Radioactive isotopes and their uses

Learning outcomesLearning outcomes CO PO Guided

learningSelf directed learning

Assessment SLT

1 Describe 3 types of radioactivity and their properties

2 Describe radioisotopes use in biological applications and methods of measuring radioactivity

3 Describe working practices when using radioactive isotopes

4 Carry out a half life calculation

5 Practise radioactivity interconversion

6 Use the concept of specific activity in calculation

Three isotopes of carbonStable• 98.9%

Stable • 1.1%

Radioactive• Negatron emitting radioisotope

Atomic number

Mass number

Review

• Isotopes of particular element have:-– Same number of proton → same atomic number– Different number of neutron → different mass

number

• May be stable or radioactive• Radioactive isotopes disintegrate

spontaneously at random to yield radiation and a decay product

Radioactive decay

• 3 forms of radioactivity1. Alpha (α) decay2. Beta (β) decay3. Gamma (γ) decay

Types of radioactivity and their properties

Radiation Range if maximum energies (MeV*)

Penetration rage in air (m)

Suitable shielding material

Alpha (α) 4 – 8 0.025 – 0.080 Unnecessary

Beta (β) 0.01 – 3 0.150 – 16 Plastic (e.g. Perspex)

Gamma (γ) 0.03 - 3 1.3 - 13 Lead

* 1 MeV = 1.6 x 10 -13 J

Alpha decay

• Involves the loss of a particle equivalent to helium nucleus

• Large and positively charge• Do not penetrate far in living tissues, but it can

causes ionisation damage• Thus, unsuitable for tracer studies

Beta decay

• Involves loss or gain of an electron or positron• It have 3 subtype:

1. Negatron (β-)2. Positron (β+)3. Electron capture (EC)

Negatron (β-)•Loss of an electron from nucleus when a neutron

transform into a proton•Most importannt form of decay for radioactive tracer

used in biology•e.g. 3H, 14C, 32P, and 35 S

Positron (β+)•Loss of a positron when a proton transform into a

neutron

Electron capture (EC)•When a protin captures and electron and transforms

into a neutron

Gamma decay

• Emission from a nucleus in a metastable state • Always follows initial alpha or beta decay.

Properties of some isotopes used commonly in life sciencesIsotope Emission(s) Half-life Main uses Advantages Disadvantages

3H β- 12.3 yr Suitable for labelling organic molecules in wide range of positions at high specific activity

Relatively safe Low effieciency of detection, high isotope effect, high rate of exchange with environment

14C β- 5715 yr Suitable for labelling organic molecules in a wide range of positions

Telatively safe, low rate of exchange with environment

Low specific activity

22Na β+(90%) + γ, EC

2.6 yr Transport studies High specific activity Hazardous

32P β- 14.3 days Labelling proteins and nucleotides High specific activity, ease of detection

Short half-life, hazardous

33P β- 25 days Labelling proteins and nucleotides Safer than 32P

Moderate half life

35S β- 87.2 days Labelling proteins and nucleotides Low isotope effect Low specific activity

36Cl β-, β+, EC 300000yr Transport studies Low isotope effet Low specifc activity, hazardous

125I EC + γ 59.9 days Labelling proteins and nucleotides High specific activity Hazardous

131I β-+ γ 8.04 days Labelling proteins and nucleotides High specific activity Hazardous

Half life• Radioactivity decay exponentially• Half life (t½) – Time taken for the radioactivity to fall from a given value to

half of that value

To calculate fraction (f) of the original radioactivity left after a particular time (t)

• Use the following relationship

f = ex, where x= -0.693t/t½

ExampleFor 35S, with a half life of 87.2 days, the fraction of radioactivity

remaining after 28 days

Solutionx = (-0.693 x 28 days) / 87.2 days = -0.222522936

f = e-0.222522936

= 0.800496646

Thus, the fraction of radioactivity remaining after 28 days is 80.0% of original activity

SI unit of radioactivity• Becquerel (Bq)– Equivalent to 1 disintegration per second (d.p.s)

• Non SI unit – curie (Ci)

1 Bq = 1 d.p.s 1Sv = 100 rem

1 Bq = 60 d.p.m 1 Gy = 100 rad

1 bq = 27 pCi 1 Gy ≈ 100 roentgen

1 d.p.s = 1 Bq

1 d.p.m = 0.0167 Bq

1 Ci = 37 MBq

1 mCi = 37 MBq

1 µCi = 37 kBq

Relationships between units of radioactivity

Specific activity

• Is a measure of the quantity of radioactivity present in a known amount of the subtance

Specific activity = radioactivity (Bq, Ci, d.p.m., etc.)amount of subtance (mol, g, etc.)

Examples

If 0.4ml of a 32P-labelled DNA solution at a concentration of 50 µmol L-1 gave count of 2490 d.p.m. What is the specific activity of the labelled DNA solution in becqueral unit?

Amount = 0.4 x 50 /1000 = 0.02 µmol

Specific activity = 2490 d.p.m/0.02 µmol = 124500 dpm µmol-1

= 2075 Bq µmol-1

Methods of measuring activity

• 4 major methods of measuring radiactivity for biological purposes:-1. Geiger-Muller (G-M) tube2. Scintillation counter3. Gamma-ray (γ-ray) spectrometry4. Autoradiography

Geiger-Muller (G-M) tube

• Operates by detecting radiation when it ionises gas between a pair of electrodes across which a voltage has been applied

• Only used for detection of γ radiation

• Routine checking for contamination of radioisotopes

Scintillation counter• Operates by detecting the scintillation

(fluorescences ‘flashes’) produced when radiation interacts with certain chemicals called fluors

• Measure and quantify β radiation

Gamma-ray (γ-ray) spectrometry

• Methods which a mixture of γ-ray emitting radionuclides rasolved by pulse-height analysis

• Pulse height (voltages) produced by a photomutiplier tube are proportional to the amount of γ-ray energy arriving at the scintillant or a lithium-drifted germanium detector

Autoradiography

• Method where photographic film is exposed to the isotope

• Used to locate radioactive tracers in thin sections of an organism or on chromatography papers and gels

• Write a blog on the title “Working practices when using radioactive isotopes”

Group work