Excitation functions of 85Rb(p,xn)85m,g,83,82,81Sr reactions up to 100 MeV: integral tests of cross section data, comparison of production routes of 83Sr and thick target yield of 82Sr

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Abstract

The β+ emitter 83Sr (T1/2=32.4 h, Eβ+=1.23 MeV, Iβ+=24%) is a potentially useful radionuclide for therapy planning prior to the use of the β emitter 89Sr (T1/2=50.5 d). In order to investigate its production possibility, cross section measurements on the 85Rb(p,xn)-reactions, leading to the formation of the isotopes 85m,gSr, 83Sr, 82Sr and 81Sr, were carried out using the stacked-foil technique. In a few cases, the products were separated via high-performance liquid chromatography. For 82Sr, both γ-ray and X-ray spectrometry were applied; in other cases only γ-ray spectrometry was used. From the measured excitation functions, the expected yields were calculated. For the energy range Ep=37→30 MeV the 83Sr yield amounts to 160 MBq/μA h and the level of the 85gSr (T1/2=64.9 d) and 82Sr (T1/2=25.5 d) impurities to <0.25%. In integral tests involving yield measurements radiostrontium was chemically separated and its radioactivity determined. The experimental production data agreed within 10% with those deduced from the excitation functions. The results of the 85Rb(p,3n)83Sr reaction were compared with the data on the production of 83Sr via the 82Kr(3He,2n)-process. In the energy range E3He=18→10 MeV the theoretical yield of 83Sr amounts to 5 MBq/μA h and the 82Sr impurity to about 0.2%. The method of choice for the production of 83Sr is thus the 85Rb(p,3n)-process, provided a 40 MeV cyclotron is available. During this study some supplementary information on the yield and purity of 82Sr was also obtained.

Introduction

In palliative therapy, radionuclides that deposit in the structure of bone, i.e. hydroxylapatite Ca10(PO4)6(OH)2, are often used. As strontium is a substituent for calcium, the longer-lived β emitter 89Sr (T1/2=50.55 d, Eβ=1.5 MeV, Iβ=100%) appears to be very suitable (cf. Lewington, 1993). However, since corpuscular radiation cannot be measured from outside the body, there is a lack of biodistribution data. The radiation dose given to the patient is therefore estimated rather empirically. A shorter-lived positron emitting analogue of 89Sr, namely 83Sr (T1/2=32.4 h, Eβ+=1.23 MeV, Iβ+=24%), offers the possibility of quantitative investigation of biokinetics via positron emission tomography (PET), allowing thereby a better estimation of the dose.

The radionuclide 83Sr has been produced earlier at Jülich via the 82Kr(3He,2n)-reaction. Besides cross section measurements (cf. Tárkányi et al., 1988), a high current gas target system was developed (Blessing et al., 1997) which was used to produce 83Sr in amounts of about 100 MBq (Rösch et al., 1996). In the present work, the possibility of production of 83Sr via the 85Rb(p,3n)-reaction was investigated and compared with the known production method 82Kr(3He,2n)83Sr. For this purpose, extensive nuclear data measurements on the 85Rb(p,xn)-reactions, leading to the formation of the isotopes 85m,gSr, 83Sr, 82Sr and 81Sr, were carried out. The decay data used in those investigations are given in Table 1 (cf. Browne and Firestone, 1986). The cross section and yield results are of interest not only for the production of the new β+ emitter 83Sr but also for 82Sr which is commonly used for the preparation of the 82Sr/82Rb generator system.

Section snippets

Experimental

Excitation functions were measured by the well-known stacked-foil technique (cf. Qaim et al., 1977; Piel et al., 1992; Hohn et al., 2001). Some of the salient features relevant to the present work are discussed below.

Cross section data

The cross sections of the 85Rb(p,xn)85m,g,83,82,81Sr reactions were measured from their respective thresholds up to 100 MeV. The data and the estimated uncertainties are given in Table 2. All values were extrapolated to 100% enrichment of the target nucleus 85Rb. The results for 85gSr describe cumulative cross sections, i.e. a sum of metastable and ground states (σm and σg).

As mentioned above, 85gSr was assayed using the 514 keV γ-ray and there could be a strong interference by the 511 keV

Acknowledgements

This work was partly carried out under a German-South African bilateral research agreement. We thank the operators of the four accelerators (CV 28 and Injector of COSY at Jülich, accelerator at Villigen, Switzerland, and the Cyclotron in Faure, South Africa) for performing the irradiations. We appreciate the technical assistance of Mr. S. Spellerberg.

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