Observation of a large parity nonconserving analyzing power in Xe.

A large parity nonconserving longitudinal analyzing power was discovered in polarized-neutron transmission through Xe. An analyzing power of 4.3±0.2% was observed in a p-wave resonance at En=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. Required Publisher's Statement Copyright held by American Physical Society. First published as JJ Szymanski et al, Observation of a large parity nonconserving analyzing power in Xe, Physical Review C, 53:6, R2576–R2580, doi: 10.1103/ PhysRevC.53.R2576. Authors J J. Szymanski, W M. Snow, J D. Bowman, B Cain, Bret E. Crawford, P P J. Delheij, R D. Hartman, T Haseyama, C D. Keith, J N. Knudsen, A Komives, M Leuschner, L Y. Lowie, A Masaike, Y Matsuda, G E. Mitchell, S I. Penttila, H Postma, D Rich, N R. Roberson, S J. Seestrom, E I. Sharapov, Sharon L. Stephenson, Y-F Yen, and V W. Yuan This article is available at The Cupola: Scholarship at Gettysburg College: http://cupola.gettysburg.edu/physfac/25 Observation of a large parity nonconserving analyzing power in Xe J. J. Szymanski, W. M. Snow, J. D. Bowman, B. Cain,* B. E. Crawford, P. P. J. Delheij, R. D. Hartman, T. Haseyama, C. D. Keith, J. N. Knudson, A. Komives,, M. Leuschner, L. Y. Lowie, A. Masaike,, Y. Matsuda, G. E. Mitchell, S. I. Penttilä, H. Postma, D. Rich, N. R. Roberson, S. J. Seestrom, E. I. Sharapov, S. L. Stephenson, Y. F. Yen, and V. W. Yuan Department of Physics, Indiana University, Bloomington, Indiana 47405 Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Duke University, Durham, North Carolina 27706 and Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708 TRIUMF, Vancouver, British Columbia, Canada V6T 2A3 Department of Physics, Faculty of Science, Kyoto University, Kyoto 606-01, Japan North Carolina State University, Raleigh, North Carolina 27695 and Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708 University of Technology, P.O. Box 5064, 2600 GA Delft, The Netherlands Joint Institute for Nuclear Research, 141980 Dubna, Moscow, Region, Russia ~Received 25 March 1996! A large parity nonconserving longitudinal analyzing power was discovered in polarized-neutron transmission through Xe. An analyzing power of 4.360.2% was observed in a p-wave resonance at En53.2 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. @S0556-2813~96!50806-X# PACS number~s!: 24.80.1y, 11.30.Er, 25.40.Ny, 27.60.1j Parity nonconserving ~PNC! effects have been observed in polarized neutron-nucleus scattering for several nuclear species @1–9#. Weak amplitudes in nuclei are typically 10–10 (5GFKF , where GF is the Fermi constant and KF is a typical Fermi momentum in a nucleus! of strong interaction amplitudes, but PNC effects in heavy nuclei are enhanced by the small level spacings of compound nuclear resonances and by the mixing of strongly excited s-wave resonances into weakly excited p-wave resonances @10,11#. Many large PNC effects have been observed, including three ;10% effects @1,5,6,9#. This mechanism is expected also to amplify parity-odd, time-reversal ~TR! noninvariant effects @10–14#. One of the motivations for searching for a PNC neutron resonance in Xe was to discover a resonance useful for a TR invariance test. Tests of TR invariance in neutron transmission involve searching for terms in the neutron forward scattering amplitude proportional to sW n•(JW3kW n) or JW •kW n(sW n•JW 3kW n)m @15–18#. In these expressions sW n is the neutral spin JW is the target spin, and kW n is the neutron momentum vector. The first expression, the threefold correlation, is both parityodd and T-odd and can thus be produced by the same mechanisms that produce CP violation in kaon decays. A TR test sensitive to the threefold correlation requires a polarized nuclear target that possesses a neutron resonance with a large PNC effect for the largest possible enhancement. Unfortunately, the nuclei in which large PNC asymmetries have been discovered so far are not easy to polarize in the required amounts. Efforts in progress to polarize La by dynamic nuclear polarization have achieved polarizations ,20% in external magnetic fields in excess of 2 T @19#. The high magnetic fields and low temperatures required place stringent design requirements on the polarized target to avoid systematic errors. In particular, the high holding fields and the spin dependence of neutron-nucleus scattering ~‘‘nuclear pseudomagnetism’’! affect the neutron polarization, producing a major source of systematic error @20#. We were encouraged to search for PNC resonances in Xe because of the interesting possibilities for the production of dense, highly-polarized Xe targets. Large quantities of highly-polarized, solid Xe with relaxation times of up to several hundred hours have been maintained by opticalpumping techniques @21,22#. The polarization can be produced at higher temperatures and lower external magnetic fields ~of order 0.1 T! than those needed for dynamic nuclear polarization. The lower field can be adjusted to cancel the pseudomagnetic effect, thereby suppressing the main source of systematic error. This paper describes the observation of a large PNC effect in a p-wave resonance in Xe. Subsidiary measurements indicate that the resonance is most likely in Xe. The measurement was carried out at the Manuel Lujan Neutron Science Center ~MLNSC!. The experiment utilized *Present address: Department of Physics, Texas A and M University, College Station, TX 77843. Present address: Department of Physics, University of New Hampshire, Durham, NH 038245. PHYSICAL REVIEW C JUNE 1996 VOLUME 53, NUMBER 6 53 0556-2813/96/53~6!/2576~5!/$10.00 R2576 © 1996 The American Physical Society flight path 2 and the apparatus that the TRIPLE Collaboration has built for PNC measurements in neutron resonances. The apparatus and techniques have been previously documented @23#. The major elements of the apparatus are ~see Fig. 1! ~1! a beam monitor to normalize the incident neutron flux @24#, ~2! a longitudinally-polarized-proton spin filter that removes neutrons with spins antiparallel to the proton polarization @25#, ~3! a spin flipper that allows rapid reversal of the neutron beam polarization @26#, ~4! the liquid Xe target, and ~5! a 55-element B-loaded liquid scintillator detector @27#. The beam polarization was reversed regularly to reduce the effect of systematic errors. The accumulated data set was divided into 44 runs, where each run was formed by 20 beam polarization sequences. The beam-polarization sequence contains 8 steps with neutron helicity states 12212112. This pattern was chosen to eliminate firstand second-order time drifts @26#. Each step takes 10 seconds and contains 200 beam pulses. A liquid Xe target was used for this measurement to provide an approximately 1.3-interaction-length attenuation to the neutron beam. The vessel containing the liquid Xe was a Cu cylinder measuring 10.2 cm diameter by 13.3 cm long. The neutron beam entered the liquid Xe through Al windows 0.25 cm thick located at each end of the Cu cylinder. The first 24 production runs were acquired with 5 cm collimation located just downstream of the polarizing cryostat and the remaining runs with 8 cm collimation. In both collimation configurations the transverse extent of the beam was wellcontained within the liquid Xe. The vessel was filled by condensing 524 liter-atm of natural Xe, with the cooling power provided through Cu cables connected to a liquid nitrogen bath. The Xe temperature was maintained at a constant value with heater wire controlled by a proportional integratordifferentiator circuit. The neutron beam polarization through the Xe target was maintained by a 60 G field parallel to the beam direction produced by a Helmholtz coil pair. The liquid Xe temperature was maintained at 166.5 K, with a typical variation of 60.2 K, which corresponds to a vapor pressure of 1080 torr and Xe density of 2.93 g/cm. The temperature was measured using diode thermometers and the pressure with a capacitance manometer. The density variations due to temperature fluctuations were negligible. Neutrons were detected in a B-loaded liquid scintillator. The scintillator is subdivided into 55 light-isolated elements @27#. Each element is read out with a photomultiplier tube with a specially-designed base that was capable of handling the typical instantaneous rate of a few MHz per tube. Individual pulses from each element are discriminated to differentiate neutron pulses from single photoelectron noise and reduce the effects of large pulses from ;2 MeV g’s produced in the reaction n1p→d1g and ;0.5 MeV g’s from n1B→B→a1Li*→Li1g. The pulses are then added using an analog summing circuit. The incoming digital pulses are stretched in the analog summer to match the pulse width to the digitization time, which results in each pulse being counted once and only once. The resulting analog voltage is passed through a 1 ms passive filter, and a 12-bit transient digitizer samples the analog voltage at 1 ms intervals. The entire arrangement has a dead time that is characteristic of each individual detector element while utilizing a single transient digitizer channel. Data are taken for 8192 1 ms channels, which corresponds to a low-energy limit of 0.25 eV at the 56 meter location of the detector array. The neutron time-of-flight transmission spectrum [T(t)5Y1(t)1Y2(t), where Y1(Y2) is the number of counts detected with 1~2! neutron helicity# and the asymmetry in the transmitted intensity [A(t)5„Y1(t)2Y2(t).../ „Y1(t)1Y2(t)...# in the vicinity of En53.2 eV are shown in Fig. 2. The PNC effect in the En53.2 eV resonance can be seen in the asymmetry spectrum. The transmission data were corrected for the effects of dead time, electronic noise, and g-ray background. The line shape of the En53.2 eV resonance was then fit to the form Y6~En!5N6C~En!exp@2nsp f~En!~16 f nP !# , where N6 is the normalization for 1 and 2 helicities, C(En) describes the energy dependence of the neutron flux, the detector efficiency, and the absorption due to the nonresonant cross section, n is the target thickness, sp max is the maximum value of the p-wave resonance cross section, f(En) is the Doppler-broadened p-wave line shape, f n is the neutron polarization, and P5(s12s2)/(s11s2) is the PNC longitudinal analyzing power of the resonance cross section. The parameters describing the resonance shape, f(En) and sp max , are extracted from a fit to the data summed over the 44 runs. The neutron polarization is determined from a subsidiary measurement described below. All other parameters are determined for each of the 44 runs. The value for the neutron polarization f n is extracted from calibration runs made with a La target, where the La target is located at the downstream end of the spin flipper, just before the Xe target. The PNC analyzing power for the En50.734 eV resonance in La is well established to be P59.55 60.35% @9#. The La measurement calibrates the relation between the proton polarization in the polarizing cryostat to the neutron beam polarization f n . The typical value is f n573%. The proton polarization is monitored during each run using an NMR measurement and a run-by-run correction to f n is made. A neutron spin-transport calculation showed that the combined fields due to the Xe cryostat Helmholtz coils and FIG. 1. Layout of the flight path 2 neutron apparatus at MLNSC. 53 R2577 OBSERVATION OF A LARGE PARITY NONCONSERVING . . .