Nuclear Data Section
International Atomic Energy Agency
P.O.Box 100, A-1400 Vienna, Austria
Memo CP-D/629
Date:
To:
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29 March, 2010
Distribution
N. Otsuka

Subject:

Total cross section in photo-nuclear reaction data

Cross sections compiled in entry M0041 give the sum of both nuclear and atomic
contribution measured by the transmission technique. The first equation of the M0041
article [1] defines their “absolute cross section” asN (E )1σ = σ nucl + σ at = log 0AN (E ), where σ nucl is the nuclear “absorption” cross section, σ at is the cross sections of pure
electromagnetic processes (photoelectric effect, Compton effect, pair production in
the Coulomb field of the nucleus and electron fields), and N 0 (E) and N(E) are thenumbers of photons in a given incident photon energy bin E with and without theabsorber in the photon beam.
Though atomic physicists often refers the cross section to “absorption cross section”,“total absorption cross section” or “attenuation cross section”, this is close to ourdefinition of “total cross section” in EXFOR. We propose to use SF3=TOT in M0044.
We also propose to exclude the atomic part from total cross sections in EXFOR. Thatis, atomic scattering like Rayleigh scattering and Compton scattering is excluded fromthe total cross section, while nuclear resonance scattering (fluorescence) is included in
the total cross sections in EXFOR. We may keep total cross sections including atomicinteraction with SF9=MSC. Because data sets in M0041 contain atomic interaction
contribution, SF9=MSC is proposed in M0044.
“Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption
Coefficients” provided by NIST (http://www.nist.gov/physlab/data/xraycoef/) is an
example of atomic interaction database which is used by nuclear reaction
experimentalists to subtract the atomic part from their measured cross section.
Springer Handbook [2] classifies the photon-atom interactions from 1 keV to 1 MeV
as follows:
Elastic process:Rayleigh scattering, nuclear Thomson scattering, Delbrück scattering, nuclear
resonance scattering
Inelastic process:Photo-excitation (incl. ionization), Compton scattering, pair production

Note that the latter is corresponding to “Nonelastic” in the EXFOR terminology.
The following addition is also proposed for LEXFOR “Total” and “Absorption”:

LEXFOR “Total”
Total cross section
Definition: the sum of all energetically possible interactions.
Note: Photo-atomic interaction contribution (e.g. Rayleigh scattering, Compton
scattering, photo-ionization) is excluded from processes considered in photo-nuclear
reaction data. (γ,tot) = (γ,n) + (γ,p) + (γ,2n) + … + (γ,f) + nuclear scattering
LEXFOR “Absorption”
Examples of cases which are not coded under absorption:…
c.) The “photoabsorption cross section” below the nucleon emission threshold must be
coded as (γ,sct), (γ,el) or (γ,inl), since only scattering is possible under the threshold.
(γ,abs) = (γ,n) + (γ,p) + (γ,2n) + … + (γ,f)
References:[1] G. M. Gurevich et al., Nucl. Phys. A338(1980)97.
[2] B. Crasemann, Springer Handobook of Atomic, Molecular, and Optical Physics,
p.915.
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