Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 38 (2005)

8(LNS Experiment : #2371, #2397, #2413, #2433)

Photoproduction of η-mesons off C and Cu Nuclei
for Photon Energies below 1.1 GeV
T. Kinoshita1, H. Yamazaki1, H. Fukasawa1, K. Hirota1＊ , T. Ishikawa1, J. Kasagi1,
A. Kato1†, T. Katsuyama1, K. Kino1‡, F. Miyahara1, T. Nakabayashi1, K. Nawa1,
K. Okamura1, Y. Saitoh1, K. Satou1, M. Sengoku1, H. Shimizu1, K. Suzuki1,
S. Suzuki1, T. Terasawa1, H. Kanda2, K. Maeda2, T. Takahashi2§, Y. Aruga3,
T. Fujinoya3, A. Iijima3, M. Itaya3, Y. Ito3, T. Iwata3, H. Kato3, T. Kawamura3,
T. Michigami3, M. Moriya3, T. Sasaki3, Y. Tajima3, S. Takita3, T. Noma3,
M. Yamamoto3, H. Y. Yoshida3, Y. Yoshida3, O. Konno4, T. Maruyama5, and T. Yorita6
1

Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826
2

3

Department of Physics, Tohoku University, Sendai 980-8578
Department of Physics, Yamagata University, Yamagata 990-8560
45
6

Ichinoseki National College of Technology, Ichinoseki 021-8511

College of Bioresource Sciences, Nihon University, Fujisawa 252-8510

Japan Synchrotoron Radiation Research Institute, Mikazuki 679-5198

The η-meson photoproduction cross sections have been measured on C and Cu targets for photon
energies between 600 and 1100 MeV to investigate the behavior of the S11(1535) resonance in a nucleus.
The excitation functions of the cross section as well as angular and momentum distributions ofη-mesons are in quantitative agreement with Quantum Molecular Dynamics (QMD) model
calculations, in which the η-meson emission processes other than the S11(1535) resonance are also
incorporated as proposed in the η-MAID model. It is shown that the excitation of the D15(1675)
resonance might play an important role for Eγ＞ 900 MeV.

§1. Introduction
The behavior of hadrons in the nuclear medium is one of the most intriguing topics in hadron andnuclear physics. Photon induced reactions are advantageous to producing hadrons deeply inside a
nucleus because photons are hardly absorbed. Modiﬁcations in appearances may always be observed.
Most of them originate simply from the basic effects of the nuclear medium, such as the Fermi motionof nucleons, Pauli blocking of the ﬁnal state and collisions with nucleons. In addition, an interesting
possibility has been proposed; i.e., mass modiﬁcation arising from partial restoration of chiral symmetry

Present address : RIKEN, Wako 351-0198＊†

Present address : Japan Cycle Development Institute, Tokaimura 319-1194‡

Present address : Center for Nuclear Study (CNS), University of Tokyo, Wako 351-0198§

Present address : High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801

9in the nuclear medium [1, 2]. The effects of the mass change of the ρ-meson have been studied inρ photoproduction on nuclear targets [3, 4] as well as in the hadron reactions [5]. However, masschange of baryons has not been studied well except for the Δ resonance [6].
The S11(1535) resonance is proposed to be a candidate of the chiral partner of the ground state
nucleon, and its resonance energy is expected to shift down by about 100 MeV in the nuclear medium
where chiral symmetry is partially restored [2]. The S11(1535) resonance is known to decay into the Nηchannel with a large branching ratio of 30-55% [7], while other nucleon resonances in this energy region
hardly decay to the Nη channel. Therefore, the excitation and decay of the S11(1535) resonance is adominant feature of the η photoproduction off the nucleon in the region of photon energies below 1000
MeV [8, 9]. It is, thus, expected that the properties of the S11(1535) resonance in the nuclear medium
can be studied through the η photoproduction off nuclei.
The measurements of A(γ,η) reactions have been reported by Röbig-Landau et al. on C, Ca, Nb and Pb
for Eγ＜ 800 MeV [10], and by Yorita et al. on C, Al and Cu for Eγ＜ 1000 MeV [11]. In both measurements,
the S11(1535) resonance is clearly observed in the excitation function, which can be reproduced by acalculation taking into account the basic effects of the nuclear medium with parameters deduced fromthe total cross section of the γp → ηp reaction. It seems, however, that the success of the interpretationof the A(γ, η) reaction with the S11(1535) resonance alone is partially due to the lack of the quality inthe previous data for Eγ＞ 800 MeV [11] as well as those of the γp →ηp reaction.
In the last several years, there were essential progresses in experimental and theoretical works on
the reaction. For the experimental side, precise measurements for Eγ＞800 MeV have improved considerably
the available data base [12-18] This led theoretical analyses to be more reliable for including
contributions of all the resonances in this energy region as well as direct η production processes. Of
particular interest is the fact that both of the analyses performed by Saghai et al. [8] and by Chiang et al.
[9] have come to the same conclusion that another S11 resonance, S11(1650), also contributes in the total
cross section of the γp →ηp reaction in such a way that the two S11 resonances interfere destructively.
All these arguments raised the interest to study the behavior of the S11 resonance again bymeasuring A(γ, η ) reactions with improved quality for the photon energies higher than 800 MeV. In
this letter, we present the experimental results and compare them with calculations based on the
Quantum Molecular Dynamics model (QMD) which is improved so as to include other processes thanthe S11(1535) resonance.

§2. Experimental Procedure
The experiment was performed at the Laboratory of Nuclear Science (LNS) in Tohoku University
by using tagged photon beams from the 1.2 GeV Stretcher-Booster Ring [19]. Two series ofmeasurements were carried out in different setups: the first one at the photon beam line 1 in theexperimental hall 2 and the second at the photon beam line 2 in the GeV-γ experimental hall. The
former tagging system is described in detail in Ref. [20] and a part of data obtained in the ﬁrst series
was reported in Ref. [21]. Photon beams of the same quality can be used at both beam lines. In thepresent work, the photon energy was covered from 600 to 850 MeV with Ee ＝ 920 MeV and from 800 to

10

4

10

3

events

10

10

2

10

1

0

200

400

600

800

invariant mass [MeV]1000

Fig.1. Invariant mass spectrum reconstructed from two photons. The solid line represents theexperimental data and the dotted line the result of the simulation.
1120 MeV with Ee ＝ 1200 MeV. The total tagged photon intensity was about 107 Hz with a duty factor
of about 80%. The size of the beam at the target position was about 6 mm (rms). The targets used were
C and Cu with thicknesses of 40 and 5 mm, respectively.
Two photons from an η-meson were detected by an electromagnetic calorimeter consisting of 206
pure CsI crystals with plastic veto counters. The shape of the crystal is truncated-trapezoidal with ahexagonal cross section and its thickness is 30 cm for 148 pieces (type-A) and 25 cm for 58 pieces (type-B);
the performance of the type-B is described in detail in Ref. [22]. In the ﬁrst series of the measurements,
they were assembled to 6 blocks and placed on three turn tables to change detector positions as reported
in Ref. [22]. In the second series, they were rearranged to 4 blocks placed in such a way that two＜φ＜17°
forward blocks covered angles 15° ＜θ＜ 72° with respect to the beam direction and angles－17°
withrespect to the horizontal plane and two backward blocks 95°＜θ＜ 125and －12°＜φ＜ 12°for both sides ofthe beam direction. The different arrangements of crystals served to check the acceptance of thedetection system.
All the data were collected using a similar data acquisition system as reported in Ref. [11]. In thepresent work, the main trigger for the data acquisition required at least one signal from the tagging
counters and two signals from the CsI detectors. The maximum counting rate of a CsI detector was
about 10 kHz and that of a tagging counter was about 200 kHz. The dead time of the data taking wasabout 8%. A time resolution for e-γ coincidences of 800 ps (FWHM) was achieved and the chancecoincidence ratio was about 3%.
The η-mesons were identiﬁed via their two photon decay with an invariant mass analysis. In Fig.1,
the invariant mass spectrum (Mγγ) measured in the present work is shown by the solid line. Two prominent
peaks corresponding to π0 and ηmesons are clearly seen on the continuum background, which is considered
to originate mainly from multi π0 events. We simulated the two π0 production process by the Monte
Carlo simulation. The result is shown by the dotted line in Fig.1. The shape is well fitted with an
exponential function, exp(aM2 γγ＋ bMγγ). In order to deduce double differential cross sections, d2σ/dθ/dp,
the invariant mass spectrum was constructed for the polar angle from 0 °to 110 °by 10 °steps and forthe momentum from 0 to 1100 MeV by 100 MeV steps. The yield of η-mesons in each spectrum was11deduced by subtracting the background events in the η mass region, which were estimated with thefunction ﬁtted to the continuum for each bin of the incident photon energy and the η-meson polar angle
and momentum. Absolute cross sections were deduced by taking into the account a thickness of thetargets, counts of tagging counters, a tagging efﬁciency, a geometrical acceptance and a branching ratio(η →γγ) [7]. The tagging efﬁciency was measured with a total absorbing lead glass detector positioned in
the direct beam. The geometrical acceptance of the detection system was calculated by the Monte Carlosimulation based on GEANT3 [23]. The systematic uncertainties of the overall normalization come fromthe photon flux (1%), the background determination (5%) and the geometrical acceptance (5%).
Consequently the overall systematic uncertainty is 7%.

§3. Results and Discussion
Differential cross sections of the (γ,η) reaction were deduced for the polar angles from 0°to 110°with
respect to the photon beam direction by integrating the double differential cross sections. We show
excitation functions of the η photoproduction cross section, which were deduced by integrating
differential cross sections for 0°＜θ＜ 110°, on C and Cu targets in Fig.52(a) and 2(b), respectively. Missing
yields for θ＞ 110 °were estimated to be 2% of the integrated values at most, and the total cross sectionin the present work is the angle integrated one. For comparisons, also plotted are the previously
reported data on C indicated with open squares up to 800 MeV [10] and with open circles up to 1000
MeV [11] and on Cu with open circles up to 1000 MeV [11]. It can be said that the present data and thereported ones are in good agreement. Moreover, the statistical accuracy is much improved for thephoton energies higher than 800 MeV. The shape of the total cross sections for C and Cu is quite similar
as expected. The cross section increases rapidly from the threshold energy (561 MeV for C and 550 MeV
for Cu), shows a broad bump structure which has the maximum at around 850 MeV, and gradually
decreases as the photon energy increases. This trend has been known from the previous investigations[11] to be basically due to the excitation of the S11(1535) resonance in a nucleus. The present data for C
and Cu may serve for detailed comparisons with model calculations.
In Fig.2(c), ratios of the cross section of Cu to that of C (σCu /σC ) are plotted against the photon
energy. One can roughly say that the observed η-mesons are mainly emitted from the surface region of
the nucleus and those emitted in the deeper region are absorbed in the nuclear medium, since the ratios
are close to 3.05 (the dotted line), corresponding to the ratio of A2/3 for Cu to C. However, there exist
non-negligible and systematic deviations from the A2/3 dependence for photon energies larger than 800
MeV; the ratio becomes about 3.5 at about 900 MeV. This requires more careful and detailed analysis.
In order to explain the present data, we have performed a QMD model calculation in a different way
from the previous one [11] as follows. At ﬁrst, the proton and the neutron are treated independently so as tosee the effect of the difference of the elementary cross sections forγp →ηp and γn →ηn. This modiﬁcation isnecessary, because a rather large difference between the total cross sections of γp →ηp and γn →ηn has
been predicted by the unitary isobar model, η-MAID [9]. The simple relationσ(γn →ηn) /σ(γp →ηp)＝2/3,
established empirically for Eγ＜800 MeV [25] and used in the previous model calculations [11, 24], might not
be correct at the higher energy region. Secondly, the effect of the interference between two S 11

12
100

(a) C(γ,η)

60

20

ab

17.5

40
20

3

(a) γp→ηp

15

1
2

4

QMD input
MAID, full calculation
single resonance

12.5

σ[µb]

σ[µb]

80

0
300 (b) Cu(γ,η)

Mainz
GRAAL
Bonn

10

σ[µb]

7.5
5

200

ab

2.5

100
3

0
20

1
2

4

17.5
15

4
3

QMD input
MAID, full calculation
MAID, double S11
MAID, D15(1675)

10
7.5
5

2
1

(b) γn→ηn

12.5

σ[µb]

σCu / σC0
5 (c) ratio

2.5

600

700

800

900

1000 1100 1200

0Eγ[MeV]

Fig.2. Cross sections ofηphotoproduction measuredon C(a) and Cu(b). The data measured
in the present work are indicated by thesolid circles, those at KEK [11] by theopen circles and at Mainz [10] by thesquares. The solid line is the result of thepresent QMD calculation. Contributions ofvarious processes are plotted by the dottedlines labeled with numbers; label 1 forthe double S11 resonance, label 2 for theP11(1710), label 3 for the D15(1675), andlabel 4 for the direct processes. The dotdashed lines are contributions due toprotons (label a) and neutrons (label b).
The calculation in the previous work by
Yorita et al. [11] is plotted by the dashedline. (c) Ratio of the cross section of theCu(γ, η) reaction to that of the C(γ, η)
reaction. The dashed line shows the ratioof A2/3 (A: mass number) and the solidline is the result of the QMD calculation.

Fig.3.

700

800

900

1000

Eγ[MeV]

1100

1200

Excitation functions of cross sections forthe elementary reaction by calculation
based on the η-MAID: (a) the γp → ηpreaction and (b) theγn→ ηn reaction. The
solid lines are used in the present QMD
calculation and the dashed lines are theresults of the full η-MAID including all
the resonances and the direct process.
In (a), the excitation function used in
the previous work by Yorita et al. [11] isalso plotted by the dotted line as well as
experimental data from Mainz [12], GRAAL[16] and Bonn [18]. In (b), contributions of
the two resonances, S11(1535) and S11(1650),
destructively interfered, are shown by the
dot-dashed line, while that of theD15(1675) is shown by the dotted line.

resonances in this energy region, S11(1535) and S11(1650), is included in the calculation as the form of
the cross section. The reason of this modiﬁcation is as follows. Saghai et al. [8] and Chiang et al. [9] analyzed
the total cross section of the γp →ηp reaction. They came to the same conclusion that the experimentallyobserved cross section below 1100 MeV is not only due to the S11(1535) resonance but also due to theS11(1650), and both resonances make a destructive interference in the cross section of η production off
the nucleon. In the present QMD calculation, it is impossible to treat directly the transition amplitudes,
and, thus, the calculated cross section including a destructive interference term is incorporated instead13of an incoherent sum of two single resonances which cannot reproduce the experimental data well.
In Fig.3, the total cross sections of the N(γ, η) reaction are shown in order to explain input quantities
to the present QMD calculations. The experimental cross sections of the γp → ηp reaction are shown in
Fig.3(a); data plotted with circles from Ref. [12], triangles from Ref. [16] and squares from Ref. [18].
Also shown are the results of the η-MAID calculation, on which we have based for the elementary crosssection. The characteristic feature of the experimental data is a broad peak followed by a ﬂat region
with a small dip at 1010 MeV. The single resonance excitation can reproduce only the broad resonanceshape but fails to reproduce the dip and ﬂat as indicated by the dotted line, which corresponds to theelementary cross section used in our previous analyses [11]. The dashed line is the result of the full ηMAID calculation which includes all the resonances in this energy region with direct η production
processes. As mentioned above, the essential point is the destructive interference of the S11(1535) andthe S11(1650) resonances, which reproduces the dip and ﬂat behavior very well. Another non-negligible
process is the excitation of the P11(1710) resonance, which slightly contributes to the ﬂat region around1100 MeV. Therefore, we have included three resonance excitations, S11(1535), S11(1650), and, P11(1710)
and the direct processes in the cross section of the γp →ηp reaction for the QMD calculation. The total crosssection of the elementary γp→ηp reaction is calculated practically by summing up the cross sections of the
double S11, the P11, and the direct processes, although the exact calculation should be the square of sumof the amplitude of each process. The solid line in Fig.3(a) is the elementary γp→ηp cross section usedin the present QMD calculation, being slightly larger than the full calculation of the η-MAID.
For the cross section of the γn→ηn reaction, no experimental data have been reported so far. We, again,
follow the η-MAID calculation as shown in Fig.3(b). The dashed line corresponds to the full calculationof the η-MAID including all the resonances and the direct processes. We select the double S11 resonance,
the D15(1675) resonance as major processes of excitations and the direct processes for the QMD input.
The η-MAID calculation predicts rather large cross sections through the D15(1675) resonance excitation,
which is essentially prohibited in the γp→ηp reaction by the Moorhouse selection rule [26]. The cross section
through the two S11 resonances is calculated with the destructive interference term. It shows a dip ataround 1000 MeV as shown by the dot-dashed line in Fig.3(b), where one sees that a large contribution
from the D15(1675) resonance, which is plotted by the dotted lines, fills the dip and appears like ashoulder of the S11(1535) resonance. The sum of the cross sections of the selected processes, which is
employed as the elementary γn → ηn reaction, is shown by the solid line. The difference from the fullcalculation is very small as in the case for the γn →ηn reaction.
Having discussed the elementary cross sections, we now return to the C(γ, η) and Cu(γ, η)
reactions. In Figs.2(a) and 2(b), the cross sections obtained in the present work are compared with the
QMD calculations. As mentioned above, the elementary cross sections of the γp→ηp and γn→ηn reactions
are treated independently; they are the solid lines in Fig.3(a) and Fig.3(b) for proton and neutron,
respectively. The η-emission probability through the resonance excitation is calculated according to theBreit-Wigner resonance formula for the P11(1710) and the D15(1675) resonances. For the S11 resonanceexcitation in the present work, however, the cross section including the interference of the two S11
resonances is used as if a resonance of the mixed state which is not described by the single Breit-Wigner

14
Table 1. Parameters of nucleon resonances used in our calculation. A are photoexcitation
helicity amplitudes of nucleon resonances and βηN is Nη decay branching ratio. Thosein the last columm are used in the previous analysis [11].
p,n1/2,3/2

N＊
S11(1535)
S11(1710)
P11(1710)
D15(1675)
S11(1535)

Mass
[Mev]
1541
1638
1721
1665
1542

Width
[Mev]
191
114
100
150
150

βηN
[%]
50
7.9
26
17
55

A

p
1/2

[10

－3

+118
+68
+23
0
+102

A
A
GeV － 1/2]
－
－
－
－
－
－
－ 58
0
－
－
p,n
3/2

n
3/2

formula is excited, and the lifetime and the decay branch of the S11(1535) resonance is applied to themixed state. This approximation seems to be allowed, since the η-meson is mainly emitted via theS11(1535) resonance. The resonance parameters in the η-MAID are used for each resonance, and aresummarized in Table 1.
It should be noticed for the QMD calculations in the present work that the effective energies of theincident channel are calculated for each photon-nucleon collision with a nucleon bound in a mean ﬁeld
potential. The effective energy which results in the reduction of the η yield for the threshold region has
been discussed [24,27] and the result of such calculation improves the reproduction of the cross sections
below 800 MeV. Thus, we have calculated the effective total energy followed as W ＝ √s － U, where W is
the effective total energy, √s is the c.m. energy of the incident photon and a nucleon in the nucleus andU is the nucleon potential calculated from the mean ﬁeld potential in Ref. [28]. As described in Ref. [11],the Fermi motion of nucleons, the Pauli blocking, collisions of nucleon resonances with nucleons andthe absorption of the η-mesons are taken into account in the calculation.
In Fig.2(a) and 2(b), the dashed line is the results of the QMD calculation in which only theS11(1535) resonance is incorporated with the assumption of σn/σp ＝ 2/3. This corresponds to the previous
calculation in Ref. [11]. For both C(γ, η) and Cu(γ, η) reactions, the calculation reproduces data up to
950MeV. However, it underestimates the yield for Eγ＞ 1000 MeV. The solid line corresponding to the newrecipe covers the deﬁcit and reproduces the data well up to 1100 MeV. Contributions of each process arealso shown in Fig.2(a) and 2(b). As can be seen, in addition to the largest contribution of the double S11
resonance indicated by the dotted line 1, the contributions of the D15(1675) resonance and the directprocesses are expected for Eγ ＞ 900 MeV. The present calculation suggests that more than 18% of the crosssection at 1000 MeV originates from other processes than the excitation of the S11 resonances. It is ofparticular interest that the D15(1675) resonance plays an important role for higher photon energies,
since only neutrons can be excited in a naive quark model. The dot-dashed lines labeled a and b in Fig.2
correspond to the contributions of protons and of neutrons, respectively. The ratio of the contribution ofneutrons to that of protons is nearly 0.67 for C and 0.84 for Cu at Eγ＜ 800 MeV, where only the S11

resonance formation process can contribute, and becomes 0.97 for C and 1.23 for Cu at around Eγ＝ 1100
MeV due to the existence of the D15(1675) resonance. The change of the contribution of neutrons toprotons may explain the change of the ratio of the total cross section σCu /σC plotted in Fig.2(c), where thecalculated ratio is also shown by the solid line. The calculation explains the trend of the ratio very well,although it fails for the lowest two points.

15(a) C target
Momentum
Angulardistribution
distribution
Eγ=750 MeV

III

100

0

0

III

Eγ=880 MeV

100

0.2

Momentum
Angulardistribution
distribution
Eγ=750 MeV

III

300

0.8

0.6

200

0.4

100

0.2

400
0

0.8
0

III

Eγ=880 MeV

300

0.6

200

0.4

100

0.2

III

Eγ=980 MeV

100

0.2

dσ/dp[µb/MeV]00dσ/dp[µb/MeV]dσ/dθ[µb/rad]

0.1

50

dσ/dθ[µb/rad]

0.2(b) Cu target

0.1

50

0
400

0
0.8

III

Eγ=980 MeV

300

0.6

200

0.4

100

0.2

400
0

0.8
0

0.1

50

0

0

III

Eγ=1090 MeV

100

0.2

III

Eγ=1090 MeV

300

0.6

200

0.4

100

0.2

0.1

50

0

400

0

0
100

θlab[deg]

500

0
1000

p [MeV]

0

0

0
100

θlab[deg]

500

0
1000

p [MeV]

Fig.4. Angular distributions, dσ/dθ, and momentum distributions, dσ/dp, of the (γ, η) reaction
measured on C (a) and Cu (b). The present results are plotted with the closed circles for Eγ＝ 750, 880, 1000, and 1100 MeV. The solid line is the result of the QMD calculation and thedashed one represents without the contribution of D15(1675), and the dot-dashed line shows
only the contribution of D15(1675) resonance multiplied by 4.
The above discussion on Fig.2 requires at least the following; the elementary cross section ofthe γn →η n reaction exceeds that of the γp →η p for Eγ ＞ 1000 MeV, due to another process besides theS11 resonance formation. Recently, Kuznetsov et al. reported the nη and pη coincidence measurements inthe D(γ, η) reaction [29]. Their result that yields of the nη coincidence events are larger than those of
the pη events at around 1000 MeV is consistent with the present interpretation.
Additional effects that might possibly give rise deviations from the pure S11 resonance formation
may be seen in the angular and momentum distributions of the emitted η-mesons. They are shown in
Fig.4 for Eγ＝ 750, 880, 980, and 1090 MeV, and compared with the QMD calculations. The results for the
C target are shown in Fig.4(a), and the Cu target in Fig.4(b). All the angular distribution data show a broad
structure peaked at around 30°. This is a characteristic of the quasi-free s-wave η production. The solid lines
in the ﬁgures are the results of the QMD calculation, and the experimental data for both angular andmomentum distributions are essentially well reproduced by the corresponding calculations. The dashed16lines shows the results without the contribution of the D15(1675) resonance and the dot-dashed lines
correspond to the contribution of the D15(1675) resonance multiplied by 4. As shown, the contribution of
the D15(1675) resonance has a different structure in these distributions because of the d-waveηemission.
Since the contribution of the D15(1675) resonance is not large, the present data, unfortunately, cannot
give a ﬁrm evidence for the excitation of the D15(1675) resonance in both reactions.
As we have discussed above, the comparison of new data with the QMD model calculations suggests
strongly that the cross sections of η photoproduction off nuclei contain non-negligible quantities
through the process other than the S11(1535) resonance formation for the photon energies above 850
MeV. Thus, in order to investigate the change of the S11 properties in the nuclear medium such as themass shift proposed in Ref. [2], one needs precise data of the cross section for the γn →ηn reaction. The
present work has shown that the use of the elementary cross sections of η-MAID can reproduce theexperimental data very well but remains an interesting subject for future investigations.

§4. Conclusion
In summary, the η photoproduction cross sections were measured on the C and Cu targets for thephoton energies between 600 and 1100 MeV. The excitation functions of the total cross section as wellas angular and momentum distributions were in quantitative agreement with the QMD model
calculations in which the cross sections proposed in the η-MAID model were used for the elementary
reactions γp →ηp and γn →ηn. The agreement suggests that there is a difference in the shape of thecross sections between proton and neutron in a nucleus. In order to discuss the change of the propertiesof the S11(1535) resonance, the cross section of the γn → ηn reaction experimentally measured is highly
desirable to be incorporated in the model calculations.

Acknowledgements
We would like to thank the accelerator staff at LNS for their support during the experiment. This
work was partly supported by the Grant-in-Aid for Scientiﬁc Research (Nos. 1040067, 07740197, and15340069) of the Ministry of Education of Japan.

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