, viz. Ce 4+ or ceric and Ce 3+ or cerous. This property is an important parameter that allows the easy separation of cerium from the other lanthanides. The ceric ion is a very powerful oxidizing agent but when associated with oxygen, which is a strongly coordinating hard ligand, it is completely stabilized as CeO 2. This form of cerium is the most widely used in industry. Cerium was named after the asteroid, Ceres, which in turn was named after the patron saint of Sicily and the Roman goddess of food and plants. It was first isolated from the impure oxide in 1803. The recognition that cerium was a unique element, and its relationship to other elements, were factors in the gradual development of the Periodic Table concept. The separation and identification of all the individual 4f elements, cerium included, caused considerable confusion a hundred years ago. This process did however lead to our eventual understanding
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of atomic structure. Cerium was introduced to the public for the first time in 1891 when gas lights were successfully installed in front of the Opern Cafe in Vienna. These lights used the Welsbach gas mantle that had fabric impregnated with thorium and cerium oxide. This form of illumination soon became widespread throughout Europe. Natural cerium is still found in lighting today, as well as in televisions, automobiles and now the radioactive isotopes of this element may also start to play a role in our daily lives. In the bulk form the element is a reactive metal, prepared by the calciothermic reduction of the fluoride, CeF 3: 2CeF3 + 3Ca → 2Ce + 3CaF2. A slight excess of calcium is used and the exothermic reaction, carried out in a tantalum crucible, is initiated at 900 ◦C. After physical separation of the upper layer of immiscible fluoride slag, vacuum distillation removes unreacted volatile Ca. Cerium can also be made by the electrolytic reduction of fused chloride. On a fresh surface the metal has a steely lustre but rapidly tarnishes in air due to the surface formation of oxide and carbonate species. For protection against oxidation the metal is usually stored in a light mineral oil. When made finely divided, e.g. on being cut, it can be strongly pyrophoric and for this reason it is used as the ferro alloy mischmetall in lighter flints and ordnance. The metal reacts steadily with water, readily dissolves in mineral acids and is also attacked by alkalies. It reacts with most non metals upon heating (27).Table 1.5 shows a synopsis of the properties of cerium.
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Table 1.5 Properties of cerium The Element Cerium
Chemical Symbol Ce Atomic Number 58 Atomic Weight 140.12 Electron Configuration [Xe]4f26s2 Valencies 3,4 Ionic Radius for (3+) 114 pm 8 coordination (4+) 97 pm Magnetic Moment 2.4 B
Cerium Metal
Crystal Structure fcc Melting Point 798 ◦C Boiling Point 3443 ◦C Density 6.77 g/cm 3 Metallic Radius 182 pm Thermal Conductivity 0.110 w/cm−K Specific Heat, 0.192 J/g K Heat of Fusion 1305 cal/mole Thermal Expansion 6.3x10 −6 K −1 Electrical Conductance 0.13 −1
1.3.4 Cerium Dioxide CeO 2
The most stable oxide of cerium is cerium dioxide, CeO 2, also called ceria or ceric oxide. (The sesquioxide, Ce 2O3, with trivalent Ce, can be prepared under strongly reducing conditions but is unstable in air and water and readily converts to the dioxide. Consequently, it is not suitable for target material.) Cerium (along with the other lanthanides) has one of the (27) highest free energies of formation for an oxide . The oxide is soluble in mineral acids but can prove difficult to dissolve unless a trace of reducing agent such as hydrogen peroxide is added. Ceria has the fluorite (CaF 2) structure, Fm3m space group, with 8 coordinate cations and 4 coordinate anions. It can be visualized as a cubic
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close packed array of metal atoms with oxygens filling all the tetrahedral holes. The structure determining OCe 4 coordination tetrahedra thereby share all edges in three dimensions.
When pure, CeO 2 is a very pale yellow probably due to Ce(IV) O(II) charge transfer transitions. The color of the oxide is sensitive not only to stoichiometry but also to the presence of other lanthanides. A slight trace (~ 0.02%) of Pr results in a buff color attributable to Ce(IV) Pr(III) transitions. (With higher values of Pr (~ 2%) the material becomes a potential red pigment (28)). Grossly non stoichiometric ceria samples are reported to be blue, related to Ce(IV) Ce(III) transitions. In addition, as the oxide is usually produced by the calcination of a precursor salt, the observed color depends on the extent of that calcination. Ceria can be sintered (at ~1400 ◦C) to high densities by the addition of trace amounts (< 1.0 %) of oxides such as TiO 2 or Nb 2O5. These additives work either by the formation of a transient liquid phase ◦ intermediate, e.g. CeTi 2O6 (m.p. 1350 C), or by suppressing oxygen deficiencies, e.g. with high valent Nb. This phenomenon might be useful for targets that behave better under bombardment conditions, provided the impurities formed from the additives can be successfully removed from the final product.
1.3.5 General Properties of Praseodymium In most compounds this element is trivalent like lanthanum, and in chemical behavior Pr(III) compounds closely resemble the analogous La(III) derivatives. Table 1.6 lists some properties of praseodymium.
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I- INTRODUCTION CHAPTER 1
Table 1.6 Properties of Praseodymium The Element Praseodymium
Chemical Symbol Pr Atomic Number 59 Atomic Weight 140.91 Electron Configuration [Xe]4f36s2 Electronegativity 1.13 Valency 3 (4) Ionic Radius, 8 coordination 113 pm Magnetic Moment 3.60 B Praseodymium Metal
Crystal Structure dhcp Melting Point 931 ◦C Boiling Point 3520 ◦C Density 6.77 g/cm 3 Metallic Radius 182.8 pm Thermal Conductivity 0.125 W/cm −K Specific Heat, 0.193 J/g K Heat of Fusion 6.89 kJ/mole Thermal Expansion 6.7 K −1 Electrical Conductance 14.7 m cm −1
Most Pr 3+ salts are pale green due to strong absorption bands in the blue from 440 to 490 nm (29). (Similar color and bands are seen in a glass matrix when Pr 3+ is present). Most praseodymium salts, when calcined in air, do not produce the sesquioxide, Ln 2O3, but a black material whose composition is best expressed as Pr 6O11 . The tetra valent state of Pr is of just sufficient stability to form this oxide preferentially with mixed Pr valencies. Charge transfer behavior leads to the enhanced stability. (The Pr O phase diagram is complex and several oxides that form a homologous series, Pr nO2n−2 , are known, each with a defect fluorite structure). The Pr(IV) ion is only stable in a few solid compounds, all oxide and fluoride based. A pale green Pr 2O3 oxide can be
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made under strongly reducing conditions but it is not stable in air (30) . Praseodymium forms ~ 4% of the lanthanide content of bastnasite but all that proportion is not recovered as a separated pure Pr material because there is currently an insufficient commercial demand. The element will be present in a small amount in almost all mixed light lanthanide derivatives, again because of the difficulty in separation. The most popular yellow ceramic pigment is a Pr doped zircon (31) that is “cleaner” and “brighter” than alternatives probably due to the Pr pigment having an optimum reflectance at ~ 560 nm. In the preparation a “mineralizer”, usually a metal halide MX, must be present to ensure complete reaction.
1.4 Review of Separation Chemistry of Cerium and Praseodymium 1.4.1 Solvent Extraction Solvent extraction involves the distribution of a solute between two immiscible liquid phases (organic phase & aqueous phase). This technique is extremely useful for very rapid and clean separations of both organic and inorganic substances. Solvent extraction plays an important role as a separation and concentration technique. Many investigations have been conducted on the separation of cerium (IV) by solvent extraction. Solvent extraction has come to be used for the initial stage of the separation process, to give material with up to 99.9% purity. The extraction separation of cerium from lanthanum takes advantage of the high distribution constant, between some media, the Ce(IV) stable in a highly oxidizing conditions (19). In 1949, it was found that Ce (IV) could readily be separated from Ln 3+ ions by extraction from a solution of nitric acid into tributyl phosphate
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I- INTRODUCTION CHAPTER 1
[(BuO) 3PO], subsequently the process was extended to separating the lanthanides, using a non polar organic solvent such as kerosene and an extractant such as (BuO) 3PO or bis (2 ethylhexyl)phosphinic acid to extract the lanthanides from aqueous nitrate solutions (32). Cerium oxidized to the +4 state was extracted with diethyl ether as a ceric nitrate complex (33). Peppard et al (34) studied the distribution constant for cerium, between nitric acid (containing potassium bromate) and di (2 ethylhexyl)phosphoric acid (dissolved in n heptane). The distribution of La(III), Ce(III) and Y(III) from potassium thiocyanate solutions by tributyl phosphate was described (35). Solvent extraction of Ce (IV) nitrato complexes has been studied in several papers mainly using TBP as an extractant. Extractability of Ce (IV) increased with HNO 3 concentration 1 in the studied range (0.5 ~ 5 mol .L ). [Ce (NO 3)4] is thought to be the main extracted Ce (IV) species (36 38). At higher nitric acid concentrations (36) H [Ce (NO 3)5] is sometimes considered . The extraction behavior of Ce(IV), Th(IV) and part of RE(III), viz., La, Ce, Nd and Yb, has been investigated using di(2 ehylhexyl)2 ethylhexyl phosphate (DEHEHP) in heptane as an extractant (39). Other extraction separations for Ce(IV) were described, in references(40 43). Extraction studies of neodymium and praseodymium with mixtures of tributyl phosphate and Aliquat 336 in xylene have been carried out. From 3.0 M aqueous ammonium nitrate solutions, negatively charged complexes of neodymium and praseodymium were extracted with Aliquat 336 in the presence of tributyl phosphate into the organic phase (44). The extraction behavior has been reported for higher concentration of TBP in kerosene from different nitrate salt solutions and nitric acid (45,46). The extraction of Y(III), La(III), Pr(III) and Nd(III)
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I- INTRODUCTION CHAPTER 1
with 8 hydroxyquinoline in chloroform from aqueous and water methanol solutions has been studied (47). The Extraction of Pr(III), Ho(III) and Er(III) in borate media of pH range 1 10 with N benzoyl N phenylhydroxylamine (BPHA) in benzene has been studied. Quantitatively the best range for separation was found to be in the pH range 7 10 (48).
1.4.2 Ion Exchange "Chromatography" is the general term for several physico chemical separation techniques, all of which have incommon the distribution of a component between a mobile phase and a stationary phase. "Chromatography" was first tried by Tswett (49), who in 1903 used a column of precipitated calcium carbonate to separate the pigments of green leaves. He extracted the dried leaves with petroleum ether and poured the extract onto the top of used column. The pigments were gradually washed down the column, and separated into different coloured zones. Since then, the name "chromatography" has been extended to the separation of several dissolved constituents whether coloured or not, by partition between an absorbent and a flowing solvent, followed by suitable detection and identification of dissolved substances as they emerge from used column.
In 1935, Adams and Holmes (50), at the National Chemical Laboratories, England, made the first organic ion exchange resin as condensation products of formaldehyde with phenol or with phenolsulfonic acid giving stable products in aqueous solutions. Other ion exchangers were also made by treating bituminous coal with fuming sulfuric acid, that were used for partial separation of yttrium, lanthanum and neodymium by Russell and Pearce (51), and of copper, cadmium, and nickel by Kozak and Walton (52).
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I- INTRODUCTION CHAPTER 1
In 1939, Samuelson (53), first investigated systematically the use of ion exchange phenomena in chemical analysis. He reported on the separation of interfering cations and anions, by absorbing iron cations on a resinous cation exchanger and replacing it in solution by hydrogen ions which are not interfering in subsequent analysis. He also showed that one could measure the total electrolyte concentration of a solution by passing it through a cation exchange resin in its hydrogen form, thus converting dissolved salts to their corresponding acids, and then titrating these acids. In 1941, Martin and Synge (54), proposed the concept of theoretical plates, which was adapted from the assumption that the chromatographic columns are similar in operation to distillation and extraction fractionating columns. They considered the column as consisting of a number of theoretical plates, within each of which equilibrium between two phases generally occurs. This theory has been further expanded by Mayer and Tompkins in 1947, so as to render possible the prediction of the number of "theoretical plates" needed to obtain a required purity of separated products (55). Ion chromatography (IC) was introduced in 1975 by Small et al.(56), as a new analytical method. Within a short period of time, ion chromatography developed from a new detection scheme for a few selected inorganic anions and cations to a versatile analytical technique for different ionic species. For sensitive detection of ions via their electrical conductance, the effluent from the separating column was passed through a " suppressor " column to reduce the background conductance of used eluent, while at the same time to increase the electrical conductance of the analyte ions. In 1979, Fritze et al. (57), described an alternative separation and detection scheme for inorganic anions, in which ion exchange resins with low capacities are used and directly coupled to a conductivity cell to deal with eluents with low
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ionic strengths. In addition, the eluent ions should exhibit low equivalent conductances, thus enabling sensitive detection of different sample components. The ion exchange method for separation of rare earths consists of in brief, absorbing the mixed rare earths on the top of a cation exchange resin column with a copper cycle and then eluting the rare earths selectivity from the resin column with a suitable aminopolycarboxylate solution. In this process the chelating properties of some of the aminopolycarboxylates mainly ethylenediaminetetraacetate are made use of for selective separation of rare earths (58). The ion –exchange process is highly successful for their separation with high yields of pure materials and hence is now universally adopted for this purpose. Ion exchange chromatography is not of real commercial importance for large scale production, but historically it was the method by which fast high purity separation of the lanthanides carried out. Subbaraman et al (59) used sodium triphosphate for the first time for the ion exchange separation of rare earths, they found that, a complexing elution on a cation exchanger with sodium triphosphate has given a more satisfactory separation of the light rare earths than the elution on an anion exchanger. Martin et al (60) studied the separation of rare earths activities from milligram amounts of salts of adjacent lower Z rare earths by Dowex 50W X12 (200 400 mesh) cation exchange resin at elevated temperatures using α hydroxyisobutyrate as eluent. A good separation of the light rare earths from each other has been achieved using milligram amounts of each available rare earth. The separations were accomplished using the technique of concentration gradient elution with Dowex 50W X8 ion exchange columns, α
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hydroxyisobutyrate solution as eluent and using fission products as radioactive tracers (61) Muraji et al (62) studied the separation of the cerium group rare earth elements by Dowex 50W X8 (100 200 mesh) cation exchange resin and using a mixed solution from an aqueous solution of diammonium hydrogencitrate and an aqueous ammonia acetic acid mixture; The two solutions were mixed in definite ratios, the separation of Sm, Nd, Pr, Ce and La was achieved. It was found that if Ln 3+ ions were adsorbed at the top of a cation exchange resin, and then treated with a complexing agent such as buffered citric acid, then the cations tended to be eluted in reverse atomic number order (32). Maoliang et al (63) described an ion exchange separation method for light rare earths which separated from fission products using α hydroxyisobutyrate with a pH value of 4.8 and concentration from 0.15 M to 0.4 M as eluent. The yield of the individual rare earths is about 95%. Kuroda et al (64) studied the separation and determination of La, Ce, Pr, Nd and Sm by dynamic ion exchange chromatography on a column of bonded phase silica by gradient elution with 0.05 to 0.5 M lactic acid (pH 3.5) in the presence of 0.01 M sodium 1 octanesulfonate. Agrawal et al (65) synthesised a six new poly(styrene p hydroxamic acids) and used these polymers as chelating ion exchange resins for the separation and determination of La, Ce, Nd and Y in the synthetic, standard and environmental samples. Anion exchange chromatography with nitric acid methyl mixed media at elevated temperature has been applied to mutual separation of
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the light lanthanides, La, Ce, Pr and Pm (66). Anion exchangers can be utilized to adsorb anionic nitrato complexes of Ce(IV). However, the number of papers published on Ce(IV) separation by anion exchange has so far been very limited. Ce(IV) nitrato complexes were adsorbed on two anion exchangers on polyvinyl pyridine (PVP) and quaternized PVP incorporated into porous silica matrix (67).
1.5 Aim of The Work Radionuclides are used both in diagnosis and therapy. In vivo diagnostic studies are performed using pure gamma emitters or positron emitters. Whereas the former are produced both at nuclear reactors and cyclotrons, the latter, being neutron deficient, can be produced only at cyclotrons via charged particle induced reactions. For internal radiotherapy, the radionuclides used consist of β , α or Auger electron emitters. They are generally produced in a nuclear reactor but occasionally cyclotrons are also used. In radionuclide production, a chemical separation of the desired product from the matrix activity is of crucial importance. A large number of separation techniques find application, some of the important ones being solvent extraction, ion exchange chromatography, distillation, thermochromatography, etc. Out of these, solvent extraction and ion exchange chromatography are commonly used. In this work it is intended to make extensive use of these two techniques in the separation of several interesting radionuclides from a cyclotron irradiated targets. A separation process for the desired radionuclide from a given matrix activity may be developed empirically. However, a more meaningful study involves the investigation of the various parameters
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influencing the separation, e.g. the effects of time, concentration of chelating or eluting agent, pH of the medium, etc. One of the goals of this work is to establish in batch experiments the basic parameters for the separation of few selected radionuclides. The main aim of the present work is to develop efficient separation methods for those radionuclides from cyclotron irradiated targets. In particular two systems will be investigated in detail: • Separation of radiocerium from irradiated lanthanum target • Separation of praseodymium from lanthanum as a simulation mode for the production of 142 Pr from α particles
activation of La 2O3 target. The major criteria for a good separation are the high yield and the high achievable purity. These will be checked via high resolution gamma ray spectroscopy. Furthermore, the chemical purity will be checked by ICP. It is expected that the developed methods of separation will provide efficient alternative routes for the production of the radionuclides under consideration.
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CHAPTER 2
EEEXPERIMENTALEXPERIMENTAL II EXPERIMENTAL CHAPTER 2
2.1 Chemicals and Reagents • Lanthanum(III) oxide (99.97 %, Koch Light Laboratories Ltd, England).
• Cerium oxide (CeO2) (99.99% Sigma, Aldrich).
• Praseodymium oxide (Pr 6O11 ) (99.97 %, Koch Light Laboratories Ltd, England). • Dowex 50W X8 (200 400 mesh, hydrogen form, Bio Rad Laboratories, Dow Chemical Co., USA). • α HIBA (HPLC grade, Fluka, Buchs, Switzerland). • Triphenyl phosphine oxide (TPPO) was obtained from Fluka Chemie GmbH, Germany. • Diethyl ether (DEE) obtained from Sisco research laboratories PVT LTD Mumbai, India. • Tributyl phosphate (TBP) from Schuchardt, Hohenbrun, Germany. • Sodium hydroxide. • 0.1 M acetate buffer pH 3 5.5. Acetate buffer solution at different pH was prepared by addition of 0.3 ml of acetic acid to deionised water. The pH of the solution was adjusted to the desired value with sodium hydroxide solution. The solution was completed to 100 ml with deionised water. • 0.1 M citrate buffer pH 3 5.5. Citrate buffer solution at different pH was prepared by dissolving 2.1g in deionised water. The pH of the solution was adjusted to the desired value with sodium hydroxide solution. The solution was completed to 100 ml with deionised water.
24 II EXPERIMENTAL CHAPTER 2
• Ammonium chloride. • Ammonia solution . • Hydrochloric acid. • Nitric acid. • Hydrogen peroxide. All reagents used in this study were of analytical grade.
2.2 Instrumentation The general instruments used throughout the work were : • Analytical balance: Bosch S2000. • Shaker: Karl Kolb, of GFL.100 oC. • Hot plate with magnetic stirrer Model 4803–02. • pH meter: Inolab level 1, Germany. • Fraction collector: Foxy Jr, U.S.A by Isco, Inc. • MGC 20 Cyclotron. The MGC 20 cyclotron is a universal machine both for applied purposes (production of radionuclides, in particular for medicine, activation analysis of a substance, wearing studies) and fundamental researches in atomic and nuclear physics. This cyclotron is a sector focusing isochronous variable energy machine designed for accelerating hydrogen and helium ions over the final energy range from 5 20 Z 2/A MeV. Fig. 2.1 shows a photo of the MGC 20 cyclotron and its external beam lines.
25 II EXPERIMENTAL CHAPTER 2
Fig. 2.1 The Egyptian cyclotron facility at Inshas
26 II EXPERIMENTAL CHAPTER 2
The central beam line consists of a target chamber provided with water cooling circuit to avoid any damage in the target caused by the beam spot. A group of rings are inserted to collimate the beam and collecting it on the target. This unit is connected directly with the cyclotron through the vacuum gate and its vacuum system is connected directly with the cyclotron vacuum system as shown in Fig. 2.2
Fig. 2.2 The central line of cyclotron.
27 II EXPERIMENTAL CHAPTER 2
For radioactivity measurement, a gamma ray spectrometer consisting of HPGe detector connected to a multichannel analyzer and PC was used. The inactive content of the separated material was determined via inductively coupled plasma optical emission spectrometry (ICP OES) Fig.
2.3 using the system ULTIMA 2ICP, jobin Yvon S. A., France .
Fig. 2.3 Inductive coupled plasma (ICP OES).
28 II EXPERIMENTAL CHAPTER 2
2.3 Chemical Separation Processes The chemical separation processes involved two systems, separation of cerium from lanthanum using two techniques, namely, solvent extraction and ion exchange chromatography and apply the optimum conditions from the two techniques to separate no carrier added 139 Ce from irradiated natural lanthanum. The second system was the separation of praseodymium from lanthanum using the same techniques.
2.3.1 Separation of Inactive Cerium from Lanthanum 2.3.1.1 Solvent Extraction Separation The primary experiments have been done on inactive cerium to determine the distribution coefficients. A stock solution containing 10 4 ppm La and 100 ppm Ce was prepared by dissolving their oxides in concentrated
HNO 3. In case of cerium, the CeO 2 was dissolved in a mixture of concentrated HNO 3 and 30% H 2O2. The solution was evaporated to incipient dryness, and the residue was dissolved in 100 ml of deionized water . To 1 ml of the stock solution, a mixture of 1 ml of 0.5M K 2Cr 2O7 and 1 ml of 1M H 2SO 4 as oxidizing agent was added. To obtain the desired molarity in a total volume of 10 ml, nitric acid was added to the mixture. The solution was transferred to a separating funnel and subjected to gentle shaking with an equal volume of each 30% (v/v) TBP diluted in chloroform, 3% (w/v) TPPO dissolved in chloroform and DEE. After disengagement, both aqueous and organic phases were monitored by ICP. The distribution coefficients of lanthanum and cerium between the two phases were calculated using the following equation:
29 II EXPERIMENTAL CHAPTER 2
C org ∨ aq K d = × ( 2 .1 ) C aq ∨ org
where C org and C aq are the concentrations of the elements in the organic and aqueous phase, respectively, and V org and V aq are the volumes of organic and aqueous solutions, respectively. The extraction process was repeated four times with another aqueous phase without elements at 9, 6 and 6M HNO 3 for DEE, 3% TPPO in chloroform and 30% TBP in chloroform respectively.
2.3.1.2 Ion Exchange Chromatography In this concern, separation of Ce from La was carried out using ion exchange technique. This was done using both the batch and column experiments. From the data obtained, the optimum conditions were applied for the separation of no carrier added radiocerium from irradiated La 2O3 target.
(A) Determination of the Distribution Coefficients To determine the distribution coefficients, a solution of 1000 ppm La and a solution of 1000 ppm Ce were prepared by dissolving their oxides in concentrated HNO 3, in case of cerium, the CeO 2 was dissolved in a mixture of concentrated HNO 3 and 30% H 2O2. The solution was evaporated to incipient dryness, and the residue was dissolved in 100 ml of deionized water. 1 ml of La solution and 0.1 ml of Ce solution were added to 8.9 ml of the medium (different concentrations of HNO 3, 0.1M of citrate buffer of different pH and 0.1 M of acetate buffer at variable pH). The total volume
30 II EXPERIMENTAL CHAPTER 2 was shaken with a fixed weight of ion exchanger Dowex 50W X8 H + form (100 mg) for 2 h to attain the equilibrium. The two phases (solid and liquid) were separated. The distribution coefficient (Kd) was obtained by the following equation:
C ads ∨ K d = × ( 2 . 2 ) C unads m
Where C ads is the concentration sorbed on the resin , Cunads is the remaining concentration in solution , V is the volume of aqueous phase in ml and m is the mass of resin in grams.
(B) Cation-Exchange Column Chromatography Many trials were done in this work for the separation of inactive cerium from lanthanum with cation exchanger using different eluants namely, citrate buffer, EDTA and α HIBA. a) 0.1 M Citrate Buffer A 100 ml stock solution containing 100 ppm La and 10 ppm Ce dissolved in 1 M HNO 3 was loaded onto a glass column, 10 cm long x 1.5 cm in diameter packed with Dowex 50W X8, the column had washed with about 400 ml of 10 6 M NaOH. A 1 ml stock sample was retained as a control. Load and wash samples were taken at a half stage of loading and washing, respectively, to test for breakthrough during these steps. Inductively coupled plasma (ICP) analysis of the wash and load samples showed no breakthrough of Ce and La from the column. About 300 ml from
31 II EXPERIMENTAL CHAPTER 2
0.1 M citrate buffer with pH 5.5 at a flow rate of (~ 1.5 ml/min) was used for elution process. The samples were collected every 17 min (~25 ml). a total of 12 sample were collected. The samples were analyzed by ICP using direct measurement of the emission lines 333.749 nm and 446.021 nm for La and Ce respectively. b) 0.1 M EDTA For the second experiment a 100 ml of stock solution containing 100 ppm La and 10 ppm Ce dissolved in 1 M HNO 3 was loaded into another column 10cm long x 1.5 cm in diameter packed with Dowex 50W X8, the column was washed with about 400 ml of 10 6 M NaOH. A 1 ml stock sample was retained as a control. Inductively coupled plasma (ICP) analysis of the wash and load samples showed no breakthrough of Ce and La from the column. The column was eluted by about 600 ml of 0.1 M EDTA of pH 8 at a flow rate of ( ~ 1.5 ml/min). The samples were collected every 17 min (~25 ml), a total of 24 sample were collected. The samples were analyzed by ICP for La and Ce. c) α HIBA The last experiment for separation of cerium from lanthanum was preformed using cation exchanger, the cation exchange resin Dowex 50W X8 in the H + form, was filled into a column (6 mm diameter, 4 cm high) and washed with 100 ml of 0.5 M NH 4Cl solution to transfer the resin to ammonium form NH4+. A solution contains 100 ppm La and 5 ppm Ce dissolved in 1 M HNO 3 was loaded onto the column with a flow rate ~ 1 ml/min. A 1 ml stock sample was retained as a control. A load sample was taken at a half stage of loading process to test the breakthrough during
32 II EXPERIMENTAL CHAPTER 2 loading. There was no breakthrough of Ce and La from the column. Cerium firstly was eluted with 0.2 M α HIBA with pH 4.75 while La still remained on the column, the samples were collected every 15 min (~15 ml). Then lanthanum was eluted with 6 M HNO 3. The samples were analyzed by ICP for La and Ce. The details of production and separation of no carrier added 139 Ce from irradiated natural lanthanum by Dowex 50W X8 using chelating agent α HIBA were discussed below.
2.3.2 Separation of Radioactive Cerium from Lanthanum Oxide Cyclotron Target. The production process of radiocerium is based on the following
(1) Preparation of the target (La 2O3 target). (2) Irradiation of the target at the MGC 20 cyclotron. 139 (3) Separation of no carrier added Ce from irradiated La 2O3 target. (4) Determination of radionuclidic purity, chemical purity and yield.
2.3.2.1 Target and Irradiation The target was prepared by pressing the lanthanum oxide salt as a pellet. The construction of the tool used for this purpose was shown in Fig 2.4. This tool was made of stainless steel. It consists of two cylinders; one of them is used as a base. Its dimensions are 40 mm diameter and 40 mm height with a big central hole, in which the second cylinder (with 37 mm diameter and 37 mm height) can be inserted. The second cylinder also involves a central hole with 13 mm diameter through which the rod of 13 mm diameter and 50 mm height pass. The rod settles on another one of 13
33 II EXPERIMENTAL CHAPTER 2 mm diameter and 6 mm height inside the second cylinder. Between these two rods, the backing foil and samples are placed. The target material 2 La 2O3 in the form of powder was pressed under a pressure of 10 tons.f /cm to 13 mm diameter pellet. The pellet was covered with a high purity Al foil of thickness 10 m.
D =13 mm
H =50 mm D=37 mm
H=37 mm b
a H=40 mm D=13 mm
H =6 mm
Rubber O Ring
D=40 mm
Fig 2.4: Pressing device used for sample preparation. (a is Al backing foil, 13 mm diameter; b is the sample after pressing, 13 mm diameter).
34 II EXPERIMENTAL CHAPTER 2
The irradiation was preformed at MGC 20 cyclotron available in Inshas, Egypt with beam current of 1 A, irradiation time was five hours and at incident proton energy of 14.5 MeV. During the irradiation the sample was cooled at the back by circulating water. Table 2.1 represents the characteristics of radionuclide formed in irradiation of La 2O3 target
* Table 2.1 Characteristics of radionuclide formed in irradiation of La 2O3 target
Nuclear Q value, Product T1/2 Energies of principal γ rays, reaction (MeV) nuclide keV (Intensity %)
(P,n) 1.06037 139 Ce 137,6 d 165.8 (80) *The target 139 La is monoisotopic.
2.3.2.2 Separation of 139 Ce from Irradiated Lanthanum Target Using Solvent Extraction Technique A stock solution of the activated sample was prepared by dissolving the irradiated matrix in a mixture of 5 ml conc HNO3 and 5 ml of H 2O2. The solution was evaporated to incipient dryness, and the residue was dissolved in 50 ml of 1 M HNO 3. To 1 ml of the solution containing the activity and lanthanum target, a mixture of 1 ml of 0.5 M K 2Cr 2O7 and 1 ml of 1 M
H2SO 4 as oxidizing agent was added. To obtain the desired molarity in a total volume of 10 ml, nitric acid was added to the mixture. The solution was transferred to a separating funnel and subjected to gentle shaking with an equal volume of each 30 % (v/v) TBP diluted in chloroform, 3 % (w/v) TPPO dissolved in chloroform and DEE. After disengagement, both aqueous and organic phases were monitored by γ ray spectrometer and ICP,
35 II EXPERIMENTAL CHAPTER 2 to calculate the distribution coefficients of cerium and lanthanum, respectively.
139 2.3.2.3 Separation of Ce from Irradiated La 2O3 with Cation Exchange Column Chromatography 139 The separation of no carrier added Ce from La 2O3 was performed using cation exchange resin Dowex 50W X8 (H form), 200 — 400 mesh, filled into a column (6 mm diameter, 4 cm high) and equilibrated with 0.5 M ammonium chloride solution. The target was dissolved in 5ml of 139 concentrated HNO 3 and loaded onto the resin. The no carrier–added Ce was eluted with 100 ml of 0.2 M α HIBA with PH 4.75, and flow rate ~ 1 ml / min. Thereafter, the macro amount of lanthanum was eluted with
100 ml of 6 M HNO 3 at the same flow rate. a) Purification of the Resulting Radionuclide
The resulting activity was loaded onto the same column (6 mm diameter, 4 cm high) packed with Dowex 50W X8 (H+ form), 200 − 400 mesh and equilibrated with 100 ml of 0.5 M NH 4Cl, the activity held to the resin and the tracer of lanthanum dropped in the solution of α HIBA, finally the Ce activity was eluted from the column with 100 ml 6 M HCl.
2.3.2.4 Determination of Radionuclidic Purity, Chemical Purity and Yield Measurement The radionuclidic purity and the absolute yield of 139 Ce were determined by γ ray spectrometry. The peak area analysis was done using
36 II EXPERIMENTAL CHAPTER 2 the software Gamma vision (Version 5.1, EG & G ORTEC). The decay data (10) of the concerned radionuclide is given in table 2.1. Measurements of the absolute photopeak detection efficiency as a function of energy were carried out with calibrated γ ray standard sources of 133 Ba, 60 Co, 137 Cs, 22 Na and 154 Eu. The total uncertainty in the yield measurement was obtained by summing the squares of the individual uncertainties (%) and taking the square root of the sum. The major individual uncertainties were: detector efficiency (5 10%) and peak area determination (2 5%). The total uncertainties for the radionuclide yield were 6 12%. The chemical impurity in the separated radiocerium was measured against lanthanum using ICP.
2.3.3 Separation of Praseodymium from Lanthanum The separation of praseodymium from lanthanum was preformed using solvent extraction and ion exchanger separation 2.3.3.1 Solvent Extraction Separation To determine the distribution coefficients, a stock solution containing 10 4 ppm La and 100 ppm Pr was prepared by dissolving their oxides in concentrated HNO 3. The solution was evaporated to incipient dryness, and the residue was dissolved in 100 ml of deionized water . To 1 ml of the stock solution, a mixture of 1 ml of 0.5M K 2Cr 2O7 and 1 ml of 1M H 2SO 4 as oxidizing agent was added. To obtain the desired molarity in a total volume of 10 ml, nitric acid was added to the mixture. The solution was transferred to a separating funnel and subjected to gentle shaking with an equal volume of each 30% (v/v) TBP diluted in chloroform, 3% (w/v) TPPO dissolved in chloroform and DEE. After disengagement, both
37 II EXPERIMENTAL CHAPTER 2 aqueous and organic phases were monitored by ICP, to calculate the distribution coefficients of lanthanum and praseodymium. The extraction process was repeated four times with another aqueous phase without elements at 9, 6 and 6M HNO 3 of DEE, 3% TPPO in chloroform and 30% TBP in chloroform respectively.
2.3.3.2 Ion Exchange Separation (A) Determination of the distribution coefficients To determine the distribution coefficients, a solution of 1000 ppm La and a solution of 1000 ppm Pr were prepared by dissolving their oxides in concentrated HNO 3, The solutions were evaporated to incipient dryness, and the residues were dissolved in 100 ml of deionized water. A 1 ml from La, 0.1 ml from Pr solutions and 8.9 ml of the medium (different concentrations of HNO 3 , 0.1M of citrate buffer of different pH, and 0.1 M of acetate buffer at variable pH) were added together to a fixed weight of ion exchanger Dowex 50W X8 (H + form), 200 — 400 mesh (100 mg). The contents were shaken for 2 h to attain equilibrium. The two phases (solid and liquid) were separated. The distribution coefficient (Kd) was obtained using equation 2.2.
(B) Cation-Exchange Column Chromatography For the separation of praseodymium from lanthanum, a glass column, 4 cm long x 6 mm in diameter, was packed with Dowex 50W X8 (H+ form), 200 — 400 mesh, and equilibrated with 0.5 M NH 4Cl to transfer the + resin to ammonium form NH 4 , then the stock solution containing
38 II EXPERIMENTAL CHAPTER 2 lanthanum and praseodymium was loaded on the column with flow rate ~ 1 ml/min. A 1 ml stock sample was retained as a control. A load sample was taken at a half stage of loading process to test the breakthrough during loading, inductively coupled plasma (ICP) analysis showed no breakthrough of Pr and La from the column. Pr was eluted using 0.2 M α HIB with PH 4.75, the samples were collected every 15 min (~15 ml), finally La was eluted from the column with 6 M HNO 3. The samples were analyzed by ICP using direct measurement of the emission lines 333.749 nm and 414.311 nm for La and Pr respectively.
39
CHAPTER 3
RESULTS AND DISCUSSION
III- RESULTS AND DISCUSSION CHAPTER 3
This chapter consists of two parts: • In the first part, the separation of cerium from lanthanum by solvent extraction and ion exchange chromatography techniques with a particular reference to produce no carrier added 139 Ce is discussed. • The second part contains the separation of praseodymium from lanthanum also by solvent extraction and ion exchange chromatography techniques as a simulation mode for the production 142 of Pr from α particles reaction on La 2O3 target.
Part I: Separation of Cerium from Lanthanum
I.1 Solvent Extraction Separation 139 I.1.1 Separation of no Carrier added Ce from Irradiated La 2O3 The predominant radionuclide present in the irradiated target in = → 139 the energy window E p 14 5. 10 MeV was Ce Fig 3.1. The procedure of chemical separation of cerium from lanthanides depends on the oxidation of cerium to the +4 state (18). The radiochemical separation of no carrier added radiocerium from bulk of lanthanum was attempted via solvent extraction using three extractants: DEE, TBP and TPPO. The details of the separation are given in the following.
40 III- RESULTS AND DISCUSSION CHAPTER 3
10 6
10 5
4 10 Ce (165.58 keV) (165.58 Ce
139
Counts 10 3
Annihilation
10 2
1 10 0 100 200 300 400 500 600 Gamma ray energy (keV)
nat Figure3.1 Gamma ray spectrum of La 2O3 target irradiated with 14.5 MeV protons.
A) Equilibrium Time The extraction kinetics of no carried added cerium was studied for the three extractants. Fig 3.2 shows that the variation of the extraction behavior depends on the time of contact between the aqueous and the organic phases. It was found that after about 3 minuts of shaking time, the equilibrium was attained.
41 III- RESULTS AND DISCUSSION CHAPTER 3
100
80
60
Ce (%) (%) Ce
139
40
Uptake of of Uptake 20 DEE TBP
0 TPPO
0 1 2 3 4 5
Time (min)
Figure 3.2 % uptake of radiocerium in the first extraction as a function of contact time with the three extractants.
B) Extraction Behavior of no-Carrier-added 139 Ce Using DEE The extraction coefficients of no carrier added Ce(IV) and bulk of La(III) were studied over a wide range of nitric acid concentration (2 11
M), the results are illustrated in Fig 3.3. The distribution coefficient (Kd) of ceric nitrate increases with increasing nitric acid concentration and
reaches a maximum value at 9 M HNO 3 and then gradually decreases. On the other hand the trivalent La did not display any extraction over the
42 III- RESULTS AND DISCUSSION CHAPTER 3
same range of acidity. Hence the optimum concentration of HNO 3 for the separation of radiocerium from lanthanum target is 9 M.
6
)
d 5 139
(K C e 4
3
2
1 [L a]
Distribution coeffecient Distribution 0
2 4 6 8 10 12 14 Concentration of HNO (m ol/l) 3
139 Figure 3.3 Distribution coefficients of Ce and La versus HNO 3 concentration over the range of 2 11 M, using DEE, shaking time 3 minutes .
For back extraction of radiocerium from the organic phase, (at
HNO 3 concentration below 3 M cerium has an observable low K d value). It is also necessary to enhance the back extraction by reduction of Ce
(IV) to Ce(III) state. Therefore a mixture of 1 M HNO 3 and 30 % of
43 III- RESULTS AND DISCUSSION CHAPTER 3
H2O2 was tested for this purpose; the back extraction efficiency amounted to 87 % of the initial activity.
C) Extraction Behavior of no-Carrier-added 139 Ce Using TBP The extraction behavior of no carrier added Ce(IV) and bulk of La(III) within the range of nitric acid concentration from 2 to 11 M are illustrated in Fig 3.4. Radiocerium and lanthanum globally have the same behavior as in the case of DEE. The only difference is that the distribution coefficient of no carrier added cerium using TBP increases with increasing nitric acid concentration till it reaches a maximum value at 6 M nitric acid, after that it decreases again. As mentioned before for back extraction of no carrier added cerium from organic phase containing TBP Ce complex, a mixture of 1 M HNO 3 and 30 % of H 2O2 was used. The efficiency of separation reached a value of 92.5 % relative to the initial activity.
44 III- RESULTS AND DISCUSSION CHAPTER 3
5 139 C e
) 4
d
3
2
1 [La]
Distribution coefficient (K coefficient Distribution 0
2 4 6 8 10 12 Concentration of HNO (mol/l) 3 139 Figure 3.4 Distribution coefficients of Ce and La versus HNO 3 concentration over the range of 2 11 M, using TBP, shaking time 3 minutes
D) Extraction Behavior of no-Carrier-added 139 Ce Using TPPO Fig 3.5 shows the relation between the distribution coefficient of no carrier added Ce(IV) in the presence of a bulk of La(III) as a function of nitric acid concentration using the TPPO as an extractant for the first
time. It is clear that the K d value of radiocerium increases within the acidity range of 2 to 6 and then decreases again over the acidity range of 7 to 11. The extracion behavior of no carrier added cerium (IV) is similar to that of no carrier added radiozirconium (IV) using 3 % TPPO
45 III- RESULTS AND DISCUSSION CHAPTER 3
in chloroform reported by Kandil et al (68). In the same manner carrier added lanthanum had negligible extractability by TPPO over the acidity range of 2 to 11 M, as in the case of carrier added yttrium (III) (68). Also as mentioned before for back extraction of no carrier added cerium from
organic phase containing TPPO Ce complex, a mixture of 1 M HNO 3
and 30 % of H 2O2 was used. 85.5% separation efficiency relative to that of initial activity was achieved.
5 1 3 9 C e
d 4
3
2
1 [L a] Distribution coefficient K coefficient Distribution
0 2 4 6 8 10 12 Concentration of HNO (m ol/I) 3
139 Figure 3.5 Distribution coefficients of Ce and La versus HNO 3 concentration over the range of 2 11 M, using 3 % TPPO in chloroform, shaking time 3 minutes
46 III- RESULTS AND DISCUSSION CHAPTER 3
E) Comparison of Investigated Methods for the Separation of Radiocerium Using Solvent Extraction Technique A summary of the results of the three investigated methods for the separation of no carrier added radiocerium from proton irradiated La 2O3 target using DEE, TBP and TPPO is given in Table 3.1. Regarding the DEE, it is observed that only 0.5 % of lanthanum was extracted from 9 M nitric acid, on the other hand TPPO extracted as much as 2.7 % of lanthanum from 6 M nitric acid. The extractant TBP displays not only a higher separation efficiency but also a high extraction value of lanthanum 4.1 %. Again with regard to DEE, further washing of organic phase improved the chemical purity of radiocerium (Table 3.2). This 4 improvement is due to the low K d of La DEE (less than 10 ) in 9 M of
HNO 3 DEE system. The non specified extracted species of La were easily stripped from the organic phase with a new portion of 9 M HNO 3. On the other hand, for the other two extractants, no improvement in chemical purity was achieved by further washing. This may be referred to the high K d values of La TBP and of La TPPO in 6 M of HNO 3 which ranges between 0.2 0.4, that reflects a slight specific extraction. We have to mention that improvement of chemical purity by further washing is normally accompanied by a loss in activity. Generally, out of the three extractants, DEE is the best one for obtaining high purity no carrier added cerium in the nitrate form.
47 III- RESULTS AND DISCUSSION CHAPTER 3
Table 3.1 Comparison of separation methods for no carrier added cerium from La 2O3 target irradiated with protons
Nitric acid Extractant Separation efficiency Extraction efficiency of concentration of 139 Ce La % % DEE 9 M 87 0.5 TPPO 6 M 85.5 2.7 TBP 6 M 92.5 4.1
Table 3.2 Effect of washing time for organic phase on the efficiency of separation 139 of no carrier added Ce from La 2O3 target irradiated with protons.
Separation efficiency of 139 Ce Extraction efficiency of La % %
Number of washing 0 1 2 0 1 2 DEE 87 68 30 0.5 0.002 0.000
TPPO 85.5 70 20 2.7 2.7 2.6
TBP 92.5 80 50 4.1 4.0 4.0
F) Optimum Conditions for Separation of 139 Ce The flow sheet of the optimized method for separation of no carrier added radiocerium from proton irradiated La 2O3 target is given in Fig 3.6. The target was dissolved in a mixture of 5 ml conc. HNO 3 plus 5 ml of H 2O2 while heating at 100 oC. The solution was evaporated to incipient dryness, and the residue was dissolved in 10 ml of a mixture of 9 M HNO 3 plus 0.05 M K 2Cr 2O7 and 0.1 M H 2SO 4 and the solution was transferred to a separatory funnel. The reaction vessel was rinsed with a new 10 ml of the same mixture and the solution was also transferred to
48 III- RESULTS AND DISCUSSION CHAPTER 3
Dissolution of La 2O3 in a mixture of 5 ml
of 14 M HNO 3 plus 5 ml H 2O2
Dissolution of residue in 10 ml of a mixture of 9 M HNO 3 plus 0.05 M K 2Cr 2 O7 and 0.1 M H2SO4
Addition of 20 ml of DEE, shaking time 3 min.
Aqueous phase La
Addition of 20 ml of DEE, shaking time 3 min. Organic phase
st Ce Aqueous phase Organic phase Added to 1 organic phase Washing with 10 ml of a mixture of
La Ce 9 M HNO 3 plus 0.05 M K 2Cr 2O7
and 0.1 M H 2SO 4 for 2 min
Addition of 15 ml of 1 M HNO 3 plus 5 ml of 30 % H2 O2, shaking time 3 minutes
Organic phase
Addition of 15 ml of 1 M HNO 3 plus 5 ml of 30 % H2O2, shaking time 3 minutes
Added to 2nd aqueous phase Aqueous phase Aqueous phase Organic phase Ce Ce
139 Figure 3.6 Flow sheet of separation of Ce from La 2O3 target
49 III- RESULTS AND DISCUSSION CHAPTER 3
the funnel. Thereafter 20 ml of DEE were added and the mixture was shaken for 3 min. The extraction process was repeated using a new portion of DEE. The two organic phases containing radiocerium were combined and first scrubbed with 10 ml of above mixture. Then 15 ml of
1 M HNO 3 plus 5 ml of 30 % H2O2 was added to the organic phase and the whole mixture was shaken for 3 min. This step was repeated with a new portion of 1 M HNO 3 plus 30 % H2O2. The optimized separation method reported above was used practically in the production of 139 Ce via the 139 La(p,n) 139 Ce reaction. In this case a La 2O3 pellet was irradiated with 14.5 MeV protons for 5 h at 1 A. The chemical separation of 139 Ce was done about one day after the end of bombardment (EOB). The results are summarized in Table 3.3. The experimentally obtained batch yield of 139 Ce is sufficient for tracer studies. However, the yield could be further increased. The target (69) could withstand currents up to 10 A and the irradiation time could be increased to about 10 h. It is thus possible to increase the batch yield of the radionuclide under consideration by a factor of 20.
Table 3.3 Thick target yield of 139 Ce Proton energy range within Irradiation Theoretical yield of Batch yield of the target MeV parameters 139 Ce radionuclide at radionuclide at EOB EOB (kBq) (kBq) = → E p 14 5. 10 1 A, 5 h 1295 762
50 III- RESULTS AND DISCUSSION CHAPTER 3
G) Production Yields A comparison between the practical yield and the theoretical yield was also undertaken. The excitation function of the 139 La(p,n) 139 Ce reaction has been recently measured by Hassan et al(70) and Vermeulen et al (71). We adopted our own data that have been done in our cyclotron MGC 20, because they appeared to us to be more consistent. Using those (1) 139 data we calculated the thick target yield of Ce from a La 2O3 target for the respective energy ranges used in the experiment. In this connection the range energy relationship given by Williamson et al (72) was used. The theoretical yield thus obtained was compared with the experimental batch yield. We found that the experimental yield of 139 Ce amounted to 86.6 % of the theoretical value before starting the chemical separation process but after the separation of 139 Ce from lanthanum oxide target the experimental yield was 59 % of the theoretical value. Considering the various factors affecting the experimental yields (73) (uncertainty in the beam current measurement, radiation damage effect, beam energy, etc.), the results appear to be satisfactory. Moreover, as mentioned before, further washing in the chemical separation process is an effective factor in activity losses. An experimental yield of 153 kBq/ A h in correlation with that obtained by Ochab and Misiak (18) was achieved. Therefore the experimental yield in this work suggests that the followed production process is satisfactory and reliable.
51 III- RESULTS AND DISCUSSION CHAPTER 3
I. 2 Ion Exchange Separation As mentioned before, the predominant radionuclide present in the irradiated natural lanthanum target at proton energy 14.5 MeV is 139 Ce Fig 3.1. Since, the properties of the lanthanide elements are similar, the radiochemical separation of these elements from each other is very difficult. In order to avoid contamination by the relatively long lived 139 radioisotope Ce (t 1/2 = 137.6 d) several trials were done in this work, particularly during the primary experiments, by employing inactive cerium instead of radioactive 139 Ce in the separation via cation exchange using different eluants, namely, citrate buffer, EDTA and α HIBA.
I.2.1 Adsorption Behaviors of Ce and La on Cation Exchanger The adsorption behaviors of cerium and lanthanum on the cation exchange resin in different media, namely, nitric acid, acetate buffer and citrate buffer were studied. Fig 3.7 shows the relationship between the distribution coefficients (K d values) of these elements and the concentration of nitric acid where in 1 M they were completely adsorbed on the resin. Their adsorbability decreased gradually with increasing the concentration up to 4 M. Over the concentration range 4—14 M the adsorbability of La and Ce was relatively low and almost constant.
52 III- RESULTS AND DISCUSSION CHAPTER 3
300
250
d La
200
150
Ce 100
50
Distribution coefficient K coefficient Distribution
0 0 2 4 6 8 10 12 14
Nitric acid concentration (mol/l)
Figure. 3.7 Distribution coefficients of La and Ce as a function of HNO 3 concentration on Dowex 50W X8 resin, shaking time 2 hours
Figure 3.8 illustrates the adsorption behavior of Ce and La in 0.1 M acetate buffer over the pH range 3 to 5.5. In the acetate medium they were strongly adsorbed within this pH range.
53 III- RESULTS AND DISCUSSION CHAPTER 3
4x10 3
d
3 3x10 La
3
2x10
Ce Distribution coefficient K 1x10 3
0
3.0 3.5 4.0 4.5 5.0 5.5
pH
Figure 3.8 Distribution coefficients of La and Ce as a function of pH of 0.1 M acetate buffers on Dowex 50W X8 resin, shaking time 2 hours.
On the other hand, on using 0.1M citrate buffer (for the same range of pH). La and Ce showed a completely different behavior Fig 3.9, where at pH 3 they were strongly adsorbed, in the pH range 3 4 their adsorbability decreased gradually, after pH 4 there was no specific adsorption. The distribution coefficients of these elements in 0.1 M citrate buffer obtained in this work were correlated with those obtained for yttrium (68).
54 III- RESULTS AND DISCUSSION CHAPTER 3
700
La 600 Ce
d 500
400
300
200
100 Distribution coefficient K
0
3.0 3.5 4.0 4.5 5.0 5.5
pH
Figure 3.9 Distribution coefficients of La and Ce as a function of pH of 0.1 M citrate buffers on Dowex 50W X8 resin, shaking time 2 hours.
I.2.2 Cation-Exchange Column Chromatography . A) 0.1M Citrate Buffer Fig 3.7 shows that the Ln 3+ ions are adsorbed on the cation
exchange resin from 1 M HNO 3. On treatment with a complexing agent such as buffered citric acid, the cations tended to be eluted in a reverse atomic number order(32). Therefore, a 0.1 M citrate buffer at pH 5.5 was used to elute La and Ce in sequence. However, in the elution process a
55 III- RESULTS AND DISCUSSION CHAPTER 3
large volume of the eluant was needed to remove Ln 3+ since the residual acid from the loading step decrease the pH of the citrate buffer to a lower 3+ values at which the K d values of Ln were higher (400—650). This difficulty was overcomed by washing the column with 10 6 M NaOH prior to the elution step. At pH 5.5, the citrate buffer displace the trivalent ions forming anionic complex which passes through the column, but the peak of cerium falls within the lanthanum peak (Fig 3.10). So there was no sufficient separation.
20 La Ce
15
10
Eluted amount (%) amount Eluted 5
0
0 2 4 6 8 10 12 Number of fraction ( fraction volume = 25 ml)
Figure 3.10 Elution profile of La and Ce using 0.1 M citrate buffer of pH 5.5 as an eluant from a column packed with Dowex 50W X8. Fraction volume = 25 ml.
56 III- RESULTS AND DISCUSSION CHAPTER 3
B) 0.1 M EDTA As described before, a column filled with Dowex 50W X8 was used. About 400 ml of 10 6 M NaOH were sufficient to adjust the pH of the column to become 8. At this pH, the EDTA could form strong complex with Ln 3+ (57), its stability is inversely proportional to the atomic number. Therefore, about 600 ml of 0.1 M EDTA of pH 8 was used for elution of cerium and lanthanum at a flow rate of ~ 1.5 ml/min. The elution profile is given in Fig 3.11. As shown in the figure, cerium peak falls within the lanthanum peak and thus the separation was not sufficient.
10
La Ce 8
6
4
2
Eluted amount (%) amount Eluted
0
0 5 10 15 20 25
Number of fraction (fraction volume = 25 ml)
Figure 3.11 Elution profile of La and Ce using 0.1 M EDTA as an eluant from a column packed with Dowex 50W X8. Fraction volume = 25 ml.
57 III- RESULTS AND DISCUSSION CHAPTER 3
C) 0.2 M α HIBA A column packed with Dowex 50W X8 and washed with 100 ml + of 0.5 M NH 4Cl to transfer the resin to NH 4 was used. A 100 ml stock
solution containing lanthanum and cerium dissolved in 1 M HNO 3 was transferred to this column. Elution was done using 100 ml of 0.2 M α HIBA of pH 4.75 at a flow rate of 1 ml /min which has the potential to allow effective separation of the lanthanides. The elution profile is given in Fig 3.12, cerium was completely eluted with 100 ml of 0.2 M α HIBA while lanthanum was still remaining on the column. A 100 ml of 6 M
HNO 3 was sufficient to elute lanthanum.
100
ααα 6 M HNO 0.2 M -HIBA 3 80
60 La (III) Ce (III)
40
Eluted amount (%) amount Eluted 20
0
0 2 4 6 8 10 12 14 16 Number of fraction (fraction volume = 15ml)
Figure 3.12 Elution profile of Ce by 0.2 M α HIBA and of La by 6 M HNO 3 in sequence from a column packed with Dowex 50 W X8. Fraction volume = 15 ml.
58 III- RESULTS AND DISCUSSION CHAPTER 3
I.2.3 Optimum Conditions for Separation of no-Carrier-added 139 Ce from Irradiated La 2O3 Using Cation Exchanger Technique The obtained results for the three investigated eluants using cation exchange resin show that α HIBA is the most suitable one. These data were considered on separating radiocerium from proton irradiated
La 2O3 target. A) Primary Chromatography Column for Separation of 139 Ce from
Irradiated La 2O3 The cation exchange resin Dowex 50W X8 (H form), 200 — 400 mesh, was filled into a column (6 mm diameter, 4 cm high) and equilibrated with 0.5 M ammonium chloride solution. The irradiated
La 2O3 target was dissolved in 5 ml of concentrated HNO 3 and loaded onto the resin. The no carrier–added 139 Ce was eluted with 100 ml of 0.2 M α HIBA with pH 4.75, flow rate ~ 1 ml / min. Approximately 97% of the Ce activity was collected in 45 ml of eluted solution with an estimated amount of La(III) reduced by a factor of ~ 7 x 10 3 (from a 300 mg La 2O3 target contaning 256 mg La, the amount of La detected in the radiocerium fraction was ~ 36 g). Thereafter, the macro amount of lanthanum was eluted with 100 ml of 6 M HNO 3 at the same flow rate.
B) Purification of the Resulting Radionuclide The separated radiocerium fraction was again loaded onto the same column packed with Dowex 50W X8 equilibrated with 100 ml of 139 0.5 M NH 4Cl. An unexpected behavior was observed, where the Ce activity was retained on the resin while lanthanum was carried with the eluting solution of α HIBA. This behavior may be related to the low concentration of La(III) and the high affinity of α HIBA to form a complex with low concentration of Ln 3+. Finally, the activity was eluted
59 III- RESULTS AND DISCUSSION CHAPTER 3
from the column with 100 ml of 6 M HCl Fig 3.13. About 92 % of the retained activity was obtained within the peak consisting of about 70 ml of the eluant while the residual activity was eluated in tailing. The purification step on chromatographic column gave an additional decontamination factor of at least 10 to La (III), reducing the amount of the remaining La to ~ 3.5 g. The optimized separation method reported above was used practically for the production of 139 Ce via the 139 La(p,n) 139 Ce reaction. The chemical separation of 139 Ce was done about one day after the end of bombardment (EOB). The experimentally obtained batch yield of 139 Ce is sufficient for tracer studies. Knowing that the target (69) could withstand currents up to 10 A and for irradiation time of 10 h, the batch yield of the radionuclide under consideration could be increased by a factor of 20.
I.2.4 Production Yields As mentioned before, a comparison of the practical yield with the theoretical yield was also undertaken. The theoretical yield at EOB for a target of thickness E p= 14.5 →10 MeV amounted to 1295 kBq for a 5 h irradiation at 1 A beam current. The experimental batch yield of 139 Ce at EOB was found to be 1121 kBq, i.e. 86.6 % of the theoretical value. However after the separation of 139 Ce from lanthanum oxide target the obtained yield was 1001 kBq, i.e. ~ 77.3 % of the theoretical value. This value is acceptable if we consider the various factors affecting the experimental yields (73) such as (uncertainty in the beam current measurement, radiation damage effect, beam energy, etc.).
60 III- RESULTS AND DISCUSSION CHAPTER 3
50
40
Ce (% ) (% Ce 139 30
20
Eluted amount of of amount Eluted 10
0 1 2 3 4 5 6 7 Number of fraction (fraction volume = 15ml)
Figure 3.13 Elution profile of 139 Ce using 6 M HCl as an eluant from a column packed with Dowex 50W X8. Fraction volume = 15 ml.
I.3 Comparison of the Investigated Separation Methods of Radiocerium A summary of the results on the four investigated methods for the
separation of no carrier added radiocerium from proton irradiated La 2O3 target is given in Table 3.4. It is apparent that the separation method using Dowex 50W X8 (cation exchanger) and α HIBA as eluent is the best, the efficiency of separation is higher and the extraction efficiency
61 III- RESULTS AND DISCUSSION CHAPTER 3
of La is rather low (0.01 %). However, the results of separation of no carrier added radiocerium from proton irradiated La2O3 target using α HIBA as eluent on cation exchanger column appear to be satisfactory. An experimental yield of 200 kBq/ A h is rather high than that obtained in solvent extraction technique using DEE as extractant (153 kBq/ A h), i.e. the batch yield has increased by 30.7 %.
Table 3.4 A summary of the results on the four methods investigated for the separation of no carrier added radiocerium from proton irradiated La 2O3 target
Nitric acid Extractant Efficiency of Extraction efficiency of concentration separation La % % DEE 9 M 87 0.5 TPPO 6 M 85.5 2.7 TBP 6 M 92.5 4.1 0.2 M α HIBA* 97 0.01 Solvent extraction method * Cation exchange method
62 III- RESULTS AND DISCUSSION CHAPTER 3
Part II: Separation of Praseodymium from Lanthanum via Solvent-Extraction and Ion-Exchange Techniques
II.1. Solvent Extraction Separation The extraction coefficients of Pr and La were studied over a wide range of nitric acid concentration (2 11 M), the results were illustrated in Figures 3.14, 3.15 and 3.16 for DEE, 3%TPPO in chloroform and 30% TBP in chloroform, respectively.
2.0
d
K
1.5
1.0
0.5
Distribution coeffecient Distribution La
0.0 Pr
2 4 6 8 10 Nitric acid concentration (mol/l)
Figure 3.14 Distribution coefficients of Pr and La versus HNO 3 concentration over the range of 2 11 M, using DEE, shaking time 3 minutes.
63 III- RESULTS AND DISCUSSION CHAPTER 3
As shown from these figures, the distribution coefficients of praseodymium and lanthanum were very low, so the separation of praseodymium from lanthanum was not possible with solvent extraction
technique using 0.05 M K 2Cr 2O7 and 0.1 M H 2SO 4 as oxidizing agent. This may attributed to that, the tetravalent state of praseodymium Pr(IV) is not stable and praseodymium behaves in chemical reactions with trivalent state Pr (III).
2.0
d 1.5 K
1.0
0.5 La
Distribution coeffecient Distribution 0.0 Pr
2 4 6 8 10 12
Nitric acid concentration (mol/l)
Figure 3.15 Distribution coefficients of Pr and La versus HNO 3 concentration over the range of 2 11 M, using 3 % TPPO in chloroform, shaking time 3 minutes
64 III- RESULTS AND DISCUSSION CHAPTER 3
2.0
1.8
d 1.6 K
1.4
1.2 1.0
0.8
0.6
0.4 Distribution coeffecient Distribution La 0.2 Pr 0.0 2 4 6 8 10 12 Nitric acid concentration (mol/l)
Figure 3.16 Distribution coefficients of Pr and La versus HNO 3 concentration over the range of 2 11 M, using 30 %TBP in chloroform, shaking time 3 minutes
As mentioned before, the chemistry of lanthanides of trivalent state are the same. To overcome this difficulty, further experiments have been carried out for separating praseodymium from lanthanum using cation exchange method and α HIBA as eluent instead of using solvent extraction technique.
65 III- RESULTS AND DISCUSSION CHAPTER 3
II.2 Ion Exchange Separation II.2.1 Adsorption Behaviors of Pr and La on Cation Exchanger The adsorption behaviors of praseodymium and lanthanum on cation exchange resin in different media, namely, nitric acid, acetate buffer and citrate buffer were analogious to that of cerium and lanthanum. Fig 3.17 showed the relationship between the distribution
coefficients (K d values) of these elements and the concentration of nitric acid. In 1 M they were completely adsorbed on the resin. Their absorbability decrease gradually with increasing the acidity upto 4 M. Over the acidity range 4—12 M the absorbability of La and Pr was relatively low and remains constant.
2000 La
d 1500
Pr
1000
500
Distribution coefficient K coefficient Distribution
0 0 2 4 6 8 10 12
Nitric acid concentration (mol/l)
Figure 3.17 Distribution coefficients of La and Pr versus HNO3 concentration over the range of 1 12 M, using Dowex 50W X8, shaking time 2 hours
66 III- RESULTS AND DISCUSSION CHAPTER 3
The adsorption behaviors of Pr and La as a function of pH in 0.1 M acetate buffer within the range from 3 to 5.5 was the same of those obtained for lanthanum and cerium. In the acetate medium they were extremely adsorbed as shown in Fig 3.18.
3000
2500 La
d 2000
1500
Pr 1000
500 Distribution coefficient K coefficient Distribution
0
3.0 3.5 4.0 4.5 5.0 5.5
pH
Figure 3.18 Distribution coefficients of La and Pr versus acetate buffer over the range of (3 – 5.5 pH) with Dowex 50W X8, shaking time 2 hours
67 III- RESULTS AND DISCUSSION CHAPTER 3
On the other hand, within the same rang of pH of citrate buffer La and Pr gave the same behavior as that of cerium and lanthanum, Fig 3.9, at pH 3 they were strongly adsorbed. By raising the pH of citrate medium till pH 4 their absorbability decreased gradually. It is clear that there is no specific adsorption over pH 4, Fig 3.19.
1000 La Pr d 800
600
400
200
Distribution coffecient K coffecient Distribution 0
3.0 3.5 4.0 4.5 5.0 5.5
pH
Figure 3.19 Distribution coefficients of La and Pr versus citrate buffer over the range of (3 – 5.5 pH) with Dowex 50W X8, shaking time 2 hours
68 III- RESULTS AND DISCUSSION CHAPTER 3
II.2.2 Cation-Exchange Column Chromatography As discussed before in the separation of cerium from lanthanum by cation exchanger, the citrate and EDTA were used as eluents but the separation was not satisfied. On the other hand, empolying α HIBA gave a satisfied results. Therfore, the separation of praseodymium from lanthanum was done using α HIBA as eluting agent. A column packed with Dowex 50W X8 was washed with 100 ml of 0.5 M NH 4Cl to + transfer the resin to NH 4 . A 100 ml of a stock solution containing 100 ppm lanthanum and 5 ppm praseodymium dissolved in 1 M HNO 3 was transferred to this column. The elution profile is given in Fig 3.20. As shown from the figure, praseodymium was completely eluted with the first 60 ml of 0.2 M α HIBA of pH 4.75 at a flow rate of 1 ml/min and lanthanum was still remaining on the column. A 100 ml of 6 M HNO 3 was sufficient to elute lanthanum. Praseodymium was collected completly (~ 100%) in third and fourth fraction (volume of 30 ml) with no lanthanum found in this volume. Therfore, there was no necessity for further purification of praseodymium.
69 III- RESULTS AND DISCUSSION CHAPTER 3
100
ααα 0.2 M HIBA 6 M HNO 3 80 La (III) Pr (III) 60
40
Eluted amount (%) amount Eluted 20
0
0 2 4 6 8 10 12 14 16 Number of fraction (fraction volume=15ml)
Figure 3.20 Elution profile of Pr by 0.2 M α HIBA and of La by 6 M HNO 3 in sequence from a column packed with Dowex 50 W X8. Fraction volume = 15 ml.
The intensive study on the separation of Pr from La by two techniques, namely, solvent extraction and ion exchange chromatography showes that, the ion exchange method is more suitable, reliable and satisfactory for the separation of Pr from La.This study is also a 142 simulation for the separation of Pr from α–induced reaction on La 2O3 target planed to be studied in the future.
70 CONCLUSION
In the present work we studied the production and separation of radiocerium from an irradiated La 2O3 target using proton beam of maximum energy 14.5 MeV. The separation process was carried out using both solvent extraction and ion exchange techniques. In the first technique the radiochemical separation of no carrier added radiocerium from bulk of lanthanum using three extractants: DEE, TBP and TPPO was studied. After separation and further purification it was found that the best result (153 kBq/ A.h) was obtained when we used DEE as an extractant. On the other hand optimization of the separation efficiency in case of ion exchange method using Dowex 50W X8 (cation exchanger) and 0.1 M citrate buffer, 0.1 M EDTA and 0.2 M α HIBA as eluents was carried out. The obtained results show that α HIBA is the most suitable eluent. An experimental yield of 200 kBq/ A.h was achieved which is higher than that obtained in solvent extraction technique using DEE by a factor of 30.7 %. The intensive study on the separation of Pr from La using both solvent extraction and ion exchange chromatography techniques showed that ion exchange method using Dowex 50W X8 as cation exchanger and α HIBA as eluent is the most reliable and suitable method. This study was a simulation for the production of 142 Pr from α induced reaction on
La 2O3 target.
71 SUMMARY
Radionuclides are finding increasing applications in almost all branches of science and technology, particularly in the field of medicine. 139 The relatively long lived radionuclide Ce (T 1/2 = 137.6 d) is useful as a standard for the calibration of γ ray detectors. This radioisotope can be produced through 139 La (p,n) 139 Ce with Q value 1.06037 MeV and has only one strong γ ray of energy 165.857 keV with 80 % intensity, which is within the optimum energy range for detection with a gamma camera. Image degradation during single photon emission tomography (SPECT) due to attenuation and Compton scattering of photons can cause clinical 142 image artifacts. The neutron rich isotope Pr (T 1/2 = 19.13 h) has good physical characteristics for internal radiotherapy. The wide use of 142 Pr almost relates to the β emission of maximum energy 2.16 MeV and an average energy of 0.83 MeV, as well as rather adequate intensity (93 %). 142 Pr has another advantage, It emits one gamma photon at 1.58 MeV with an intensity of 3.7% which does not interfere with internal medical applications due to its relatively low intensity and low specific γ dose constant. The main aim of the present work was to develop efficient separation methods for those radionuclides from cyclotron irradiated targets. In particular two systems were investigated in detail: • Separation of radiocerium from irradiated lanthanum target • Separation of praseodymium from lanthanum as a simulation mode for the production of 142 Pr from α particles
induced reaction on La 2O3 target. The thesis is composed of three chapters.
72 SUMMARY
Chapter 1: Introduction • Some details about the employed cyclotron and its constituents are given. • An overview of the importance and production routes of relevant radionuclides is given. • The chemistry of lanthanum, cerium and praseodymium is also given. At the end of this chapter, a literature survey on the separation chemistry of cerium and praseodymium is outlined. • The main aim of the present work is to develop efficient separation methods for those radionuclides from cyclotron irradiated targets.
Chapter 2: Experimental Method The experimental part defines the chemicals used and includes separation procedures for Ce and Pr after an extensive study on the distribution coefficients of La, Ce and Pr using different aqueous solutions by ion exchange chromatography and different extractants by solvent extraction techniques. Also this chapter includes detailed description for production and separation of no carrier added 139 Ce from irradiated natural lanthanum by Dowex 50W X8 using chelating agent α HIBA and using solvent extraction techniques with three extractants, namely, tri butyl phosphate (TBP), tri phenyl phosphine oxide (TPPO) and diethyl ether (DEE). The quality control of the product is an integral part of this aspect of work.
73 SUMMARY
Chapter 3: Results and Discussion This chapter consists of two parts.
• First part, the separation of cerium from lanthanum by solvent extraction and ion exchange chromatography techniques with particularly reference to produce no carrier added 139 Ce.
• The second part contains a separation of praseodymium from lanthanum also by solvent extraction and ion exchange chromatography techniques as a simulation mode for the production 142 of Pr from α particle reaction on La 2O3 target .
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ا ا ا ا ت ا ة وع ا وا و ا ا . و م ا م ١٣٩ ذو ة ا ا ة ( ة ا = ١٣٧ ٦ م ) و ا ج ه ا ا ا ا و ن ا م ١٣٩ . و ا ا ث ١٦٥ ٨٧٥ ك أ. . ف و ٨٠ % وه ق ا ا ة و ا م آ اف ا ث ا ن ا د وا ى ث رج ا رة ا ء ا ث ا ن وا آ ا ه وا رة آ ن وا ان ن أ ء اآ ا رة . . ا از د م – ١٤٢ ( ة ا = ١٩ ١٣ ) ا و ت ا ا ة ج ا ا ا . و ا ام ا ا از د م – ١٤٢ ا ا ث ت ا ٠ ٨٣ م أ. . ف و ة ٩٣ ٩٣ % و ا أ أن ا ا ا ا و ١ ٥٨ م أ. . ف و ٣ ٧ % ا ا ت ا ا ا ا ة ا و ا ا ا . . ا ف ا ه ا ا ه ا ل ا ق ا ات ا اه اف ا ون . و ه ا ا ص درا آ • ا م ا ه ف ا م • ا از د م ا م آ م آ ج ا از د م – ١٤٢ ت ا ه ف أ آ ا م . . ن ا ث ل : : ا ا ول : ى ا ا ون ا م و ا ة اه و ق ا ج ا ات ا ا را وآ ء ا م وا م وا از د م و ه ا ا ا را ا ء ا م وا از د . م . م ا ا : ا ء ا و ا اد ا ا وا ا ا م وا از د م درا و ا ز ا م وا م وا از د م او دة ا ا م ا دل ا ا و ا و ت درا ا ت . آ ه ا ا آ ا ج و ا م – ١٣٩ ا ( ون ا ) ا م ا ا ا ا ت Dowex 50W X8 ا ام α HIBA آ ا ص وذ ا ام ا دل ا ا و ا . و ا ام ا ت ا ام ث ت ه ( TBP), (TPPO), (DEE) . . ن ا دة ا ه ء أ ه ا ا ا . . ا ا : ا وا ن ه ا ا . . • ا ء ا ول ، و ا م ا م ا ت ا ت وا دل ا ا و ا ج ا م – ١٣٩ . . • ا ء ا ى ا از د م ا م ا ام ت ا ت وا دل ا ا و ا آ م آ ج ا از د م – ١٤٢ ت ا ه ف اآ ا م . .