Question: How do you calculate the probability of photoelectric effect?
About the article "Photoelectric effect", section "Cross section".

Candidate passages:
Video camera tubes in the early days of television used the photoelectric effect, for example, Philo Farnsworth's "Image dissector" used a screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal. 
Background radiation doses in the immediate vicinities of particles of high atomic number materials, within the human body, have a small enhancement due to the photoelectric effect. 
The following examples show how to use the module and make function calls to get information about protein disorders.
The first two examples make library function calls to calculate the probability of disorder for every residue in the sequence provided.
The third and fourth examples demonstrates how easily one can get the disordered regions of the protein.
Example 1: Calculate the probability of disorder for every residue in the sequenceExample 2: Calculate the probability of disorder for every residue in the sequence for all proteins from the FASTA input fileExample 3: Get the disordered regions of the protein for a single protein sequenceExample 4: Calculate the disordered regions for the proteins from FASTA file 
  Calculate group and grand means  Calculate k=number of groups, n=number of observations per group, N=total number of observations (k x n)  Calculate SS-total (or total variance) as: (Each score - Grand mean)^2 then summed  Calculate SS-treat (or treatment effect) as: (Each group mean- Grand mean)^2 then summed x n  Calculate SS-error (or error effect) as (Each score - Its group mean)^2 then summed  Calculate df-total: N-1, df-treat: k-1 and df-error k(n-1)  Calculate Mean Square MS-treat: SS-treat/df-treat, then MS-error: SS-error/df-error  Calculate obtained f value: MS-treat/MS-error  Use F-table or probability function, to look up critical f value with a certain significance level  Conclude as to whether treatment effect significantly affects the variable of interest  
The photoelectric effect is one interaction mechanism between photons and atoms.
It is one of 12 theoretically possible interactions.
At the high photon energies comparable to the electron rest energy of , Compton scattering, another process, may take place.
Above twice this () pair production may take place.
Compton scattering and pair production are examples of two other competing mechanisms.
Indeed, even if the photoelectric effect is the favoured reaction for a particular single-photon bound-electron interaction, the result is also subject to statistical processes and is not guaranteed, albeit the photon has certainly disappeared and a bound electron has been excited (usually K or L shell electrons at gamma ray energies).
The probability of the photoelectric effect occurring is measured by the cross section of interaction, σ.
This has been found to be a function of the atomic number of the target atom and photon energy.
A crude approximation, for photon energies above the highest atomic binding energy, is given by:formula_19Here Z is atomic number and "n" is a number which varies between 4 and 5.
(At lower photon energies a characteristic structure with edges appears, K edge, L edges, M edges, etc.)
The obvious interpretation follows that the photoelectric effect rapidly decreases in significance, in the gamma ray region of the spectrum, with increasing photon energy, and that photoelectric effect increases steeply with atomic number.
The corollary is that high-Z materials make good gamma-ray shields, which is the principal reason that lead (Z = 82) is a preferred and ubiquitous gamma radiation shield.