Many governments and international organizations are concerned about high activity source security and provide institutional support for the adoption and development of non-radioisotopic alternatives. The most common substitution for radioactive sources is x-ray tubes and electron accelerators, some of which can operate in e-beam mode and x-ray mode. The main advantages of these alternative technologies is their ability to be turned on and off for safety and their minimal security risks. These machines cannot produce residual radioactivity, and therefore do not result in radioactive waste. Historically, such accelerators were complex and rather expensive. However, lately, numerous models have been developed for various applications, and many of them are compact, versatile, and much more reliable.
One of the most widespread sources of x-rays is the x-ray tube - a simple tool, developed over a century ago, but still widely used in medicine, industry, and science. An x-ray tube consists of a cathode, which emits electrons. Accelerated by an electric field, they bombard the metal anode. As the incident beam of electrons pass through the anode, they are primarily slowed down by the Coulomb field surrounding nuclei. The difference between the electron's initial energy and its final energy is emitted in the form of photons, also known as bremsstrahlung radiation. The bremsstrahlung energy spectrum of photons is continuous, ranging from nearly zero up to the applied voltage. X-ray tubes are simple, compact, and robust; however, their energy output is limited to a few hundred keV, which is less than the energy of gamma rays emitted by cesium-137 and cobalt-60.
Electron accelerators also use electron guns (cathodes) to produce a collimated electron beam, which is accelerated with either DC voltage or RF cavities. Large machines can accelerate electrons to hundreds of GeV, however for industrial purposes, electrons' energy should not exceed 10 MeV and should be even less for some applications. This energy can be achieved with rather compact units. High energy electrons can be used "as is", which is suitable for irradiation of low density or thin products. Alternatively, they can be converted into x-rays using a converter - a thin plate of high Z material, such as tungsten or tantalum. The resulting x-rays are highly penetrable, but the efficiency of the conversion process does not exceed more than a few percent. The mean energy of bremsstrahlung photons can be roughly estimated as one third of the electron beam energy. Thus, a 9 MeV electron beam results in mean photon energy to be about 3 MeV, and a 4 MeV electron beam will be "gamma-energy-equivalent" to the cobalt-60 source.
To provide high dose rates required for high product throughput, one would need reliable accelerators delivering both high energy (as close to 10 MeV as local regulations allow) and high power (tens of kW) beams. Only a handful of companies in the world produce such machines, and they are still relatively complex and expensive. To replace industrial cobalt irradiators, more than one accelerator should be installed in each facility – both to provide the throughput and to offer backup in case of maintenance or shutdown.
