Measurement of cross section asymmetry of 12C(,N) and 16O(,N) reactions
below pion photoproduction threshold
D. Burdeinyi, G. Bochek, V. Ganenko, A. Deev, V. Morochovskij
NSC “Kharkov Institute of Physics and Technology", Kharkov 61108, Ukraine
A proposal submitted to the 2012 MAX-lab Nuclear Physics PAC
1 Introduction
The proposal is a part of ongoing MAX-lab program on investigations of the nuclei structure and mechanisms of the
photonuclear reactions and represents development of the approved propositions [1] on studying of the light nuclei
photodisintegration with linearly polarized photons and it is of their practical implementation phase. Now the proposal
is aimed at obtaining experimental data on the cross section asymmetry () of reaction of carbon and oxygen
photodisintegration with one nucleon emission, namely, the reactions:


12
C( γ ,p)11B and 12C( γ ,n)11C
(1)
 14
 15
16
16
O( γ ,p) N and O( γ ,n) O,
(2)
with separation the ground and some low-laying states of the final nuclei. The measurements are planned to be
performed in the photon energy range E~30-70 MeV using linearly polarized tagged photon beam that has been
produced at the MAX-lab facility [2,3].
The cross section asymmetry is defined as
d ||  d 

,
(3)
d ||  d 
where d || and d  are the reaction cross sections for photons polarized, respectively, parallel and perpendicular to
the reaction plane. Because the reaction cross section on unpolarized bremsstralung (d/d) is related with the cross
sections on the polarized photons d || and d  by the relation d/d=( d || + d  )/2, one could simultaneously
obtain information about the reactions cross sections (d/d), as well.
The reactions (1,2) in the intermediate energy range (the energy interval from Giant Resonance till threshold of the
pion photoproduction) when final state of the nuclei are determined have being studied by many authors during many
years, predominantly at MAX-lab and Mainz at photon energies less E~<100 MeV and a large body of experimental
data on such processes cross sections has been obtained with unpolarized photons at E~60 MeV, see, e.g. [4-11] and

reference in their. However, the asymmetry in this energy range has been measured only for the reaction 12C( γ ,p)11B at
Sendai [12] and recently at MAX-lab [13]. In this proposal we plan to extend the asymmetry measurements on the
processes (1,2) and to get information on () at the kinematical conditions that have been realized at MAX-lab with
unpolarized photons: 12C(,p)11B [4,6,7], 12C(,n)11C [5], 16O(,p)15N [8,10], 16O(,n)15O [9].
Polarization observables and the cross section asymmetry, as well, are more sensitive to details of the reaction
mechanisms and nuclear structure. So new data on asymmetry of the reactions (1,2) together with the data on the cross
sections [4-11] will considerably extend possibilities for various theoretical approaches verification and will allow one
to study the reactions mechanisms and sub-nuclear degrees of freedom in the nuclei, e.g., such as meson exchange
current (MEC), more accurately.
1 Motivation
The intermediate energy interval that will be covered in the measurements (also is known as quasi-deuteron (QD))
is, the energy range, where photons incoming to nucleus may interact with one nucleon or with nuclear clusters,
involving two (or more) nucleons. So, from this point of view the (,N) reactions can be produced by some
mechanisms. One of them may be, by analogy with (e,e’p) process, due to direct knockout single nucleon by the photon
from the nucleus (DKO mechanism). Another possible mechanism is, so-called, quasi-deuteron mechanism when the
photon interacts with pn-cluster after which one of the nucleons returns to a bound state of the nucleus.
There are well known arguments on two-body absorption of the photons in the intermediate energy range – (i) the
DKO mechanism is suppressed because there is large-momentum mismatch between the photon and the emitted
nucleon; (ii) the DKO mechanism predicts small ratio of the (,n) and (,p) reaction cross sections, whereas experiment
has shown that cross sections of the (,p) and (,n) reactions on carbon and oxygen nuclei are of the same order of
magnitude and have similar shape of angular distributions of the emitted nucleons, [5,9,11].
The quasi-deuteron models, in which photon absorption takes place predominantly on pn-pairs and then one of the
produced nucleons is reabsorbed by the nucleus, predict a similar behaviour of the (,p) and (,n) cross sections and
satisfactory describe the experimental data [14]. The reaction cross section is factorable function of terms relating to the
photon-pair interaction and the pn-pair momentum distribution, thus the shape of cross section is determined by the pn-
pair momentum distribution and the two-body effects taking into account phenomenologically due to including
experimental d(,p) cross section.
On the contrary to the QD models also more fundamental microscopic models have been developed in which the
nucleus is considered as a system of interacting baryons, and the interactions between nucleons is performed through
meson exchange currents. The microscopic models allow one to divide the total photon absorption into various
contributing processes of one- and two-body absorptions and takes into account effects of NN correlations and meson
exchange currents (MEC). One of the first microscopic models proposed by Gari and Hebach [15] includes meson
exchange current effects through the use of Siegert's theorem, gives a reasonable description of (,p) reactions.
Later another microscopic model (Random Phase Approximation (RPA)) was developed by the Gent group
[16,17]. It was based on a Hartree Fock description of the nucleus where the nuclear excitations are treated as a
combination of particle-hole configurations and MEC effects were considered. These models have been applied for
analysis the existing experimental data on the (,N) reactions and as a whole satisfactory describe the experimental data
on the cross sections. For illustration in Figs. 1,2 it is shown the data on the cross section of the 12C(,N) and 16O(,N)
reactions from Ref. [5,9] obtained at MAX-lab and their detail comparison with theoretical calculations on the base of
RPA model.
Fig. 1. Differential cross sections for 12C(,N) reactions from [5].
Theoretical calculations [17] are shown as a solid line (,n) and a
dotted line (,p), where the calculation is a coherent sum of RPA
and HF+OPEC, the latter is shown for (,n) only as a dashed
line.
Fig. 2. Differential cross section for 16O(,N) reactions from [9].
The solid and dot-dashed curves denote the HF-RPA
calculations for (,n) and (,p), respectively. The dotted and
dashed curves denote the HF calculations for (,n) and (,p),
respectively.
In the figures it is presented the angular distributions of the differential cross sections for 12C(,p)11B and
12
C(,n)11C reactions, Fig. 1, and 16O(,p)15N and 16O(,n)15O, Fig. 2, when final nuclei 11B, 11C, 15N and 15O are in the
ground or in one of the low-lying exited states. The photon energy is E~60 MeV.
As one can see, there is the overall close similarity of the (,n) and (,p) cross sections, which can be natural
confirmation of the phenomenological QD model and strongly suggests that photon absorption on NN exchange
currents is an important mechanism in the intermediate energy region.
In the Fig. 1 it is shown the theoretical calculations of Gent group [17] where RPA and HF+OPEC models are used
to calculate 1h and 2h-lp components, respectively, of the A = 11 states. The HF+OPEC is the contribution from 2h-1p
transition with taking into account one pion exchange current and using the Hartree-Fock wave functions, which is very
small. As a whole the coherent sum of RPA and HF+OPEC contributions reproduce the 12C(,p)11B cross sections with
enough good accuracy, whereas the calculations of the 12C(,n)11C cross sections worse agree with experiment.
Also in spite of the broad similarity between (,n) and (,p) cross sections however there is exist differences in
detail at forward angles, e.g., when the final nuclei are in the ground state. Thus one may suggest that the reaction
mechanism is somewhat different for (,n) and (,p) processes.
In Fig. 2 the curves labeled HF are the calculation of the contribution of the direct-knockout with taking into
account final-state rescattering, that were made using the Skyrme interaction. They produce rather different results for
(,n) and (,p) reactions. The former is far below the data, while the latter, in which quasi-free knockout is significant, is
a factor 2 low the experimental data. However, one can say that the RPA calculation which includes the effects of the
single-nucleon current, the multi-step processes, arising from NN correlations and MEC as a whole rather good
describes the 16O(,p)15N reaction data at photon energy E~60 MeV and somewhat poorer the 16O(,n) process. And
the (,p) strength arises largely from meson exchange currents [8] with a smaller contribution from multi-step processes.
On the contrary for (,n) strength at this energies (E~60 MeV) arises predominantly from final-state rescattering with a
relatively minor contribution from MEC [9].
As known, polarization observables are very sensitive to the reaction mechanisms. A definitive clarification on the

role of the direct mechanism in intermediate energy photoreactions can be obtained directly from an experiment on ( γ ,p) and



( γ ,n) reactions with polarized photons. Calculations [18] performed for 16O( γ ,p0)15N and 16O( γ ,n0)15O reactions at
energies E~60, 80 and 100 MeV have shown quite different behaviour of the cross sections d || and d  for the DKO

mechanism, namely, for the ( γ ,p) reaction the cross section d || is much larger than the d  , that produces large

asymmetry, but in the case of 16O( γ ,n0)15O reaction the asymmetry is small, ~0.

In Fig. 3 it is shown calculations of the angular dependence asymmetry of the reactions 16O( γ ,p0)15N and

16
O( γ ,n0)15O performed at photon energy E~60 MeV on the base of microscopic RPA theory with the Skyrme force
Sk3 for the effective nucleon-nucleon interaction [19]. The exchange-current contributions are implicitly included by
the use of Siegert theorem and only electric dipole and quadrupole transition amplitudes are included in the calculations.

Fig. 3. 16O( γ ,p0)15N and
16O(

γ ,n0)15O angular distribution at E~60 MeV. Upper part: d || (full line) and d  (dashed tine) in
RPA-Sk3, dash-and-dot line is the d || in HF-Sk3. All angular distributions have been divided by a factor of 2. Lower part: angular
dependence of the asymmetry in RPA-Sk3 (full line) and HF-Sk3 (dashed line).


The calculations predict very large asymmetry for16O( γ ,p0)15N and 16O( γ ,n0)15O reactions, ~0.8 in a wide
angular interval 300-1500, similar in form and comparable in magnitude, and the same asymmetric angular trend is
expected in the energy range from 40 to 80 MeV.
The dashed-dotted curve shows calculation obtained without lplh residual interaction (the Hartree-Fock (HF) limit)
which demonstrates very different behaviour of the photoproton and photoneutron asymmetries, if a knockout

mechanism would be acting. The asymmetry of the ( γ ,n0) reaction becomes small, ~0, in accordance with the results

of Ref. [18], whereas asymmetry of the ( γ ,p0) reaction stays large enough although there is noticeably change in the
asymmetry magnitude.
Recently in more detail calculations of the asymmetry of the single proton emission from 12C, 16O and 40Ca nuclei
have been presented in [21] for the photon energies E80 MeV. The calculations were performed on the base of a
nuclear model developed by authors to investigate electromagnetic excitations of the nucleus in inclusive single and
double coincidence experiments. The starting point of the approach is the continuum shell model implemented with the
optical potential to take into account the final state interaction (FSI), and using this model to describe nuclear excited
states and shirt range correlations (SRC). The MEC were also included by considering one-pion exchange diagrams,
including the seagull and pionic terms and Δ currents. Although the calculations were produced and photon energy
some more than discussed before E~60 MeV the main features of the reactions should be similar for both energies and
can use the predictions for estimation possible effects related with final state interaction (FSI) and MEC effects.

In Figs. 4 it is presented the angular dependences of asymmetries of the ( γ ,p) reactions at the 12C, 16O and 40Ca nuclei
for photon energy E=80 MeV that have been calculated by using different optical potentials (Schwandt et al. (Sc),
Comfort and Karp (CK), Abdul-Jalil and Jackson (AJ), for detail see references in [21]) to describe the emitted proton
wave function, and Woods-Saxon potential (WS) was considered for the hole states. These calculations have been done
by considering one-body (OB) currents only, and without taking into account shirt range correlations (SRC) of
nucleons. One can see that all the calculated asymmetries are large ~0.6-0.8 in the angular interval 300-1500 and
having in general similar behaviors for all potential there are also differences in the detailed structure of the asymmetry
angular distributions between the various potentials.

Fig. 4. Asymmetry angular distribution from [21] of the ( γ ,p)
reactions for target nuclei 12C, 16O and 40Ca and the hole states of
the remaining nuclei, calculated by using OB currents only with
various optical potentials: the Sc potential (solid), the CK
potential (dotted), the AJ potential (dashed), the real WS
potential (dashed-dotted).
Fig. 5. Asymmetry angular distributions from [21] resulted from
various MEC terms contribution: OB currents only (thin solid
lines), the seagull and pionic currents (dashed-dotted lines), the
Δ currents with the coupling constants: fN=0.299, fN=1.69
(thick full lines), fN=0.373, fN=2.15 (dashed lines), fN=0.12,
fN=2.15 (dotted lines). The Sc potential is used.
It should be noted hat the shape of angular distributions of the asymmetries is characteristic of the hole states
angular momentum. In Fig. 4 the results obtained for the 1p3/2 state of 16O and 12C nuclei, have similar structures, but
they are very different from those of the 1p1/2 state for 16O and these effects could be able to test by asymmetry
measurements. Results of the calculations of the angular distributions of asymmetries resulted from various MEC terms
contributions are presented in Fig. 5. They show that the seagull and pionic terms of the MEC produce small effects. On
the contrary, the effects of the Δ currents are remarkable. These effects of the Δ currents are very sensitive to the values
of the coupling constants and become more evident at large emission angles, where including the Δ currents change
asymmetry sign. The energy dependence of the MEC relative effects are shown in Fig. 6 where it is presented
asymmetries calculated at θ=1200. Calculations show that even at photon energy the E=60 MeV the including of the
MEC with the Δ currents strongly modified the asymmetry.
2,0
1,8
1,6
Asymmetry
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
30
40
50
60
70
80
90
E MeV

Fig. 6. Energy dependence of asymmetries of the ( γ ,p) reactions
for the proton emission angle θ=1200, from [21]. The full thin
lines have been obtained with the OB currents only, the dotted
lines by adding the SRC, the dashed lines by including MEC and
the thick continouse lines by considering both MEC and SRC.
The dashed-dotted lines include OB, seagull and pionic currents
and SRC.
Fig. 7. Cross section asymmetry of the 12C(,p0) reaction at
proton emission angle θp=900. Circles are the data obtained at
MAX-lab [13], squares are the data [12]. The lines are the
predicted asymmetry from the [12], black – RPA theory
calculations, the red - the quasi-deuteron model.
So, the calculations [21] show that asymmetries are extremely sensitive to the presence of MEC, in particular to the
Δ currents, which produce both quantitatively and qualitative modifications of the angular distributions. Measurements
of this observable would provide clean information about MEC in medium-heavy nuclear systems.

At present there are only two experiments [12,13] where asymmetry for the reaction 12C( γ ,p) are measured at
photon energies between 40 and 70 MeV and proton emission angle θp=900, Fig. 7. The experiments demonstrate large
asymmetry of the reaction, ~0.8, that is agreed with the calculation on the base of the RPA theory [19], where the hole
states of the residual nucleus are assumed to be in the 1p 3/2 and 1s1/2 shells.

Any measurements on the ( γ ,n) process in the intermediate energy range are absent. It is interesting question if


the asymmetry of the ( γ ,p) and ( γ ,n) process are the similar as the cross sections
3. Experimental technique
The measurements are planned to carry out using the tagged polarized photon beam of the MAX-lab facility [2,3]
and at the same kinematical conditions that were in the previous MAX-lab experiments on measurements the cross
sections of the (,p) reaction [6,7,8,10] and (,n) reactions [5,9]. The asymmetry for the (,p) and (,n) reactions will be
measured simultaneously for each nucleus, using different thickness of the targets for these channels. Because one can
get the cross sections from the measured reaction’s yields one can control results on asymmetry.
The protons are detected under angles 600, 900 and 1200 by CsJ/SSD detectors, the neutrons under the same angles
by the liquid scintillation Nordball detectors. Scheme of the detectors lay out is shown in Fig. 8.
Neutron
detectors
Proton
detectors
Veto
detector
s
Photon
beam
Target
(,p)
SSD1
SSD1
Target
(.n)
CsI
Fig. 8. Scheme of the detectors lay out.
polarizzation
The targets parameters and the detector placement will be taken similar to used in previous MAX-lab experiments
to provide energy resolution value being in the previous MAX-lab experiments that was enough for separation ground
state and first exited states of the final nuclei.
Photon beam. The polarized photon beam will be produced due to coherent bremsstrahlung (CB) of electrons with
energy E0~200 MeV in a diamond crystal 0.1 mm thick
0,8
[2,3]. The main tagger with SAL 62 channel hodoscope
0,7
will be used. It provides energy resolution
0,6
~1 MeV/channel and covers the photon range E~220,5
78 MeV at one (340) setting that allows one to accept all
0,4
energy interval of the measurement.
0,3
At the crystal orientation, when the point (022) gives
0,2
0,1
main contribution to the coherent cross section, expected
0,0
polarization is shown in Fig. 9 for collimator hole 4 mm
-0,1
(the collimation angle c~0.4). =mc2/E0, m is the
-0,2
electron mass. The measurements will be produced at two
-0,3
coherent peak positions, E,d=40 and 60 MeV. So, the
-0,4
10 20 30 40 50 60 70 80 90 100 110 120
photon polarization in the coherent maximum at energy
E, MeV
E,d~60 MeV is expected to be P~0.4 for the collimator
4 mm, and it decreased up to P~0.23 at the collimator
Fig. 9. Expected polarization as a function of CB peak position.
E0=192.66 MeV, collimator 4 mm (collimation angle c~0.4.), 12 mm (collimation angle c~1.2), at the E,d~40 MeV
diamond 0.1 mm. Calculations were produced by ANB code [21] P~0.52 and 0.25 for collimator 4 and 12 mm, respectively
The tagging efficiency at the collimator holes 12 mm and 4 mm are ~0.35 and 0.12, respectively. If it is restricted
by the counting rate of the tagging array ~0.5106 s1 channel1 we can get the photon beam intensity on the target
dN/dE~1.75105 /s/MeV and ~0.6105 /s/MeV, respectively.
The photon polarization will be controlled in the course of the measurements with help of reaction of the deuteron
d(,p)n disintegration. At that the target CD2 target 1 mm thick will be used.
Targets. We plan to use a carbon graphite target (A=12.01, =2.21 g/cm3) 0.5 mm thick placed under angle m=450


to the photon beam for 12C( γ ,p)11B reaction measurements and the carbon target 5 cm thick for 12C( γ ,n)11C reaction.
The targets provide a number of the nuclei per cm2, respectively:
NC=LNA/Acosm0.052.216.0221023/12.01/0.7071 0.7841022nucl/cm2 for the (,p) channel;
NC=LNA/A52.216.0221023/12.015.5411023nucl/cm2 for the (,n) channel.

The H2O targets (A=18.01, ~1.0 g/cm3) will be, respectively, 0.5 mm thick for measurements 16O( γ ,p)15N reaction

and 9 cm thick for 16O( γ ,n)15O reaction. These targets provide a number of the nuclei per cm2, respectively:
NO=LNA/Acosm0.051.06.0221023/18.01/0.70710.23641022nucl/cm2 for the (,p) channel;
NO=LNA/A91.06.0221023/18.013.0091023nucl/cm2 for the (,n) channel.
Detector system.
Proton detectors. The protons will be detected by two SSD/CsI telescopes. The SSD/CsI telescope consists of two
single-sided silicon strip (E) detectors and a CsI counter functioned as (E) detector and was used in the previous
experiment [13]. The protons were identified using standard E-E technique. SSD detectors have an active diameter
64 mm and thickness 500 mkm, 64 strips on each side with a width of 1 mm. It will be placed on the distance ~15 cm
from the target and will cover the solid angle ~250 msr. One telescope will be used for measurements under angle
θ=1200, another under angles 0=600 and 900.
One can use instead of the second SSD/CsI any other E-E telescope with approximately similar parameters.
The neutron detectors. The neutrons will be detected with 16 liquid scintillator neutron Nordball detectors [22].
Ten detectors are of hexagonal cross section and have volume Vd=3.3 l, six detectors are pentagonal and have volume
Vd=2.6 l. The detectors thickness is 16 cm, thus the squares of the front surface of the detectors are S~3300/16=206.25
and 162.5 cm2 for hexagonal and pentagonal detectors, respectively.
The detectors will be combined into 2 blocks (9 detectors in one block A and 7 in the other, block B) which will be
placed on the distance in accordance with the distances being in the experiments [5,9] to provide energy resolution ~12 MeV. The A detector will be placed under angle 120 0 to the photon beam on the distance ~3.2 m from the target and
will cover solid angle ~9206.25/3202~ 0.018 sr, the block B will be placed under angles 60 0 and 900 on the distance
~3.2 m and will cover solid angle ~7162.5/3202~ 0.011 sr.
The expected neutron detection integral efficiency is n~0.15 for.
The gamma-quanta background will be suppressed by the pulse shape discrimination (PSD) technique.
4. Estimation of the counting rate
The asymmetry is determined from the relation

1 N ||  N  ,
P N ||  N 
(4)
where P is the photon beam polarization, N|| and N are the reaction yields in the planes parallel and perpendicular to
the photon polarization. The expected proton counting rate has been calculated using the formula,
NN=(d/dN)NTN(dN/dE)Eliv
with following values of the parameters for the reactions with proton emission:
-NT0.7841022nucl/cm2 for the 12C target L=0.5 mm thick;
-NT0.23641022nucl/cm2 the 16O target L=0.5 mm thick;
- intensity of the tagged photon beam on the target:
-for collimator 4 mm (the tagging efficiency n~0.12) is dN/dE~0.6105 /s/MeV;
-for collimator 12 mm (the tagging efficiency n~0.35) dN/dE~1.75105 /s/MeV;
- photon energy interval E=4 MeV;
-the solid angle of the proton telescope is 0.25 sr;
-liv~0.8 is the live time.
(5)
The expected neutron counting rate has been calculated using the relation
NN=(d/dN)NTN(dN/dE)Enliv
Where:
-NT5.5411023nucl/cm2 for the 12C target L=5 cm thick;
(6)
-NT3.0091023nucl/cm2 for the 16O target L=9 cm thick;
- the solid angle of the block A is ~0.018 sr and block B ~0.011 sr;
- the neutron detector’s efficiency n~0.15;
-liv~0.8 is the live time.
The measurements are planned to perform in two stages, during the first one the reactions



12
C( γ ,n0)11C are measured, in the second stage the reaction 16O( γ ,p0)15N and 16O( γ ,n0)15O.

C( γ ,p0)11B and
12


1st stage. The reaction 12C( γ ,p)11B and 12C( γ ,n)11C measurements
With the mentioned above parameter’s values the reaction’s yields are:

for 12C( γ ,p0)11B, L=0.5 mm,
Np1.354(d/dp) p/mkb/hour for collimator 4 mm
Np3.949(d/dp) p/mkb/hour for collimator 12 mm,
(7)
(8)

for 12C( γ ,n0)11C, L=5 cm
block A
Nn1.034(d/dn) n/mkb/hour for collimator 4 mm
Nn3.016(d/dn) n/mkb/hour for collimator 12 mm,
(9)
(10)
block B
Nn0.632(d/dn) n/mkb/hour for collimator 4 mm
(11)
Nn1.843(d/dn) n/mkb/hour for collimator 12 mm,
(12)
where (d/dp) is in mkb units. Expected values of the counting rate, statistics and the data accuracy are presented in
the Table 1. The reaction cross sections were taken from [5,9], polarization was calculated with ANB code [21]. The
measurements will be produced at two coherent peak positions, E ,d=40 and 60 MeV.

Table 1. Estimation of the needed beam time for the 12C( γ ,N) reaction for the collimators 4 and 12 mm.
N||+N is the total statistics for two directions of the beam polarization.
E,d=60 MeV

C( γ ,p0)11B, L=0.5 mm
12
,
deg
40
60
90*
120
Coll,
mm
4
12
4
12
4
12
4
120
Np
/h/mkb
1.354
3.949
1.354
3.949
1.354
3.949
1.354
3.949
d/d
mkb/sr
21
27
10
2
Np
hour1
28.43
82.93
36.56
106.6
13.54
39.49
2.708
7.898
12C(
,
deg
40
60
90
120
Coll,
mm
4
12
4
12
4
12
4
12
Np
/h/mkb
0.632(B)
1.843(B)
0.632(B)
1.843(B)
0.632(B)
1.843(B)
1.034(A)
3.016(A)
T,
hours
50
50
40
40
80
80
120
120
N||+N
counts
1422
4146
1462
4264
1083
3159
325
948
P


0.4
0.23
0.40
0.23
0.40
0.23
0.40
0.23
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.052
0.052
0.052
0.059
0.061
0.068
0.110
0.125
P


0.40
0.23
0.40
0.23
0.40
0.23
0.40
0.23
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.071
0.080
0.063
0.070
0.103
0.116
0.104
0.12
P


0.52
0.5
0.039

γ ,n0)11C, L=5 cm
Np,
T,
N||+N
hours
hour1
counts
13.27
60
796
38.71
60
2323
27
17.06
60
1024
49.77
60
2986
10
6.32
60
379
18.43
60
1106
2
2.068
180
372
6.032
180
1086
Total beam time for E,d=60 MeV, T=180 hours
d/d
mkb/sr
21
*Measurements under this angle have been performed.
Table 1 (continue)
E=40 MeV

12C(
γ ,p0)11B, L=0.5 mm
,
deg
40
Coll,
mm
4
Np
/h/mkb
1.354
d/d
mkb/sr
50
Np
Hour1
67.7
T,
hours
20
N||+N
counts
1354
60
90*
120
,
deg
40
60
90
120
12
4
12
4
12
4
12
3.949
1.354
3.949
1.354
3.949
1.354
3.949
Coll,
mm
4
12
4
12
4
12
4
12
Np
/h/mkb
0.632(B)
1.843(B)
0.632(B)
1.843(B)
0.632(B)
1.843(B)
1.034(A)
3.016(A)
66.56*
40.68*
16
197.5
90.12
262.9
55.08
160.7
21.66
63.19
20
3950
15
1352
15
3944
25
1377
25
4016
50
1083
50
3160

12C(
γ ,n0)11C, L=5 cm
Np,
T,
N||+N
hours
hour1
counts
31.6
30
948
92.15
30
2765
66.56
42.07
30
1262
122.69
30
3681
40.68
25.71
50
1286
74.99
50
3749
16
16.54
80
1323
48.25
80
3859
Total beam time for E,d=40 MeV, T=110 hours
d/d
mkb/sr
50
0.25
0.52
0.25
0.52
0.25
0.52
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.055
0.039
0.055
0.039
0.056
0.043
0.062
P


0.52
0.25
0.52
0.25
0.52
0.25
0.52
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.046
0.067
0.040
0.057
0.040
0.057
0.039
0.056
*Measurements under this angle have been performed.

The beam time for taking statistics for the 12C( γ ,N) reaction is 110 hours for measurements at E,d=40 MeV and 180
hours for measurements at E,d=60 MeV or total beam time is 290 hours (2.5) weeks.


2d stage. The reactions 16O( γ ,p0)15N and 16O( γ ,n0)15O measurements
The expected reaction yield is

for 16O( γ ,p0)15O, L=0.5 mm

for 16O( γ ,n0)15O, L=9 cm
block A
Np0.408(d/dp) p/mkb/hour for collimator 4 mm
Np1.191(d/dp) p/ mkb/hour for collimator 12 mm,
(13)
(14)
Nn0.562(d/dn) n/mkb/hour for collimator 4 mm
Nn1.638(d/dn) n/mkb/hour for collimator 12 mm,
(15)
(16)
Nn0.343(d/dn) n/mkb/hour for collimator 4 mm
Nn1.001(d/dn) n/mkb/hour for collimator 12 mm,
(17)
(18)
block B

Table 2. Estimation of the needed beam time for the 16O( γ ,N) reaction for the collimators 4 and 12 mm.
E,d=60 MeV

O( γ ,p0)15O, L=0.5 mm
Np,
T,
N||+N
hours
counts
hour1
4.896
50
245
14.29
50
715
4.08
50
204
11.9
50
595
1.836
100
184
5.355
100
536
0.816
120
98
2.38
120
286

16
O( γ ,n0)15O, L=9 cm
Np,
T,
N||+N
hours
counts
hour1
16
,
deg
Coll,
mm
Np
/h/mkb
40
4
12
4
12
4
12
4
12
0.408
1.191
0.408
1.191
0.408
1.191
0.408
1.191
,
deg
Coll,
mm
Np
/h/mkb
d/d
mkb/sr
40
4
12
4
12
4
0.343B
1.001B
0.343B
1.001B
0.343B
7
60
90
120
60
90
d/d
mkb/sr
12
10
4.5
2
10
4.5
2.401
7.00
3.43
10.01
1.544
60
60
50
50
100
144
420
172
500
154
P


0.40
0.23
0.40
0.23
0.40
0.23
0.40
0.23
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.128
0.144
0.14
0.158
0.147
0.166
0.202
0.228
P


0.40
0.23
0.40
0.23
0.40
0.5
0.5
0.5
0.5
0.5
0.168
0.189
0.152
0.172
0.167
120
12
4
12
1.001B
0.563A
1.638A
,
deg
Coll,
mm
Np
/h/mkb
40
4
12
4
12
4
12
4
12
0.408
1.191
0.408
1.191
0.408
1.191
0.408
1.191
,
deg
Coll,
mm
Np
/h/mkb
40
4
12
4
12
4
12
4
12
0.343(B)
1.001(B)
0.343(B)
1.001(B)
0.343(B)
1.001(B)
0.562(A)
1.638(A)
60
90
120
60
90
120
2
4.502
100
450
0.23
1.126
120
135
0.40
3.284
120
394
0.23
Total beam time for E=60 MeV, T=210 hours
d/dp,
mkb/sr
45*
45
35
10
d/dp,
mkb/sr
45*
45
35
10
Table 2 (continue).
E,d=40 MeV

16
O( γ ,p0)15O, L=0.5 mm
Np,
T,
N||+N
hours
counts
hour1
18.36
50
918
53.55
50
2678
18.36
50
918
53.55
50
2678
14.28
70
999.6
41.65
70
2916
4.08
120
490
11.91
120
1428

16
O( γ ,n0)15O, L=9 cm
Np,
T,
N||+N
hours
hour1
counts
15.44
70
416
45.02
70
1213
15.44
70
416
45.02
70
1213
12.05
50
625
35.01
50
1833
5.62
120
674
16.38
120
1967
0.5
0.5
0.5
0.181
0.172
0.190
P


0.52
0.25
0.52
0.25
0.52
0.25
0.52
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.0.47
0.067
0.0.47
0.067
0.045
0.065
0.064
0.093
P


0.52
0.25
0.52
0.25
0.52
0.25
0.52
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.070
0.100
0.070
0.100
0.057
0.082
0.055
0.079
Total beam time for E,d=40 MeV, T=190 hours
*Cross section has been obtained by extrapolation.

The beam time for taking statistics for the 16O( γ ,N) reaction is 190 hours for measurements at E,d=40 MeV and
210 hours for measurements at E,d= 60 MeV, so total beam time is 400 hours or 3.5 weeks if four angle will be
measured.
For three angles 600, 900 and 1200 the needed beam time is 120 hours for measurements at E,d=40 MeV and 120
hours for measurements at E,d=60 MeV, so full beam time is 240 hours or 2 weeks. One week is needed for background
measurements from empty target for estimation the contribution from the target container wall and . So, total beam time

for the 16O( γ ,N) reaction is 3 weeks.
The sum beam time for both reactions is ~6 weeks.
Summary




1. The new data on the cross section asymmetry of the 12C( γ ,p)11B, 12C( γ ,n)11C, 16O( γ ,p)14N and 16O( γ ,n)15O will
be obtained in the energy range 40-60 MeV and angles of the nucleon emission 400, 600, 900 and 1200. Data on the
asymmetry in this energy range are practically absent.


Simultaneous measurements of the ( γ ,p) and ( γ ,n) reaction asymmetry will decrease the systematic errors of the
relative value of these processes asymmetry.
2. The measurements will be produced at the same kinematical conditions where the cross sections of the reactions
have been produced earlier at the MAX-lab, so the new data on the cross sections of these reactions will add to the
existing MAX-lab data and increase the cross section data accuracy.
3. The new data will allow one to test existing theoretical approaches in more detail and study reaction mechanisms
in more detail, as well, particularly, data on the asymmetry at large angles of the nucleon emission are very sensitive to
the MEC and will allow one to study non-nucleon degrees of freedom in the nuclei.
1.
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