Search

The TRIPLE (Time Reversal and Parity at Low Energies) collaboration uses a polarized epithermal neutron beam and a capture γ-ray detector to study parity violation in neutron–nucleus reactions. In order to preserve the spin polarization of the neutrons as they travel the 60-m path to the target, the beam pipes are wrapped with wire to produce a solenoidal magnetic field of about 10 G along the beam direction. The flanges and bellows between sections of the beam pipe cause gaps in the windings which in turn produce radial fields that can depolarize the neutron spins. A computer code has been developed that numerically evaluates the effect of these gaps on the polarization. A measurement of the neutron depolarization for neutrons in the actual spin-transport system agrees with a calculation of the neutron depolarization for the TRIPLE system. Features that will aid in designing similar spin-transport systems are discussed.

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.

We have developed a large-area 10 B -loaded liquid scintillation detector for parity-violation studies in neutron resonances with high instantaneous neutron fluxes from the LANSCE short-pulse spallation source. The detector has an efficiency of 95%, 85% and 71% at neutron energies of 10, 100 and 1000 eV, respectively. The neutron mean capture time in the detector is (416±5) ns . We describe the detector and the current-mode signal processing system, that can handle neutron rates up to 500 MHz.

Parity violation has been studied recently for neutron resonances in {sup 117}Sn. The neutron resonance spectroscopy is essential for the analysis of the parity violation data. We have measured neutron resonances in {sup 117}Sn for neutron energies from 1 to 1500 eV using the time-of-flight method and the (n,{gamma}) reaction. The sample was enriched to 87.6{percent} {sup 117}Sn. Neutron scattering and radiative widths were determined, and orbital angular momentum assignments were made with a Bayesian analysis. The {ital s}-wave and {ital p}-wave strength functions and average level spacings were determined. {copyright} {ital 1999} {ital 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) 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}

Parity nonconserving asymmetries have been measured in $p$-wave resonances of ${}^{106}\mathrm{Pd}$ and ${}^{108}\mathrm{Pd}$. The data analysis requires knowledge of the neutron resonance parameters. Transmission and capture $\ensuremath{\gamma}$-ray yields were measured for ${E}_{n}=20$--2000 eV with the time-of-flight method at the Los Alamos Neutron Science Center (LANSCE). A total of 28 resonances in ${}^{106}\mathrm{Pd}$ and 32 resonances in ${}^{108}\mathrm{Pd}$ were studied. The resonance parameters for ${}^{106}\mathrm{Pd}$ are new for all except one resonance. In ${}^{108}\mathrm{Pd}$ six new resonances were observed and the precision improved for many of the resonance parameters. A Bayesian analysis was used to assign orbital angular momentum for the resonances studied.

A beam of nearly monochromatic γ-rays was produced via intracavity Compton backscattering in the OK-4/Duke storage ring FEL. The OK-4 FEL operated in the near-UV range (345–413 nm) with electron energies of 260–550 MeV. The energy of the produced γ-rays varied from 3 to 16 MeV. In the near future we plan to increase the energy of the γ-rays to 50 MeV by increasing the electron energy and reducing FEL wavelength. Results from direct measurements of the γ-ray flux and energy resolution (using a 3 mm diameter lead collimator) (Litvinenko et al., Phys. Rev. Lett. 78 (1997) 4569) are very close to our theoretical predictions. The energy resolution of the γ-rays was ∼1%. In this paper we present a comparison of our measurements with theoretical predictions and our plans for future improvements. We discuss upgrades of our system to attain an energy resolution down to 0.1% and to increase the flux. A brief description of the experimental program utilizing the OK-4/Duke monochromatic γ-ray source is presented.

We have constructed a 0.4 mole solid 3 He target, cryogenically polarized at 12 mK in a field of 7 T. The 0.04 atoms/b target reached a polarization of 38% in 35 h. Such a target may be applied to any experiment which is tolerant of the large ambient magnetic field and which produces target heating of less than a microwatt. High energy neutron and photon scattering experiments meet these requirements. The target's figure of merit for neutron transmission measurement exceeds that of polarized gas targets by greater than 35. At the Triangle Universities Nuclear Laboratory we have used the target to measure the total cross section differences Δσ T and Δσ L for incident polarized neutrons of energies 2–8 MeV. The cross section difference is sensitive to the excited state structure of the n- 3 He system. The results have been compared to a recent R-matrix analysis of R = 4 scattering and reaction data, and provide support for the 4 He level scheme derived from that analysis.

A nearly monochromatic beam of 100% linearly polarized g rays has been produced via Compton backscattering inside a free electron laser optical cavity. The beam of 12.2 MeV g rays was obtained by backscattering 379.4 nm free-electron laser photons from 500 MeV electrons circulating in a storage ring. A detailed description of the g-ray beam and the outlook for future improvements are presented. [S0031-9007(97)03322-X]

We report measurements of the depolarization of epithermal neutrons (1.7–59 eV) in magnetic domains of a 2.0-cm-diam cylindrical single crystal of ferromagnetic holmium. The neutrons were polarized by a dynamically polarized proton target and polarization analyzed using a statically polarized rare-earth spin filter. Based on the dependence of the depolarization on neutron energy and crystal orientation, we determined the domains to be laminar, or needlelike, with the long axis slightly deflected from the c crystalline axis and having an average width of 59 μm.

We describe an apparatus for studies of parity violation in neutron-nucleus scattering. This experiment requires longitudinally polarized neutrons from the Los Alamos Neutron Scattering Center over the energy range from 1 to 1000 eV, the ability to reverse the neutron spin without otherwise affecting the apparatus, the ability to detect neutrons at rates up to 500 MHz and an appropriate data acquisition system. We will discuss the neutron polarizer, fast neutron spin reverser, detector for transmitted neutrons and high rate data acquisition system.

We report the results of a high-resolution study of neutron resonances below 500 eV in {sup 166}Ho. We find nine previously unreported weak resonances at energies 24.79, 75.08, 119.98, 187.94, 210.86, 227.87, 235.94, 248.60, and 264.98 eV. Bayesian probability analysis of the widths indicates they are formed primarily by {ital s}-wave capture and are therefore negative-parity resonances. If they have measurable {ital d}-wave neutron widths, they are candidates for the fivefold correlation test of time reversal. {copyright} {ital 1996 The American Physical Society.}

Measurements of polarized-neutron--polarized${\mathrm{\ensuremath{-}}}^{3}$He scattering are reported. The target consisted of cryogenically polarized solid $^{3}\mathrm{He}$, with thickness 0.04 atom/b and polarization \ensuremath{\sim}0.4. Polarized neutrons were produced via the $^{3}\mathrm{H}$(p\ensuremath{\rightarrow},n\ensuremath{\rightarrow}${)}^{3}$He or $^{2}\mathrm{H}$(d\ensuremath{\rightarrow},n\ensuremath{\rightarrow}${)}^{3}$He polarization-transfer reactions. The longitudinal and transverse total cross-section differences \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}}$ and \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}}$ were measured for incident neutron energies 2--8 MeV. The results are compared to phase-shift predictions based on four different analyses of n${\mathrm{\ensuremath{-}}}^{3}$He scattering. The best agreement is obtained with a recent R-matrix analysis of A=4 scattering and reaction data, lending strong support to the $^{4}\mathrm{He}$ level scheme obtained in that analysis. Discrepancies with other phase-shift parametrizations of n${\mathrm{\ensuremath{-}}}^{3}$He scattering exist, attributable in most instances to one or two particular partial waves. \textcopyright{} 1996 The American Physical Society.

A large parity nonconserving longitudinal analyzing power was discovered in polarized-neutron transmission through Xe. An analyzing power of 4.3\ifmmode\pm\else\textpm\fi{}0.2% was observed in a $p$-wave resonance at ${E}_{n}=3.2$ eV. The measurement was performed with a liquid Xe target of natural isotopic abundance that was placed in the polarized epithermal neutron beam, flight path 2, at the Manuel Lujan Neutron Science Center. This apparatus was constructed by the TRIPLE Collaboration, and has been used for studies of parity symmetry in compound nuclear resonances. Part of the motivation of the experiment was to discover a nucleus appropriate for a sensitive test of time-reversal invariance in polarized-neutron transmission. The large analyzing power of the observed resonance may make it possible to design a test of time reversal invariance using a polarized-Xe target.

The development of intense, short-wavelength FEL light sources has opened opportunities for new applications of high-energy Compton-backscattered photons. These applications range from medical imaging with X-rays to high energy physics with photon colliders. In this paper we discuss the practical aspects applications using polarized Compton backscattered γ-rays in the 5–150 MeV range from the Duke storage-ring-driven FEL. Such applications include: nuclear physics, cancer therapy, radiographic imaging, radiation effects testing, and positron production for material science studies.

The spin-dependent total cross-section difference {Delta}{sigma}{sub {ital T}} has been measured at energies of 7.43, 9.57, and 11.60 MeV for neutrons, polarized transverse to the beam momentum, transmitted through a transversely polarized proton target. In addition, the energy at which {Delta}{sigma}{sub {ital T}} crosses through zero has been determined in the energy range 3.65--6.25 MeV. The measurements were carried out with a polarized TiH{sub 2} target and polarized neutrons from the {sup 3}H(p,n){sup 3}He and {sup 2}H(d,n){sup 3}He reactions. The cross-section differences {Delta}{sigma}{sub {ital T}} are sensitive to the strength of the {sup 3}{ital S}{sub 1}--{sup 3}{ital D}{sub 1} mixing parameter {epsilon}{sub 1}. Phase-shift analyses are used to determine {epsilon}{sub 1} values from the data. The values are in good in agreement with {ital NN} potential models and do not support the low {epsilon}{sub 1} values found in previous studies around 15--20 MeV.

We describe the construction and operation of a solid 3 He polarized nuclear target which we have used for measurements of the spin dependence of the n- 3 He interaction at MeV energies. The target, which contains 0.4 mole of 3 He was polarized to 38% at 12 mK in a field of 7 T. The target is suitable for nuclear physics measurements which are insensitive to the large magnetic field and produce beam heating of tenths of microwatts. We discuss refinements and paths to improved solid 3 He targets at higher polarizations and lower fields.

A summary of the polarized and aligned nuclear targets which have been constructed and used at the Triangle Universities Nuclear Laboratory is given. Statically polarized targets, typically operating at a temperature of 12 mK and a magnetic field of 7 T, have provided significant nuclear polarization in 1 H, 3 He, 27 Al, 93 Nb and 165 Ho. A rotating, aligned 165 Ho target is also in use. A 3 He melting curve thermometer has been developed for use in statically polarized targets. A dynamically polarized proton target is under construction.

A simple and reliable method for determining the concentration of SF 6 in an accelerator gas mixture with N 2 and CO 2 is described. The technique makes use of the low velocity of sound in SF 6 (approximately 1 3 that of air). The sound velocity of the mixture is determined by measuring the spacing of acoustic resonances in a tube filled with the gas. Data from standard gas mixtures containing 0–10% SF 6 are presented, showing that the technique is accurate to approximately 0.5% absolute.

We described a facility for the production and analysis of the polarization of epithermal neutrons and show its application to the measurement of neutron depolarization in magnetic materials. The polarization of the neutrons is analyzed using the spin dependent transmission of neutrons at energies of compound nuclear resonances in a polarized rare earth target. The technique is applicable to the characterization of magnetic domains in thick samples or materials having large internal magnetizations. We illustrate the use of the technique in measuring the orientational dependence of depolarization in a single crystal of ferromagnetic holmium.

Calculations of polarized neutron-polarized [sup 3]He scattering are presented for the transverse and longitudinal total cross-section differences [Delta][sigma][sub [ital T]] and [Delta][sigma][sub [ital L]] in the energy range [ital E][sub [ital n]]=0.1 to 10 MeV. Predictions made from phase-shift analyses, [ital R]-matrix analysis, and a multichannel resonating group-model calculation are compared. The predictions are in qualitative agreement and show that polarized neutron-polarized [sup 3]He scattering is sensitive to the level structure of [sup 4]He in the excitation energy range 20--30 MeV.

The neutron total cross section for natural indium was measured for ${\mathit{E}}_{\mathit{n}}$=25--500 eV with the time-of-flight method at the Los Alamos Neutron Scattering Center. The neutron capture reaction was studied on a highly enriched sample (99.99%) of $^{115}\mathrm{In}$. A total of 47 previously unreported resonances were observed. The combination of the two measurements allowed assignment of the new resonances to $^{113}\mathrm{In}$ or $^{115}\mathrm{In}$. Resonance parameters were extracted for all of the neutron resonances observed.

Measurements have been made of \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}}$ for polarized neutrons incident on a polarized-proton target from 3.65 to 11.60 MeV. In the energy range near 10 MeV, \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}}$ is very sensitive to the nucleon-nucleon tensor interaction. Comparison of the data to potential-model predictions indicate that the tensor interaction is weak, resulting in values of the $^{3}$${\mathit{S}}_{1}$${\mathrm{\ensuremath{-}}}^{3}$${\mathit{D}}_{1}$ mixing parameter ${\mathrm{\ensuremath{\epsilon}}}_{1}$ which are smaller than predicted by any nucleon-nucleon potential model. A smaller tensor force will bring the predictions of local potential models for the triton binding energy into closer agreement with the experimental value.

The parity-nonconserving longitudinal asymmetry [ital P] has been measured in nonresonant regions in [sup 232]Th for neutron energies in the range 6--86 eV. The measured value of [ital P][sub nonres]=(9.8[plus minus]21.7)[times]10[sup [minus]6] is consistent with statistical descriptions of parity violation.

The epithermal polarized-neutron beam facility used for tests of fundamental symmetries is described. The initial unpolarized beam is obtained from the spallation source at the Los Alamos Neutron Scattering Center. Characteristics of the polarized and unpolarized beams are described, as well as design and performance of a fast-spin reversal system, neutron flux monitor and target cooler. Experimental results for polarized-neutron transmission experiments are given to illustrate the overall quality of the system.

We describe a search for the time reversal non-invariant five-fold correlation (s·Ixk)(I·k) in the scattering of 2 MeV polarized neutrons from an aligned165Ho target. Here s is the neutron spin,I is the target spin, andk is the neutron momentum vector. Differences in transmission were sought when s was reversed. The target was a single crystal of holmium, cryogenically aligned with a specially built dilution refrigerator. Rotating the target axis of alignment while cold permits the parity-even, five-fold correlation to be identified among other spin-dependent correlations in the forward elastic scattering amplitude. An analyzing power was found to be (1±6)×10−4, consistent with time reversal invariance.We obtain a bound of 5×10−3 for αT, the ratio of T-odd to T-even couplings in the effective nucleon-nucleon interaction.

Temperature measurement and control are important in brute force polarization experiments. We discuss the installation and use of3He melting curve thermometers in a crystat used to polarize a TiH2 target. Comparison is made between the melting curve thermometers and the60CoCo nuclear orientation thermometer, which is often used in such experiments. The melting curve thermometers provide increased temperature resolution and sensitivity, and were used in a feedback heating system to control temperature to ±5.5 μK at 16.5 mK. The3He melting curve and the60CoCo temperature scales are found to agree within 2% at 15 mK. The present status of the melting curve scale and the effect of a magnetic field on melting curve thermometry are also discussed.

The depolarization of epithermal neutrons in a thick single crystal of ferromagnetic holmium has been measured and analyzed with a model of neutron precession through a highly correlated array of magnetic domains. The large parity violation in the 0.734-eV p-wave resonance in $^{139}\mathrm{La}$ was used to analyze the neutron polarization, and represents an application of parity violation in nuclear resonances to a measurement in condensed-matter physics.

Parity violation effects have been studied at 40 neutron p-wave resonances of the even-even nuclei238U and232Th. Of these 11 show parity violation effects larger than 2 standard deviations, making parity violation a rather common phenomenon. Parity mixing up to 10% has been found. The root-mean squared matrix elements for parity violation derived from these resonances are M=0.58 (+0.50/-0.25) meV for238U, respectively 1.39 (+0.35/-0.38) meV for232Th.

The parity nonconserving longitudinal analyzing power [ital P] was measured for the [ital E][sub [ital n]]=0.88 eV neutron resonance in [sup 81]Br. The value of [ital P]=+1.77[plus minus]0.33 %; the sign is determined relative to the 0.73-eV resonance in [sup 139]La.

Parity nonconservation (PNC) was studied for 16 p-wave resonances in $^{238}\mathrm{U}$ by measuring the helicity dependence of the total cross section for epithermal neutrons scattered from $^{238}\mathrm{U}$. A statistical analysis yields a root-mean-square PNC matrix element M=0.${56}_{\mathrm{\ensuremath{-}}0.20}^{+0.41}$ meV, which corresponds to a spreading width of ${\mathrm{\ensuremath{\Gamma}}}^{\mathrm{PV}}$=0.9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ eV. Under plausible assumptions this gives a value of 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ for \ensuremath{\Vert}${\mathrm{\ensuremath{\alpha}}}_{\mathit{p}}$\ensuremath{\Vert}, the ratio of strengths of the P-odd and P-even effective nucleon-nucleon interactions.

Parity nonconservation was studied for seventeen states in {sup 239}U by measuring the helicity dependence of the total cross section for epithermal neutrons scattered from {sup 238}U. The root-mean-squared parity-violating matrix element for the mixing of {ital p}-wave and {ital s}-wave states was determined to be {ital M}=0.58{sub {minus}0.25}{sup +0.50} meV. This corresponds to a parity-violating spreading width of {Gamma}{sup PV}=1.0{times}10{sup {minus}7} eV. Under plausible assumptions this gives a value of 4{times}10{sup {minus}7} for {vert bar}{alpha}{sub {ital P}}{vert bar}, the ratio of strengths of the {ital P}-odd and {ital P}-even effective nucleon-nucleon interactions.

Weak levels in the compound nucleus {sup 114}Cd were located by neutron time-of-flight spectroscopy techniques. Neutron capture measurements were performed with both a natural cadmium target and a highly enriched {sup 113}Cd target. A total of 22 new resonances were located in the neutron energy interval 20-500 eV and were assumed to be {ital p}-wave. Resonance parameters, {ital E}{sub 0} and {ital g}{Gamma}{sub {ital n}}, are given for the newly identified levels. The {ital p}-wave strength function was determined to be 10{sup 4}{ital S}{sub 1}=2.8{plus minus}0.8 and the average level spacing {l angle}{ital D}{sub 1}{r angle}=14 eV. Comparison of the reduced widths with a Porter-Thomas distribution is consistent with having missed 15% of the {ital p}-wave levels.

We describe the design and operation of a cryogenic aligned nuclear target built to search for time reversal violation in neutron transmission. The apparatus includes a single crystal 165Ho target in which the nuclei are aligned by the hyperfine effect at T < 0.3 K. The aligned target may be precisely rotated to identify the angular signature of the time reversal violating effect. As a test of the system we have measured the nuclear deformation cross section of 165Ho.

Spin-spin cross sections were measured for 7.5-MeV polarized neutrons incident on cryogenically polarized $^{93}\mathrm{Nb}$. Measurements were made with spin orientations transverse and longitudinal to the beam direction, yielding ${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}}^{\mathit{s}\mathit{s}}$=-5.2\ifmmode\pm\else\textpm\fi{}5.3 mb and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}}^{\mathit{s}\mathit{s}}$=-2.2\ifmmode\pm\else\textpm\fi{}6.9 mb, respectively. The results are consistent with recent valence-nucleon folding model calculations. The tensor spin-spin potential is found to be small, ${\mathit{V}}_{12}$=-67\ifmmode\pm\else\textpm\fi{}190 keV. Volume integrals of spin-spin potentials are derived from depolarization and transmission experiments on $^{13}\mathrm{C}$, $^{15}\mathrm{N}$, $^{27}\mathrm{Al}$, and $^{93}\mathrm{Nb}$. The results for the central spin-spin potential are shown to be consistent with the folding model predictions.

Parity nonconservation has been studied in the transmission of longitudinally polarized epithermal neutrons incident upon a {sup 139}La target. Previous experiments have demonstrated the existence of a large value of the longitudinal asymmetry in the 0.734-eV {ital p}-wave resonance of the compound nucleus {sup 140}La, but report conflicting values for the measured asymmetry. We report the results of an experiment at the Los Alamos Neutron Scattering Center (LANSCE) which measured the asymmetry by two independent methods. One of these methods utilizes a conventional polarizer and a single lanthanum target. The other method uses an unpolarized beam with two lanthanum targets, and therefore does not require a separate measurement of the beam polarization. The asymmetry values determined from these two methods (0.1015{plus minus}0.0045 and 0.0955{plus minus}0.0035, respectively) are compared to previously measured values. Measurement of the asymmetry of the 0.734-eV resonance in {sup 139}La provides a highly accurate method of determining the polarization of an epithermal neutron beam.

The parity-even time-reversal-odd five-fold correlation has been studied in 2 MeV polarized neutron transmission through a rotating, cryogenically-aligned 165Ho target. The analyzing power was found to be (1 ± 6) × 10−4, consistent with time reversal invariance. We obtain a bound of 5 × 10−3 for αT, the ratio of T-odd to T-even couplings in the effective N−N interaction.

Parity-nonconserving longitudinal analyzing powers were measured for 23 {ital p}-wave neutron resonances in {sup 232}Th. Seven resonances show effects of greater than 2{sigma} statistical significance (95% confidence)---the largest sample yet measured in a single nucleus. All seven analyzing powers have positive sign. Strong sign correlations are not a feature of the conventional statistical model of parity mixing between compound nuclear states. The results suggest that the mechanism of parity violation in resonance reactions is more complicated than previously assumed.

Parity non-conservation was studied for seventeen states in 239 U by measuring the helicity dependence of the p-wave resonance cross section for epithermal neutrons scattered from 238 U. The root-mean-squared parity-violating matrix element for the mixing of p-wave and s-wave states was determined to be M = 0.58 +0.50 −0.25 meV. This corresponds to a parity-violating spreading width of Γ PV = 1.0 × 10 −7 eV. Under plausible assumptions this gives a value of 4 × 10 −7 for αp, the ratio of strengths of the P-odd and P-even effective nucleon-nucleon interactions. The application of these techniques in studies of Time Reversal Symmetry is discussed.

The formalism for evaluating parity and time reversal violating terms in total cross sections of polarized targets and low energy resonance neutrons is reviewed. A complete set of symmetry violating terms (P-odd, T-even; P-odd, T-odd; and P-even, T-odd) is obtained by analyzing the dependence of the cross section on the statistical tensors describing the beam and target. Results are tabulated in numerical and closed analytic form, using the j-spin representation. P-odd, T-even experiments are classified, with emphasis on experiments with an unpolarized beam and a polarized target where the effect of induced polarization is found to be small. Different combinations of partial neutron widths are shown to enter when P-odd, T-odd effects are compared to P-odd, T-even effects. The results for P-even, T-odd experiments for single level and two level mixing are summarized.

Two-dimensional pulse height and time-of-flight (TOF) spectra were measured at the IBR-30 pulsed neutron source to make the isotopic identification of the parity-violating {ital p}-wave resonance at {ital E}{sub 0}=3.2 eV recently observed in Xe. Analysis of the TOF spectra corresponding to the characteristic low-energy gamma-rays established the {sup 131}Xe isotope as the origin of the 3.2-eV resonance. This new finding poses a challenge for the development of a polarized solid {sup 131}Xe target for a test of time-reversal invariance with neutrons. {copyright} {ital 1996 The American Physical Society.}

Parity violation in p-wave neutron resonances of the palladium isotopes 104, 105, 106, and 108 has been measured by transmission of a longitudinally polarized neutron beam through a natural palladium target. The measurements were performed at the pulsed spallation neutron source of Los Alamos Neutron Science Center. The rms weak interaction matrix elements and the corresponding spreading widths were determined for ${}^{104}\mathrm{Pd},$ ${}^{105}\mathrm{Pd},$ and ${}^{106}\mathrm{Pd}.$

We have measured neutron resonances in the palladium isotopes 104, 105, and 110 for neutron energies from 1 to 2100 eV. Many new p-wave resonances have been observed. Their neutron widths and, in several cases, the radiative widths were measured. The average level spacings and the s-wave and p-wave neutron strength functions were determined. The time-of-flight method was used for both neutron total cross section measurements and total $(n,\ensuremath{\gamma})$ reaction yield measurements at the pulsed spallation neutron source of Los Alamos Neutron Science Center. Well established resonance spectroscopy for these isotopes is essential for the analysis of parity violation data that were recently measured in palladium.

Parity violating effects in the nucleus are normally very small, but in certain compound nuclear resonances such effects are enhanced to several percent, as in the 0.734 eV p-wave resonance in 139 La. We have used this enhanced parity violation to measure the depolarization of neutrons traversing a thick single crystal of ferromagnetic holmium. The depolarization was dependent on the relative orientation of the neutron polarization and the holmium c crystalline axis. The effect was explained in terms of neutron precession in an array of highly correlated domains magnetized in the c axis direction. The results show the high degree of correlation of magnetic domains in the single crystal and demonstrate the usefulness of epithermal neutrons in measuring magnetic properties of thick crystals. The use of other parity violating resonances should allow polarization analysis of neutrons over a range of neutron energies.

Polarized neutron transmission experiments show remarkable sensitivity to the presence of small symmetry violating forces in nuclear interactions. Parity-nonconserving (PNC) analyzing powers of a percent or more have been measured on p-wave neutron resonances in nuclei ranging from 81Br to 238U. A brief summary is given of the study of PNC effects in neutron resonances. Details of ongoing polarized neutron transmission experiments are discussed and some sample data presented. Several development projects are currently under way, which taken together should improve the sensitivity of these experiments by a factor of ∼ 20. Similar large PNC effects can be observed in (n, γ) resonances with much smaller samples. A design for a near 4π gamma ray detector to be constructed of CsI(pure) is described.

Parity nonconservation (PNC) was studied in p-wave neutron resonances of {sup 103}Rh in the neutron energy range 30 to 490 eV. The helicity dependence of the neutron total cross section of rhodium was determined by capture measurements with the time-of-flight method at the Manuel Lujan Neutron Scattering Center at the Los Alamos National Laboratory. A total of 32 p-wave resonances were studied and statistically significant longitudinal asymmetries were observed for resonances at E{sub n}=44.5, 110.8, 321.6, and 432.9 eV. A statistical analysis treating the PNC matrix elements as random variables yields a weak spreading width {Gamma}{sub w}=(1.42{sub {minus}0.59}{sup +1.21}){times}10{sup {minus}7}heV. {copyright} {ital 1999} {ital The American Physical Society}

Parity violation in p-wave neutron resonances of ${}^{121}\mathrm{Sb},$ ${}^{123}\mathrm{Sb},$ and ${}^{127}\mathrm{I}$ has been measured by transmission of a longitudinally polarized neutron beam through natural antimony and iodine targets. The measurements were performed at the pulsed spallation neutron source of the Los Alamos Neutron Science Center. Five statistically significant parity violation effects were observed in ${}^{121}\mathrm{Sb},$ one effect in ${}^{123}\mathrm{Sb},$ and seven effects in ${}^{127}\mathrm{I}.$ The weak interaction rms matrix elements and the corresponding spreading widths were determined.