Radiation Detection

6088 Unique Visitors
11722 Page-views

Our universe is made up of two elements: mass and energy. These two entities, at their most basic level, comprise a single reality that manifests itself as mass and energy at different moments. Through Einstein’s famous mass-energy relation, E = mc2, they are inextricably linked. Energy, like matter, can move from one point in space to another in the form of particles or waves.

These energy carriers always come from somewhere and travel across space until they are absorbed by or annihilated by some material. This transfer of mass and energy over space is referred to as “radiation.”

Radiation has played a key part in technical advancements in a range of industries since its potential was realised. We all benefit from the use of radiation in medical diagnoses and treatment, for example. On the other side, atomic explosions and radiation exposure have exposed the globe to the dangers of radiation.

Whether we consider radiation to be a threat or a boon, its research is critical to our survival and development. If we carefully consider the benefits and risks associated with the use or misuse of radiation, we may see that its benefits definitely outweigh its drawbacks. Radiation has limitless potential, and when used properly, it may be extremely useful to humanity.

Ionizing and non-ionizing radiation, particles and waves, dangerous and non-hazardous radiation, and so on are all types of radiation. None of these classifications, on the other hand, draw solid lines between the attributes of the individual particles that make up the radiation; rather, they depict the bulk behaviour of particle beams. For example, simply because an electron belongs to the category of ionising particles does not mean it always ionises the atoms with which it interacts.

The most prevalent method of interaction leads atoms to ionize when a large number of electrons come into contact with a large number of atoms. Radiation is sometimes classed based on its wave and particle properties, which has puzzled physicists. If light had continuous wave qualities, scientists found it impossible to explain the relationship of energy radiated by a black body on the wavelength of emitted radiation. Max Planck answered the enigma by developing a hypothesis in which light waves were quantized and propagated in little wave packets rather than being continuous.

Term photon was later given to this wave packet. This hypothesis, as well as the mathematical model that accompanied it, was exceedingly successful in explaining the black body spectrum. When Einstein explained the photoelectric effect, a photon with the correct amount of energy knocks off a bonded electron from an atom, he further reinforced the concept.

Electromagnetic energy is emitted and absorbed in discrete bundles, according to Max Planck. The amount of energy carried by such a bundle (i.e., a photon) is proportional to the radiation’s frequency.

Sources of radiation
Natural and Man-made radiation sources are the two types of radiation sources.

Commonly Used Radioactive Isotopes

Cosmic, terrestrial, and interior sources of radiation are the three types of natural sources of radiation. Most of these sources only expose us to a small amount of radiation, which isn’t known to do any measurable harm to our bodies. However, as we’ll see later in this section, some potentially harmful compounds in our environment, such as radon, are a matter for concern because they can provide high integrated doses.

Sources of cosmic radiation

Radiation from a number of sources, including burning (e.g., our Sun) and exploding (e.g., supernovae) stars, fills outer space. These bodies emit massive amounts of radiation, some of which makes its way to Earth. Fortunately, the Earth’s atmosphere protects us from the harshest effects of this radiation. The ozone layer, for example, blocks UV wavelengths from the Sun. However, not all dangerous radiation is blocked, and some of it reaches the Earth’s surface, causing skin burns and cancer in those who are exposed to sunlight for long periods of time. Where the ozone layer has been reduced, the situation is far worse.

There is also background radiation of low-energy photons on top of these localised sources of radiation. This radiation is assumed to be a leftover from the big bang, which formed the universe. Because the photon spectrum peaks in the microwave portion of the electromagnetic spectrum, it’s called cosmic microwave background radiation. Despite the fact that these photons reach the Earth’s surface, they are not considered dangerous due to their low energy.

Aside from photons, there are a variety of other particles that are constantly generated in space. However, due to magnetic deflection or the Earth’s upper protective atmosphere, the vast majority of them never make it to the surface.

Muons, electrons, and neutrinos are some of the particles formed when other cosmic particles interact with atoms in the upper atmosphere. These low-energy particles are constantly bombarding the Earth’s surface, but due to their low contact probabilities, they are not thought to pose a substantial health risk.

Because of their low interaction cross sections, some muons and neutrinos created directly by light objects in space make it to Earth.

Terrestrial radiation sources
This form of radiation can be found in small amounts all around us and is almost impossible to avoid. Our environment, the water we drink, the air we breathe, and the food we eat all play a role in our health.

We are all tainted with trace amounts of radiation-emitting isotopes in the food we eat. Despite the fact that these isotopes are exceedingly dangerous, they do not cause cancer.

Unless they are present in larger concentrations, they do not do any significant harm to our bodies,

compared to typical concentrations

Uranium and its decay products such as thorium, radium, and radon are the primary sources of terrestrial radiation. Despite the notion that the overall natural concentration of these radioactive elements is safe for humans, h igher quantities of uranium and thorium in surface soil have increased radiation levels to harmful levels in several parts of the world.

Unfortunately, man has exacerbated the problem by detonating nuclear weapons and dumping nuclear waste.

The two radon isotopes, 222Rn and 220Rn, as well as their daughter products, are the most frequent radioactive elements present in our environment. The main source of worry for these -emitting isotopes is inhalation or digestion, in which case short-range -particles continually cause harm to interior organs, leading to cell alterations and eventually cancer.

Internal radiation sources
Our bodies contain traces of radioactive materials that expose our tissues to modest doses of radiation on a regular basis. Potassium-40 and carbon-14 isotopes are the main sources of internal radiation. However, the absorbed dose and tissue damage caused by this radiation are negligible.

Man-made sources
Scientists began working on generating sources that might be utilised to produce radiation in controlled laboratory circumstances soon after the discovery of radiation and recognition of its potential. These sources are designed for a specific function and emit a single type of radiation. Medical X-ray machines, airport X-ray scanners, Physics and Engineering of Radiation Detection, isotopes used in nuclear medicine, particle accelerators, and lasers are all examples of such sources.

Medical diagnosis and therapy expose the public to the highest levels of radiation out of all of these sources.

Patients should not be subjected to repeated X-rays unless it is absolutely necessary.

Radiation is also emitted as a by-product by various consumer products.

Sources of Natural Radiation

Television, smoke detectors, and building materials are examples of such sources.

As a result, there are several naturally occurring and man-made isotopes that produce various types of radiation. Some of these radioisotopes have found applications in a number of sectors based on their half-lives, types of radiation they produce, and energy.

Radiation  Detection is done by following tools:

1.Gas-filled detectors
Radiation travelling through a gas can ionise the gas molecules if the energy delivered by the radiation is greater than the gas’s ionisation potential. An external electric field can be used to make the charge pairs formed move in opposite directions, resulting in a detectable electrical pulse. The so-called gas-filled detectors were built using this method. A gas-filled detector typically has a gas cage as well as positive and negative electrodes. Depending on the design and mode of operation of the detector, the electrodes are kept at a high potential difference that can range from a few hundred volts to a few thousand volts.

The externally applied electric field is perturbed by the generation and movement of charge pairs caused by the passage of radiation in the gas, resulting in an electrical pulse at the electrodes. The ensuing charge, current, or voltage pulse at one of the electrodes can then be monitored, providing useful information about the particle beam, such as its energy and intensity, when combined with correct calibration.

It’s obvious that such a system would work well if a huge number of charge pairs were not only produced, but also readily gathered at the electrodes before recombining to form neutral molecules.

We can control the formation of charge pairs and their kinematic behaviour in the gas by changing the gas, the detector shape, and the applied voltage.

Gas-filled detectors’ operating regions

A detector can operate in a variety of modes depending on the bias voltage provided, with each mode differing in the amount of charges produced and their movement inside the detector volume. The application determines which mode to use; in general, detectors are tuned to work only within the range of the applied voltage that is typical of that mode.

Regions of operation of gas filled detectors are following:

1. The charges created by the passage of radiation quickly recombine to form neutral molecules in the absence of an electric field. When the bias voltage is applied, some of the charges begin to drift toward the electrodes on the opposite side. The recombination rate drops as the voltage is increased, while the current passing through the detector increases. The current recorded at the detector’s output does not adequately reflect the energy deposited by the incoming radiation due to significant recombination in this region. As a result, using the detector in this region is pointless in terms of monitoring radiation properties.
2. Ion chamber region- In the recombination region, the collection efficiency of electron ion pairs increases with applied voltage until all the charges created are collected. The so-called ion chamber region begins at this point. In this region, increasing the high voltage has no effect on the measured current because the electrodes efficiently collect all of the charges created. The saturation current is proportional to the energy deposited by the incident radiation and is monitored by the related electronics in this region. Ionization chambers are detectors that are designed to work in this region.
   In the ion-chamber region, it is nearly impossible to totally exclude the potential of charge pair recombination. Ionization chambers with plateaus with negligibly tiny slopes can be created with suitable design.
3. Proportional region- Primary ionisation is the process of producing electron ion pairs by the passage of radiation. Secondary ionisation occurs when the charges created during primary ionisation have sufficient energy to produce additional electron ion pairs. It is also possible for these charges to ionise further if they have enough energy. Obviously, this process can only take place if there is a high enough electric potential between the electrodes for the charges to reach extremely high velocities. Although the energy obtained by the ions increases when the bias voltage is increased, the electrons are the ones who produce the majority of future ionizations due to their tiny mass.

The proportional detectors use this multiplication of charges at high fields to raise the height of the output signal. The multiplication of charges in such a detector occurs in such a way that the output pulse is proportionate to the deposited energy. An electric field of several kV/cm is not uncommon for a detector operating in the proportional range. This high electric field not only speeds up charge collecting, but it also triggers avalanche multiplication, which is the rapid multiplication of charges by primary charges generated by incident radiation. The output pulse amplitude increases as a result of the charge multiplication.

The output pulse amplitude remains proportional to the bias voltage up to a particular bias voltage. A detector working in this region is therefore called as a proportional
counter.

• Region of limited proportionality- As the bias voltage is raised, more and more charges are generated inside the detector’s active volume. Due to the fact that heavier positive charges move far slower than electrons, a cloud of positive charges forms between the electrodes. The electric field is shielded by this cloud, which minimises the effective field experienced by the charges. As a result, there is no longer any guarantee that the total number of charges created will be proportional to the original number of charges. As a result, this area is known as the region of limited proportionality.
• Geiger Mueller region- Increased voltage may cause a significant avalanche in the gas, resulting in a huge number of charge pairs. As a result, the readout electronics detect a big pulse of many volts. The so-called Geiger Mueller region begins at this point. Individual incident particles can be counted in this region because each particle creates a breakdown and a pulse above the noise level. The detectors used in this region are not suitable for spectroscopy since the output pulse is neither proportional to the deposited energy nor dependent on the kind of radiation.

Such detectors also have a substantial amount of dead time. The term “dead time” refers to the period of time when the detector is virtually turned off. This can occur as a result of the detector’s internal processes or the speed with which the related electronics can process the signals. If there is a sufficient concentration of positive charges, the internal electric field can be reduced to the point where avalanche multiplication is no longer possible. If charge pairs are produced by radiation during this interval, the charges are not doubled, and no pulse is formed. As soon as the relevant electrode has collected the majority of the positive charges, the detector resumes operation.

The intense multiplication of charges in a GM detector is sometimes referred to as a gas breakdown. The vast number of ions produced during the avalanche move much more slowly than electrons, therefore they take longer to reach the cathode. When these hefty positive charges collide with the cathode wall, additional ions from the cathode material can be released into the gas. This procedure has an efficiency of less than 10% in most cases. The second Townsend coefficient is called as. is not high enough to induce a considerable rise in charge population at moderate voltages. The probability of secondary ion emission increases with higher voltages, worsening the output pulse’s linearity with applied voltage.

• Further voltage increases may cause gas discharge. The current reaches extremely high levels at this point, with only the external circuitry acting as a limit. That is, the pulse’s height becomes independent of the number of electron ion pairs present at the start. In this area, geiger tubes are used.
• Continuous discharge- If the high voltage is pushed even higher, the aforesaid breakdown process can progress to a continuous discharge process. This continuous discharge begins as soon as a single ionisation occurs and is uncontrollable unless the voltage is reduced. Electric arcs can form between the electrodes in this area, causing the detector to be damaged. Radiation detectors are obviously unable to function in these settings.

Ionization chambers: One of the first forms of radiation detectors was the ionisation chamber. They are still one of the most extensively used detectors due to its simple design and well-understood physical processes. Normally, chambers are operated in the middle of the plateau zone to minimise excessive current changes when the power source voltage varies slightly. This assures stability and reduces the need for extremely steady power supply, which are typically fairly costly.

An ion chamber’s mechanical design consists of three main components: an anode, a cathode, and a gas enclosure. The geometry of these pieces vary depending on the application. Because of their ease of design and manufacture, durability, radiation hardness, and low cost, ionisation chambers are widely employed in a number of applications.

Diagnostic X-ray readings, portable dosage monitoring, radiation intensity monitoring, and use in smoke detectors are just a few of their common uses. It’s important to remember that there’s no such thing as a universal detector that can be utilised in any situation. While some detectors, such as ionisation chambers, can be utilised in a number of applications, the majority are designed and built to meet specific needs. As a result, discussing the benefits and drawbacks of detectors is fairly subjective. We shall, however, take a broad look at the pros and downsides of ionisation chambers due to their versatility.

Their advantages are
● Insensitivity to applied voltage: Because the ionization current in the ion chamber region is essentially independent of the applied voltage, small fluctuations and drifts in high-voltage power supplies have no effect on the system resolution. This also means that lower-cost power supplies can be used to bias the detector safely.

● Proportionality: The energy deposited by incident radiation is directly proportional to the saturation current.
● Less vulnerability to gas deterioration: Because ionisation chambers do not have gas multiplication, modest changes in gas quality, such as an increase in the concentration of electronegative pollutants, do not have a significant impact on their performance. This is true for low-resolution devices operating in moderate to high-radiation domains at the very least.

Despite the fact that ionization chambers are among the most extensively used detectors, they have their own set of restrictions, the most significant of which are described below.

● Low current: In most radiation situations, the current flowing through an ionisation chamber is quite tiny.

2.Liquid- filled detectors

When radiation flows through a liquid, charge pairs are created, which can then be directed toward electrodes to generate a pulse. It can be utilised as an active medium for radiation measurements if the liquid has a good relation between the deposited energy and the number of charge pairs formed. As it turns out, a variety of liquids have a reasonable proportionality and can thus be utilised as detection medium. In a liquid, the chance of charge recombination should be substantially larger than in a conventional gas. This is correct, but we must also keep in mind that increased density ensures the formation of a greater number of charge pairs.

In large area detectors, such as liquid calorimeters for high-energy physics investigations, liquefied argon is the most widely utilised detection medium.

In a liquid, the underlying mechanism of creating a charge pair is the same as in a gas. The method is a little more complicated in the case of liquids since the energy levels in the liquid phase are significantly different from those in the gaseous phase. Each molecule in a gaseous state can be thought of as a unique entity with its own discrete energy levels, at least to a decent approximation. The issue is more complicated in liquids, because molecules in close proximity are vulnerable to each other’s electromagnetic fields.

The presence of energy bands is a result of this physical proximity. As a result, in order to comprehend the formation of a charge pair in a liquid, we must consider the entire liquid rather than its individual molecules.

Solid-state detectors

Despite the fact that gas-filled detectors have shown to be incredibly effective in a wide range of applications, their use is restricted due to a variety of problems. For example, in low-radiation settings, the minimal number of electron ion pairs that may be formed in a gas is a severe difficulty for high-resolution systems. The number of target atoms per unit volume in the gas that the incident radiation meets is one cause for this inefficiency. This means that if we employ liquids or solids instead of gases, the likelihood of charge pair creation increases. However, it turns out that, in addition to density, the mechanism of charge pair generation is influenced by a number of other parameters.

However, when compared to gases, one sort of solid has been discovered to have considerably superior charge pair generation capabilities. The electrical conduction properties of these so-called semiconductors are intermediate between those of conductors and insulators. Another material that has been discovered to have excellent charge pair generation potential is diamond. Solid-state detectors refer to all detectors that utilise solids as active sensing media, a phrase that is occasionally used exclusively for semiconductor detectors.

Semiconductor detectors- Semiconductors are crystalline solids with covalent bonds that hold the atoms together. Because their electrical conduction qualities are intermediate between those of insulators and conductors, they are referred to as semiconductors. Two of the most often utilised semiconductor materials are germanium (Ge) and silicon (Si). The bulk of semiconductor detectors have so far been made of silicon, but as the search for more radiation-tolerant semiconductors continues, this trend may change. GaAs has proven to be a promising material for use as a detecting medium.

Some semiconductors are not suitable for use in radiation detectors. Many factors influence the decision, including resistivity, charge mobility, drift velocity, purity, operating temperature, and cost. The most often used material in particle detectors has traditionally been silicon, however this is changing. Other materials that are often utilized include germanium (Ge), gallium arsenide (GaAs), and cadmiumzinctellurium (CdZnTe). The demand for a new generation of radiation-hard silicon detectors is driving scientists to create increasingly complicated semiconductor architectures.