The Most Tightly Bound Nuclei

The most tightly bound of the nuclei is ⁶²Ni, a case made convincingly by M. P. Fewell in an article in the American Journal of Physics. Though the championship of nuclear binding energy is often attributed to ⁵⁶Fe, it actually comes in a close third. The four most tightly bound nuclides are listed in the table below with a tabulation of the binding energy B divided by the mass number A. The curve adapted from Fewell shows those nuclides that are close to the peak.

Nuclide B/A (keV/A) ^62Ni 8794.60 +/- 0.03 ^58Fe 8792.23 +/- 0.03 ^56Fe 8790.36 +/- 0.03 ^60Ni 8780.79 +/- 0.03

 

The most tightly bound nuclides are all even-even nuclei. The curve drawn through the cluster of nuclei above is just to show the nature of the trend with mass number. A similar kind of trend is observed with even-odd nuclides at a lower range of binding energy, and then by odd-odd nuclei at the least-bound extreme. The peaks of all three groups occur around A = 60.

The high binding energy of this group of elements around A=60, typically called "the iron group" by astrophysicists, is significant in the understanding of the synthesis of heavy elements in the stars. It is curious that the abundance of ⁵⁶Fe is an order of magnitude higher than that of ⁶²Ni. Fewell discusses this point, and indicates that the reason lies with the greater.

 

LISTENING

You are going to listen to the staff report “Mexican Plant to Host July Global Response Exercise“. Mind the proper names.

Laguna Verde nuclear power plant in Alta Lucero

"Joint Radiation Emergency Management Plan"

the IAEA´s Incident and Emergency Centre (IEC)

the Inter-Agency Committee for Response to Nuclear Accidents (IACRNA)

Cernavoda nuclear power plant in Romania

the World Health Organization (WHO)the World Meteorological Organization (WMO)

Mexico and the Mexican neighboring countries

2. Speed listening. Note only the essential details of what you hear:

9. The Olympics……………………….

10. China and the IAEA……………………..

11. A training workshop………………………..

12. Anita Nilsson said………………………………

13. IAEA´s work in Beijing………………………………………

14. Chinese authorities…………………………………………

15. Nuclear security measures……………………………………

16. Help to Member states…………………………………………

 

3. General information: Complete the chart with the basic ideas:

 

What?	Where?	When?	Who?	How?	Why?

4. Gap filling: Listen once again and complete the gaps in the summary of the passage below with the correct word or phrase you hear:

 

On the threshold of the Summer Olympic Games Chine and the IAEA are working together at strengthening _____________ and minimizing threats. For that purpose _______________ was held in Beijing. The IAEA’s role was to assist in integrating ___________into the existing Chinese security system. The spheres which are covered by the conducted advisory missions and training exercises are _____________.

The IAEA has already rendered technical assistance to different international public events among which are ____________________.

Member States of the IAEA get comprehensive help from the Office of Nuclear Security in form of ______________. This is vitally important for protecting __________________and fighting against__________.

 

5. Decide whether these statements are true, false or the information is not given:

1. In recent years safety record of nuclear power has caused concern and necessity to improve.

2. 60 IAEA Member States and 10 international organizations will take part at the Conference in Mexico.

3. The 2-day exercise is to assess efficiency of communications between possible crisis partners.

4. During the emergency drill different emergency response bodies will show their preparation to deal with radiological accidents.

5. The similar event in Romania showed necessity in arranging such activities.

6. Emergency Conventions set rules how information exchange and assistance should be realized in the conditions of radiological emergency.

7. When the event at Laguna Verde finishes, the IAEA will publish a statement that there was nothing dangerous for the public.

 

Work in pairs or groups. Discuss the topic mentioned in the staff report “Mexican Plant to Host July Global Response Exercise“.

PRESENTATION

Make up a presentation “BINDING ENERGY”

(See appendix 4)

SECTION 5

MODES OF RADIOACTIVE DECAY

LEAD-IN

Describe the following processes.

1. Alpha decay

2. Beta-minus decay

3. Beta-plus decay

4. Electron capture

5. Internal conversions

6. Isomeric transitions

READING

TEXT 1

Before reading the following text, work in small groups (3-4 students) and discuss the questions below basing on your possible knowledge of the topic. Then read the text and check your guesses.

1. Do you know how many and what radionuclides occur in nature?

2. What factors do affect the half-life (temperature, pressure, or gravitational, magnetic, or electrical fields, etc.)?

3. Does radionuclide disappear after decaying?

4. Particles of what mode of decay have higher velocity?

 

Radioactive Decay

The nucleus of each atom has a specific number of protons and neutrons and is either stable or unstable, de­pending on the relative number of each. The most stable atoms are those that have an equal number of protons and neutrons. Atoms that are unstable are radioactive. An atom that is radioactive can also be called a radionuclide. Of the known nuclides (approximately 2,000), only 264 are stable, and of the known radionuclides (approximate­ly 1,700), only 70 occur in nature. The rest are man-made. Unstable atoms undergo a process called radioac­tive decay to reach a more stable state.

While a radionuclide is going through the process of decay, energy is released from the atom in one of three modes: alpha, beta, or gamma radiation. These modes may take several steps, involving only the nucleus or the entire atom. Each radionuclide has one or more charac­teristic modes of decay. The particular mode of decay determines the type of energy, or radiation, released from the atom, and consists of either subatomic parti­cles, protons, or both.

Radionuclides are unstable to varying degrees. The more unstable a radionuclide is, the faster it decays. The quantity of a radioactive substance is expressed as disin­tegrations per second, in units of Curies (Ci) named for Marie Curie, or if Systeme International is used, Becquerels (Bq) named for Henri Becquerel. The rate at which a radionuclide decays depends upon its half-life, the expected time required for half of the nuclei to decay to a stable state. The half-life is typically not affected by temperature, pressure, or gravitational, magnetic, or electrical fields.

When radioactivity was first discovered, it was thought that all the energy given off by the radionuclide was basically the same, with differences only in pene­trating power. However, research conducted by Becquer­el and Pierre Curie proved that there were three distinct modes of radioactive decay, which differed not only in their ability to penetrate, but also in their velocity, as well as their susceptibility to magnetic fields.

Alpha and beta radioemissions are actually particulate matter that is thrown out from the nucleus. An alpha particle is two protons and two neutrons, or in other words, it is a helium atom without the electrons. After an alpha particle is emitted, the atomic mass de­creases by four, and the number of protons and neutrons decrease by two. Alpha decay occurs in radionuclides with an atomic number greater than 83 and a mass number greater than 209. Alpha particles interact with negatively charged electrons in the environment, which consequently use up the energy in the particle, slowing it down and greatly diminishing its penetrating power. Even a sheet of paper can stop an alpha particle. The di­rection of an alpha particle is only slightly affected by a magnetic field because the particle has a balanced charge. When a radionuclide decays by alpha radiation, it does not just disappear. Instead, the radionuclide trans­mutes into another radionuclide or nuclide. For example, uranium-238 transmutes into several other radionuclides, including radium-226 and radon-222, before ending up as lead-206, a stable nuclide.

Beta radiation, which also involves particulate emis­sions, can be either be negatively charged or positively charged. Beta particles are actually created in the nucleus by either a proton changing into a neutron (positron emission) or a neutron changing into a proton (negatron emission). A beta particle has a higher velocity than an alpha particle, and its path is markedly deflected by a magnetic field. When a negatron is emitted from an atom, the atomic mass of the atom is unchanged, the number of protons increases by one, and the number of neutrons de­creases by one. The mass remains unchanged when a positron is emitted, the number of neutrons increases by one, and the number of protons decreases by one.

An atom usually becomes excited from either of the above-mentioned decay processes and sheds excess en­ergy in the form of a gamma ray photon. With gamma emissions, the atomic mass, number of protons (atomic number), or the number of neutrons, remains unchanged. The velocity of a gamma ray is almost that of light and is not affected by magnetic fields.