Parity-violating neutron spin rotation in a liquid parahydrogen target.Our understanding of hadronic parity violation is far from clear despite nearly 50 years of theoretical and experimental progress. Measurements of low-energy parity-violating observables in nuclear systems are the only accessible means to study the flavor-conserving weak hadronic interaction. To reduce the uncertainties from nuclear effects, experiments in the few and two-body system are essential. The parity-violating rotation of the transverse neutron polarization vector about the momentum axis as the neutrons traverse a target material has been measured in heavy nuclei and few nucleon nucleon, term applying to both the proton and the neutron, the two constituents of atomic nuclei. The nucleon may be considered a single particle, of which the proton and the neutron are two different states. See atom; elementary particles. systems using reactor cold neutron sources. We describe here an experiment to measure the neutron spin-rotation in a parahydrogen target (n-p system) using pulsed cold-neutrons from the fundamental symmetries beam line at the Spallation Neutron Source The Spallation Neutron Source (SNS) is an accelerator-based neutron source being built in Oak Ridge, Tennessee, USA, by the U.S. Department of Energy (DOE). SNS is being designed and constructed by a unique partnership of six DOE national laboratories: Argonne, Lawrence Berkeley, under construction at the Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle, LLC. ORNL is located in Oak Ridge, Tennessee, near Knoxville. . Key words: cold neutrons; neutron spin rotation; nucleon-nucleon interaction; parity-violation; spallation neutron source; weak hadronic interaction. 1. Introduction The weak hadronic interaction in nuclear systems is not well characterized. The leading theoretical approach pictures the parity-violating nucleon-nucleon (NN) interaction as occurring through meson meson (mē`zŏn) [Gr.,=middle (i.e., middleweight)], class of elementary particles whose masses are generally between those of the lepton class of lighter particles and those of the baryon class of heavier particles. exchange with one strong interaction vertex and one vertex described by phenomenological weak couplings. Desplanques, Donoghue, and Holstein (DDH DDH Decision Diffie-Hellman DDH Developmental Dysplasia of Hip DDH Dorothy Day House DDH Document Drafting Handbook DDH Dublin Dental Hospital DDH Destroyer-Carrying Helicopter DDH Dissociated Double Hypertropia ) estimated the nucleon-nucleon-meson weak couplings using quark model In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks, i.e., the quarks (and antiquarks) which give rise to the quantum numbers of the hadrons. These quantum numbers are labels identifying the hadrons, and are of two kinds. and symmetries arguments and determined a reasonable range of values including their "best guess" values for each of the seven coupling amplitudes [1]. The experimental pursuit of identifying the strength of these couplings has not produced a consistent set of data. A comprehensive discussion of the parity-violating weak hadronic interaction, including the motivation for pursuing this field of study, its role in a broader context of strong and weak quark-quark interactions, and the current status of the theoretical and experimental approaches can be found in these proceedings. See paper by W. M. Snow in this Special Issue. An experimental and theoretical program of study was developed by the community to characterize the weak hadronic interaction in low-energy nucleon-nucleon systems. The experimental plan involves a series of measurements in two and few nucleon systems (to minimize nuclear structure effects) to determine the weak-interaction amplitudes either in the vector meson In high energy physics, a vector meson is a meson with total spin 1 and odd parity (usually noted as JP=1−). Compare to a pseudovector meson, which has a total spin 1 and even parity. exchange model or expressed as a function of transition amplitudes between S-wave and P-wave states. The neutron spin rotation measurements in helium (five-body system) and hydrogen (two-body system) are included in this set. In these proceedings is a description of the spin rotation in liquid helium Liquid helium experiment, including a discussion of the preliminary measurement, the apparatus modifications being carried out and the plans for taking data. See paper by C. D. Bass in this Special Issue. The transverse spin polarization Spin polarization is the degree by which the spin, i.e. the intrinsic angular momentum of elementary particles, is aligned to a given direction. This property may pertain to the spin, hence to the magnetic moment, of conduction electrons in ferromagnetic metals, such as iron, vector is a linear combination of plus and minus helicity Helicity (quantum mechanics) A fundamental quantized variable used in quantum mechanics to specify the relative orientations of spin and linear momentum of massless particles. states, [[sigma].sub.[+ or -]]. The parity-violating (PV) neutron spin rotation about the momentum axis as long-wavelength ([lambda] > 1 [Angstrom angstrom (ăng`strəm), abbr. Å, unit of length equal to 10−10 meter (0.0000000001 meter); it is used to measure the wavelengths of visible light and of other forms of electromagnetic radiation, such as ultraviolet ]) neutrons traverse the target medium arises from the spin-dependent part ([[sigma].sub.[+ or -]] * p term) of the weak interaction forward scattering amplitude. As a result of the weak interaction, each helicity state propagates with a different phase so that the transverse polarization vector rotates. The magnitude of the rotation is directly proportional (Math.) proportional in the order of the terms; increasing or decreasing together, and with a constant ratio; - opposed to See also: Directly to the parity non-conserving (PNC PNC Purdue University North Central (Westville, Indiana) PnC Point 'n Click PNC Police National Computer PNC People's National Congress (Guyana) PNC People's National Congress ) part of the forward scattering amplitude, [f.sub.PNC](0), and is given by [[phi].sub.PNC] = 4 [pi][rho]l[f.sub.PNC](0) (1) where [[phi].sub.PNC] is the parity non-conserving neutron spin rotation, [rho] and l are the density and length of the target material, respectively [2]. The forward scattering amplitude is proportional to the real part of the weak neutron-nucleus matrix element. Therefore, the size of the PV spin rotation is a direct measure of the strength of the weak nuclear interaction. An axial magnetic field will rotate a transverse spin vector spin vector Physics A vector quantity used to represent the spin angular momentum of a particle. Its magnitude represents the magnitude of the spin, while its direction lies along the axis of rotation of the particle, following the right hand rule. about the momentum axis. For example, a total spin rotation of 10 rad is expected over 1 m in the earth's magnetic field Earth's magnetic field (and the surface magnetic field) is approximately a magnetic dipole, with one pole near the north pole (see Magnetic North Pole) and the other near the geographic south pole (see Magnetic South Pole). of 50 [micro]T. The total spin rotation from the weak interaction and the background magnetic field is presented in Fig. 1. To measure small rotations on the order of [10.sup.-7] rad, the background magnetic fields magnetic fields, n.pl the spaces in which magnetic forces are detectable; created by magnetostrictive ultrasonic scalers to cause the tips of instruments such as ultrasonic scalers to vibrate. must be as small and homogeneous as possible. It is important to note that the flight path or time a neutron spends in a residual axial field will determine the magnitude of the total background rotation. [FIGURE 1 OMITTED] Using the vector meson exchange model of the weak nucleon-nucleon interaction developed by DDH and a model dependent representation of the strong interaction, Avishai and Grange calculated the neutron spin rotation through a liquid parahydrogen target and obtained [3] [[phi].sub.PNC](n-p) = [-3.12[f.sub.[pi]]-0.23[h.sub.[rho].sup.0]-0.23[h.sub.[omega].sup.0]-0.25[h.sub.[rho].sup.2]] X [10.sup.-6] rad/m (2) where [f.sub.[pi]] is the isovector pion exchange amplitude, [h.sub.[rho].sup.0] and [h.sub.[rho].sup.2] are, respectively, the isoscalar and isotensor [rho]-meson exchange amplitudes and [h.sub.[omega]] is the isoscalar-meson exchange amplitude. Given that [f.sub.[pi]], [h.sub.[rho].sup.0], and [h.sub.[rho].sup.2] are of the same order of magnitude A change in quantity or volume as measured by the decimal point. For example, from tens to hundreds is one order of magnitude. Tens to thousands is two orders of magnitude; tens to millions is three orders of magnitude, etc. and [h.sub.[omega].sup.0], is smaller than the others, Eq. (2) clearly demonstrates the strong dependence of [[phi].sub.PNC](n-p) on the isovector pion exchange amplitude. A measurement of the n-p spin rotation observable would determine [f.sub.[pi]] in a complementary way compared to another parity-odd observable in the n-p system, the gamma asymmetry in polarized A one-way direction of a signal or the molecules within a material pointing in one direction. neutron capture Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than charged particles which are repelled by on protons. (For a discussion of this measurement, please see the article by S. Page et al. in this Special Issue.) The coupling [f.sub.[pi]] is of particular interest because of its long range. The dominant contribution is from the neutral current part of the weak interaction. At present, the set of low-energy weak interaction data indicates different ranges for the isovector amplitude. Using the DDH theoretical "best values," the spin rotation observable in the (n-p) system is -9 X [10.sup.-7] rad/m. Using a reduced value for [f.sub.[pi]], consistent with the parity doublet dou·blet n. A pairing of two lenses to optically correct a chromatic and spherical aberration. measurement in [.sup.18]F, the prediction becomes -6 X [10.sup.-7] rad/m. For a 20 cm target, a sensitivity of less than 6 X [10.sup.-8] radians is needed to separate these two predictions. The spin rotation apparatus is based on a crossed neutron polarizer polarizer an appliance for polarizing light. and analyzer designed by Heckel et al. [4] for solid targets and is represented schematically in Fig. 2 for the liquid helium target measurement. For an ideal polarimeter polarimeter: see polarization of light. , a measurement of the count rate asymmetry for analyzer alignment plus and minus is proportional to the sine of the integrated rotation angle, [phi]. The polarimeter is designed to minimize and separate parity-conserving (magnetic field induced) rotations from the parity-violating signal. Magnetic fields are reduced with the use of high permeability material shielding and trim coils. The front and rear coils serve to preserve the neutron spin in the transition between high field regions and the very low field (< 10 nT) target region. A more sophisticated means of performing the traditional "target in" and "target out" comparisons to remove background signals is achieved with two target positions separated by a coil that rotates the spin ideally [pi] radians about the vertical (initial polarization) axis. As a result, rotations in front of the [pi]-coil are negative relative to the rotations behind the [pi]-coil. By alternately filling and emptying the target chamber in front then behind the [pi]-coil, the parity-violating signal follows the target material and is modulated in sign between negative for target in front and positive for target in back while the background is ideally constant. A comparison of these two configurations allows for a subtraction subtraction, fundamental operation of arithmetic; the inverse of addition. If a and b are real numbers (see number), then the number a−b is that number (called the difference) which when added to b (the subtractor) equals of the constant parity-conserving background rotations to the extent that the background does not change with target position. [FIGURE 2 OMITTED] In a realistic apparatus, target dependent magnetic fields exist for example the diamagnetic di·a·mag·net·ic adj. Of or relating to a substance that is repelled by a magnet. di  a·mag properties of the target material alters the field in the chamber region. Target dependent neutron scattering The term "Neutron Scattering" encompasses all scientific techniques whereby the deflection of neutron radiation is used as a scientific probe. It falls into two basic categories - elastic and inelastic scattering. effects introduce changes in beam divergence The beam divergence of an electromagnetic beam is the increase in beam diameter with distance from the aperture from which the beam emerges in any plane that intersects the beam axis. or path length that behave like target dependent magnetic field rotations. These two types of parity-conserving rotations will mimic the target dependent parity violating signal, cannot be subtracted away, and must be reduced below the desired sensitivity of [10.sup.-7] rad/m. Compared to the liquid helium and solid targets, the scattering rate in the hydrogen target is quite high requiring more effort to control the scattering systematics systematics: see classification. . There are two states of the hydrogen molecule, [H.sub.2], depending on the relative alignment of the proton spins: the spin 1 ortho-hydrogen and the spin 0 parahydrogen configurations. The neutron scattering cross section is about a factor of 20 higher for the ortho configuration compared to the para configuration. In addition, at relatively high energies (E > 15 meV) spin-flip scattering is allowed on the ortho-hydrogen molecule which adversely affects the rotation angle. To minimize scattering systematic effects, the para-hydrogen molecule is used. Note that the neutron mean free path in para-hydrogen is about 20 cm compared with 1 m or greater in liquid helium. We plan to mount the hydrogen spin rotation apparatus at the spallation neutron source in Oak Ridge Oak Ridge, city (1990 pop. 27,310), Anderson and Roane counties, E Tenn., on Black Oak Ridge and the Clinch River; founded by the U.S. government 1942, inc. as an independent city 1959. , TN. The pulsed nature of the beam provides important advantages for characterizing systematics. With the pulsed time structure, neutrons can be counted as a function of time that corresponds to an average neutron velocity so that the faster neutrons arrive first and the slower, longer wavelength neutrons arrive later. The time structure can be used to filter the higher energy neutrons and the low-energy, long wavelength neutrons that tend to have a greater beam divergence. As can be seen from Eq. (1), the PV spin rotation is independent of neutron velocity (energy) while the scattering and magnetic field rotations are energy dependent. The background false (parity conserving) signals can be characterized by measuring the count-rate asymmetry, or spin rotation angle, as a function of energy, while a measurement of a constant PV signal as a function of time puts a convincing upper limit on these false contributions. Significant changes to the spin rotation polarimeter from the helium apparatus include: 1) a new cryostat cryostat /cryo·stat/ (kri´o-stat) 1. a device by which temperature can be maintained at a very low level. 2. in pathology and histology, a chamber containing a microtome for sectioning frozen tissue. appropriate for a 20[degrees] liquid parahydrogen including important safety features, 2) an upgraded position sensitive detector and 3) a change in [pi]-coil running conditions from static to dynamic. With the danger and risks associated with known ratios of hydrogen to oxygen, extensive safety features and procedures must be instituted for a hydrogen target system and cryostat. We will put to use the experience gained with the design and certification of the 30 cm long, 201 liquid para-hydrogen target for the np-d[gamma] parity violation experiment. (Please see the paper by S. Page in this Special Issue.) The target will be operated at 17 K where the equilibrium ortho to para hydrogen ratio is 0.03%. If needed, a catalyst of paramagnetic par·a·mag·net·ic adj. Relating to or being a substance in which an induced magnetic field is parallel and proportional to the intensity of the magnetizing field but is much weaker than in ferromagnetic materials. material can be used to convert the ortho-hydrogen to the para configuration. The target length is constrained by the scattering systematics and set to one mean-free-path of 20 cm. The collection plates of the segmented [.sup.3]He ionization ionization: see ion. ionization Process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules (ions) by the removal or addition of negatively charged electrons. chamber used in the helium measurement is divided into four quadrants providing useful neutron count-rate information in the upper, lower, right and left regions of the beam [5]. The PV signal must be independent of geometry as well as energy, and position sensitivity provides a handle on the scattering systematics. We propose to develop a neutron detector that can collect the entire beam with a 1 cm position resolution. The design combines the [.sup.3]He conversion of neutrons to charged particles (protons and tritons) as in an ionization chamber, with the fast timing and position sensitive charge collection techniques of the MicroMegas detector. The MicroMegas technique [6] uses a wire mesh grid to locally amplify the electron charge that is subsequently detected on wire strips or pixels. Several detector modules will be placed in series along the beam axis providing 100% collection efficiency. (A discussion of this proposal can be found by D. M. Markoff et al. in this Special Issue.) The [pi]-coil, smears the polarization of the neutrons in a polychromatic polychromatic /poly·chro·mat·ic/ (-krom-at´ik) many-colored. pol·y·chro·mat·ic or pol·y·chro·mic or pol·y·chro·mous adj. Having or exhibiting many colors. neutron beam, producing a count-rate asymmetry that corresponds to a rotation about one-half the true rotation angle. A neutron precesses about the [pi]-coil magnetic field by an amount proportional to the time spent in the field. Therefore, the precession angle was inversely proportional to the neutron velocity, or linearly dependent upon the wavelength. For a continuous polychromatic beam, the coil current was optimized to rotate the majority of the neutrons by an angle as close to 180[degrees] as possible, while some neutrons are necessarily over-rotated and others are under-rotated. The measured angle was an energy averaged value of the spread in transverse components of the polarization vector. A measurement of the polarization product, P, gives the effective reduction in the rotation angle, that is, sin([phi]) = P sin([phi])[.sub.ideal]. Polarization products for a monochromatic monochromatic /mono·chro·mat·ic/ (-kro-mat´ik) 1. existing in or having only one color. 2. pertaining to or affected by monochromatic vision. 3. staining with only one dye at a time. beam approach that of 90%, compared to about 50% on a polychromatic beam [7]. At the SNS SNS sympathetic nervous system. where the neutron beam velocity is a linear function of time, we can ramp the coil current to provide a 180[degrees] rotation for each time-slice of the beam. This effectively sets the coil for a nearly monochromatic beam of width much less than 0.5 [Angstrom] in neutron wavelength. Unlike in the continuous beam case, the current can be reversed during off periods of the pulsed beam incurring no additional dead time. We have discussed the spin rotation apparatus and experimental approach to achieving a measurement sensitivity of [10.sup.-7] rad/m, and the systematic effects that impact this measurement. We have included a comparison to the spin rotation apparatus for a liquid helium target currently being upgraded for a second measurement and the importance of the time-structured beam at the Spallation Neutron Source for the success of the n-p measurement. With an expected flux of 8 X [10.sup.9] n/[cm.sup.2]/sec at the SNS, we can achieve a measurement with a statistical sensitivity of 2 X [10.sup.-8] rad in a 20 cm target with about 40 days of data, assuming scattering losses in the apparatus and spending 1/3 of the time acquiring systematic check data. 2. References [1] B. Desplanques, J. F. Donoghue, and B. R. Holstein, Unified treatment of the parity violating nuclear force, Ann. of Phys. 124, 449, (1980). [2] F. C. Michel, Parity nonconservation in nuclei, Phys. Rev. B 133, B329-349 (1964). [3] Y. Avishai and P. Grange, J. Phys. G: Nucl. Phys. 10, L263-L270 (1984). [4] M. Forte, B. R. Heckel, N. F. Ramsey, K. Green, G. L. Greene, J. Byrne, and J. M. Pendelbury, First measurement of parity non-conserving neutron spin rotation: the tin isotopes. Phys. Rev. Lett. 45 (26): 2088, 92 (1980). [5] S. D. Penn, E. G. Adelberger, B. R. Heckel, D. M. Markoff, and H. E. Swanson, Development of a low-noise [.sup.3]He ionization chamber for measuring the energy spectrum of a cold neutron beam, Nucl. Inst. Meth. A457, 332 (2001). [6] S. Andriamonje, D. Cano-Ott, A. Delbart, J. Derre, S. Diez, I. Giomataris, E. M. Gonzales-Romero, F. Jeanneau, D. Karamanis, A. Lepretre, I. Papadopoulos, P. Pavlopoulos, and D. Villamarin, Experimental studies of a Micromegas neutron detector, Nucl. Inst. Meth. A481, 120 (2002). [7] B. R. Heckel, Parity Non-conserving Neutron Spin Rotation: The Tin Isotopes, PhD. thesis, Harvard University (1981). About the author: Diane M. Markoff is a research assistant professor in the Physics Department at the North Carolina State University History
Diane M. Markoff (1) North Carolina State University, Department of Physics, Raleigh, NC Accepted: August 11, 2004 Available online: http://www.nist.gov/jres (1) Present address: North Carolina Central University History NCCU was chartered in 1909 and opened in 1910 as the National Religious Training School and Chautauqua under the leadership of President James E. Shepard. , Durham, NC. |
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