Section 9: Natural Transmutations: Alpha, Beta, Gamma

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Section 9: Natural Transmutations: Alpha, Beta, Gamma Nuclear Stability In Physics 2204, you learned that there are only four basic forces in nature (that we know about!

force) and the strong nuclear force. The 4 part picture below may help you understand the roles of these two forces as nuclei either stay in one piece (stable) or split apart (unstable). 1.

1 Section 9: Natural Transmutations: Alpha, Beta, Gamma Nuclear Stability In Physics 2204, you learned that there are only four basic forces in nature (that we know about!): the gravitational force the electromagnetic force the strong nuclear force the weak nuclear force In this section we will deal with the electric force (which is included in the electromagnetic force) and the strong nuclear force. The 4 part picture below may help you understand the roles of these two forces as nuclei either stay in one piece (stable) or split apart (unstable). 1. Here you see two neutrons attracted to each other. Their arms represent the strong nuclear force. Obviously, the strong nuclear force is not caused by charge because neutrons have no charge. 2. The same strong nuclear force exists between protons and neutrons. 3. Here there is a conflict: The protons are bound together by the strong nuclear force. However, all protons have the same positive charge, and you know that "like charges repel". These two protons are being repelled by the electric force. If a nucleus were made up entirely of protons, it would be in great danger of disintegrating because the repelling electric force would overcome the strong nuclear force. 4. Mr. Neutron has placed himself between the two protons. All three particles are bound together by "the glue" of the strong nuclear force, and the protons are separated a little bit so the repelling electric force has a smaller effect. 1

2 It is the presence of neutrons that provides stability for the nucleus. In fact, for protons numbering up to 20, one neutron for each proton does a fine job of keeping the nucleus stable. For example, look at the element 12 Mg in the periodic table. Remember that the 12 means there are 12 protons in the nucleus. Note that the unified atomic mass for magnesium is , which suggests the number of neutrons must also be 12 or very close to 12. Also, look at the element 20 Ca. The unified atomic mass of calcium is , again telling us that the number of neutrons must be equal to or very nearly equal to the number of protons. However, as the atomic number Z increases, the electric repelling force among the many protons would cause the nucleus to disintegrate to some degree if it were not for many more neutrons than protons. For example, look at 47 Ag, which is silver. The unified atomic mass of silver is This means that the nucleus of a silver atom has about 60 neutrons (as compared to only 47 protons). Neutrons contribute to the strong nuclear force of attraction between the particles; the presence of a large number of neutrons contributes to the stability of the nucleus. However, when a nucleus becomes large (Z > 82), the short range strong nuclear force which acts only between neighboring particles cannot counterbalance the repelling electric force that each proton exerts on all other protons. (The nuclear force only acts over a very small distance m, whereas the electric force is unlimited in range.) Therefore, in large nuclei (where Z>82) the electric force gains the upper hand. 2

SUMMARY The stable nuclei are those in which the far reaching electric repelling force does not over power the strong but short range strong nuclear force.

3 SUMMARY The stable nuclei are those in which the far reaching electric repelling force does not over power the strong but short range strong nuclear force. Since neutrons contribute to the strong nuclear force of attraction between the particles, the presence of a large number of neutrons contributes to the stability of the nucleus. However, when a nucleus becomes large (Z>82), the short range strong nuclear force that acts only between neighboring particles cannot counterbalance the repelling electric force that each proton exerts on all other protons. The picture below may be of some help. You should not get the idea that an unstable nucleus suddenly goes spring, with pieces going in all directions. Usually a limited number of particles or a limited amount of energy is emitted from the nucleus. This can happen in steps. That is, particles or energy might be emitted from a parent nucleus resulting in a new element called a daughter nucleus which itself might be unstable and subsequently emit more particles or energy. When such disintegration occurs spontaneously, the process is called radioactivity. The changing of one element into another by the process of radioactivity is called transmutation. 3

4 BUT WHAT IS RADIOACTIVITY? It is the spontaneous breakdown of an unstable nucleus. It results in the emission of particles or electromagnetic radiation. It is found naturally and in artificially produced sources. Radioactivity cannot be detected by human senses. All naturally occurring elements with atomic numbers greater than 83 are radioactive, as well as some isotopes of lighter elements. 4

5 Natural Transmutations: Alpha, Beta, Gamma Certain elements in nature emit three different types of radiation: alpha (α), beta (β) and gamma (γ). By passing the radiation through a magnetic field certain characteristics could be determined Characteristics 1. ALPHA(") Particles The alpha particle was much more massive than the others Alpha particles consist of 2 protons and 2 neutrons. This accounts for its mass and its charge. The atomic notation for an alpha particle is has the same form as However, that the alpha particle has no electrons. Consequently, alpha particles must be helium nuclei. 2. Beta ($) Particles Beta particles have mass and charge exactly equal to that of an electron. In other words, beta particles are electrons. They are usually designated with a negative sign like this: β. The negative sign is important because as time went on another particle was discovered. It has the same mass as an electron, but the charge of a proton! That is, it has a positive charge. Such a particle is called a positron, and is designated by the symbol β Gamma Radiation Gamma (γ) radiation, which is short wave (or high frequency) electromagnetic radiation. 5

6 Alpha particles positively charge particles ejected at a high speed but only have a range of a few centimetres in air Stopped by an ordinary sheet of thin aluminium foil or Paper Beta particles Stream of high energy electrons Ejected with various speeds, sometimes approaching the speed of light Able to penetrate several millimetres of aluminium Gamma particles Electromagnetic radiations with a very short wavelength Wavelengths and energies can vary Can penetrate at least 30 cm of lead or 2 km of air 6

7 α, (β) and γ emissions are directed into a magnetic field. Use LHR and RHR to determine the deflection of the Alpha and Beta particles. 7

8 Ionizing Ability Recall that an ion is an atom which has become charged because an electron was added or removed from the atom. Ionizing ability can be thought of as the ability to strip electrons from atoms. Being the nucleus of a helium atom, an α particle is much more likely to knock an electron out of orbit when it hits a target atom. β particles are much less likely to ionize target atoms because the β particles are thousands of times lighter than α particles. Relatively speaking, therefore, alpha particles have the greater ionizing ability. Since gamma radiation has no mass and no charge, it has the lowest ionizing ability of the three emissions. 8

9 How can these particles be seen? Although we can t actually see α, β, and γ radiation we can see the effects of the radiation. For example, suppose you look in the sky after a jet flown overhead and off into the horizon. Although you cannot actually see the jet anymore, you can see the condensation trail left behind from the jet. Radioactive emissions also make trails if allowed to pass through a gas or liquid. If a gas is used as a target, the set up called a cloud chamber. If a liquid is used, the set up is called a bubble chamber. When the cloud chamber or bubble chamber is placed in a magnetic field, the particles can be identified by the size and shape of their tracks. The reason for this is because as the emissions rip through, they ionize a path of atoms in the gas or liquid. The gas molecules (or bubbles if a liquid is used) are attracted to the ionized path in a big way, and tracks can be seen. Each type of particle leaves a different path: For alpha particles the tracks are short and fat, for beta particles the tracks are skinnier, and for gamma rays the tracks are long and thin and difficult to detect. In your textbook: In your text, on p. 772: d #11. 9

10 1. Which emission has the greatest ionization ability? a) α b) β+ c) β d) γ 2. Between/among which of the following atomic particles does the strong nuclear force act? a) neutrons only b) protons only, c) all atomic particles d) between neutron/neutron, proton/proton, and proton/neutron 3. Which of the following forces most contributes to the instability of a nucleus? a) the strong nuclear force b) the weak nuclear force c) Coulomb's electric force d) the gravitational force 4. Which emission is a stream of electrons? a) α b) β+ c) β d) γ 5. From which of the following will an alpha particle be repelled? a) gamma ray b) neutron c) positive charge d) negative charge 10

SCIENCE 10: (7.1) ATOMIC THEORY, ISOTOPES AND RADIOACTIVE DECAY Name: Date: Block: (Textbook Reference pp in BC Science 10) into an

SCIENCE 10: (7.1) ATOMIC THEORY, ISOTOPES AND RADIOACTIVE DECAY Name: Date: Block: (Textbook Reference pp. 286-301 in BC Science 10) Natural background radiation: It has the ability to interact with an