X-rays are considered ionising radiation because they have enough energy to ionise atoms and molecules, i.e., to remove electrons from them. This ionising capability arises due to their high energy and short wavelength. The implications of X-rays being ionising are significant, especially in terms of biological effects. When X-rays interact with biological tissue, they can cause ionisation of the molecules within the cells. This can lead to direct damage to the DNA and other critical cellular structures, which can cause mutations and potentially lead to cancer. Therefore, while X-rays are invaluable in medical imaging and treatments, their use must be carefully controlled. Appropriate shielding, limiting exposure time, and using the lowest effective dose are essential for minimising the risks associated with their ionising nature. It's also why safety protocols and regulations are stringently applied in both medical and industrial contexts where X-rays are used.

The accelerating voltage in an X-ray tube significantly influences the properties of the X-rays produced. Higher accelerating voltages increase the kinetic energy of the electrons travelling from the cathode to the anode. This increased energy results in X-rays with higher frequencies and shorter wavelengths, which are more penetrating. Specifically, a higher voltage decreases the minimum wavelength of the X-ray spectrum produced (as per the relationship lambda_min = hc / (eV)), leading to a 'harder' (more penetrating) X-ray spectrum. Additionally, higher voltages generally increase the intensity of the X-ray beam. However, it's important to note that excessively high voltages can lead to increased patient dose in medical applications and may require more stringent shielding measures. Hence, the choice of voltage must balance the need for penetrating power with safety and image quality considerations.

X-rays cannot be focused using traditional optical lenses like visible light. This is due to the fact that X-rays have much shorter wavelengths compared to visible light. When X-rays encounter materials used for traditional lenses, such as glass or plastic, they are either absorbed or pass through without significant refraction. To 'focus' or manipulate X-rays, different techniques are employed. One common method is the use of X-ray mirrors, which operate under the principle of total external reflection at very shallow angles, known as grazing incidence. These mirrors can focus X-rays but require very smooth surfaces and precise angles. Another method involves the use of crystal diffractors, which exploit the diffraction of X-rays through the crystal lattice to focus or redirect them. These methods are more complex and limited compared to the focusing of visible light and are predominantly used in specialised applications like X-ray crystallography and high-resolution X-ray imaging systems.

The vacuum inside an X-ray tube plays a crucial role in the efficient generation of X-rays. In the absence of a vacuum, the free electrons emitted from the cathode would collide with air molecules, leading to unnecessary energy loss and reduced kinetic energy upon reaching the anode. This would result in a decrease in the efficiency of X-ray production. Additionally, air molecules could become ionised and produce secondary electrons, leading to unwanted background radiation and reducing the clarity of the X-rays produced. A vacuum ensures a clear path for the electrons to travel from the cathode to the anode without interference, maximising the transfer of kinetic energy into X-ray photons upon collision with the target. Furthermore, the vacuum prevents oxidation and degradation of the internal components of the tube, particularly the cathode and anode, thereby extending the lifespan of the tube.

Tungsten is commonly used as the target material in X-ray tubes due to several of its advantageous properties. Firstly, it has a high atomic number (74), which increases the efficiency of X-ray production, particularly characteristic X-rays, as higher atomic number materials emit X-rays more efficiently. Secondly, tungsten has a high melting point (approximately 3422°C), making it capable of withstanding the intense heat generated during X-ray production. This is crucial since a significant portion of the kinetic energy of the electrons is converted into heat. Additionally, tungsten's thermal conductivity is relatively high, which helps in dissipating this heat effectively. Its low vapour pressure also minimises the risk of the target material evaporating and contaminating the vacuum inside the tube. These properties make tungsten an ideal candidate for producing high-quality X-rays while ensuring the longevity and reliability of the X-ray tube.