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The α-clusterization of an atomic nucleus has been studied for a long time, even from the earliest days of nuclear physics. Our understanding of the α-cluster structure is, however, mainly limited to the self-conjugate A = 4n (n = 2, 3, 4, · · · ) light nuclei and some of the neutron-rich radionuclides such as 10Be, 11B, and 14C. In order to study the α-cluster structure of a neutron-deficient 22Mg nucleus, we have measured the 18Ne(α, α)18Ne scattering in inverse kinematics by using radioactive 18Ne beams and the 4He gas target at the Center for Nuclear Study radioactive ion beam separator of the University of Tokyo. Recoiling α particles from the scattering were detected by using silicon strip detectors, which constitute ΔE-E telescopes for particle identification. By adopting a thick target method, we were able to investigate a wide range of excitation energies Ex = 9.9 - 16.5 MeV in 22Mg in this work.

We conducted a measurement of 18Ne + α scattering in inverse kinematics , in order to search for experimental evidence of α -cluster structure in 22Mg above the alpha threshold ( E thr = 8 . 14 MeV ) and determine the astrophysical rates of the 18Ne( α , p)21Na reaction under the conditions of break-out from the Hot-CNO cycle. A high intensity 18 Ne beam at 2.54 AMeV was successfully produced at CRIB (CNS Radioactive Ion Beam separator) of the Center for Nuclear Study, the University of Tokyo. This paper presents the in-flight production of the radioactive 18Ne beam by using the production reaction 3He(16O, 18Ne)n with a windowed cryogenic gas target, which was employed in the mentioned resonant α -scattering experiment.

Studies on nuclear astrophysics, nuclear structure, and other interests have been performed using the radioactive-isotope (RI) beams at the low-energy RI beam separator CRIB, operated by Center for Nuclear Study (CNS), the University of Tokyo. The elastic resonant scattering is a striking tool to study astrophysical reactions and nuclear clusters. In particular, when it is coupled with a thick target and inverse kinematics, the measurement can be very efficient and even feasible with RI beams. By measuring resonant scattering, we can study the properties of resonant states which could play an important role in the astrophysical reaction, or have an exotic nuclear structure. The 10Be+α elastic scattering has been measured at CRIB, and three resonances which are in agreement with the prediction of a linear-chain structure by Suhara-En’yo were observed, giving a strong indication of the existence of such an exotic structure. Measurements based on the indirect technique of the reaction measurement, such as the Trojan horse method, have also been performed at CRIB.

Background: Type I x-ray bursts are the most frequently observed thermonuclear explosions in the galaxy, resulting from thermonuclear runaway on the surface of an accreting neutron star. The $^{30}\mathrm{S}(\ensuremath{\alpha},p)$ reaction plays a critical role in burst models, yet insufficient experimental information is available to calculate a reliable, precise rate for this reaction.Purpose: Our measurement was conducted to search for states in $^{34}\mathrm{Ar}$ and determine their quantum properties. In particular, natural-parity states with large $\ensuremath{\alpha}$-decay partial widths should dominate the stellar reaction rate.Method: We performed the first measurement of $^{30}\mathrm{S}+\ensuremath{\alpha}$ resonant elastic scattering up to a center-of-mass energy of 5.5 MeV using a radioactive ion beam. The experiment utilized a thick gaseous active target system and silicon detector array in inverse kinematics.Results: We obtained an excitation function for $^{30}\mathrm{S}(\ensuremath{\alpha},\ensuremath{\alpha})$ near ${150}^{\ensuremath{\circ}}$ in the center-of-mass frame. The experimental data were analyzed with $R$-matrix calculations, and we observed three new resonant patterns between 11.1 and 12.1 MeV, extracting their properties of resonance energy, widths, spin, and parity.Conclusions: We calculated the resonant thermonuclear reaction rate of $^{30}\mathrm{S}(\ensuremath{\alpha},p)$ based on all available experimental data of $^{34}\mathrm{Ar}$ and found an upper limit about one order of magnitude larger than a rate determined using a statistical model. The astrophysical impact of these two rates has been investigated through one-zone postprocessing type I x-ray burst calculations. We find that our new upper limit for the $^{30}\mathrm{S}(\ensuremath{\alpha},p)^{33}\mathrm{Cl}$ rate significantly affects the predicted nuclear energy generation rate during the burst.

International audience; Studies on nuclear astrophysics, nuclear structure, and other interests have been performed using the radioactive-isotope (RI) beams at the low-energy RI beam separator CRIB, operated by Center for Nuclear Study (CNS), the University of Tokyo. A type of measurement to study astophysical reactions at CRIB is by the elastic resonant scattering with the thick-target method in inverse kinematics. An example is the α resonant scattering with 7Be beam, related to the astrophysical 7Be(α,γ) reactions, which is relevant in the hot p-p chain and νp-process in supernovae. Other α resonant scattering measurements with 30S, 10Be, 15O, and 18Ne beams have been performed at CRIB, using the thick-target method. There have also been measurements based on other experimental methods. The first Trojan horse method (THM) measurement using an RI beam has been performed at CRIB, to study the 18F(p, α)15O reaction at astrophysical energies via the three body reaction 2H(18F, α15O)n. The 18F(p, α)15O reaction rate is crucial to understand the 511-keV γ-ray production in nova explosion phenomena, and we successfully evaluated the reaction cross section at novae temperature and below experimentally for the first time.

The Trojan Horse Method (THM) is an indirect method that allows to get information about a two body reaction cross-section even at very low energy, avoiding the suppression effects due to the presence of the Coulomb barrier. The method requires a very accurate measurement of a three body reaction in order to reconstruct the whole kinematics and discriminate among different reaction mechanisms that can populate the same final state. These requirements hardly match with the typical low intensity and large divergence of radioactive ion beams (RIBs), and experimental improvements are mandatory for the applicability of the method. The first reaction induced by a radio activeion beam studied by applying the THM was the 18F(p,α)15O. Two experiments were performed in two different laboratories and using different experimental set-ups. The two experiments will be discussed and some results will be presented.

International audience; For the first time the Trojan Horse Method was applied to infer information about a neutron induced reaction on a radioactive nucleus at low energies relevant for astrophysics. In particular, the ^18F(n, \(\alpha \))^15N reaction has been studied by using the ^18F+d reaction. A ^18F beam was produced at the CRIB facility at RIKEN, Japan, and was focused onto a CD_2 target. The beam had energy of 48 MeV, an average intensity of 10^5 pps and an average purity of 80%. A new designed experimental set-up covering a large solid angle was used to detect the ejected particles. The reaction cross section has been for the first time investigated in the energy range from 0 up to 2.5 MeV and several resonant states of ^19F have been populated.

The $^{11}\mathrm{C}{(\ensuremath{\alpha},p)}^{14}\mathrm{N}$ reaction is an important $\ensuremath{\alpha}$-induced reaction competing with $\ensuremath{\beta}$-limited hydrogen-burning processes in high-temperature explosive stars. We directly measured its reaction cross sections both for the ground-state transition $(\ensuremath{\alpha},{p}_{0})$ and the excited-state transitions $(\ensuremath{\alpha},{p}_{1})$ and $(\ensuremath{\alpha},{p}_{2})$ at relevant stellar energies 1.3--4.5 MeV by an extended thick-target method featuring time of flight for the first time. We revised the reaction rate by numerical integration including the $(\ensuremath{\alpha},{p}_{1})$ and $(\ensuremath{\alpha},{p}_{2})$ contributions and also low-lying resonances of $(\ensuremath{\alpha},{p}_{0})$ using both the present and the previous experimental data which were totally neglected in the previous compilation works. The present total reaction rate lies between the previous $(\ensuremath{\alpha},{p}_{0})$ rate and the total rate of the Hauser-Feshbach statistical model calculation, which is consistent with the relevant explosive hydrogen-burning scenarios such as the $\ensuremath{\nu}p$ process.

The level structure of 30 P up to 8.25 MeV was investigated via in-beam γ -ray spectroscopy using the 28 Si( 3 He,p γ ) 30 P reaction at 9 MeV at the University of Tsukuba Tandem Accelerator Complex in Japan. An energy level scheme was deduced from γ - γ coincidence measurements. 47 new transitions have been observed from the previously known states (mostly resonances), thereby reducing the uncertainties in the excitation energies of 17 states from 3 to 10 keV to values of γ -ray angular distributions and γ - γ directional correlation of oriented nuclei (DCO ratios) were made for several observed levels of 30 P.

A cryogenic gas target system was developed for the radioisotope (RI) beam production at CNS Radio Isotope Beam separator (CRIB). Hydrogen gas was cooled to 85–90 K using liquid nitrogen and used as a secondary beam production target having a thickness of 2.3 mg / cm 2 . An intense 7Be beam ( 2 × 10 8 particles per second) was successfully produced using this target. We observed a density-reduction effect at the gas target for high-current primary beams with about 7.5 W heat deposition. One main feature of the target system is forced circulation of the target gas. We have found that the circulation of the target gas at a rate of 55 standard liters per minute (slm) was effective in eliminating the density reduction. The extent to which the forced flow can prevent the density reduction had not been known well. In this work, the relation between the density reduction and the forced circulation rate was quantitatively studied.

We performed the first measurement of 30 S+α resonant elastic scattering to experimentally examine the 30 S(α, p) stellar reaction rate in type I x-ray bursts. These bursts are the most frequent thermonuclear explosions in the galaxy, resulting from thermonuclear runaway on the surface of accreting neutron star binaries. The 30 S(α, p) reaction plays a critical role in burst models, yet very little is known about the compound nucleus 34 Ar at these energies nor the reaction rate itself. We performed a measurement of alpha elastic scattering with a radioactive beam of 30 S to experimentally probe the entrance channel. Utilizing a gaseous active target system and silicon detector array, we extracted the excitation function from 1.8 to 5.5 MeV near 160° in the center-of-mass frame. The experimental data were analyzed with an R -Matrix calculation, and we discovered several new resonances and extracted their quantum properties (resonance energy, width, spin, and parity). Finally, we calculated the narrow resonant thermonuclear reaction rate of 30 S(α, p) for these new resonances.

Measurement of nuclear cross sections at astrophysical energies involving unstable species is one of the most challenging tasks in experimental nuclear physics. The use of indirect methods is often unavoidable in this scenario. In this paper the Trojan horse method is applied for the first time to a radioactive ion beam-induced reaction studying the ${}^{18}\mathrm{F}(p,\ensuremath{\alpha}$)${}^{15}\mathrm{O}$ process at low energies relevant to astrophysics via the three-body reaction ${}^{2}\mathrm{H}{(}^{18}\mathrm{F},{\ensuremath{\alpha}}^{15}\mathrm{O})n$. The knowledge of the ${}^{18}\mathrm{F}{(p,\ensuremath{\alpha})}^{15}\mathrm{O}$ reaction rate is crucial to understand the nova explosion phenomena. The cross section of this reaction is characterized by the presence of several resonances in $^{19}\mathrm{Ne}$ and possibly interference effects among them. The results reported in literature are not satisfactory and new investigations of the ${}^{18}\mathrm{F}{(p,\ensuremath{\alpha})}^{15}\mathrm{O}$ reaction cross section will be useful. In the present work the spin-parity assignments of relevant levels have been discussed and the astrophysical $S$ factor has been extracted considering also interference effects.

The level scheme above the proton threshold in 26Si is crucial for evaluating the 25Al(p, γ)26Si stellar reaction, which is important for understanding the astrophysical origin of the long-lived cosmic radioactivity 26Al(T 1/2 = 7.17 × 105 y) in the Galaxy. The excited states in 26Si have been studied using an in-beam γ-ray spectroscopy technique with the 24Mg(3He, nγ)26Si reaction. γ-rays with energies up to 4.6 MeV emitted from excited states in 26Si have been measured using large volume HPGe detectors. The spin-parity of one of the most important states reported recently at 5890.0keV has been assigned as 0+ by γ-γ angular correlation measurements in this work.

The αp-process is a sequence of (α, p)(p, γ) reactions important to the nuclear trajectory to higher masses in type I X-ray bursts. Specifically, the αp-process is schematically pure helium-burning, and thus unlike pure hydrogen-burning processes, does not require slow β+ decays. Explosive helium burning is responsible for the observed short rise-times of X-ray bursts but ultimately gives way to the rp-process as the Coulomb barrier increases. Because the stellar reaction rates of these (α, p) reactions are poorly known over the relevant astrophysical energies, we performed systematic studies of the 18Ne(α,p), 22Mg(α,p) and 30S(α,p) reactions at the Center for Nuclear Study (CNS) low-energy radioactive ion beam separator, called CRIB. We produce the radioactive beams in-flight and scan the center-of-mass energy down into the Gamow Window using a thick target in inverse kinematics. The helium target gas also serves as part of the detector system, an active target, which was newly designed for these measure...

International audience; The abundance of 18F in Nova explosions is considered to be an important piece of information for the understanding of this astrophysical phenomenon. It is then necessary to study the nuclear processess that both produce and destroy this isotope in Novae. Among these latter reactions, the 18F(p,α)15O is one of the most important 18F destruction channels. Here we report on an experiment performed using the CRIB apparatus of the Center for Nuclear Study of the University of Tokyo. This was the first experiment that used the Trojan Horse method applied to a Radioactive Ion Beam induced reaction.

Background: The resonance structure in ${}^{11}$C is of particular interest with regard to the astrophysical ${}^{7}$Be($\ensuremath{\alpha},\ensuremath{\gamma}$) reaction, relevant at high temperature, and to the $\ensuremath{\alpha}$-cluster structure in ${}^{11}$C.Purpose: The measurement was made to determine the unknown resonance parameters for the high excited states of ${}^{11}$C. In particular, the $\ensuremath{\alpha}$-decay width can be useful information to discuss the $\ensuremath{\alpha}$-cluster structure in ${}^{11}$C.Methods: New measurements of ${}^{7}\mathrm{Be}+\ensuremath{\alpha}$ resonant scattering and the ${}^{7}$Be($\ensuremath{\alpha},p$)${}^{10}$B reaction in inverse kinematics were performed for center-of-mass energy up to 5.5 MeV, and the resonances at excitation energies of 8.9--12.7 MeV in the compound ${}^{11}$C nucleus were studied. Inelastic scattering of ${}^{7}\mathrm{Be}+\ensuremath{\alpha}$ and the ${}^{7}$Be($\ensuremath{\alpha},p$${}_{1}$)${}^{10}$B${}^{*}$ reaction were also studied with a simultaneous $\ensuremath{\gamma}$-ray measurement. The measurements were performed at the low-energy radioactive ion beam facility CRIB (CNS Radioactive Ion Beam separator) of the Center for Nuclear Study, the University of Tokyo.Results: We obtained excitation functions of ${}^{7}$Be($\ensuremath{\alpha},{\ensuremath{\alpha}}_{0}$)${}^{7}$Be (elastic scattering), ${}^{7}$Be($\ensuremath{\alpha},{\ensuremath{\alpha}}_{1}$)${}^{7}$Be${}^{*}$ (inelastic scattering), ${}^{7}$Be($\ensuremath{\alpha},p$${}_{0}$)${}^{10}$B, and ${}^{7}$Be($\ensuremath{\alpha},p$${}_{1}$)${}^{10}$B${}^{*}$. Many resonances including a new one were observed and their parameters were determined by using an $R$-matrix analysis.Conclusions: The resonances we observed possibly enhance the ${}^{7}$Be($\ensuremath{\alpha},\ensuremath{\gamma}$) reaction rate but with a smaller magnitude than the lower-lying resonances. A new negative-parity cluster band, similar to the one previously suggested in the mirror nucleus ${}^{11}$B, is proposed.

Studies on nuclear reactions relevant for astrophysics have been performed using the radioactive-isotope (RI) beams at the low-energy RI beam separator CRIB, operated by Center for Nuclear Study (CNS), the University of Tokyo. A type of measurement to study astrophysical reactions at CRIB is by the elastic resonant scattering with the thick-target method in inverse kinematics. We introduce the α resonant scattering with 7 Be beam, related to the astrophysical 7 Be(α, γ) reactions, which is relevant in the hot p-p chain and νp-process in supernovae. Other α resonant scattering measurements with 30 S, 10 Be, 15 O, and 18 Ne beams have been performed at CRIB, using the thick-target method.

Nucleosynthesis is one of the keys in studying the mechanism of core-collapse supernovae, which is an interesting challenge for modern science. The νp-process, which is similar to an explosive hydrogen burning process, has been proposed as the most probable process in the very early epoch of type II supernovae. Here, we discuss our experimental efforts for the νp-process, the first extensive direct measurements of the (α,p) reactions on bottle-neck proto-rich nuclei in light mass regions. Other challenges for the νp-process study are also discussed.

We determined the total reaction rate of the {sup 11}C({alpha},p){sup 14}N reaction relevant to the nucleosynthesis in explosive hydrogen-burning stars. The measurement was performed by means of the thick target method in inverse kinematics with {sup 11}C RI beams. We performed the identification of the ground-state transition and excited-state transitions using time-of-flight information for the first time.

The quasi-elastic scattering angular distribution of the proton drip line nucleus F-17 on a C-12 target was measured at 60 MeV. The experimental data have been compared with the theoretical analysis based onto optical model and continuum discretized coupled channels (CDCC). The couplings between breakup and elastic scattering channels, and between inelastic and elastic scattering channels resulted very weak. In order to explore the breakup effects the total reaction cross-section was deduced from the angular distribution of the quasi-elastic scattering data, and then compared with the existing data for the other weakly and tightly bound nuclei on C-12 target using a universal function. From this comparison, we concluded that the breakup effect is not important for weakly bound projectiles on the light target as obtained also with the CDCC analysis.

Several reactions have been experimentally studied, including the 12N(d,n)13O and the ones induced by the 3He+12C entrance channel. The former was carried out at the CRIB facility of University of Tokyo, aiming to indirectly determine the astrophysical reaction rates of the 12N(p,γ)13O reaction. For the 3He+12C entrance channel, many excited states of several nuclei are populated and the angular distribution of each state is being analyzed.

We determined the 11C(α,p)14N reaction rate relevant to the nucleosynthesis in explosive hydrogen-burning stars. The measurement was performed by means of the thick target method in inverse kinematics with 11C RI beams. We derived the excitation functions for the ground-state transition and excited-state transitions using time-of-flight information for the first time. The present reaction rate is compared to the previous one.

The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations., Comment: 22 pages, 12 figures

We performed an experiment of the 21Na(αp)24Mg reaction for the first time by a direct measurement with a thick target method. The studied energy was covered for high temperature conditions in X-ray bursts and core-collapse supernovae. The 21Na(αp)24Mg reaction could make a branch of the extended αp-process and could be important to understand the production of 22Na during the hydrogen explosion at high temperature, like in X-ray bursts, and the 44Ti production from core-collapse supernovae.

A new measurement of {alpha} resonant scattering on {sup 7}Li was performed over the excitation energy of 10.2-13.0 MeV in {sup 11}B at the low-energy RI beam facility CNS Radioactive Ion Beam separator (CRIB) of the Center for Nuclear Study (CNS), University of Tokyo. The excitation function of {sup 7}Li+{alpha} at 180 deg. in the center-of-mass system was successfully measured for the first time with the inverse kinematics method, providing important information on the {alpha} cluster structure in {sup 11}B and the reaction rate of {sup 7}Li({alpha},{gamma}), which is relevant to the {sup 11}B production in the {nu} process in core-collapse supernovae. The excitation function of the {sup 7}Li({alpha},p) reaction cross section for 11.7-13.1 MeV was also measured.

The structure of proton-unbound {sup 30}S states strongly determines the thermonuclear {sup 29}P(p,{gamma}){sup 30}S reaction rate at temperatures characteristic of explosive hydrogen burning in classical novae and type I x-ray bursts. Specifically, the rate had been previously predicted to be dominated by two low-lying, unobserved, levels in the E{sub x}= 4.7-4.8 MeV region, with spin and parity assignments of 3{sup +} and 2{sup +}. In recent experimental work, two candidate levels were observed with energies of 4.699 MeV and 4.814 MeV, but no experimental information on their spins and parities was obtained. We have performed an in-beam {gamma}-ray spectroscopy study of {sup 30}S with the {sup 28}Si({sup 3}He, n{gamma}){sup 30}S reaction. By constructing the decay schemes of proton-unbound states with a {gamma}-{gamma} coincidence analysis of their decay {gamma} rays, their J{sup {pi}} values were inferred from a comparison to the known decay schemes of the corresponding mirror states in {sup 30}Si. For the two aforementioned states, our results strongly corroborate the spin-parity assignments assumed in recent evaluations of the {sup 29}P(p,{gamma}){sup 30}S reaction rate.