Category: 2. Radio Isotopes

Nowadays radiotracer has become an indispensable and sophisticated diagnostic tool in medicine and radiotherapy purposes.

Diagnostic purpose

The most common radioactivity isotope used in radioactive tracer is technetium (⁹⁹Tc). Tumors in the brain are located by injecting intravenously ⁹⁹Tc and then scanning the head with suitable scanners.

¹³¹I and most recently ¹³²I and ¹²³I are used to study malfunctioning thyroid glands. Kidney function is also studied using compound containing ¹³¹I. ³³P is used in DNA sequencing. Tritium (³H) is frequently used as a tracer in biochemical studies. ¹⁴C has been used extensively to trace the progress of organic molecule through metabolic pathways.

A most recent development is positron emission tomography (PET), which is a more precise and accurate technique for locating tumors in the body. A positron emitting radionuclide (e.g., ¹³N, ¹⁵O, ¹⁸F, etc.) is injected to the patient, and it accumulates in the target tissue. As it emits positron which promptly combines with nearby electrons, it results in the simultaneous emission of two γ-rays in opposite directions. These γ-rays are detected by a PET camera and give precise indication of their origin, that is, depth also. This technique is also used in cardiac and brain imaging.

Compound X-ray tomography or CT scans. The radioactive tracer produces gamma rays or single photons that a gamma camera detects. Emissions come from different angles, and a computer uses them to produce an image. CT scan targets specific area of the body, like the neck or chest, or a specific organ, like the thyroid.

The sustainability of radioisotope production is one of the critical areas that receive great attention. There are more than 160 different radioisotopes that are used regularly in different fields; these isotopes are produced either in a medium or in high-flux research reactors or particle accelerators (low or medium energy). Some of the radioisotopes produced by the reactor and particle accelerators and their applications are given in Table.

Reactor radioisotope	Half-life	Applications
Radioisotopes produced by reactors
Bismuth-213	45.59 min	It is an alpha emitter (8.4 MeV). Used for cancer treatment, e.g., in the targeted alpha therapy (TAT)
Cesium-131	9.7 days	It emits photon radiation in the X-ray range (29.5–33.5 keV). Used in brachytherapy of malignant tumors
Cesium-137	30 years	Used in medical devices (sterilization) and gauges (661.64 keV)
Chromium-51	28 days	Used in Diagnosis of gastrointestinal bleeding and to label platelets (320 keV)
Cobalt-60	5.27 years	Used for controlling the cancerous growth of cells (1173.2 keV)
Dysprosium-165	2 h	Used for synovectomy treatment of arthritis (95 keV)
Erbium-169	9.4 days	Used for relieving arthritis pain in synovial joints (8 keV)
Holmium-166	26 h	Diagnosis and treatment of liver tumors (81 keV)
Iodine-125	60 days	Used in cancer brachytherapy and radioimmunoassay (35 keV)
Iodine-131	8 days	Widely used in treating thyroid cancer and in imaging the thyroid, diagnosis, and renal blood flows (284 keV)
Iridium-192	74 days	Used as an internal radiotherapy source for cancer treatment. Strong beta emitter for high-dose rate brachytherapy (317 keV)
Iron-59	46 days	Used in studies of iron metabolism in the spleen (1095 keV)
Lead-212	10.6 h	Used in TAT for cancers (239 keV)
Molybdenum-99	66 h	Used as the parent in a generator to produce technetium-99 m (740 keV)
Palladium-103	17 days	Used to make brachytherapy permanent implant seeds for early-stage prostate cancer. Emits soft X-rays (362 keV)
Potassium-42	12.36 h	Used for potassium distribution in bodily fluids and to locate brain tumors (1524 keV)
Radium-223	11.4 days	Used to treat prostate cancers that have spread to the bones
Rhenium-186	3.71 days	Used for therapeutic purpose to relief pain in bone cancer. Beta emitter with weak gamma for imaging (137 keV)
Samarium-153	47 h	Effective in relieving the pain of secondary cancers lodged in the bone, sold as Quadra met. Beta emitter (103 keV)
Selenium-75	120 days	Used to study the production of digestive enzymes (265 keV)
Sodium-24	15 h	Used for studies of electrolytes within the body (2754 keV)
Ytterbium-169	32 days	Used for cerebrospinal fluid studies in the brain (63 keV)
Radioisotopes produced by accelerators
Cobalt-57	272 days	Used as a marker to estimate organ size and for in vitro diagnostic kits (122 keV)
Copper-64	13 h	Used for PET imaging studies of tumors and also cancer therapy (511 keV)
Copper-67	2.6 days	Beta emitter, used in therapy
Fluorine-18	110 min	Used as fluorothymidine (FLT)
Gallium-67	78 h	Used for tumor imaging and locating inflammatory lesions (infections)
Indium-111	2.8 days	Brain studies, infection, and colon transit studies
Iodine-123	13 h	Used for diagnosis of thyroid function
Rubidium-82	1.26 min	Convenient PET agent in myocardial perfusion imaging
Strontium-82	25 days	Used as the parent in a generator to produce Rb-82
Thallium-201	73 h	Used for location of low-grade lymphomas

Table.

Some of the radioisotopes produced by the reactor and particle accelerators and their applications.

Radiotracers are widely used in medicine, agriculture, industry, and fundamental research. Radiotracer is a radioactive isotope; it adds to nonradioactive element or compound to study the dynamical behavior of various physical, chemical, and biological changes of system to be traced by the radiation that it emits. The tracer principle was introduced by George de Hevesy in 1940 for which he was awarded the Nobel prize.

The applications of radioisotopes have played a significant role in improving the quality of life of human beings. The whole world is aware of the benefits of the radiation, but the phobia of nuclear weapons on Hiroshima and Nagasaki (August 6 and 9, 1945) and the nuclear accidents occurred in Chernobyl in Russia (April 25–26, 1986) and Fukushima in Japan (March 2011) was so deep in the mind of the common man that we can still struggle to come out of it. Major problems arrived by workers in nuclear fields are due to lack of legalization, shortage of resources, and knowledge about nuclear society and safe guards. To minimize these problems the international organization such as International Atomic Energy Agency (IAEA) and International Commission on Radiation Protection (ICRP), identify requirements and provide the infrastructure that can support nuclear technology.

The decay series of uranium and the type of radiation and range of energy of decay products are shown in Table 2. The important daughter product of uranium series is radon and its progenies. Radon is a naturally occurring radioactive gas. This was discovered by F.E. Dorn in 1900. It is found everywhere as part of our environment (i.e., in soil, water, and air). The ubiquitous radioactive gas is formed by radioactive decay of radium (²²⁶Ra), which is the daughter product of uranium decay series. The half-life of radon is 3.82 days; it decays by emission of alpha particle to form radon decay products or progeny, which are divided into short-lived and long-lived progeny. These are the significant contributor of natural radiation. On the basis of the epidemiological studies, it has been established that the enhanced levels of indoor radon in dwellings can cause health hazards and may lead to serious diseases like lung cancer in human beings .

Thorium decay series

The decay series of thorium and types of radiation with range of energies of decay products are as shown in Table 3. The important daughter products in this series are thoron and its progenies. The thoron progeny has relatively long half-life than that of radon progeny; therefore thoron progeny would give a significant dose to the lungs [11, 12, 13]. The decay of thorium ²³²Th leads to the subsequent formation of thoron (²²⁰Rn), its half-life 55 seconds. It is more abundant than ²³⁸U, but the short half-life of ²²⁰Rn allows only a fraction to escape into the atmosphere. The ²²²Rn is the one of the most significant isotope and it contributes significant dose to publics as ionizing radiation.

There are two important sources of radiation: they are natural and man-made.

Natural background radiation

The radiation that exits all around us is called natural background radiation. All living organisms including man have been continuously exposed to ionizing radiations emitted from different sources, which always existed around us. The sources of natural radiation are cosmic rays and naturally occurring primordial radionuclides such as ²³⁸U, ²³²Th, ²³⁵U, and their decay products as well as the singly occurring natural radionuclides like ⁴⁰K and ⁸⁷Rb, which are present in the earth crust, soil, rocks, building materials, ore, and water in the environment. Background radiation is a constant source of ionizing radiation present in the environment and emitted from a variety of sources. Natural radiations originated from three major sources: terrestrial, extraterrestrial, and internal (intake of natural radionuclides and their daughter product) sources of radiation.

Terrestrial sources of radiations

Terra means earth; the radiation originated from the earth crust is called terrestrial radiation. The primordial radionuclides (²³⁸U, ²³²Th, and ⁴⁰K) present in varying amounts in soil, rocks, water, and atmosphere are the sources of terrestrial radiation. The bulk of the natural radiation is mainly due to ⁴⁰K and ²³⁸U, ²³²Th, and their decay products. Natural uranium consists of three isotopes ²³⁴U, ²³⁵U, and ²³⁸U. ²³⁸U is present in an abundance of 99.28% with a half-life of 4.5 × 10⁹ years and ²³⁵U in abundance 0.72% with a half-life of 0.7 × 10⁹ years. Thorium is one of the important natural primordial radionuclides with a half-life of 1.4 × 10⁹ years. It is about four times more abundant in nature than uranium. Average crustal abundance of ²³²Th is 7.2 ppm. All substances found in the terrestrial system contain variable amounts of ²³⁸U and ²³²Th; they undergo radioactive decay until they become stable isotopes. The two main important radioactive series are given in Tables 2 and 3.

The bulk of natural radiation comes from the primordial radionuclides such as ²³⁸U, ²³⁵U and ²³⁵Th. They decay into other radioactive isotope as a part of radioactive series. These series are naturally occurring radioactive series, which have existed since the earth was formed. The nuclei in each series decay by emitting α, β and γ particles until stable (lead). These radioisotopes are chemically bound to minerals in rocks and soils and pose no biological hazards except radon, thoron and its progeny. Radon and thoron are noble radioactive gases, the higher concentrations of these gases and progenies are inhaled to produce lung cancer. According WHO and UNSCEAR, radon and their progeny are the second leading lung cancer after tobacco smoking.

Parent nuclide	Half-life T1/2	Decay mode (% branch)	Decay energy (MeV)	% Intensity	Daughter nuclide	γ-emission energy (keV)	% γ-emission intensity
238U	4.5 × 109 years	α (100)	4.198	79.0	234Th	49.55	0.063
4.151	20.9	113.50	0.0102
234Th	24.10 days	Β (100)	0.199	70.3	234Pa	63.28	4.1
0.104	19.2	92.37	2.4
0.103	7.6	92.79	2.39
234Pa	1.17 m	β (99.84)	2.269	98.2	234U	1001.03	0.837
1.224	1.007	766.38	0.294
IT (0.16)	*	234Pa	73.92	*
234Pa	6.70 h	β (100)	0.642	19.4	234U	131.30	0.029
0.472	33.0	946.00	0.021
234U	2.5 × 105 years	α (100)	4.7746	71.38	230Th	53.20	0.123
4.7224	28.42	120.90	0.0342
230Th	7.5 × 104 years	α (100)	4.6870	76.3	226Ra	67.672	0.373
4.6205	23.4	143.872	0.0483
226Ra	1600 years	α (100)	4.7843	94.45	222Rn	186.21	3.59
4.601	5.55	262.27	0.0050
222Rn	3.8235 days	α (100)	5.4894	99.92	218Po	511.00	0.076
218Po	3.10 m	α (99.98)	6.0024	100.0	214Pb	**
β (0.02)	*	218At
218At	1.60 s	α (100)	6.0024	100.0	214Bi	*
214Pb	26.8 m	β (100)	0.671	48.9	214Bi	351.93	35.1
0.728	42.2	295.22	18.2
1.023	6.3	241.99	7.12
214Bi	19.9 m	β (99.98)	3.272	18.2	214Po	609.31	44.6
1.542	17.8	1764.50	15.1
1.507	17.02	1120.29	14.7
α (0.02)	5.452	53.9	210Tl	1238.11	5.78
5.516	39.2	2204.21	4.98
214Po	164.30 μs	α (100)	7.6868	99.99	210Pb	799.7	0.0104
210Tl	1.30 m	β (100)	4.209	30.0	210Pb	*
1.863	24.0
210Pb	22.3 years	β (100)	0.0166	0.0631	210Bi	46.54	4.25
0.0631	16.0
210Bi	5.013 days	β (100)	1.1615	100	210Po	**
210Po	138.376 days	α (100)	5.3043	99.99	206Pb	803.10	0.00122
206Pb	Stable end product

Table 2.

Decay series of uranium (²³⁸U)

No gamma rays observed.

Parent nuclide	Half-life T1/2	Decay mode (% branch)	Decay energy (MeV)	% Intensity	Daughter nuclide	γ-emission energy (keV)	% γ-emission intensity
232Th	1.4 × 1010 years	α (100)	4.0123	78.2	228Ra	63.81	0.263
3.9472	21.7	140.88	0.021
228Ra	5.75 years	β (100)	0.0392	40.0	228Ac	13.52	1.6
0.0128	30.0	16.24	0.72
0.0257	20.0	12.75	0.30
228Ac	6.15 h	β (100)	1.158	29.9	228Th	911.20	25.8
1.731	11.66	968.97	15.8
2.069	8.0	338.32	11.27
228Th	1.9116 years	α (100)	5.4232	72.2	224Ra	84.373	1.22
5.3404	27.2	215.98	0.254
224Ra	3.66 days	α (100)	5.6854	94.92	220Rn	240.99	4.10
5.4486	5.06	292.70	0.0062
220Rn	55.6 s	α (100)	6.2881	99.87	216Po	549.76	0.114
216Po	0.145 s	α (100)	6.7783	100	212Pb	804.9	0.0019
212Pb	10.64 h	β (100)	0.335	82.5	212Bi	238.63	43.3
0.574	12.3	300.09	3.28
0.159	5.17	115.18	0.592
212Bi	60.55 m	β (64.06)	2.248	86.57	212Po	727.33	6.58
1.521	6.81	1620.50	1.49
0.627	2.92	785.37	1.102
α (35.94)	6.0508	69.19	208Tl	39.86	1.06
6.0899	27.12	288.20	0.337
5.7675	1.78	452.98	0.363
212Po	0.299 μs	α (100)	8.7849	100.0	208Pb	**
208Tl	3.053 m	β (100)	1.796	48.7	208Pb	2614.53	35.64
1.286	24.5	583.19	30.4
1.519	21.8	510.77	8.13
208Pb	Stable end product

Table 3.

Decay series of thorium (²³²Th)

Depending on its effects on matter and its ability to ionize the matter, radiation is classified in two main categories: ionizing and nonionizing radiations.

Ionizing radiation

Radiation passing through the matter which breaks the bonds of atoms or molecules by removing the electron is called ionization radiation. It passes through the matter or living organisms, and it produces various effects.

Ionizing radiation is produced by radioactive decay, nuclear fission, and fusion, by extremely hot objects, and by particle accelerators. The emission of ionizing radiation is explained in Section 2.1. The ionizing radiation is again divided into two types: direct and indirect ionizing radiation.

Direct ionizing radiation

Directly ionizing radiation deposits energy in the medium through direct Coulomb interaction between the ionizing charged particles and orbital electrons of atoms in the medium, for example, α, β, protons, and heavy ions.

Indirect ionizing radiation

Indirectly ionizing radiation deposits energy in the medium through a two-step process; in the first step, charged particles are released in the medium. In the second step, the released charged particles deposit energy to the medium through direct coulomb interaction with orbital electron of the atoms in the medium, for example, X-rays, photons, γ rays, and neutrons.

Nonionizing radiation

Nonionizing radiation is part of the electromagnetic radiation where there is insufficient energy to cause ionization. But it has sufficient energy only for excitation and not to produce ions when passing through matter . Radiowaves, microwaves, infrared, ultraviolet, and visible radiation are the examples of nonionizing radiations. Nonionizing radiation is essential to life, but excessive exposures will cause biological effects.

The ionizing radiations such as α, β, and γ except neutron are originated from unstable nuclei of an atom in an element undergoing radioactive decay.

Alpha radiation

Some naturally occurring heavy nuclei with atomic number 82 < Z < 92 and artificially produced transuranic element Z > 92 decay by alpha emission, in which the parent nucleus loses both mass and charge. The alpha particle is emitted in preference to other light particles such as deuteron (²H), tritium (³H), and helium (³He). Because energy must be released in order for decay to take place at all. The alpha particle has very stable and high binding energy, has tightly bound structure, and can be emitted spontaneously with positive energy in alpha decay, whereas ²H, ³H, and ³He decay would require an input energy. The parent nucleus (Z, A) is transformed XZA→XZ−2A−4+αXZA→XZ−2A−4+αE1

It has less penetrating and high ionizing power.

Beta radiation

Beta particles are fast electron or positron; these are originated from weak interaction decay of a neutron or proton in nuclei, which contains an excess of the respective nucleon. In a neutron-rich nucleus, neutron can transform itself in to a proton by emission of beta particles and antineutrino. Similarly, in the nuclei with rich proton, it transforms into neutron by emission of neutrino and positron. These radiations are high penetrating and less ionizing power:

n → p + e− + ν− E2

Similarly in the nuclei with rich proton, the decay is

p → n + e+ + E3

Gamma radiation

The emission of gamma rays is usually the most common mode of nuclear excitation and also occurs through internal conversion.

X-ray radiation

X-rays arises from the electron cloud surrounding the nucleus. They were discovered by Roentgen in 1895. X-rays are produced in X-ray tube by fast moving electron which is suddenly stopped by target.

Neutron radiation

It is a neutral particle that produces ionization indirectly by emission of γ-rays and charged particles when interacting with matter. These charged particles produce the ionization. It has more penetrating than gamma ray and can be stopped by thin concrete or paraffin barrier. They are produced by nuclear reaction and spontaneous fission in nuclear reactors. The characteristic emission of α, β, γ, and neutron sources is given in Table.

Source/isotope	Half-life	Energy (MeV)
α
241Am 210Po 242Cm 243Am 239Pu	433 years 138 days 163 days 7.4 × 103 years 2.4 × 104 years	5.486 5.443 5.305 6.113 6.070
β
H13H13 14C 36Cl 63Ni 204Tl	12.26 years 5730 years 3.08 × 105 years 92 years 3.81 years	0.0186 0.156 0.714 0.067 0.766
γ
60Co 137Cs 22Na C2760C2760	5.2 years 30 years 2.6 years 5.2 years	0.662 1.277 1.173 1.332
X-rays
41Ca 44Ti 49V 55Fe	8 × 105 years 48 years 330 days 2 k.6 years	3.690 keV 4.508 4.949 5.895
Source	Half-life	Energy MeV	Yield × 106
Neutron
239Pu/Be 210Po/Be 238Pu/Be 241Am/Be	24,000 years 138 days 87.4 years 433 years	5.14 5.30 5.48 5.48	65 73 79 82

Table.

Characteristics of some α, β, and γ emitters and neutron (sources).

The atom is the basic building block of matter. The concept of atoms and molecules was first introduced by John Dolton in 1811, and he proposed the atomic theory. The atom consists of positively charged nucleus and surrounded by a number of negatively charged electron, so that atom as a whole is electrically neutral. The electron had been discovered by J. J. Thomson in 1897.

The nucleus consists of positive-charged proton and neutral-charged neutron referred as nucleons. The nucleus and proton were discovered by Rutherford in 1911, and neutron was discovered by James Chadwick in 1932. The number of proton present in the nucleus is called atomic number (Z), and total number of neutrons and protons present in the nucleus is called mass number (A). The atomic number of an element is the same, but different mass numbers are called isotope of an element. If the nucleus contains either excess of neutrons or protons, the force between these constituents will be unbalanced leading to unstable nucleus. An unstable nucleus will continuously vibrate and will attempt to reach stability by undergoing radioactive decay.

The number of neutrons determines whether the nucleus is radioactive or not. The radioactive isotopes of an element are called radioisotopes; they are natural and artificially produced by nuclear reactors and accelerators. The discovery of radioisotope was one of the result works on the radioactive element. The way in which isotope arises in the radioactive element can be understood in terms of effects of radioactive decay on the atomic number Z and mass number A. In the year 1902, Rutherford and Soddy established that radioactivity is directly connected to the state of atomic nucleus.

The unstable nuclei of an element can undergo the variety of processes resulting in the emission of radiation in two forms, namely, radioactivity and nuclear reactions. In a radioactive decay, the nucleus spontaneously disintegrates to different species of nuclei or to a lower energy state of the same nucleus with the emission of alpha (α), beta (β), and gamma (γ) radiation is called radioactivity. The radioactivity was discovered by Henry Becquerel in 1896. Alpha, beta, and their ionizing property were discovered by Rutherford in 1899, and gamma was discovered by Villard in 1900. In nuclear reaction, the nucleus interacts with another particle or nucleus with subsequently emission of radiation as one of its final products. In some cases, the final product is also radioactive. The radiation emitted in both these processes may be electromagnetic (X-rays and γ-rays) or particle-like α, β, and neutrons. The nuclear reactions were discovered by Rutherford in 1917.

Radiation and radioactivity existed long before life evolved on the earth and are indispensable parts of the environment. We are continuously exposed to natural and artificial radiations. In addition to these, some of the radionuclides such as polonium and radium are present in our bones; our muscle contains radiocarbon and radiopotassium, radon, thoron, and their progeny in our lungs, and they emit ionizing radiation.

The radiation coming from the sun is due to the nuclear fusion; it is very essential for the existence of life on earth. Therefore we live in a natural radioactive world. All organisms including human beings on the earth are getting benefits from radiation in a direct way without realizing it. Therefore, without radiation life does not exist. Scientific understanding of radiation and radioactivity and their benefits and effects on humans, that’s back almost century to the pioneering work of Roentgen (1895) and Becquerel (1896). Further investigation by M. Curie and P. Curie (1898) and Rutherford (1911) showed that radioactivity is exhibited by heavy elements such as uranium, thorium, and radium. The discovery of isotope was one of the results of work on the radioactive elements. The name “isotope” was first suggested by Soddy in 1913.

The radioactive decay law was also proposed by him. More than thousand natural radioisotopes are present in our nature. At present more than 200 radioisotopes were produced from nuclear reactors and accelerators. The application of radioisotopes in medical, industry, and research field has served human civilization over a several decades. The radioisotopes have been a valuable gift to many braches of medicine and biology. Shorter half-lives of radioisotopes are used in medicine because they decay quickly and they are suitable for medical diagnosis and therapy. The World Health Organization (WHO) and International Atomic Energy Agency (IAEA) jointly coordinated a research program on radioactive tracers in cardiovascular diseases and searched for clues to this widespread health problem.

. There are numerous applications of radioisotopes in medical fields; one of the revolutionized techniques is radioimmunoassay; this is used to detect and quantify minute levels of tissues components such as hormones, enzymes, or serum proteins by measuring the components ability to bind to an antibody or other proteins in competition with a standard amount of the same component that had been radioactivity tagged in the laboratory. For this technique, Rosalyn Sussman Yalow was awarded Nobel Prize in 1977.

The precise dose is a life-and-death matter; therefore the IAEA has several program components to assist institutions in the members of the countries and aspect of radiation therapy and diagnosis. The IAEA in cooperation with the WHO offers on intercomparison service to check and improve accuracy of radiation dosimetry due to increase in the effectiveness of the radiotherapy. The release of radioisotopes from nuclear fuel cycles, naturally occurring radioactive materials (NORM) from mining activity, mishandling of radioisotopes in industries and laboratories, and accidental release of radioactive materials could enter into the atmosphere. Therefore, it is necessary to require an urgent decision for protective actions. Therefore, the main objective is to focus on the applications and effects of radioisotopes and radiological protection.