2 radioactivity and nuclear transformation

Lecture 8

Shahid Younas

NUCLEAR TRANSFORMATION

INTRODUCTION

Lecture 8

Why don't all the protons in the nucleus repel each other

and cause the nucleus to blow apart?

Nuclear Force

INTRODUCTION

Lecture 8

Nuclei are composed of

combinations of nucleons.

Certain combinations bear a high

degree of stability while others are

relatively unstable.

INTRODUCTION

Lecture 8

One radioactive nuclide may have a high N/P ratio while for

another the ratio may be low.

A radionuclide may have an odd number of protons and an even

number of neutrons, while for another the reverse may be true.

INTRODUCTION

Lecture 8

Do the radioactive nuclides different from each other?

Yes, of course

INTRODUCTION

Lecture 8

It is not surprising, then, that various modes of decay are possible,

depending upon the nature of the nuclide and the type of instability.

INTRODUCTION

Lecture 8

Unstable nuclei are said to be radioactive

because they emit radiation as they undergo

spontaneous decay.

INTRODUCTION

Lecture 8

This radiation is emitted either from the nucleus or orbital electrons.

The nature of this radiation is a function of the mode of decay of a

particular nuclear species.

INTRODUCTION

Lecture 8

Transformation literally means going beyond

your form.

Nuclear Stability Tests

Lecture 8

Magic Numbers:

Number of nuclear particles in a completed shell of protons or

neutrons.

Magic Numbers for P+ → 2, 8, 20, 28, 50, 82, (114)

Magic Numbers for No → 2, 8, 20, 28, 50, 82, 126

If a nucleus has two magic numbers, it is very stable.


Lecture 8

Magic Numbers

Example:

4He2

p+ → 2

no → 2

Both are magic numbers, very

stable.

Which is more stable?

102Sn51 or 101Sn51

102Sn51 → p+ = 51, no = 51

101Sn51 → p+ = 51, no = 50

101Sn51 has a magic number and 102Sn51

doesn't.


Lecture 8

The Even-Odd Rules

1. There is instability when there is an odd number of

neutrons or protons in the nucleus

2. Nuclei with an even number of protons and an even

number of neutrons are very likely to be stable.


Lecture 8

Which is more likely to be stable, 62Ga31 or 64Ge32?

64Ge32 because it has both an even number of protons and an even

number of neutrons.


Lecture 8

Band of Stability

The region in which stable

nuclides lie in a plot of number

of protons against number of

neutrons.


Lecture 8

“Band of Stability”

Any nuclide falling in the

band of stability will be stable

and therefore not radioactive.

its edges are not as smooth as

in the diagram.

DECAY BY α- PARTICLE EMISSION

Lecture 8

The α-particle consists of 2 protons and 2 neutrons and is a

relatively large particle

For a nucleus to be capable of releasing so large a particle, the

nucleus must be relatively large itself.

Nuclei which decay by α emission have a Z >83 and A > 150.


Lecture 8

Which is the origin of alpha particles?

Nucleus or Orbit


Lecture 8

α decay is followed by gamma and characteristic x-ray emission.

Alpha particles are emitted with discrete energy ranges from 2 to

10 MeV.

These are not used in Medical imaging however potential source

of hazard in radiation protection.


Lecture 8

Generic Reaction equations,


Lecture 04

α-EMISSION: DECAY OF (Rn222)


Lecture 8

In Route 1, the decay is

directly to the ground

state. There is no excited

or metastable state

formed and no gamma

rays are released.


Lecture 8

In Route 2, there is decay

by alpha to a metastable

state of the daughter,

followed by emission of a

gamma ray and transition

to the ground state.


Lecture 8

There is alpha decay to a highly

excited state of the daughter,

followed by gamma ray emission

either directly to the ground state

or indirectly to the ground state

through sequential emission of

two separate gamma rays.


Lecture 8

Do you know about Recoil / Transition Energy?

What happens to your shoulder when you fire a bullet.

The emitting particle transfers a small part of the energy to the

parent or daughter element. This gives an recoil to that element.


Lecture 8

BETA MINUS (β-) NEGATRON DECAY

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N/Z ratio is too large i.e. neutron-rich region.

The neutron is converted into a proton, which remains in the

nucleus.


Lecture 8

Generic decay equation is,


Lecture 8

Do you know about isobaric transitions?

Decay in which mass number remains constant is called as isobaric

transition.


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Beta-minus decay decreases the N/Z ratio, bringing the daughter

closer to the line of stability;


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Emitted β particles are poly-energetic with Emax = Eβ- + Ev


Lecture 8

DECAY SCHEME OF Au198:

Three steps are involved same

as alpha decay.


Lecture 8

COMPARISON OF ALPHA- AND BETA- DECAY SCHEMES

Alpha decay schemes and Beta decay schemes are mirror images of

each other.

BETA PLUS (β+) POSITRON DECAY

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Neutron poor i.e. low N/Z ratio.

Increases the neutron number by one.


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Decreases the atomic number by 1

Isobaric transition.


Lecture 8

Accelerator produced radionuclides often decay by β+ mode.


Lecture 8

Accelerator produced radionuclides often decay by β+ mode.

18F9 β+ 18O8

95Tc 43 → 95Mo42 + 01e


Lecture 8

Energy distribution between the positron and neutrino is similar to

that between the negatron and the anti-neutrino in B minus decay.

Positron decay is also poly-energetic.

Negatron are physically identical to electron.

Opposite magnetic moment than electron.


Lecture 8

Do you know the difference between neutrino and anti-neutrino?

Both are neutral particles but different magnetic moment.


Lecture 8

FATE OF POSITRONS IN

MATTER

When they come to rest

they react violently with

their electrons.

Two 511 keV energy

photons emitted.


Lecture 8

FATE OF POSITRONS IN

MATTER

According to E= mc2 is the

energy equivalent of the

rest mass of both particles.

RADIOACTIVITY & NUCLEAR TRANSFORMATION

Lecture 8

Be less curious about people and more

curious about ideas.

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2 radioactivity and nuclear transformation