Elsevier

Applied Radiation and Isotopes

Volume 49, Issue 12, 1 December 1998, Pages 1537-1540
Applied Radiation and Isotopes

Radiochemistry and radionuclide applications (AER)
Production of 76Br by a low-energy cyclotron

https://doi.org/10.1016/S0969-8043(97)10129-4Get rights and content

Abstract

We present a production method of a positron-emitting bromine isotope 76Br (T1/2=16.2 h) using a low-energy proton reaction 76Se(p, n)76Br. A pressed copper (I) selenide pellet containing enriched 76Se is used as a target. Irradiation of a 180 mg/cm2 target with 16→8 MeV protons gives a yield of 65–70 MBq/μAh EOB. In our target set-up with a simple water cooling, the target can withstand 14 μA. Bromine-76 is separated with an efficiency of 65–75%, within 60–75 min using dry distillation at 1090°C. The loss of target material is less than 1% per separation cycle and the target can be re-used several times. Thermal chromatography is applied to separate 76Br from traces of volatilized selenium. The method allows a simple and economical production of 76Br with the use of accelerators commonly available at PET-centers.

Introduction

Positron emission tomography (PET) is a powerful diagnostic tool, which provides superior spatial resolution and an opportunity to obtain quantitative information concerning distribution of radioactivity in vivo. So far, this method has mainly been associated with short-lived positron emitters with half-lives less then 2 h, e.g. 11C and 18F. However, to follow biomedical processes of slower kinetics, positron emitters with longer half-lives are needed. We are developing production techniques of several non-conventional positron emitters at low-energy cyclotrons (Lundqvist et al., 1995; Tolmachev and Lundqvist, 1996; Tolmachev et al., 1997a, Tolmachev et al., 1997b). Here we present the development of a routine production method for 76Br.

76Br has a fairly high abundance of positron emission (54%) and a half-life of 16.2 h which is long enough to follow the kinetics of e.g. antibody distribution during two full days (Lövqvist et al., 1997). The labelling chemistry of bromine is similar to that of iodine, and is relatively well studied (Maziere and Loc'h, 1985). Due to the scarcity of “cold” bromine sources in the laboratory environment, 76Br might be obtained at a high specific activity, which makes it an attractive label for receptor ligands (see e.g. Martinot et al., 1991; Martinot et al., 1994). The high energies of emitted positrons (maximum energies of 3.38 MeV and 3.94 MeV in 26% and 6% of the decays, respectively) may effect the PET measurements. However, in a separate study Lövqvist et al. (1996) showed only a minor increase of the spatial resolution (full width of half maximum) using 76Br in comparison with 18F. The resolution of the tomograph used (Scanditronix GE 4096+, which is a 2D tomograph with septa) was predominantly determined by the size of the BGO crystals. The impact of scattered radiation from high-energy gamma was found to be small, too. A somewhat larger region of interest had to be used to account for the total radioactivity of a point source using 76Br compared with 18F. This was attributed to a fraction of high energy positrons that were distributed in an area around the bulk of events determining the spatial resolution.

76Br was produced earlier via the nuclear reactions:Br(p, xn)76Kr76Br(Scott-Robson et al., 1991; De Jong et al., 1979) or75As(3He, 2n)→ 76Br(Loc'h et al., 1994).

The use of both methods have been limited since high-energy protons (more than 50 MeV) and 3He particles are not readily available in the PET community. However, as indicated by data published by Kovàcs et al., 1985, one way to produce 76Br by low-energy cyclotrons available at most PET centers is to use the 76Se(p, n)76Br reaction (proton energy range 16→8 MeV). Unfortunately, this production route requires the use of expensive 76Se-enriched selenium target material. Dry distillation techniques, that leave the target material almost intact might then be more cost effective and practical than wet chemical separation and recovery methods. Several methods for dry distillation of bromine isotopes from selenium-containing targets are described in the literature. Janssen et al. (1980)used an enriched target with the composition of Na2SeO3.88·0.38Na20 for the production of 77Br. Dry distillation methods were applied for the production of 75Br by Kovàcs et al. (1985) and Vaalburg et al. (1985). They used enriched targets of metallic selenium and metal selenides, respectively. The last method gave the smallest losses of target material and was, with some modifications, applied in the development of a routine production method of 76Br using a low-energy proton reaction.

Section snippets

Target material

76Se-enriched Cu2Se was prepared by mixing 301 mg 76Se metal powder enriched up to 96.5% (Stabis, Moscow, Russia) with 492 mg Cu metal powder (Mallinkrodt, Germany) in a quartz ampoule. The ampoule was evacuated, sealed, and placed in an oven. The temperature in the oven was kept at 417°C during 4 days. The formed copper selenide was removed from the ampoule after cooling. It was pulverized and placed in another evacuated, sealed ampoule, and kept at 533°C for 21 h, at 418°C for 6 days, and at 125°C

Results and discussion

A stream of He during the irradiation was necessary to prevent the oxidation of the target material and thus loss of target material. At beam currents higher than 14 μA (current density >28 μA/cm2) the target material started to get somewhat affected. Occasionally hot spots in the beam could partly melt the target. Usually production runs for 1–2 h were made late in the evening and the separations in the morning in order to let co-produced 63Zn decay. The targets were fetched manually and

Conclusions

The 76Se(p, n)76Br reaction using a 76Se-enriched Cu2Se target enables a simple and cost-effective routine production of 76Br in a low-energy cyclotron. In the present study, the application of thermal chromatography in conjunction with dry distillation allowed the separation of selenium-free 76Br from a Cu2Se target enriched in 76Se. The target remains intact with only minor loss of material (<1% per run) and is directly ready for the next irradiation.

Acknowledgements

This work was supported by the Swedish Medical Research Council (project No B95-14P-09822-04A) and the Medical faculty, Uppsala University.

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