1. Yasuhiro Togano Rikkyo University
Investigation of Stellar 26Si(p,g)27P Reaction via Coulomb Dissociation
Yasuhiro Togano
Rikkyo University
Firstly I would like to thank the organizers for giving me the opportunity to talk my study. Today, I talk about the experimental result of Coulomb dissociation of 27P and astrophysical implications of 26Si(p,g)27P reaction obtained from present study.
The 26Si(p,g)27P reaction is supposed to be relevant to the 26Al production.

2. 26Al: constraint for nucleosynthesis models
Ground state of 26Al decays to 26Mg with 1.8 MeV g-line.
This g-ray is observed in the interstellar medium.
26Al is a well known nuclide as a gamma ray emitter in the interstellar medium.
The ground state of 26Al decays to 26Mg with 1.8 MeV gamma ray. And this gamma-ray is observed in the interstellar medium.
The amount of 26Al constrains the nulceosynthesis models. For the estimate of 26Al in the models the nuclear database around 26Al is very important including the unstable nuclei.
Nuclear database around 26Al is needed including the unstable nuclei

3. 26Si(p,g)27P reaction
Proton capture reaction of the unstable nuclei 26Si.
One of the reactions which occur in the rp-process.
26Si predominantly b decays to the isomeric state of 26Al (26Alm).
26Al in high temperature (T9~0.4): thermal equilibrium between isomeric state and ground state.
This figure shows the nuclear chart around 26Al. Arrows show the reaction flow by (p,g) reactions or beta decays. Shaded square shows the stable nuclei. 26Al is drawn separately because it have a isomeric state at 228 keV.
The 26Si(p,g) reaction is the proton capture reaction of the unstable nuclei 26Si.
And it is relevant to the rapid proton-capture process.
The 26Si predominantly beta decays to the isomeric state of 26Al. In the high temperature conditions the thermal equilibrium between isomeric and ground states in 26Al is achieved.
Thus the 26Si destruction by proton capture is important to estimate the amount of this isomer and the effect of the equilibrium.
26Si destruction by proton capture is important to estimate the amount of 26Alm and the effect of equilibrium.

4. Resonances in 26Si(p,g)27P reaction
Level structure at low energy was studied via transfer reactions.
28Si(7Li,8He)27P, 32S(3He,8Li)27P
No experimental information on Gg (p,g) cross section
Coulomb Dissociation
In the past studies, the resonance structure at low energy was studied via 2 transfer reactions, 28Si(7Li,8He) and 32S(3He,8Li)
This is the level scheme of the 27P and 26Si+p system measured on these studies.
The three states were found in the studies.
Two states which locates at low energy, whose resonant energies are 340keV and 772keV, are astrophysically important because their energies are close to the Gamow-window of the novae and X-ray bursts.
However, there is no experimental information on the cross sections through these resonances.
So, we applied the Coulomb dissociation method on 27P to determine the cross sections through these resonances.
772 keV
340 keV

5. Coulomb dissociation Inverse reaction of (p,g) reactions : (g,p)
Large cross section : 104 ~ 105 times larger than the (p,g)
Higher beam energies : thicker target available
One can determine Gg values from CD cross sections
Coulomb dissociation is a inverse reaction of the (p,g) reactions. Coulomb dissociation cross section is much larger than one of (p,g) reaction. And by using the intermediate energy beams thicker targets are available. Thus we can determine the (p,g) reaction cross section of the nuclei far from the stability.
This figure shows schematic view of the Coulomb dissociation process. The final state of the (p,g) reaction is used as a beam. And bombards a high-Z target. usually it is lead. The beam particle absorbs the virtual photon from the nuclear Coulomb field of the target. The beam is excited, and is breakup to the proton and residual heavy ion. This process can be regarded as a photodisintegration by virtual photons.
Photodisintegration by virtual photons

6. Production of 27P beam: RIPS
36Ar 115 MeV/u
462 mg/cm2 Be target
Fragmentation
27P 57 MeV/u
Purity: ~1%
Intensity: ~2.8kcps
The experiment have been performed at RIPS beam line in RIKEN. 57MeV per nucleon 27P was produced by the fragmentation of 115MeV per nucleon 36Ar. The target for the fragmentation is 462mg/cm2 thick Be.
The 27P was separated using these two dipoles, and resultant purity and intensity are 1% and 2.8kcps, respectively.
Experimental
Area

7. Setup for 27P dissociation
Lead target: 125 mg/cm2
Relative energy between 26Si nuclei and protons were measured
Momentum vectors of products
278cm F2 F3
48.2cm
This figure shows the setup for 27P dissociation.
The target thickness is hogehoge, and momentum vectors of beams and reaction products, 26Si and proton were measured using these detectors.

8. Relative energy between 26Si and proton
Relative energy = collision energy in (p,g) reaction
Relative energies between 26Si and proton correspond to the collision energy in the (p,g) reaction.
This figure shows the relative energy spectrum.
Solid curve shows the best fit using the response functions calculated by the Monte-Carlo simulations and dashed curves are each components.
And this figure is the level scheme.
This peak corresponds to the first excited state in 27P, and this is second, and this is third.
(Third excited state is very small but the this component is needed to fit this peak correctly.)
Using cross sections of these three peaks, we extract the radiative widths of each state.

9. Gamma decay width of the states
1st excited state
g.s.(1/2+) st (3/2+)
Gg = Gg(E2) + Gg(M1)
GgC.D. = Gg(E2) = (9.3 ± 1.7) × 10-5 eV
Gg(M1) = Gg(E2) × d d2: E2/M1 mixing ratio
d2: based on shell model: ambiguity = 60%
Gg(1st) = (4.6±2.7)×10-3 eV
2nd , 3rd excited states
g.s.(1/2+) nd, 3rd(5/2+)
GgC.D. = Gg
Gg(2nd) = (2.7 ± 0.5) × 10-4 eV
Gg(3rd) = (3.4 ± 0.9) × 10-4 eV
E2/M1
The transition between ground state and 1st excited state is induced by E2 and M1 multipolarities. It means that g-decay width have E2 and M1 component. The gamma decay width derived from Coulomb dissociation experiment is E2 component because Coulomb dissociation is very sensitive to the E2 transition. From present study, E2 component is determined to be *** plus minus *** times 10 to the minus fifth electron volt.
To deduce the total gamma decay width, E2/M1 mixing ratio delta is needed. But delta for 27P is not measured. We calculate the delta using the known ratio for mirror nuclei 27Mg and a shell model.
Transition to the 2nd and 3rd excited state are induced by the E2 multipolarity, so gamma decay width of the states are directly determined from Coulomb dissociation cross section. E2

10. Astrophysical reaction rate for 26Si(p,g)27P
From these experimental results, we calculate the astrophysical reaction rate of the 26Si(p,g)27P. This figure shows the temperature dependence of the reaction rate. This solid line denotes the present result and dotted line represents the margin of the experimental error. Dashed line shows the direct capture component. This component is calculated on the basis of the shell model. From this figure it is revealed that the resonant capture is dominant above 0.1 GK. and resonant capture through the 2nd and 3rd excite state has negligible contribution to the reaction rate.
Resonant capture through 1st excited state in 27P is dominant at T > 0.08 GK

11. Competition of 26Si(p,g) and 26Si b-decay
Novae
Heavy : 26Si(p,g)27P reaction
dominates at around
peak temperature
Xp=0.5
J. Jose et al. ApJ 520, 347
C. Iliadis et al. ApJ Supl. 142, 105
From the estimated reaction rate, we evaluate the competition between 26Si(p,g) and 26Si beta decay. This figure shows which process is dominant in specific astrophysical environment. This axis is temperature of stars, and this axis is the density. The solid and dot-dashed curves represents the condition that two process are in equilibrium and its margin of error, respectively. Dashed lines show the profile of novae and X-ray bursts. From this figure, in the heavy novae, we can conclude that the reaction is dominant in high temperature phase and 26Si beta decay hardly occurs in X-ray bursts.

12. Reaction flow around 26Si and its effect on 26Al
25Al(p,g)26Si: No experimental determination of cross section.
Estimate by Parpottas et al. (PRC 70, ) ×
By superimposing the same plot for the 25Al, the reaction flow around 26Si can be investigated. There is no experimental determination of cross section for 25Al(p,g)26Si, by using the estimate by him, we extract the beta-decay-(p,g) reaction equilibrium environment is obtained. This red zone shows the hoge hoge equilibrium for 26Si beta decay and (p,g) reaction with error associated with present study. And this blue curve shows the same plot for 25Al.left side…. Right side… The blue curve almost overlaps with the red region.
This means that most of produced 26Si by 25Al(p,g) reaction are destructed by the 26Si(p,g) reaction in any stellar temperature and density, and almost no flow to the 26Si beta-decay branch. Thus, the 26Alm production by 26Si beta decay hardly occur in any stellar environment.

13. Summary
We have performed an experiment to study the 26Si(p,g)27P reaction by Coulomb dissociation method.
The g-decay width of three states below 1.5 MeV were determined.
Astrophysical reaction rate of the 26Si(p,g)27P was estimated using experimental results.
Resonant capture via the first excited state is the dominant process in most of the stellar environment.
Most of 26Si is destructed by the proton capture and 26Alm is mainly synthesized by 25Mg(p,g)26Alm reaction.
State
1st
2nd
3rd
Gg [eV]
(4.6±2.7)×10-3
(2.7±0.5)×10-4　
(3.4±0.9)×10-4
Let me summarize my talk…

14. Collaborators list
T. Gomi, T. Motobayashi, Y. Ando, N. Aoi, H. Baba, K. Demichi,
Z. Elekes, N. Fukuda, Zs. Fulop, U. Futakami, H. Hasegawa,
Y. Higurashi, K. Ieki, N. Imai, M. Ishihara, K. Ishikawa,
N. Iwasa, H. Iwasaki, S. Kannno, Y.Kondo, T. Kubo,
S. Kubono, M. Kunibu, K. Kurita, Y. U. Matsuyama,
S. Michimasa, T. Minemura, M. Miura, H. Murakami,
T. Nakamura, M. Notani, S. Ota, A. Saito, H. Sakurai, M. Serata,
S. Shimoura, T. Sugimoto, E. Takeshita, S. Takeuchi, K. Ue,
K. Yamada, Y. Yanagisawa, K. Yoneda, and A. Yoshida
Rikkyo Univ., RIKEN, ATOMKI, TITECH, Tohoku Univ.
Univ. of Tokyo, CNS, Kyoto Univ.