Particle-induced X-ray emissionFrom Wikipedia, the free encyclopedia

Particle-induced X-ray emission or proton-induced X-ray emission (PIXE) is a technique used in the determining of the elemental make-up of a material or sample. When a material is exposed to an ion beam, atomic interactions occur that give off EM radiation of wavelengths in the x-ray part of the electromagnetic spectrum specific to an element. PIXE is a powerful yet non-destructive elemental analysis technique now used routinely by geologists, archaeologists, art conservators and others to help answer questions of provenance, dating and authenticity.

The technique was first proposed in 1970 by Sven Johansson of Lund University, Sweden, and developed over the next few years with his colleagues Roland Akselsson and Thomas B Johansson.[1]

Recent extensions of PIXE using tightly focused beams (down to 1 μm) gives the additional capability of microscopic analysis. This technique, called microPIXE, can be used to determine the distribution of trace elements in a wide range of samples. A related technique, particle-induced gamma-ray emission (PIGE) can be used to detect some light elements.

Contents

  [hide] 

1 Theoryo 1.1 X-ray emission

o 1.2 Proton backscattering

o 1.3 Proton transmission

2 Protein analysiso 2.1 Data analysis

o 2.2 Limitations

o 2.3 Advantages

o 2.4 Scanning

3 Cell and tissue analysis 4 Artifact analysis 5 Proton beam writing 6 References 7 External links

Theory[edit]

Three types of spectra can be collected from a PIXE experiment:

1. X-ray  emission spectrum.2. Rutherford backscattering  spectrum.3. Proton transmission spectrum.

X-ray emission[edit]

Quantum theory states that orbiting electrons of an atom must occupy discrete energy levels in order to be stable. Bombardment with ions of sufficient energy (usually MeV protons) produced by an ion accelerator, will cause inner shell ionization of atoms in a specimen. Outer shell electrons drop down to replace inner shell vacancies, however only certain transitions are allowed. X-rays of a characteristic energy of the element are emitted. An energy dispersive detector is used to record and measure these X-rays.

Only elements heavier than fluorine can be detected. The lower detection limit for a PIXE beam is given by the ability of the X-rays to pass through the window between the chamber and the X-ray detector. The upper limit is given by the ionisation cross section, the probability of the K electron shell ionisation, this is maximal when the velocity of the proton matches the velocity of the electron (10% of the speed of light), therefore 3 MeV proton beams are optimal.

Proton backscattering[edit]

Protons can also interact with the nucleus of the atoms in the sample through elastic collisions, Rutherford backscattering, often repelling the proton at angles close to 180 degrees. The backscatter give information on the sample thickness and composition. The bulk sample properties allow for the correction of X-ray photon loss within the sample.

Proton transmission[edit]

The transmission of protons through a sample can also be used to get information about the sample.

Protein analysis[edit]

Protein analysis using microPIXE allow for the determination of the elemental composition of liquid and crystalline proteins. microPIXE can quantify the metal content of protein molecules with a relative accuracy of between 10% and 20%.[2]

The advantage of microPIXE is that given a protein of known sequence, the X-ray emission from sulfur can be used as an internal standard to calculate the number of metal atoms per protein monomer. Because only relative concentrations are calculated there are only minimal systematic errors, and the results are totally internally consistent.

The relative concentrations of DNA to protein (and metals) can also be measured using the phosphate groups of the bases as an internal calibration.

Data analysis[edit]

Analysis of the data collected can be performed by the programs Dan32,[3] the front end to gupix.[4][5]

Limitations[edit]

In order to get a meaningful sulfur signal from the analysis, the buffer should not contain sulfur (i.e. no BES, DDT, HEPES, MES, MOPSO or PIPES compounds). Excessive amounts of chlorine in the buffer should also be avoided, since this will overlap with the sulfur peak; KBr and NaBr are suitable alternatives.

Advantages[edit]

There are many advantages to using a proton beam over an electron beam. There is less crystal charging from Bremsstrahlung radiation, although there is some from the emission of Auger electrons, and there is significantly less than if the primary beam was itself an electron beam.

Because of the higher mass of protons relative to electrons, there is less lateral deflection of the beam; this is important for proton beam writing applications.

Scanning[edit]

Two-dimensional maps of elemental compositions can be generated by scanning the microPIXE beam across the target.

Cell and tissue analysis[edit]

Whole cell and tissue analysis is possible using a microPIXE beam, this method is also referred to as nuclear microscopy[citation needed].

Artifact analysis[edit]

MicroPIXE is a useful technique for the non-destructive analysis of paintings and antiques. Although it provides only an elemental analysis, it can be used to distinguish and measure layers within the thickness of an artifact.[6]

Proton beam writing[edit]

Proton beams can be used for writing (proton beam writing) through either the hardening of a polymer (by proton induced cross-linking), or through the degradation of a proton sensitive material. This may have important effects in the field of nanotechnology.

Gas detectorFrom Wikipedia, the free encyclopedia

Portable gas detector

A gas detector is a device that detects the presence of gases in an area, often as part of a safety system. This type of equipment is used to detect a gas leak and interface with a control system so a process can be automatically shut down. A gas detector can sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.

Gas detectors can be used to detect combustible, flammable and toxic gases, and oxygen depletion. This type of device is used widely in industry and can be found in locations, such as on oil rigs, to monitor manufacture processes and emerging technologies such as photovoltaic. They may be used in firefighting.

Gas leak detection is the process of identifying potentially hazardous gas leaks by sensors. These sensors usually employ an audible alarm to alert people when a dangerous gas has been detected. Common sensors include infrared point sensors, ultrasonic sensors, electrochemical gas sensors, and semiconductor sensors. More recently, infrared imaging sensors have come into use. All of these sensors are used for a wide range of applications and can be found in industrial plants, refineries, waste-water treatment facilities, vehicles, and homes.

Contents

  [hide] 

1 History 2 Types

o 2.1 Electrochemical

o 2.2 Infrared point

o 2.3 Infrared imaging

o 2.4 Semiconductor

o 2.5 Ultrasonic

o 2.6 Holographic

3 Calibrationo 3.1 Challenge (bump) test

4 Oxygen concentration 5 Hydrocarbons and VOCs

o 5.1 Considerations for detection of hydrocarbon gases/risk control

6 Ammonia 7 Combustible 8 Other 9 Household safety 10 Industrial applications 11 Research 12 Manufacturers 13 See also 14 References

History[edit]

Gas leak detection methods became a concern after the effects of harmful gases on human health were discovered. Before modern electronic sensors, early detection methods relied on less precise detectors. Through the 19th and early 20th centuries, coal miners would bring canaries down to the tunnels with them as an early detection system against life-threatening gases such as carbon dioxide, carbon monoxide and methane. The canary, normally a very songful bird, would stop singing and eventually die if not removed from these gases, signaling the miners to exit the mine quickly.

Before the development of electronic household carbon monoxide detectors in the 1980s and 1990s, carbon monoxide presence was detected with a chemically infused paper that turned brown when exposed to the gas. Since then, many electronic technologies and devices have been developed to detect, monitor, and alert the leak of a wide array of gases.

As the cost and performance of electronic gas sensors improved, they have been incorporated into a wider range of systems. Their use in automobiles was initially for engine emissions control, but now gas sensors may also be used to insure passenger comfort and safety. Carbon dioxide sensors are being installed into buildings as part of demand-controlled ventilation systems. Sophisticated gas sensor systems are being researched for use in medical diagnostic, monitoring, and treatment systems, well beyond their initial use in operating rooms. Gas monitors and alarms for carbon monoxide and other harmful gases are increasingly available for office and domestic use, and are becoming legally required in some jurisdictions.

Originally, detectors were produced to detect a single gas. Modern units may detect several toxic or combustible gases, or even a combination.[1] Newer gas analyzers can break up the component signals from a complex aroma to identify several gases simultaneously.[2]

Types[edit]

Gas detectors can be classified according to the operation mechanism (semiconductors, oxidation, catalytic, infrared, etc.). Gas detectors come packaged into two main form factors: portable devices and fixed gas detectors.

Portable detectors are used to monitor the atmosphere around personnel and are worn on clothing or on a belt/harness. These gas detectors are usually battery operated. They transmit warnings via audible and visible signals, such as alarms and flashing lights, when dangerous levels of gas vapors are detected.

Fixed type gas detectors may be used for detection of one or more gas types. Fixed type detectors are generally mounted near the process area of a plant or control room, or an area to be protected, such as a residential bedroom. Generally, industrial sensors are installed on fixed type mild steel structures and a cable connects the detectors to a SCADAsystem for continuous monitoring. A tripping interlock can be activated for an emergency situation.

Electrochemical[edit]Main article: Electrochemical gas sensor

Electrochemical gas detectors work by allowing gases to diffuse through a porous membrane to an electrode where it is either chemically oxidized or reduced. The amount of current produced is determined by how much of the gas is oxidized at the electrode,[3] indicating the concentration of the gas. Manufactures can customize electrochemical gas detectors by changing the porous barrier to allow for the detection of a certain gas concentration range. Also, since the diffusion barrier is a physical/mechanical barrier, the detector tended to be more stable and reliable over the sensor's duration and thus required less maintenance than other early detector technologies.

However, the sensors are subject to corrosive elements or chemical contamination and may last only 1–2 years before a replacement is required.[4] Electrochemical gas detectors are used in a wide variety of environments such as refineries, gas turbines, chemical plants, underground gas storage facilities, and more.

Infrared point[edit]Main article: Infrared point sensor

Infrared (IR) point sensors use radiation passing through a known volume of gas; energy from the sensor beam is absorbed at certain wavelengths, depending on the properties of the specific gas. For example, carbon monoxide absorbs wavelengths of about 4.2-4.5 μm.[5] (This wavelength is approximately a factor of 10 larger than that of visible light, which ranges from 0.39 μm to 0.75 μm for most people.) The energy in this wavelength is compared to a wavelength outside of the absorption range; the difference in energy between these two wavelengths is proportional to the concentration of gas present.[6]

This type of sensor is advantageous because it does not have to be placed into the gas to detect it and can be used for remote sensing. Infrared point sensors can be used to detect hydrocarbons [7]  and other infrared active gases such as water vapor and carbon dioxide. IR sensors are commonly found in waste-water treatment facilities, refineries, gas turbines, chemical plants, and other facilities where flammable gases are present and the possibility of an explosion exists. The remote sensing capability allows large volumes of space to be monitored.

Engine emissions are another area where IR sensors are being researched. The sensor would detect high levels of carbon monoxide or other abnormal gases in vehicle exhaust and even be integrated with vehicle electronic systems to notify drivers.[8]

Infrared imaging[edit]Main article: Thermographic camera

Infrared imaging sensors include active and passive systems. For active sensing, IR imaging sensors typically scan a laser across the field of view of a scene and look for backscattered light at the absorption line wavelength of a specific target gas. Passive IR imaging sensors measure spectral changes at each pixel in an image and look for specific spectral signatures that indicate the presence of target gases.[9] The types of compounds that can be imaged are the same as those that can be detected with infrared point detectors, but the images may be helpful in identifying the source of a gas.

Semiconductor[edit]

Semiconductor sensors detect gases by a chemical reaction that takes place when the gas comes in direct contact with the sensor. Tin dioxide is the most common material used in semiconductor sensors,[10] and the electrical resistance in the sensor is decreased when it comes in contact with the monitored gas. The resistance of the tin dioxide is typically around 50 kΩ in air but can drop to around 3.5 kΩ in the presence of 1% methane.[11] This change in resistance is used to calculate the gas concentration. Semiconductor sensors are commonly used to detect hydrogen, oxygen, alcohol vapor, and harmful gases such as carbon monoxide.[12] One of the most common uses for semiconductor sensors is in carbon monoxide sensors. They are also used in breathalyzers.[13] Because the sensor must come in contact with the gas to detect it, semiconductor sensors work over a smaller distance than infrared point or ultrasonic detectors.

Ultrasonic[edit]

Ultrasonic gas detectors use acoustic sensors to detect changes in the background noise of its environment. Since most high-pressure gas leaks generate sound in the ultrasonic range of 25 kHz to 10 MHz, the sensors are able to easily distinguish these frequencies from background acoustic noise which occurs in the audible range of 20 Hz to 20 kHz.[14]The ultrasonic gas leak detector then produces an alarm when there is an ultrasonic deviation from the normal condition of background noise. Ultrasonic gas leak detectors cannot measure gas concentration, but the device is able to determine the leak rate of an escaping gas because the ultrasonic sound level depends on the gas pressure and size of the leak.[15]

Ultrasonic gas detectors are mainly used for remote sensing in outdoor environments where weather conditions can easily dissipate escaping gas before allowing it to reach leak detectors that require contact with the gas to detect it and sound an alarm. These detectors are commonly found on offshore and onshore oil/gas platforms, gas compressor and metering stations, gas turbine power plants, and other facilities that house a lot of outdoor pipeline.

Holographic[edit]

Holographic gas sensors use light reflection to detect changes in a polymer film matrix containing a hologram. Since holograms reflect light at certain wavelengths, a change in their composition can generate a colorful reflection indicating the presence of a gas molecule.[16] However, holographic sensors require illumination sources such as white light orlasers, and an observer or CCD detector.

Calibration[edit]

All gas detectors must be calibrated on a schedule. Of the two form factors of gas detectors, portables must be calibrated more frequently due to the regular changes in environment they experience. A typical calibration schedule for a fixed system may be quarterly, bi-annually or even annually with more robust units. A typical calibration schedule for a portable gas detector is a daily "bump test" accompanied by a monthly calibration.[17] Almost every portable gas detector requires a specific calibration gas which is available from the manufacturer. In the US, the Occupational Safety and Health Administration (OSHA) may set minimum standards for periodic recalibration.[citation needed]

Challenge (bump) test[edit]

Because a gas detector is used for employee/worker safety, it is very important to make sure it is operating to manufacturer's specifications. Australian standards specify that a person operating any gas detector is strongly advised to check the gas detector's performance each day and that it is maintained and used in accordance with the manufacturers instructions and warnings.[18]

A challenge test should consist of exposing the gas detector to a known concentration of gas to ensure that the gas detector will respond and that the audible and visual alarms activate. It is also important inspect the gas detector for any accidental or deliberate damage by checking that the housing and screws are intact to prevent any liquid ingress and that the filter is clean, all of which can affect the functionality of the gas detector. The basic calibration or challenge test kit will consist of calibration gas/regulator/calibration cap and hose (generally supplied with the gas detector) and a case for storage and transport. Because 1 in every 2,500 untested instruments will fail to respond to a dangerous concentration of gas, many large businesses use an automated test/calibration station for bump tests and calibrate their gas detectors daily.[19]

Oxygen concentration[edit]

Oxygen deficiency gas monitors are used for employee and workforce safety. Cryogenic substances such as liquid nitrogen (LN2), liquid helium (He), and liquid argon (Ar) are inert and can displace oxygen (O2) in a confined space if a leak is present. A rapid decrease of oxygen can provide a very dangerous environment for employees, who may not notice this problem before they suddenly lose consciousness. With this in mind, an oxygen gas monitor is important to have when cryogenics are present. Laboratories, MRIrooms, pharmaceutical, semiconductor, and cryogenic suppliers are typical users of oxygen monitors.

Oxygen fraction in a breathing gas is measured by electro-galvanic fuel cell sensors. They may be used stand-alone, for example to determine the proportion of oxygen in anitrox mixture used in scuba diving,[20] or as part of feedback loop which maintains a constant partial pressure of oxygen in a rebreather.[21]

Hydrocarbons and VOCs[edit]

Further information: Volatile organic compound#VOC sensors

Detection of hydrocarbons can be based on the mixing properties of gaseous hydrocarbons – or other volatile organic compounds (VOCs) – and the sensing material incorporated in the sensor. The selectivity and sensitivity depends on the molecular structure of the VOC and the concentration; however, it is difficult to design a selective sensor for a single VOC. Many VOC sensors detect using a fuel-cell method.

VOCs in the environment or certain atmospheres can be detected based on different principles and interactions between the organic compounds and the sensor components. There are electronic devices that can detect ppm concentrations despite not being particularly selective. Others can predict with reasonable accuracy the molecular structure of the volatile organic compounds in the environment or enclosed atmospheres[22] and could be used as accurate monitors of the chemical fingerprint and further as health monitoring devices.

Solid-phase microextraction (SPME) techniques are used to collect VOCs at low concentrations for analysis.[23]

Considerations for detection of hydrocarbon gases/risk control[edit]

Methane is lighter than air (possibility of accumulation under roofs) Ethane is slightly heavier than air (possibility of pooling at ground levels/pits) Propane is heavier than air (possibility of pooling at ground levels/pits) Butane is heavier than air (possibility of pooling at ground levels/pits)

Ammonia[edit]

Gaseous ammonia is continuously monitored in industrial refrigeration processes and biological degradation processes, including exhaled breath. Depending on the required sensitivity, different types of sensors are used (e.g., flame ionization detector, semiconductor, electrochemical, photonic membranes[24]). Detectors usually operate near the Lower Exposure Limit of 25ppm;[25] however, ammonia detection for industrial safety requires continuous monitoring above the fatal exposure limit of 0.1%.[26]

Combustible[edit]

Catalytic bead sensor Explosimeter Infrared point sensor Infrared open path detector

Other[edit]

Flame ionization detector Nondispersive infrared sensor Photoionization detector Zirconium oxide  sensor cell Catalytic sensors Metal oxide semiconductor Gold film Colorimetric Detector Tubes Sample collection and chemical analysis Piezoelectric microcantilever Holographic Sensor Thermal Conductivity Detector Electrochemical gas sensor

Household safety[edit]

There are several different sensors that can be installed to detect hazardous gases in a residence. Carbon monoxide is a very dangerous, but odorless, colorless gas, making it difficult for humans to detect. Carbon monoxide detectors can be purchased for around US$20–60. Many local jurisdictions in the United States now require installation of carbon monoxide detectors in addition to smoke detectors in residences.

Handheld flammable gas detectors can be used to trace leaks from natural gas lines, propane tanks, butane tanks, or any other combustible gas. These sensors can be purchased for US$35–100.

Industrial applications[edit]

This section requires expansion.

(December 2013)

Research[edit]

The European Community has supported research called the MINIGAS project that was coordinated by VTT Technical Research Center of Finland.[27] This research project aims to develop new types of photonics-based gas sensors, and to support the creation of smaller instruments with equal or higher speed and sensitivity than conventional laboratory-grade gas detectors.[27]

Manufacturers[edit]

Dräger General Monitors Crowcon Detection Instruments Honeywell Analytics Industrial Scientific Corporation Mine Safety Appliances Oldham

Ion implantationFrom Wikipedia, the free encyclopedia

An ion implantation system at LAAStechnological facility in Toulouse, France.

Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into a solid. This process is used to change the physical, chemical, or electrical properties of the solid. Ion implantation is used insemiconductor device fabrication and in metal finishing, as well as various applications in materials science research. The ions alter the elemental composition of the target, if the ions differ in composition from the target, stop in the target

and stay there. They also cause many chemical and physical changes in the target by transferring their energy and momentum to the electrons and atomic nuclei of the target material. This causes a structural change, in that the crystal structure of the target can be damaged or even destroyed by the energetic collision cascades. Because the ions have masses comparable to those of the target atoms, they knock the target atoms out of place more than electron beams do. If the ion energy is sufficiently high (usually tens of MeV) to overcome the coulomb barrier, there can even be a small amount of nuclear transmutation.

Contents

  [hide] 

1 General principle 2 Application in semiconductor device fabrication

o 2.1 Doping

o 2.2 Silicon on insulator

o 2.3 Mesotaxy

3 Application in metal finishingo 3.1 Tool steel toughening

o 3.2 Surface finishing

4 Other applicationso 4.1 Ion beam mixing

5 Problems with ion implantationo 5.1 Crystallographic damage

o 5.2 Damage recovery

o 5.3 Amorphization

o 5.4 Sputtering

o 5.5 Ion channelling

6 Hazardous materialso 6.1 High voltage safety

7 See also 8 References 9 External links

General principle[edit]

Ion implantation setup with mass separator

Ion implantation equipment typically consists of an ion source, where ions of the desired element are produced, an accelerator, where the ions are electrostatically accelerated to a high energy, and a

target chamber, where the ions impinge on a target, which is the material to be implanted. Thus ion implantation is a special case of particle radiation. Each ion is typically a single atom or molecule, and thus the actual amount of material implanted in the target is the integral over time of the ion current. This amount is called the dose. The currents supplied by implanters are typically small (microamperes), and thus the dose which can be implanted in a reasonable amount of time is small. Therefore, ion implantation finds application in cases where the amount of chemical change required is small.

Typical ion energies are in the range of 10 to 500 keV (1,600 to 80,000 aJ). Energies in the range 1 to 10 keV (160 to 1,600 aJ) can be used, but result in a penetration of only a few nanometers or less. Energies lower than this result in very little damage to the target, and fall under the designation ion beam deposition. Higher energies can also be used: accelerators capable of 5 MeV (800,000 aJ) are common. However, there is often great structural damage to the target, and because the depth distribution is broad (Bragg peak), the net composition change at any point in the target will be small.

The energy of the ions, as well as the ion species and the composition of the target determine the depth of penetration of the ions in the solid: A monoenergetic ion beam will generally have a broad depth distribution. The average penetration depth is called the range of the ions. Under typical circumstances ion ranges will be between 10 nanometers and 1 micrometer. Thus, ion implantation is especially useful in cases where the chemical or structural change is desired to be near the surface of the target. Ions gradually lose their energy as they travel through the solid, both from occasional collisions with target atoms (which cause abrupt energy transfers) and from a mild drag from overlap of electron orbitals, which is a continuous process. The loss of ion energy in the target is called stoppingand can be simulated with the binary collision approximation method.

Accelerator systems for ion implantation are generally classified into medium current (ion beam currents between 10 μA and ~2 mA), high current (ion beam currents up to ~30 mA), high energy (ion energies above 200 keV and up to 10 MeV), and very high dose (efficient implant of dose greater than 1016 ions/cm2).[citation needed]

All varieties of ion implantation beamline designs contain certain general groups of functional components (see image). The first major segment of an ion beamline includes a device known as an ion source to generate the ion species. The source is closely coupled to biased electrodes for extraction of the ions into the beamline and most often to some means of selecting a particular ion species for transport into the main accelerator section. The "mass" selection is often accompanied by passage of the extracted ion beam through a magnetic field region with an exit path restricted by blocking apertures, or "slits", that allow only ions with a specific value of the product of mass and velocity/charge to continue down the beamline. If the target surface is larger than the ion beam diameter and a uniform distribution of implanted dose is desired over the target surface, then some combination of beam scanning and wafer motion is used. Finally, the implanted surface is coupled with some method for collecting the accumulated charge of the implanted ions so that the delivered dose can be measured in a continuous fashion and the implant process stopped at the desired dose level.[1]

Application in semiconductor device fabrication[edit]

Doping[edit]

The introduction of dopants in a semiconductor is the most common application of ion implantation. Dopant ions such as boron, phosphorus or arsenic are generally created from a gas source, so that the purity of the source can be very high. These gases tend to be very hazardous. When implanted in a semiconductor, each dopant atom can create a charge carrier in the semiconductor after annealing. A hole can be created for a p-type dopant, and an electron for an n-type dopant.

This modifies the conductivity of the semiconductor in its vicinity. The technique is used, for example, for adjusting the threshold of a MOSFET.

Ion implantation was developed as a method of producing the p-n junction of photovoltaic devices in the late 1970s and early 1980s,[2] along with the use of pulsed-electron beam for rapid annealing,[3] although it has not to date been used for commercial production.

Silicon on insulator[edit]

One prominent method for preparing silicon on insulator (SOI) substrates from conventional silicon substrates is the SIMOX (Separation by IMplantation of OXygen) process, wherein a buried high dose oxygen implant is converted to silicon oxide by a high temperature annealing process.

Mesotaxy[edit]

Mesotaxy is the term for the growth of a crystallographically matching phase underneath the surface of the host crystal (compare to epitaxy, which is the growth of the matching phase on the surface of a substrate). In this process, ions are implanted at a high enough energy and dose into a material to create a layer of a second phase, and the temperature is controlled so that the crystal structure of the target is not destroyed. The crystal orientation of the layer can be engineered to match that of the target, even though the exact crystal structure and lattice constant may be very different. For example, after the implantation of nickel ions into a silicon wafer, a layer of nickel silicide can be grown in which the crystal orientation of the silicide matches that of the silicon.

Application in metal finishing[edit]

Tool steel toughening[edit]

Nitrogen or other ions can be implanted into a tool steel target (drill bits, for example). The structural change caused by the implantation produces a surface compression in the steel, which prevents crack propagation and thus makes the material more resistant to fracture. The chemical change can also make the tool more resistant to corrosion.

Surface finishing[edit]

In some applications, for example prosthetic devices such as artificial joints, it is desired to have surfaces very resistant to both chemical corrosion and wear due to friction. Ion implantation is used in such cases to engineer the surfaces of such devices for more reliable performance. As in the case of tool steels, the surface modification caused by ion implantation includes both a surface compression which prevents crack propagation and an alloying of the surface to make it more chemically resistant to corrosion.

Other applications[edit]

Ion beam mixing[edit]

Ion implantation can be used to achieve ion beam mixing, i.e. mixing up atoms of different elements at an interface. This may be useful for achieving graded interfaces or strengthening adhesion between layers of immiscible materials.

Problems with ion implantation[edit]

Crystallographic damage[edit]

Each individual ion produces many point defects in the target crystal on impact such as vacancies and interstitials. Vacancies are crystal lattice points unoccupied by an atom: in this case the ion

collides with a target atom, resulting in transfer of a significant amount of energy to the target atom such that it leaves its crystal site. This target atom then itself becomes a projectile in the solid, and can cause successive collision events. Interstitials result when such atoms (or the original ion itself) come to rest in the solid, but find no vacant space in the lattice to reside. These point defects can migrate and cluster with each other, resulting in dislocation loops and other defects.

Damage recovery[edit]

Because ion implantation causes damage to the crystal structure of the target which is often unwanted, ion implantation processing is often followed by a thermal annealing. This can be referred to as damage recovery.

Amorphization[edit]

The amount of crystallographic damage can be enough to completely amorphize the surface of the target: i.e. it can become an amorphous solid (such a solid produced from a melt is called a glass). In some cases, complete amorphization of a target is preferable to a highly defective crystal: An amorphized film can be regrown at a lower temperature than required to anneal a highly damaged crystal.

Sputtering[edit]

Some of the collision events result in atoms being ejected (sputtered) from the surface, and thus ion implantation will slowly etch away a surface. The effect is only appreciable for very large doses.

Ion channelling[edit]

A diamond cubic crystal viewed from the<110> direction, showing hexagonal ion channels.

If there is a crystallographic structure to the target, and especially in semiconductor substrates where the crystal structure is more open, particular crystallographic directions offer much lower stopping than other directions. The result is that the range of an ion can be much longer if the ion travels exactly along a particular direction, for example the <110> direction in silicon and other diamond cubic materials[citation needed]. This effect is called ion channelling, and, like all the channelling effects, is highly nonlinear, with small variations from perfect orientation resulting in extreme differences in implantation depth. For this reason, most implantation is carried out a few degrees off-axis, where tiny alignment errors will have more predictable effects.

Ion channelling can be used directly in Rutherford backscattering and related techniques as an analytical method to determine the amount and depth profile of damage in crystalline thin film materials.

Hazardous materials[edit]

In the ion implantation semiconductor fabrication process of wafers, it is important for the workers to minimize their exposure to the toxic materials used in the ion implanter process. Such hazardous elements, solid source and gasses are used, such as arsine and phosphine. For this reason, the semiconductor fabrication facilities are highly automated, and may feature negative pressure gas bottles safe delivery system (SDS). Other elements may include antimony, arsenic,phosphorus, and boron. Residue of these elements show up when the machine is opened to atmosphere, and can also be accumulated and found concentrated in the vacuum pumps hardware. It is important not

to expose yourself to these carcinogenic, corrosive, flammable, and toxic elements. Many overlapping safety protocols must be used when handling these deadly compounds. Use safety, and read MSDSs.

High voltage safety[edit]

High voltage power supplies in ion implantation equipment can pose a risk of electrocution. In addition, high-energy atomic collisions can generate X-rays and, in some cases, other ionizing radiation and radionuclides. Operators and maintenance personnel should learn and follow the safety advice of the manufacturer and/or the institution responsible for the equipment. Prior to entry to high voltage area, terminal components must be grounded using a grounding stick. Next, power supplies should be locked in the off state and tagged to prevent unauthorized energizing.

Other types of particle accelerator, such as radio frequency linear particle accelerators and laser wakefield plasma accelerators  have their own hazards.

Accelerators for Health and MedicineThe most direct way that particle accelerators impact most of our lives will be through their applications in medicine. With the advancement of accelerator science, new techniques for the treatment of cancer and diagnosis of various diseases have contributed significantly to major steps forward in the last decades. 

Treating Cancer

One of the most important of all applications of particle accelerators is their application in the treatment of cancer. Cancer takes on many forms, so the treatment for cancer must also take on many forms. The main types of cancer treatment are:

surgery, where the cancerous tissue is surgically removed,

chemotherapy, where powerful cancer-killing medicine is given to the patient,

radiotherapy, where the cancer is destroyed by energy deposited by radiation.

Accelerator based treatments fall into the category of radiation therapy. Presently about 50% of all patients with cancer will undergo radiation therapy often in conjunction with other treatments such as chemotherapy or surgery. The most common form of radiation therapy is external beam radiotherapy where a beam of radiation is fired into the body by a particle accelerator. The different types of radiation used are discussed below. 

X-Ray therapy

X-ray therapy is the most common form of radiotherapy. High energy X-rays - generated by firing electrons at a material like tungsten - are directed into a cancerous tumour. The radiation damages the DNA of the cancerous cells which ‘kills’ the cells. A high dose in the tumour is essential in the curing process by radiation. Unfortunately the X-ray radiation doesn’t only harm the cancerous cells; it also damages healthy cells at locations in the

patient’s body that cannot be avoided by the X-ray beam. This can lead to unpleasant side effects for the patient such as pneumonitis, dry mouth due to loss of saliva production and damage to the skin. The aim of all new techniques in radiotherapy is to deliver a high dose of radiation to the tumour (to get a high cure rate) whilst reducing the radiation dose in healthy tissue (to reduce side effects).

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New radiotherapy techniques being developed that use particle accelerators, such as proton therapy,  show clear signs of reducing side effects due to a reduction of the radiation dose in healthy tissues and for some types of cancer an increase in  the possibility of curing the cancer; see section below on proton beam therapy.

For more information see:Web: ESTRO, the European SocieTy for Radiotherapy and OncologyWeb (NHS): RadiotherapyWeb (National Cancer Institute):  Radiotherapy factsheet 

Electron beam therapy

This technique uses the same type of accelerator that is used to generate X-rays, however the X-ray  producing target is removed so that the electrons are fired directly into the tumour. Electron beam therapy is useful for treating tumours near the surface of the skin as the electrons quickly lose their energy when they interact with atoms in the body (see the graphbelow). Electron beam therapy causes far less damage to healthy tissue in the body as the beam does not pass all the way through the body.For more information see:Lecture [2008]: Electron Beam Therapy Dosimetry, Planning & Techniques 

Hadron beam therapyA hadron is a type of particle made up of quarks. Hadron therapy falls into three categories:

proton therapy

neutron therapy

ion (e.g. Carbon or Helium Ion) therapy.

The three types of hadron therapy differ in the type of particle beam that is used in the treatment. The dose deposition and its biological consequences depend on the particle type and its energy. The different therapies are used to treat different forms of cancer at different locations in the body as the particles penetrate and deposit their energy in different ways.

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The dose deposition of high energy neutrons is much like that of X-rays. The therapeutic difference is the stronger biological effect (cell killing per amount of deposited dose) of neutrons. For a limited amount of cancers and at certain locations in the body, this can be of an advantage.

The advantage of proton and ion-beam therapy over current X-ray radiotherapy is that the particles do not penetrate the whole body; they are fired into the body with a certain energy which dictates the range of the particles. The range is the depth that the protons or ions can penetrate into the body. Once protons or ions reach this depth they stop, whereas X-ray radiation passes through the whole body, thus also damaging cells on the way in and on the way out. Using protons or ions, this means that, for a certain dose in the tumour, the patient is exposed to less radiation in regions outside the tumour and therefore less healthy tissue is damaged in the treatment.

Ions such as carbon have a larger biological effect than protons. This can be exploited in certain cases. But to accelerate ions to sufficient energies, much larger accelerators, beam transport systems and gantries are needed than for protons. Until the 1990s proton therapy was performed in nuclear physics laboratories that were equipped with a particle accelerator and in this period most of the pioneering proton therapy research was performed and most of the dose delivery concepts were developed.

For more information on hadron therapy in general see;Web: PTCOG, Particle Therapy Co-Operative GroupVideo: Particle Accelerators for Tumour Therapy [UK], 5min26sArticle [2011] (CERN Courier): Hadron Therapy Collaborating for the Future Article [2010] (CNRS): Hadron Therapy, a Treatment for X-Ray Resistant Cancers Book: Reviews of Accelerator Science and Technology, Vol. 2: Medical Applications of Accelerators (2009), (ed A. W Chao, W. Chou), World Scientific Publishing CompanyBook: Proton   Therapy   Physics   (ed H. Paganetti), CRC Press, Taylor & Francis Group, 2012Book: Proton and   Carbon Ion Therapy  (ed C. Ma and A. Lomax), CRC Press, Taylor & Francis Group, 2013  

For more information on proton treatment see;Web (PSI): Proton Therapy at PSIWeb: CATANA, The First Italian Protontherapy Center for the ocular melanoma treatment  Article [2011] (Fermilab Today ): Advancements in Proton Therapy Cause for CelebrationArticle [2008] (Symmetry Magazine):  The Power of Proton Therapy Article [2012] (Medical Physics Web): Future Prospects for Proton Therapy 

For more information on neutron therapy

Article [2005] (Symmetry Magazine):  Neutrons for Cancer Treatment Lecture [2012] (Fermilab): Fighting Cancer with Neutrons 

For more information on heavy ion therapyArticle [2011] (Symmetry Magazine): Kicking Cancer with Carbon IonsWeb: Carboniontherapy.orgWeb: CNAO Foundation, National Centre of Oncology in Italy 

Relationship between different kinds of particles (at a given energy) and their ability to penetrate inside the human body : In depth distribution of the absorbed dose, in the case of 20 MeV electrons, 8 MeV photons, 190 MeV protons: Electrons and photons mainly affect the first layers of tissue, while protons release most of the energy to a precise depth, variable with the beam energy. The green line shows the distribution of the dose received by the patient in the case of treatment of a tumor with a thickness of 6 cm and located between 18 and 24 cm depth, irradiated with proton beams with controlled different energies. The dose is concentrated along the lesion, with limited damage to the surrounding tissues. Image credit:INFN /Asimmetrie , page 21. 

Cancer research

Accelerator scientists are working on developing new techniques, such as a novel technique using synchrotron radiation from the ALICE accelerator at Daresbury Laboratory in the UK, to detect certain cancers early, refine earlier techniques and improve accelerator technology so that treatments like proton therapy can become economically accessible for more hospitals around the world.

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For more information see;Article [2012] (Symmetry Magazine): Accelerator Beams for Early Cancer DetectionArticle [2009] (Symmetry Magazine): Shrinking the Cost of Zapping CancerArticle [2010] (Symmetry Magazine): Accelerator Physicists Strive to Lower Cost of Cancer TreatmentArticle [2011] (Symmetry Magazine): SLAC Physicists Using Physics Simulation Tool to Make Cancer Therapy SaferArticle [2013] (STFC): Breakthrough in the Development of a Diagnostic test for Oesophageal Cancer 

Medical ImagingMedical imaging is an incredibly important area of medicine with applications in both diagnosis and research. Most of us will have an image of some part of our body taken for diagnosis in our lifetime, whether it is an X-ray image of our teeth at the dentist’s or a full body Magnetic resonance imaging (MRI) scan. The role of accelerators directly (as an X-ray source for radiography) and indirectly (in the development of the physical principles behind MRI imaging) on the development of new imaging techniques cannot be overstated.

Positron Emission Tomography (PET)

Positron Emission Tomography (PET) is a medical imaging technique which produces a detailed 3D image. A radioactive isotope is introduced to the body. The isotope undergoes radioactive decay within the body which emits a positron. As this particle travels through the body, it interacts with other particles in the body and slows to the point where it can annihilate with an electron producing a pair of gamma photons. These photons are then picked up by a detector which will send the data to a computer for the production of an image of the photon origins.

The physics behind this technique was first understood by particle physicists who also played a role in the development of the technique. The detectors used by PET scanners were developed first for particle physics experiments. Particle accelerators are also used in the synthesis of the radioisotopes used in this technique; this is discussed in the ‘Accelerators for Isotope Production’ section below.

PET scans can be combined with an MRI or Computerised Tomography (CT) scan to help make the images easier to interpret. A PET scan measures important body functions, such as blood flow, oxygen use, and sugar (glucose) metabolism, while CT imaging provides excellent anatomic information.  The PET technique has the advantage of the use of atoms (PET isotopes) that are typical in biology, e.g.: carbon, oxygen, nitrogen and Fluor. It is also possible to determine the concentration of the isotopes in the 3D image in an absolute way, this makes the PET technique very powerful in many applications.

PET/CT imaging: At right

is a CT scan, at left is a

PET scan, at center is a

combined PET/CT scan.

Image credit: John Prior,

CHUV, Switzerland

For more information see;Web (NHS): PET scan 

Proton Computerised Tomography (CT) scan

Particle physicists and medical researchers are working together to develop a new medical imaging technique called a proton computerised tomography (pCT) scan, which is like a conventional computerised tomography (CT) scan but uses protons rather than X-rays. This new technique will reduce the patient’s exposure to harmful radiation whilst producing more accurate 3D images than those produced by current CT scans. A pCT scan also has great potential in improving the effectiveness of proton beam therapy which could improve proton therapy, and reduce side effects related to still existing range uncertainties of the proton beam.

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For more information see;Article [2012] (Symmetry Magazine): Pursuing Protons for Medical ImagingArticle [2011] (Science Daily): Proton Imaging Provides More Accuracy, Less Radiation to Paediatric Cancer PatientsArticle [2012] (University of California Santa C ruz ): Image of Hand Shows Progress Towards Proton RadiographyArticle [2010] (Northern Illinois University): Proton Centre on Verge of Major Medical Breakthrough 

Magnetic Resonance Imaging (MRI)

Since its development in the 1970’s magnetic resonance imaging (MRI) scanning has saved countless lives due to its revolutionary ability to image soft tissue within the body. The basic physical principles were first discovered by particle physicists in the 1930’s and later the field of particle physics played another crucial role in the development of the powerful superconducting magnets required for the technique to work.

MRI an Accelerator Born

Technology: Above is a still image

from a GIF produced by artist Andy

Ellison of an MRI scan of a husk of

corn. Image credit: Andy Ellison

For more information see;Article [2008] (Symmetry Magazine): Deconstruction: MRI 

Medical Materials Produced with Accelerators

Particle accelerators have become an important tool in the production of customised materials for industry (see section on industrial applications). The medical industry is no exception. Indeed specialized materials such as prosthetic implants or hydrogels are often produced using accelerators. 

Heart valves

Bombarding materials with ions can produce materials with desirable properties. This technique has been used by scientists in the USA to produce a durable material to be used in artificial heart valves.

For more information see;Article [2009] (Symmetry Magazine): Heart Valves

 

Hydrogels

Particle accelerators firing beams of electrons play an important role in the production of materials known as hydrogels. Hydrogels are used to make dressings for wounds like burns which are more comfortable for the patient than standard bandages and can be used in drug delivery.

Hydrogels: Particle

accelerators play a

vital role in producing

hydrogels for use in

medicine. Image

credit: Kathy F.

Atkinson, University

of Delaware

For more information see;Article [2010] (Symmetry Magazine): Hydrogels 

Accelerators for Isotope ProductionScientists have found many uses for radioactive isotopes, notably in medicine where they can be used in medical imaging and radiotherapy.

A particle accelerator is used to produce isotopes by accelerating protons or other ions to close to the speed of light and smashing them into atoms to initiate a nuclear reaction, in which radioactive isotopes are produced.  In a chemical process, these isotopes are included into molecules that can be brought into the patient’s body by e.g. inhalation, orally or by an injection.  These molecules are used in specific processes in the patient’s body. The emitted radiation can be detected with special detectors and the obtained 3D image of the spread of the molecule then gives interesting diagnostic information. Apart from this diagnostic application, there is also a therapeutic application of radioactive isotopes. Isotopes that emit radiation with a very short range can be used to irradiate tissue, for e.g. the treatment of certain types of cancer.

Manmade

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isotopes made using

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medical imaging and radio

therapy - Image

credit:TRIUMF

Industrial applicationsIt is estimated that over 24,000 particle accelerators have been built in the last 60 years for industrial applications. These accelerators are used in either the production or preparation ofmore than US$500B (390B€) worth of products worldwide annually. Accelerators in various forms are used for a wide range of tasks in the production or preparation of many different end products, ranging from electronics to shrink wrap. The sections below explore in more detail some of the products produced using industrial particle accelerators.Ion Implantation

Ion implantation for electronics (semi-conductor materials)Semiconductors are one of the key components of almost all electrical devices from mobile phones to desktop computers. They are materials that conduct a small amount of electricity, more than an insulator but less than a conductor, hence the name. Semiconductors are usually made from silicon - sometimes germanium - that has been doped. Doping is the process of adding impurities to the silicon so that an electric current flows through the material (silicon crystal is an insulator). The doping of silicon is done by a process known as ion implantation. In this process, a beam of ions is fired at a target material. The ions then penetrate, and come to rest within the material at a penetration depth related to the energy of the beam. The development of ion implantation technology leads to better and cheaper semiconductor production, which in turn drives down the cost of electronics and improves the quality of the product.

Ion implantation for electronics: Many digital electronics rely on ion

implanters to build fast transistors and chips. Image credit: Shutterstock.com

For more information see;Web (Case Technology):  Ion ImplanterWeb (The Industrial Physicist [ceased publication]): Ion Implantation in Silicon TechnologyArticle (Symmetry Magazine) [2013]: Semiconductors

Video: Semiconductors-Ion implantation: 3D Animation2min28s 

Ion implantation for hardening surfaces (metals, ceramics and biomaterials)In addition to being used in the production of semiconductors, ion implantation can also be used to harden surfaces. When used for surface hardening, the ions of the doping agent material are fired at the target material and only penetrate the material a very short distance, essentially remaining at the surface of the material. Ions are chosen that will complement the atomic structure of the target material, making it stronger. This process is used to create hard surfaces for materials that are used for example in objects like artificial heart valves and other prosthetic implants. (See also section on health and medicine).

For more information see;Web (azom.com): Surface Hardening by Ion ImplantationWeb (Bodycote): Ion Implantation 

See also the section on surface hardening using electron beams below. 

Electron Beam Material Processing

Electron Beam Welding (EBW)As the name suggests, electron beam welding (EBW) uses a focused beam of electrons to fuse together two materials. As the beam collides with the metal, the kinetic energy of the electrons is transferred into heat. The heat from the collisions causes the metal to melt and flow together, which joins the two pieces when the molten metal cools. EBW can weld very thin pieces of incompatible alloys with minimal thermal deformations resulting from the process.

Space-Age Part Fabrication: A technique developed by NASA, called

electron beam freeform fabrication (EBF3) creates small metal parts for

spacecraft in a fully automated process involving an electron beam.

Image credit: NASA.

For more information see;R. Hamm & M. Hamm Eds., Industrial Accelerators and their Applications (Singapore: World Scientific, 2012) ISBN-13 978-981-4307-04-8, p 57-85Web (Bodycote): Electron Beam WeldingDoc (European Federation for Welding) [2007]: Electron Beam Welding 

Electron Beam Machining (EBM)

Electron beam machining (EBM) is similar to EBW insofar as electrons are accelerated to high velocities and focused into a beam. The electron beam is then focused onto a metal, the electrons collide with the metal and the kinetic energy of the electrons is converted to heat. The difference between EBM and EBW is the energy of the electron beam: it is significantly higher in EBM than in EBW so that the metal evaporates, whereas EBW melts the metal. This means that EBM can be used to either, cut, or drill through a metal rather than fuse two pieces of metal. EBM is used to drill holes in the manufacture of air craft engines and nuclear reactor pressure differential devices, and to aid in the etching of microprocessors for the electronics industry.

For more information see;Lecture (rcsaini.blogspot.com): Electron Beam MachiningWeb (engineershandbook.com): Electron Beam Machining 

Electron Beam Heat Treating (Surface Hardening)Certain metal alloys possess a property known as hardenability; this is the amount that the material is hardened after going through a heat treatment process.  Heat treatment is an industrial process where the physical properties of a material are altered by the application of an extreme temperature, hot or cold. Electron beam heat-treating is a process, which heats the surface of a hardenable metal alloy up to a certain temperature specific to that material. The material is then cooled quickly which causes the material to harden. The process by which the electron beam heats the metal is the same as for EBW and EBM, the kinetic energy of the electron beam is converted into heat as the electrons collide with the surface. To see how electron beam surface hardening can be used in automobile manufacturing, see the case study: ‘Accelerators for Building a Car’.For more information see;R. Hamm & M. Hamm Eds., Industrial Accelerators and their Applications (Singapore: World Scientific, 2012) ISBN-13 978-981-4307-04-8, p 76-78Doc (Fraunhofer Institute for Surface Engineering and Thin Films): Electron Beam Hardening and Hard Coating for Highly Stressed Tools and Components J. Davis, Surface Hardening of Steel: Understanding the basics (ASM International, 2002) ISBN: 978-0-87170-764-2, p267-271 [link via google books] 

Electron Beam Material Irradiation

Cross-linking polymers

Polymers are molecules made up of a long chain of repeating atomic structures. For example, polyethylene which is the most common form of plastic is a polymer made up of a long chain of carbon atoms bonded together, each with two hydrogen atoms bonded to them.

In the production of shrink wrap, an electron beam is fired at the polyethylene: electrons collide with the polyethylene molecules causing the hydrogen-carbon bonds to break, leaving gaps in the polymer where new atoms can bond to the carbon atoms. Carbon atoms

from two separate polyethylene molecules are now free to bond to each other. As carbon-carbon bonds are very strong, this prevents the plastic from melting when heated, the plastic instead shrinks. This process is known as polymer cross-linking and is utilized to create materials used for products like shrink wrap, wire and cable insulation, plastic foam, hydrogels, vulcanized rubber and composite materials used in car and plane manufacture.

Shrink Wrap: Shrink wrap packaging is used to package many products

and is a common sight in supermarkets, but did you know that particle

accelerators play a vital role in its production. Image credit: Vicki &

Chuck Rodgers/Flickr

 

Hardening materials (X-ray curing of material composites)Another method of cross-linking polymers is by using X-ray or gamma ray radiation. The decision to use either electron beam or X-ray/gamma-ray treatment is made depending on the type of polymer being cross-linked and the size of the object being treated. Traditionally, curing of materials (curing is the name given to the process of hardening a material by cross-linking polymers) such as vulcanization of car tires and the production of composites (materials made of several different substances) used in car and plane manufacture was done by exposing the ingredients to very high temperatures. The substances were essentially placed in a mold and cooked in a big oven. Curing materials using electrons or X-rays is much quicker than traditional heat curing techniques, greatly increasing the throughput of factories producing composite car and plane parts in addition to producing materials with more favorable properties than the traditional method.

Hardening materials: Replacing steel with X-ray cured carbon composites can

reduce car energy consumption by 50%. Image credit: INFN/ Domenico

Santonocito, Italy

For more information see;Article (Symmetry Magazine) [2010]: Heat-shrink tubing, A wiry protectorArticle (Symmetry Magazine) [2009]: An electron zap turns flimsy plastic into sturdy shrink wrapDoc (UDRI): Curing and Bonding of Composites Using Electron Beam ProcessingWeb (about.com): Electron Beam CuringArticle (Sterigenics) [2010]: Radiation Crosslinking of Polymers 

See also the case study: Accelerators for building a car 

Treating waste and medical materialsBeams of electrons fired from small particle accelerators are used in factories that produce medical apparatus to sterilize them. The beam of electrons kills any microbes without

damaging the equipment or the packaging, an advantage this method holds over other sterilization techniques.

For more information see;Article [2010] (Symmetry Magazine): Sterilizing Medical SuppliesWeb (IBA Sterilization): E-Beam Sterilization for Medical Devices 

Food preservation

The same process is used for food preservation. However for irradiation of food items, it is essential to select a dose high enough to kill or prevent the bacteria or insect from reproducing, and low enough that it does not damage the food. Many food products are treated using an electron beam: fruit and vegetables, cereals, spices, fish, fresh meats…

Many foods such as fruits and vegetables are treated using electron

beams to help keep them fresher for longer. Image credit: World Bank

Photo Collection/Flickr