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The Beginning of Nuclear Sciences 1 The Beginning of Nuclear Sciences 1 Chapter 1 The Beginning of Nuclear Sciences Discoveries of X-rays by Wilhelm Conrad Röntgen in November 1895 and radioactivity by Henri Becquerel in February 1896 had profound effect on the fundamental knowledge of matter. These two discoveries can be treated as the beginning of the subject ‘Radiochemistry’. Röntgen found that some invisible radiation produced during the operation of cathode ray tube caused luminescence on a card board coated with barium platinocyanide which was placed at a distance. He called this radiation X-rays. He established that X-rays can penetrate opaque objects like wood and metal sheets. Henri Becquerel, in his investigation to establish a relation between fluorescence and emission of X-rays, stumbled upon the discovery of radioactivity. Becquerel discovered that crystals of a fluorescent uranium salt emitted highly penetrating rays which were similar to X-rays and could affect a photographic plate. Based on subsequent experiments, it was observed that uranium salts, fluorescent or not, emitted these rays and the intensity of the rays was proportional to the amount of uranium present in these salts. These rays were called uranic rays. Marie Curie christened the phenomenon as ‘Radioactivity’. One of the interesting observations by Marie and Pierre Curie was that uranium ores were more radioactive than pure uranium and also more radioactive than a synthetically prepared ore similar to the ore. Foresight and further work by Curies led to the discovery of new elements polonium and radium. It is worth recording the monumental efforts made by Curies to isolate significant quantities of radium. Starting from two tonnes of pitchblende residue, from which much of the uranium had been removed, they obtained 100 mg of radium chloride. Madam Curie determined the atomic weight of radium as 226.5 and also prepared radium metal by electrolysis of the fused salt. All the separation work was carried out in a cow shed!! This accomplishment represented the culmination of their scientific faith and perseverance. a, b, g-radiations The emanations from radioactive substances were found to have three components, called a, b and g radiations. From the nature of their deflection in electric and magnetic fields, a-particles were recognised as having positive charge, b-particles as having negative charge and g-radiation as electromagnetic radiation. The a-particles were found to be less 2 Fundamentals of Radiochemistry penetrating compared to b-particles and g-radiation. Later, Rutherford conclusively proved, by spectroscopic measurements, that a-particles were doubly charged helium ions. Transformation Hypothesis Observation of emanations from thorium salts and compounds of radium, led Rutherford and Soddy to conclude (i) a radioactive element undergoes transformation and an atom of a new element is formed, (ii) the radiations are accompaniment of these changes and (iii) radioactive process causes a subatomic change within the atom. These conclusions were drawn when the existence of nucleus was not known, neutron was not discovered, isotope concept was not proposed and the source of energy of the emitted radiations was a big puzzle!! Radioactive Decay Law In 1902, Rutherford and Soddy proposed the theory of radioactive disintegration. They proposed that “the disintegration of the atom and expulsion of a charged particle leaves behind a new system lighter than before and possessing physical and chemical properties quite different from those of the original parent element. The disintegration process, once starts, proceeds from state to state with measurable velocities in each case”. This would mean that the rate of decay of an active species in unit time is proportional to the total number of atoms of that species present at that time. Rate of disintegration (-dN/dt) continuously changes as the number of atoms (N) are changing (decreasing). dN - =Nl (1.1) dt where l is a proportionality constant known as the decay constant. Solving eqn. 1.1, one obtains -lt N=N0 e (1.2) where N0 is the number of atoms present initially. In 1905, E. Von Schweilder formulated the -lt radioactive decay lawN=N0 e based on the decay probability (P) of a particular atom in a given time interval (Dt) (Details are given in Chapter 4). It may be noted that the observable is the radioactivity, rather than number of atoms. Radioactivity is the product of number of atoms (N) and the decay constant (l). Radioactive Equilibrium When a radioactive atom (parent) decays, it transforms into another atom of a different element (except in g transition). e.g. 14C decays by emitting b- and the product is 14N is formed, which is stable. In this case, 14C decays exponentially and 14N grows exponentially with time. On the other hand, there are many cases where the product The Beginning of Nuclear Sciences 3 (daughter) is also radioactive. In such cases, the daughter grows with time, not exponentially, but reaches a maximum value of radioactivity. As long as the combined parent-daughter system is undisturbed, depending on the decay constants of parent and daughter, the activities of both will be in a constant ratio. This condition is called ‘radioactive equilibrium’ condition. e.g. 226Ra undergoes a-decay and its daughter 222Rn also undergoes a-decay but at a faster rate compared to 226Ra. After about 12-13 days, activity of 222Rn and 226Ra will be in constant ratio (near to 1). More details are given in Chapter 4. Natural Radioactive Elements Elements (isotopes) having atomic number greater than bismuth are radioactive. For example, 238U undergoes a-decay with 234Th as the product, 234Th further undergoes b- decay Fig. 1.1 The uranium series [Nuclear data are taken from J.K. Tuli, Nuclear Wallet Cards, 5th ed (1995), Brookhaven National Laboratory, Upton, New York]. 4 Fundamentals of Radiochemistry Fig. 1.2 The thorium series [Nuclear data are taken from J.K. Tuli, Nuclear Wallet Cards, 5th ed (1995), Brookhaven National Laboratory, Upton, New York]. Fig. 1.3 The actinium series [Nuclear data are taken from J.K. Tuli, Nuclear Wallet Cards, 5th ed (1995), Brookhaven National Laboratory, Upton, New York]. The Beginning of Nuclear Sciences 5 with 234Pa as the product. The chain continues until the stable end product 206Pb is reached (Fig. 1.1). All the radioisotopes present in this chain form a family or series. This family is known as uranium series or 238U series or 4n + 2 series since the mass number of all the members has a reminder of 2 after dividing by 4. Similarly, Thorium series, starting from 232Th and ending at 208Pb is known as 4n series (Fig. 1.2). Actinium series, starting from 235U and ending at 207Pb is known as 4n+3 series (Fig. 1.3). Other radioisotopes present in nature are shown in Table 1.1. In addition, tritium (3H) and 14C are continuously produced by cosmic ray induced nuclear reactions in the atmosphere. Another series of radioisotopes Table 1.1 - Naturally occurring radioactive substances other than members of 4n, 4n+2 and 4n+3 series Active Type of Half-life Isotopic Stable substance Disintegrationa (y) abundance Disintegration (%) Products 40K b-, EC, b+ 1.277 x 109 0.0117 40Ca, 40Ar 50V EC, b- 1.4 x 1017 0.250 50Ti, 50Cr 87Rb b- 4.75 x 1010 27.835 87Sr 113Cd b- 9.3 x 1015 12.22 113In 115In b- 4.41 x 1014 95.7 115Sn 123Te EC >1x1013 0.908 123Sb 138La EC, b- 1.05 x 1011 0.0902 138Ba, 138Ce 144Nd a 2.29 x 1015 23.8 140Ce 147Sm a 1.06 x 1011 15.0 143Nd 148Sm a 7x1015 11.3 144Nd 152Gd a 1.08 x 1014 0.20 148Sm 176Lu b- 3.73 x 1010 2.59 176Hf 174Hf a 2.0x1015 0.162 170Yb 187Re b- 4.35 x 1010 62.60 187Os 190Pt a 6.5x1011 0.011 186Os aThe symbols EC, b- and b+ stand for electron capture, negatron decay and positron decay, respectively; these decay modes are described in Chapter 5. [Nuclear data are taken from J.K. Tuli, Nuclear Wallet Cards, 6th ed (2000), Brookhaven National Laboratory, Upton, New York]. 6 Fundamentals of Radiochemistry Fig. 1.4 The 4n + 1 series [Nuclear data are taken from J.K. Tuli, Nuclear Wallet Cards, 5th ed (1995), Brookhaven National Laboratory, Upton, New York]. starting from 237Np and ending with 209Bi, known as 4n+1 series is an artificial series (Fig. 1.4). Uranium-233, a member of this series, is a fissile isotope. Artificial Radioactive Elements Rutherford discovered the first nuclear reaction when he was investigating the possibility of a induced transmutation on various targets. In 1919, he postulated the transmutation of 14N as per the following reaction. 14 +®+4 17 1 7 NHeOH2 8 1 (1.3) In 1932, similar studies by his student Chadwick involving a bombardment of boron lead to the discovery of neutron. 10 +®+4 13 1 5 BHeNn2 7 0 (1.4) In the same year positron, an antiparticle of electron, was discovered by Anderson. In 1934, Irene and Federic Joliot Curie, in their studies on production of positrons by bombardment of aluminium with a-particles, found that the product formed continued to emit positrons even after the a-source was removed (Eqns. 1.5 and 1.6). They separated phosphorus as posphine, measured its half-life and concluded that the product formed was a radioactive element. 27 +®+4 30 1 13 Al2 He15 P0 n (1.5) 30 ®+30 b+ 15 PSi14 (1.6) The Beginning of Nuclear Sciences 7 This heralded the beginning of artificial production of radioisotopes. With the construction of accelerators and nuclear reactors, a large number of radioisotopes have been produced.
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