Dr. Arshad Kamal (Assistant Professor) Department of Preparatory Year College of Engineering and...

Dr. Arshad Kamal

(Assistant Professor)

Department of Preparatory Year

College of Engineering and Islamic Architecture

Umm-Al Qura University, Makkah

Kingdom of Saudi Arabia

Dr. Arshad KamalDepartment of Preparatory Year

Umm-Al Qura University, Makkah 1

CHAPTER 30

NUCLEAR PHYSICS

Topics

30.1 Radioactivity

30.2 Half-Life

Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura University,

Makkah

2

3Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura Univer-

sity, Makkah

Before we start....

- At the end of this presentation, you will be a genious about these following issues (at least I hope so ) :

- Radioactive (a type of exponentional) Decay Law

- Concept of Half- life

- How to solve half-life problems

4

What is Nuclear Physics?

Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura University,

Makkah

• Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei.

• The most infamous application of nuclear physics was probably the development of the atom bomb in the 1940s, but nuclear physics has many more applications, including highly beneficial ones.

• The most commonly known applications of nuclear physics are nu-clear

power generation and nuclear weapons technology, but the research has provided application in many fields, including those in nuclear medicine and magnetic resonance imaging, ion implantation in materials engineering and radiocarbon dating in geology and archeology.


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Early Pioneers in Radioactivity

Rutherford:

Discoverer

Alpha and Beta rays

1897

Roentgen:

Discoverer of

X-rays 1895

The Curies:

Discoverers of Radium

and

Polonium 1900-1908

Becquerel:

Discoverer of Radioac-

tivity

1896

6Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura University,

Makkah

Basic Properties of Nucleus

The nucleus is made up of protons and neutrons, collectively known as nucleons. A proton has a positive electrical charge equal in magnitude to the electronic charge and a mass about 1840 times that of the electron. Neutrons are about 0.1% more massive than protons. They are electrically neutral. A nucleus is specified by its atomic number Z and its mass number A. Z is the number of protons, and A is the total number of nucleons. Neutron number N = A-Z. The mass number A=Z+N determines approximately the mass of the

nucleus in atomic mass units.

7Dr. Arshad KamalDepartment of Preparatory Year

Umm-Al Qura University, Makkah

A particular nucleus defined by A and Z is known as a nu-clide.

The standard notation for nuclei is represented by U92238

This nucleus has 238 nucleons, out of which 92 are protons and 146 are neutrons.

U is the chemical symbol for the 92nd element, Uranium.

Nuclear species, or nuclides, which have the same atomic number but different neutrons numbers are called isotopes.

Three distinct types of forces play important roles in nuclei.

Nuclei are held together by very strong, short-ranged nuclear forces among the nucleons.


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Electrical forces are smaller in magnitude, but they become progressively more important as the number of protons in the nucleus increases.

The weak interactions are responsible for the beta decay process. In the beta decay, neutrons in nuclei are converted into protons as they emit electrons and neutrinos.

Gravitational forces are the weakest forces so they are not important in nuclear physics.


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What do we mean by Radioactivity?Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electro-magnetic waves.

There are numerous types of radioactive decay. The general idea:An unstable nucleus releases energy to become more stable

The phenomenon of self-emission of radiation is called radioactivity and the substances which emit these radiations are called radioactive

substances. All the naturally occurring elements with atomic number Z greater than 82 (i.e. heavier than lead) are radioactive . Examples: Radium, Polonium, Thorium, Uranium, Actinium, etc.


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The radiations from a radioactive material such as Uranium were madeto pass through a strong magnetic field as shown in the figure. The direction of the magnetic field is perpendicular to the plane of the paperand it is represented by crosses (x) in the diagram.

Under the action of magnetic field, the beam/radiation separated into three distinct parts as shown in the figure.

These three kinds of radiations were named Alpha (α), Beta (β) and

Gamma (γ) radiations.


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Where do these particles come from ?

These particles generally come from the nuclei of atomic isotopes which are not stable.

The decay chain of Uranium produces all three of these formsof radiation.

Let’s look at them in more detail…

Alpha Particles (a)

Uranium

U238

92 protons146 neutrons

Thorium

Th234

Note: This is theatomic weight, whichis the number ofprotons plus neutrons


+ nnp

p

a (4He)2 protons2 neutrons

-The alpha-particle (a) is a Helium nucleus which are positively charged. - relatively easy to stop, can only penetrate light materials such as paper - large size- double positive charge- It’s the same as the element Helium, with the electrons stripped off !

Beta Particles (b)

CarbonC14

6 protons8 neutrons

NitrogenN14

7 protons7 neutrons

+e-

electron(beta-particle)

We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. In symbolic notation, the following process occurred:

n p + e ( + )nYes, the same neu-trino we saw previ-

ously

-faster than alpha particles, penetrate light materials, 10 mm of wood, 3 mm of Al

Gamma particles (g)Gamma rays are high-frequency electromagnetic radiations(i.e. pho-tons which do not carry any charge.

In much the same way that electrons in atoms can be in an excited state, so can a nucleus.

NeonNe20


(in excited state)


(lowest energy state)

+

gamma

NeonNe20

- very fast and able to penetrate most materials such a 7 cm of lead

- can be harmful to our bodies, suffer structural damage

- It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

- very fast and able to penetrate most materials such a 7 cm of lead

- can be harmful to our bodies, suffer structural damage

- It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

Gamma Rays

NeonNe20 +

The gamma from nuclear decayis in the X-ray/ Gamma ray

part of the EM spectrum(very energetic!)

NeonNe20

How do these particles differ ?

Particle Mass*(MeV/c2)

Charge

Gamma (g) 0 0

Beta (b) ~0.5 -1

Alpha (a) ~3752 +2

* m = E / c2* m = E / c2


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The three main decays are Alpha, Beta and Gamma


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Where are the Sources of Radioactivity?

Naturally Occurring Sources:Radon from the decay of Uranium and Thorium Potassium -40 – found in minerals and in plantsCarbon 14 – Found in Plants and Animal tissue

Manmade Sources:Medical use of Radioactive IsotopesCertain Consumer products –(e.g. Smoke detectors)Fallout from nuclear testingEmissions from Nuclear Power plants


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Radioactivity – Is it a Health Problem?

The Alpha, Beta and Gamma particles all add energy to the body’s tissues. The effect is called the Ionizing Energy. It can alter DNA.

Even though Alpha particles are not very penetrative if the decaying atom is already in the body (inhalation, ingestion) they can cause trouble.

While gamma has the highest penetrating power, so gamma radia-tions

are more harmful. When they fall on the human body, they kill the living tissues, cause radiation burns and can induce cancer.


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Radioactivity – Is it a Health Problem?

Biological effects of nuclear radiation

1. Short term recoverable effects,2. Long term irrecoverable effects and3. Genetic effects

The first two effects are limited to the individuals who are actu-ally

exposed to the radiations, while the third effect appears in the later generations.


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Half-Life (T) is the time required for half a sample of radioactive nuclei to decay

impossible to predict the moment any nucleus will decay

nuclear decay is random process but it is possible to predict the time it will take for half the nuclei to decay

different radioactive isotopes have different half lives varying from

nanoseconds to billions of years

can predict the approximate age of a sample


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Radioactive Decay LawSuppose at time t = 0, there are N0, nuclei, then one half-life later at t= T, an average of N0 /2 will remain.At t=2T when two half life have elapsed, N0 /4, nuclei will be left.At t = 3T, N0 /8 will be left, and so on. When the elapsed time is not an exact integer multiple of the half life,

we can find the number of nuclei remaining as follows:

The change ∆N in the number of nuclei N present occurring in a short time ∆t is proportional to N and ∆t, so we can write

∆N = - λN ∆t The negative sign is included because the number N of radioactive nuclei decreases with time due to decays.


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What was that?!!! In the previous equation you have seen a symbol like: λ λ is a constant of proportionality, called the decay constant

It differs according to the isotope it is in.

The greater λ is, the greater the rate of decay

This means that the greater λ is, the more radioactive the isotope is said to be.


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Still confused about the equation...• Don’t worry! If you are still confused about

why this equation is like this, here is some of the important points....


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Figure shows a plot of N versus t which is the radioactivity decay curve. When t = T, N/ N0 = ½, according to half-life definition,When t = 2T, N/ N0 = ¼; ………….values of N/ N0 can be read fromthe graph or calculated using an electronic calculator.

Exponential decay formula


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How to calculate half-life?The decay constant and half-life has the relationship :

Example 30.1 Iodine 131 is used in the treatment of thyroid disorders.Its half-life is 8.1 days. If a patient ingests a small quantity of I-131 and none is excreted from the body, what fraction N/ N0 remains after8.1 days, 16.2 days, 60 days?

λ = ln(2)/T = 0.693/T


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Some representative half-lives are given in Table 30.1


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Homework

Problems: 30.1, 30.2, 30.3, 30.5 and 30.7

Dr. Arshad Kamal (Assistant Professor) Department of Preparatory Year College of Engineering and...

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