Observation of nonresonant coulomb break-up of 156 MeV6Li projectiles

Abstract

[3] specific investigations of photodissociation processes of light projectiles in the Coulomb field of a heavy target nucleus as a source of astrophysically interesting information on radiative capture processes at low relative energies. In particular, it has been suggested to take profit of the enhanced dissociation cross section at higher projectile energies due to the more intensive and harder equivalent photon spectrum experienced by the projectile when passing the Coulomb field. Furthermore, a kinematically complete observation of the break-up fragments emitted in a narrow angle cone provides the access to rather low relative energies, precisely determined by the correlated laboratory energies and emission angles. Experimentally the approach requires measurements at angles far below grazing in order to minimize the observation of events induced by the nuclear field.In order to explore the feasibility of the experiments and to study possible disturbances and interpretational complications arising from the presence of the target nucleus which works as a catalyst of the photodissociation, we have started experimental investigations of the 6Li -, a + d break-up at 156 MeV and identified the nonresonant Coulomb break-up under conditions well above the Coulomb barrier [4]. Though 6Li break-up proceeds predominantly via an E2 mode with comparatively smaller cross sections than the E1 7Li case, the case of 6Li provides an excellent test ground because (i) the sequential break-up via the long-living 3. + resonance state of 6Li is well understood and can be used to check the Coulomb break-up mechanism, (ii) at relative energies Ead > 1 MeV data for the capture reaction a + d -~ 6Li + y are available [5] for comparison and (iii) due to the identical charge-to-mass ratio of projectile and fragments post-acceleration effects obscuring the relative energy at the break-up locus are minimized.Recently a Heidelberg group [6] explored the direct 6Li break-up at 60 MeV and reported conspicuous and peculiar "forward-backward" asymmetries for the nonsequential emission of the break-up fragments from the ct + d center-of-mass system. The extent of these asymmetries can be hardly explained by the Coulomb break-up theory, at least not within the standard "quasi-sequential" approach. In view of this communication we considered carefully the measured data of our current experiments, and this note comments briefly the aspect of possible "forward-backward" asymmetries.Our experiments at the 156 MeV 6Li beam of the Karlsruhe Isochronous Cyclotron used the magnetic spectrograph "Little John" equipped with a special focal plane detector. The detector consists of two identical parts each covering half of the momentum acceptance plane of the spectrograph. Each part consists of a position sensitive proportional counter for momentum determination, followed by an ionization chamber and a scintillator for particle identification. This set-up allows the coincident detection of the break-up fragments emitted from the 4 mg/cm2 208Pb target in ne...

Investigation of the sequential break-up mode6Li?6Li*(31 +)??+d of 156 MeV6Li projectiles on208Pb in the very forward angular hemisphere

“…A particular interest arises from a novel approach [3] of nuclear astrophysics using Cou-lomb break-up as an access to nuclear transition matrix elements which govern the "time-reversed" process: lowenergy radiative capture reactions of astrophysical interest. Of course, there is less doubt [4][5][6] that the breakup is dominated by the Coulomb force for disintegration events when the fragments are scattered in the angular region distinctly below the grazing angle. Nevertheless the question to which extent the interference with a potentially remaining nuclear amplitude affects the pattern of the differential cross sections is of quantitative nature.…”

Coulomb Breakup of Nuclei—applications to Astrophysics

“…In order to approach the experimental requirements the pilot experiment (31,32), exploring the <Li ~ a+d breakup at low relative energies, used a specially designed spectrometer setup and applied the split-focal plane detector technique (36,37) The spectrograph technique strongly mmtmtses problems of conventional detector telescopes: geometric limitations, overloading; reducing the coincidence detection efficiency by the dominance of elastic scattering events. It can be extended for to charged-particleneutron coincidence measurements by inserting a straight-through port in the focal plane (36).…”

E1-E2interference in the Coulomb dissociation of8B

“…1 a comparison of our calculations with the data [17] for the triple differential cross sections for the reaction 6 Li + Pb → α + d + Pb at 156 MeV beam energy as a function of the relative energy between the alpha particle and the deuteron for θ α = θ d =3…”

“…The data have been taken from [17]. Positive E αd implies velocity of α particle is larger than that of deuteron, while negative E αd corresponds to larger deuteron velocity than that of α particle.…”