S. Cierjacks

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).

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.

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.

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.

Search for emission of nucleons and nucleon pairs following muon capture in selected light nuclei

Preliminary results of measurements of proton energy spectra and search for coincidentp, n pair emission following negative muon capture in12C,16O,27Al are given. A very clear signal for proton emission is found, which can be exploited in future detailed studies.

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.

Direct three-nucleon pion-absorption processes in 3He

A new reaction mechanism has been discovered in pion absorption on 3 He in which all three nucleons of the target are involved. From detailed measurements it has been shown that this type of reaction has a constant matrix element over the entire phase space of the 3N final state. This feature strongly supports a direct three-nucleon absorption process. Sequential processes like knock-out of one nucleon with subsequent absorption of the pion on the other two nucleons can be excluded. The fraction of the new reaction mechanism was found to be approximately 20% of the total pion absorption cross section in 3He. Possible relations of the new process with a three-body force are briefly mentioned.