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Applied Radiation and Isotopes 64 (2006) 1521–1527

www.elsevier.com/locate/apradiso

Production and separation of no-carrier-added radioactive tracers ofyttrium, strontium and rubidium from heavy-ion irradiated germaniumtarget: Applicability to the standardization of a separation technique for

production of positron-emitting radionuclide 86Y$

Sujit Pala,�, Sankha Chattopadhyayb, M.K. Dasb, M. Sudersananc

aAnalytical Chemistry Division of Bhabha Atomic Research Centre (BARC), Variable Energy Cyclotron Centre (VECC), 1/AF, Bidhan Nagar,

Kolkata-700 064, IndiabRadiopharmaceuticals Laboratory of Board of Radiation & Isotope Technology (BRIT), Variable Energy Cyclotron Centre (VECC), 1/AF,

Bidhan Nagar, Kolkata-700 064, IndiacAnalytical Chemistry Division, Mod. Lab., BARC, Trombay, Mumbai-400 085, India

Received 24 April 2006; accepted 4 May 2006

Abstract

Among various positron-emitting radionuclides, certain radioisotopes of Y, Sr and Rb have important applications in diagnostic and

therapeutic nuclear medicine. In the present work, an attempt has been made to produce some of those radioisotopes by irradiating a

natural Ge-target material with heavy-ion oxygen (16O+6) projectiles. An effective radiochemical separation scheme was developed to

isolate Y, Sr and Rb radiotracers from the irradiated Ge-matrix in no-carrier-added form with a view to applying those radiotracers for

standardization of a technique for the radiochemical separation of Y from natural Sr target. The standardized separation technique

could be utilized for the production of the positron-emitting 86Y from an enriched 86Sr target irradiated at a medical cyclotron.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Heavy-ion irradiation; Oxygen projectile; Natural germanium oxide; Separation of Y, Rb and Sr radiotracers; No-carrier-added radiotracer

Y-86

1. Introduction

In recent years, use of positron emission tomography(PET) has gained considerable significance in diagonisticand therapeutic nuclear medicine, and for metabolicstudies and drug evaluation using cyclotron producedpositron-emitting (b+) radioisotopes. (Qaim et al., 1993,2003; Stocklin et al., 1995; Perlman and Stone, 1998;Machulla, 1999; Schlyer, 2004). Among various positron-emitting radionuclides, 86Y, 83Sr and 82mRb are potentiallyuseful b+-emitters that find important applications inPET (Rosch et al., 1996a, b, 1999; Finn et al., 1999; Reischl

e front matter r 2006 Elsevier Ltd. All rights reserved.

radiso.2006.05.003

in part at the 5th International Conference on Isotopes

, Belgium, 25–29 April 2005.

ing author. Tel.: +91 33 233 71230; fax: +91 33 235 91782.

ess: psujit@veccal.ernet.in (S. Pal).

et al., 2002; Kettern et al., 2002; Kastleiner et al., 2002;Kovacs et al., 1991; Blessing et al., 1997).The production of these important radionuclides in-

volves development of efficient and suitable radiochemicaltechniques for fast separation of the radioisotopes ofinterest from the target material after their production ina cyclotron, in addition to studies of nuclear reactioncross-sections (Rosch et al., 1993a, b; Blessing et al.,1997; Kettern et al., 2002; Reischl et al., 2002; Kastleineret al., 2002; Qaim, 2003) and development of highcurrent targetry systems for irradiation (Finn et al., 1995;Qaim, 1989; Schlyer, 2003). Furthermore, since expen-sive highly enriched target materials are generally usedfor irradiation and subsequent production of desiredradionuclides for biomedical applications, the targetrecovery is also an important feature of the productionprocess.

ARTICLE IN PRESSS. Pal et al. / Applied Radiation and Isotopes 64 (2006) 1521–15271522

For the last few decades, applications of acceleratorproduced heavy-ion beams have become increasinglyimportant in various areas of research in fundamentalnuclear physics and technology, chemistry and condensedmatter studies (Lanford, 1985). Heavy ions (ZX3) ofsufficient energy may induce different nuclear reactions ona particular target nuclide leading to the production of alarge number of radioisotopes through multiple radio-activation pathways. These heavy ion induced multiplereaction products can be used for the production of manyradionuclides and effectively applied in different fields(Ambe et al., 1991a, b; Ambe, 2000). The radiotracersproduced under no-carrier-added conditions have highspecific activities which is a pre-requisite for manyimportant applications, specially in the field of lifesciences and in making labeled compounds (Bonardiet al., 2002, 2004).

In the present study, an attempt has been made to applyheavy-ion activation for the production of several Y, Srand Rb radionuclides by irradiation of a natural germa-nium target using medium energy 16O6+ heavy-ionprojectiles (�7MeVamu�1) at the Variable Energy Cyclo-tron Center (VECC), Kolkata, India. An effective radio-chemical separation scheme was developed for the isolationof those radioisotopes from the bulk germanium targetmatrix in no-carrier-added form. The aim of the presentseparation work was carried out with a view to utilizingthese radiotracers for the standardization of a radio-chemical separation technique that could be applied for theroutine production of no-carrier-added 86Y from enriched86Sr targets using a medical cyclotron for radiopharma-ceutical applications.

2. Experimental

2.1. Irradiation

A thick target of germanium was prepared by depositing25mg cm�2 GeO2 (BDH, England) on a 25.4 mm thick highpurity aluminum backing foil (15mm� 15mm; Good-fellow, UK) by centrifugation technique. The target waswrapped in a thin piece of aluminum foil (7.6 mm) andirradiated for 24 h with 115MeV 16O6+ heavy-ion beamproduced at the Variable Energy Cyclotron at Kolkata,India. An average beam intensity of about 100 nA wasmaintained throughout the irradiation. The beam profilewas roughly 10mm� 10mm. Water cooling of the targetwas provided in order to dissipate the heat producedduring irradiation. The 16O6+-beam energy degradation inthe 7.6 mm aluminum foil, used to cover the target, wascomputed using the statistical computer code TRIM(Ziegler, 1985) and was found to be 5.4MeV. Thus, theeffective beam energy on the target during irradiation wasapproximately 110MeV. After irradiation, the germaniumtarget was allowed to cool for about 1 d prior to carryingout radiochemical separation of the radioisotopes ofinterest.

2.2. Measurementof g-ray spectra

All g-ray spectroscopic measurements of the activatedsample, solutions before and after each radiochemicalseparation, were carried out using an HPGe detector [30%efficiency relative to a 300 � 300 NaI(Tl) detector], having1.74 keV resolution at 1332 keV, coupled to a PC-basedmulti channel analyzer. The presence of all the radio-isotopes of interest was monitored with the different g-linesof the respective radioisotopes and following their half-lives.

2.3. Chemical separation

In order to separate the radioisotopes of Y, Sr and Rb,from the irradiated GeO2 target, the matrix activity wasfirst removed. This was followed by the separation ofY-radioisotopes by co-precipitation with iron carrier andby ion-exchange chromatography, respectively, in order toisolate radio-yttrium from inactive iron in no-carrier-addedform. Finally, the Sr and Rb radioisotopes were separatedfrom each other by cation-exchange technique using HBrmedium. All the reagents, used for chemical separation,were of analytical grade and procured from E. Merck,India.After 24 h cooling, the irradiated GeO2 target was

dissolved cautiously in 100 ml of 5% NaOH solution takingcare to minimize the dissolution of aluminum backing foil.The radioactive solution was acidified with a few drops ofconc. HNO3 to separate the bulk of the Ge by hydrolysis asinsoluble GeO2 (Mirzadeh and Lambrecht, 1996). Thesolution was centrifuged to remove the white residue ofGeO2 and the supernatant liquid was counted on HPGedetector to verify the production of different isotopes ofinterest of Y, Sr and Rb before proceeding for furtherradiochemical separation of each individual radioelement.

2.4. Separation of Y from Sr and Rb

The solution, containing the radioisotopes of Y, Sr andRb, was spiked with 500 ml of Fe(III) carrier, prepared bydissolving iron filings (purity499.99%) (H. Cross Co.,USA) in 1:1 HCl and diluting with water (Fe conc.:1mgml�1). The iron was precipitated as Fe(OH)3, byadding few drops of concentrated ammonia. Yttriumradioisotopes were co-precipitated almost quantitativelyalong with the Fe(OH)3. After centrifugation, the pre-cipitate was isolated and the supernatant liquid, containingthe radioisotopes of Sr and Rb, was preserved. Theprecipitate, containing Fe(OH)3 and the co-precipitatedyttrium radioisotopes, was dissolved in a few drops of 6MHCl and diluted to 10ml with water.To separate the no-carrier-added Y-radioisotopes from

the Fe-carrier, a Dowex 2� 8 (Sigma Chemical Co., USA),200–400 mesh, anion-exchange resin column of length60mm and diameter 10mm was used. The resin waswashed thoroughly with water before being loaded into a

ARTICLE IN PRESSS. Pal et al. / Applied Radiation and Isotopes 64 (2006) 1521–1527 1523

column. After loading, the resin column was equilibratedwith 6M HCl. The Fe(III) solution containing the radio-tracers of yttrium in 6M HCl medium, as obtained above,was loaded onto the column and eluted with 10ml of 6MHCl. The flow rate during elution was maintained at 4–5dropsmin�1. During elution, the activity of the eluate wasmonitored by g-spectroscopy after each 20 drops (�1ml)and was found to contain only Y-radioistopes in a no-carrier-added form without any Sr or Rb isotope activities.The elution was continued until no radioyttrium was foundin the eluate. The eluted solutions, containing radio-yttrium, were pooled together and dried under a heatinglamp and finally dissolved in 1ml of 2M HCl for use in thetracer studies. The inactive Fe(III) carrier, remaining in theanion-exchange column, was eluted with 2M HCl to checkthe residual g-activities of any of the radionuclides.

2.5. Separation of Rb from Sr

To separate Rb from Sr radioisotopes in the filtrate,saved after co-precipitation of Y-radioisotopes withFe(III), Dowex cation exchange resin in HBr mediumwas used. Before loading into the ion exchange column, theammonical filtrate solution was acidified with a few dropsof 6M HCl and dried under an infra red lamp to removeHCl. The residual mass was then converted into bromideby repeated heating with conc. HBr acid (49%; 9M) andfinally the residue was reconstituted in 2ml of 9M HBr.A Dowex 50� 4 resin (Aldrich Chemical Co., Inc., USA),400 mesh, was thoroughly washed with water andequilibrated with 9M HBr before loading into the column[45mm (l)� 6mm (dia)]. The solution, containing Rb andSr radioisotopes, was loaded onto the column and elutedwith 15ml of 9M HBr (flow rate: 4–5 dropsmin�1) tocollect the Rb radioisotopes in no-carrier-added form.After separation of Rb, the column was washed with 10mlof 4M HBr solution to elute Sr from the column in no-carrier-added form. During elution processes, g-spectro-scopy was used to monitor the separation of Rb and Srradioisotopes through their activity measurements. Afterseparation of Rb and Sr isotopes from each other, theeluted solutions of Rb and Sr tracers were pooled,respectively, dried under a heating lamp and dissolved in2M HCl to be used for further tracer studies.

The flow sheet of the full radiochemical separationprocedure developed is presented in Fig. 1.

3. Results and discussion

Prior to carrying out the heavy-ion irradiation experi-ment, the feasibility of the production of relevant radio-nuclides of Y, Sr and Rb from natural Ge by 16O6+-induced nuclear reactions was calculated theoreticallyusing the Monte Carlo simulation computer code PACE

2 (Gavron, 1980) with the assumption of compoundnucleus formation. This theoretical computation revealedthat all the natural isotopes of germanium [70Ge(21.23%),

72Ge(27.66%), 73Ge(7.73%), 74Ge(35.94%) and 76Ge(7.44%)]would simultaneously produce a large number of radioactivenuclides of Y [80,81,82,83,84,85,86,87,87m,88Y], Sr [79,80,81,82,83,85Sr]and Rb [77,78,79,80,81,82,83,84Rb] with favorable reaction crosssections through 115MeV 16O6+ heavy-ion induced nuclearreactions via different production routes like (16O, xn) or(16O, xpyn). Therefore, during the actual irradiation experi-ment, the beam energy of 16O6+ heavy-ion projectiles wasselected at 115MeV for irradiation of the germanium target.The nuclear characteristics of some of the useful radio-nuclides of Y, Sr and Rb that might be produced fromnatural Ge target by 16O-induced heavy-ion nuclear reactionsare shown in Table 1 (Reus and Westmeier, 1983). Theproduction of all these radioisotopes were detected in thepresent study.To confirm the production of Y, Sr and Rb radio-

nuclides in the 16O6+-irradiated germanium matrix, it isnecessary to remove the activity produced in the matrixduring irradiation, prior to performing g-ray spectroscopy.Natural germanium metal was not used as an irradiationtarget because its dissolution in halogen acids is difficult.Instead, natural GeO2 was used as target material due to itsrapid solubility in alkali. For the separation of germanium,sulfide precipitation and co-precipitation with hydroxidesof Group III elements are well-known techniques(Mirzadeh and Lambrecht, 1996) but in the presentseparation procedure acid hydrolysis of GeO2 in presenceof conc. HNO3 (Mirzadeh and Lambrecht, 1996) followedby centrifugation was found to be a quick and effectivemethod to isolate radioisotopes of interest from the bulkmatrix activities. The g-spectra of the solution, obtainedafter removal of germanium matrix activity, indicate thepresence of the different photopeaks of the radioisotopes ofY, Sr and Rb (as shown in Table 1). Production of theseindividual radioisotopes, from natural germanium throughvarious 16O6+ heavy-ion induced nuclear reaction chan-nels, was confirmed through their characteristic g-energiesand half-lives.

3.1. Separation of Y from Sr and Rb

The separation of the Y radionuclides from those of Srand Rb was performed by co-precipitation on inactive Fe(III)-carrier (Rosch et al., 1993a; Finn et al., 1999). Theion-exchange chromatography procedure, known as one ofthe best methods to separate radioactive tracers in carrierfree form (Stevenson and Nervik, 1961), was adopted toisolate Y from Fe (III)-carrier as well as Rb from Sr. Thedifferent adsorption behavior pattern of Fe and rare earthin HCl-Dowex anion exchange resin system at 6M acidconcentration was applied in this work to separateY-radionuclides from inactive Fe (Kraus and Nelson,1958; Stevenson and Nervik, 1961). The solution contain-ing Y and inactive Fe(III)-carrier in 6M HCl medium,when passed through Dowex 2� 8 anion exchange resin,the column retained Fe and released Y in no-carrier-addedform. The g-ray spectra of the no-carrier-added solution of

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Fig. 1. Flow sheet diagram of the separation procedure for no-carrier-added radioisotopes of Y, Sr and Rb from 16O6+ heavy-ion irradiated GeO2 target.

Table 1

Nuclear characteristics of Y, Sr and Rb radionuclides produced by 16O6+

heavy-ion irradiation on natural Ge target

Radionuclide

produced

Half-life Principal g-energy,keV (intensity, %)

86Y 14.74 h 1076.6(82.5);

627.7(32.6); 1153.0

(30.5)87mY 12.9 h 381.1 (78.1)87Y 3.35 d 484.9 (92.2); 388.4

(84.8)88Y 106.6 d 898.0(94.0); 1836.0

(99.4)

83Sr 32.4 h 762.7 (29.7); 381.5

(17.3)85Sr 64.84 d 514.0 (99.3)

83Rb 86.2 d 520.4 (46.1); 529.5

(30.0); 552.5 (16.3)84Rb 32.87 d 881.6 (67.8)

S. Pal et al. / Applied Radiation and Isotopes 64 (2006) 1521–15271524

Y- isotopes (Fig. 2) indicate the presence of all the g-lines ofradioisotopes of Y (86,87,88Y) and absence of Rb and Srradionuclides. The g-ray spectra of ammonical filtratesolution, obtained after separation of Y from Sr/Rb by Fe-hydroxide co-precipitation, show that the filtrate is freefrom Y-radioisotopes but contains all the radioisotopes of83Rb and 83,85Sr. The g-counting of Fe(III)-chloridesolution, obtained by eluting the anion exchange columnwith 2M HCl, after Y-separation, has indicated no residualg-activities of either of these radionuclides and that provescomplete separation of Y from Rb/Sr activities.

3.2. Separation of Rb from Sr

The ion-exchange separation of Rb and Sr from eachother was carried out with Dowex 50� 4 cation exchangeresin in 9M HBr medium. In presence of higher molarity ofHBr, adsorbability of Sr is increased in the Dowex 50� 4ion-exchange resin column compared to Rb but at lower

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300 350 400 450 500 550 600

Cou

nts

per

Cha

nnel

0

1000

2000

3000

4000

5000

6000

7000

γ-Energy (keV) 610 710 810 910 1010 1110 1210

0

50

100

150

200

250

86Y

[10

76.6

keV

]

86Y

[62

7.7

keV

]

88Y

[89

8.0

keV

]

86Y

[11

53.0

keV

]

87Y

[38

8.4

keV

]

87Y

[48

4.9

keV

]

Fig. 2. Partial g-ray spectra showing the photopeaks of Y radioisotopes after separation from Sr and Rb radioisotopes by ion-exchange chromatography.

480 500 520 540 560 580

0

Sr -radioisotopes

γ-Energy (keV)

480 500 520 540 560 580

Cou

nts

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Cha

nnel

100

200

300

400

500

100

200

300

400

500 Rb-radioisotopes

85Sr

[511

.0 k

eV]

[51

1 ke

V]

83R

b[529

.5 k

eV]

[520

.4 k

eV]

83R

b

83R

b

[514

keV

]

[552

.3 k

eV]

(i) (ii)

Fig. 3. Partial g-ray spectra showing the photopeaks of (i) Rb and (ii) Sr radioisotopes after separation by ion-exchange chromatography in HBr medium.

S. Pal et al. / Applied Radiation and Isotopes 64 (2006) 1521–1527 1525

molarity of HBr the adsorption of Sr decreases in the sameresin column (Nelson and Michelson, 1966). It is reportedby Nelson and Michelson (1966) that adsorbability of Rbdecreases with increasing molarity of HBr in the sameDowex 50� 4 resin. This adsorption behavior of Rb/Srtowards Dowex 50� 4 resin in different molarity of HBracid was exploited in the present experiment to separatethese two radionuclides from each other in no-carrier-added form. It was observed during our experiment that in9M HBr medium, Sr is totally adsorbed in the resincolumn allowing Rb to be released and subsequently Sr iseluted 4M HBr. The individual g-spectra of Rb and Srradioisotopes, as shown in Fig. 3(i) and (ii), respectively,confirm clearly the separation of Rb and Sr radionuclidesfrom each other.

3.3. Application of heavy-ion produced no-carrier-added

radiotracers

The impetus for the present experimental study was notonly to provide a method for simultaneous production andradiochemical separation of no-carrier-added radiotracersof Y, Rb and Sr from an irradiated Ge-matrix but also touse these radioisotopes to standardize a separation

procedure that could be adopted in future for routineproduction of 86Y from an enriched 86Sr target. This is animportant radionuclide for PET imaging which is generallyproduced by proton irradiation of enriched 86Sr (as86SrCO3) using the well established 86Sr(p, n)86Y nuclearreaction (Rosch et al., 1993a, b; Finn et al., 1999; Ketternet al., 2002; Reischl et al., 2002; Garmestani et al., 2002).The radiochemical separation technique of radioyttriumfrom the Sr-target system has to be designed in such amanner that the no-carrier-added 86Y is separated not onlyfrom the target material but also from the 83Rb and 85Srradioisotopes which might be co-produced as impuritiesduring proton activation of Sr at high incident energies(Rosch et al., 1993a, b). Furthermore, the recovery of theexpensive enriched 86SrCO3-target material after irradia-tion and 86Y isolation is also an important criterion of theradiochemical separation procedure.In view of this, the no-carrier-added radionuclides of Y,

Sr and Rb, produced by the heavy-ion irradiation ofnatural GeO2, were added to about 200mg of naturalSrCO3 to design a radiochemical separation method toisolate Y-radionuclides in no-carrier-added form as well asto study the recovery of Sr. A combination of co-precipitation with iron(III) and anion-exchange technique

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Fig. 4. Flow sheet diagram of the radiochemical separation of Y radioisotopes from SrCO3.

γ - Energy (keV)500 520 540 560 580 600

Cou

nts

per

Cha

nnel

0

750

1500

2250

3000 85Sr

[51

1 ke

V]

83R

b

83R

b

83R

b[5

14 k

eV]

[520

.4 k

eV]

[529

.5 k

eV]

[552

.3 k

eV]

Fig. 5. Partial g-ray spectra showing the presence of Rb radioisotopes

after recovery of Sr as SrCO3.

S. Pal et al. / Applied Radiation and Isotopes 64 (2006) 1521–15271526

was used to isolate Y-radioisotopes from SrCO3. Afterseparation of the Y-radioisotopes, the Sr was recovered byprecipitation as the carbonate with (NH4)2CO3 solution inalkaline medium. The flow sheet of the radiochemicalseparation procedure is given in Fig. 4.

The g-spectrometry of the Y-radionuclides, separated fromSrCO3, has shown a complete separation of no-carrier-addedY with a yield of about 88% and free from any Rb and Sractivities. The Sr was recovered almost in its originalchemical form by precipitation as carbonate. By applyingradiometric, as well as gravimetric methods, the recoveryyield of SrCO3 was found to be about 85%. However, theRb isotopes were found to accompany the SrCO3 precipitateas seen from the g-spectrometric results (Fig. 5).

In actual production of 86Y through proton irradiation,recovered enriched 86SrCO3 target material would contain83Rb produced by 86Sr(p, a)83Rb reaction with a half-life of86.2 d (Rosch et al., 1993a,b). Instead of allowing therecovered 86Sr-target material to cool for days to decay83Rb activities to negligible level prior to its reuse, Rbcould be radiochemically separated from 86Sr by adoptingthe developed separation method as shown in the Fig. 1.

4. Conclusion

In the present work, heavy-ion activation of GeO2 with16O6+ projectiles has been found to be useful to produce Y,Sr and Rb radionuclides. The developed radiochemicalseparation technique is suitable to isolate these radio-isotopes in no-carrier-added state with high radiochemicalpurity from the heavy-ion irradiated matrix elements. Theheavy-ion bombardment are not used for producingradionuclides in sufficient quantities for radiopharmaceu-tical applications due to low radionuclide yield. However,through the present experimental study the heavy-ionproduced Y, Sr and Rb radioisotopes were successfully

applied for standardizing a radiochemical separationtechnique that could be used for the production of the86Y radioisotope from enriched 86SrCO3 target by protonbombardment as well as for post-irradiation recovery ofexpensive enriched 86SrCO3 target material.

Acknowledgments

The authors gratefully acknowledge Dr. Bikash Sinha,Director, Variable Energy Cyclotron Center (VECC),Kolkata, India for his support and the operational staffof Cyclotron for their cooperation during heavy-ionirradiation experiments.

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