P. Weber

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).

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.

Two-nucleon absorption of π+ and π− on 3He across the Δ-resonance region☆

Abstract The angular distributions of nucleons from a kinematically complete measurement of pions absorbed in 3He on nucleon pairs with isospin T = 0 and T = 1 at incident pion energies of 64, 119, 162 and 206 MeV are presented. The T = 0 data are quite similar to pion absorption data in deuterium but scale with the number of nucleon pairs. The T = 1 cross sections are an order of magnitude smaller and the angular distributions show a significant asymmetry. This asymmetry together with the measured energy dependence indicates interference effects between amplitudes with and without Δ isobars in the intermediate states.

Measurement of the ratio (π−3He → π0t)(π−3He → dn) and of the respective partial K X-ray yields

Abstract The reactions π −3 He → π 0 t) and π −3 He → dn have been measured for stopped pions. The ratio (π −3 He → π 0 t ) (π −3 He → dn ) was determined to be 1.15 ± 0.12. In addition the pionic K X-ray were measured in coincidence for both reaction channels resulting in K X-ray yields of Y π 0 K = (35 ± 4)% and Y d K = (45 ± 5)% respectively. Using also results from previous measurements, branching ratios per stopped pion for all absorptive channels as well as their absolute 1s rates were determined.

A position-sensitive detector system with subnanosecond timing and its utilization in the measurement of pion absorption

A complex position-sensitive, time-of-flight detector was designed at the SIN to be used in the kinematically complete measurements of pion absorption in light nuclei, for the detection of both neutrons and charged particles. Design parameters and TOF properties resulting from the computer calculations and suitable test measurements are given for the system consisting of 200 × 10 × 5 cm3 NE 102A scintillator bars, XP 2230 photomultipliers and discriminator. Position dependence of the timing parameters of the 2 m long NE 102A bar viewed from the ends by photomultipliers has been clearly revealed in our model and experimentally verified.

Pion Absorption in 3He and 4He Nuclei

The absorption of positive and negative pions across the delta resonance was studied in the helium isotopes. New results on primary reaction mechanisms involving 2 and 3 nucleons were obtained.

Radiative decay of the Delta resonance: Analyzing powers for pi -p

The polarized target asymmetry [ital A][sub [ital N]] for [pi][sup [minus]][ital [rvec p]][r arrow][gamma][ital n] has been measured by coincident detection of the [gamma] and the neutron at several angles across the [Delta] resonace energy region. A high-resolution NaI [gamma] spectrometer together with good time-of-flight information for the neutron resulted in excellent separation of this radiative capture reaction from the much higher cross-section charge exchange reaction [pi][sup [minus]][ital [rvec p]][r arrow][pi][sup 0][ital n]. Statistical uncertainties are [Delta][ital A][sub [ital N]]=0.02[minus]0.04, representing a significant improvement over previous results. The new data are discussed in the context of recent theoretical models.

Radiative decay of the Δ resonance: Analyzing powers forπ−p→→γn

The polarized target asymmetry [ital A][sub [ital N]] for [pi][sup [minus]][ital [rvec p]][r arrow][gamma][ital n] has been measured by coincident detection of the [gamma] and the neutron at several angles across the [Delta] resonace energy region. A high-resolution NaI [gamma] spectrometer together with good time-of-flight information for the neutron resulted in excellent separation of this radiative capture reaction from the much higher cross-section charge exchange reaction [pi][sup [minus]][ital [rvec p]][r arrow][pi][sup 0][ital n]. Statistical uncertainties are [Delta][ital A][sub [ital N]]=0.02[minus]0.04, representing a significant improvement over previous results. The new data are discussed in the context of recent theoretical models.