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RADIOCHEMICAL METHODS OF ANALYSIS

Early Pioneers in Radioactivity

Roentge

n:Discoverer of X-

rays 1895

Becquer

el:Discoverer of

Radioactivity

1896

The

Curies:Discoverers of

Radium and

Polonium 1900-

1908

Rutherfo

rd:Discoverer Alpha

and Beta rays

1897

All atoms nuclei, except hydrogen, is made up of a collection of protons and neutrons.

The chemical properties of an atom are determined by its atomic number, Z, the number of protons.

The sum of neutrons and protons is the mass number, A.

The nuclei of isotopes of an element contain the same number of protons, but have different numbers of neutrons.

Radioactive isotopes (radionuclide's), undergo spontaneous disintegration, which ultimately leads to stable isotopes.

Radioactive decay of isotopes occurs with the emission of electromagnetic radiation in the form of x-rays or gamma ray. Accompanying this emission is the formation of electrons, positrons and the helium nucleus.

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Some of the most chemically important types of radiation from radioactive decay are listed in the table. Four of these types - alpha particles, beta particles, gamma-ray photons and X-ray photons can be detected and counted.

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Product Symbol Charge Mass Number

Alpha particle +2 4

Beta particles

Negatron - -1 1/1840 (~0)

Positron + +1 1/1840 (~0)

Gamma ray 0

X-Ray 0 0

Neutron n 0 1

Neutrino v 0 0

Alpha Decay:

Alpha decay is a common radioactive process encountered with heavier isotopes.

The alpha particle is a helium nucleus having a mass of 4 and a charge of +2.

• Alpha particles are charged particles so they are deflected by electric and magnetic radiation.

• These are heavier particles so their mean velocity is less than other nuclear emissions that have same kinetic energy

• Size of alpha particles prevents them from penetrating them into substances and they are easily stopped by a few centimeters of air, or by the skin.

• They have defined energy in narrow range.

• Alpha particles rapidly lose their kinetic energy as they penetrate into the matter. Owing to the rapid loss, alpha particles are highly effective in ionizing matter through which they pass. They interact with electron and converted into helium atom 5

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α-

Decay…………………..

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Researchers are currently trying to use the damaging nature of alpha

emitting radionuclides inside the body by directing small amounts

towards a tumor. The alphas damage the tumor and stop its growth

while their small penetration depth prevents radiation damage of the

surrounding healthy tissue. This type of cancer therapy is called

unsealed source radiotherapy

Alpha decay can provide a safe power source for radioisotopes, e.g;

thermoelectric generators used for space probes and artificial heart

pacemakers. Alpha decay is much more easily shielded against than

other forms of radioactive decay. Plutonium-238, a source of alpha

particles, requires only 2.5 mm of lead shielding to protect against

unwanted radiation.

Most smoke detectors contain a small amount of the alpha emitter

americium-241

Applications: α-

Decay…………………..

Beta Decay:

Beta decay is a radioactive process in which, the atomic number changes but the mass number stays the same.

Three types of decay are encountered: negatron formation, positron formation and electron capture. Example of these three process are:

negatron formation

positron formation

electron capture

Negatrons (-) are electrons that form when one of the neutrons in the nucleus is converted to a proton.

A positron (+), with the mass of the electrons, forms when the proton in the nucleus is converted to neutron.

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v CuZn 65

29

65

30

v NC -14

7

14

6

raysXVe Cr 48

23

0

1

48

24

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β-

Decay…………………..

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• Beta particles are deflected by electric and magnetic field.

• Beta particles have the medium penetrating power.

• Bata particles can penetrate about 500 times the distance penetrated

by alpha particles of same energy.

• They can be stopped by 3mm layer of lead.

• Kinetic energy is continuous and have broad spectrum of energy.

• They move with the speed of light.

• Positron when react with matter they are slowed down and eventually

annihilated by reacting with electrons to form two equivalent gamma rays.

This is called back to back emission. Entire mass of gamma rays

is converted into gamma rays. Gamma emission at 0.511 Mev is indicative

of positron emission.

e- + e+ → 2 γ ( gamma rays)

β-

Decay…………………..

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Applications of positrons:

It is used in biological sciences to see the spectacular effects on brain

scanning, this technique is known as positron emission tomography.

Applications of negatron:

P32 is powerful tool in research of molecular biology and genetics.

Tritium and C14 is used in labeling of organic compounds.

S35 is used to label methionene to study proton synthesis.

Gamma-Ray Emission:

Gamma rays are produced by nuclear relaxations.

Gamma-ray emission is the result of a nucleus in an excited state returning to the ground state in one or more quantized steps with the release of monoenergetic gamma rays.

Generally, the lifetime of the excited states is very small, of the order of 10-16 to 10 -13 seconds. The γ radiation is emitted immediately after a preceding α or β decay.

The gamma-ray emission spectrum is characteristic for each nucleus and is thus useful for identifying radioisotopes.

Gamma radiation is highly penetrating12

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ϒ-

Decay…………………..

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• They have no charge so they remain unaffected by electric and

magnetic field.

• Gamma rays usually are emitted along with other particles.

• Gamma rays are the most energetic form of electromagnetic

radiation, with a very short wavelength of less than one-tenth

of a nanometer.

• Gamma particles move with the speed of light.

• They have more penetration power than alpha and beta radiations.

• Gamma radiations from Co60 penetrate into 15cm steel.

Properties:

ϒ-

Decay…………………..

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Gamma rays are also used for diagnostic purposes in nuclearmedicine in imaging techniquesIn industry principle useinclude casting and weld.

103Ru44 →103Rh45 +

β- + γ131I53 →

131Xe54 + β-

+ γ

Electron Capture Decay

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• An electron from the closest energy level falls into the nucleus,

which causes a proton to become a neutron.

• A neutrino is emitted from the nucleus.

• Another electron falls into the empty energy level and so on

causing a cascade of electrons falling. One free electron, moving

about in space, falls into the outermost empty level, this cascade of

electrons falling creates a characteristic cascade of lines, mostly in

the X-ray portion of the spectrum. This is the fingerprint of electron

capture.

•4) The atomic number goes DOWN by one and mass number remains

unchanged

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Electron Capture Decay…………..

Electromagnetic spectrum

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The Nuclear Stability Belt

Kinds of Radioactivity

The three main decays are

Alpha, Beta and Gamma

The blue grid below represents a quantity of C14. Each time you click,

one half-life goes by and turns red.

C14 – blue N14 - red

As we begin notice that no

time has gone by and that

100% of the material is C14

Half

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

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The grid below represents a quantity of C14. Each time you click,

one half-life goes by and you see red.

C14 – blue N14 - redHalf

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

After 1 half-life (5730 years), 50% of

the C14 has decayed into N14. The ratio

of C14 to N14 is 1:1. There are equal

amounts of the 2 elements.

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The blue grid below represents a quantity of C14. Each time you click,

one half-life goes by and you see red .

C14 – blue N14 - redHalf

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

Now 2 half-lives have gone by for a total

of 11,460 years. Half of the C14 that was

present at the end of half-life #1 has now

decayed to N14. Notice the C:N ratio. It

will be useful later.23

The blue grid below represents a quantity of C14. Each time you click,

one half-life goes by and you see red.

C14 – blue N14 - redHalf

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

3 12.5% 87.5% 1:7

After 3 half-lives (17,190 years) only

12.5% of the original C14 remains. For

each half-life period half of the material

present decays. And again, notice the

ratio, 1:724

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What is the half life

represented in this

graph?

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Radioactivity and Half-LifeHalf-life is the time taken for half of a radioisotope to decay. This value is constant for that

particular radioisotope.

Usually questions about radioactivity involve

Half-life

The time over which the radioactivity has been measured

The quantity or intensity of the radiation.

Worked example 1.

A radioisotope has a half-life of 4 days. The radioisotope had an initial count rate of 800 counts per

minute. What will be the count rate after 16 days?

At start count rate = 800 counts min-1.

After 4 days = 400 counts min-1.

After 8 days = 200 counts min-1.

After 12 days = 100 counts min-1.

After 16 days = 50 counts min-1.

Worked example 2.

Thallium-208 has a half life of 3.1 minutes and decays by beta emission to form a stable isotope.

(a) What mass of a 2.08 g sample of Tl-208 will remain unchanged after

9.3 minutes?

(b) How many atoms will have decayed.

(c) Identify the stable isotope formed by the decay of Tl-208 by beta emission.

(a) At start mass = 2.08g

After 3.1 days = 1.04g

After 6.2 days = 0.52g

After 9.3 days = 0.26g

(b) At start no. of atoms = 2.08/208 x 6.02 x 1023 = 6.02 x 1021

After 3.1 days no. of atoms of Tl-208 left undecayed= 3.01 x 1021

After 6.2 days no. of atoms of Tl-208 left undecayed= 1.505 x 1021

No. of atoms of Tl-208 which have decayed= 6.02 x 1021 -0.7525 x 1021.

5.2675 x 1021

(c) Pb-208 has been formed.

After 9.3 days no. of atoms of Tl-208 left undecayed= 0.7525 x 1021

Calculations for you to try.

1. Th-234 has a half-life of 24.1 days. What mass of a 20.4g sample will remain after 96.4 days?

At start mass = 20.4 g

After 24.1 days mass left = 10.2 g

After 48.2 days mass left = 5.1 g

After 72.3 days mass left = 2.55 g

After 96.4 days mass left = 1.275 g

2. A sample of Pu-242 has a mass of 1.21 g.

(a) How many atoms of Pu-242 are there in the sample?

(b) How many atoms of Pu-242 will remain after 3 half lives.

(c) Use the half life in the data book for Pu-242 to calculate the time it would

take to reduce the number of Pu-242 atoms in the sample to 1/8 of its original

value. (a) No of atoms = 1.21/242 x 6.02 x 1023. = 3.01 x 1021

(b) No of atoms at start = 3.01 x 1021

After 3 half lives no. of atoms = 1/8 x 3.01 x 1021

= 3.76 x 1020

(c) 1/8 of original value means that 3 half lives have passed.

3 x 3.79 x 105 years = 1.137 x 106 years

Calculations for you to try.

3. The count rate due to carbon-14 in ancient wooden timber was found to be

100 counts per minute. A sample of modern wood had a count rate of 1600 counts per minute.

Given that carbon-14 has a half life of 5570 years, calculate the age of the ancient timber.

At start count rate = 1600 counts min-1.

After 1 half-life = 800 counts min-1.

After 2 half-life = 400 counts min-1.

After 3 half-life = 200 counts min-1.

After 4 half-life = 100 counts min-1.

Age of timber = 4 x 5570 = 22 280 years.

4. A radioisotope used in a laboratory has a half life of 6.75 hours.

It had a count rate of 2000 counts per minute at 8.00 a.m. on Monday.

What would be the count rate at 11 a.m. the following day?

Between 8.00 a.m. and 11.00 am the next day 27 hours have passed.

Number of half-lives in 27 hours = 27/6,75 = 4

Count rate 2000 1000 500 250 125 counts min-1.