The NPDGamma Experiment

J.D. Bowman, M. Gericke, G.S. Mitchell, S.I. Penttila, G. Peralta, P.-N. Seo, W.S. Wilburn (P-23) representing the NPDGamma Collaboration
Excerpted from LA-14202-PR

Introduction

The nature of weak interactions between strongly interacting hadrons is not well understood. The NPDGamma (^→n + p → d + γ) experiment¹ currently being commissioned at the Los Alamos Neutron Science Center (LANSCE), will study the parity-violating weak interaction between the most common hadrons: protons and neutrons. The hadronic weak interaction is observed in nuclei and nuclear processes², but interpretation of these experiments is difficult because of the complicated many-body dynamics of a nucleus. The goal of the NPDGamma experiment is to measure the parity-violating directional gamma-ray asymmetry in the reaction (^→n + p → d + γ) to an accuracy of 5 × 10^-9, which is approximately 10% of its predicted value.^3,4 Such a result, in a simple system, will provide a theoretically clean measurement of the weak pion-nucleon coupling, thus resolving a long-standing question in nuclear physics.

In early 2004, all elements of the NPDGamma experiment were successfully commissioned, with the exception of the liquid-hydrogen target. Following final construction of the target and verification of appropriate safety measures for its operation, it will be installed and commissioned in summer 2005. This will allow the experiment to begin taking production data.

Theory Background

The flavor-conserving weak interaction between hadrons is the most poorly tested aspect of electroweak theory.⁴ While much is known about quark-quark weak interactions at high energies, the low-energy weak interactions of hadrons (particles made of quarks, such as the nucleons—the proton and the neutron) are not well measured. At low energies, the effects of the weak interaction are typically obscured by other processes, making their experimental study challenging. The weak nucleon-nucleon interaction has been expressed in various forms and models.^3,4,5 Regardless of the formalism, pion exchange is particularly interesting because it is the longest-range component of the interaction, and is therefore presumably the most reliably calculable. The hadronic exchange of neutral currents, which is expected to dominate the weak pion exchange between nucleons, has not been isolated experimentally in an unambiguous way. For both of these reasons, the coupling constant H_π¹ for pion exchange in the weak nucleon-nucleon interaction is of special interest.

An accurate measurement of H_π¹ in a simple nucleon-nucleon system is needed to resolve previous experimental inconsistencies. A two-nucleon system such as in the ^→n + p → d + γ process is sufficiently simple that the measured asymmetry of the emitted gamma rays can be related to the weak meson-nucleon-nucleon coupling with negligible uncertainty to nuclear structure. The relationship between the parity-violating asymmetry A_γ; and H_π¹,where A_γ is the correlation between the direction of emission of the gamma ray and the neutron polarization, is calculated to be A_γ ≈ -0.045 H_π¹. The goal of NPDGamma is to measure A_γ to a precision of ± 5 × 10^-9, which will determine H_π¹ to ± 1 × 10^-7. Such a result will clearly distinguish between the values for H_π¹ extracted from experiments in nuclear systems as well as between predictions by various theories of the weak interaction of hadrons in the nonperturbative quantum chromodynamics regime.

Experiment

To determine H_π¹ with an uncertainty of 1 × 10^-7, we must achieve a statistical uncertainty of 0.5 × 10^-8 on A_γ. This means that the experiment must detect a few × 10¹⁷ of the 2.2 MeV gamma rays from the ^→n + p → d + γ reaction. In addition, possible systematic errors in the experiment require careful attention. The tiny parity-violating signal in the reaction will be isolated by flipping the neutron spin. The real asymmetry will change sign under spin reversal, while spin-independent false asymmetries will not. The weak interaction is the only fundamental particle interaction that can produce a parity-violating signal; parity violation is simply described as a difference between a physical process and its mirror image. For example, in the ^→n + p → d + γ reaction, if more gamma rays are emitted in the same direction as the neutron spin, rather than in the opposite direction, then that is a parity-violating signal and must be caused by the weak interaction. The experiment then consists of observing the direction of emission of the gamma rays from many ^→n + p → d + γ captures, and if there is an asymmetry in their distribution with respect to the neutron polarization direction, the effect of H_π¹ has been observed.

The requirements for the experiment are the following: a large number of polarized, cold neutrons; a method of flipping the neutron polarization; a proton target; and a detector system for the 2.2 MeV gamma rays.

The experiment consists of a pulsed, cold neutron beam, transversely polarized by transmission through polarized helium-3, with polarization reversal achieved on a pulse-by-pulse basis by a radio frequency spin flipper (RFSF). The neutrons are incident on a liquid para-hydrogen target. The 2.2 MeV gamma rays from the capture reaction will be detected by an array of cesium iodide scintillators coupled to vacuum photodiodes and operated in current mode. A drawing of the setup for the experiment is shown in Figure 1.

Cold Neutron Beam and Polarizer

The experiment requires a high flux of cold neutrons, with energies below 15 meV. While such neutrons are available from cold moderators at both reactors and spallation neutron sources, the nature of the neutron flux from a pulsed spallation source provides a very powerful diagnostic tool for a number of possible systematic effects for this experiment. At LANSCE, the cold neutron source consists of a liquid-hydrogen moderator coupled to the 20 Hz pulsed neutron source. At cold-neutron energies, it is possible to use neutron guides to transport neutrons. The neutron guide possesses a reflectivity that is close to unity for neutrons incident at glancing angles below a (well-known) critical angle, and the reflectivity falls sharply above this angle. The function of the neutron guide is to conserve the high-cold-neutron flux available near the moderator.

For the experiment, a new beam line and neutron guide, Flight Path 12 (FP12), have been built at the Lujan Neutron Scattering Center.⁶ One element of this beam line is a frame overlap chopper, a large rotating disk coated with gadolinium oxide. A photo of the chopper before it was covered with shielding is shown in Figure 2. The purpose of the chopper is to keep each pulse’s slowest neutrons from being passed by the faster neutrons from the following pulse. The chopper was commissioned in early 2004 and its effect is shown in Figure 3. The chopper is a key element of the flight path because it allows us to unambiguously associate a neutron’s time of flight with its energy.

In order to observe parity violation (in the distribution of gamma rays with respect to the neutron polarization direction), the experiment requires polarized neutrons. Cold neutron beams can be polarized in several ways, but the best technology for NPDGamma is a helium-3 spin filter. Helium-3 spin filters are compact, possess a large phase-space acceptance, and produce a negligible fraction of gamma-ray background. In addition, they do not require strong magnetic fields or produce field gradients. This is important for the control of systematic errors in the experiment. The thickness of the spin filter can be optimized for polarization versus transmission.

Neutron Spin Flipper

For NPDGamma the neutron spins are flipped on a 20 Hz pulse-by-pulse basis with a RFSF. The RFSF is a shielded solenoid that operates according to the well-known principles of nuclear magnetic resonance. In the presence of a homogeneous constant magnetic field and an oscillating magnetic field in a perpendicular direction, the neutron spin will precess, and the amplitude of the oscillating field can be selected to precess the spin by 180° as the neutron travels through the spin-flipper volume. The spin flip is introduced on a pulse-by-pulse basis by simply turning the radio frequency field on and off. The solenoid produces only negligible external magnetic fields and field gradients, an important property given the possible sensitivity of the detector apparatus to magnetic-field-induced gain shifts. The spin-flipper efficiency was measured in commissioning data in early 2004 and determined to be 95% ± 5% over the extent of the neutron beam.

Proton Target

In the liquid-hydrogen (proton) target, it is essential that the polarized neutrons retain their polarization until they capture. Many of the neutrons will scatter in the target before they are captured, and the spin dependence of the scattering is therefore important. The ground state of the hydrogen molecule (known as para-hydrogen) has spin of zero (J=L=S=0), and the first excited state, the lowest ortho-hydrogen state, is at 15 meV above the para- state. A large fraction of the cold neutrons possess energies lower than 15 meV. Because these neutrons cannot excite the para-hydrogen molecule into its first excited state, only elastic scattering and capture are allowed, and spin-flip scattering is forbidden. The neutron polarization therefore survives the scattering events that occur before the capture. Higher-energy neutrons will undergo spin-flip scattering and therefore lose their polarization. The liquid-hydrogen target must be in the para- state. For liquid hydrogen held at 20 K and atmospheric pressure, the equilibrium concentration of para-hydrogen is 99.8%, low enough to ensure a negligible population of ortho-hydrogen. The target is under final construction and will be installed in the FP12 cave in the summer of 2005. Production data-taking will begin following this experiment.

Cesium-Iodide Gamma-Ray Detector Array

Finally, the experiment must detect the 2.2 MeV gamma rays from the neutron capture. Given the small size of the expected asymmetry and the goal precision of the experiment, the number of events required to achieve sufficient statistical accuracy in a reasonable time immediately leads to the conclusion that the 2.2 MeV gamma rays must be counted in current mode. This means that instead of observing individual events in the detector, many are seen at once, and the sum of their presence is detected (as a total voltage or current from the detector electronics, rather than as individual pulses). It is important to demonstrate in a current-mode measurement that the electronic noise is negligible compared to the shot noise due to the discrete nature of the energy deposited by each gamma ray and the number of photoelectrons produced by each event. In addition, the detector must cover a large solid angle with a large and time-independent efficiency that is unaffected by neutron spin reversal and radiation damage. Segmentation of the detector is required to resolve the angular dependence of the expected parity-violating signal and discriminate false effects. The fully constructed detector array of 48 cesium-iodide scintillator crystals is shown installed in the FP12 cave in Figure 4.⁷ The noise performance of the detectors and their preamplifier electronics has been measured in the laboratory, and it corresponds well to predictions based on the fundamental limit of Johnson noise. This allows the detectors to accumulate data at the counting statistics limit and to quickly demonstrate that no false experimental effects exist in the electronics. As evidence of this, Figure 5 shows a histogram of asymmetry values accumulated with gamma rays produced in an aluminum target. Aluminum is present in much of the experimental apparatus, and if it were to produce a gamma-ray asymmetry, it could obscure the effect we are looking for from our proton target. We have placed a sufficient limit on possible asymmetry arising from neutron capture in aluminum. Figure 6 shows results of gamma-ray asymmetry by a detector pair for a chlorine (CCl₄) target. The sine and cosine dependence of the asymmetry arises due to the physical locations of the detector pairs. Chlorine produces a known asymmetry of ~ 20 parts per million⁸, and this shows that our apparatus functions sufficiently to make a precision measurement of a very small asymmetry.

Summary

A sensitive measurement of the parity-violating gamma-ray asymmetry in the reaction ^→n + p → d + γ can give definitive information on one of the most important and interesting components of the weak nucleon-nucleon interaction. Commissioning results have demonstrated the performance of the essential components of the experiment; this includes published results for the FP12 moderator performance and measurements of parity-violating asymmetries in neutron capture on nuclear targets (chlorine, aluminum, and others) to a precision of 2 × 10^-6, limited only by counting statistics. The experimental design incorporates a number of powerful diagnostics to isolate systematic effects. Commissioning of the final assembly of the experiment, including the hydrogen target, will begin in 2005. The NPDGamma experiment to search for the parity-violating gamma-ray asymmetry in the reaction ^→n + p → d + γ will achieve a sensitivity which is likely to obtain a nonzero result, providing an experimental and unambiguous measure of the hadronic weak interaction in a simple and calculable system.

References

1. W.M. Snow et al., “Measurement of the parity violating asymmetry A_γ in ^→n + p → d + γ,” Nuclear Instruments and Methods A 440, 729–735 (2000).
2. G.E. Mitchell et al., “Parity violation in compound nuclei: Experimental methods and recent results,” Physics Reports 354, 157–241 (2001).
3. B. Desplanques, J.F. Donoghue, and B.R. Holstein, “Unified treatment of the parity-violating nuclear force,” Annals of Physics 124, 449–495 (1980).
4. E.G. Adelberger and W.C. Haxton, “Parity violation in the nucleon-nucleon interaction,” Annual Review of Nuclear and Particle Science 35, 501–558 (1985).
5. S.-L. Zhu et al., “Nuclear parity-violation in effective field theory,” ePrint Archives preprint nucl-th/0407087 (29 Dec 04).
6. P.-N. Seo et al., “A measurement of the flight path 12 cold H₂ moderator brightness at LANSCE,” Nuclear Instruments and Methods A 517, 285–294 (2004).
7. M. Gericke et al., “A current mode detector array for gamma asymmetry measurements,” accepted for publication in Nuclear Instruments and Methods A (2005).
8. G.S. Mitchell et al., “A measurement of parity-violating gamma-ray asymmetries in polarized cold neutron capture on ³⁵Cl, ¹¹³Cd, and ¹³⁹La,” Nuclear Instruments and Methods A 521, 468–479 (2004).

Acknowledgment

This work was supported in part by the U.S. DOE 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. Assistance in the construction and design of this experiment has been provided by the Manuel Lujan, Jr. Neutron Scattering Center (LANSCE-12) and Neutron and Nuclear Science (LANSCE-3).

For further information, contact Gregory Mitchell at 505-665-8484, gmitchell@lanl.gov.