When I read some textbooks I find that the definition of cross section is the probability. Of incident neutron interact with a matter .but I find many other (cross section of fission, scattering, absorption..) And I find another definition of probability of interaction in distance $l$ which is given by


Where $\lambda=\frac{1}{\Sigma} $ And $\Sigma$ is the macroscopic cross section $\Sigma=N \sigma$, $N$ is the density of nuclear and $\sigma$ is the croos section

My question is what is the difference between the two definition?and if we want to know if an incident particle is interact with nuclear who is the better and give experiment result?

For example if we send neutron into $Po^{209}$ what is all the cross section that we should take it into consideration

  • 1
    $\begingroup$ Before answering your question, can you please comment on the limit where $l\rightarrow\infty$ because $P(l)\rightarrow -\infty$ in that case! Shouldn't $0\leq P(l)\leq 1$? Should it be $P(l)=1-e^{-\frac{l}{\lambda}}$? $\endgroup$
    – Newbie
    Feb 1, 2022 at 2:25
  • $\begingroup$ @Newbie is correct. Please see my answer. $\endgroup$
    – John Darby
    Feb 1, 2022 at 3:52
  • $\begingroup$ @JohnDarby Just upvoted it. Good answer. $\endgroup$
    – Newbie
    Feb 1, 2022 at 3:54

2 Answers 2


For nuclear reactions the following definitions are typically used.

The cross section $\sigma$ for a particular reaction can be defined as "the fraction of the incident particles which undergo the specific reaction, divided by the number of target nuclei per unit area of a thin target". [Evans, The Atomic Nucleus] The cross section is the effective area per single target nucleus for the reaction. [Glasstone and Edlund, The Elements of Nuclear Reactor Theory] The cross section has units of area, typically expressed in barns where one barn = $10^{-24} cm^2$. The cross section is sometimes called the "microscopic cross section" to distinguish it from the "macroscopic cross section" defined below.

The product of the (microscopic) cross section $\sigma$ and the density of target nuclei $N$ is called the "macroscopic cross section" $\Sigma = \sigma N$. $\Sigma$ is the probability of the reaction per unit path length and has units of inverse distance. The macroscopic cross section is typically expressed in $cm^{-1}$.

For incident particles with density $n$ and velocity $v$, the flux is defined as $\phi = nv$. The reaction rate for a particular reaction with target nuclei is $\Sigma \phi$ reactions per $cm^3 \over sec$.

The probability that an incident particle will have a first interaction over a distance $l$ is $\int_{0}^{l}e^{-\Sigma x} \Sigma dx = 1 - e^{-\Sigma l}$. $\Sigma$ is sometimes denoted as $1/\lambda$ where $\lambda$ is the mean free path for the reaction in $cm$.

If you bombard $Po^{209}$ neutrons, $\sigma_a$ is the basic dimension needed for the reaction type $a$, but you also need to know $N_{Po^{209}}$ and $\phi$ for your particular condition to evaluate the reaction rate $\Sigma_{a} \phi$.


The problem seems to be that the word cross section is used for different things in different fields and maybe even falsely by some people. I'm personally used to the definition in particle physics, where the cross section has the units of an area and has to be multiplied with multiple other quantities which describe the experiment (e.g. beam flux and target density) to gain a reaction rate which may be nearest to a probability. Therefore people may call it a probability.

The definition your formulas are coming from is different. It comes from the "Absorption cross section". If you just search for "cross section" on Wikipedia, it will give you a range of different meanings.

For your example I would guess you are looking for the neutron cross section which has it's own article.

  • $\begingroup$ If we throw a neutrons to Po and use the formula above and say that 5% of particle are interact and 95% are pas is that true (or the formula use to other thing $\endgroup$ Mar 29, 2020 at 15:35
  • $\begingroup$ I have no idea from where you have your numbers, but I would say that you should NOT use above formula, as it is from a different definition of a cross section. $\endgroup$
    – paleonix
    Mar 29, 2020 at 15:42
  • $\begingroup$ So can you give the true formula to know the probality of interact neutron with material $\endgroup$ Mar 29, 2020 at 16:05
  • $\begingroup$ Like I said, you will find it, if you search "Neutron cross section" on Wikipedia. $\endgroup$
    – paleonix
    Mar 29, 2020 at 16:24
  • $\begingroup$ $V=\sigma vdt $ do you mean this formula $\endgroup$ Mar 29, 2020 at 16:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.