Total photoabsorption on quasi free nucleons at 600 – 1500 MeV N.Rudnev, A.Ignatov, A.Lapik, A.Mushkarenkov, V.Nedorezov, A.Turinge for the GRAAL collaboratiion Institute for Nuclear Research RAS, Moscow, New experimental data on total photo-absorption cross sections for proton, deuteron and carbon nuclei at E  = MeV are presented. Measurement has been done at the GRAAL facility. Two independent methods (subtraction of background and summing of partial channels) were applied to provide the high experimental accuracy. It is shown that the difference between total photo absorption cross sections for the quasi free proton and quasi-free neutron does not exceed 5% in absolute scale, F15 (1680) resonance at E  = 1000 MeV is clearly seen in both cross sections. A comparison of quasi free and free proton partial meson photo-production cross sections allows to conclude that nuclear media effects are caused not only by Fermi motion but also by the two nucleon correlations in the final state.

Scheme of the GRAAL experiment

Previous experiment on free proton data Total photoabsorption cross section on the free proton obtained by the subtraction method. Full dots, triangles, and circles represent the GRAAL, Mainz, and Armstrong data, respectively. Partial  0 photo-production cross section on the proton, measured with different wave length laser light. Green and red points correspond to 340 and 514 nm, respectively. Curve represents the prediction of SAID.

Subtraction method Total yield from the full target (open circles), empty target (full triangles) and their difference (full squares). Selection of a proton and charged pion in BGO detector

Separation of the events for one charged pion photo-production on quasi-free nucleon Red – experiment, green – simulation Angle between calculated and measured directions of the nucleon (reaction  n=>p  - ) Difference between calculated and measured energies of the forward nucleon (reaction  n=>p  - ). Here and later the black vertical lines specify the cuts for event selection

Separation of the events for one neutral pion photo-production on quasi-free nucleon Red – experiment, green – simulation Invariant mass of two  quanta in BGO detector (reaction  p=>p  0). Missing mass of two g-quanta in BGO detector (reaction  p=>p  0 ).

Separation of the events for  meson photo-production on quasi-free nucleon Red – experiment, green – simulation Separation of the events for double  0 photo-production on quasi-free nucleon, Red – experiment, green – simulation Invariant mass of two  -quanta in BGO detector (reaction  p=>p  ). Invariant masses of two pairs of  -quanta (reaction  p=>p  0  0 ). Rectangle marks area of the selected events.

Cross section evaluation method Photon flux (a), yield (b), measurement efficiency (c) (reaction  p=>p   ). Cross section (d) is obtained by division of the yield on the flux, and normalized on the measurement efficiency and thickness of the target.

Partial cross sections for one and double pion and  meson photo-production on free and quasi-free proton and quasi-free neutron Red – free proton, green – quasi-free proton, blue – quasi-free neutron. Specific media modification in different channels indicates that two nucleon correlations plays important role in addition to Fermi motion.

Total photo-absorption cross section for the deuteron, obtained by subtraction (open points) and summing method (full points) in comparison with literature Armstrong data (full triangles) Results obtained by two different methods coincide within 5% error bars in the energy region up to 1.2 GeV. Above this energy, there is seen a noticeable disagreement, which is explained by the triple meson production channels contribution which was not included in the present evaluation of partial channels. We see the similar relative behavior, especially presence of the F15 (1680) resonance in the cross sections, obtained by subtraction and summing methods in contradiction with the Armstrong data seen.

Total photo-absorption cross section for the 12 С. Crosses correspond to GRAAL data, obtained by the subtraction method. Full and open points are taken from Bianchi e. a. and Mirazita e. a. data, respectively. “Universal curve” is marked by the full line. Good agreement with earlier published results and the “universal curve” is seen.

Comparison of the total photo-absorption cross sections for the proton (full triangles), deuteron (open points) and carbon (full points). The carbon cross section is in 30% less than the free proton and deuteron one.

Total photo-absorption cross sections on bound proton (green points) and neutron (red points) obtained by the summing method (deuteron target) Slight (~5%) but systematic difference in the absolute scale above 1 GeV can be caused by the neutron efficiency uncertainty.

The ratio of free and bound proton photo absorption cross sections

Comparison of the total photo-absorption cross sections on free and quasi-free proton (actinide nuclei) Dotted line – free proton Dashed line and points – nuclei from 7 Li to 238 U Solid line – actinide nuclei (CEBAF data) Dotted line and triangles – free proton (Armstrong and GRAAL data, respectively) Full points – GRAAL data for carbon Carbon and actinide nuclei cross sections are very close in absolute scale.

CONCLUSION 1.Total cross sections obtained by subtraction and summing methods coincide within 5% error bars in the energy region up to 1.2 GeV. The discrepancy above this energy is explained by the triple meson production. 2. For neutron and proton we see the similar relative behavior and agreement in absolute scale, especially the presence of the F15 (1680) resonance in both cross sections in contradiction with the Armstrong (1972) results. 3.It is seen, that not only Fermi motion but also two nucleon correlations in the final state interaction is responsible for the modification of cross sections in the nuclear media. 4. Carbon cross section is practically coincides with the “universal curve” but lies in 30% below than the proton and deuteron one..