Fundamental Neutron Physics Beamline at the Spallation Neutron Source at ORNL

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1 Fundamental Neutron Physics Beamline at the Spallation Neutron Source at ORNL N. Fomin 1, G. L. Greene 1,2, R. Allen 2, V. Cianciolo 2, C. Crawford 3, P. Huffman 4, E. Iverson 2, R. Mahurin 5, M. Snow 6, Others 3 1 University of Tennessee, Knoxville, TN, USA 2 Oak Ridge National Lab, Oak Ridge, TN, USA 3 University of Kentucky, Lexington,USA 4 North Carolina State Univeristy, Raleigh,USA 5 University of Manitoba, Winnipeg,Canada 6 Indiana University, Bloomington,USA Abstract The Spallation Neutron Source at Oak Ridge National Lab is the most intense pulsed neutron source in the world. The recently commissioned Fundamental Neutron Physics Beamline, with a cold and a monochromatic UCN guides will be the site of the next generation of measurements of the Hadronic Weak Interaction, Neutron Beta decay, as well as the Electric Dipole Moment of the neutron. 1. The Spallation Neutron Source The Spallation Neutron Source (SNS) is the most intense pulsed neutron source in the world, designed to deliver a sub-microsecond proton pulse onto a mercury target at a repetition rate of 60 Hz with time-averaged proton power of 1.4 MW. The released spallation neutrons are moderated by supercritical hydrogen and the resulting slow neutrons are used for a variety of experiments. The SNS can support upto 18 beamlines (and up to 24 instruments), all conducting experiments simultaneously. The normal conducting linac consists of six drift tube linac (DTL) tanks, which bring the linac energy upto 86.8 MeV, and four coupled cavity linac (CCL) structures, which supply additional acceleration to bring the energy to MeV. This is followed by a superconducting linac consisting of 11 medium-beta (β=0.61) cryomodules and 12 high-beta (β=0.81) cryomodules. The medium-beta cryomodules each contain 3 cavities and provide 10.1 MV/m, bringing the protons upto an energy of 379 MeV. The high-beta cryomodules (consisting of 4 cavities each) can provide upto 15.9 MV/m 1 resulting in a maximum proton energy of 1.3 GeV, although currently the SNS operates at around 875 MeV. The H pulses arrive at the ring injection point via a 150-m long beam line which is used for energy collimation (bending magnets) and transverse halo collination (straight section). The ring 1 Not yet, they can t. Preprint submitted to Elsevier June 18, 2013

2 consists of four straight sections as well as four arcs, giving a circumference that is 1/1000th of the linac pulse length. The incoming H beam is stripped of its two electrons by passing through a thin carbon or diamond foil, and is merged in phase space with the circulating proton beam. Currently, the ring design allows operation at up to 1.0 GeV, but RF systems and injection kickers have been designed to support 1.3 GeV operation with minimal upgrades. Figure 1: A cutaway view of the mercury target, surrounded by four moderators enclosed in a beryllium reflector. Each moderator is viewed by several beamlines. See text for details. Spallation neutrons are moderated by undergoing repeated elastic scattering with hydrogen and beryllium atoms. The configuration at the SNS includes four moderators, two of which are viewed from both sides of the target. The moderators are surrounded by a water-cooled 2 beryllium inner reflector, which is then surrounded by water-cooled stainless steel [1, 2]. The configuration can be seen in Fig. 2. The viewed faces of all the moderators are 10 cm (horizontal) by 12 cm (vertical). The FnPB views the bottom downstream moderator, where the hydrogen is delivered through the bottom of the vessel via a jet, which forces the hydrogen to circulate. This moderator is fully coupled unpoisoned hydrogen (parahydrogen) at 20K (viewed from one side only), with a curved viewed surface and maximum moderator thickness of 60 mm (average of 55 mm). The moderator is surrounded by 20 mm of light water acting as a premoderator. 2 The system was designed for heavy water cooling, but heavy water is expensive. We re currently light-water cooled. 2

Figure 2: The mercury target vessel surrounded by 4 moderators inside of a beryllium reflector. FnPB views the bottom downstream moderator (bottom right in the figure).

13A is the monochromatic line (left) and will extend into the external building to serve the nedm experiment.

3 Figure 2: The mercury target vessel surrounded by 4 moderators inside of a beryllium reflector. FnPB views the bottom downstream moderator (bottom right in the figure). See text for details. 2. Fundamental Neutron Physics Beamline - Overview Figure 3: A top view of the Fundamental neutron Physics Beamline(s). 13A is the monochromatic line (left) and will extend into the external building to serve the nedm experiment. 13B is the polychromatic beamline (right), shown with NPDGamma installed in the experimental area. The Fundamental Neutron Physics Beamline (FnPB) will deliver neutrons to two experimental areas via a split beamline. Beamline 13A, or the UCN line, delivers 8.9 Å neutrons to an 3

Figure 4: Schematic view of the Beamline 13 components, including bender guide, choppers, monochromator assembly, secondary shutters (A and B), as well as straight (A) and expanding (B) guides.

4 Figure 4: Schematic view of the Beamline 13 components, including bender guide, choppers, monochromator assembly, secondary shutters (A and B), as well as straight (A) and expanding (B) guides. external building that will house the neutron Electric Dipole Moment experiment. Beamline 13B, a.k.a. the cold beamline, is polychromatic and will serve a suite of nuclear physics experiments in the FnPB cave in the SNS target building. Fig. 4 shows a top view of the completed beamlines and the experimental areas Shielding The SNS has strict radiological and magnetic field requirements that must be met on the boundaries between neighboring beamlines as well as on the roof of any given instrument. The bender guide results in the loss of the line of sight at 7.5 meters, the location of the second bandwidth chopper. In order to satisfy the SNS requirement of <0.25 mrem/hr combined gamma and neutron radiation at the boundary with neighboring beamlines and in the instrument hall (assuming that bandwidth choppers are open and 2 MW beam is being delivered), an extensive shielding package was designed. The individual shield blocks were designed so that once the whole shielding package was assembled, no neutrons would have line of sight through it into the cave. The shielding configuration starts with cm of steel around the guide, which staggers periodically along the beamline, followed by high density (HD) concrete to the location of the third bandwidth chopper [3]. Downstream of 9 meters, the shield blocks are made of regular concrete. The height of the beamline shielding is 4.45 meters from the instrument floor to a distance of 7.4 meters, and 3.96 meters beyond that, to the experimental cave. The shielding making up the roof of the enclosure is 45 cm of regular concrete. In additional to the radiological shielding requirements, the SNS guidelines also dictate that the magnetic field should be <50 mg at the boundaries with neighboring beamlines. The walls and ceiling of the experimental cave are lined with A1010 steel to serve as magnetic shielding. There s 2.54 cm of shielding on both walls, meters into the experimental enclosure; an additional 5.1 cm of shielding on the beam-right wall, starting at 60.2 cm into the experimental enclosure and continuing for 3.66 meters; cm of shielding on the floor starting 1.46 meters 4

5 into the experimental enclosure and continuing for 6.35 meters, and finally the roof is lined with cm of shielding for 7.28 meters inside the experimental enclosure. Both beamlines also have beam stops. The beam stop for the cold beam made of heavy concrete and it s cm 3 in size. The beam stop rests on a pedestal made of regular concrete that is 91.4 cm in height (same other dimensions). The beam stop designed to stop the full neutron beam at 2 MW. There s a cutout that s cm 3 on the upstream face, centered at 1.83 meters from the ground and also centered in the left-right direction. At the back of the cutout, there are two pieces of lithium carbonate (Li 2 CO 3, with trace amounts of hydrogen and zinc) shielding, embedded in a silicon matrix (SiO(CH 3 ) 2 Zn). The beam stop for the UCN line is lithium carbonate as well. The experimental enclosure itself is shaped like a pie slice with width of blah at the upstream end and blah at the downstream end Neutron Guide The beamline begins 1 m downstream of the moderator face with rectangular guides that are 10 cm in width and 12 cm in height. The first element is a m long core guide with m=3.6. This is followed by 1.8 m of shuter guide with m=3.8 for all but the beam right side of the guide. These two sections were manufactured by Swiss Neutronics [4]. The first section of FnPB guide is a piecewise bender that starts 4.3 m downstream of the moderator face. The guide bends beam left and is subdivided into four segments. All of the supports for the cold guide system are mounted to the concrete shielding, which is cast directly on the instrument floor. This first section of guide is 2.9 m long and is subdivided into five channels; it continues the 117-m meter radius bend that starts with the shutter guide. All concave internal surfaces as well as top, bottom, and outer pieces have a supermirror coating of m 3.5, and the convex internal surfaces have an m 2.0. This gives a neutron reflectivity of>0.65 when averaged over all the pieces. 3 Starting at 7.5 m downstream of the moderator, the beamline is made of straight guide, 10 cm in width, and 12 cm in height and continues for 7.5 m. All surfaces of the straight guide have reflectivities of 3.6. The neutron guide ends at 15 m from the face of the moderator Choppers A neutron chopper is a rotating disk coated with a neutron-absorbent material, containing one or more apertures to allow neutrons to pass through. Choppers can be used to select neutrons of a particular wavelength by phasing them so that the aperture is aligned with the beam when neutrons of a desired energy (corresponding to a particular time of flight to the chopper) arrive at the chopper. Neutrons are transmitted for a time t, defined by the rotational velocity and the size of the aperture. The FnPB includes two choppers at 5.5 m and 7.5 m downstream of the moderator, as well as housing to accomodate two more choppers at 9 m and 10.5 m. The axis of rotation for all four (potentially) is parallel to the neutron beam. The FnPB choppers spin at 3600 RPM as the neutron pulses are delivered at 60 Hz. The chopper disk is made of carbon fiber composite with a coating containing 10 B (minimum of 0.13 g/cm 2 ) to absorb neutrons. The physical parameters of the choppers are listed in Table Leaving aside questions of how one averages some number of 3.5s and some number of 2.0s to get 0.65, I don t think that the idea of averaging reflectivity across pieces on the inside and outside makes much sense. I d pull that sentence completely. 5

6 Table 1: Parameters of the FnPB choppers Chopper 1 Chopper 2 Axis to beam center 25.0 cm 25.0 cm Outer diameter 63.7 cm 63.7 cm Cutout Angle Cutout inner radius 18.6 cm 18.6 cm 10 1 Unchopped Chopped 10 0 N/cm 2 /Å/MW s (x10 8 ) Wavelength, Å Figure 5: Unchopped calculated (red solid) spectrum, and chopped (blue dashed) to give Å neutrons, as required by NPDGamma [5], the first approved experiment to run on the beamline. There is something wrong with this data. How can there be different bandwidths from each of the three chopper openings? 2.4. Monochromator Crystals The nedm experiment, which will be served by the UCN beamline, requires 8.9 Å neutrons. In order to select these from the polychromatic beam, a system of crystals was designed and built. Neutrons of a wavelengthλcan be filtered out by undergoing Bragg diffraction from a set of crystal planes if the Bragg condition (2d sinθ=nλ,where d is the lattice spacing) is satisfied. Two sets of intercalated pyrolytic graphite (IPG) crystal monochromators [6] were designed and built [7]. Graphite intercalation compounds consist of a periodic sequence of graphite basal planes and intercalate layers stacked along the layer normal, referred to as staging, where a stage-n compound is a structure in which any two successive intercalate layers are separated by n carbon planes. The intercalate material is evaporated from a hot molecular beam source onto a cooled graphite substrate and is diffused into it. This process has been long established for 6

7 potassium and bromine, but has not previously been performed for rubidium. The first monochromator is potassium intercalated in graphite, and consists of an array of 24 crystals, each having dimensions 20 mm x 45 mm for a total area of 120 mm x 180 mm. It intersects the full beam (100 mm x 120 mm) and reflects out neutrons of 8.9Å, as well asλ/n wavelengths. In order to remove λ/n wavelength neutrons, 2 sets of normal (un-intercalated) graphite crystals were inserted. One of these is oriented to reflect 4.5Å (λ/2) neutrons and the other is oriented to reflect 3Å (λ/3) neutrons in first order Bragg reflection. The mosaic width (5 ) of these crystals were selected so that precise wavelength tuning is not required. It should be noted that neutrons of 2.25Å (λ/4) are also reflected in second order by theλ/2 filter. The second monochromator, rubidium intercalated in graphite, consists of an array of 35 crystals, each having dimensions 20 mm x 45 mm for a total area of 140 mm x 225 mm. This monochromator reflects the beam of 8.9Å neutrons (as well as any remaining λ/n neutrons) into the UCN ballistic guide, and avoids the need for a bender. Figure 6: Monochromator crystal assemblies (A: potassium intercalated in graphite, B: rubudium intercalated in graphate, C: graphite filter) as well as their housing. On the right, crystal assembly A can be retracted into the top of the housing when BL13A is not taking data. See text for details. Monochromator housing begins 6.5 m downstream of the moderator. Fig. 2.4 shows the arrangement of the crystals inside the monochromator housing. Note that the lid is designed to allow for the first crystal assembly to be retracted when it s not being used in order to avoid neutron damage as well as neutron depletion of the cold beam. 7

2.5. Cold Beamline Figure 7: Secondary shutter located on the cold line. See text for details. The secondary shutter on the cold line (Fig. 2.5) starts 10.5 m downstream of the moderator focal point.

8 2.5. Cold Beamline Figure 7: Secondary shutter located on the cold line. See text for details. The secondary shutter on the cold line (Fig. 2.5) starts 10.5 m downstream of the moderator focal point. It is well beyond the end of the loss of sight to the moderator. The secondary shutter is a steel drum with a rotating cylinder inside, containing 0.5 m of neutron guide (open position) and 0.48 m of steel 180 away (closed position). The steel is followed by a lithium tile 2 cm thick on the downstream end of it, creating a beamstop when the shutter is closed. The lithium absorber in the secondary shutter is made of 6 Li phosphate. The weight of the tile is 218 grams and contains grams of 6 Li. The spectral flux was measured at the end of the cold beamline, m from the moderator. A 3 He proportional counter, efficiency-calibrated in a variety of ways [8], was used in combination with an aperture near the center of the exit of the guide. As the neutrons have made several bounces off the walls of the guide, the flux spectrum is presumed to have no position dependence over the area of the guide exit. The energy of the neutrons was calculated based on time of flight from the moderator to the 3 He detector and calibrated via the identification of aluminum Bragg edges from windows along the beamline. This measured spectrum is compared to the flux spectrum calculated using a MCSTAS model of the beamline [9] in Figure 8. Fig. 8 shows both spectra out to long wavelegths UCN beamline The UCN beamline continues downstream of the monochromator assembly with am 8 m section of ballistic guide. It starts with a rectangular cross section 14 cm tall and 12 cm wide and expands to 30 cm by 20 cm, respectively. The top and bottom of the guide have reflectivity of m=3.6 and the sides have m=2.2. The transverse distribution of neutrons passing through the monochromator assembly was measured at the end of the 8 m long ballistic section of the UCN guide. Several techniques 8

9 10 FNPB13 measured calculation N/cm 2 /Å/MW s (x10 8 ) Wavelength, Å Figure 8: Measured and calculated spectra for the cold guide at the FnPB. were employed, including using the previously mentioned absolutely calibrated 3 He detector in combination with neutron-sensitive image plates, as well as a CCD camera. [10] 6 5 n/cm 2 /Å/s/MW (x10 5 ) Å λ/2 λ/ Wavelength (Å) Figure 9: Measured spectrum for the UCN guide at the FnPB. A chopper upstream of the monochromator was used in order to separate neutrons of different wavelengths. 9

10 3. Science Program The FnPB will be home to a series of experiments that aim to answer fundamental questions about the weak force. They include studies of the hadronic weak interactions between nucleons, neutron beta decay as well as a search for time resersal invarance violation, in the form of a neutron electric dipole moment (nedm) measurement Hadronic Parity Violation The first two experiments on the FnPB will test the strength of the hadronic weak interaction between nucleons. This interaction was first parametrized in terms on meson exchange in the DDH model [? ] over thirty years ago. However, the meson exchange couplings are still largely unknown today. The first experiment to take data is NPDGamma, which aims to measure a parity violating asymmetry in the direction of gamma rays emitted from cold neutron capture on parahydrogen. [? ]. This asymmetry is directly related to the long range pion-nucleon weak coupling, h 1 π. The high flux at the FnPB will allow for a measurement with precision, which will test the DDH model as well as a more recent prediction from lattice QCD [? ]. Following NPDGamma will be a measurement the parity violating asymmetry of the correlation between the longitudinal polarization of incoming cold neutrons and the outgoing momentum of protons after nuclear breakup in the reaction n+ 3 He p+t+765kev [? ]. This asymmetry is sensitive to a compination of the meson couplings, and in combination with other HWI measurements, can be used to isolate the individual couplings Neutron beta decay There are many avenues available to search for physics beyond the Standard Model, and neutron beta decay is one of them. By measuring beta correlation correlation parameters, in conjuction with measurements of the neutron lifetime, we can perform tests of the unitarity of the CKM matrix. The Nab experiment [? ] will perform a precise measurement of a, the electronneutrino correlation parameter, and b, the Fierz interference term in neutron beta decay, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the 10 3 accuracy level inδa/a, and will provide an independent measurement ofλ=g A /G v, the ratio of axial-vector to vector coupling constants of the nucleon. The Nab collaboration will also measure b for the first time in neutron decay, which will provide an independent limit on the tensor weak coupling. The spectrometer design lends itself to possible further use in other beta decay experiments Time Reversal Violation Search The neutron EDM search is another way to test the Standard Model, specifically through time reversal violation tests of CPT symmetry. Over the last 50 years, many models that would give rise to a non-zero nedm have been ruled out, but new ones have been put forth. The nedm experiment proposed for the FnPB [? ] aims to lower the current limit by two orders of magnitude, making it the world s most precise measurement, able to test the baryogenesis hypothesis for the matter/anti-matter imbalance in the universe. 4. Summary The FnPB is a dual-use commissioned beamline at the Spallation Neutron Source. It will be the site of numerous experiments to study the hadronic weak interaction between nucleons as well as to perform precision tests of the Standard Model. 10

11 References [1] Spallation neutron source project completion report (2006). [2] E. B. Iverson, P. D. Ferguson, F. X. Gallmeier, I. I. Popova, Detailed SNS neutronics calculations for scattering instrument design: SCT configuration, Tech. Rep. SNS DA0001-R00, Oak Ridge National Laboratory (July 2002). [3] P. D. Ferguson, private communication. [4] S. Neutronics, how to cite this? [5] J. D. Bowman, Precision measurement of a γ in n+ p d+γ, SNS proposal (2005). [6] A. Freund, On the wavelength dependence of neutron monochromator reflectivities, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 238 (1985) 570. [7] P. Courtois, C. Menthonnex, R. Hehn, K. Andersen, V. Nesvizhevsky, O. Zimmer, F. Piegsa, P. Geltenbort, G. Greene, R. Allen, P. Huffman, P. Schmidt-Wellenburg, M. Fertl, S. Mayer, Production and characterization of intercalated graphite crystals for cold neutron monochromators, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 634 (1, Supplement) (2011) S37 S40. [8] E. B. Iverson, B. J. Micklich, D. V. Baxter, R. G. Cooper, P. D. Ferguson, D. W. Freeman, F. X. Gallmeier, S. E. Hammons, C. M. Lavelle, I. Popova, Neutronic measurements to commission the SNS, in: Proceedings of ICANS XVII, the Seventeenth Meeting of the International Collaboration on Advanced Neutron Sources, [9] P. R. Huffman, L. Greene, G., R. Allen, V. Cianciolo, P. Koehler, D. Desai, R. Mahurin, A. Yue, G. R. Palmquist, M. Snow, W., Beamline performance simulations for the fundamental neutron physics beamline at teh spallation neutron source, Journal of Research of the National Institute of Standards and Technology 110 (2005) [10] A. C. Hamilton, E. B. Iverson, Diagnostic use of neutron-sensitive image plates at ORNL neutron facilities, in: Proceedings of the Tenth International Meeting on Nuclear Applications of Accelerators AccApp 11, 2012, p

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