Total Hadronic Photoabsorption cross sections on Hydrogen, Deuteron and Carbon Nuclei from 0.6 to 1.5 GeV

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15 Νοε 2013 (πριν από 3 χρόνια και 1 μήνα)

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Total Hadronic Photoabsorption cross sections on Hydrogen, Deuteron
and Carbon Nuclei from 0.6 to 1.5 GeV


N. Rudnev {1
}
, V.Bellini {2,3}, .P. Bocquet {4
}
, M. Capogni {
a
,
b}
,M. Casano {
b}
, A.
D’Angelo {5
,6
}
, J.
-
P. Didelez {7
}
, R. Di Salvo {5
}
, A. Fantini
{
5,6}
, D.Franco {5,6} , G. Ge
r-
vino {8
}
, F. Ghio {
9}
, B. Girolami {
9}
,
G.Giardina {2,3},
M. Guidal {
7}
, A. Giusa {2,3
j}
,
E
E
.
.


Hourany

{7
}
, A. Lapik {
1}
, P. Levi Sandri {10}, A. Lleres {7
}
, G.Mandaglio {2,11},
F.Mammoliti {2,3}, M.Manganaro {2,11}, D. Moriccia
ni {5
}
, A. Mushkarenkov {1
}
,V. N
e-
dorezov {1
}
, C. Perrin {4
}
, C.Randieri {2,3}, D. Rebreyend {4
}
, G. Russo {2,3
}
, C. Schaerf
{5,6
}
, M.
-
L. Sperduto {2,3
}
, M.
-
C. Sutera {2
}
, A. Turinge {1
}, V.Vegna {5,6}.


1
-

Institute for Nuclear Research, 117312 Moscow, R
ussia


2


INFN, Sezione


di Catania, I
-
95123 Catania, Italy

3


Dipartimento di Fisica e Astronomia, Universita di Catania, , I
-
95123 Catania, Italy

4
-

IN2P3, Laboratoire de Physique Subatomique et de Cosmologie, 38026 Grenoble,France

5


INFN, Sezione
d
i Roma ”Tor Vergata”, I
-
00133 Roma, Italy

6


Dipartimento Di Fisica ”Tor Vergata”, I
-
00133 Roma, Italy

7
-

IN2P3, Institut de Physique Nuclґeaire d’Orsay, 91406 Orsay, France

8


Dipartimento di Fisica Sperimentale, Universia di Torino and INFN


Sezione

di Torino, I
-
00125,
Torino, Italy

9


Instituto Superiore di Sanita, I


00161, Roma anf INFN


Sezione di Roma , I
-
00185, RomaI, Italy

10


INFN Laboratori Nazionali di Frascati, I
-
00044 Frascati, Italy

11


Dipartimento di Fisica, Universita di Messina,

I
-
98166, Messina, Italy.



* Reference person rudnev@cpc.inr.ac.ru




Final total cross sections are given for the GRAAL experiment at ESRF on hadronic photon a
b-
sorption in hydrogen, deuteron and carbon at incident energies from 0.6 to 1.5 GeV. Measureme
nt has
been done using back scattered gamma beam and large aperture detector LAGRAN

E. Two independent
methods (subtraction of background and summing of partial cross sections) were applied to improve the
experimental accuracy. It is shown that total phot
oabsorption cross section for the proton and neutron
(deuteron target) coincide within 5% of error bars in absolute scale, being in contrary to the previous li
t-
erature data where these cross sections were found to be noticeably different. Moreover, F
15 (16
80)

res
o-
nance at E


near 1 GeV is clearly seen in both cross sections whereas earlier it was evidenced for the pr
o-
ton only. This indicates possible
existence

of the “door
-
way state” which is identical for the proton and
neutron as the first step of the photon


nucleon int
eraction. Also, one can expect that the free proton and
free neutron total cross sections are identical as they are identical for the deuteron target. Carbon data i
n-
dicate large difference (about 30%) between bound and free nucleon in studied energy region
. This means
evidently that nuclear modification of the total photoabsorption cross sections is caused dominantly by
the nucleon correlations inside a nucleus whereas the Fermi motion effects are small in this energy region.
Obtained results can be used f
or revision of actinide nuclei photoabsorption data indicating non linear
electrodynamics

effects in photo
-
excita
t
ion of such nuclei
.
Exotic narrow resonances have been not
observed nor in the partial nor in the total cross sections.


1.INTRODUCTION



The

completed results of the GRAAL experiment on photoabsorption of hydrogen, de
u-
terium and carbon targets for the energy range from 0.6 to 1.5 GeV (above
Р
33

(1520) resonance)
are presented. Portions of the data, reported earlier for the free proton [1], have shown good
agreement with the literature results [2] but new results for the neutron photoabsorption show
contradiction with published data [3] for th
is energy region. Other published results for the total
proton and neutron photoabsorption are not available yet at E

above 800 MeV in the nucleon
resonance energy region.


Nuclear photoabsorption cross sections for different A (f
rom Li to U) were measu
red in
[4
-
6]. New GRAAL results complete them essentially with higher accuracy.
Approximated
Armstrong data using multi
-
pole analysis [7] are widely used as reference data for comparison.


Almost all the existing data on the total photo
-
absorption cross s
ections in the nucleon
resonance energy region were obtained with bremsstrahlung tagged gamma beams. To reduce
the electromagnetic backgrounds different methods were used. For example, Cherenkov counters
were applied to eliminate the events caused by the e
lectromagnetic processes like Compton
scattering and e
+
e
-

-

pair production.
At GRAAL facility we used the Compton back scattered
laser photons, so the low energy gamma tail was significantly reduced by the collimation.


2. EXPERIMENTAL PROCEDURE



The e
xperimental GRAAL scheme is shown in fig.1.








Fig.1.


Basic elements of the GRAAL facility :

1


interaction region of laser photons and electrons, 2


tagging system, 3


laser hutch, 4


gamma beam collimating system, 5


large aperture detector
LAGRAN

E for charged and ne
u-
tral particles , 6


target, 7

plane MWPCs, 8


double plastic scintillation wall, 9


electroma
g-
netic shower calorimeter, 10


beam monitors, 11


total photoabsorption spectrometer.



The GRAAL apparatus has been described i
n several papers, see for example [
8
-
10
]. The
LAGRAN


detector is located approximately 30 meters downstream of the Compton intera
c-
tion region (6 meter straight section of the 6 GeV electron storage ring (ESRF)). An argon laser
of 514 nm (green line) or 3
51 nm (UV line) gives rise to the maximal energies for back scattered
photons E = 1.1 GeV and 1.5 GeV, respectively.


Collimated back scattered beams have a low
-
energy tail much lower than Bremsstrahlung
beams. The central LAGRAN

E part is organized aroun
d the cryogenic Liquid Hydrogen (LH)
or Liquid Deuterium (LD) target (6 cm length, 5 cm diameter). Solid Carbon (SC) target has a
thickness of 3 mm which is equivalent to 4 cm of LD. Central detector part is composed of a
tracking system based on two cyli
ndrical MWPCs, a barrel of 32

E plastic scintillators and a
high resolution BGO ball.

The forward part (
θ

25
0
) consists of two plane MWPCs, a double wall of plastic sci
n-
tillators for TOF identification and a sandwich shower detector (SD). The backward reg
ion
(
θ

155
0
) is covered by two discs of plastic scintillators, separated by 2 mm of Pb, to discrim
i-
nate between neutral and charged particles. Therefore, the solid angle is equal to almost 4


for
neutral and
charged

particles.


Background conditions were discussed in details recently [1] for the experiment with the
free proton target. It was evidenced that the backward scattering technique used at GRAAL pr
o-
vides the minimal electromagnetic background and, respectively, the high

systematic accuracy.
As background conditions do not depend on the target type we’ll not repeat the details presented
in paper [1]. We conclude only that for charged particles, when the plastic scintillation barrel is
included in the analysis, the contrib
ution of the background falls down approximately from 20%
to 5%. For any partial reaction channel (when a kinematics selection is applied) it usually b
e-
comes less than 1%.


An important feature of the GRAAL beam is possibility to change the gamma beam
ener
gy range varying the laser wave length during the experiment. For example, the gamma
energy ranges (E


= 500
-
1100 MeV and 800


1500 MeV for green and UV lines, respectively)
were used. Measurement of the yield in the overlapping energy regions allows to evaluate the
systematic errors precisely. Fig.2 shows an example of such measurements for the

0

pho
to
-
production channel where the accuracy of 5% can be demonstrated.



Fig.2.

Partial

0

photo
-
production cross

section on the proton, measured with

different wave length laser light.

Full dots and circles correspond to 514

and 340 nm, respectively. C
urve represents

the prediction of multi
-
pole analysis
MAID
-

2008
.






It should

be noted that the background contributions were rather stable in the long term
measurements. The long term stability was provided due to the high quality of the ESRF electro
n
beam and the high performance of the collimating system.


3. SUBTRACTION METHOD


The subtraction

method is based on the fact deduced from the experiment that the ele
c-
tromagnetic background is coming outside of the target whereas the contribution from th
e target
itself is negligible. Therefore, this background can be subtracted from the total yield using the
empty target measurement. Evidently, the total hadron yield is equal to


)
Ω(E
)
(E
σ
N
N
=
)
Y(E
γ
γ
tot
γ
p
γ




(1)

Here
,
N
p

is the number of protons in the target (liquid hydrogen target of 6
-
cm thickness corr
e-
sponds to
23
10
2.568

proton/cm
2
) ;
N

is the gamma flux, (which is typically equal to about
6
10
2



s) integrated over exposition time;
)
(E
σ
γ
tot
is the total photoabsorption cross section;
and
γ
Ω(E
)
is the

measurement efficiency evaluated by simulations.


Total number of hadron events collected during one day was great enough (
7
10
2

) to
provide the statistical error 2% in each energy bin of 16
-
MeV width. Neither identificati
on of
events, nor kinematics selection was applied here.

The gamma flux was measured simultaneously by two monitors: total photoabsorption
detector ("spaghetti") and thin plastic scintillation counters ("thin monitor") in coincidence with
the tagging sys
tem.
The response of the tagging system was studied by means of the spaghetti
and thin monitor and was the same for both data taking and the flux monitoring.

Simulated global BGO efficiency
γ
Ω(E
)

is

presented in Table 1 for two

different thres
h-
olds (100 and 160 MeV) for comparison (the experimental threshold was approximately equal to
160 MeV). As is seen from Table 1, the global efficiency for the hadron events (the same for LP
and LD targets) very weakly depends on the thresho
ld in this region. This is a favorable feature
of the total cross section measurement since the distribution of the total energy deposited in the
BGO has a steep rise in the region of the experimental threshold. Careful analysis shows that the
systematic e
rrors due to this effect do not exceed 1%.






Table 1

Simulated at 100 and 160 MeV thresholds as functions of E

.global BGO efficiencies


E


GeV



0.55

0.65

0.75

0.85

1.05

1.15

1.25

1.35

1.45

MeV)
Ω(E
γ
100


0.86

0.88

0.90

0.89

0.88

0.90

0.90

0.90

0.90

MeV)
)
Ω(E
γ
160


0.84

0.86

0.87

0.86

0.87

0.86

0.87

0.87

0.87



The to
tal yield was measured with the hard trigger (160 MeV energy release in BGO),
and only the cluster size (number of simultaneously illuminating neighboring crystals) was li
m-
ited (MCLUS
<
8).

Neither kinematics nor other selection cuts were applied here.


Fig.3 demonstrates r
esults obtained by the s
ubtraction method
results
for the carbon
target.








Fig.3.

Total yield from the
12
C target

(open circles
-

upper curve), empty

target (full triangles


down curve)

and their difference (full squares


a
verage

curve).









Total cross section is the difference between two
total hadron
yields from full and empty
target normalized on the corresponding fluxes. Taking into account the target
thickness, we
obtain the
total cross section in absolute scale
.


4.

SUMMING METHOD



Summing method supposes measurement of partial meson photoproduction cross
section
which contribute signifi
ca
nt
ly to the to
t
al photoabsorption one. Similarly to the total hadron
yield the partial reaction one (integrated over the solid

angle) is determined by the equation:


)
Ω(E
)
(E
σ
)
(E
N
N
=
)
(E
Y
γ
γ
part
λ
γ
p
γ
part



, (2)


where
)
(E
σ
γ
part

is the partial cross section. Other parameters are the same as described in the
subtraction method presented abo
ve.

A partial reaction yield was separated from the total one by means of the kinematics rel
a-
tions (energy and momentum conservation laws). The number of partial channels which contri
b-
ute significantly to the total photoabsorption cross sections at E

< 1 GeV on the proton and ne
u-
tron is limited by the following reactions:




-
>

π
n

π
o
p

π
π

p

π
π
0
n

π
0
π
0
p

ηp



-
>

π

p

π
o
n

π
π

n

π

π
0
p

π
0
π
0
n

ηn


Other reactions including tripple pion production, kaons photoproduction and Compton
scattering contribute less than 2% in this energy region.

Evaluation algorithm for these partial channels was described elsewhere [1]. The result is
available
due to alm
ost

i-
ment. At first, here we took into account the single and double



meson photo
-
production on the proton and neutron using the deuteron target.
The products of

0

decay (2

)
were

detected in BGO
-
detector, nucleons and charged mesons were measured in both BGO and
front scintillation wall detectors. Time of flight measurement for the forward detector was a
p-
plied to separate nucleons and charged mesons.
Simulation was done for each p
artial reaction
providing the algorithm for data analysis
. The contribution of neighbor channels to the any s
e-
lected yield does not exceed 1%.

An example of the proton and pion selection for the reaction

n
=>
p



using two
-
dimension distributions is shown in fig.4.







Fig.4.


Selection of a proton and charged pion


in BGO detector







There where four criteria of event selection for the reactions

p
=>
p

0

and

n
=>
n
,



invariant mass of two

-
quanta
, missing mass of the nucleon, difference between ca
l-
culated and measured angles of nucleon and difference between calculated energy of nucleon
and its energy measured by TOF.


Selection of events for

-
meson photo production was done practically by the s
ame way
as for

0

photo production, but
the cuts on the invariant mass
(only 2 gammas of

decay

were
considered) were applied corresponding to the mass of

-
meson.



Selection of events for double


0

photo production
was done similarly to the

single


0

p
hoto production
. The only difference was to use the two dimension distributions of invariant
masses of two pars of

-
quanta corresponding to two

0
.


Events with one neutral and one charged meson were selected using the invariant masses
of

0

, at first
.

Then the energy and momentum of the missing mass were calculated. Energy of
charged meson was measured, so the missing mass for the neutral and charged meson was equal
to the mass of nucleon. Then the angle and energy of the nucleon was evaluated and com
pared
with the measured values.


The most difficult case was to separate the reaction of two charged mesons photo
production. If

nucleon hits the BGO detector we can’t measure the energies of all three particles.
Therefore, in order to decrease backgroun
d, the events with nucleon, coming in forward
direction where taken into account. Energy and momentum of the nucleon and its missing mass
were calculated. Energy of one charged meson was evaluated, so the nucleon missing mass and
this meson was equal to th
e mass of second meson. Then angle of second meson was calculated
and compared with the measured value.

Principal problem of present analysis is determination of the measurement efficiency for
all partial reactions which was done by simulations. This is b
ased on the computer program
chain which includes LAGGEN (LAGrange GENerator), LAGDIG (LAGrange DIGitation) and
PREAN (PRE
-
Analysis). LAGGEN contains the event generator to extract the energy and ang
u-
lar distributions for reaction products taking into acco
unt the results of multi
-
pole analysis and
previous experimental data. It would be emphasized that necessary literature data for all partial
channels which contribute noticeably to the total absorption cross section are available. Differe
n-
tial cross sectio
ns for reactions with two particles in final state can be extracted with a high acc
u-
racy from literature; for three particle production they were supposed isotropic in accordance
with existing experimental and multi
-
pole analysis data. The goal of this wor
k was to use these
data and to obtain the reliable results for absolute values of the partial and total cross sections.
So, the first step of simulations was to get the geometrical efficiencies which are defined by the
probability of the reaction product
particle to reach the detector.

On the second stage the simulation was done using LAGGEN again which includes the
MAID [9] and GEANT3.21 package [10], and
to evaluate the probability for the particle which
touches the detector to be measured, taking into

account the ionization, energy losses, size of
the cluster etc. Then, the code LAGDIG was used to provide the conversion of the traces of
particles in detectors to the digital outputs (QDC, TDS), taking into account the thresholds of the
detectors. Afte
r this procedure, the outputs were calibrated by means of the PREAN code,
exactly in the same way as corresponding experimental data, and respective cuts were identical.

Finally, the measurement
efficiency for any partial reaction was calculated as ratio o
f
simulated events (obtained in accordance with the described above algorithm) to the total
number of events simulated for selected reaction using the event generator.

The result of simulations is shown in table 2. Some multiple meson and kaon production
processes above 1.0 GeV can contribute to the yield, but they were ignored. The BGO crystal
threshold is equal to 10 MeV. The geometry partial efficiencies shown in parentheses are d
e-
fined as the probability of all the particles to reach the detector and
provide

a total energy release
greater than the BGO threshold of 160 MeV. It would mentioned that the data presented in table
2 are related to almost 4

detector and can be useful for another such kind facilities.

Table 2.

Simulated BGO efficiency for selected partial channels on the proton and neutron..


In parentheses the geometry efficiency is shown (see text).

.


E


GeV

p >


n


p >


p


p >




p


p >




n


p >




p


p >

p

0.550.12(0.68)

0.44(0.72)

0.13(0.33)

0.031(0.29)

0.13(0.24)


0.650.13(0.64)

0.42(0.71)

0.15(0.34)

0.037(0.29)

0.13(0.24)


0.750.12(0.59)

0.35(0.64)

0.16(0.34)

0.038(0.29)

0.12(0.23)

0.008(0.00)

0.850.11(0.55)

0.2
5(0.56)

0.17(0.33)

0.034(0.28)

0.12(0.23)

0.052(0.10)

0.950.11(0.54)

0.19(0.52)

0.15(0.31)

0.031(0.26)

0.11(0.22)

0.069(0.14)

1.050.10(0.49)

0.13(0.50)

0.15(0.29)

0.027(0.25)

0.11(0.22)

0.066(0.14

1.150.09(0.44)

0.09(0.46)

0.16(0.28)

0.022(0.23)

0.10(0.
21)

0.062(0.14)

1.250.08(0.41)

0.06(0.41)

0.17(0.26)

0.019(0.21)

0.10(0.21)

0.059(0.13)

1.350.07(0.40)

0.05(0.38)

0.17(0.24)

0.017(0.19)

0.10(0.20)

0.049(0.12)

1.450.06(0.38)

0.04(0.36)

0.17(0.22)

0.016(0.17)

0.10(0.18)

0.041(0.11)


n >


p


n >


n


n >




n


n >




p


n >




n


n >

n

0.65











0.75












0.85












0.95












1.05












1.15


























1.35












1.45
















Outputs for each of tw
elve mentioned above reactions were collected in several dozen
million events. For the same runs the corresponding fluxes were evaluated as was described in
[1].

Naturally, the simulation results depend on different specialized experimental problems
whic
h were solved for different reactions. For example, for



production channels, we took into
account the overlapping neutral clusters from the



decay. For partial channels with a recoil
neutron in the final state, we have to take into account the scattering neutrons in the BGO dete
c-
tor and to select the prima
ry scattering events. Neutral clusters corresponding to the primary
sca
t
tered neutron events were identified by the complanarity condition.


We would note that energy scale in presentation of all cross section has a bin of 20 MeV
whereas the energy resol
ution of the tagging system is equal to 16 MeV at 1 GeV. This was done
to avoid artificial wrong resonant structure which appears usually in the initial stage of analysis
when the energy bin and microstrip energy width are identical. Of course, it is possi
ble to check
the number of events carefully by hand in any energy bin to avoid the artificcial structure. Here
we prefered to use the energy bin of 20 MeV which seems to be optimal because it very slightly
decreases the energy resolution but improves sign
ificantly the systematic errors.


5. EXPERIMENTAL RESULTS



FREE PROTON


GRAAL results on the total photoabsorption cross section for the free proton obtained by
two independent methods
are shown

in fig.5. There is no visible difference between these two
results in the energy region up to 1 GeV. It is seen also that above 1 GeV, the subtraction
method gives an excess in comparison with the summing method and this difference is
increasing with the photon energy. This means evidently that other reactions
except mentioned
in the present analysis (mostly triple meson production) contribute significantly at high energies
(above 1 GeV). This result confirms high quality of the data obtained by the subtraction method
which will be presented as the final GRAAL

result on the total photoabsorption cross section on
the free proton. Total error bars evaluated by the comparison of two methods results do not
exceed 5%.







Fig.5.



Total photoabsorption cross section for

the free proton obtained at GRAAL by

the s
ubtraction method (open points) and


summing method (black points).



Comparison of the GRAAL results (subtraction method) with available literature data is
shown in fig.6. One can see a good agreement between different results in all energy regions.








Fig.6.


Total photoabsorption cross section for

the free proton obtained by GRAAL ,

Armstrong [3] and Mainz [6] are

represented by full, open points and

triangles, respectively.












It would be noticed that Armstrong data for the free proto
n photoabsorption cross section
are widely used as reference ones for comparison with nuclear photoabsorption cross sections. In
such case they are usually presented by the approximation curve basing on the multi
-
pole fit
which takes into account nucleon r
esonances and non resonance background [7].. In fig.7 one
can see a good agreement of the GRAAL and Armstrong approximation data for the free proton
except

some points on the left edge of the D13 resonance.





Fig.7

Total photoabsorption for the free

proton. Points correspond to

GRAAL data, curve is the result

of approximation [7] of the

Armstrong data.






NEUTRON



Situation with the neutron photoabsorption cross section is much more difficult because
we have no free neutron target. But basing o
n the summing method we could evaluate the total
photoabsorption for the bound neutron (deuteron target) and compare it with a bound proton
cross section obtained with the same target under the same conditions (tagging and detector
efficiency etc). Unexpec
tedly, the result was quite surprising as compared with an existing
knowledge.


But first of all we see in fig.8 that subtraction and summing method gives the identical
values (within 5% of error bars) at E

< 1 GeV for the neutron cross section, as it was found for
the free proton. Subtraction method was applied to the deuteron target, than cross section was
normalized on factor 2 (number of nucleons in the deuteron). Summing method was done taking
int
o

acc
ount 12 partial channels (these results are presented below in fig.9).



In fig.8 one can see a noticeable differe
nce with the Armstrong data [4] for the deuteron
target.

Unfortunately, all the GRAAL beam measurements with the deuteron target were
performe
d with the UV laser only, so the energy range was a little bit less than for the free
proton runs but it is enough to conclude that F15 resonance is clearly seen in both cross sections
(proton an n
eutron). Also, we would mention that absolute values would be significantly
corrected as the reference ones in accordance with the new GRAAL data.






Fig.8


Open and full points correspond to

the summing and subtraction method,

respectively. Triangles

represent to the

Armstrong data [3
] (deuteron target).












Fig.9 shows the partial cross sections which were measured as contributions to the total
photo
-
absorption cross section on the proton and neutron (deuteron target).






Fig.9.


Partial
cross sections f


meson

photo
-
production on bound proton

(open dots) and neutron (stars)

measured with deuteron target.

Free proton data are shown for

comparison (open dots).













Now it is interesting to compare results obtained by the summ
ing method for bound pr
o-
ton and neutron (deuteron target). In fig.10 one can see that both cross sections are coincide in
limits of 5% error bars in spite of the fact that partial cross section for p
roton and neutron are di
f-
ferent
. Probably, this indicates

the existence of so called “door
-
way” states which are identical
for the proton and neutron
in spite of the difference in electric charge
(terminology is taken from
the nuclear giant resonance physics).
Indeed
, such
hypothesis

produced many questions imme
d
i-
ately

about
quantum

numbers
etc.

How
the
partial channels including non resonant contribution

are follows
? Nevertheless, it is difficult to assume that
equality

for total proton and neutron cross
sections could be
originated

by chance.

Also, one can expe
ct now with a high reliability that total photoabsorption cross section
for the free neutron and for the free proton are identical because they are identical for the bound
nucleons in the deuteron target. If we assume this statement we can compare the fre
e neutron
cross section which is equal to the free proton cross section (see fig.11).















Fig.10.


Total photo
-
absorption cross section

for the bound proton (open points) and

neutron (full points) obtained by the

summing method (deuteron tar
get).








We would note that correction on Fermi motion is not applied for the deuteron target
here. Armstrong did such correction obtaining the neutron cross section by subtraction of the
proton cross section from the deuteron one. The Armstrong result

for the neutron is not in a
agreement with new GRAAL result (see fig.11) and this disagreement is large..





Fig.11.


Total photoabsorption for the

n
eutron
.

Points


GRAAL data.

Curve is the Armstrong approximated

result obtained from the deuteron cr
oss

section by subtraction of the proton one.

Total error bars are presented.







CARBON


Total photo
-
absorption cross section for the
12
C nuclei is shown in Fig.12. GRAAL results were
obtained by the subtraction method only.

Good agreement with earlier
published results and the
“universal curve” is seen. Error bars are not shown here but we would remember about 5% sy
s-
tematic accuracy for the GRAAL data for
hydrogen

and deuterium.












Fig.11.


Total photo
-
absorption cross section

for
12
С. Cross
es correspond to

GRAAL data, full and open points

taken from Bianchi [4] e. a. and

Mirazita [5] e. a.data, respectively.

“Universal curve” is marked by the

full line.







One can see that F15 resonance is smeared completely as compared with the deu
teron target.
This result was found earlier [ 4,5] but we would pay attention on very strong difference in abs
o-
lute values (about 30%) for carbon and deuteron and proton cross sections(see fig.12). Evidently
this can be explained by Fermi motion effects an
d indicate probably an in important role of intra
nuclear interactions in the final state.





Fig.12.


Comparison of the total photo
-
absorption

cross sections for the proton (open points), deuteron

(full triangles) and carbon (full points).











ACTINIDE NUCLEI




New GRAAL results for the proton and neutron allow to revise the existing data for
heavy actinide nuclei where some interesting effects have been evidenced earlier [12, 13]. Now
we can specify the features looking in the fig.13 as follo
wing. In the

resonance region the t
o-
tal photoabsorption cross section of fissioning actinide nuclei exceeds the “universal curve” on
20%,. This effect was found for the first time in Novosibirsk [12] and was confirmed with higher
precision and larger energy range at
JLAB [13]. It indicates
probably the

contribution of high
order
electrodynamics

excitation

mechanisms

for heavy (high Z) nuclei. At E


>

1 GeV one can
see the opposite picture, namely the free nucleon cross section exceeds the “universal curve” and
fissio
ning nuclei one more than in 30%.




Fig.13.

Total photoabsorption cross section for the

actinide nuclei (CEBAF data [
13
])is shown

by solid line. Dotted line corresponds to the

free proton (Armstrong [
2

]), experimental


points are the results for di
fferent nuclei

with A = 7


240 (universal curve).









This figure is taken from paper [
13

] ; the only difference is related to the dotted line. In paper
[
13

] it was calculated taking into
account

the difference between proton and
neutron

photoa
b-
sorption cross section. Now, in accordance with new GRAAL results, there is no difference b
e-
tween proton and neutron, so the free nucleon cross section is used for
comparison
.


High experimental accuracy for actinide nuclei and reference nucleon cross sect
ions a
l-
low to conclude that experimental data can not be explained in frame of the traditional
knowledge. Namely
,

in
Delta

resonance region it is seen that actinide cross section exceeds the
nucleon reference one in 20%, about. This means, probably, that p
hotoabsorption of actinide n
u-
clei is not exhausted by the meson production mechanism only, or there contribute significantly
non linear quantum
electrodynamics

effects like inelastic e+e
-

pair production etc. At first, ind
i-
cation on low energy and momentum

transfer nuclear excitations at intermediate photon energies
was done in [
12

]. Now it can be studied in
details

in closed future basing on modern high
qua
l
ity photonuclear facilities.


As seen in fig.13, above the Del
ta resonance region, situation i
s
contrary. Here the nucl
e-
on cross section exceeds the actinide one in 30%, about. This effect also can not be explained by
the existing knowledge and contradicts with the well known vector dominance model At first,
the photon energy of 1 GeV is too small to

produce significantly vector meson (

,


etc) to pr
o-
vide the shadowing due to
harmonization

of photons. At second, the A dependence would be
seen but all nuclear cross section normalized on the number of nucleons are rge same in this e
n-
ergy region in accordance with the experiment.


So, new
results open new fundamental questions which would be interesting to
study
.


CONCLUSIONS




Now, we summarize new results on the total photoabsorption cross sections which are
important for future developments, including sum
rules
, nucleon
modifications

in

nuclear media,
new mechanisms of nuclear excitations at intermediate energies etc. They are the following:

-

total photoabsorption cross sections for proton and neutron (deuteron target) are identical
within 5%of systematic error bars. F15


resonance nea
r 1 GeV is seen in b
oth cross sections.
This means, probably
that the

free neutron cross section is equal to the free proton one. Also this
indicates existence of the door
-
way states in the first step of photon


nucleon interaction which
is the sane for t
he proton and neutron.

-

Carbon photoabsorption cross section
normalized

on the number of nucleons is in 30% less
then the free nucleon one in studied energy
region
. Fermi correction is not sufficient to explain
modification of cross sections in nuclear m
edium.
In this sense s
pecial attention would be paid to
the heavy (actinide) nuclei.



Author thanks


This work is supported by RFBR, grant
08
-
02
-
00648
-
а



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