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I know there are different types of direct reactions like inelastic scattering and such. But is the main difference between direct and compound reactions that one excites (Or at least can) the whole nucleus (Compound), while the other does not ? And if so, what is the extra energy used for in direct reactions, if not exciting the nucleus, compared to compound reactions ?

It ha been baffling me for a while now, and I'm not entirely sure I understand.

Thanks in advance.

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I would argue that this is like saying that the main difference between roses and rosemaries is the smell, in the sense that they differ in many things while both being flowers, and you are arbitrarily selecting a difference to be the outstanding one.

The differences between direct reaction (DR) and compound nucleus reaction (CNR) are: the time duration: CNR takes longer, momentum distribution: CNR is more isotropic, energy distribution: CNR is not restricted to kinematics constrictions, final particles, CNR is richer phenomena, at least compared to low energies DR.

All this measurable differences supported the argument that a different reaction mechanism occurs, which lead Bohr to propose his CNR model. And indeed this idea that a compound nucleus is formed which is excited with all incoming energy can explain very well the features observed.

But we have to remember that the "compound nucleus" is an abstraction and therefore the process occurring physically can be somewhat different. This means that the mechanism is not fully determined by the model that explains the results, since its predictions are limited and not based in fundamental assumptions, because this one is a "working model" rather than a fundamental one. Indeed this model obviates the quantum behavior explicitly.

Summarizing: it is important in this case to be clear what the phenomenological differences are, and how this simple model can account for them. But the details of the phenomenon are not exhausted by the model and cannot by summed up in finding a "main difference". This could only be done if the detailed evolution of the reaction could be known and we could establish a set of incoming energy, angular momentum and such conditions that could completely determine the reaction.

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compound reaction:=the typical reaction time of compound reaction which is 10^-14 to 10^-15 second direct reaction:=the typical reaction time of direct reaction which is of the order of 10^-21 to 10^-22 second

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