Physics practical report: determining the range of a radiation and the energy content
Introduction
Most of the radiations emitted by sources usually emanate from radioactive isotopes. These isotopes may be naturally radioactive or the emission may be induced by an external source of energy. Natural isotopes are mainly elements whose atomic mass exceeds 208. For this reason, the atomic size is very large creating an aspect of imbalance in charge within the atom. The highly positively charged nucleus does not feel the same magnitude of negative charge effect from the electron cloud surrounding the nucleus. This makes the nucleus to disintegrate naturally to release some energy as it scales down the number of neutrons and protons to have a stable nucleus. The most common form of radiations are: alpha (α) radiation which has an equivalent structure to a helium ion, beta (β) radiation which has charge equivalence to that of electrons and gamma (ϒ) radiations which have no charge 1.
The above form of radioactive decay is known spontaneous or natural decay of the isotope. The second form of emission is known as induced or artificial radioactive radiation. This involves supply of energy to the isotope making its nucleus to be unstable thus disintegrating to release the radiations. The most common element used for artificial radiation is cobalt 60 element which when illuminated with UV rays of favorable wavelength release gamma radiations.
Of the three radiations, the penetrative power varies with the size of the particle emitted. This means that the large the size of the particle released, the lower the penetrative power. As an interpretation, alpha radiations have the lowest penetrative power followed by beta and lastly the gamma radiations 1.
- Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons
These radiations usually have harmful effects when exposed to the human tissues hence are shielded. Alpha particles can be deflected by a sheet of paper while beta radiations can penetrate the paper but are stopped by thin aluminum sheet. Gamma radiations have the highest penetrative power thus penetrates both the paper and the aluminum but is stopped by a thick block of lead.
Larger isotopes have a higher the probability of disintegrating releasing the different radiations. Alpha particles are released by heavier isotopes while the less heavy isotopes disintegrate to release beta particles 1.
In any statistical work, error analysis must be considered. This is also done with this experiment so as to ensure that error propagation is done and probably the best proximity to the exact value obtained. This will be done on both the counts and the distances 1.
Apparatus and procedure
Unlike beta and gamma radiations, alpha particles have extremely short ranges of radiation in air. Almost half of the particles’ energy is used to penetrate the GM tube which is only 30-40mm in length.
- Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons
R=NN0
Where N is the count rate when the distance between the source and detector is not zero and N0 is the count rate when the source and detector at attached to one another (distance between the source and detector is zero).
Apparatus
- Geiger Muller equipment and its control unit.
- Radioactive isotopes (sealed) which include: 241Am 90Sr and 137Cs
- Measuring equipment (meter rule).
- Timing equipment.
- Lead blocks and aluminum sheets.
- Tweezers.
- Retort stand or holding equipment.
Procedure
- Use tweezers to place a radioactive source in the holder and place it just below the end of GM tube. Take this as N0 but do not touch the GM with anything.
- Determine a standard time duration which you will be taking the counts recorded by the GM equipment maybe 100 milliseconds. Record the number of counts against the distance between the GM and the source in the holder in a data table. Use the duration that you chose in your experiment as the standard duration.
- Determine the counts for different distances N and using the above formula determine the respective ranges in air of the radiation.
- The sources placed in the holder are Am, Sr and Cs and for each source repeat the above three steps.
Diagram
Results
Results for NN0
Graph (vertical scale has been multiplied by ten)
Discussion
The second particle to look at is the beta particle. From the graph plotted it is evident that beta particle traverse a longer distance in air but is still definite and very small compared to the distance traversed by gamma radiations. The reason that makes these particles move a longer distance is their lightweight. They are equivalent to electrons thus also suffer effects suffered by electrons. Due to these properties, the particles are stopped by a thin layer of aluminum 2.
Lastly, the other aspect to consider is the energy change underwent during the disintegration of the nucleus. This process usually emits a lot of energy which some is absorbed by the particles (beta or/ and alpha) while the rest is released in form of a radiation. This radiation is known as gamma radiation. Since this radiation is in form of energy and does not have any particulate nature, it does not suffer any opposition while traversing through air. This can be claimed to be the reason behind its indefinite range in air. Due to this high penetrative power and the high energy content of the radiation, it has harmful effects. This makes it to be shielded from reaching the people handling equipment releasing the radiation using a thick block of lead 2.
Assignment questions
- After the release of a beta particle, the mass of the isotope is not affected but the atomic number increases by 1. In case the particle is captured or during an electron capture the atomic number is not affected but the atomic number decreases by one. In case of an alpha particle radiation, the atomic mass decreases by four while the atomic number of the isotope decreases by two.
- Taking a look at part three of this section, the estimated amount of energy released in form of gamma radiations is the different between the two energy levels of the alpha particles. This can be taken to be 0.18MeV which can be estimated to2.88*10-14 J. Looking at the second part, the estimated energy is equal to the energy of gamma radiation and the energy of the alpha particles. This can be estimated to be 4.78MeV.
Conclusion
Having covered the above procedures, several aspects can be drawn from the experiment. The most significant are: radioactive emission can be induced or spontaneous (natural) 3. , the isotopes can undergo a chain reaction before achieving a stable nucleus 3, alpha particles have an equivalent structure as that of a helium atom thus bulky making them to have very low penetrative power 3, beta particles are equivalent to electrons thus light making them to have higher penetrative power than alpha but are stopped by a sheet of aluminum (absorbed) 3 and gamma radiation is just energy only and is released during either beta or alpha decay. It has no mass and very high penetrative power accounting for lack of a definite range in air 3.
- Geoffrey K. Penetrative power, retrieved from http://www.lbl.gov/abc/experiments/Experiment4.html
- Three types of radiation - Alpha, Beta and Gamma retrieved from http://www.blackcatsystems.com/GM/experiments/ex7.html
References
Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons
Geoffrey K. Penetrative power, retrieved from http://www.lbl.gov/abc/experiments/Experiment4.html
Three types of radiation - Alpha, Beta and Gamma retrieved from http://www.blackcatsystems.com/GM/experiments/ex7.html
