A measurement of parity-violating gamma-ray asymmetries in polarized cold neutron capture on 35Cl, 113Cd, and 139La

https://doi.org/10.1016/j.nima.2003.11.192Get rights and content

Abstract

An apparatus for measuring parity-violating asymmetries in gamma-ray emission following polarized cold neutron capture was constructed as a 1/10th scale test of the design for the forthcoming n+pd+γ experiment at LANSCE. The elements of the polarized neutron beam, including a polarized 3He neutron spin filter and a radio frequency neutron spin rotator, are described. Using CsI(Tl) detectors and photodiode current mode readout, measurements were made of asymmetries in gamma-ray emission following neutron capture on 35Cl, 113Cd, and 139La targets. Upper limits on the parity-allowed asymmetry sn·(kγ×kn) were set at the level of 7×10−6 for all three targets. Parity-violating asymmetries sn·kγ were observed in 35Cl, Aγ=(−29.1±6.7)×10−6, and 139La, Aγ=(−15.5±7.1)×10−6, values consistent with previous measurements.

Introduction

The NPDGamma experiment [1], [2] is under construction at the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory. This includes construction of Flight Path 12 at the Manuel Lujan Jr. Neutron Scattering Center at LANSCE, which will be a pulsed cold neutron beamline dedicated to fundamental physics. The goal of the experiment is to measure the parity-violating directional gamma-ray asymmetry Aγ in the reaction n+pd+γ to an accuracy of 5×10−9, which is approximately 10% of its predicted value [3]. This measurement will provide a theoretically clean determination of the weak pion-nucleon coupling and resolve the long-standing controversy over its value [4], [5], [6]. The experiment will consist of a pulsed, cold neutron beam, transversely polarized by transmission through polarized 3He, incident on a liquid para-hydrogen target. The 2.2MeV gamma-rays from the capture reaction will be detected by an array of CsI(Tl) scintillators coupled to vacuum photodiodes and operated in current mode. In Fall 2000, an engineering run was completed using prototypes of all major components to measure parity-violating asymmetries in neutron capture on several nuclei. Accuracies of order 7×10−6, limited by counting statistics, were obtained after several hours of running using Cl, Cd, and La capture targets. This paper will discuss the results of this engineering run and its implications for the design of the NPDGamma experiment.

Parity violation permits a term in the differential cross-section for the (n,γ) reaction proportional to sn·kγ, where sn is the neutron spin direction and kγ is the photon momentum vector. A constant, Aγ, measures the size of this term in the differential cross section for gamma-ray emission. The cross section is proportional to 1+Aγcosθ, where θ is the angle between the neutron polarization and photon momentum. The parity violation arises due to weak interactions inside and between the nucleons in the nucleus, which introduces new opposite-parity components into the initial and final states and allows mixing and interference between electromagnetic transitions from opposite parity states [4]. For example, in the n+pd+γ reaction weak effects allow a small amount of E1 transition to interfere with the primary M1 amplitude. In systems with Z>1 the interference is typically much more complicated, involving many states and many transitions. Parity-allowed asymmetries in the differential cross section with nontrivial angular distributions such as sn·(kγ×kn), where kn is the neutron momentum vector, are also possible [7]. A general analysis of the various angular and polarization correlations in (n,γ) reactions is given in Ref. [8].

The sn·kγ correlation has been observed in 35Cl and 139La in previous experiments [9], [10]. While parity violation is observed in neutron capture on 113Cd in p-wave resonances at epithermal neutron energies, for cold neutron capture the process is dominated by a strong s-wave resonance and no parity violation is expected. The origin of the parity-violating effect in 139La is known to be due to mixing with a narrow p-wave resonance at 0.734eV. The huge (∼10%) parity-odd effects at resonance in this [11] and many other heavy nuclei [12] are now understood in terms of two mechanisms: dynamical enhancement, which comes from the close spacing of two levels of opposite parity in the compound resonance regime; and kinematic enhancement, which is due to the difference in widths of the s and p resonances involved in the interference. The size of the effect for cold neutron energies below the 139La resonance is as expected from the tail of this resonance. The origin of the parity-violating effect in 35Cl is thought to be due to the mixing of two opposite-parity levels, a Jπ=2p-wave level at 398eV and a Jπ=2+ subthreshold s-wave resonance at −130eV. The presence of a p wave in the intermediate state in combination with final state effects in the reaction can also give rise to the parity-allowed sn·(kγ×kn) correlation. It was therefore possible that a significant parity-allowed asymmetry in gamma-ray emission following polarized cold neutron capture in 35Cl or 139La might exist.

There are several motivations to measure parity-violating and parity-allowed asymmetries on Cl, La, and Cd targets in preparation for the n+pd+γ experiment. Foremost, the measurement of the parity-violating correlations can be used to test a 1/10th scale version of the planned apparatus at the few parts per million level, with the Cd target as a null test. In addition, the discovery of a large parity-allowed sn·(kγ×kn) asymmetry would be useful for a detector alignment scheme for the NPDGamma experiment. Knowledge of the detector element angles to a precision of 20mrad with respect to the neutron spin direction (determined by a magnetic holding field) is required in order to suppress systematic effects associated with parity-allowed neutron spin-correlated gamma-ray signals leaking into the orthogonal direction associated with the parity violation signal. Finally, the measurement also provided an opportunity to check calculations of the neutron moderator brightness and to measure relative intensity fluctuations in the neutron beam and limit this potential source of extra noise into the n+pd+γ measurement.

Section snippets

Description of setup

This section describes the apparatus used in Fall 2000 to measure directional asymmetries in gamma-ray emission following polarized cold neutron capture on nuclear targets. A schematic picture of the setup is shown in Fig. 1.

Asymmetry measurements

Asymmetry measurements were made on three targets. The neutron energy range analyzed in each case was 2.5–40meV, corresponding to 8–32ms time of flight, or 1.4–5.7Å.

Asymmetries were formed using matched eight-step spin sequences (↑↓↓↑↓↑↑↓) of consecutive pulses and a calculation of the geometric mean asymmetry within the sequence. This 8-step sequence is chosen to cancel linear and quadratic time-dependent drifts in detector efficiencies. The raw experimental asymmetries are calculated as

Summary and prospects

The parity-violating sn·kγ neutron capture asymmetry measurements reported here are consistent with previous experimental results [9], [10] and of somewhat comparable precision. However, they were made in a fraction of the run time (8h per target) due to the large flux available with the pulsed beamline at LANSCE. In the n+pd+γ experiment the parity-violating asymmetry in 35Cl will be used to periodically monitor the performance of the apparatus.

No parity-allowed sn·(kγ×kn) asymmetries were

Acknowledgements

The authors would like to thank Mr. G. Peralta and Dr. J.M. O'Donnell for their technical support during this experiment. This work was supported in part by the U.S. Department of Energy (Office of Energy Research, under Contract W-7405-ENG-36), the National Science Foundation (Grant No. PHY-0100348), and the Natural Sciences and Engineering Research Council of Canada.

References (24)

  • W.M. Snow

    Nucl. Instr. and Meth. A

    (2000)
  • W.M. Snow

    Nucl. Instr. and Meth. A

    (2003)
    W.M. Snow, et al., Nucl. Instr. and Meth. A, accepted for...
  • E.G. Adelberger et al.

    Ann. Rev. Nucl. Part. Sci.

    (1985)
  • V.A. Vesna

    JETP Lett.

    (1982)
  • V.W. Yuan

    Phys. Rev. C

    (1991)
  • P.D. Ferguson, G.J. Russell, E.J. Pitcher, ‘Reference moderator calculated performance for the LANSCE upgrade project,’...G. Muhrer, P.D. Ferguson, G.J. Russell, E.J. Pitcher, ‘As-built Monte Carlo model of the Lujan target system and...J.B. Donahue, et al., ‘LANSCE short-pulse spallation source target upgrade,’ Proceedings of the 1997 Particle...
  • L.W. Alvarez et al.

    Phys. Rev.

    (1940)
  • A. Abragam

    Principles of Nuclear Magnetism

    (1961)
  • R. Golub et al.

    Am. J. Phys.

    (1994)
  • J.J. Szymanski

    Nucl. Instr. and Meth. A

    (1994)
  • S.D. Penn

    Nucl. Instr. and Meth. A

    (2001)
  • B. Desplanques et al.

    Ann. Phys.

    (1980)
  • Cited by (30)

    • Monte Carlo calculation of the average neutron depolarization for the NPDGamma experiment

      2019, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
    • Nuclear Data Sheets for A=140

      2018, Nuclear Data Sheets
    • Monte Carlo calculation and verification of the geometrical factors for the NPDGamma experiment

      2018, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
    • Nuclear Data Sheets for A = 36

      2012, Nuclear Data Sheets
    • Nuclear Data Sheets for A = 140

      2007, Nuclear Data Sheets
    View all citing articles on Scopus
    1

    Current address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

    2

    Current address: Indiana University, Bloomington, IN 47405, USA.

    View full text