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## Group Activity: Positron Decay and Electron Capture

#### Positron Decay

A positron is an elementary particle that has the same mass as an electron but instead of a minus one charge it has a plus one charge. If a positron ever meets an electron face to face they both dissappear and their combined mass is changed completely into energy in the form of high-energy light called gamma rays. In beta decay involving the electron’s “evil twin” the positron a proton in the nucleus breaks down to become a neutron, emitting the positron in the process. There is a medical scanning technology called PET: Positron Emission Tomography. It creates images of the inside of the body and is especially useful for studying the patterns of activity in the brain. Here is an example of an beta decay process involving a positron:

2312Mg  —> 2311Na  + 0+1β

The atom on the left side of the equation is the one that decays. Positron decay is very similar to ordinary beta decay but can be thought of as its mirror image. In this process the mass number stays the same and the atomic number decreases by one. Here is another example of positron beta decay:

5025Mn  —> 5024Cr  + 0+1β

For the following nuclides, write the equation for positron beta decay.

1. 68 35Br
2. 2515P
3. 2214Si
4. 3822Ti
5. 7  5B
6. 11  7N
1. 3419K
2. 4727Co
3. 5430Zn
4. 5931Ga
5. 12  8O
6. 15  9F

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#### Electron Capture

In electron capture an electron that is orbiting the nucleus is absorbed by a proton in the nucleus. The proton becomes a neutron. The resulting nucleus has an unchanged atomic mass number but a decrease in the atomic number. In this electron capture is very similar to beta positron decay. Here is an example of an electron capture process:

8136Kr  + 0-1e  —> 8135Br

The nuclide that decays is the one on the left-hand side of the equation. Notice that the electron is added on the left side: this indicates that the electron was added to the nucleus. No radiation is emitted unless the resulting nucleus is in an excited state. (In which case a gamma ray may be emitted). Here is another example:

23393Np  + 0-1e  —> 23392U

For the following nuclides, write the equation for electron capture.

1. 3718Ar
2. 8038Sr
3. 5727Co
1. 7333As
2. 24799Es
3. 20084Po
Identify the type of decay and complete the nuclear equation.
1. 8235Br  —> 0-1β + _____
2. 15167Ho  —> 14765Tb + _____
3. 23594Pu   +  _____ —> 23593Np
4. 20380Hg  —> 20381Tl + _____
5. 19278Pt  —> 4  2He2+ + _____
1. _____   —> 4  2He2+  + 14058Ce
2. 9943Tc  —> 0-1β + _____
3. 20  9F  —> 2010Ne + _____
4. 14460Nd  —> 4  2He2+ + _____
5. 9744Ru   +_____ —> 9743Tc

Radiation is feared by many people because of the damage it can do to the human body. For example, there has been much discussion about the irradiation of food to preserve it. By exposing food, such as strawberries, to gamma rays we can kill off any mold or bacteria which would cause the strawberries to spoil. The concern is that it won’t be just the mold and bacteria that are damaged by the gamma rays but also the cells of the strawberries. Radiation of all kinds can cause damage to the tissue of organisms by breaking chemical bonds and causing cellular machinery to malfunction. So strawberries that have been irradiated stay fresh longer but may contain altered biological molecules which may (or may not) be dangerous. The strawberries do not become radioactive as a result of such treatment.

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But what happens to people who are exposed to radiation? It depends on the type of radiation, the energy of the radiation, the ionizing ability of the radiation and the chemical properties of the radioisotope.

1. Type of Radiation: Alpha radiation (α or2He) is stopped by the skin and can be blocked entirely by clothing, paper, or glass. Beta radiation (β or0-1e) is more penetrating and can enter the body to a distance of about 1 cm. It can be blocked by sheets of aluminum metal. Gamma rays are the most penetrating radiation of all and require heavy shielding to be exluded.
2. Energy of Radiation: The more energy in a given dose of radiation, the more damage it can cause. If a lot of radioactive material is present or if the material has a short half-life then more decay events occur per unit time and more energy is deposited in tissue upon exposure. Dosages of radiation exposure are measured in rads (radiation absorbed dose). One rad equals 0.01 J per kg of body tissue.
3. Ionizing Ability: Gamma radiation is very penetrating but causes very little ionization. Alpha and beta radiation can cause a lot of damage by causing biological molecules to lose electrons. This makes these molecules unable to function and can lead to futher damage as the ionized molecules meet other molecules and cause them in turn to be ionized. These types of radiation are not very penetrating but if appropriate radioisotopes (plutonium, polonium-210) are ingested they can wreak havoc.
4. Chemical Properties: If a radioisotope is ingested the amount of damage it does depends on how long it stays in the body. For example8536Kr (krypton-85) is a beta-emitter but since it is a noble gas and is chemically unreactive it passes quickly out of the body. On the other hand,9038Sr (strontium-90) is also a beta-emitter but it stays in the body for long periods because it is chemically similar to calcium and becomes incorporated into the bones. This can lead to leukemia and bone cancer.

There are two types of damage associated with radiation exposure: somatic damage and genetic damage. Genetic damage is when the radiation causes changes in the DNA of reproductive cells. This can produce harmful mutations that cause offspring to die or suffer from malfunctioning organs. Somatic damage is damage to the tissue of an organism. It can cause immediate sickness, death or (if the doses are small) cancer. If a massive dose is received the effects might appear immediately but chronic exposure to small doses can lead to long-term sickness or the appearance of cancer many years later.

One famous case of radiation poisoning happened very recently under circumstances that were very suspicious. Alexander Litvinenko—a vociferous critic of the Russian government who lived in London— died in November of 2006 of what was later identified as the ingestion of polonium-210 (21084Po). This isotope is an alpha-emitter and decays to 20682Pb. Mr. Litvinenko died within 3 weeks of his ingestion of an estimated 10 μg (1.0 × 10-5 g).

1. How does radiation cause damage to living tissue?
2. What factors determine how much damage radiation can cause?
3. What are the different kinds of damage that radiation can cause? What makes them different?
4. Which would be worse: exposure of the skin to the alpha-emitter23892U or the ingestion of the beta-emitter9038Sr? Why?
Last updated: Dec 12, 2013       Home