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ISOTOPES - BASICS AND CLASSIFICATION
Rutherford first demonstrated the existance of atomic nucleus. Since then a number of discoveries have been made by various investigators and a numerous facts have come to over knowledge about the atom, nuclei, and its constituents and properties.
Total 109 elements have been discovered which have different physical and chemical properties. Out of 109 elements, 92 occur naturally and 17 have been prepared artificially. These elements are found in the form of solids and gasses except mercury - a metal and bromine - a non metal that are liquid at room temperature.
Thousands and lacs of physical and chemical combinations (mixtures and compounds) have been discovered with amazing properties. Depending upon the specific properties of various atoms and molecules, a number of applications have been discovered in a variety of fields. Isotope Hydrology is comparatively a new field of research and deals with the use of different atoms of elements and molecules of compounds for various hydrological investigations. Modern isotope research is based on the discovery of the natural abundance of carbon-14 and tritium by W.F. Libby (1946) and the experimental and theoretical work carried out by H. C. Urey (1947). The development of sophisticated nuclear instrumentation has given a new direction to the investigations and research in the field of isotope hydrology.
2.0 ATOM :
Atom consists of a positively charged nucleus surrounded by a cloud of negatively charged particles called electron which revolve around it. The diameters of atoms are of the order of 10-8 cm while nuclei of atoms are about 10-12cm (1000 times less). In nucleus, most of the atom's mass is concentrated. The nucleus contains different types of particles that interact with each other but proton and neutron are considered to be the main constituents. The proton is a positively charged particle while neutron is a neutral particle. The combination and distribution of positive and negative charge of an atom makes it neutral in normal conditions. The details of prominent atomic particles are given below:
Proton - positively charged; mass - 1.672648 x 10-27 Kg ; Electron - negatively charged; mass - 9.10986 x 10-31 Kg or 1/1836 times of mass of proton; Neutron - neutral (no charge); mass - 1.6749543 x 10-27 Kg
Conveniently, the atomic masses of particles, atoms and molecules are expressed in atomic mass unit (amu) instead of real mass. Originally 1 amu was equivalent to the mass of a proton but later on for practical reasons, the atomic mass unit has been redefined as 1/12 times the mass of 12C atom, i.e. 1 amu = 1.6605655 x 10-27 Kg.
The atomic masses of proton, electron and neutron can be expressed in terms of amu as: 1 proton - 1.0072765 amu ; 1 neutron - 1.0086649 amu ; 1 electron - 0.0005486 amu.
If we now add up the atomic masses for 12C atom consisting 6 protons and 6 neutrons, it comes to 12.09564 amu compared to atomic mass 12.000 amu of 12C. The difference of 0.09564 amu is called the mass defect and is treated as binding energy of the atom particularly nucleus to keep the particles together. This difference of mass converted to energy can be explained using the Einstein's equation E= mc2, where E is the energy, m is the mass and c is the velocity of light (2.997925 x 108 m/s). The equivalence between mass and energy is expressed as,
1 amu = 931.5 MeV (million electron volt)
Where, 1eV = 1.602189 x 10-19 Joule or 1.602189 x 10-12 erg (1 Joule = 10 7 erg ).
Thus, the binding energy of 12C atom can be determined by multiplying 0.09564 amu with 931.5 MeV = 89.08866 MeV or 7.42 MeV per nucleon.
The number of proton and number of electron are always same in a stable atom. This number is called the atomic number of an atom and is denoted by a symbol (Z). The number of neutron (N) plus either the number of protons or electrons (normally number of neutrons and number of protons) is called the atomic mass or weight of an atom and its denoted by a symbol (A), i.e. A = Z+N.
Thus an element (X) with its nucleus constituents can be denoted as [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image002.png[/IMG]. In general, it is sufficient to denote an element only by notation [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image004.png[/IMG] or even AX also. The distribution of electrons in different shells (K, L, M, N....) is governed by the law 2n2 where n is the shell number i.e. 1,2,3. It has been found that elements having 2 or 8 electrons in the outermost shell of their orbit remains fully satisfied with the arrangement of electron, proton and neutron and normally do not react chemically with other elements. On the other hand, all other elements differing in number of electrons in the outermost orbit of their atoms (other than 2 or 8) show a wide variety of chemical properties. Therefore, the number of electrons or atomic number of an element, is the governing factor of the chemical properties of an atom (element) and because of this systematic, all elements could be arranged in a periodic table. The atomic structure along with other details of a number of light elements is shown in table 1.1.
There are three important terms i.e., isotopes, isobars and isotones that are used to differentiate and distinguish the atoms of a same element and atoms of different elements showing similarities in physical and chemical properties. Isotopes are the atoms of an element having same atomic number (Z) but different atomic weight (A). In other words, the atoms of an element having different number of neutrons (N) but same number of protons or electrons are called isotopes. For example, hydrogen has three isotopes having the same atomic number of 1 but different atomic masses or weights of 1, 2 and 3 respectively i.e.,
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image006.png[/IMG] - only one proton in nucleus and one electron revolving around the nucleus in an orbit.
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image008.png[/IMG] - one neutron added to the nucleus of [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image006.png[/IMG] atom.
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image010.png[/IMG] - one more neutron added to the nucleus of [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image012.png[/IMG].
Table 1.1: Electronic configurations of atoms of various light elements.
Similarly oxygen has eleven isotopes, 12O,13O, 14O, 15O, 16O, 17O, 18O, 19O, 20O, 21O and 22O, but except 16O, 17O, and 18O all other isotopes are radioactive and their existance in nature is very small ( half life vary from 150 seconds to few femo seconds - of the order 10-15 seconds ) therefore, we normally talk about only three isotopes of oxygen i.e., 16O, 17O, and 18O. The carbon also has three isotopes 12C, 13C and 14C.
Isobars are the atoms of different elements having same atomic weight (A) but different atomic number (A). For example, [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image015.png[/IMG] and [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image017.png[/IMG] are isobars. On the other hand atoms having same number of neutrons but different atomic number (Z) and atomic weight (A) are called isotones. For examples, [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image008.png[/IMG] and [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image020.png[/IMG] are the isotones.
It has been established that atomic forces are the strongest forces but these range only at a very small distances. These forces keep atomic particles (mainly protons and neutrons) intect together to form a nucleus and atom as a whole. However, it has been found that the stability of nucleus or atom depends upon the mutual number of protons and neutrons i.e. if an element contains even number of protons and neutrons, its abundance will be more and it will have more stable isotopes. The elements having magic numbers 2, 8, 20, 28, 50, 82 and 126 of neutron (N) or protons (Z) have a relatively high stability and consequently the large natural occurrence. The largest stable isotopes 208Pb (Z=82 and N=126) is double magic. On the other hand, elements having uneven Z and N are unstable. However, in case of light elements, the slight increase in number of neutrons than protons does not create any instability. In fact, it is the ratio of neutrons to protons that is the deciding factor for nuclear instability. For example, one can see that in case of 1H and 2H, the atoms are stable while in case of 3H, only one number of neutrons is added but the ratio of neutron to proton becomes double, therefore, 3H is unstable. Figure 1.1 gives a clear picture of isotopes (stable and unstable), isobars and isotones in the the periodic table.
Fig. 1.1 The isotopes of an element (equal Z) are found in a horizontal row, isobars (equal A) along diagonal lines, isotones (equal N) in vertical columns. The natural radioactive isotopes of H, Be, and C are marked grey.
3.1 Classification of Isotopes
Isotopes can be classified in two important categories, (i) stable isotopes and (ii) unstable isotopes.`
Stable isotopes are the atoms of an element, which are satisfied with the present arrangement of proton, neutron and electron. On the other hand, unstable isotopes are the atoms of an element which do not satisfy with the present arrangement of atomic particles and disintegrate by giving out alpha ( a ), beta ( b ) particles and/or gamma (g) radiation etc. and transform into an another type of atom. This process continued till the stable nuclide (element) is formed. Because of disintegration or the property of giving out radiation, the unstable isotopes are also called radioactive isotopes. For example, 1H and 2H are stable isotopes while 3H is unstable. Similarly 12C and 13C are stable isotopes while 14C is unstable. On the other hand, isotopes of oxygen (16O, 17O and 18O) are stable.
Isotopes can also be classified as natural and artificial isotopes, i.e., the isotopes that occurs naturally are called natural isotopes while those produced in a reactor or laboratory under controlled conditions are known as artificial isotopes. Normally the artificially produced isotopes are radioactive while stable and radioactive, both types of isotopes occur naturally.
Another category of isotopes has been devised that is called environmental isotopes. These isotopes have different types of categories i.e. naturally occurring stable and radioactive isotopes and radioisotopes introduced into the atmosphere due to anthropogenic activities etc. The environmental radioisotopes whether naturally occurring due to cosmic ray interaction with various gaseous molecules or anthropogenically produced and become the part of hydrological cycle are safe in normal conditions and do not pose any threat to human health.
The following diagram gives a clear picture about the classification of various isotopes.
3.1.1 Stable Isotopes
As described earlier, the atoms of an element which do not decay with time or take infinite time to decay are called stable isotopes of that element. Over 2000 isotopes of 92 naturally occuring elements have been identified out of which several hundred are stable isotopes. But for hydrological investigations, we talk much about hydrogen and oxygen stable isotopes. As we know water molecule is made up of two hydrogen atoms and one oxygen atom therefore, many combinations (18) are possible out of which 1H1H16O, 1H1H16O, 1HD16O, 'HD18O, 1H1H17O and 1HD17O are important. The natural occurrence of few very important types of water molecules is given below:
H216O ~ 997640 ppm(99.7640 %)
H218O ~ 2040 ppm (0.204 %)
HD16O ~ 320 ppm (0.032 %)
There are few other stable isotopes (3He, 6Li, 11B, 13C, 15N, 34S, 37Cl, 81Br and 87Sr) which have been found useful in many hydrological studies. These stable isotopes are popularly called environmental stable isotopes as they are available in the environment and introduced in the hydrological cycle naturally. Thus the investigator does not require them to either purchase or inject into the system for carrying out hydrological studies. The details of these isotopes with other useful information are given in table 1.2.
Measurements of stable isotopes are done in terms of abundance ratios i.e. atomic mass of heavy atom to the atomic mass of light atom. For example heavy water 2H216O(D216O ) has a mass of 20 compared to normal water 1H216O which has a mass of 18. Similarly heavier stable molecule of water D218O has a mass 22. This is because of the variation in the number of neutrons. However, the absolute abundance ratio of isotopes is not usually measured in natural waters and in other components. Only the relative difference in the ratio of the heavy isotopes to the more abundant light isotope of the sample with respect to a reference is determined. The difference is designated by a Greek letter d and is defined as follows:
d = (Rsample - Rreference ) / Rreference (1.1)
Where R's are the ratios of the 18O/ 16O and D/H isotopes in case of water.
The difference between samples and references are usually quite small, d values are therefore, expressed in per mille differences (‰) i.e. per thousand, d (‰ ) = d x 1000.
d (‰ ) = [(Rs - Rr ) / Rr ] x 10 3 = [ (Rs / Rr) - 1 ] x 10 3 (1.2)
If the d value is positive, it refers to the enrichment of the sample in the heavy-isotope species with respect to the reference and negative value corresponds to the sample depleted in the heavy-isotope species.
The reference standards normally considered are SMOW (Standard Mean Oceanic Water) and VSMOW (Vienna Standard Mean Ocean Water)
(18O/ 16O)SMOW = 1.008 (18O/ 16O) NBS-1 (1.3)
(D/H) SMOW = 1.050 (D/H)NBS-1 (1.4)
Craig evaluated the isotopic ratios of SMOW as;
18O/ 16O = (1993.4 ± 2.5) x 10-6 and D/H = (158 ± 2) x 10-6 (1.5)
VSMOW has the same 18O content as defined in SMOW but its D-content is 0.2 ‰ lower.
Over the period of use, the old standards have been consumed. Therefore, other reference standards have been developed in due course of time. These are SLAP (Standard light anatarctic precipitation), NBS-1 and NBS-1A (National Bureau of Standard) and GISP (Greenland ice sheet precipitation).
The relation between dD and d18O that has been observed in global precipitation is expressed mathematically by the equation;
dD‰ = 8 d18D + 10 (1.6)
Table-1.2: Stable isotopes with their natural abundance and reference standards used for ratio measurements.
Isotope Ratio % natural Reference Commonly measured phases
Abundance (abundance ratio)
2H 2H /1H 0.015 VSMOW (1.5575 . 10-4) H2O, CH20, CH4, H2, OH-
3He 3He/4He 0.000138 Atmsopheric He (1.3 . 10-6) He in water or gas, crustal
6Li 6Li/7Li 7.5 L-SVEC (8.32 . 10-2) Saline waters, rocks
11B 11B/10B 80.1 NBS 951 (4.04362) Saline waters, clays,
13C 13C/12C 1.11 VPDB (1.1237 . 10-2) CO2, carbonate, DIC,
15N 15N/14N 0.366 AIR N2 (3.677.10-3) N2, NH4+, NO3-, N-
18O 18O/16O 0.204 VSMOW (2.0052 . 10-3) H2O,CH20,CO2, sulphates
VPDB (2.0672 . 10-3) NO3-, carbonates,silicates
34S 34S/32S 4.21 CDT (4.5005 . 10-3) Sulphates, sulphides,
37Cl 37Cl/35Cl 24.23 SMOC (0.324) Saline waters, rocks,
81Br 81Br/79Br 49.31 SMOB Developmental for saline
87Sr 87Sr/86Sr 87Sr=7.0 Absolute ratio measured Water, carbonates,
86Sr=9.86 sulphates, feldspar
The relation between dD and d18O can be written in a standard form (equation for straight line) i.e.;
dD‰ = A d18O + d (1.7)
Where A is the slope and d is the intercept of dD - d18O line of fresh global meteoric waters. One can develop regional and local meteoric water lines on the pattern of standard relationship between dD and d18O valid on regional or local levels.
In early days, the use of radioisotopes was in vogue. Mostly, the radioisotopes, artificially produced in reactor/laboratory, were used as tracers. The radioisotope of hydrogen (tritium) in the form of water molecule (3H2O) and denoted by symbol 3H or T is still widely used for various hydrological studies. There are other variety of artificially produced radioisotopes like 60Co, 82Br, 131I, 137Cs, 198Au, 226Ra/241Am etc. that are used for various hydrological investigations.
However, with the introduction of sophisticated instrumentation, the radioisotopes that occur in traces in the environment and past and parcel of hydrological cycle are used. This has reduced the use of artificial radioisotopes tremendously which may have an unwanted impact of health hazards in the mind of users as well as in the public. The most widely used environmental radioisotopes are given in table 1.3 with their half life, decay mode, principal sources and commonly measured phases.
22.214.171.124 Radioactivity and Radiation:
Radioactivity: The process of decay of an unstable atom in order to attain a stable nuclear configuration by giving out a, b particles and g or x rays is known as radioactivity. The process of radioactivity may complete in a fraction of second or it may continue for millions of years depending upon the type of decaying atom. During the process of radioactivity, an unstable atom may be converted into a different type of stable atom in one stroke or there may be a series of chain of daughter nuclei before finally stable nuclide is formed. The decaying atom is called the parent nuclide while the intermediate transformations and finally stable nuclide are called daughter and grand daughter nuclides respectively.
The details of various radioisotopes with their half lives, decay mode, principle sources etc. are given in table 1.3.
Radiation: During the process of decay the unstable nuclides emit different types of particles/radiations with different energies. The energy although is measured in erg and joule but, keeping in view of the very low energy of these radiations, small unit of energy like electron volt ( eV) is used to measure it. Other unites of energy that are generally used are kilo electron volt (KeV) and million electron volt (MeV). [ 1eV = 1.602189 x10-12 erg]
The different types of particles/radiationds that are emitted out by unstable nuclides naturally or in artificially created conditions are described here in brief.
Alpha particle: Alpha particles are called doubly ionized helium atom (4He++). It comprise of two protons and two neutrons therefore, it is the heaviest particle that decays from the nucleus. It is denoted by a symbol a and in the event of its decay, the parent looses atomic mass by 4 and positive charge by 2 i.e. atomic number by 2 and transforms into a new element (daughter). The transformation takes place due to a day is e shown below:
As the a particle is relatively heavy therefore, when it is emitted out it imparts recoil energy to the nucleus. In consequence the total a decay energy is the sum of recoil energy, the kinetic energy of the a particle and the energy of any gamma rays emitted. This
can be understood by taking an example of natural decay of [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image027.png[/IMG] to [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image029.png[/IMG].
Where the helium nucleus is the alpha particle and the total alpha decay energy is Q.
Table-1.3: Details of various radioisotopes with their half lives, decay mode, principal sources and commonly measured phase.
Isotope Half-life Decay Principal Sources Commonly measured
(years) mode phases
3H 12.43 b- Cosmogenic, weapons testing H2O, CH2O
14C 5730 b- Cosmogenic, weapons testing, DIC, DOC, CO2, CaCO3 CH2O
36Cl 301,000 b- Cosmogenic and subsurface Cl-, surface Cl-salts
39Ar 269 b- Cosmogenic and subsurface Ar
85Kr 10.72 b- Nuclear fuel processing Kr
81Kr 2,10,000 ec Cosmogenic and subsurface Kr
129I 1.6 x 107 b- Cosmogenic, subsurface, I- and I in organics
222Rn 3.8 days a Daughter of 226Rn in 238U Rn gas
226Ra 1600 a Daughter of 230Th in 238U Ra2+ , carbonate, clays
230Th 75,400 a Daughter of 234U in 238U Carbonate, organics
234U 2,46,000 a Daughter of 234Pa in 238U UO22+, carbonate, organics
238U 4.47 x 109 a Primordial UO22+, carbonate, organics
b- - beta emission.; a - alpha emission.; ec - electron capture.
The kinetic energy of the alpha particle is 4.2 MeV. The energy spectrum of alpha particles emitted out by a radioactive nuclide is represented by the discrete energy peaks rather than a continuous energy distribution. The alpha decay has been found more common among the nuclides having atomic number more than 58 (cerium). However, it also occurs in case of few light atomic number nuclides e.g. [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image033.png[/IMG], [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image035.png[/IMG] and [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image037.png[/IMG]. In most of the cases the emission of alpha particles is followed by the emission of gamma rays.
An alpha particle loses its energy in a medium by successive coulambian interactions with the orbital elections with the medium which results in the excitation and ionisation of electrons and also the division of nucleus or emission of nucleonic particles. The electrons released in the ionisation process also interact and cause further excitations and ionisation in the material. The alpha particles initially posses high energies of the order of a few MeV but, most of their energy is lost in the ionisation of the material. Therefore, the range of penetration in materials is very small i.e., 3.5cm in air and .004 cm in aluminum.
Beta Particle: As stated above, beta particle (b) is a negatively charged particle and also called negatron. Each beta decay is accompained by the emission of anti-neutrino (n-) and is also followed, in most of the cases, by the emission of gamma rays. When the excess neutron breaks up in the nucleus into proton and electron, the later is expelled out from the nucleus as beta particle. Therefore, we can say that free electrons having some kinetic energy are called beta particles. The decay of neutron may be shown as given below:
N à p+ + b- + n- + Q (1.10)
Where n- is an anti neutrino - a particle with a relativistic mass i.e. mass because of its motion (zero mass at rest). Q represents the total reaction energy that is shared between the beta particle and anti neutrino. Although Q is specific, but the energy spectrum of the beta particles is continuous from zero to Emax = Q rather than discrete energy peaks in case of alpha decay. The energy spectrum of beta decay is shown in fig. 1.2.
The beta decay results in an increase of atomic number (Z) by 1 while neutron number (N) decreases by 1. However, the mass number (A) remains same of daughter nuclide in case of beta decay. It can be shown by the following equation.
Thus X and Y will be isobars having same atomic mass (A) but different atomic number. Z and Z+1 respectively. If the daughter is itself radioactive and decays by a beta emission, a second isobaric daughter is formed and so on until at last a stable daughter is formed.
The example of natural decay of 40K into 40Ca can be depicted for beta day by the following equation:
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image041.png[/IMG]+ b- + n- + Q (1.12)
Fig. 1.3: Branced decay schclene of 40K nuclide with the emission of b-, b+ and Ec together with the formation of 2g by annihileation of 9n emission b+ particle.
Beta particles are comparatively lighter than alpha particles, therefore, these travel at much higher velocity than the alpha particles of the same energy. This is the reason that why electrons or beta particles lose less energy in the interaction with the atoms of the medium and causes less ionisation effects. Because of this fact, beta particles penetrates much greater than that of alpha particles of same energy and in a same medium. However, beta particles lose their energy upto 10 MeV by excitation and ionisation of electrons of the medium as in the case of alpha particles. The range of penetration (R) of beta particles in materials with respect to their energy can be calculated using the following relation.
R (cm2/g) = 0.542 Emax - 0.130 ~ 1/2 Emax (MeV) (1.13)
The observations of few common beta emitters and their ranges in aluminum are given below:
Isotope Beta energy (Max) Range in aluminium mg/cm2
3H .018 0.23
14C .155 2.0
204Tl 0.77 3.0
90Y 2.20 11.0
Positron: Positron is a positively charged particle having mass equal to an electron, that is why positron is called a positively charged electron. The positrons have energy spectrum similar to those of beta particles. On the similar lines to negatron decay each positron decay is accompanied by the emission of neutrino (n) whose kinetic energy is the difference between the total reaction energy and the energy of the positron. In most of the cases, the positron decay is also followed by the emission of gamma rays. The emission of positron from the nucleus is understood due to the transformation of proton into a neutron inside the nucleus. The simple equation of positron emission can be written as:
P+ à n+ e+ + n + Q (1.14)
In case of positron emission, electron capture process takes place and positron combine with the available electron. This results in the emission of 2 gamma radiation (g) i.e.;
e+ + e- à 2g (1.15)
The atomic number (Z) of the daughter element decreases by 1 while the neutron number (N) increases by 1. The atomic masses of parent and daughter element remains same in positron emission. Thus, the daughter element will be an isobar. The decay equation can be shown as given below:
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image050.png[/IMG]+ b+ + n + Q (1.16)
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image051.png[/IMG]Fig. 1.4: Position of [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image055.png[/IMG] to stable [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image055.png[/IMG] in two set of positions having end point enteries 1.809 and 4.1 MeV. (Data from Holden and Walker 1972).
Similar to beta emission, if the daughter is itself radioactive in case of positron decay, it also decays by positron and a second isobaric daughter is formed and so on until at last a stable daughter is produced. The example of positron decay may be represented by the natural decay of [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image057.png[/IMG]to [IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image055.png[/IMG].
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image059.png[/IMG]+ e+ + n + Q (1.17)
Electron Capture: In the process of electron capture, the proton in the nucleus captures one of its extranuclear electrons to form a neutron and a neutrino. The probability of capturing electron is greater from the K shell because of its least distance from the nucleus. However, depending upon from which atomic shell the electron is caught by the nucleus, electron capture is called K capture, L capture and so on.. In this process, the atomic number (Z) of the daughter element decreases by 1 while neutron number (N) increases by 1. However, the atomic mass remains same as in the case of beta and positron emission. This can be represented by the equations as given below:
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image060.png[/IMG] p+ + e- n + n + Q (1.18)
In the process of electron capture, the atom is left in an excited state and returns to the ground state by emitting low energy gamma rays or X rays.
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image064.png[/IMG] + g (1.20)
Neutron Emission: Neutron is the heaviest and neutral constituent of nucleus. Neutron emission does not takes place on its own as it normally happens in the case of emission of other neucleonic particles. Alpha particles from an element (radioactive) are bombarded to an another element (non-radioactive) which ultimately emitts neutrons. The emission of neutrons is therefore, depends upon the proper combination of two different nuclides (one of them is radioactive). This may be better understood from the following example. A combination of 226Ra and 9Be give rises the emission of neutrons from 9Be nuclide. 226Ra emitts alpha particles which interact with 9Be nuclei and make them unstable which results in the emission of neutrons. The following equations show the process of neutron emission.
This type of decay is called induced decay. The emission of neutron with the bombardment of alpha particles can be indicated using the following notation.
It indicates that alpha particle goes in and two neutrons come out. However, there may be one or three neutrons emission depending upon the type of nuclides. In case of neutron emission the target atom is transformed into an atom of another element and is not an isobar of the target.
During induced decay, neutrons are emitted out with high velocities and thus called "fast" neutrons. In many cases, these fast neutrons must be slowed down to sustain the chain reaction. This is achieved by a 'moderator' with which the fast neutrons can collide with out being absorbed by the nuclei of the moderator. The slowed down neutrons are also called thermalized neutrons. In reactors, D2O (heavy water) or graphite is used as moderator. Although ordinary water (H2O) also serves the purpose in the so called "swimming pool reactors".
Gamma Radiation: Gamma radiation (g) is a high frequency electromagnetic radiation with no mass. In other words, we can say that gamma rays (radiation) are made of high energy photons which move with the velocity of light. The emission of gamma radiation follows particle emission. Thus, gamma rays are not emitted out alone but these are emitted by the excited nuclides as a consequence of emision of particles like, alpha, beta and positron etc. An example of g ray emission from 60Co is given below.
[IMG]file:///C:%5CUsers%5CRanveer%5CAppData%5CLocal%5CTemp%5Cmsohtmlclip1%5C01%5Cclip_image072.png[/IMG] g (1.24)
In some cases, however beta decay is directly to the ground state of the daughter nuclear, without the emission of (g) radiation. The gamma rays are similar to X-rays and their energy spectrum overlaps. However, the basic difference between these two is that gamma rays are emitted from the excited nucleus while X-rays are emitted from the excited electron. The gamma rays are quite penetrating radiations and could penetrate through several centimeters of lead. Radioisotopes like 60Co, 82Br, 131I, 134Cs, 137Cs and 198Au can be used as gamma ray sources for various purposes.