GALLIUM-67

Uses

Gallium behaves in the body in a similar way to ferric iron. It is commonly used as a trivalent citrate compound for nuclear medicine imaging, and is a valuable agent in the detection and localization of certain neoplasms and inflammatory lesions.

2.19. GALLIUM-67

Decay mode

Gallium-67 decays to stable ⁶⁷Zn by electron capture. Its decay emissions include gamma rays of 93.3 keV (37.0%), 184.6 keV (20.4%) and 300.2 keV (16.6%).

Electron emission products of ⁶⁷Ga

Photon emission products of ⁶⁷Ga

Nuclear reactions

Gallium-67 is commonly produced by using enriched ⁶⁸Zn targets through the ⁶⁸Zn(p, 2n)⁶⁷Ga nuclear reaction [2.19.1].

Fraction Energy (MeV)

0.010853 0.093175

0.032844 0.092117

0.268110 0.083652

0.602060 0.007530

1.648900 0.000990

Fraction Energy (MeV)

0.022420 0.208950

0.028560 0.091266

0.044768 0.393530

0.065838 0.009570

0.159940 0.300220

0.164720 0.008616

0.197060 0.184580

0.322970 0.008639

0.357000 0.093311

Excitation functions

The excitation functions for ⁶⁷Zn(p, n)⁶⁷Ga and ⁶⁸Zn(p, 2n)⁶⁷Ga are shown in Figs 2.19.1 and 2.19.2, respectively.

Target material

The target material is enriched ⁶⁸Zn.

Target preparation

The enriched ⁶⁸Zn may be pressed or electroplated onto a copper plate.

Electroplating may be accomplished using a constant current; the electrolyte is ZnCl₂ solution (with pH5–6), and a small amount of hydrazine hydrate (NH₂NH₂) is added as a depolarizer [2.19.2]. The electroplated material should have a shiny metallic coloration, evenly distributed with no obvious dendrite formation and well adhered to the surface.

In a typical use of a powder target, a zinc target can be prepared by pressing metal powder uniaxially at 523 MPa under vacuum by means of a

Cross-section (mb)

FIG. 2.19.1. Excitation function for the ⁶⁷Zn(p, n)⁶⁷Ga reaction.

2.19. GALLIUM-67

suitable punch and die set, machined from a high carbon and high chromium steel used for tools. The metal disc, of 95% theoretical maximum density, is annealed/sintered in an oven for 0.5 h at 400°C.

Processing

There are several methods for separation of ⁶⁷Ga from a zinc target. Two commonly used methods entail ion exchange separation and solvent/solvent extraction.

Ion exchange method

After irradiation, the ⁶⁸Zn target is dissolved in concentrated HCl, followed by separation of ⁶⁷Ga from ⁶⁸Zn using a Dowex 50W-X8 resin. Target solution is loaded on the Dowex column, which has been previously conditioned with 10M HCl. Continued elution with 10M HCl removes the enriched ⁶⁸Zn (saved for later processing for recovery of enriched material).

Gallium-67 is eluted with 3.5M HCl, and the solution is evaporated to near dryness. A few drops of 30% hydrogen peroxide are added, and the solution is

Cross-section (mb)

FIG. 2.19.2. Excitation function for the ⁶⁸Zn(p, 2n)⁶⁷Ga reaction.

then evaporated very carefully to dryness. Gallium-67 is recovered as gallium citrate by dissolving the residue in a solution of 2–4% sodium citrate solution.

Solvent/solvent extraction method

Irradiated ⁶⁸Zn target is dissolved in 7.5M HCl, followed by extraction of

67Ga in di-isopropyl ether (DIPE). Zinc-68 remains in the acidic aqueous layer.

After three repeated extractions of ⁶⁷Ga with DIPE, and scrubbing of the organic layer with 7.5M HCl to remove residual ⁶⁸Zn, ⁶⁷Ga is back-extracted from DIPE with a small volume of sterile water. Extraction is repeated twice to recover all the ⁶⁷Ga. To the solution containing ⁶⁷Ga is added 0.5 mL of 2.5%

sodium citrate, which is then carefully evaporated to dryness. The residue is dissolved in saline containing a sufficient amount of sodium citrate for manufacture of radiopharmaceuticals.

Enriched material recovery

The recommended method for recovery of the enriched material is electrochemical separation [2.19.3].

In the wet chemical method, the acidic aqueous layer containing ⁶⁸Zn from target processing is allowed to decay for some time and is pooled together from multiple targets. Zinc-68 is precipitated as zinc sulphide with Na₂S solution, followed by dissolution in concentrated HCl. Zinc-68 is also precipi-tated from acid solution as zinc hydroxide by adding sufficient NaOH to attain a basic pH (8–10). Controlled drying realizes ⁶⁸Zn as an oxide recycled to prepare additional targets.

Specifications

The radionuclidic impurity levels in the separated gallium citrate depend upon the radioisotopic enrichment of the target material and the efficiency of the separation method. Co-production of high energy ⁶⁶Ga (through the (p, 2n) reaction) should also be taken into account while processing the irradiated target. Sufficient time must be allowed post-irradiation for the short lived gallium isotopes to decay to an acceptable level (pharmacopoeia standard).

The final ⁶⁷Ga products must have low levels of contaminants that are within acceptable human toxicity and pharmacopoeial limits.

2.19. GALLIUM-67

It is essential to ensure through the chromatographic method a lower than acceptable level of free ⁶⁷Ga (<5%), and to assure more than 95% as gallium citrate.

REFERENCES TO SECTION 2.19

[2.19.1] INTERNATIONAL ATOMIC ENERGY AGENCY, Charged Particle Cross-section Database for Medical Radioisotope Production: Diagnostic Radioisotopes and Monitor Reactions, IAEA-TECDOC-1211, IAEA, Vienna (2001).

[2.19.2] VAN DEN WINKEL, P., et al., Vrije Universiteit Brussel, Brussels, personal communication, 1995.

[2.19.3] INTERNATIONAL ATOMIC ENERGY AGENCY, Standardized High Current Solid Targets for Cyclotron Production of Diagnostic and Therapeutic Radionuclides, Technical Reports Series No. 432, IAEA, Vienna (2005) CD-ROM.

BIBLIOGRAPHY TO SECTION 2.19

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SZELECSÉNYI, F., BOOTHE, T.E., TAKÁCS, S., TÁRKÁNYI, F., TAVANO, E., Evaluated cross section and thick target yield data bases of Zn + p processes for practical applications, Appl. Radiat. Isot. 49 (1998) 1005–1032.

SZELECSÉNYI, F., BOOTHE, T.E., TAVANO, E., PLITNIKAS, M.E., TÁRKÁNYI, F., Compilation of cross sections/thick target yields for ⁶⁶Ga, ⁶⁷Ga and

68Ga production using Zn targets up to 30 MeV proton energy, Appl. Radiat. Isot. 45 (1994) 473–500.

TAKÁCS, F., TÁRKÁNYI, F., HERMANNE, A., Institute of Nuclear Research of the Hungarian Academy of Sciences, personal communication, 2005.

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