Radiation Activity Instructions
Of the types of radiation produced in nuclear decay reactions, the three most common are alpha particles (?), beta particles (?), and gamma rays (?). All of them are considered to be forms of ionizing radiation, that is, when they strike atoms or molecules in their paths they cause electrons to be knocked away, forming ions or free radicals. While this is generally not much of a concern with inanimate materials, these highly reactive species can cause undesirable effects in living tissue.
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Alpha particles are fast moving particles that contain two protons and two neutrons, which make them identical to helium nuclei, with a mass number of 4 and a charge of 2+. They are written using either the Greek letter ? or the symbol for the helium nucleus, He-4.
Beta particles are high-energy electrons, with a mass number of 0 and a charge of 1–. They are identical to other electrons in the atom, but do not exist in the nucleus until a neutron decays into a proton and an electron. They are written with either the Greek letter ?.
Gamma rays are high-energy photons (similar to X rays but higher energy). They are given the Greek letter ?.
Because cells in our bodies can be damaged by ionizing radiation, appropriate shielding must be used when working with radioactive sources. There are three ways to minimize your exposure to ionizing radiation when working with radioactive materials:
- Use proper shielding. Since alpha particles are the heaviest of the forms of radiation and carry the greatest charge, they interact the most strongly with matter and are absorbed within a very short distance. A piece of paper, a lab coat, even the outer, dead layer of our skin are sufficient protection against alpha particles. (If inhaled or ingested, however, alpha particles can cause serious damage to internal organs.)
Beta particles, having a smaller mass and charge, are able to penetrate matter to a greater depth, as far as 4-5 mm into body tissue. While beta particles are absorbed before they can reach internal organs, they can be damaging to the skin. Heavy clothing and gloves, or metals such as aluminum, provide adequate protection from beta particles.
Gamma rays are the most penetrating of all, passing right through body tissue and many materials. They require much more dense materials such as lead or concrete to stop them.
Table 1 lists the approximate penetration depths for the types of radiation in various materials.
Table 1. Approximate Penetration Depths of Radiation in Various Materials
(2) Minimize the time you spend exposed to radioactive materials. If you are in an area with radioactive sources for twice as long, you will get twice as much exposure.
(3) Keep your distance. This is the relationship we will examine in part 3 of this laboratory exercise.
We often see reports of archaeological studies that claim to have found a scroll from the time of the Romans, a mummy from 1000 B.C., or a piece of bone from a human that is 10,000 years old. How do we determine the ages of these objects? One method is to use a form of radiological dating that relies on the decay of carbon-14, a beta emitter.
Carbon-14 is created in the upper atmosphere, which is constantly being bombarded by cosmic rays of very high energy. These rays consist of electrons, neutrons, and atomic nuclei. One of the important reactions is the capture of a neutron by 14N to create 14C and a proton:
The 14C reacts with oxygen in the atmosphere to form carbon dioxide, which eventually makes its way into the biosphere where it is taken up by plants during photosynthesis. Animals eat the plants—and we eat both animals and plants—ultimately exhaling the 14C again as CO2. In this way a steady concentration of 14C is maintained in living tissue. (Although 14C emits ionizing radiation, and can therefore damage cells, the natural abundance of 14C is very small: there exists only one atom of 14C in every 1012 atoms of carbon.)
Once an organism dies the 14C is no longer replenished, and the amount of 14C steadily decreases as it undergoes ? decay. Thus when a sample of that scroll, the mummy, or the bone is measured for its 14C content, there is less of it than in a living organism. If we assume that the person who was mummified had the same amount of 14C in her tissues when she was alive as we do today, and we know the rate at which 14C decays, we can determine her age.
The half-life of a radioisotope is the time it takes for one-half of a sample to decay. Carbon-14 has a half-life of 5730 years, which means that after that period of time, only half of the 14C that was originally present in an organism would remain. After another 5730 years, only half of that quantity would be present, or one-quarter would remain, and so on.
Similarly, the radioactivity of a sample (as measured by a Geiger counter) will decrease by the same fraction. Note that if a sample is either too young or too old, this method will not work. If a piece of bone is only a couple years old, there will have been too little decay of 14C for us to be able to measure a decrease. Likewise, if it is more than about 40,000 years old, there will be too little 14C left to measure accurately.
In this experiment, you will
- Identify the type of radiation produced by various nuclides by examining how much that radiation penetrates shielding materials
- Examine the effectiveness of shielding against gamma rays by three materials.
Estimate the age of various samples using radiocarbon dating.
Part 1. Types of Radiation
- On your computer, go to:http://www.uccs.edu/vgcl/nuclear-chemistry/experiment-2-types-of-radiation.html
- Click on the apparatus to start the lab. The experimental setup includes a number of radioactive sources, different types of shielding, and the apparatus to measure the radioactivity. On the right is a Geiger counter that will measure radiation in counts per second. On the left is a holder for the radioactive source, and in the middle is a holder for the shielding material.
- Click on the drop-down list of radioactive sources and choose a nuclide. Drag the source into the sample holder. Record the name of the source and uses/applications of this nuclide.
- Click on the Geiger counter switch to turn it on. Record the activity of the sample in counts/sec. (The needle on the gauge may move around a bit. You should try to get an average reading. Note that the scale on the gauge is not linear.)
- Click the switch again to turn it off.
- Click on the drop-down list of shielding materials and choose one. Drag the shielding into its holder in the apparatus. Again click on the Geiger counter switch, and again record the activity in the appropriate column in the worksheet.
- Repeat step 7 for the other types of shielding material.
- Repeat steps 3-7 for all of the radioactive sources.
- Use table 1 to identify the type of radiation (alpha, beta, or gamma) given off by each nuclide.
Part 2. Shielding and Gamma Rays
- On your computer, go to:http://www.uccs.edu/vgcl/nuclear-chemistry/experiment-3-shielding.html
- Click on the apparatus to start the lab. The experimental setup includes a number of radioactive sources, several 1-mm thick sheets of three different types of shielding, and the apparatus to measure the radioactivity.
- Click on the drop-down list of radioactive sources and choose strontium-85, a gamma emitter. Drag the source into the sample holder. Click on the Geiger counter switch to turn it on. Record the activity of the sample in counts/sec. Click the switch again to turn it off.
- Click on the drop-down list of shielding materials and choose a single sheet (1 mm) of one of them. Drag the shielding into its holder in the apparatus. Again click on the Geiger counter switch, and again record the activity in the appropriate column in the worksheet.
- Repeat step 4 for the other types of shielding materials and thicknesses.
Part 3. Radiation Intensity vs. Distance
- On your computer, go to:http://www.uccs.edu/vgcl/nuclear-chemistry/experiment-4-radiation-and-distance.html
- Click on the apparatus to start the lab. The experimental setup includes the radioactive source (gallium-67, a gamma emitter), a ruler to measure the distance from the source to the detector, and the apparatus to measure the radioactivity. Click on the drop-down list to retrieve the source and drag the source into the sample holder.
- Click on the 1-cm mark on the ruler to position the source holder at 1 cm distance from the detector. Click on the Geiger counter switch to turn it on. Record the activity of the sample in counts/sec. Click the switch again to turn it off.
- Click at a point on the ruler to move the source and record the activity for all distances in the data table.
Part 4. Radiocarbon Dating
- On your computer, go to:http://www.uccs.edu/vgcl/nuclear-chemistry/experiment-5-radiocarbon-dating.html
- Click on the apparatus to start the lab. The experimental setup includes a number of samples of 14C isolated from objects discovered in archaeological excavations, and the apparatus to measure the radioactivity.
- Click on the drop-down list of samples and choose one. Drag the source into the sample holder. Click on the Geiger counter switch to turn it on. Record the activity of the sample in counts/sec. Click the switch again to turn it off.
- Repeat step 3 for the other samples.
- To determine the age of the sample:
- Open the link on the screen (http://www.uccs.edu/vgcl/nuclear-chemistry/experiment-5-radiocarbon-dating.html) titled “Worksheet” located directly above the drop-down list of samples.
- ALL YOU NEED TO DO TO THIS WORKSHEET IS TYPE VALUES INTO THE “Initial Activity:” box above the graph titled Decay Curve for Carbon-14 and interpret the graph. Ignore the rest of the worksheet.
- For one of the present-day materials, type its activity in the “Initial Activity” cell and hit Enter. This will generate a decay curve for 14C based on that starting activity.
- For each of the archaeological objects made of that material, estimate its age using the activity and the 14C decay curve. Enter the age into your data table in evernote.
- Repeat steps c and d for the other present-day materials.