I can’t always be gentle with the reader and clearly define and explain in simple English why we are interested in what nuclear chemists find interesting. That being said, I’ll jump into this next topic.

I’ve come across the term, **Interaction Barrier**, several times now and I’ve finally figured out what it means. At first I thought the interaction barrier was a nicer way to say the coulomb barrier. I thought it was a term that the learned nuclear scientist used to describe the point where the attractive nuclear force and the coulomb potential would equal each other. Unfortunately I was wrong, I was misled to this conclusion by several recent (recent being defined as anytime from today to 5 years ago) pieces of literature that I have read, but I’ll be nice and not point them out here.

The formal definition of the interaction barrier is (as defined by Mitch rewording Reiner Bass):

The interaction barrier is the threshold bombarding energy in the center-of-mass system which is needed classically for two fragments to undergo a nuclear reaction. Noting that a nuclear reaction is defined liberally and involves any inelastic process.

The informal definition that you can use to make yourself sound smart (which others do retardedly often ):

The interaction barrier, unlike the coulomb barrier which assumes 2 charged-spheres in a static and rigid touching configuration, describes the minimum distance R_{int} two fragments need to be to undergo a dynamic inelastic non-negligible mass-energy exchange. Where R_{int} approximately equals R_{1} + R_{2} + 3fm, where R_{1} and R_{2} are the classical nuclear radii one would calculate for a projectile and target nucleus.

A figure plotting the coulomb potential (solid-line) and the actual nuclear+coulumb potential (dash-line) is shown below. The figure is for ^{20}Ar + ^{208}Pb.

As can be seen in the figure, the interation barrier (**B**_{int}) shows the boundary where the interesting physics begins. While the calculated coulomb barrier shows some useless point far above and behind the actual fusion barrier. No wonder “sub-coulomb barrier” reactions seem to occur so readily in our field! They may be sub-coulomb, but the only important barrier is the fusion barrier! This might stand out to others, but took me sometime to realize the importance.

If you are interested in more physics of nuclear reactions with heavy ions. See this book. . I’ve been secretly borrowing it from Walt for 2-weeks now, shhhh….. If you want the book too, please purchase through this link: Nuclear Reactions With Heavy Ions, it’ll give me a few quarters.

Mitch Rating for Interaction Barrier:

Complicated Soundingness: 3/10

Usefulness: 6.5/10

Next Week’s Term: Sommerfeld Parameter

Mitch

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