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, 117312
New experimental data on total photo-absorption cross sections for proton,
deuteron and carbon nuclei at E g = 600 -1500 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 Eg = 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 p0 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 gn=>pp- )
Difference between calculated and measured
energies of the forward nucleon
(reaction gn=>pp-).
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 g-quanta in BGO
detector (reaction gp=>pp0).
Missing mass of two g-quanta in BGO detector
(reaction gp=>pp0).
Separation of the events for h - meson
photo-production on quasi-free nucleon
Red – experiment, green – simulation
Invariant mass of two g-quanta in BGO
detector (reaction gp=>ph).
Separation of the events for double p 0
photo-production on quasi-free nucleon,
Red – experiment, green – simulation
Invariant masses of two pairs of g-quanta
(reaction gp=>pp0p0). Rectangle marks area of
the selected events.
Cross section evaluation method
Photon flux (a), yield (b), measurement efficiency (c) (reaction gp=>pp0 ). 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 h 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 12С.
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 7Li to 238U
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.