Re: FLUKA: photonuclear cross section data base

[Alberto Fasso' <Alberto.Fasso@cern.ch>]

Dear Baerbel,

I will try to answer your questions.

> 1) Pb :
> Comparing with experimental data from the EXFOR data base (NEA) 
> for 208-Pb and 206-Pb I found a good agreement with the FLUKA 
> parameterization and I assumed, that the shape of the cross 
> section curves describes the total photonuclear cross section. 
> >>>  Is this right, or not ?? 

Yes and no. In principle FLUKA uses total photonuclear cross 
sections, but experimental data exist only for a few light nuclides.
For heavy nuclei, the total cross section is practically identical
to the (gamma,Sn) cross section, i.e.:
(gamma,n)+(gamma,np)+(gamma,2n)+(gamma,3n)+ ...+(gamma,fission)
  [note that this is different from the (gamma,Tn) cross section
  (neutron yield cross section), which is:
   gamma,n)+(gamma,np)+2(gamma,2n)+3(gamma,3n)+ ...+nu(gamma,fission)]
The reason that sigma(gamma,Sn)=sigma{gamma,total) is that for large Z the
Coulomb barrier is too high to allow emission of charged particles.

So, in FLUKA we use at the moment:
  a) Experimental total cross section data if they exist:
     3He, 7Li, 9Be, 12C, 13C, 14N, 15N, 16O, 17O, 18O, 19F, 23Na,
     24Mg, 25Mg, 26Mg, 27Al, 29Si, 30Si, natSi, 32S, 34S, natCa
  - Otherwise:
    For Z > 29:
    b) a Lorentz parametrization with published Lorentz parameters
       (peak energy, peak height, width), if they exist. They are
       all those reported in the Atlas of Dietrich and Berman 
       Atomic Data and Nuclear Data Tables 38, 199 (1988), except
       Pr, Au and Pb, for which we have used the parameters
       published in Berman et al., Phys. Rev. C36, 1286 (1987). 
    -  Otherwise:
      c) a Lorentz parametrization with parametrized Lorentz parameters
         (it sounds funny, but Berman and Fultz (Rev. Mod. Phys. 47,
         713 (1975) have published some general formulas giving the 3   
         Lorentz parameters as a function of A and Z.)
    For Z <= 29, total cross section obtained as:
    d) sum of partial cross sections: 4He 
    e) difference sigma(nat. composition) minus sigma(available isotopes): 
       28Si  
    f) sigma(gamma,Sn) or sigma(gamma,Tn) corrected by PEANUT:
       6Li, 10B, 11B, 31P, 35Cl, 37Cl, 40Ar, natK, 42Ca, 45Sc, 48Ti, 50Ti,
       51V, natCr, 55Mn, natFe, 59Co, 60Ni, 63Cu, natCu 
    g) the parametrized formula of Berman and Fultz for all nuclides not
       listed above, with an empirical correction for low-Z. This can be
       very bad in some cases (e.g. 58Ni), and we will try to improve it.

Please note that the quality of these data can be: 
  good: cases a), b), d)
  fair: c), e), f)
  poor: g)

> Unfortunately I could not found any evaluated or experimental
> photoneutron cross section data for the isotope 204-Pb (with an
> abundance of 1.4 %) to compare these data with the FLUKA 
> paramerization. This cross section seems to be too small.
> This Pb-isotope is responsible for the major contribution of 
> activity of the lead radiator and it is very important to use the 
> correct cross section values. There, I want to ask the authors, 
> how the photonuclear (photoneutron) cross section parameterization
> was obtained for 204-Pb ? 
> Could you, please, comment this question ? 

The answer for 204Pb is case c) above.

> 2)Hg :
> We compared the results of the neutron emission for the lead 
> radiator with results for a mercury radiator. The neutron flux
> for Hg is approximately four times smaller as for Pb. Looking at 
> the photonuclear (photoneutron) cross sections it seems, that in 
> the case of Hg nuclides the cross sections are normalized
> proportional to the percentage of the natural abundance of the 
> Hg-isotopes ? In the Berman Compilation I found, that the maximum 
> cross section of nat. Hg amounts to 600 mb. I could not found any
> other cross section data for Hg, e.g. in the NEA data bank. 
> How good are the cross section parameterizations for the
> Hg-isotopes used in the program FLUKA ?

In this moment I don't have the Dietrich-Berman Atlas at hand: if Hg is
not in that list, then also for it we use the "parametrized Lorentz
parameters" of case c).

Best regards,

    Alberto

-- 
Alberto Fassò
CERN-EP/AIP, CH-1211 Geneve 23 (Switzerland)
Phone: (41 22) 767 2398    Fax: (41 22) 767 9480   Alberto.Fasso@cern.ch