If the proton energy is >30 MeV, the particles tend to be relativistic, i.e., their mass and their cycle time in orbit increase. A constant frequency of the accelerating electric field would cause the ions to come out of phase. This can be compensated for either by increasing the magnetic field as a function of the cyclotron radius (isochronic cyclotrons) or by decreasing the radiofrequency during acceleration (synchrocyclotrons). Such accelerators tend to be more complex and expensive and, for this reason, 30 MeV is a typical energy for commercial accelerators that need to have large beam currents and to be both reliable and cost-effective.
Commercial accelerators usually run beam currents of several milliamperes. Since it is technically difficult to extract such high beam currents due to heating problems in the separating foils, most commercial accelerators use internal targets. Many patients in nuclear medicine undergo single photon emission computed tomography (SPECT) investigations. Besides reactor-produced \(^{99}\text{m} \text{Tc}\), commercial cyclotrons commonly produce \(^{67}\text{Ga}\), \(^{111}\text{In}\), \(^{123}\text{I}\), and \(^{201}\text{Tl}\). In addition, some PET radionuclides, such as \(^{124}\text{I}\), are becoming commercially available. Increasing demand for the \(^{68}\text{Ge}/^{68}\text{Ga}\) generator has also led to commercial production of the cyclotron-produced mother nuclide \(^{68}\text{Ge}\).
Only a few radionuclides of medical interest require production energies above 30 MeV. A limited number of high energy accelerators with high beam currents, usually at national physics laboratories, have the capacity for the production of, for example, \(^{52}\text{Fe}\) and \(^{61}\text{Cu}\) and other isotopes used for research activities.
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