Questions and Answers on Nuclear Reactions

I try to keep questions submitted by groups as the are. Unclear or trivial questions are not replied. Reply has not been proof read.

Q
Favourable nuclear reaction
Which type of nuclear reactions is more naturally favourable?
A
What is vavourable is not defined. Reaction types cannot be determined by us. Nature takes its course.

Q
Units for cross section
7b1: Nuclear Reactions Cross Sections:

rate = cross sections * N * I where N: number of atoms per unit area m-2 and I: flux, particles s-1 m-2
When calculating the cross section, the unit depends on the unit used for I (flux). Which one (cm or m) is the more common unit being used?
A
The unit for I is usually number of particles cm-2.

Q

There are many radioactive rays traveling inside the earth atmosphere, and if nuclear reactions can be induced by these radioactive media, how come we are still able to find some "high-atomic number" elements? How come the earth does not consist of only one basic element (lowest-atomic number element)?
A
Only very high energy gamma rays induce nuclear reactions, but the cross sections are small.

I guess you wonder why do we not have very high-atomic-weight elements. They have to last for us to observe. The question about abundances of elements on Earth is related to the formation of these elements and the formation of the Earth as a planet.

Q
Radiation for cancer treatment
Medical applications is listed as one use of nuclide production. I often hear of radiation treatment for cancer. What type of radiation is used in this situation? What reactions are used to produce these radioactive particles?
A
There are many ways radiation is used in medicine. Some examples are given at the last Module. There are many way for radiation treatment of cancer. The whole idea is to kill cancer cells by gamma rays. Gamma source focuses on cancer cells can be made from external and from seeds implanted in the area where cancer is.

Q
Feynman diagrams
The interaction of electrons positrons and nuetrinos are part of the Feynman diagrams. Does this indicate that the interaction of Masons should product a definable mass that decays in a predicatable way?
A
Feynman diagrams are not used to predict what will happen, but they are used to indicate what has happening or has happened.

Q

Is it possible that there are still nuclides that have yet to be discovered or have all possible holes in the periodic table been filled?
A
There are probably some nuclides to be discovered, but all elements have been discovered. As indicated in this module, many elements have been made. Of course, some researchers are still trying to make elements with atomic number greater than 112. Making is different from discovery, and I think you may also mean make instead of discovery.

Q
Neutron sources
Under the topic of neutron sources we learn that neutrons are made from different neutron elements. Why are gamma ray sources separate from light needed to avoid irradiation by alpha and beta particles?
A
Two types of neutron sources are given in the lecture. One source is stimulated by alpha and the other by gamma rays. Energy of these neutrons are different, and thus, we prevent alpha induced neutron so that we get only those neutrons induced by gamma rays.

Q
Neutron induced nuclear reaction
As neutrons (which carry high kinetic energy) approach the nucleaus, they either bounce off or knock other nucleon out of the nucleus. How does this affect the nuclear reaction?
A
If a neutron replace another neutron in a nucleus, it is a scattering. If a neutron knocks out any nucleon from a nucleus, it has induced a nuclear reaction. The product can be deduced from the nucleons knocked out.

Q
Coulomb barrier for electron
When a positively charged particle approaches a nucleus, Coulomb barrier will be formed. What about electron? Will Coulomb barrier be formed when the electrons passing through the electron cloud before in contact with the nucleus?
A
Electrons carry negative charge, and theoretically, there is an attraction between the electron and the nucleus. Electron capture is a beta decay process, and electrons are seldom used to induce nuclear reactions.

Q
Number of isotopes of an element
Some elements have a number of isotopes, Astatine has over 20. What classifies an isotope as being an isotope? For example, must it exist for a specific period of time to be accepted? Is there a way of determining the "limit" to the number of isotopes (not only stable ones) that are possible for a given element?
A
There are many ways to look at the number of isotopes of an element. The number of stable isotopes of an element count only the stable one. They are usually present naturally. For each element, there are many that have been made. These are usually radioactive, because they no longer exist in nature. As a matter of fact, Astatine has no stable isotopes. Some exist in nature as part of the radioactive decay series, and others are all man-made.

Q

What theories are there for the "spikes" (i.e. energy states) in the nuclear cross section at certain neutron energies? For instance, with electrons in atomic orbits, the energy levels were attributed to the wavelength of the electron forming a standing wave in the orbit.
A
The spikes are due to high cross section at these energies. The theory is due to the structure of the nuclei. Certain energies compliment them well, and the reaction possibility is high.

Q
Elastic scattering
With the nuclear reaction type called "Elastic Scattering", one neutron reacts with a nucleus and one neutron exits the nucleus (not necessarily the same one) and the original element remains after the reaction. What are the practical applications of an "Elastic Scattering" reaction?
A
I cannot think of an application for elastic scattering, because there is essentially no net reaction.

Q
High-energy particles and nuclear reactions
Module 7 talks about nuclear reactions in terms of the more familiar particles of neutrons and electrons. In module 5, we studied the particles in depth and were introduced to the many particles (or energy states) such as pions and kaons, etc. What role do these more abstract particles play in nuclear reactions?
A
Many particles introduced in Module 5 are rare. Utilizing them for nuclear reaction is impractical. Besides, their half lives are very short, making them even more impractical. Thus, I have not included nuclear reactions of these particles. However, they have some special applications in terms of research.

Q

How are the "magic numbers" nucleons determined for stable nuclides? What is it that sets these nuclides apart from nuclides that do not contain a magic number of nucleons?
A
The magic number of protons or neutrons for any nuclide is not determined. They are simply there. These nuclides does not have to be stable nuclides. If a nuclide does not have a magic number of proton or magic number of neutron, it is not one of those that have.

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