R.C. Byrd

Measurements and calculations of the performance of a multi-element neutron detector

Abstract Large, multi-element scintillation detectors are increasingly being used in neutron experiments, particularly at medium energies, to provide the coincidence requirements needed for kinematic constraints and background rejection. These arrays involve specialized techniques for event processing and calibration, and their development has created the need for new simulation programs. We describe the design and testing of such a program, including detailed discussions of procedures for event reconstruction detector calibration, and resolution measurements for segmented detectors. The discussions are illustrated using a prototype array composed of three 51×16×4 cm3 planes, each consisting of four scintillator bars 51 cm in length. Comparisons are made to measurements obtained with tagged beams of neutrons and protons at energies of 100–200 MeV. Very good agreement is obtained for efficiencies, resolutions, energy losses, inter-element velocities, and angular distributions. The results allow detailed analyses of the effect of charge-particle vetos and the assumptions about carbon breakup reactions in organic scintillators.

Neutron detection and applications using a BC454/BGO array

Neutron detection and multiplicity counting has been investigated using a boron-loaded plastic scintillator (BC454)/bismuth germanate (BGO) phoswich detector array. Boron-loaded plastic combines neutron moderation (H) and detection ({sup 10}B) at the molecular level, thereby physically coupling increasing detection efficiency and decreasing die-away time with detector volume. Separation of the phoswich response into its plastic scintillator and bismuth germanate components was accomplished on an event-by-event basis using custom integrator and timing circuits, enabling a prompt coincidence requirement between the BC454 and BGO to be used to identify neutron captures. In addition, a custom time-tag module was used to provide a time for each detector event. Time-correlation analysis was subsequently performed on the filtered event stream to obtain shift-register-type singles and doubles count rates.

A test of charge symmetry in n-p scattering at En = 183 MeV

Abstract Measurements of the spin-dependent left-right asymmetries for n-p elastic scattering at E n = 183 MeV have been obtained by scattering polarized neutrons from polarized protons. If charge symmetry holds in the n-p system, the two analyzing powers A n and A p must be equal. Our preliminary experimental result for the average value of A n −A p in the range 82.2° θ cm ≤ 116.1° is (32.1 ± 6.1 ± 6) × 10 −4 where the first error is the statistical uncertainty and the second is the systematic uncertainty. This result provides clear evidence of charge symmetry breaking in the nuclear force, and agrees well with meson-exchange predictions based on the Bonn nucleon-nucleon potential.

Cross section and analyzing power for quasifree (p, n) reactions

Abstract Cross sections and analyzing powers have been measured for 12C(p, n) at 494 MeV and 795 MeV and for Pb(p, n) at 795 MeV. The data span an energy loss range of at least 200 MeV, vhich is sufficient to clearly reveal the large peak associated with quasifree neutron knockout. The kinematics, cross section and analyzing power for this peak are compared to the corresponding observables for free nucleon-nucleon scattering and to the observables calculated with a relativistic Fermi-gas model.

Measurements and calculations of the gamma response of neutron scintillators

Abstract By adapting routines from existing simulation programs for the interaction of neutrons and gammas with organic scintillators, a code has been produced to calculate the response of multi-element detectors to neutrons below 200 MeV and gammas below 10 MeV. The approach for the gamma calculations is similar to the macroscopic model used for neutron interactions. To test the approximations concerning gamma multiple-scattering and electron range-energy relations, measurements were made for the response of plastic scintillators to gammas at energies up to 1.3 MeV. The measurements focus on count rates and light-output distributions, and their analysis includes discussions of energy-to-light conversion and detector resolution. All measurements and calculations are in good agreement. The conclusions are discussed in terms of calculations of detection efficiencies for neutrons at energies of 1–200 MeV.

Charge symmetry tests: Final charge symmetry violation results from IUCF

In the context of ‘‘Charge Symmetry Tests’’, we report final results from an n‐p scattering experiment completed at IUCF providing definitive evidence for charge‐symmetry‐breaking (CSB) in the strong interaction. Our results are well described by meson‐exchange calculations which include ρ−ω mixing in addition to the n‐p mass difference effect on one‐boson exchange.

Ratio of Gamow-Teller to Fermi Strength Observed in 13,14C(p,n) at 492 and 590 Mev

It has been recognized for a number of years that certain spin-isospin components of the nucleon-nucleus effective interaction can be inferred from (p,n) reactions to states of known nuclear structure. For L = 0, S = 0 and L = 0, S = 1 transitions, the 0-degree (p,n) cross section can be related respectively to Fermi and Gamow-Teller beta decay matrix elements1. If these transitions occur in the same nucleus, the ratio of isovector spin-flip to non-spin-flip effective interactions can be measured without regard for absolute normalization. The best reaction to measure this is 14C(p,n) which goes by a pure Gamow-Teller transition to the 1+ state at 3.95 MeV in 14N, and Fermi transition to the 2.31 MeV 0+ state. This work extends the ratio measurements made at lower energies (ref. 1, 2, 3) to 492 and 590 MeV.