Among the natural radiations that concern most are radon, cosmic radiation, galactic, and solar particles. The alpha (α) radiation emitted by radon is same as the α-radiation radiated from plutonium (Pu). It is a naturally occurring radioactive gas. You cannot see it, smell it, or taste it. The primary routes of potential human exposure to radon are inhalation and ingestion. Humans, particularly in underground and work areas such as mines and buildings, are exposed to elevated concentration of radon and its decay products. The next natural radiation that affects the environment is the galactic and solar particles that have free access to spacecraft outside of the magnetosphere and were identified in early 1960 during the malfunction of the spacecraft electronics.

Figure 2.2 shows EGRET gamma ray surveyed by NASA, USA. Some galactic cosmic rays interact with the interstellar medium and produce gamma (γ) rays [2]. The terrestrial and cosmic rays induce single events and produce an annual dose equivalent to 3.1 millisievert (mSv) [3, 4]. Table 2.1 shows the list of the radiation sources that affect the environment and the surrounding.

Levels of natural or background cosmic radiation, however, can vary greatly from one location to another depending on the altitude. The biological effects due to natural or background cosmic radiation are so small that they may not be counted for.

So far, we have discussed mostly about the biological effects of ionizing radiation on living animals in context of human species. But when we are talking about the effect of ionizing radiation on the environment and the surrounding, we must consider the plants and vegetations that are part and parcel to our environment. An obvious question would be: How the ionizing radiation affects the plants and the vegetations that constitute the major part of the environment? Real and his coworkers [31] have reported a comprehensive review on the effects of ionizing radiation exposure to plants, fish, and mammals.

Figure 2.8 shows radiation effects on plants in a field near Pripyat in the Ukrainian Soviet Socialist Republic. On April 26, 1986, reactor number four at Chernobyl Nuclear Power Plant near Pripyat in the Ukrainian Soviet Socialist Republic exploded. The plume drifted over extensive parts of Western Soviet Union, Eastern Europe, Western Europe, Northern Europe, and eastern North America with light nuclear rain falling as far as Ireland. After the disaster, three square miles of pine forest in the immediate vicinity of the reactor turned ginger brown and died, earning the name of the Red Forest [32–34].

Effects of ionizing radiation on plants and animals are considered at two levels: (1) on the individual and (2) on the population [35]. The population effect arises from the individual effects, and it is only the statistical aspect, which is important at the population level. For plants, population effects are loss of foliage, reduced growth, and loss of reproductive ability and ultimate extinction of the species. In the case of animals, it is generally considered on the basis of the health and overall survival of the animals suffering from chronic diseases.

Higher plants, which are more sensitive to radiation, are affected by ionization energy less than 10–1,000 Gy. Extensive studies on pine-birch forests have shown that trees of the same species and age have different dose sensitivities depending on external conditions such as light interception, soil fertility, and wind erosion. For example, several studies on the general threshold in the oak–pine forest at Brookhaven have shown that approximately 0.5 Gy/day (d) is detrimental to the trees. Most of the species are found to be affected when the radiation label reaches approximately 3 Gy/d. Several plants such as algae, fungi, and symbiotic lichens are resistant to gamma-ray radiation. However, older plants and mostly the woody species are seen to be affected with a dose of 10 Gy/d or more. The Erigeron canadensis are seen to show detrimental effects at 6.4 Gy/d, where most of the Mediterranean species are found more resistant to ionizing radiation. However, the sub-Mediterranean species are not as much resistive to ionizing radiation compared to the Mediterranean species. Deserts shrubs are seen to be affected when the ionization goes beyond 0.001–0.1 Gy/h and above 0.07 Gy/d are seen to be detrimental to all vegetation.

It has been found that gamma radiation produces enhanced protease activities in some seeds like peanut and wheat and induces oxidative stress, which damages the protein quantity of the seeds. Gamma radiation also damages the enzyme of the citrus fruits when the dose exceeds 15 Gy. Considering reproductive viability, even though the size and rate of pollen are temporarily affected with a low dose of 0.7 Gy/d, they get restored within 3 years for irradiation up to 12 Gy [36].

From the studies of the radiation exposure and the consequences that are so far presented, it seems that mammals are the most sensitive to ionizing radiation, and a dose of 5–15 Gy can be lethal dose to them. At the other extreme are the bacteria and protozoa that are affected with a dose ranging from 10³ to 10⁴ Gy. Classical Andrews lymphocyte depletion curves (1–4) and accompanying clinical severity ranges correspond roughly the following graph (Fig. 2.9) [38]:

Table 2.4 shows the comparative radiosensitivity of groups of organisms.

Besides being its detrimental effects on health of the living cells and tissues of creatures, nuclear radiation induces defects in several semiconductor devices and electronics leading to enhanced generation/recombination currents. As a result, charge collection signal is reduced, and there is a drift in operating point.

The vertical primary population intensities of galactic cosmic rays (GCR) and solar particles affect the electronics of the spacecraft. As a result, military space electronics require special technology to study the primary population intensities of GCR and solar particles that cause variations in secondary neutron and proton levels [2]. During irradiation, the incident particles collide with the nucleus and transfer part of the energy. Experimental observations show that there is an exact linear relationship between ionizing energy deposited by the incident particle in lattices of the semiconductor materials and the measured degradation [39]. However, long-term exposure to ionizing energy results in nonlinearities, and these complicate the determination of displacement damage factors.

Radiation-induced defects in silicon (Si)- and germanium (Ge)-based junction devices (e.g., CMOS, bipolar transistors, photodiodes) are well established [40, 41]. Irradiated energetic particles incident on the device lose their energy to ionizing process and result in the production of electron–hole pairs and displaced atoms. The effectiveness of the radiation-induced displacement damage depends upon the bombardment conditions and on the time after irradiation [42]. A key theme that emerged from all these studies by various workers suggests that the effects on electrical properties of the semiconductor materials depend on the irradiated rays (fission or neutrons or gamma rays) [43, 44].

As a matter of fact, the metal oxide semiconductor (MOS) technology, particularly complimentary MOS (CMOS) technology, which is the dominant commercial technology, has been severely affected by ionizing radiation [27, 28]. On the other hand, silicon–on–insulator (SOI) technology has been developed for radiation-hardened applications for many years. However, bipolar amplification caused by floating body effects can significantly reduce the single-event upset (SEU) hardness of SOI devices [29].

In bipolar junction transistors (BJTs, npn, and pnp), current gain is noticed when base emitter junction is irradiated [30, 31]. Earlier study by Goben demonstrated the damage in bipolar transistor by neutrons [32].

Many medical modalities like computer tomography (CT), positron emission tomography (PET), single–photon emission tomography (SPECT), magnetic resonance imaging (MRI), and ultrasound are inherently digital. These digital technologies offer significant improvements in image quality and dose utilization [33]. However, most of these digital circuitry (flat–panel detectors (FPD), charge–coupled devices (CCD), and thin–film transistor (TFT) array) used in medical imaging system are based on solid-state integrated circuit (IC) technology, similar in many ways to the imaging chips used in visible wavelength digital photography and video [45]. These photonic imagers are also being used in space systems, where they are exposed to space radiation environment. Ionizing radiation during X-ray and γ-ray exposure affects the normal behavior of these devices [46].

Aerospace is involved in collaborative research efforts to study the novel approaches for hardening commercially available integrated circuits (ICs) against single-event latch-up. The picosecond laser facility is also being used to study the effectiveness of various design strategies for mitigating the effects of single-event transients in digital electronic circuits. Figure 2.10 shows the use of a large-diameter laser beam allowing for rapid identification of regions of complex integrated circuits (ICs) that are susceptible to single-event effect.

It has been observed that when a cosmic ray passes through the drain region of an NMOS transistor, a short circuit is momentarily created between the substrate (normally grounded) and the drain terminal (normally connected to the positive power of the supply voltage) resulting a spike of current. The amount of charge that is collected from the ion track can burn out the device or may cause malfunctioning of the device. Figure 2.11 shows the path of the cosmic ray through the drain of an NMOS transistor.

During 1990s, many groups were involved in studying the radiation effects in charge-coupled devices (CCDs, arrays of MOS capacitors) including CMOS [47] thin-film transistors (TFTs), passive pixel arrays such as charge-induced devices (CID), photodiodes (including avalanche), infrared (IR) detectors, and other semiconductor devices that are being used in space program or in medical imaging system [48–51].

High dose of irradiation causes radiation damage and general alteration of the operational and detection properties of a detector (scintillator). There are three main aspects of radiation stability in plastic scintillator such as polymer hardness (optical stability), dopant stability, and stability in fiber waveguide structures. The amount of light intensity losses in the scintillator (which is a bulk effect of the polymer) depends on the dose and the building material of the scintillator. The light yield loss and the transmittance loss in plastic scintillators are about ~20 % and 2–6 %, respectively, for a dose of 10⁵Gy [52]. In scintillation fibers, radiation degrades scintillation core, polymer cladding, and core/cladding interface [53]. On the other hand, garnet-containing inorganic scintillators of terbium (Tb) and lutetium (Lu) show increased resistance to radiation damage until the dose reaches 10⁵Gy or higher [54].

Beta particles are energetic electrons emitted from the nuclei of many natural and man-made radioactive materials. Low-energy beta particles are less penetrating than the alpha particles, requiring special techniques for detection. From the detection standpoint, unfortunately, high-energy beta and gamma radiations are not the primary decay products of the most likely radioactive materials. Rather, the major potential source of beta and/or gamma radiations is from fission products.

Alpha (α) and beta (β) particles are detected and measured by ionization chamber. Absolute measurements of (α) particle are straightforward and involve evaluation of the effective solid angle subtended by the counteractive volume (which is close to 2π for initial source flow counts). On the other hand, absolute (β) activity measurements are often carried out in a 4π flow counter. When both alpha (α) and beta (β) particles are present in the emitting source, the spectra of α- and β-particles will be different [55–57]. The β-spectrum is generally broader and shorter in pulse height. The corresponding count curve will show two distinct plateaus: In the first plateau, αs are counted, and in the second, both αs and βs are counted. It has been observed that the beta plateau is generally shorter and shows a greater slope than alpha plateau. The obvious reason is that the beta particle pulse height distribution is broader and less well separated from the low-amplitude noise.

Figure 2.12 shows the spectra of alpha and beta particles, when the pulse height (H) is plotted against differential number of pulses within the differential amplitude increment (dN/dH). The right-hand-side curve shows the plateaus when counting rate is plotted against the applied voltage (V).

Experimental detection of α-particles from the radioactive decay of natural bismuth (²⁰⁹Bi) has been reported by P. de Marcillac et al. [55]. They have detected an energy release of 3,137 ± 1 (statistical) ± 2 (systematic) keV and a half-life of (1.9 ± 0.2) ~ 1,019 years, which are in agreement with expected values. It has been observed that the nuclear structure of naturally occurring ²⁰⁹Bi isotope gives rise to an extremely low decay probability and it generates low-energy particles that are difficult to detect. Therefore, their detection has no quantitative use but is used as a safety precaution.

The key process of detecting gamma ray is by ionization. During ionization, it gives up part or all of its energy to an electron. The energized electrons then collide with other atoms and liberate electrons. The liberated charge is collected either directly, using gas ionization type (ion chambers, proportional counters, or Geiger–Muller (GM) counters) counters, or indirectly (as with scintillation detector), in order to record the presence of gamma ray and measure its energy. Details of these counters and the scintillators will be discussed later.

X–rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus. To detect X-rays, we can use either a photographic plate or photostimuable phosphor. Proportional counters are the most common X-ray detectors used, although CCD chips are rapidly gaining popularity. Besides, Geiger counter based on the ionization of gases is also being used.

Scintillators like NaI:Tl can convert X-ray photons into visible photons and can be detected by a photomultiplier tube (PMT). Other solid-state direct detectors like lithium-doped silicon (Si(Li)) and lithium-doped germanium (Ge(Li)) have also been used to detect X-rays. However, detectors like CdTe, CZT, and HgI₂ have also been used to detect X-rays. Recently, these materials are deposited on a TFT array or flat-panel array for medical imaging using X-rays.

Thermal neutrons are used in nondestructive testing (NDT) because of their advantages over X-ray and gamma-ray testing [86, 87]. As a matter of fact, NDT testing by thermal neutrons provides complementary information about the interactions of photons and neutrons with matter. Photons interact with the atomic electrons, while neutrons interact with the atomic nucleus itself. Neutrons can penetrate into the high-density materials that are opaque to X-rays thus allowing the inspection of objects obstructed by a dense material. Moreover, the attenuation of thermal neutrons (relative to X-rays) is pronounced for lighter elements such as hydrogen, carbon, and their compounds that comprise the base for many organic materials. The nuclides that are frequently used for thermal neutron detection are helium (He³), lithium (Li⁶), boron (B¹⁰), cadmium (Cd¹¹³), and gadolinium (Gd¹⁵⁵ and Gd¹⁵⁷) [88–90].

The energy of a slow neutron is extremely low (energy below the cadmium cutoff of about 0.5 eV) with the reaction Q-value, and it loses its initial kinetic energy (KE) during conversion process. Slow neutrons are detected through nuclear reactions which result in energetic charged particles such as protons and alpha (α) particles [91]. The neutron detectors are composed of two detectors—the target detector and the detector for detection and measurements. Since cross section for neutron interactions in most materials is a strong function of the energy of the incoming neutrons, different techniques have been developed to detect and measure neutrons in different energy regions. For the detection and measurements of slow neutrons in an efficient way, we have to consider several factors:

The instrument, in principle, is the simplest of all the gas-filled detectors, where ions are formed by direct ionization and measured by the application of electric field. Unlike proportional counter and the Geiger tubes that are operated in pulse mode, ion chamber is operated either in current mode or pulse mode [97, 98].

Figure 2.13 shows the picture of an ionization chamber, which is an electrically closed vessel containing an internal electrode.

Working Principle: Ions are formed in the gas chamber either by direct interaction with the incident particle or through a secondary process in which some of the particle energy is first transferred to an energetic electron. Experimental observation shows that when a particle having energy of 1 MeV is stopped within the gas, it creates almost 30,000 ion pairs. The deposited energy is proportional to the number of ion pairs formed provided the average energy lost by the incident particle per ion pair formed (W) is constant [99].

The total number of energy of the incident particle, which is converted into the information carriers, is seen to fluctuate. An empirical constant known as Fano factor is multiplied with the predicted variance to convert it into the experimentally observed variance. The empirical constant is only significant when the detector is operated in pulse mode [100, 101].

The incident particle having sufficient energy ionizes the gas inside the chamber and creates positive ion and free electron. In course of time, the electron and the positive ion gain thermal energy and can diffuse away from the regions of higher density. During thermal motion, collision between positive ion and free electron results in recombination, and a state of charge neutrality exists. However, in presence of electric field, the free electron and positive ion gain momentum, which is known as drift velocity [102]. The drifting of the charges actually constitutes an electric current, and a dc ion chamber is used to measure the ionizing current.

At high voltage, the ionization current will be saturated, the electric field will be large everywhere, and the recombination will be negligible. The percent by which the measured ion current falls short of true saturation is seen to increase as a function of the measured ion current. It is not unusual to observe some imbalance in the steady-state concentrations of the two charges when electron drifts toward anode followed by the drift of positive charge toward cathode. As a result, a gradient will exist for a species that is free to migrate, and some net diffusion must take place in the direction of decreasing concentration. The direction of diffusion will be opposite to the direction of charge carrier flow by the electric field, and the effect will reduce the measured ion current. The ratio of the change in ionizing current to the total ionizing current can be modeled after Rossi and Staub as [103]
where ϵ is the ratio of the average energy of charge carrier with electric field present to that without electric field, k is Boltzmann constant, T is temperature in absolute, e is electronic charge, and V is the applied voltage.

The free–air ionization chamber has become the most popular instrument to measure ionization of gamma rays [104–106]. The parallel plate electrodes inside the chamber receive the collimated gamma (γ)-ray radiation. There are three small electrodes on the top. The central electrode on the top receives the signal, and it is kept at negative potential with respect to the bottom electrode. The two other small electrodes on the top are grounded. Figure 2.14 shows the schematic of a free-air ionization chamber.

The drawback of this system is the loss of ionization due to the creation of secondary electrons at high energy (>100 keV) which creates some difficulties during measurements. In order to minimize the loss of ionization measurements, gamma-ray (γ) energy is kept below 100 keV. At higher energies, γ-ray exposure measurements are carried out in cavity chambers surrounded by a small volume of air. The secondary electrons created by collisions make the surrounding volume of air large. To improve the situation, the volume of air is compressed, and the walls are made sufficiently thick—a condition called electronic equilibrium is established. It has been observed that the exposure rate R (C/kg-s) of an air equivalent ion chamber is directly proportional to the saturated ion current (I _s) in amps and inversely proportional to the mass (m) in kilogram (kg), and the relation can be expressed mathematically as
where Q is the charge due to ionization, V is chamber volume (m³), P = air pressure in the chamber, P ₀ is the saturated pressure (760 mmHg), T air temperature inside the chamber, and T ₀ is standard temperature (273.15 K).

Again, Q, the ionization charge, is equal to (eT)/(W), where e is the electronic charge; T is the kinetic energy of the charged particle; and W is the mean energy required to create an ion pair in a gas. Now, we will introduce a new term called kerma (ℜ), which is defined as the kinetic energy released per unit mass, and we will define it as
where (1 − g) is the mean correction for energy given to the radiated photon. The value of W _air = 33.97 eV with a relative standard uncertainty of 0.15 % [107].

Mostly, the ionization chambers are used in current mode, and the average rate of ion formation within the chamber is measured. However, an ionization chamber can also be operated in pulse mode in which separate radiation quantum gives rise to distinguishable single pulse. The equivalent circuit of an ion chamber operated in pulse mode is shown in Fig. 2.15.

Proportional counter is a gas-filled (mostly 90 % argon and 10 % methane) ionization counter that works on the same principle as the Geiger–Muller (GM) counter. One of the most important applications of PCs as detection and spectroscopic instruments is for low-energy radiation measurements (α-counting, β-counting, mixed α- and β-counting, and for the measurements of γ- and X-rays). They are widely used to detect neutrons. Figure 2.16 shows an earlier version (1952) of a 4-pi gas flow proportional counter. Schutmeister and Meyer are generally credited for the construction of the first true 4-pi detectors in 1947–1948.

According to the design, the PC can be position–sensitive, parallel–plate avalanche, multi–wire proportional, or gas proportional scintillation type. It relies on the phenomenon of gas multiplication due to secondary ionization, which is a consequence of increasing electric field, and acceleration of the electron liberated by the secondary ionization process. During its subsequent drift, it undergoes collisions with other neutral gas molecules and creates additional ionization. However, positive and negative ions achieve very little energy between collisions and do not take part in secondary ionization.

Figure 2.17 shows the schematic of a simplified proportional counter circuit, which operates in the proportional region. The circuit illustrates that the walls act as one of the electrodes (cathode) and a long fine wire at the center of the chamber acts as an another electrode (anode). The geometry of the electrodes and the voltage on them are chosen in such a way that in most of the volume of the counter, the electric field strength is not enough to produce a Townsend avalanche [108].

The gas multiplication process takes the form of a cascade and is known as Townsend avalanche (Fig. 2.18). Now, we can define the Townsend avalanche coefficient (α) as the fractional increase in the number of electrons (n) per unit path length (x). Mathematically, it is expressed as

As the applied voltage is increased, a nonlinearity in the pulse form is noticed. On the other hand, when the applied voltage is low, the field is insufficient to prevent recombination of the original ion pairs. However, over some region of the voltage–pulse amplitude curve, gas multiplication is linear, which means that the collected charges are proportional to the original numbers of ion pairs created by the incident radiation. This is the true proportionality region and represents the mode of operation of conventional proportional counter. Assuming the linearity between the Townsend coefficient (α) and the electric field (E), the multiplication factor (M) can be written as
where K is constant, b = cathode radius, a = anode radius, V = applied voltage, and p = gas pressure [109].

The charge, which is developed in a pulse during measurement (assuming no nonlinearity), is proportional to the number of ion pairs created (n ₀) by the incident radiation. The amplitude of the pulse, however, is seen to fluctuate from pulse to pulse, and the expression for variance in amplitude of the pulse is (Qq/Q)² and can be written as
where is the single-electron multiplication variation and (σ _n0/n ₀)² is the ion pair fluctuation. Following Furry distribution and assuming that A is reasonably large, the expected distribution in the number of electrons produced in a given avalanche reduces to a simple exponential form as
when the multiplication factor A (the mean value of A) is large due to electric field E, (σ _A/A)² ≈ 1/(1 + θ) ≈ c, where θ is a parameter related to the fraction of electrons where energy exceeds a threshold energy for ionization (0 < θ < 1) and c is cathode radius [60].

By proper functional arrangement, modifications, and biasing, proportional counter can be used as spectroscopic instruments for soft X-rays and gamma (γ) rays. It can also be used for neutron radiation detection in mixed radiation fields. As a matter of fact, low-energy X-ray spectroscopy is one of the most important applications of proportional counter. The energy of the absorbed photoelectrons formed by photon interaction within the gas is directly related to X-ray energy. However, proportional counter is not used when the gamma rays are having higher photon energies.

Position–sensitive proportional counter (PSPC) is based on the principle of charge division. The anode is a high-resistive wire and is connected to two amplifiers at two ends. Electrons drift along radial field lines from their place of formation. The position of the avalanche is a good indicator of the axial position at which the original pair is formed. By summing the amplitude of the two amplifiers, a conventional output pulse is produced with amplitude proportional to the total charge (Q _A + Q _B). A position signal is generated by dividing the output signal of a single amplifier by summed signal {(Q _A)/(Q _A + Q _B)} to give a pulse that will indicate relative position along length.

Figure 2.19 shows the diagram of position localization in proportional counters using charge division {(Q _A)/(Q _A + Q _B)}. It is proportional to the position of the particle track. In the Fig. 2.19, HV is the high voltage and G is the ground. The input impedance I _imp of the amplifiers is made high, compared to the resistance of the anode wire, R [61], which is made sufficiently large so that the collected charge on one side divided by the total charge on the two opposite sides of the amplifiers follows a simple relation to the position of the interaction [37, 62, 63]. An alternative method is developed on the basis of the relative rise time from the preamplifiers placed at either end of the resistive wire [64].

It consists of two parallel plate electrodes separated by a distance inside a closed chamber with a low-pressure gas inside. When a high voltage is applied, a charge particle that traverses the gap between the plates is multiplied through the usual gas amplification process and leaves a trail of ions and electrons. The numbers of ions that are formed and traveled through the gap reflect the amount of energy lost by the particle in transit. Experimental observation shows that for gaps of the order of 1 mm, the fast component rise time is about 2 ns under high field [65].

It is an instrument that can detect a single particle of ionizing radiation with an operating voltage in the Geiger plateau. It consists of a tube filled with low-pressure (0.1 atm) inert gas and an organic vapor or a halogen gas. Inside the chamber, there are two electrodes, between which a potential difference of several hundred volts is created without any flow of current. Figure 2.20 shows the schematic of an experimental setup with a Geiger–Muller counter.

The electrons from the gas discharge are collected as pulses within a microsecond. The pulses should have two slopes, the fast one corresponding to the collection of electrons and the slower one will correspond the drifting time of the positive ions. A single ion pair formed within the fill gas of the GM tube can trigger a full Geiger discharge, and the counting efficiency can reach 100 %.

A portable GM meter used to survey gamma (γ) radiation consists of high-voltage supply and pulse-counting meter. The higher field in GM tubes enhances the intensity of avalanche process. The most important achievement of a GM counter is that all pulses from a GM tube are of the same amplitude regardless of the number of original ion pairs that initiated avalanche process. Therefore, a GM tube cannot be used for direct radiation spectroscopy rather than as a simple counter of radiation-induced events [66]. Another disadvantage of GM tube is that besides lack of energy information, it has a large dead time that limits its application as radiation detector. Figure 2.21 shows a picture of a Geiger–Muller (GM) counter with an attached probe head.