110 Measurement of Parity Violation in np Capture: the ...2. The NPDGamma Experiment The NPDGamma experiment [6] is the first of a new program of fundamental electroweak symmetry exper-iments

Volume 110, Number 3, May-June 2005Journal of Research of the National Institute of Standards and Technology

195

[J. Res. Natl. Inst. Stand. Technol. 110, 195-203 (2005)]

Measurement of Parity Violation in np Capture:the NPDGamma Experiment

Volume 110 Number 3 May-June 2005

Shelley A. PageUniversity of Manitoba,Winnipeg, MB Canada R3T 2N2J. D. BowmanLos Alamos National Laboratory,Los Alamos, NM 87545R. D. CarliniThomas Jefferson NationalAccelerator Facility,Newport News VA 23606T. CaseUniversity of California,Berkeley, CA 94720-7300T. E. Chupp and K. P. CoulterUniversity of Michigan,Ann Arbor, MI 48109-1120M. DabaghyanUniversity of New Hampshire,Durham, NH 03824D. DesaiUniversity of Tennessee,Knoxville, TN 37996-1200S. J. FreedmanUniversity of California,Berkeley, CA 94720-7300T. R. GentileNational Institute of Standards andTechnology,Gaithersburg, MD 20899-0001M. T. GerickeLos Alamos National Laboratory,Los Alamos, NM 87545R. C. GillisUniversity of Manitoba,Winnipeg, MB Canada R3T 2N2G. L. GreeneUniversity of Tennessee,Knoxville, TN 37996-1200F. W. HersmanUniversity of New Hampshire,Durham, NH 03824

T. Ino and S. IshimotoKEK National Laboratory,Tsukuba, Ibaraki 305-0801 JapanG. L. JonesHamilton College,Clinton, NY 13323B. LaussUniversity of California,Berkeley, CA 94720-7300M. B. Leuschner and B. LosowskiIndiana University Cyclotron Facility,Bloomington, IN 47408-1398R. MahurinUniversity of Tennessee,Knoxville, TN 37996-1200Y. MasudaKEK National Laboratory,Tsukuba, Ibaraki 305-0801 JapanG. S. MitchellLos Alamos National Laboratory,Los Alamos, NM 87545H. NannIndiana University Cyclotron Facility,Bloomington, IN 47408-1398S. I. PenttilaLos Alamos National Laboratory,Los Alamos, NM 87545W. D. RamsayUniversity of Manitoba,Winnipeg, MB Canada R3T 2N2S. SantraIndiana University Cyclotron Facility,Bloomington, IN 47408-1398P.-N. SeoLos Alamos National Laboratory,Los Alamos, NM 87545E. I. SharapovJoint Institute for Nuclear Research,Dubna, RussiaT. B. SmithUniversity of Dayton,Dayton OH 45469-1679

W. M. SnowIndiana University Cyclotron Facility,Bloomington, IN 47408-1398W. S. Wilburn and V. YuanLos Alamos National Laboratory,Los Alamos, NM 87545andH. ZhuUniversity of New Hampshire,Durham, NH 03824

The NPDGamma experiment will measurethe parity-violating directional gamma rayasymmetry Aγ in the reaction n→ + p → d + γ.Ultimately, this will constitute the firstmeasurement in the neutron-proton systemthat is sensitive enough to challenge moderntheories of nuclear parity violation, provid-ing a theoretically clean determination of theweak pion-nucleon coupling. A new beam-line at the Los Alamos Neutron ScienceCenter (LANSCE ) delivers pulsed coldneutrons to the apparatus, where they arepolarized by transmission through a largevolume polarized 3He spin filter and cap-tured in a liquid para-hydrogen target. The2.2 MeV gamma rays from the capture reac-tion are detected in an array of CsI(Tl) scin-tillators read out by vacuum photodiodesoperated in current mode. We will completecommissioning of the apparatus and carryout a first measurement at LANSCE in2004-05, which would provide a statistics-limited result for Aγ accurate to a standarduncertainty of ±5 × 10–8 level or better,improving on existing measurements in theneutron-proton system by a factor of 4.Plans to move the experiment to a reactorfacility, where the greater flux would enableus to make a measurement with a standarduncertainty of ±1 × 10–8, are actively beingpursued for the longer term.

Key words: hadronic weak interaction;neutron capture; parity violation.

Accepted: August 11, 2004

Available online: http://www.nist.gov/jres

1. Introduction

Despite the many remarkable successes of theStandard Model, hadronic weak interactions remainrelatively poorly understood. On the theoretical side,calculations based on W and Z exchange betweenquarks are notoriously unreliable in the hadronic sectorat low energies, where quarks are unavoidably boundby the strong interaction into nucleons and mesons. Onthe experimental side, studies of the weak nucleon-nucleon interaction are confined to measurements ofparity-violating observables which constitute a uniquebut usually tiny signature of the weak interaction. Thissignature can be strongly enhanced by nuclear structureeffects in heavier nuclei, but complications of theenhancement mechanisms have precluded a consistentdetermination of the most basic parameters of the weaknuclear potential despite a sizeable body of experimen-tal data.

Theoretical descriptions of the weak nucleon-nucle-on interaction are traditionally based on an effectivemeson exchange model in which the coupling constantsfor parity violating π, ρ, and ω meson exchanges set thescale for weak interaction effects and are uncertain towithin factors of 2 to 3 on theoretical grounds [1]. Pionexchange mediates the only long range component ofthe interaction, and ought to be the dominant contribu-tion to observed parity-violating effects in a wide rangeof nuclear systems. Early attempts to measure the weakpion nucleon coupling, f 1

π , were motivated by theunusually large sensitivity of the weak pion coupling toneutral currents, i.e., to the very existence of the Zboson as a carrier of the weak force, later demonstratedby the production of Z’s in high energy collisions at theEuropean Center for Nuclear Research (CERN). Themost precise limits on f 1

π were obtained from measure-ments of the circular polarization of gamma rays froma well known parity-mixed doublet in 18F. Surprisinglysmall results were obtained in the 18F experiment, find-ing f 1

π to be no larger than 10 % of the best theoreticalestimate available, and consistent with zero [2]. On theother hand, a measurement of the anapole moment of133Cs [3] indicates a surprisingly large value of f 1

π ,which cannot be reconciled with the 18F data.

The neutron-proton system is the only two nucleonsystem that is sensitive to f 1

π and can provide a cleanmeasurement free of nuclear structure uncertainties.The up-down γ-ray asymmetry relative to the neutronspin direction in the reaction n→ + p → d + γ at very lowenergy is sensitive almost exclusively to f 1

π , and this isthe motivation for the NPDGamma experiment atLANSCE. Previous measurements [4] of Aγ failed to

reach sufficient precision to test model predictions, andset only upper bounds that were less definitive than the18F experiments noted earlier. Advances in techniquesfor producing high intensity beams of polarized, coldneutrons now make possible for the first time a meas-urement of Aγ and hence f 1

π to within 10 % of modelpredictions. The asymmetry is predicted to be [5]Aγ = –0.11 f 1

π = –5 × 10–8 and we expect to be able tomake a measurement to a standard uncertainty of ±1 ×10–8 or better, with systematic uncertainties at or below5 × 10–10. The current experimental situation is illustrat-ed in Fig. 1.

2. The NPDGamma Experiment

The NPDGamma experiment [6] is the first of a newprogram of fundamental electroweak symmetry exper-iments to be run at the Lujan Center spallation neutronsource at LANSCE, which currently provides the high-est intensity pulsed cold neutron source in the world forfundamental neutron physics. To measure the parity-violating gamma ray asymmetry in n→ + p → d + γ, weneed an intense source of polarized neutrons in themeV energy range, a hydrogen target, a high-efficiency,large solid angle γ-ray detector, and a means of revers-ing the spin of the neutron beam without altering anyother experimental conditions. At LANSCE, a 120 µA,800 MeV proton beam pulsed at 20 Hz impinges on atungsten spallation target; MeV neutrons emergingfrom the target are cooled in a liquid hydrogen moder-ator and transported via a supermirror guide to theexperimental apparatus (Fig. 2), where they emerge


196

Fig. 1. Experimental and theoretical limits on the weak pion-nucle-on coupling f 1

π. The expected precision of an initial run of theNPDGamma experiment at LANSCE, assuming 1000 h of data,together with the statistics-limited final result planned for the exper-iment, are shown.

from the (9.5 × 9.5) cm2 guide at 21 m from the source.The m = 3 supermirror guide [7] enhances the total neu-tron flux in the desired energy range 0 meV to 15 meVwith respect to the Maxwellian distribution of neutronsemerging from the moderator. The pulsed nature of thebeam enables the energies of the neutrons to be deter-mined from their times of flight, which is an importantadvantage for diagnosing and reducing many types ofsystematic uncertainty.

Neutrons are polarized in the vertical direction byselective transmission through a polarized 3He gas cellwhich acts as a spin filter, producing an energy depend-ent polarization spectrum. Figure 3 shows a MonteCarlo simulation of the anticipated neutron beam inten-sity and polarization distributions at a target location 15m from the source, for a 200 µA proton beam and a 5bar cm 3He spin filter cell polarized at 65 %. (Theseconditions, from the experimental proposal in 1999, aresomewhat more optimistic than the present runningconditions, as discussed in Sec. 3.) The neutron beamintensity is measured with a 3He ionization chamberupstream and downstream of the polarizer cell, andagain at the end of the beamline with a third ion cham-ber. The transmission of the 3He cell serves as an on-

line measurement of its polarization and hence that ofthe neutron beam. A uniform vertical guide field, Bo =1 mT preserves the neutron beam polarization as it istransported to the liquid hydrogen target, where the


197

Fig. 2. Schematic of the NPDGamma apparatus on flight path 12 at LANSCE (see text).

Fig. 3. Monte Carlo simulation (NPDGamma proposal) of the neu-tron flux and polarization distributions versus time of flight, assum-ing a 10 cm diameter, 500 cm3 3He polarizing cell at 5 bar cm and50 % polarization, achieved by spin exchange optical pumping of3He.

incident neutrons are captured to produce the 2.2 MeVgamma rays of interest.

Low energy neutrons depolarize rapidly in ortho-hydrogen, while those below 15 meV retain their polar-ization in a parahydrogen target; hence, it is importantto ensure that the liquid hydrogen target is prepared andmaintained with the very low equilibrium ortho-hydro-gen concentration of 0.1 %. On-line monitoring of thetarget transmission provides a check on the target con-ditions, since the scattering cross-sections for ortho andpara hydrogen differ by about a factor of 20 in the ener-gy range of interest. Approximately 60 % of the beamneutrons will be captured in the target; those that scat-ter through the target walls will be absorbed in a 6Liliner surrounding the target vessel, and the remaining15 % will be transmitted to the ionization chamber atthe end of the beamline for diagnostic purposes.

The 2.2 MeV γ rays from neutron capture in the tar-get are detected with an array of 48, (15 cm × 15 cm ×15 cm ) CsI(Tl) crystals [8] surrounding the target. Thedetectors are read out in current mode by vacuum pho-todiodes coupled via low noise current-to-voltage pre-amplifiers to transient digitizers sampling the diode sig-nals every 10 µs. The time of flight information fromthe CsI detectors allows the γ-ray asymmetry Aγ withrespect to the neutron spin direction to be deduced as afunction of incident neutron energy from the angulardistribution: (1 + Aγ cosθ), where θ is theangle between the neutron spin and the direction ofemission of the gamma ray. Aγ should be constant for allof the cold neutrons in the beam, but the experimentalasymmetry will reflect the energy dependence of thebeam polarization as illustrated in Fig. 3. DuringNPDGamma data taking, a beam chopper upstream ofthe apparatus eliminates frame overlap by blockingvery slow neutrons from the tail of the preceding beampulse, and also cuts off each beam pulse after 33 ms inorder to permit a beam-off background measurement inthe γ detectors to be made as part of the normal datataking cycle.

A resonant radio frequency (RF) spin flipper, con-sisting of a 30 cm diameter by 30 cm long solenoidwhose magnetic field amplitude is tailored as a functionof time of flight to flip neutron spins of all energies, islocated upstream of the target. With the use of a spinflipper, the up-down γ-ray asymmetry can be deter-mined for each detector, effectively imposing full up-down geometrical symmetry on the apparatus. The spinflipper reverses the direction of the neutron spin on suc-cessive beam pulses according to an 8-step [+ – – + – ++ –] reversal pattern, which cancels systematic drifts ofdetector efficiencies and electronic gains to 2nd order.

Note that the asymmetry measurements are insensitiveto noise at 60 Hz and harmonics, because the beampulses are synchronized to 60 Hz. The spin flipper cur-rent supply is switched to a dummy load on alternatebeam pulses when the flipper is “off” to keep the exper-imental conditions as constant as possible under thetwo spin flipper operational states. The spin flipper effi-ciency has been measured in tests runs to be in excessof 98 % across a large area as appropriate to the beamsize in our experiment, as illustrated in Fig. 4.

The statistical uncertainty in the measurement of Aγ

is ultimately determined by counting statistics, set bythe beam intensity, the detector solid angle, and thecounting time. The gamma ray detectors and low noisepreamplifiers have been designed to ensure that sourcesof instrumental noise are small compared to this limit.The preamplifier noise alone is 100 times smaller thancounting statistics at the rates that will be encounteredduring the Aγ measurements.

An exhaustive Monte Carlo study of possible sys-tematic uncertainties has been carried out in the courseof preparing this experiment. Care has been taken toidentify all possible sources of uncertainty, to minimizethe sensitivity of the apparatus, and to work out a pro-gram of ancillary measurements to quantify individualuncertainty sources. The overall conclusion of thesestudies is that it should be possible to measure Aγ to astandard uncertainty of ±0.5 × 10–8 with systematicuncertainties no larger than 10 % of the statisticaluncertainty quoted above. Several key features of theexperimental design should be noted here, namely:

1. Three independent magnetic field reversals canbe employed to manipulate the neutron spin andshould give identical results for Aγ (3He cell, RFspin flip, holding field in the experimental area);

2. the pulsed beam allows systematic effects to beisolated by their different time of flight depen-dences;


198

d 1d 4ð

ωΩ

=

Fig. 4. RF spin flipper efficiency, measured in a test run on FP11Aat LANSCE.

3. the use of vacuum photodiodes for detector read-out reduces the gain sensitivity to magnetic fieldsas compared to conventional photomultipliertubes by four orders of magnitude, and the detec-tor gains are essentially independent of bias volt-age;

4. the depolarization of the beam above 15 meVallows a number of systematics associated withinteraction of polarized neutrons in the target tobe isolated;

5. the very small value of the electronic noise com-pared to counting statistics (1/100) makes it pos-sible to test for instrumental effects with a stan-dard uncertainty of 10–9 on a timescale of 1 day.

Systematic uncertainties arising from interactions ofthe neutron spin are potentially the most serious for theexperiment, including a number of reactions that takeplace in the hydrogen target in parallel with the n→ +p → d + γ reaction. Spin dependent effects can lead toeither up-down or left-right asymmetries which couldleak into the up-down signal from which Aγ is deduced.To limit contributions from left-right asymmetries at orbelow 5 × 10–10, we require a means of determining thedetector alignment with respect to the neutron spindirection to 20 mr or better. The alignment will be ver-ified by scanning the detector array transverse to thebeam by a few millimeters horizontally and verticallywith the target in place and measuring the effective γyield in each detector as a function of the array posi-tion.

The parity-conserving transverse analyzing power Ay

in np elastic scattering gives rise to a left-right centroidshift of the beam—this is mitigated by the very lowenergy of the beam with an estimated analyzing powerAy ≈ 2 × 10–8, and the symmetry and alignment toler-ances of the detector array yield an estimated false up-down asymmetry of 2 × 10–10 from this effect.Asymmetries associated with the n→ + p → d + γ reac-tion itself include effects of a small circular polariza-tion of the gamma rays, and a left-right asymmetry aris-ing from the correlation sn·(kγ × kn), where sn is the neu-tron spin direction, kγ and kn are the gamma ray andneutron propagation directions respectively—bothhave been examined and will lead to false asymmetriesat the 10–10 level or smaller in the Aγ measurement. Aparity violating neutron spin rotation will be experi-enced by the beam as it propagates through the liquidhydrogen; estimates of the size of this effect amount to6 µrad across the 30 cm target, but the scale of thiseffect is negligible compared to our alignment require-ment of ±20 mrad between the neutron spin direction

and the up-down symmetry axis of the gamma raydetectors.

A false asymmetry associated with the correlationsn·kβ in neutron beta decay, where kβ is the propagationdirection of the beta particle, is reduced by the fractionof neutrons that decay in the target (10–7) and the frac-tional gamma yield for a typical electron from thedecay, yielding an estimate for the false asymmetrybelow 10–12. A small contamination of deuterium in thetarget can produce gamma rays via n→ + d → t + γ with aparity violating asymmetry estimated from theoreticalcalculations of the weak meson-nucleon coupling con-stants; this is a small asymmetry to begin with and isfurther reduced by the relative cross sections and targetabundances to an estimated false asymmetry of 10–10.The reaction n→ + 6Li → 7Li* → α + t will take place inthe Li-loaded plastic neutron-absorbing target liner,which is needed to prevent neutron damage to thegamma detectors; a parity violating sn·kα correlation,where kα is the propagation direction of the alpha parti-cle, followed by (α, n) reactions, can lead to falseasymmetries in the CsI detectors, estimated at the 2 ×10–11 level. Electromagnetic Mott-Schwinger scatteringof polarized neutrons in the hydrogen target can lead toa left-right asymmetry of order 10–8 which will lead toa false up-down asymmetry of order 10–10 under exper-imental conditions.

Effects associated with the polarization of the beambut not arising from the hydrogen target can be isolatedby comparing target full and target empty runs. Onesuch effect is a potential Stern-Gerlach steering of theneutrons in the vertical direction, arising from inhomo-geneities in the vertical 10 G guide field. Estimates ofthis effect for a 10–5 mT (0.1 G) field change over thedimensions of the apparatus predict a false asymmetryof 10–10, which reverses when the direction of the guidefield is reversed. Another class of effects leads togamma ray asymmetries from beta decays of polarizednuclei produced by interactions of the beam with mate-rials upstream of the hydrogen target. Numerical esti-mates of false asymmetries from neutron capture on27Al, 26Mg, 7Li, 19F, 18O, and 208Pb have been made; thelargest effect is a false asymmetry from 27Al at 6 × 10–10

with all others an order of magnitude or more smallerthan this.

3. Progress Towards Data Taking

In previous test runs carried out using prototype elec-tronics and data acquisition systems on flight path 11 A,we demonstrated that the noise in the asymmetry signal


199

measured with LEDs illuminating the photodiodes wasat least 1/10 the noise in the γ-ray asymmetries pro-duced by polarized neutron capture in a CCl4 target. Wealso measured the cold neutron flux, which comparedwell to a Monte Carlo simulation carried out byLANSCE accelerator staff for the present source condi-tions. Fluctuations in the cold neutron flux relative tothe intensity of the primary proton beam were alsomeasured and found to be at least a factor 1/10 the max-imum tolerable consistent with the statistical goal of theexperiment. We also measured a known parity violatingasymmetry following neutron capture on 35Cl [9] to be:Aγ = (–29.1 ± 6.7) × 10–6. This test run result, takenwith a 1/10 scale apparatus and lower neutron flux thanthat of flight path 12, which we will use for theNPDGamma measurements, was consistent with a pre-viously published value but acquired in only a fewhours of running time. During NPDGamma data taking,we will periodically measure this asymmetry with a Cltarget to monitor the consistency of the detector per-formance.

The new FP12 beamline for NPDGamma was com-pleted late in 2003 [7], and the experimental apparatus,exclusive of the liquid hydrogen target, was installed inthe new FP12 cave early in 2004 (Fig. 5). We have justcompleted a very successful first commissioning run,during which the beamline, chopper, beam monitors,magnetic guide field, 3He polarizer and analyzer cells,RF spin flipper, full CsI detector array and data acqui-sition systems were exercised. Following a tune up andcommissioning of each element of the apparatus, timewas devoted to measurements of parity violating asym-metries from solid targets that will contribute to back-grounds in the NPDGamma measurements, e.g., fromthe aluminum target vessel, and measurements with aboron target were used to confirm that the full arrayoperates at the counting statistics limit [8]. Highlightsof the commissioning run are discussed briefly below.

Figure 6 shows the measured neutron time of flightdistribution using the 3He ionization chamber mountedat the end of the flight path 12 neutron guide.Agreement with the Monte Carlo calculation as nor-


200

Fig. 5. Photograph of NPDGamma apparatus installed in the new FP12 experimental cave, 2004. From left to rightare shown the end of the neutron guide, the 3He polarizer assembly, the RF spin flipper, and the CsI detector array.The horizontal racetrack coils provide a uniform vertical guide field for the neutron spins. Present but not visible inthis picture are two 3He ionization chambers for precision monitoring of the beam current and 3He polarizer char-acteristics, mounted on the downstream end of the neutron guide and immediately downstream of the 3He polariz-er call, respectively. The liquid hydrogen target, whose vessel is completely surrounded by the gamma detectorarray, will be installed in the summer of 2004.

malized to the measured moderator brightness [7] isexcellent. Note that the measured peak flux is signifi-cantly lower than the goal simulation shown in Fig.3—this is attributable to a lower moderator brightnessas well as a lower production beam current in reality, ascompared to initial forecasts when the LANSCEupgrade and the NPDGamma experiment were firstproposed. Ultimately, the FP12 neutron flux will limitthe precision in Aγ that we can obtain in a reasonableamount of running time at LANSCE.

The experimental counting rate asymmetry is givenby ε = Pn Aγ where Pn is the beam polarization; the per-formance of the 3He spin filter cell therefore has a cru-cial influence on the statistical precision of the Aγ

result. The large volume 3He cells for NPDGammahave been fabricated at NIST [10] and are approximate-ly 10 cm diameter and 5 bar cm thick. The cells arepolarized by spin exchange with polarized Rb vaporwhich is directly optically pumped using a diode laserarray. 3He polarizations in excess of 60 % have beendemonstrated in bench tests of these large cells. Itshould be noted that the spin filter technique for a thickpolarizer cell can lead to neutron beam polarizationsthat are much higher than the 3He polarization andapproaching 100 % for the slowest neutrons in thebeam, as illustrated in Fig. 2. With precision beammonitors located upstream and downstream of the 3Hepolarizer cell, the neutron beam polarization can be

inferred directly from a comparison of the polarizedand unpolarized cell transmissions—this allows for acontinuous on line measurement of the neutron beampolarization during NPDGamma data taking. An exam-ple under commissioning run conditions (the 3He polar-ization was not yet fully optimized) is shown in Fig. 7.

During the commissioning run, we repeated our ear-lier measurements of the parity violating up-down γasymmetry following neutron capture on 35Cl and con-firmed the earlier results to much higher precision in afew hours of integration time. The NPDGamma detec-tor array is such a precise instrument that we were ableto use this small parity-violating asymmetry to tune theRF spin flipper, although use of a polarized 3He analyz-er cell and a third beam monitor downstream of the cellis the preferred method. A preliminary result from thebackground studies using a solid aluminum target isshown in Fig. 8, which illustrates the superb perform-ance of the CsI detector array. The γ-ray asymmetryhistogram is seen to be Gaussian over at least fourorders of magnitude, with only very loose cuts on theincident neutron beam intensity. The conclusion fromthis preliminary analysis is that the parity violatingasymmetry following neutron capture on aluminum issmall enough that it will result in a negligible back-ground correction with the liquid hydrogen target run-ning.


201

Fig. 6. Neutron beam time of flight spectrum measured with a 3He ionization monitor mountedon the end of the flight path 12 neutron guide. The beam is intentionally blocked for t > 30 ms bya frame overlap chopper in order to provide an interleaved background measurement using theCsI detectors. The Monte Carlo simulation shown for comparison does not include distortionsassociated with aluminum Bragg edges due to windows in the beam.

4. Summary and Future Outlook

The NPDGamma experiment has just completed avery successful first commissioning run on flight path12 at LANSCE. We have demonstrated that the full CsIdetector array, instrumented for current mode readout,achieves a statistical uncertainty consistent with count-ing statistics. This summer, the liquid hydrogen targetwill be installed, followed by first data taking as earlyas possible during the 2005 run cycle. We have deter-mined that the physics asymmetry Aγ can be measuredat LANSCE with the full NPDGamma apparatus with astatistical uncertainty of ±4 × 10–4 per beam pulse with120 µA proton beam on the spallation target[9]—unfortunately this falls short of our original ±1 ×10–4 per beam pulse goal that would enable us to meas-ure Aγ to 10 % of its predicted value in one calendaryear of running time. Our test measurements [7] haveshown that expectations of the available neutron fluxfrom the upgraded LANSCE facility were too opti-mistic by almost a factor of four, with roughly equalcontributions from reduced moderator brightness andreduced production beam current.

In view of the ultimate limit on the statistical accura-cy that we can hope to achieve at LANSCE, theNPDGamma collaboration plans to complete commis-sioning of the apparatus and carry out a first measure-ment in 2005-2006, which would provide a statistics-limited result for Aγ accurate to a standard uncertaintyof ±5 × 10–8 or better, improving on existing measure-ments in the neutron-proton system by a factor of four,as shown in Fig. 1. In the longer term, our aim is tomove the experiment to the Fundamental NeutronPhysics Beam Line at the Spallation Neutron Source,which would enable us to make a measurement with astandard uncertainty of 10–8.

4. References

[1] B. Desplanques, J. F. Donoghue, and B. R. Holstein, Ann. Phys.124, 449 (1980).

[2] S. A. Page et al., Phys. Rev. C 35, 1119 (1987); M. Bini et al.,Phys. Rev. Lett. 55, 795 (1985).

[3] C. S. Wood et al., Science 275, p. 1759 (1997).[4] J. F. Cavaignac, B. Vignon, and R. Wilson, Phys. Lett. B 67, 148

(1977).[5] R. Schiavilla, J. Carlson, and M. Paris, Physical Review C 67,

032501 (2003).[6] J. D. Bowman et al., Experiment proposal: Measurement of the

Parity-Violating Gamma Ray Asymmetry Aγ in the Capture ofPolarized Cold Neutrons by Para-Hydrogen, Los AlamosReport LA-UR-99-5432 (1999).


202

Fig. 7. Neutron beam polarization deduced from 3He polarizer celltransmission measurements with the NPDGamma beam monitorsduring the 2004 commissioning run: where To andT are the unpolarized and polarized transmissions and Pn is the neu-tron beam polarization. (The 3He polarization was not yet optimizedwhen these data were taken.)

Fig. 8. Preliminary measurement of the up-down parity violating γasymmetry following neutron capture on aluminum. The histogramshows the asymmetry calculated from eight successive beam pulseswith a spin reversal sequence arranged to minimize the sensitivity tofirst and second order gain drifts. Note the ideal Gaussian distribu-tion of the data, over four orders of magnitude, with only very loosecuts placed on the neutron beam intensity. The measured asymmetryis small enough to proceed to hydrogen target data taking forNPDGamma without making a significant correction for this back-ground effect.

2n o1 ( / )P T T= −

[7] P.-N. Seo et al., New Pulsed Cold Neutron Beam forFundamental Nuclear Physics at LANSCE, this Special Issue;P.-N. Seo et al., NIM A 517, 285 (2004).

[8] M. T. Gericke et al., Commissioning of the NPDGammaDetector Array, this Special Issue.

[9] G. S. Mitchell et al., NIM A 521, 468-479 (2004).[10] T. R. Gentile, 3He Spin Filters for Slow Neutron Physics, this

Special Issue.

About the author: Shelley A. Page is a Professor inthe Department of Physics and Astronomy, Universityof Manitoba, Winnipeg, Canada R3T 2N2.


203

Documents

110 Measurement of Parity Violation in np Capture: the ...2. The NPDGamma Experiment The NPDGamma experiment [6] is the first of a new program of fundamental electroweak symmetry exper-iments