Recoil proton tagged knockout reaction for He

We report for the first time the discrimination of the core fragment knockout and valence nucleon knockout reaction mechanisms at medium energy range, by the use of the recoil proton tagging technique. Intense ⁸He beams at 82.3 MeV/u were supplied by the RIPS beam line at RIKEN, and impinged on both hydrogen and carbon targets. Recoil protons were detected in coincidence with the forward moving core fragments and neutrons. The core fragment knockout mechanism is identified through the polar angle correlation and checked by various kinematics relations. This mechanism may be used to extract the cluster structure information of unstable nuclei. On the other hand, with the selection of the tagged valence nucleon knockout mechanism, a narrower peak of ⁷He ground state is obtained. The extracted neutron spectroscopic factor Sn=0.512(18) is relatively smaller than the no-tagged one, and is in good agreement with the prediction of ab initio Green?s function Monte Carlo calculations.     

A study of the proton resonant property in 22Mg by elastic scattering of 21Na + p and its astrophysical implication in the 18Ne(α, p)21Na reaction rate

Proton resonances in ²²Mg have been investigated by the resonant elastic scattering of ²¹ Na + p . The ²¹Na beam with a mean energy of 4.00 MeV/nucleon was separated by the CNS radioactive-ion-beam separator (CRIB) and bombarded a thick ( CH_{2 n} target. The energy spectra of recoiled protons were measured at scattering angles of θ_c.m. ≈ 172^° , 146^° , respectively. Several excited states observed before have been confirmed including two states (at 6.616, 6.796 MeV) observed at TRIUMF. A new state at 7.06 MeV has been observed, and another new one at 7.28 MeV is tentatively identified due to its low statistics. The proton resonant parameters were deduced from an R -matrix analysis of the differential cross-section data with a SAMMY-M6-BETA code. The astrophysical implication for the ¹⁸ Ne(α, p)²¹ Na reaction has been briefly discussed.     

Emission order of light particles and light fragments from hot nuclei followed by binary fission

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of ⁴⁰Ar+¹⁹⁷Au at E _beam /A=25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, we found that high velocity deuterons are emitted prior to high velocity Li fragments but low velocity deuterons are emitted after low velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as protons and deuterons. Furthermore, the emission orders are found to depend only weakly on the mass asymmetry of fission fragments. Received: 3 June 1997 / Revised version: 23 July 1997     

Study of proton resonances in 18Ne via resonant elastic scattering of 17F+p and its astrophysical implication

The proton resonant properties in ¹⁸Ne, which determine the reaction rate of the key stellar ¹⁴O(α,p)¹⁷F reaction, have been studied by using a technique of proton resonant elastic scattering of ¹⁷F+p. A 4.22 MeV/nucleon ¹⁷F radioactive ion (RI) beam was produced via a projectile-fragmentation reaction, and separated by a Radioactive Ion Beam Line in Lanzhou (RIBLL). By bombarding a thick (C_H2) _n target, the energy spectra of the recoiled protons were measured by two ΔE-E silicon telescopes at the center-of-mass scattering angles of θ _c.m.≈175°±5°, θ _c.m.≈152°±8°, respectively. Several proton resonances in ¹⁸Ne were observed clearly. A further R-matrix analysis of the experimental data is under way to determine the resonant parameters. The present work reports the preliminary results briefly.     

Emission of intermediate-mass fragments in the interaction of 16O with 59Co, 93Nb and 197Au

In this paper we study the emission of ⁸Be_gs, B and N fragments in the interaction of ¹⁶O ions with ⁵⁹Co, ⁹³Nb and ¹⁹⁷Au at incident energies varying from 6 to 25 MeV/nucleon. The spectra of these fragments, as well as those of C fragments studied in a previous paper, are dominated at forward angles by a component originating from break-up of ¹⁶O. At the higher incident energies break-up occurs after quite a sizeable projectile energy loss. Another mechanism which dominates at large emission angles, favours the emission of low-energy fragments and is attributed to the coalescence of nucleons during the cascade of nucleon-nucleon interactions by means of which the excited nuclei produced in the primary two-ion interaction thermalize. Received: 21 January 2003 / Accepted: 27 March 2003 / Published online: 5 June 2003     

Polarization in quasielastic (p,2p) scattering on a 4He nucleus at 1 GeV

The polarization of secondary protons from the (p, 2p) reaction induced by 1-GeV protons incident to a ⁴He nucleus was measured in a kinematically complete experiment. By using a two-arm magnetic spectrometer, two secondary protons from this reaction were recorded in coincidence at unequal scattering angles of Θ₁ = 18°?24.21° and Θ₂ = 53.22° over a broad range of the recoil-nucleus momentum K _B between 0 and 150 MeV/c. It was found that the measured polarization of either secondary proton is less than that observed in free elastic proton-proton scattering. The magnitude of this difference is determined by the mean binding energy of s-shell protons rather than by the effective nuclear density. The polarizations measured in inclusive quasielastic and elastic scattering of 1-GeV protons on ⁴He nuclei are presented for scattering angles in the range Θ₁ = 18° ? 24.21°.     

One-neutron knockout from Ne isotopes

One-neutron knockout reactions of ^24–28Ne in a beryllium target have been studied in the Fragment Separator (FRS), at GSI. The results include inclusive one-neutron knockout cross-sections as well as longitudinal-momentum distributions of the knockout fragments. The ground-state structure of the neutron-rich neon isotopes was obtained from an analysis of the measured momentum distributions. The results indicate that the two heaviest isotopes, ²⁷Ne and ²⁸Ne, are dominated by a configuration in which a s_1/2$s_{1/2}$ neutron is coupled to an excited state of the ²⁶Ne and ²⁷Ne core, respectively.     

Fragment spin distribution in 400 MeV40Ar+93Nb collisions investigated by light-particle anisotropies

In and out-of-plane angular correlations between light particles and projectile-like fragments from deeply inelastic collisions of 400 MeV⁴⁰Ar+⁹³Nb were measured. At backward angles, the equilibrium emission of a particles and protons from the target-like fragment was identified. By analysing the out-of-plane angular correlations in the framework of a semi-classical statistical model, an average spin of 30±2? and an average alignment ofP _zz =0.80±0.15 for the target-like fragment were deduced.     

Positive pion production at threshold on A 10B target

The angular distribution of low-energy positive pions (E_π ≈ 11 MeV) produced by 154 MeV protons on a ¹⁰B target has been measured between 16.2° and 54.8° c.m. angles. Only pions corresponding to the ground state of ¹¹B were observed. The data have been compared with theoretical angular distributions calculated in the framework of the one-nucleon mechanism using the DWBA formalism.     

Study of proton resonances in 18Ne via resonant elastic scattering of 17F + p and its astrophysical implication in the stellar reaction of 14O(α, p)17F

The stellar ¹⁴O (α, p) ¹⁷F reaction is thought to be one of the most important breakout reactions from the Hot CNO cycles into the rp-process in Type I X-ray bursters. In the present work, the properties of proton resonances in ¹⁸Ne have been investigated efficiently by utilizing a technique of proton resonant elastic scattering with a ¹⁷F radioactive-ion (RI) beam and a thick proton target. A 4.22 MeV/nucleon ¹⁷F RI beam, which was produced via a projectile-fragmentation reaction and experiencing a series of energy degradation, was separated by a Radioactive Ion Beam Line in Lanzhou (RIBLL) and bombarded a (CH₂)_n target. Energy spectra of the recoiled protons were measured by two sets of ΔE-E silicon telescope at center-of-mass scattering angles of θ _c.m. ≈ 175° ± 5°, θ _c.m. ≈ 152° ± 8°, respectively. Several proton resonances in ¹⁸Ne were observed, and their resonant parameters have been determined by an R-matrix analysis of the differential cross-sections. A doublet structure around 7.10 MeV has been identified and thought to be one state at 7.05 MeV (2⁺) and another one at 7.12 MeV (4⁺). The presently calculated total reaction rates of ¹⁴O (α, p) ¹⁷F are, at least, a factor of 1.2 ～ 1.9 larger than the previous ones in a temperature region of 1.7 ～ 3.0 GK mainly owing to the contribution from the 7.05 MeV (2⁺) state. This result implies that this breakout reaction may play a more important role than previously expected.     

Hydrocarbon ions with triplet ground states

The N.M.R. spectra of meta dichloro and meta dibromo benzenes are analysed in the nematic phase of 4-methoxy benzylidene-4-amino-α-methyl cinnamic acid-n-propyl ester at room temperature.

The direct couplings are found to be of negative sign. The molecules orient preferentially with the aromatic plane along the direction of the magnetic field and the C ₂-symmetry axis perpendicular to it.

The relations between the various direct couplings and the inter-proton distances are reported. The ratio of the distance between the meta protons along the axis perpendicular to the C ₂ symmetry and the ortho protons is determined precisely. The other ratios could not be determined to any reasonable precision. The reasons are discussed.

The angles between the C-H bonds of the equivalent protons and the C-C bonds in both the molecules are determined on the assumption of the C-H bond lengths and the ring geometry of benzene.     

Mößbauereffekt anFe 57 nach Neutroneneinfang in Fe56

Mößbauer effect measurements with the 14.4 keV gamma rays of Fe⁵⁷ following thermal neutron capture in Fe⁵⁶ are reported. During the transition from the capture level at 7.6 MeV to the first excited state at 14.4 keV recoil energies up to 549 eV are imparted to the Fe⁵⁷ nuclei. Thus the Mößbauer gamma rays are emitted by nuclei displaced from their normal lattice positions. The measurements have been performed in order to get informations about the final position of the recoiled nuclei in metallic iron and in ordered Fe-Al alloys. Mößbauer spectra of recoil atoms in metallic iron did not show any significant deviation from the spectra of atoms on normal lattice positions. In ordered FeAl alloys of CsCl structure changes in the isomer shift due to recoil effects have been found. In a Fe₃Al alloy of DO₃ type order additional changes in the internal magnetic field have been observed. All results are discussed in terms of possible final positions for the primary recoiled iron atoms in the lattice.     

Cross sections and reaction mechanisms of (p,pxn) reactions on 208Pb in the 24–52 MeV range

The cross sections for the reactions ²⁰⁸Pb(p, pn)^207mPb, ²⁰⁸Pb(p, p2n)^206mPb have been measured for 24, 28, 36, 44 and 52 MeV incident protons. The experimental results are shown to be consistent with the clean knockout mechanism for the (p, pn) reaction and with knockout of one neutron followed by evaporation of another for the (p, p2n) reaction. We deduce a theoretical formula for the cross section for these reactions.     

Polarization in the quasielastic scattering of 1-GeV/c protons on light nuclei

Polarization measurements in the A(p, 2p)B reactions on ⁶Li, ⁷Li, and ²⁸Si nuclei at a proton-beam energy of 1 GeV were performed in a kinematically complete experiment. By using a two-arm magnetic spectrometer, two secondary protons were recorded in coincidence at asymmetric scattering angles of θ₁=15°?26° and θ₂=58.6° for residual-nucleus momenta in the range K _B=0–150 MeV/c. Either arm of the spectrometer was equipped with polarimeters based on proportional chambers. The data coming from this experiment are analyzed within the distorted-wave impulse approximation. It is shown that the polarization of recoil protons formed at angle θ₂ in the interaction featuring a proton from the P shell of the ⁷Li nucleus can be described under the assumption of an effective intranuclear-proton polarization by using the single-particle shell-model wave function of the nucleus. Our data on the polarizations of the two protons from the reaction (p, 2p) on a ²⁸Si nucleus also suggest the effective polarization of the protons in the D shell of the ²⁸Si nucleus. It is found that, for high recoil-nucleus momenta of K _B≥90 MeV/c, the effective polarization of the protons in the P shell of the ⁶Li nucleus—this polarization was discovered in studying the polarization of recoil protons in the reaction ⁶Li(p, 2p)⁵He—cannot be described within the shell model assuming LS coupling. As might have been expected, the polarization of recoil protons knocked out from the S shells of the ⁶Li and ⁷Li nuclei comply well with the predictions obtained in the impulse approximation with allowance for the depolarization effect alone.     

Large-angle proton emission in the 9Be(p, 2p) reaction at 300 MeV

A ⁹Be(p, 2p) coincidence experiment performed to further elucidate the reaction mechanism for the production of energetic wide-angle protons in intermediate-energy proton-induced reac- tions is reported. Detectors in a coplanar geometry were used to measure coincidences between trigger protons at 90° to the beam and forward-angle protons on the opposite side of the beam. The incident proton energy was 300 MeV. We report both the inclusive spectra for the trigger protons and the differential mean multiplicities for the coincidence events.The outgoing proton energies were measured using NaI detectors. Trigger protons were grouped into 10 MeV bins covering the kinetic energy range from 55 to 155 MeV. The forward protons were measured over a kinetic energy range of 65–280 MeV and an angular range of 14–60° with respect to the beam.The present results are compared with two previous experiments which covered a more restrictive kinematical range. Calculations are performed with both phase-space and direct knockout models, and compared with experiment. Observation of angle and energy correlation effects suggested by knockout models indicate that such direct mechanisms provide a significant contribution to energetic wide-angle inclusive proton spectra.     

Study of the out-of-plane emission of protons and light fragments in symmetric heavy-ion collisions

Midrapidity protons from²⁰⁹Bi+²⁰⁹Bi collisions were measured with the Kaon Spectrometer at SIS at incident energies of E _Lab /A=400, 700 and 1000 MeV. Additionally, light fragments were analysed at 400 MeV. We have investigated the azimuthal emission pattern of the particles relative to the reaction plane as function of transverse momentum, bombarding energy and impact parameter. We observe an enhanced emission of particles perpendicular to the reaction plane at all bombarding energies. The ratio of the number of particles emitted out-of-plane/in-plane increases strongly with the particles transverse momentum. The anisotropy decreases with increasing beam energy. Composite particles show a much stronger effect than protons.     

Quasi-free proton-scattering at 164 MeV

Inclusive proton spectra have been measured over the angular range 25° – 150° for 164 MeV protons on ²⁷Al, ⁵⁸Ni, ⁶²Ni and ²⁰⁸Pb. At 25° and 30° a peak attributable to quasi-free scattering, accounting for ≈20% of the total number of fast protons at these angles, is present. No quasi-free peak is evident in the spectra at larger angles. These results are in disagreement with previously published work at this energy but in line with results reported at other energies.     

The sign of the magnetic moment of the first 2+ state in 20O

The measured precission of the ²⁰O(2⁺) spin in the magnetic experienced on recoiled in polarized iron implies a negative g-factor. A supplementary measurement on the ¹⁶O(3^?) state yields for the static field at oxygen in iron the limits 0 < H_static < +90 kG.     

Proton diffusion in the room-temperature phase of [(NH4)1−xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation in the rotating frame

Proton diffusion in the room-temperature phase (phase II) of [(NH₄)_1?xRb_x]₃H(SO₄)₂ (0≤x≤1) has been studied by means of ¹H spin-lattice relaxation times in the rotating frame, T_1ρ. The ¹H T_1ρ values were measured at 200.13 MHz in the range of 380–490 K. The ammonium protons and the acidic protons have independent T_1ρ values in the higher temperature range of phase II, suggesting that the spin diffusion between the two species is ineffective. The translational diffusion of the acidic protons is the most dominant mechanism to relax both the ammonium protons and the acidic protons in phase II. The ¹H T_1ρ values in phase II are analyzed theoretically and the motional parameters are obtained. The results of NMR well explain the macroscopic proton conductivity.     

Quasi-free scattering in 7(p,pd)5He and 12C(p,pd)10B reactions at 670 MeV

The reactions ⁷Li(p, pd)⁵He and ¹²C(p, pd)¹⁰ were investigated at 670 MeV in quasi-free scattering geometry at large angles. In transitions to low-lying as well as to highly excited states of the residual nuclei the energy distributions of the scattered protons are in agreement with the mechanism of quasi-free scattering on quasi-deuterons in the p- and s-shells of ⁷Li and ¹²C.