J. Krug

The COSY-TOF barrel detector

Abstract A barrel-shaped scintillator hodoscope was developed as part of the time-of-flight spectrometer TOF operated at an external beam line of the cooler synchrotron COSY. The COSY-TOF spectrometer is used mainly to perform kinematically complete experiments on light meson production and proton–proton bremsstrahlung in proton–proton collisions for laboratory energies up to 2.5 GeV. The newly developed scintillator hodoscope (BARREL1), one of several detector segments of the COSY-TOF spectrometer, consists of one layer of 96 scintillator bars of 15 mm thickness arranged to form a barrel with 3 m in diameter and 2.85 m in length. For any given interaction process the TOF spectrometer allows to determine the velocity vectors of all charged ejectiles emerging from the target by measuring the time of flight between the start and the stop detector as well as the point of impact on the stop detector. For the BARREL1 detector the position information is obtained by two-sided light read-out of the scintillator bars. For minimum ionizing particles the spatial resolution was measured to be Δ z( FWHM )=8 cm , corresponding to a polar angle resolution in the range 0.57°≤Δ Θ ≤2.86°, depending on the hit position. The binning in azimuthal angular direction amounts to Δ Φ =360°/96=3.75°. For the time-of-flight resolution of minimum ionizing particles a value of Δ t( FWHM )=0.61 ns was obtained.

Cross section and analyzing power A y in the breakup reaction H 2 ( p →, pp ) n at 65 MeV: Collinearity configurations

Kinematically complete breakup cross section and proton analyzing power data for four different collinearity configurations (neutron at rest in the c.m. system) have been measured in the reaction [sup 2]H([ital [rvec p]],[ital pp])[ital n] at [ital E][sub [ital p]][sup lab]=65 MeV. The experimental data are compared with rigorous solutions of the Faddeev equations using the Argonne AV14, Bonn B, Nijmegen, and Paris potentials. While the overall agreement is quite good there exist distinct discrepancies between theoretical and experimental cross section and analyzing power data in some regions of phase space.

Analyzing power in n+d elastic scattering at 67 MeV

The analyzing power Ay of 2H(n, n)2H elastic scattering at En = 67 MeV has been measured in the angular range 30° < θ c.m. < 165°. The data are in good agreement with the results of our rigorous three-nucleon calculations employing the PARIS and the BONN B potential.

Ghost anomaly and first excited state of 8Be in the 9Be(d, tα)4He reaction at 7 MeV

Abstract The three-body reaction 9 Be + d → t + α + α has been investigated in kinematically complete experiments at E d = 7.0 MeV for θ t = −40° and θ t = 40°, θ α = −72.5° to −162.5. The experimental spectra are dominated by sequential processes via intermediate excited states of 8 Be and 7 Li. R -matrix theory was employed to describe the line shape of the first excited state and the ghost anomaly of the ground state of 8 Be. Use was made of the αα phase shifts. The best fits were obtained assuming a channel radius of a αα = 4.5 fm. The average intensity of the ghost anomaly for E αα = 0.35–1.0 MeV is 4.3%/MeV ( θ t = 40°) and 8%/MeV ( θ t = 100°). For the first excited state of 8 Be we find E x = 3.00 MeV and Γ x = 1.23 MeV(FWHM).

Analyzing power of the 2H(n, np)n breakup reaction at 67 MeV in selected kinematical configurations

Abstract The angular distribution of the analyzing power A y for the 2 H( n , np)n breakup reaction at 67 MeV was measured in two kinematical regions: np quasi-free scattering and np final-state interaction. The results are compared with rigorous three-nucleon calculations using the Paris potential and allowing for charge dependence in the 1 S 0 state. According to the actual experimental conditions energy averaging was simulated in the theoretical calculations. In both configurations agreement between experimental and calculated values is found.

Resonant charged particle emission from 11B+p and 12C states at x = 20−23 MeV

Abstract Absolute differential cross sections of the reactions 11 B(p, p i ) 11 B( i ) , i = 0−3, and 11 B(p, α 0 ) 8 Be have been measured at about 20 angles for up to 37 bombarding energies E p = 4.5−7.5 MeV corresponding to excitation energies E x ( 12 C ) = 20.1−22.8 MeV . Several compound resonances are observed for which from their angular distributions the following spin and parity assignments were derived: E x = 20.25 MeV ( J ) π = 1 + ), 20.63 (3 − ), 21.52 (3 − ), 21.99 (1 − ), and 22.47 (3 − ). The positions and widths of the resonances were determined by R -matrix formulae. In all cases the sum of the proton partial widths virtually exhausts the total width indicating dominant 1p1h configurations. The resonances at 20.25, 20.63 and, tentatively, 21.99 MeV are identified to be the T z = 0 analog states of low-lying T = 1 multiplets. A strong J π = 2 + background was found throughout the whole energy range which was taken to be the low-energy tail of the isoscalar giant quadrupole resonance.

Differential cross section for elastic n+d scattering and n+d break-up quasi free-scattering at 67 MeV

Abstract The differential cross sections for the n+d elastic scattering and for the 2 H(n,np)n break-up reaction in the np quasi-free scattering configuration was measured at 67 MeV. The experimental data are compared with results of the calculations based on a rigorous solution of the three-nucleon Faddeev equations using meson-exchange potentials. Good agreement is found between experiment and theory for the n+d elastic-scattering cross section while in the case of np quasi-free scattering the theoretical predictions overestimate the data at neutron angles larger than 40° by about 30%.

Facility for neutron induced few body reactions at Bochum University

Abstract A facility is described which is designed for the measurement of neutron induced three-body breakup. It has been used for the breakup of deuterium and of the nucleus 9 Be. Neutrons are produced by a pulsed beam of deuterons from the Bochum 4MV Dynamitron-Tandem accelerator by bombarding a thick tritium-titanium target or a deuterium gas target. The outgoing beam is collimated by a 4π shielding to a solid angle of about 1 msr. In most cases, a liquid scintillator (NE232 or a mixture of NE232/NE213) serves as a target for the neutron beam. Scattered neutrons are detected by NE213-detectors of different sizes. For testing purposes the differential elastic n-d cross section and simultaneously the response of NE232 have been measured at 22.4 and 7.9 MeV.

Intermediate structure effects in the reaction23Na(p, α)20Ne

Abstract Excitation functions of the reaction 23 Na(p, α) 20 Ne have been taken at eight angles between θ L = 30°and170° for proton energies E p = 4.0–8.0MeV. The average step size was Δ E = 12keV. An autocorrelation analysis of the strongly fluctuating α 0 yield curves revealed the presence of two coherence widths at a mean excitation energy of E x ( 24 Mg) = 17.5MeV. The smaller width Γ = 58±4keV was the only one observed in a simultaneously recorded α 1 yield curve and is identified with the fine structure or compound nuclear width. No indications of a J -or T -dependence were found. The larger width of about 150 keV is interpreted in terms of intermediate structure effects. A comparison with autocorrelation analyses of several hundred synthetic excitation functions was performed.

Neutron-neutron scattering length in the reaction9Be (n, nn)8Be at 10.3 MeV

In a kinematically complete experiment on9Be(n, nn)8Begs at 10.3 MeV, the absolute differential cross section of then-n FSI configuration at Θlab=30° has been measured. It was our particular object to determine the singlet scattering length of then-n interaction via its final-state interaction being embedded in a multi-nucleon background. An extended and thorough analysis yielded1ann=−16.5±1.0 fm which is consistent with the results of the most recent and most precise kinematically complete measurements with three hadrons in the final state. These, however, altogether contradict the values deduced from π−d→nnγ.