Program - Brookhaven National Laboratory

the detector setup compared to the campaign at 96 MeV had to be made. The second ∆E detectors have
been replaced by 1000 µm thick SSBDs and the size of the crystals used as E detectors was increased to a
total length of 100 mm and a diameter of 50 mm. The ∆E − E technique is used to identify the light ions,
and cutoff energies as low as 2.5 MeV for protons and 4.0 MeV for alpha particles are achieved. Suppression
of events induced by neutrons in the low-energy tail of the neutron field is achieved by time-of-flight
techniques. The data are normalised relative to elastic np scattering measured in one of the telescopes
at 20 degrees. Preliminary double-differential cross sections for oxygen are presented and compared with
theoretical reaction model calculations.
[1] U. Tippawan, et al, Studies of neutron-induced light-ion production with the MEDLEY facility, International
Conference on Nuclear Data for Science and Technology, Nice, France, April 22-27, 2007, editors
: O.Bersillon, F.Gunsing, E.Bauge, R.Jacqmin, and S.Leray, EDP Sciences Proceedings, 2008, p.1347,
http://nd2007.edpsciences.org/. [2] R. Bevilacqua et al, Medley spectrometer for light ions in neutroninduced
reactions at 175 MeV, Nuclear Instruments and Methods in Physics Research A 646 (2011) 100-107.
PR 87
The New Approach to Analysis and Evaluation of Reliable Partial Photoneutron Reactions
Cross Sections
B.S. Ishkhanov, V.N. Orlin, V.V. Varlamov, Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow
State University, 119234 Moscow, Russia.
Accurate and reliable information on cross sections of partial photoneutron reactions primarily (γ,n), (γ,2n)
and (γ,3n) is very important for both basic research and many applications. The most modern and actual
now of those is monitoring of the beam luminosity in ultra-relativistic heavy-ion colliders by measuring
neutron emission (in (γ,n) and (γ,2n) reactions) rates in mutual electromagnetic dissociation of colliding
nuclei. The majority of such kind data were obtained using quasimonoenergetic annihilation photons at
Livermore (USA) and Saclay (France). Data are included into various reviews, atlases and databases, are
used in many modern codes (GEANT, IMPIRE, TALYS). Unfortunately there are significant (till 100 %)
systematical disagreements between those data - generally (γ,n) reaction cross sections are larger at Saclay,
but (γ,2n) reaction cross sections vice versa at Livermore. In various investigations (for example, [1, 2]) it
was shown that the main reasons are shortcomings of neutron multiplicity sorting methods used at Saclay
and the method was proposed for overcoming of those disagreements - recalculation of unreliable Saclay
data putting them into accordance with reliable Livermore data. But at the same time there are many
doubts that Livermore data are reliable because in many cases behavior of (γ,n) reaction cross section
is very strange - above the Giant Dipole Resonance (GDR) maximum it falls rapidly going into the region
of negative values, then returns back to positive values showing clear positive maxima and after that
again goes to negative values. That means that the task of investigation of those cross sections reliability
and authenticity remains till now very actual and important. To solve it the simple, objective and absolute
criteria were proposed at the first time - transitional photoneutron multiplicity functions Fi(γ,in)
= σ(γ,in)/σ(γ,xn) equal correspondingly to ratios of cross sections of (γ,n), (γ,2n) and (γ,3n) reactions
to that of total photoneutron yield reaction (γ,xn) = (γ,n) + 2(γ,2n) + 3(γ,3n) + ... Follow their definitions
those functions Fi could not have absolute values higher than 1.00, 0.50, 0.33,... correspondingly.
Larger values mean definite incorrectness of neutron multiplicity sorting and therefore - non-reliability and
non-authenticity of data. The main consequence of that is appearing of non-physical negative values in
cross section. Systematic analysis of experimental data for (γ,n), (γ,2n) and (γ,3n) reaction cross sections
obtained at Livermore for nuclei 90 Zr, 115 In, 112,114,116,117,118,119,120,122,124 Sn, 159 Tb, 165 Ho, 181 Ta, 197 Au,
208 Pb revealed that majority of those data are not reliable and authentic because in many energy ranges
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