Measurement of the absolute differential cross section fornpelastic scattering at 194 MeV

Abstract

We describe a double-scattering experiment with a novel tagged neutron beam to measure differential cross sections for np back-scattering to better than ±2% absolute precision. The measurement focuses on angles and energies where the cross section magnitude and angle-dependence constrain the charged pion-nucleon coupling constant, but existing data show serious discrepancies among themselves and with energy-dependent partial wave analyses (PWA). The present results are in good accord with the PWA, but deviate systematically from other recent measurements. PACS numbers: 25.40.Dn, 25.10.+s, 28.20.Cz The neutron-proton elastic scattering database at intermediate energies is plagued by experimental inconsistencies and cross section normalization difficulties [1, 2, 3]. These problems have led the most sophisticated partial wave analyses (PWA) of the data [4, 5, 6] to ignore the majority (including the most recent) of measured cross sections, while the literature is filled with heated debates over experimental and theoretical methods [7, 8], including proposed radical "doctoring" (angle-dependent renormalization) to "salvage" allegedly flawed data [9]. Meanwhile, an empirical evaluation of a fundamental parameter of meson-exchange theories of the nuclear force-the charged πNN coupling constant, f 2 c-hangs in the balance [8]. We report here the results of a new experiment, carried out utilizing quite different techniques from earlier measurements in an attempt to resolve the most worrisome experimental discrepancies. The present experiment involves a kinematically complete double-scattering measurement to produce and utilize a "tagged" intermediate-energy neutron beam [10], thus greatly reducing the usual systematic uncertainties associated with the determination of neutron flux. Products from the second scattering were detected over the full angle range of interest simultaneously in a large-acceptance detector array, to eliminate the need for cross-normalization of different regions of the angular distribution. The use of carefully matched solid CH 2 and C targets permitted frequent measurement and accurate subtraction of quasifree scattering background, thereby minimizing reliance on kinematic cuts to isolate the free np-scattering sample. These methods, combined with multiple internal crosschecks built into the data analysis procedures, have allowed us to achieve systematic error levels in the absolute cross section below 2%. In addition to addressing the previous discrepancies, the results provide a useful absolute cross section calibration for intermediate-energy neutron-induced reactions.

Working Group Summary: Chiral Dynamics in Few-Nucleon Systems

“…17 S. Vigdor gave an overview of a recent experiment aimed at resolving this discrepancy. 18 In this double-scattering experiment, the flux of the neutron beam was accurately determined by counting the outgoing protons in the reaction 2 H(p, n)2p that produced the beam. The results of this absolute np differential cross section measurement at backward angles and a lab energy of 194 MeV are consistent with the predictions of the Nijmegen PWA.…”

Reference Cross Sections Relevant for Neutron Detectors

“…Recently, the entire problem has been addressed at IUCF with a novel technique [7,8]. Tagged neutrons have been produced at 194 MeV using the D(p,n) 2 He reaction, where 2 He denotes two protons in a 1 S 0 state, i.e., by detecting two correlated low-energy protons, the energy and direction of the outgoing neutron was identified event-by-event.…”

“…Proton recoils from these tagged neutrons impinging on a scintillator target were detected in a large scintillator array equipped with wire chamber tracking. [7,8], compared with data from TSL Uppsala [5] and with PWA calculations [9] at two relevant energies. The figure is taken from ref.…”

“…[8], i.e., "present experiment" refers to ref. [8]. The error bars on the present results are statistical (including background subtraction), while the shaded band represents all systematic uncertainties, including those in the overall normalization.…”

Spin observables and spin structure functions: Inequalities and dynamics

“…A summary of most of this work can be found in a review by Leluc and Lehar [6] 1 . More recent measurements have been performed, e.g., by the PINTEX collaboration at the Indiana Cyclotron [7][8][9]; by the EDDA collaboration at COSY [10][11][12]; at JINR, Dubna [13]; etc.…”