1 Introduction to Nuclear Physics

Sanjay Kumar Chamoli

Learning Outcomes

From this module students may get to know about the following:

The aim and scope of nuclear physics.
The development of nuclear physics.
Present understanding of nuclear physics.
Useful terms and basic units of measurements used in nuclear physics

1. Aim and scope of nuclear physics

The nuclear physics is the branch of physics in which the study of atomic nucleus, its constituents and the interactions happening among them is done. Right from the discovery of radioactivity in 1996 by Henri Becquerel, the nuclear physics has evolved with time and now its applications can be seen in many fields. The most common application of nuclear physics is in power generation, but its use in many other fields like medical sector, agriculture sector, archaeology, industries and geology has now made the study of nuclear physics important. Though the aims and objectives of studying nuclear physics are not limited to few domains, but the importance of learning nuclear physics can be understood in the following ways :

1.1 Understanding the origin of elements: The knowledge of nuclear physics, helps us in understanding the origin of our world and the elements in it. Our Universe is believed to be formed in the ‘Big Bang’ happened some 14 billion years ago. In the early Universe only the light elements like hydrogen and helium along with trace amounts of lithium and beryllium were formed. Then as the cloud of cosmic dust and gases from the Big Bang cooled, stars formed, which then grouped together to form galaxies. The other elements heavier than the hydrogen and helium found in nature were created in nuclear reactions in these stars and in huge stellar explosions known as supernovae. Few examples are :

1.2 Understanding the origin of various properties of elements : The nuclear physics not only accounts for the formation of elements in the universe, it also explains as how the elements gain various properties like mass, charge spins etc. To our basic understanding, the nucleus consists of protons and neutrons. The protons are positively charged while the neutrons are neutral. So the charge on the nucleus is just the algebraic sum of the charge on the protons inside it. Also as the proton and neutron are massive particles (mp ~ mn), so almost all the mass of elements in the universe is due to mass of the nuclei. The proton and neutron are fermionic particles with spin 1/2, so all the nuclei have spins. The arrangement of protons and neutrons inside the nucleus is governed by the strong interaction acting between them inside the nucleus. So depending upon the number of protons and neutrons and their distribution inside, the nuclei have different shapes and sizes. So nuclear physics in general can account for almost all the observed properties of elements in the universe.

1.3 Understanding the origin the forces in Universe: The four basic forces in the Universe are the strong force, electromagnetic force, weak force and the gravitational forces. These forces vary in strength and their range of applications. The modern days understanding of nuclear and particle physics suggest that all these four forces at the fundamental level work through some mediating particle. The strong forces work via exchange of ‘gluons’, The weak force via ‘W/ G bosons, electromagnetic force via ‘photons’ and the gravitational force via ‘graviton’. Working at the fundamental level, the scientists have found that all these four forces are not different but have a single origin. Through Grand Unified Theory (GUT) they have been able to unify the strong force, electromagnetic forces and the weak forces successively.

1.4 Improving the quality of life on earth : The nuclear physics is not only unfolding the mysteries of the University but is helping to improve the quality of life on earth. Nowadays the nuclear techniques are being used by almost every industry. The biggest example of nuclear application is in power generation in Nuclear Power plant. The other big area using the nuclear techniques is the medical sector, where a number of sophisticated diagnostic and therapeutic procedures based on the nuclear techniques have been developed.

2. Development of Nuclear Physics : The beginning of the nuclear Physics is considered from 1996 when Henry Becquerel discovered radioactivity. Since then the nuclear physics have undergone a long journey and a number of discoveries, concepts have been developed.

2.1 Discovery of nucleus ; Rutherford’s gold-foil experiment: The nucleus was discovered by Ernest Rutherford in 1911, on the basis of his famous alpha particle –gold foil scattering experiment.

Fig. 1 : the picture of the experimental setup of Rutherford’s gold foil experiment.

In his famous Gold foil experiment , Rutherford used a narrow beam of energetic alpha particles to pass through a thin gold foil, and the scattered alpha particles were recorded on a movable zinc sulphide screen after the foil. In his experiment he made the following observations:

Almost all (~ 99 %) the alpha particles did pass through the foil but (scattering angle 00).
Some alpha particles were deflected off at different angles as observed on the screen of the detector. (scattering angle 00 – 1800).
Very few of the alpha particles (one or two) even bounced backwards after hitting the gold foil (scattering angle 1800).

On the basis of these observations, Rutherford made the following conclusions:

Since most of the alpha particles passed straight through the gold foil without any deflection, most of the space within the atoms is empty.
Since some of the alpha particles (which are big in size) were deflected by large angles or bounced backwards, they must have approached some positively charged region responsible for the deflection. This positively charged region is now called the nucleus.
Since the alpha particles are heavy charge particles and are deflected by the central volume of charge, it shows that almost all the mass of the atom must be within the central volume.

2.2 Composition of nucleus ; the proton-electron theory: At the time when nucleus was discovered by Rutherford, only two fermionic particles, proton and electron were known. So it was thought that the nucleus was composed of protons and electrons, i.e. a nucleus X with mass A and charge Z was supposed to be composed of 2-protons and 1-electron. This theory was successful in explaining most of the experimental findings of that time, including the emission of alpha, beta and gamma radiations in radioactivity. However, later on it was turned out to be limited in its approach and therefore had to be discarded due to its inability to explain the following observed facts:

a. According to the Heisenberg uncertainty principle, if electron has to be within the nucleus, then its de-Broglie wavelength had to be of the order of the size of the nucleus.(a few fermi) –

So the Kinetic energy of the electron :

b. Due to violation of angular momentum coupling rule:

So there should be two protons and one electron. As proton and electron both are fermions with spin ½ particles.

But the measured ground state spin of deuteron is : J = 1

c. Magnetic moment value of the nucleus :

So, as the measured magnetic moment of nuclei are very much less than the magnetic moment of the electron, the electron cannot reside inside the nucleus.

d. stability of nucleus with both proton and electron inside: If electron has to be within the nucleus, then a very strong force, even stronger than the electromagnetic force would be needed to bound them. Yet no evidence of any strong force between the proton and the atomic electrons exist.

2.3. Discovery of neutron : The neutron was discovered in 1932 as the result of a series of experiments on the nuclear reactions made by physicist in different countries. The historic among them are the following :

Working in Germany, W.G. Bothe and H. Becker in 1930, found that when samples of boron or beryllium were bombarded with alpha particles, they emitted invisible, uncharged radiations that resembles the gamma rays. Its interaction with matter showed that it carried energies ~ 10 MeV (much energetic than the gamma rays previously observed.

and showed that this radiation was able to knock protons out of paraffin. But they misinterpreted the phenomenon as scattering of gamma rays on protons (a process similar to Compton effect – scattering of -rays on electrons).

Chadwick in 1932 at Cambridge (U.K.) studied the same reaction but used ionisation chamber to measure ionisation and the length of track. He used several target materials (H, He, Li, etc.) on the way of neutral radiation from Be and observed that the particles ejected from hydrogen behaved like protons with speeds up to 3.2 109 cm/s. He also noticed that the particles ejected from the heavier targets had larger ionising power and were in each case recoil ions of the element. On the basis of his observations, he concluded that if the ejection of a proton is due to the scattering of photon on nucleus, then to speed up proton up to 3.2 109 cm/s, a 52 MeV photon is needed. This exceeded all known energies of photons, emitted by nuclei. All difficulties disappeared when he assumed that incident particles are neutral particles with the mass equal to that of proton (which he proved mathematically too on the basis of his kinematic calculations). Chadwick called this neutral particle as neutron and published its findings in a letter to Nature Journal in 1932, on the basis of which he got the Nobel Prize in 1935.

2.4 The proton-neutron theory of nucleus : The discovery of neutron by Chadwick gave way to the proton-neutron model of atomic nucleus. According to this theory the nucleus of an atom having atomic number Z and mass number A consists of Z protons and A-Z neutrons. The isotopes of the nucleus differ only in the number of the neutrons they contain. Thus the nucleus of the Hydrogen (1H1) consist of one proton while that of deuteron (2D1, an isotope of hydrogen) consists of one proton and one neutron. A more general term ‘nucleons’ refers to both kinds of nuclear particles.

3.0 Our present understanding of the nuclei : The nuclear physics is now more than 100 years old. In this long duration it has got so much maturity that people are exploring the ways to use it rather than exploring the world of nuclei. Our current understanding of the world of nuclei suggest that there are around 6000 possible (stable/unstable) combinations of proton & neutrons (nuclei) and out of which roughly about 3000 have been discovered (or at least have been produced in the laboratories) All the known nuclei, when arranged in terms of the number of neutrons and protons in them in a chart (Segre chart) shown below, the following observation can be made. :

Stable nuclei : The nuclei which do not decay by itself via any mode are called stable nuclei. In the Segre chart, the nuclei shown by ‘black dot’ are stable nuclei. A hypothetical line joining all the stable nuclei is called ‘line of stability’ or the ‘beta stability line’ (the unstable nuclei in the neighborhood of the stable nuclei decay preferentially by beta particle emission and attain the stable configuration).

Unstable nuclei : The nuclei falling in the ‘ blue region’ or ‘green region’ are called unstable nuclei. Since in these nuclei, the neutron to proton ratio is more (or less) than what is required for the stability, so they decay to other nuclei via -particle (for heavy nuclei) or (and) particle (for intermediate and lighter nuclei) emission until they attain some stable structure.

Fig. 2 : The chart of nuclei (Segre Chart).

Neutron dripline :The number of isotopes any nucleus can have is governed by the interplay between the electromagnetic force acting between the protons and the strong nuclear forces acting between p-p, n-n & n-p inside the nucleus. So the number of isotopes for a given nucleus is limited, i.e there is a limit upto which the neutrons can be added to a given nucleus. The nucleus after which no more neutron can be added to it (neutron separation energy, Sn = 0) is called the drip-line nucleus. The hypothetical line joining all the drip-line nuclei towards the neutron axis in the Segre chart is called ‘neutron dripline’.

Proton dripline: The number of isobars for a given mass number is limited, i.e there is a limit upto which the protons can be added to a given nucleus. The nucleus after which no more proton can be added to it (proton separation energy, Sp = 0) is called the drip-line nucleus. The hypothetical line joining all the drip-line nuclei towards the proton axis in the Segre chart is called ‘neutron dripline’.

4.0 Useful terms in nuclear physics: Following terms are most commonly used in nuclear physics:

Nuclide : A nuclear species, with a given proton number Z and neutron number N

Isotopes : Nuclides of same Z and different N

Isotones : Nuclides of same N and different Z

Isobars : Nuclides of same mass number A (A = Z + N)

Isomer : Nuclide in an excited state with a measurable half-life

Nucleon : Neutron or proton

Mesons : Particles having mass between electron mass (m0) & proton mass (MH).

Positron : Positively charged electron of mass m0

Photon : Quantum of E-M radiation, commonly apparent as light, x ray, or – ray

5.0 Some basic units used in nuclear physics: The following are the most common used units in nuclear physics

Summary:

The world of nuclear science is very fancy. Since its inception, the nuclear physics has solved number of scientific mysteries of the Universe and has significantly contributed in understanding our World today. Now the Nuclear Physics has evolved to such a level so that focus has now a bit shifted from solving the mysteries of world to using it for improving the quality of life on earth. Nowadays the nuclear techniques are being preferentially used in various sectors of human activities, In medical sector, the nuclear techniques are used for diagnosis as well as threptic purposes. In agriculture sector, the nuclear techniques are in use for pest control, getting improved varieties of seed, enhancing the production and preserving the agriculture produce. In industries, the nuclear techniques are used for a variety of reasons like testing of products, improving the quality of products (Vulcanization, Galvanization, etc.). In archaeology, the nuclear techniques are in use for finding the age of the fossils while in geology, the technique is use in the detection and exploration of minerals remotely. Now days in almost every field, the nuclear techniques are in use either as the only choice or an alternate to other conventional techniques due to their great precession, simplicity, inexpensive and time saving nature.

References:

ntroduction to Nuclear Physics, 2nd Edition, W.N.Cottingham & D.A. Greenwood.
Concepts of Modern Physics, Arthur Beiser, McGraw-Hill Publication.
Introduction to Nuclear and Particle Physics, A.Das & T. Ferbel, World Scientific Publication.

Web Links

Did You know?

The ‘Father of Nuclear Physics’, Ernest Rutherford was born in New Zealand. He made a number of discoveries and inventions. Apart from his discovery of nucleus, he also discovered the concept of radioactive half-life and proved alpha and beta radiation in different elements.

Rutherford invented a new form of radio receiver while he was doing his research.

While working with J.J. Thomson at Cambridge University, Rutherford conducted experiments which led to the discovery of electrons

Rutherford experimented with uranium and explored its radioactive qualities. On the basis of his observations, he discovered alpha, beta and gamma rays and their properties. For his this discovery, he was awarded the Nobel Prize in 1908.

In 1909, Rutherford conducted the Gold Foil Experiment which is one of his most famous works. On the basis of his observations in the experiment, he discover that atoms consist of positively charged nucleus, where the mass is centered.

Rutherford held the world record for detecting electromagnetic waves by half a mile and during the First World War, he worked on a top secret project of solving the problems of submarine detection by sonar.

To honor Rutherford for his great contributions in Science, the element ‘Rutherfordium’ is named after him and also the unit of radioactivity is named as ‘Rd’ which stands for ‘Rutherford’.

Interesting facts about James Chadwick :

Chadwick’s early research, was concerned with gamma-ray absorption; first with its use as a precision test of radium standards and then with applications of the method devised for standardization. He investigated the excitation of gamma rays by beta rays (electrons) and then by alpha rays (helium nuclei). In both cases the excitation was confirmed. In Berlin with Geiger, Chadwick set out to determine by direct observation, using a primitive Geiger point counter, the relative intensities of the discrete lines observed by Rutherford and Robinson in radioactive beta ray spectra. Although he was able to identify a few of the most intense of the observed lines, he also found a continuous spectrum alongside the discrete one. He tried changing the detection apparatus, but this merely confirmed the conclusion. The result came as a complete surprise and could not readily be explained theoretically, but it was a clear indication of Chadwick’s experimental skill. Both spectra, and the relation between them, became an important problem in atomic and nuclear physics.

Chadwick’s last major work before leaving Cambridge was to demonstrate the nuclear photoelectric effect in the form of the disintegration of deuterium under gamma ray illumination. This work also led to the first accurate figure of the mass of the neutron, and to speculation as to the significance of slow neutrons. It was not published, however, and a few months later Enrico Fermi observed and realized the significance of the same phenomenon.

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