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X-rays are developed from the electrons that originate from the outer cloud of an atom. This electron is then converted into highly energised photons when they are energised from a higher energy level to a lower level, releasing excess energy. This occurs because fast electrons decelerate suddenly, and their large amount of kinetic energy is transferred into photons of electromagnetic radiation. This process occurs in an X-ray tube that accelerates the electrons via the potential difference between the electrodes. These are then directed to the material they impact.
The electromagnetic spectrum is the entire range of electromagnetic waves. It is composed of different types of waves with different wavelengths and energy.
The electromagnetic spectrum shows how X-rays are a form of ionising radiation, Wikimedia CommonsX-rays can be categorised into two types: low-energy X-rays and high-energy X-rays.
- Low-energy X-rays emit a low amount of energy when electrons transfer into lower energy levels.
- High-energy X-rays are obtained using a different method, which we discuss below.
We've got plenty more info on X-rays! Check out our explanations: Absorption of X-Rays and X-Ray Image Processing.
What are high-energy X-rays?
High-energy X-rays, also known as HEX-rays, are a type of X-ray that has an energy about one order of magnitude greater than standard X-rays or gamma rays.
High-energy X-rays have a great energy spectrum and are emitted when accelerated electrons pass near their atomic nuclei and are deflected by the electric field.
Atomic nuclei are comprised of positively charged protons and neutrons with a neutral charge. They are held together by a fundamental force known as a strong force. The electrons are located outside of the nuclei in a cloud orbiting the nuclei, and they are held together by an electrostatic force. The positive charge in the nucleus and the negative charge of the orbiting cloud of electrons creates an electric dipole, and an electric field is created as a result.
How are high-energy X-rays formed?
High-energy X-rays are formed by modern machines that produce synchrotron radiation. Synchrotron radiation is electromagnetic radiation emitted when charged particles are accelerated, and the vector of their acceleration is perpendicular to the vector of their velocity. These particles are accelerated using particle accelerators or subjecting electrons to large magnetic fields.
Synchroton radiation (or magnetobremsstrahlung radiation) is the radiation emitted by a charged particle when it travels in a curved trajectory due to strong magnetic fields present in particle accelerators.
Particle accelerators are machines that accelerate charged particles to very high speeds in different directions using powerful electromagnetic fields created by superconducting magnets. This acceleration is confined in circular beams so that the charged particles collide. These collisions of particles produce large amounts of energy. Particle accelerators are used for research in particle physics or to generate high-energy X-rays and gamma rays.
The largest particle accelerator in the world is located at the CERN laboratory (European Organization for Nuclear Research) and is known as the Large Hadron Collider (LHC). It consists of superconducting magnets that accelerate particles to speeds nearly equal to the speed of light. It is the most powerful and largest accelerator with a 27-kilometre circumference!
Machines that generate synchrotron radiation are found in various laboratories, such as at the European Synchrotron Radiation Facility (the ESRF). High-energy X-rays can penetrate deep into matter, which is useful for research in physics, material science, and cancer treatment.
Characteristics of high-energy X-rays
High-energy X-rays formed by synchrotron radiation have a continuous spectrum of photon energies with an energy range between 80keV and 1000keV (kiloelectronvolts). These X-rays have properties that are different to conventional X-rays. Here are some of these characteristics:
- Conventional X-rays have an energy about one order of magnitude lower than high-energy X-rays.
- High-energy X-rays have a low absorption ability.
- High-energy X-rays have a high spatial resolution and high penetrating power.
- The synchrotron radiation emitted by high-energy X-rays is polarised.
- High-energy X-rays can damage living cells due to their high energy.
Functions of high-energy X-rays
Due to their ability to penetrate deep matter, high-energy X-rays are used for
- structural investigations of various types of materials,
- inspection purposes (like cargo inspection for airport security),
- material science imaging (such as spectroscopy, which is a technique that provides information about the composition, stresses, and strain of material),
- industry applications (such as sterilisation of items or food) and
- medical treatment (such as radiation therapy, which is used to shrink cancerous cells).
How are high-energy X-rays detected?
When high-energy X-rays (which have energies much larger than 30keV) are incident on a material, the penetration ability of the material decreases due to a reduced mass attenuation coefficient in the materials. The mass attenuation coefficient expresses the ability of a material to be penetrated by energy, and it decreases at higher energies. Therefore, the higher the mass attenuation coefficient of the detector’s material, the higher its penetration ability.
For the detection of high-energy X-rays, the detectors used are made of high-density materials with a higher mass attenuation coefficient. Here are some X-ray detection techniques:
- High-energy X-ray detectors are used in cargo inspection and consist of a scintillator coupled with photodiodes working as photovoltaics. The distribution of scintillation is varied through the beams.
- Scientific cameras detect high-energy X-rays using direct and indirect methods.
- Direct detection entails the direct absorption of soft X-ray photons with silicon sensors producing electron-hole pairs. This method is limited to the range of photon energies that silicon can absorb.
- Indirect detection uses scintillator materials to convert high-energy X-rays into visible photons that are then detected by a fibre-optic camera. This method ensures that the sensor is protected from damage by high-energy photons.
- XEXITEC ASIC detectors detect high-energy X-rays by using high-density semiconductors, such as cadmium telluride, and consist of an array of pixels capable of measuring X-rays with energies in the range of 200kEV. The X-rays interact with the detector, forming charge clouds of electron and hole pairs. The charge is drifted across the detector, creating a detecting signal. This system eliminates the need for cryogenic cooling systems that are usually required by similar technologies.
Scintillators are materials that are illuminated when they are exposed to ionising radiation as they absorb the energy of the radiation.
High Energy X-Rays - Key takeaways
- High-energy X-rays are about one order of magnitude greater in energy than conventional X-rays.
- High-energy X-rays are formed by particle accelerators that accelerate particles in large magnetic fields.
- High-energy X-rays have higher energy, higher penetrating power, and higher spatial resolution than conventional X-rays.
- High-energy X-rays are used in industries for sterilisation, inspection, and medical treatment.
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Frequently Asked Questions about High Energy X-Rays
What is the highest energy X-ray?
High-energy X-rays usually have energies between 80 and 1000 kiloelectronvolts (keV).
What are high-energy X-rays used for?
High-energy X-rays are used for various applications, including structural and security inspection, material science research, medical treatment, and industry sterilisation.
How do high-energy X-rays treat cancer?
High-energy X-rays treat cancer by shrinking or killing cancerous cells due to their high energy.
Why are high-energy X-rays used in radiotherapy?
High-energy X-rays are used in radiotherapy because they can damage living cells due to their ability to penetrate matter. Because of their high energy, high-energy X-rays can shrink or kill cancerous cells.
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