H.J. Weyer

Pion absorption in light nuclei

Abstract The recent developments in the study of pion absorption are reviewed. The dominance of two-nucleon absorption is discussed for stopped and in-flight pion absorption. Emphasis is put on the latest developments in the knowledge of multi-nucleon absorption. The strong affinity of pion- and photon-induced reactions is discussed.

Kinematically complete measurement of the absorption of stopped pions in 3He

Abstract The reactions π −3 He → nnp and π −3 He → nd have been measured in a kinematically complete experiment with stopped pions. The results show strong preference for collinear events. About three quarters of all absorption processes occur as quasi-free absorption on np- or pp-pairs with a ratio of R = 10.1 ± 1.5. The remaining quarter shows energy sharing among all three nucleons and is located in the final-state interaction regions. Branching ratios for all observed final states are given.

A large-area position-sensitive time-of-flight counter for energetic neutrons and charged particles☆

Design parameters and performance characteristics of a large position-sensitive scintillation counter developed for neutron and charged particle detection in the range from 10 MeV to 300 MeV are given. The time-of-flight (TOF) counter consists of thirty bars of 5 × 10 cm2 cross section and 2 m length each arranged in a suitable counter matrix covering an overall effective detection area of ∼1 × 2 m2. The individual bars have pulse height responses over the length uniform to 5% except close to the ends, position resolutions of almost 5 cm and intrinsic time dispersions of 450 ps. Typical integral neutron detection efficiencies are 19% for 50 MeV neutrons and 13% for 170 MeV neutrons at a pulse height threshold of 5 MeV electron-equivalent energy (MeVee).

Pionic and muonic X-ray measurements on light isotopes

Abstract New data on strong interaction effects in the pionic atoms of 3 He, 16 O, 18 O and Cl are presented. Energies and lorentzian widths of the 1s and partly of the 2p levels have been determined from the X-ray spectra. For the first time significant isotope effects could be observed in the 1s level width. Absolute and relative intensities are presented for K and L X-ray transitions respectively trom pionic and muonic spectra. Thus by fitting cascade parameters p-state absorption probabilities could be determined even for the lightest isotopes. The results are compared with those of others and with recent theoretical calculations. The nuclear γ-ray of the transition 5 2 + → 3 2 − in 13 C was determined to be 169.356 ± 0.020 keV.

Isospin dependence of pion absorption on nucleon pairs in 3He

Abstract The ratio of quasifree pion absorption on nucleon pairs with isospin I = 0 and I = 1 has been measured in 3He. The ratios R(T π , θ lab ) = σ[ 3 He (π + , pp )]/σ[ 3 He (π − , np )] were determined for 120 MeV pions at proton angles of 77° and 54° relative to the incident beam and for 165 MeV pions at 54°. The corresponding results are: R(120, 77) = 22.2 ± 7.5, R(120, 54) = 17.0 ± 5 and R(165, 54) = 25.0 ± 7, respectively. Together with our previous results on pions at rest the data provide for the first time a homogeneous set of R values from kinematically complete experiments over a wide energy range.

What is the mechanism for three-nucleon absorption of pions in 3He?

Abstract Experimental data for 3 He(π + , pp)p at T π =120 MeV in a kinematical region dominated by three-nucleon absorption, where all three protons have obtained an appreciable amount of energy, are analyzed by means of variables specifically sensitive to different reaction mechanisms. The data are compared with predictions for two-step mechanisms and a genuine three-nucleon absorption mechanism. No evidence is found for such two-step processes as scattering or charge exchange preceding a two-nucleon absorption or as two-nucleon absorption followed by “Hard FSI”. The results, however, agree with a genuine three-nucleon absorption process, consistent with the recently measured phase-space-like behavior of this process.

Three-nucleon processes and the total pion absorption cross section in 3He

Abstract Pion absorption processes involving three nucleons have been investigated in 3 He with positive and negative pions of 64, 119, 162 and 206 MeV kinetic energy. The most important of these processes is characterized by a constant phase-space density and has comparable integrated cross sections for π + and π − with a Δ-resonance-like dependence on the pion energy. This process constitutes up to one third of the total absorption cross section. The total absorption cross section has been determined by adding up the measured cross sections for all the observed absorption channels.

Pion absorption in flight on 4He

Abstract We have measured double and triple coincidences following π + and π − absorption on 4 He at various pion energies. Differential and integrated cross sections are determined for all possible absorption channels. We study and compare the two-nucleon absorption on nucleon pairs with isospin I = 0 and I = 1. In the dNN final-state two absorption mechanisms are identified, one with only one spectator nucleon and one with phase-space behavior. The existence of a three-nucleon absorption component with a spectator nucleon is established and a four nucleon phase-space-like contribution is also identified and found to be small. The total absorption cross section is directly determined by adding the integrated cross sections of all measured reaction channels.

LARGE-SOLID-ANGLE STUDY OF PION ABSORPTION ON 3HE

Measurements have been made of [pi][sup +] absorption on [sup 3]He at [ital T][sub [pi]][sup +]=118, 162, and 239 MeV using the Large Acceptance Detector System. The nearly 4[pi] solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The total absorption cross section is reported. In addition, the total cross section is divided into components in which only two or all three nucleons play a significant role in the process. These are the first direct measurements of the total and three nucleon absorption cross sections.

A LARGE ACCEPTANCE DETECTOR SYSTEM (LADS) FOR STUDIES OF PION ABSORPTION

Abstract A Large Acceptance Detector System (LADS) has been designed and built at the Paul Scherrer Institute to study multiparticle final states following pion-nucleus absorption. It consists of a 28-sector cylinder of plastic scintillators of 1.6 m active length and 1.4 m diameter, two cylindrical wire chambers, and two 14-sector plastic scintillator end-caps which close each end. The nearly 4π solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The design and the performance of the LADS detector are presented.