Physics Assignment 11

8/14/2019 Physics Assignment 11

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NUCLEUS OF AN ATOM

The composition of the nucleus of an atom

Matter is made up of very small particles called atom. Each atomhas a very small and very dense core called nucleus.Most of the

mass of atom is contained in the nucleus. The electrons move in

orbits around the nucleus. A nucleus consists of number of proton

and neutrons. Proton and neutrons also known as nucleons. A

proton has a unit of positive charges. A neutron is an uncharged

particles of about the same mass of the proton. An atom is neutral

because it contains an equal number of negatively charged

electrons. So the net charged is zero.


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The proton number (Z)

The proton number is defined as the number of protons in the

nucleus. The number of electrons is equal to the number of

protons. An element is identified by its proton number.

Nucleon number(A)

Nucleon number is defined as the total number of protons and

neutrons in a nucleus.

Number of neutrons(N) = A-Z

Nuclide and its notationA nuclide is a type of atom whose nuclei have specific numbers of

protons and neutrons (both are called nucleons). Therefore,

nuclides are composite particles of nucleons.

According to the standard model, up and down quarks are the basic

components of nucleons. Thus, nuclides can also be considered

composite particles of quarks

The term isotopes is often used to mean nuclides, because anuclide is usually an isotope of an element. Strictly speaking,

isotopes are atoms with the same number of protons but different

number of neutrons in their nuclei.


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Notation for a nuclide

The notation for a nuclide with mass numberA and atomic number

Zis representd by a symbol of its elementE.

Notation of a nuclideAEZ

For example

235U92

Stable and Unstable Nuclides

There are stable and radioactive nuclides. Stable nuclides exist for

an indefinite period of time, and they are the constituents of

ordinary material. Unstable nuclides emit subatomic particles, with4a, b, g, n, p being the most common. Few undergo nuclear fission.

However, unstable nuclides with long half-lives are also present in

nature.

Stable nuclides are not radioactive. They remain unchanged for an

indefinite period.

Unstable nuclides are radioactive, and they emit alpha, beta,

gamma, or proton and they eventually convert to stable nuclides.

The study of nuclides is an experimental and observatory science.

It involves data gathering, classification, organization, observation,and theorization about nature.
http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclidestable.htmlhttp://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclideunstable.htmlhttp://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclidestable.htmlhttp://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclideunstable.html


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Isotope

Atoms of the same element can have different numbers of

neutrons; the different possible versions of each element are calledisotopes. For example, the most common isotope of hydrogen has

no neutrons at all; there's also a hydrogen isotope called

deuterium, with one neutron, and another, tritium, with two

neutrons.

Hydrogen Deuterium Tritium


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As a result of their having different numbers of neutrons, anelement's isotopes differ in mass. Atomic mass has very little

bearing on chemical reactions; therefore the chemical reactions of

an element's different isotopes are almost identical.

The physical properties of atoms, however, do depend on mass.

This enables isotopes to be separated from one another by methods

such as diffusion and fractional distillation.

Isotopes of an element contain the same number of proton and thesame number of electron. So isotopes have the same chemical

properties. However they have different physical properties

because their mass is different. Some isotopes exist naturally and

some of them can also be made artificially.


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RADIOACTIVE DACAY

Radioactivity

An unstable nucleus spontaneously emits particles and energy in a

process known as radioactive decay.The termradioactivity

refers to the particles emitted. When enough particles and energyhave been emitted to create a new, stable nucleus radioactivity

ceases.


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Radioactive emissions

Alpha Radiation

Alpha rays have the least penetrating power, move at a slower

velocity than the other types, and are deflected slightly by a

magnetic field in a direction that indicates a positive charge.

Beta Radiation

Beta rays are more penetrating than alpha rays, move at a very

high speed, and are deflected considerably by a magnetic field in a

direction that indicates a negative charge

Gamma Radiation

Gamma rays have very great penetrating power and are not

affected at all by a magnetic field. They move at the speed of light

and have a very short wavelength (or high frequency); thus they

are a type of electromagnetic radiation


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Radioactive decay


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Alpha Decay

The reason alpha decay occurs is because the nucleus has too

many protons which cause excessive repulsion. In an attempt

to reduce the repulsion, a Helium nucleus is emitted. Theway it works is that the Helium nuclei are in constant

collision with the walls of the nucleus and because of its

energy and mass, there exists a nonzero probability of

transmission. That is, an alpha particle (Helium nucleus) will

tunnel out of the nucleus. Here is an example of alpha

emission with americium-241:

Alpha Decay of Americium-241 to Neptunium-237. Adapted from Alpha

Decay.
http://www.triumf.ca/safety/tsn/tsn_6_3/subsubsection3_4_2_4.htmlhttp://www.triumf.ca/safety/tsn/tsn_6_3/subsubsection3_4_2_4.htmlhttp://www.triumf.ca/safety/tsn/tsn_6_3/subsubsection3_4_2_4.htmlhttp://www.triumf.ca/safety/tsn/tsn_6_3/subsubsection3_4_2_4.html


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Beta Decay

Beta decay occurs when the neutron to proton ratio is too

great in the nucleus and causes instability. In basic beta

decay, a neutron is turned into a proton and an electron. Theelectron is then emitted. Here's a diagram of beta decay with

hydrogen-3:

Alpha Decay of Hydrogen-3 to Helium-3. Adapted from

Stability of Nuclei.

Gamma Decay

Gamma decay occurs because the nucleus is at too high an

energy. The nucleus falls down to a lower energy state and,
http://www.triumf.ca/safety/tsn/tsn_6_3/subsection3_4_1.html#betahttp://www.triumf.ca/safety/tsn/tsn_6_3/subsection3_4_1.html#beta


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in the process, emits a high energy photon known as a

gamma particle. Here's a diagram of gamma decay with

helium-3:

Gamma Decay of Helium-3

Example of use equations to represent changes in thecompositions of the nucleus when particles are emitted


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Half life

Half-life is the period of time it takes for a substance undergoing

decay to decrease by half. The name originally was used to

describe a characteristic of unstable atoms (radioactive decay), but

may apply to any quantity which follows a set-rate decay.The time required for half the nuclei in a sample of a specific

isotopic species to undergo radioactive decay.

The only thing which can alter the half-life is direct nuclear

interaction with a particle from outside, e.g., a high energy

collision in an accelerator.

Determine half life from a decay curve

Once an atom has decayed, it is no longer that type of atom. (The

carbon atom that decays is no longer a carbon atom.) If we start

with a certain number of atoms - say 100 million of them, thenafter a certain period we will notice that the number of atoms

remaining is decreasing. The number remaining depends on how

stable the atom is (or isn't)! We end up with a graph which curves

down towards zero.
http://en.wikipedia.org/wiki/Radioactive_decayhttp://en.wikipedia.org/wiki/Radioactive_decay


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Decay graph

This remarkable graph halves in equal time intervals called

HALF-LIVES(T1/2).


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Example of solve problems involving half lives

PROBLEM; Strontium - 90 has a half-life of 28.1 years. A sample has an

initial activity of 400 MBq . What will be the activity after 92 years?

Solution;

From the graph, about 38 MBq. OR (not on syllabus )

Using the formula, let No = 400MBq , T1/2 = 28.1 years , t = 92 years.

Using N = N0t/T

1/2

2

]= 400

292/28.1

= 400

23.274

= 400 = 41.3 MBq

9.67


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The formula is more precise than the graph and gives the activity at 41.3

MBq.

Greater precision could be gained using the graph if, say, the graph was

recalibrated for after two half lives, starting at 100 MBq, then looking up a

time of (93 - 56.2 ) years. One could do this because the SHAPE of the

graph does NOT change despite starting point or calibration in half-life.

RADIOISOTOPES

Many of the chemical elements have a number of isotopes. The

isotopes of an element have the same number of protons in their

atoms (atomic number) but different masses due to different

numbers of neutrons. In an atom in the neutral state, the number of

external electrons also equals the atomic number. These electrons

determine the chemistry of the atom. The atomic mass is the sum

of the protons and neutrons. There are 82 stable elements and

about 275 stable isotopes of these elements.

When a combination of neutrons and protons, which does not

already exist in nature, is produced artificially, the atom will be

unstable and is called a radioactive isotope or radioisotope. There

are also a number of unstable natural isotopes arising from thedecay of primordial uranium and thorium.

Overall there are some 3800 radioisotopes. At present there are up

to 200 radioisotopes used on a regular basis, and most must be

produced artificially


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Example of radioisotopes

Iodine--123

Carbon--11

Phosphorous--32

Iodine--131Thallium--201

Gallium--67

Chromium51

The nucleus of a radioactive isotope spontaneously decomposes,

emitting a particle (such as a proton, neutron, alpha particle, etc...)

and electromagnetic radiation (such as gamma rays). Because of

the loss of nuclear particles, the element in question can turn into anew element.

Consider the example of carbon-14, which is radioactive and

undergoes radioactive decay and changes from carbon-14 to

nitrogen-14, while emitting an electron and an anti-neutrino.

Applications of radioisotopes

Stable isotopes are tools used by researchers worldwide in the

diagnosis of disease, to understand metabolic pathways in humans,

and to answer fundamental questions in nature. They help

researchers find answers by allowing them to look at a problem in

a new way, from a different perspective. They help to better

understand a process, trace a compound from a particular source,

measure the concentration of a chemical in a sample, or measure

the rate of a related process. Stable isotopes already play an

important role in research today and will become even more

important to research in the future.


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Physics Assignment 11