Shima, T

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PoS(NIC-IX)176Photonuclear reactions of light nuclei studied withhigh-intensity real photon beamsTatsushi Shima∗Research Center for Nuclear Physics, Osaka UniversityE-mail: shima@rcnp.osaka-u.ac.jpPhotonuclear reactions of light nuclei play crucial roles in various processes of nucleosynthesis instars and in the early universe. They are also important in relation to the explosion mechanism ofType-II supernovae and associating neutrino-induced nucleosynthesis, because they can provideuseful information about the analogous processes of the neutrino-induced nuclear reactions. Anew experimental method by means of a laser-Compton scattered γ-ray beam together with anactive-target technique has been developed for studies of photonuclear reactions relevant for nu-clear astrophysics. The method has been successfully used for high-precision measurements ofthe absolute cross sections of the photodisintegrations of 4He and 12C in the γ-ray energy rangeup to 30 MeV, where serious discrepancy has existed among the previous data. In this contribu-tion the new experimental results on the photonuclear reactions of 4He and 12C and a new γ-raysource at the NewSUBARU synchrotoron radiation facility will be presented.International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX25-30 June 2006CERN∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)176Photonuclear reactions of light nuclei studied with high-intensity real photon beams Tatsushi Shima1. IntroductionNuclear electromagnetic processes of radiative-capture reactions and photodisintegrations oflight nuclei are considered to play crucial roles in various processes of nucleosynthesis occurringin stars and in the early universe. For quantitative studies of those nucleosynthesis processes, re-liable data of the relevant nuclear reactions are indispensable. Photonuclear reactions will provideuseful tools to obtain information about nuclear electromagnetic processes of many-body systemsas well as of two-body ones. Also they would be useful to study the analogous processes of theneutrino-induced nuclear reactions [1, 2], which are considered to play critical roles in the dynam-ics of Type-II supernova explosions (SNes) [3, 4, 5] and in neutrino-induced nucleosynthesis [6, 7].Recent developments of quasi-monochromatic γ-ray sources and highly sensitive particle de-tectors provide unique opportunities to perform high-precision measurements on nuclear reactionscaused by low-energy γ-rays. In this paper recent experimental work on photonuclear reactions of4He and 12C and a new high-performance γ-ray source at the NewSUBARU synchrotron radiationfacility will be shown.2. Experimental methodThe cross section σ of a photonuclear reaction is experimentally obtained asσ =Rε ·n ·L ·Φγ(2.1)using measured values of the counting rate R, number density n of the target nucleus, effectivelength L of the target, efficiency ε of the detector, and incident γ-ray intensity Φγ . To measure thecross section accurately, one has to reduce the systematic errors of n, L ε and Φγ as well as thestatistical error of R. Therefore we designed a new experimental method as shown in Fig. 1.3. Laser Compton-scattered γ-ray beamA laser Compton-scattered (LCS) γ-ray beam is generated by head-on collision of a laser lightand an electron beam circulating in a storage ring. The γ-ray energy Eγ is given by the kinematicalcondition of the Compton scattering asEγ =4Elaser · γ21+ γ2θ 2 +4Elaser · γ/mec2, (3.1)where Elaser, γ , and θ are the energy of laser photons, the relativistic factor of the electron,and the scattering angle of the photon, respectively. me denotes the rest mass of the electron.By selecting the 180◦ backward component of the scattered photons using a γ-ray collimator, aquasi-monochromatic γ-ray beam is obtained. Polarized γ-rays can be easily generated by using apolarized laser light. A LCS γ-ray beam has the following features;· quasi-monochromatic (∆Eγ/Eγ = 5−10%) and variable (a few ∼ a few tenth MeV) energy,· little background γ-rays due to bremsstrahlung,· small angular dispersion (∆θ < ∼0.3 mrad),2PoS(NIC-IX)176Photonuclear reactions of light nuclei studied with high-intensity real photon beams Tatsushi Shimaxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxe-Nd - YLF laserMirrorCollimatorTime Projection     ChamberBGOdetectorColliding regionElectron Storage Ring "TERAS"( Emax= 800MeV, Imax=300mA )Gamma-ray      beampHElectric fieldDrift regionAnode plane3Figure 1: Experimental setup for measurement of photonuclear reaction using LCS γ-ray beam.· considerably high intensity (103−105 photons/MeV/s), and· large polarization (∼100%, linear and/or circular).The above features of LCS γ-ray beams are great advantages for precise spectroscopic studies ofthe photonuclear reactions relevant for astrophysical nucleosynthesis.4. Time projection chamber with active targetSince the energy regions of astrophysical interest are in general not so much higher than thethreshold energies of the reactions, an active target is considered to be useful for detecting thereaction products with high efficiency. Therefore we employed a time projection chamber (TPC)whose operational gas serves as an active target. It enables us to measure low-energy chargedparticles from various reaction channels of the photodisintegration with detection efficiencies ofnearly 100% and a solid angle of 4pi . In addition, the TPC can measure the tracks and energylosses of the emitted charged particles. Those data are quite useful for precise identification ofthe true events and measurement of angular distributions of the reaction fragments. Detail of thepresent TPC is given in Ref. [8].5. Photodisintegration of 4HeIt has been claimed that the heating of the outgoing shockwave by delayed neutrinos wouldplay the key role in Type-II SNes [3, 4, 5]. The interaction between neutrinos and nuclei is stillone of the uncertain inputs for theoretical models of Type-II SNes. 4He is considered as one of the3PoS(NIC-IX)176Photonuclear reactions of light nuclei studied with high-intensity real photon beams Tatsushi Shimaprincipal ingredients of the matter surrounding the collapsing core [9], and therefore the neutrino-induced nuclear reactions of 4He rates would be of special importance. The neutrino-inelasticscattering reaction of 4He is also supposed to make important contribution in production of lightelements as well as heavy ones via the rapid process of nucleosynthesis in the neutrino-driven windof Type-II SNe [6, 7, 10]. So it is necessary to obtain information about the responses of nuclei toneutrinos. Especially, nuclear responses to weak neutral current have been less understood mainlydue to little experimental information. The photonuclear reactions will be a unique tool to inves-tigate nuclear responses to weak neutral current because of the analogous structures between theoperators of the weak interaction by neutral current and the electromagnetic interaction [2]. Thecross section of the spallation reaction of 4He by weak neutral current has been recently calculatedin the energy region of supernova neutrinos (∼8−12 MeV) by using a microscopic theory basedon the Lorentz integral transform (LIT) method [11], which suggests 5−15% enhancement in thereaction rate compared to the previous estimation with the systematic property of the giant dipoleresonance [1]. Since those theoretical methods are also applied to photonuclear reactions, they canbe tested by comparing the experimental data of the photonuclear reaction cross sections with thevalues estimated with those theories. So far the photonuclear reaction cross sections of 4He havebeen measured using bremsstrahlung photons, tagged photons, pair-annihilation γ-rays, and LCSγ-rays, as well as the inverse 3H(p,γ)4He and 3He(n,γ)4He reactions. However, there have beendiscrepancy of∼50−100% between the values measured with different experimental methods, andthat requests further experimental efforts to measure the absolute photodisintegration cross sectionsof 4He with more improved accuracy. Therefore we have performed a new measurement [12] us-ing a LCS γ-ray beam at National Institute of Advanced Industrial Science and Technology (AIST)[13] and the above mentioned TPC.In our experiment the systematic errors associating the experimental parameters in Eq. 2.1 arequoted as follows. The detection efficiency ε for true photodisintegration events was evaluated tobe better than 98% by comparing the measured pulse height spectra of the charged fragments withthe Monte Carlo simulation. And also it was confirmed by measuring the photodisintegration ofdeuteron, whose cross section has been well known. Therefore the error of ε is not greater than2%. The number density n of the 4He target was determined from the measured temperature andpressure of the TPC gas according to the equation of state of real gas, and its error is smaller than0.2%. The length L of the effective region was defined from the position distribution of the reac-tion point of the true events along the γ-ray beam axis. Therefore its error amounts to 2%, which isdue to the accuracy of the position measurement. The incident γ-ray intensity was determined bythe analysis of the γ-ray pulse height spectrum measured with a BGO scintillator whose intrinsicefficiency was larger than 99.5%, and the error is less than 1%. In summary, the total systematicerror of the present method amounts to be less than 5%.Fig. 2 represents the measured cross section of the 4He(γ, p)3H and 4He(γ,n)3He reactionstogether with other available data, including the result of the most recent (γ,n) experiment withbremsstrahlung photons [14], and the recent theoretical calculations. The present result is consis-tent with the previous data measured with monochromatic γ-ray beams. The present cross sectionof the 4He(γ,n)3He reaction is in rather good agreement with the one obtained with the Faddeev-type calculation [16], while the present 4He(γ, p)3H and 4He(γ,n)3He cross sections are smaller4PoS(NIC-IX)176Photonuclear reactions of light nuclei studied with high-intensity real photon beams Tatsushi Shima4He(γ,p)3H0124He(γ,n)3HeCross Section [mb]Eγ [MeV]20 25 30 35 40 45 50012Figure 2: Two-body photodisintegration cross sections of 4He. The open circles denote the present data,while the other symbols indicate the previous ones (see the references in [12]). Upper panel; (γ, p) crosssections, open upward triangles; Gorbunov, open downward triangles; Arkatov et al., crossed squares; Bern-abei et al., filled squares; Feldman et al., open diamonds; Hoorebeke et al., gray squares; Hahn et al.. Lowerpanel; (γ ,n) cross sections, open upward triangles; Gorbunov, open downward triangles; Arkatov et al., opensquares; Berman et al., diagonal crosses; Ward et al., filled circles; Komar et al., crosses; Nilsson et al..The error bars of the previous data are not shown for clarity. The long-dashed curves are the cross sectionscalculated using the LIT method with the MTI-III potential [17]. The short-dashed curve represents thecalculated (γ ,n) cross section based on the AGS formalism [15, 16].than the ones [17] calculated with the semirealistic Malfliet-Tjon (MT) nucleon-nucleon (NN) po-tential and the LIT method which was used in the calculation for the neutral current ν-4He reactionrate in the energy region above kT ∼8 MeV [11]. It should be noted a new calculation for the totalphotoabsorption on 4He with the LIT method was performed using the realistic Argonne V18 NNpotential and the Urbana IX three-nucleon force [18], and predicted a smaller peak cross sectionthan the previous one with the semirealistic MT NN potential. Therefore further efforts in both the-oretical and experimental studies of the photodisintegration of 4He is quite important to investigatethe role of the neutral current ν-4He interaction in the neutrino heating in Type-II SNes.6. 12C(γ,2α)4HeThe 12C(γ,2α)4He reaction is interesting in connection with the 3α reaction, which plays acrucial role in stellar helium burning [19, 20, 21]. The existing experimental data are, however,quite scarce [22, 23, 24], and therefore we performed a new measurement in the similar manner asfor the measurement of the photodisintegration of 4He described above. Fig. 3 shows an example ofthe tracks of the 12C(γ,2α)4He reaction events detected with the present TPC. The true events canbe accurately identified, because they are characterized by three tracks of the α particles emitted5PoS(NIC-IX)176Photonuclear reactions of light nuclei studied with high-intensity real photon beams Tatsushi Shima  	     Figure 3: Example of the tracks of α particles emitted in the 12C(γ,2α)4He reaction events. The box showsthe edge of the effective region of the TPC in section. The γ-ray beam axis is indicated by an arrow.Eγ [MeV]Cross Section [mb]10 15 20 25 30 3500.10.20.30.40.5Figure 4: The 12C(γ,2α)4He reaction cross sections. Open circles; present work (preliminary), closedtriangles; Maikov et al. [22], open triangles; Murakami et al. [23], diagonal crosses; Kotikov et al. [24].at the reaction point on the γ-ray beam axis. Fig. 4 shows our preliminary result together with theprevious ones. The previous three measurements were all performed with a similar method usingbremsstrahlung photons and nuclear emulsion chambers, and the measured excitation functions arecharacterized by the peaks corresponding to the resonance excitations of 8Be in the 12C nucleus.The absolute value of the cross section from the most recent work is however smaller by a factorof ∼2−6 than the values from two older measurements as shown in Fig. 4. Our result is found tobe consistent with the older data by Maikov et al. [22] and Murakami et al. [23].7. SummaryLCS γ-ray beams have a lot of advantages as mentioned above, and are considered to beuseful for experimental studies of photonuclear reactions relevant for nuclear astrophysics. To per-form high-precision measurements of photonuclear reactions, new LCS γ-ray sources with higherintensity and better quality have been developed at several new-generation synchrotron radiationfacilities in the world. At the NewSUBARU of the University of Hyogo in Japan, a LCS γ-raysource in the energy of 16−38 MeV has been newly developed by using the electron beam of6PoS(NIC-IX)176Photonuclear reactions of light nuclei studied with high-intensity real photon beams Tatsushi Shima1.0−1.5 GeV and the 1064 nm photons from a Nd:YVO4 laser [25]. A unique feature of the γ-raysource at NewSUBARU is its excellent stabilities in the intensity and the energy distribution dueto top-up mode operation, in which the electrons are continuously injected into the ring in order tokeep the beam current constant. It is of great advantage in high-precision measurements of pho-tonuclear reactions at low energies. And now it is planned to install a near-infrared laser basedon the optical-parametric oscillation (OPO) technique [26, 27] in order to cover the γ-ray energyrange from 5 to 85 MeV. The following experimental studies are currently ongoing or planned atthe NewSUBARU LCS γ-ray facility ;(1) photodisintegrations of light nuclei relevant for few-nucleon physics and nuclear astrophysics,(2) (γ,α) reactions relevant for α-process and γ-process,(3) (γ,n) reactions of heavy nuclei relevant for p-process,(4) isomers populated by (γ,n) reactions in relation to heavy element synthesis and nuclearcosmochronology, and(5) photo-excitations of isobaric analog states to simulate nuclear spin excitations by neutrinos.AcknowledgmentsThe author would like to thank Profs. Y. Nagai, H. Ejiri, H. Utsunomiya, T. Mochizuki, and S.Miyamoto for fruitful discussions. This work was supported in part by Grant-in-Aid for SpeciallyPromoted Research of the Japan Ministry of Education, Science, Sports and Culture and in part byGrant-in-Aid for Scientific Research of the Japan.References[1] S.E. Woosley, D.H. Hartmann, R.D. Hoffman, and W.C. Haxton, Astrophys. J. 356 (1990) 272.[2] H. 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Photonuclear reactions of light nuclei studied with high-intensity real photon beams

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