The first direct neutron–neutron scattering experiment using the YAGUAR pulsed reactor has yielded initial results. They show a unforeseen significant thermal neutron background as a result of radiation-induced desorption within the scattering chamber. Thermal neutrons are mostly scattering not from other neutrons but instead from the desorbed gas molecules. Analysis of the obtained neutron time-of-flight spectra suggests neutron scattering from H2 molecules. The presented desorption model agrees with our experimental value of the desorption yield ηγ=0.02molecules/gamma. Possible techniques to reduce the effect of the desorption background are presented.

The motivation and design is outlined for the experiment to measure the neutron–neutron singlet scattering length directly with thermal neutrons at the pulsed reactor YAGUAR. A statistical accuracy of 3% can be reached, though achieving the goal of an overall accuracy of 3–5% for the nn-scattering length depends on the background level. Possible sources of background are discussed in depth and the results of extensive modeling of the background are presented. Measurements performed at YAGUAR to test these background calculations are described. The experimental results indicate an anticipated background level up to 30% relative to the expected nn effect at the maximal energy burst of the reactor. The conclusion is made that the nn experiment at YAGUAR is feasible to produce the first directly measured value for the neutron–neutron scattering length.

The Direct Investigation of ann Association (DIANNA) is finalizing the design of a direct measurement of the nn-scattering length to be performed at the YAGUAR reactor in Snezhinsk, Russia. Extensive modeling of the neutron field, nn-scattering kinematics, and sources of detector background have verified the plan for a 3% measurement of ann. Measurements of the neutron flux support the neutron field modeling. Initial test measurements of the neutron field inside the underground channel have confirmed calculations of the thermal neutron background.

A direct neutron–neutron scattering length, ann, measurement with the goal of 3% accuracy (0.5 fm) is under preparation at the aperiodic pulsed reactor YAGUAR. A direct measurement of ann will not only help resolve conflicting results of ann by indirect means, but also in comparison to the proton–proton scattering length, app, shed light on the charge-symmetry of the nuclear force. We discuss in detail the analysis of the nn-scattering data in terms of a simple analytical expression. We also discuss calibration measurements using the time-of-flight spectra of neutrons scattered on He and Ar gases and the neutron activation technique. In particular, we calculate the neutron velocity and time-of-flight spectra after scattering neutrons on neutrons and after scattering neutrons on He and Ar atoms for the proposed experimental geometry, using a realistic neutron flux spectrum—Maxwellian plus epithermal tail. The shape of the neutron spectrum after scattering is appreciably different from the initial spectrum, due to collisions between thermal–thermal and thermal–epithermal neutrons. At the same time, the integral over the Maxwellian part of the realistic scattering spectrum differs by only about 6 per cent from that of a pure Maxwellian nn-scattering spectrum.

In order to resolve long-standing discrepancies in indirect measurements of the neutron-neutron scattering length an. and contribute to solving the problem of the charge symmetry of the nuclear force, the collaboration DIANNA (Direct Investigation of an, Association) plans to measure the neutron-neutron scattering cross section sigma(nn). The key issue of our approach is the use of the through-channel in the Russia reactor YAGUAR with a peak neutron flux of 10(18) /cm(2)/s. The proposed experimental setup is described. Results of calculations are presented to connect sigma(nn) with the nn-collision detector count rate and the neutron flux density in the reactor channel. Measurements of the thermal neutron fields inside polyethylene converters show excellent prospects for the realization of the direct nn-experiment.

We propose to perform the first measurement of the neutron–neutron scattering cross section in the through-channel of the pulsed aperiodic reactor YAGUAR (Snezhinsk, Russia). Such a measurement directly determines the neutron–neutron scattering length, and by comparison with the proton–proton scattering length bears upon the issue of charge symmetry of the nuclear force. The proposed experimental set-up, as well as modelling of the neutron density and of the frequency of neutron–neutron collisions is described. Experimental results are reported on the formation and optimization of the thermal neutron field inside the through-channel of the reactor YAGUAR. The instantaneous value of 1.1 × 1018 cm−2 s−1 obtained for the thermal neutron flux density is large enough to perform the first direct neutron–neutron scattering length measurement.

The TRIPLE Collaboration studies of space-parity symmetry in the compound nucleus show numerous examples of parity violation in Br, Rh, Pd, Ag, Cd, In, Sn, Sb, I, Cs, Xe, La, U, and Th. The longitudinal cross section asymmetries have measured values in the range of 10−3–10−1 for neutron energies from several eV up to 300–2000eV, depending on the target. The high density of states leads to enhancement of the parity violation by factors as large as 106 relative to parity violation in pp scattering. The high degree of complexity of the levels permits the use of statistical methods for determination of the root mean square weak matrix element M for each nucleus. This report is focused on the experimental results of the TRIPLE Collaboration studies. Parity violation has been observed in 75 resonances of 18 nuclides. The experimental data and analysis are presented for each nuclide studied. A nonstatistical anomaly (the sign correlation effect) was observed in thorium. Statistical analysis techniques were developed and successfully applied to determine the rms weak matrix elements and the weak spreading widths Γw. The value of Γw obtained from our analysis is about 1.8×10−7eV, which is in qualitative agreement with theoretical expectations. The individual weak spreading widths are consistent with a constant or slowly varying mass dependence and there is evidence for local fluctuations.

This paper gives a brief overview of Parity violation effects at neutron p-wave resonances observed by the TRIPLE collaboration at Los Alamos together with some information about neutron resonance capture gamma spectroscopy experiments carried out by the Geel collaboration in relation to these parity violation experiments.

The analysis of parity nonconservation (PNC) measurements performed on Th-232 by the TRIPLE Collaboration has been extended to include the neutron energy range of 250 to 1900 eV. Below 250 eV all ten statistically significant parity violations have the same sign. However, at higher energies PNC effects of both signs were observed in the transmission of longitudinally polarized neutrons through a thick thorium target. Although the limited experimental energy resolution precluded analysis in terms of the longitudinal asymmetry, parity violations were observed and the cross section differences for positive and negative neutron helicities were obtained. For comparison, a similar analysis was performed on the data below 250 eV, for which longitudinal asymmetries were obtained previously. For energies below 250 eV, the p-wave neutron strength functions for the J = 1/2 and J = 3/2 states were extracted: S-1/2(1) = (1.68 +/-i 0.61) x 10(-4) and S-3/2(1) = (0.75 +/- 0.18) x 10(-4). The data provide constraints on the properties of local doorway states proposed to explain the PNC sign effect in thorium.

Parity nonconservation (PNC) was studied in p-wave resonances in indium by measuring the helicity dependence of the neutron total cross section in the neutron energy range 6.0-316 eV with the time-of-flight method at LANSCE. A total of 36 p-wave neutron resonances were studied in {sup 115}In, and statistically significant asymmetries were observed for nine cases. An analysis treating the PNC matrix elements as random variables yields a weak matrix element of M=(0.67{sub -0.12}{sup +0.16}) meV and a weak spreading width of {gamma}{sub w}=(1.30{sub -0.43}{sup +0.76})x10{sup -7} eV. (c) 2000 The American Physical Society.

A new search has been performed for parity violation in the compound nuclear states of ${}^{94}\mathrm{Nb}$ by measuring the helicity dependence of the neutron total cross section. Transmission measurements on a thick niobium target were performed by the time-of-flight method at the Manuel Lujan Neutron Scattering Center with a longitudinally polarized neutron beam in the energy range 32 to 1000 eV. A total of 18 p-wave resonances in ${}^{93}\mathrm{Nb}$ were studied with none exhibiting a statistically significant parity-violating longitudinal asymmetry. An upper limit of $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ eV (95% confidence level) was obtained for the weak spreading width ${\ensuremath{\Gamma}}_{w}$ in ${}^{93}\mathrm{Nb}.$

Parity nonconservation (PNC) has been studied in the neutron p-wave resonances of ${}^{106}\mathrm{Pd}$ and ${}^{108}\mathrm{Pd}$ in the energy range of 20 to 2000 eV. Longitudinal asymmetries in p-wave capture cross sections are measured using longitudinally polarized neutrons incident on $\ensuremath{\sim}20\ensuremath{-}\mathrm{g}$ metal-powder targets at LANSCE. A CsI $\ensuremath{\gamma}$-ray detector array measures capture cross section asymmetries as a function of neutron energy which is determined by the neutron time-of-flight method. A total of 21 p-wave resonances in ${}^{106}\mathrm{Pd}$ and 21 p-wave resonances in ${}^{108}\mathrm{Pd}$ were studied. One statistically significant PNC effect was observed in ${}^{106}\mathrm{Pd},$ and no effects were observed in ${}^{108}\mathrm{Pd}.$ For ${}^{106}\mathrm{Pd}$ a weak spreading width of ${\ensuremath{\Gamma}}_{w}{=34}_{\ensuremath{-}28}^{+47}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7} \mathrm{eV}$ was obtained. For ${}^{108}\mathrm{Pd}$ an upper limit on the weak spreading width of ${\ensuremath{\Gamma}}_{w}<12\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7} \mathrm{eV}$ was determined at the 68% confidence level.

Parity nonconservation (PNC) was studied in {ital p}-wave resonances in Ag by measuring the helicity dependence of the neutron total cross section. Transmission measurements on natural Ag were performed in the energy range 32 to 422 eV with the time-of-flight method at the Manuel Lujan Neutron Scattering Center at Los Alamos National Laboratory. A total of 15 {ital p}-wave neutron resonances were studied in {sup 107}Ag and nine {ital p}-wave resonances in {sup 109}Ag. Statistically significant asymmetries were observed for eight resonances in {sup 107}Ag and for four resonances in {sup 109}Ag. An analysis treating the PNC matrix elements as random variables yields a weak spreading width of {Gamma}{sub w}=(2.67{sub {minus}1.21}{sup +2.65}){times}10{sup {minus}7} eV for {sup 107}Ag and {Gamma}{sub w}=(1.30{sub {minus}0.74}{sup +2.49}){times}10{sup {minus}7} eV for {sup 109}Ag. thinsp {copyright} {ital 1999} {ital The American Physical Society}