high-density (such as tungsten) target. the electrons or x-rays can be used to treat both benign and malignant disease. the linac produces a reliable, flexible and accurate radiation beam. the versatility of linac is a potential advantage over cobalt therapy as a treatment tool. in addition, the device can simply be powered off when not in use; there is no source requiring heavy shielding – although the treatment room itself requires considerable shielding of the walls, doors, ceiling etc. to prevent escape of scattered radiation. prolonged use of high powered (>18 mev) machines can induce a significant amount of radiation within the metal parts of the head of the machine after power to the machine has been removed (i.e. they become an active source and the necessary precautions must be observed). aerial view of the little linac model in 2019 a little linac model kit, containing 82 building blocks, was developed for children undergoing radiotherapy treatment for cancer. the hope is that building the model will alleviate some of the stress experienced by the child before undergoing treatment by helping them to understand what the treatment entails. the kit was developed by professor david brettle, institute of physics and engineering in medicine (ipem) in collaboration with manufacturers best-lock ltd. the model can be seen at the science museum, london. application for medical isotope development the expected shortages of mo-99, and the technetium-99m medical isotope obtained from it, have also shed light onto linear accelerator technology to produce mo-99 from non-enriched uranium through neutron bombardment. this would enable the medical isotope industry to manufacture this crucial isotope by a sub-critical process. the aging facilities, for example the chalk river laboratories in ontario, canada, which still now produce most mo-99 from highly enriched uranium could be replaced by this new process. in this way, the sub-critical loading of soluble uranium salts in heavy water with subsequent photo neutron bombardment and extraction of the target product, mo-99, will be achieved. disadvantages the device length limits the locations where one may be placed. a great number of driver devices and their associated power supplies are required, increasing the construction and maintenance expense of this portion. if the walls of the accelerating cavities are made of normally conducting material and the accelerating fields are large, the wall resistivity converts electric energy into heat quickly. on the other hand, superconductors also need constant cooling to keep them below their critical temperature, and the accelerating fields are limited by quenches. therefore, high energy accelerators such as slac, still the longest in the world (in its various generations), are run in short pulses, limiting the average current output and forcing the experimental detectors to handle data coming in short bursts.