Neutrons Caused By The Deuteron Rupture Experimental Study Of The Reaction

Few body problems have received special attention from the early days of nuclear physics. Great progress has been made during the last decades in few nucleon system through tremendous research on it, both theoretically and experimentally. Most of the experimental data of three nucleon(3N) system can be well predicted by nowadays theory based on modern realistic N—N potentials such as CD-Bonn, Paris, Nijm I and II. One of the most interesting 3N system is the nd system. The exact solutions of the rigorous Faddeev-type calculation can be done because there is no coulomb force involved in nd system. Therefore, great progress has been made in theoretical predictions for nd reaction from the last decade, both in elastic and breakup channel. The high precision of the theoretical calculation calls for the new requests on experimental data and improves the experiments.However, a few striking discrepancies still exist, mainly as the following: (1) The vector analyzing power puzzle at low energy, i.e, large discrepancies exist between experiment and theory in polarized experiment. (2) Space star anomaly. the breakup cross section is not described correctly if the three nucleons fly apart in a symmetric star oriented perpendicular to the beam direction. (3) The neutron-neutron quasi-free scattering(QFS). the measured cross sections for neutron-neutron QFS are about 20% larger than predicted. (4) The neutron-neutron scattering lengthα_nn. At present the determinedα_nn from different experiments can not agree with each other. This basic parameter for N—N interaction still remains unsolved up to now.In this work, we have done detailed experimental studies in the following two aspects.One is the experimental study of n—n quasi-free scattering in the nd breakup reaction at 25.0 MeV. Few precise experimental data are available up to now due to the difficulties of measurement. While measured data with high quality are necessary to probe the n—n interaction and to test the nowadays theoretical calculation for 3N system based on modern realistic N—N potentials.The experiment was performed on the HI-13 tandem accelerator in China Institute of Atomic Energy(CIAE). The 25.0 MeV neutrons were produced by T(d, n)⁴He reaction. Comparing to the D(d, n)³He reaction, the advantage of this neutron source is that the energy of the breakup neutrons are much lower than that of D(d, n) reaction. The highest energies of the breakup neutrons from T(d, n) reaction and D(d, n) are about 20 and 6 MeV lower than the energies of mono-energetic neutrons, respectively. Aφ10×75 mm tritium gas target filled with tritium gas at a pressure of 2.2 bar was used to produce neutrons. A pre-chamber withφ10×30 mm filled with 0.3 bar helium gas was employed to ensure the safety operation of tritium gas target. The source neutrons were shielded and collimated at zero degree and then luminated a cylindrical (φ20×20 mm) CD₂ target. Two BC501A neutron detectors (φ180×100) were positioned symmetrically atθ_n=±42.2°(angle between the neutron beam and outgoing neutrons) and 80 cm from the CD₂ target, to detect the two neutrons from QFS. The absolute neutron beam fluence was determined by n-p scattering, putting a polyethylene foil between gas cell and CD₂ target and detecting the recoiled proton at 30 degree with aâ–³E-E telescope. The distance between the polyethylene foil and the E detector(center to center) was 30 cm. Thus the absolute cross sections of QFS were normalized by n-p scattering. Another ST-451 liquid scintillator was positioned at 60 degree and 2.5 m from the CD₂ target to detect the elastic scattered neutrons from C and D of the CD₂. Thus the absolute cross sections also can be normalized by n—C elastic scattering. The results from these two normalization were used to check each other. This improve the reliability of the result. The absolute cross sections were measured with an accuracy of a few percent(about 5%).The large amount of background was suppressed by the coincidence measurement of the two QFS neutrons. The accidental and sample out background was also measured in detail. The calibration and background subtraction were performed and analyzed carefully.The measured data were analyzed by detailed Monte-Carlo simulation based on rigorous three-body calculations with realistic nucleon-nucleon potentials.The measured cross sections are (16.0±4.6)% larger than the theoretical predictions based on CD-Bonn. The discrepancy between experiment and theory was discussed. The measured data were also compared with the theoretical predictions with other realistic N—N potentials. We conclude that in n—n QFS regard, the present theory can not reproduce the experimental data correctly. The other experiment in this work is the experimental study of n—n final-state-interaction(FSI) in nd breakup reaction at 17.4 MeV. The ¹S₀ stateα_nn was determined by precise measurement of the absolute cross sections of protons from FSI at around 0 degree. The 17.4 MeV neutrons were produced by D(d, n)³He reaction with a deuterium gas target. The gas target was 12 mm in diameter and 42 mm in length, filled with deuterium gas at a pressure of 4.2 bar. Both ends of the gas cell were closed with 10-μm-thick Havar foil. A 0.5-mm-thick cooled gold disc close behind the gas target was served as beam stop and Faraday cup. A 15-cm-long cylindrical reaction chamber, containing the scattering target and a detector telescope, was placed at zero degree with respect to the deuteron beam directly behind the beam stop. The target consisted of a 10-mm-long steel cylinder lined with graphite on the inside, and had a diameter of 10 mm. It was closed at both ends with nominally 75-μm-thick Kapton foils and filled with deuterium gas at a pressure of 20.9 bar. The distance from neutron source to reaction chamber and from reaction chamber to E detector was 9.0 and 9.1 mm, respectively. The telescope was housed in a carbon collimator and shielded from all sides by graphite against neutron-induced charged-particle background. The steel wall of the vacuum chamber was cooled to improve the resolution of detectors and decrease the radiation damage. The effective thickness of the reaction target was also increased.The recoiled deuteron and FSI proton events were clearly selected by coincidence measurement ofâ–³E and E detectors. The backgrounds were subtracted and cor-rected(The background from deuteron breakup reactions induced by energy degraded neutrons can not be subtracted by measurement. The energy degraded neutrons were mainly produced by n—p scattering in Kapton foil) carefully.The theoretical calculations based on CD-Bonn, Nijm I and Bonn-B potentials were incorporated with detailed Monte-Carlo simulation. The simulated data were fitted to the measured ones to get the minimum x² The determinedα_nn of this work isα_nn =-(16.5±0.9) fm. In this experiment, the absolute yield of the breakup spectrum is reproduced very well over the entire energy range investigated, unlike in most previous such experiments. This means that the background subtraction and correction were done correctly in this work.The result ofα_nn = - (16.5±0.9) fm in this work agrees with the result of Bonn(α_nn = - (16.2±0.3 fm)) and the average value before 1990(α_nn= - (16.7±0.5 fm)) with the complete configuration measurement in nd breakup reaction. It is also comparable with the previous incomplete configuration measurements(α_nn≈- 15.5 fm). However, this result conflicts with theα_nn determined from D(Ï€^-,nn)γreaction and the recentα_nn result measured in TUNL with complete configuration in nd breakup reaction(≈-18.7 fm). This means that it is necessary to do more study in this regard.