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DISSERTATION Ultracold Neutron Converters angestrebter akademischer Grad Doktorin der Naturwissenschaften (Dr. rer. nat.) Verfasserin: Malgorzata Kasprzak Matrikel-Nummer: 0448516 Dissertationsgebiet: Physik Betreuer: Univ.-Prof. Dr. Eberhard Widmann Wien, am 20. Oktober 2008 Abstract The aim of the work described in this thesis was to investigate the properties of the ultra- cold neutron (UCN) converter materials deuterium D2,oxygenO2 and heavy methane CD4 (with emphasis on D2) in the temperature range between 8 K and room temperature. Those investigations are part of the research and development project made in connection with the new high intensity ultracold neutron source based on solid D2 (sD2)asUCNconverter and being built at Paul Scherrer Institute (PSI), Villigen, Switzerland. The development of high intensity UCN sources is important for improving the accuracy of experiments in- vestigating fundamental properties of the neutron, e.g. the search for the electric dipole moment. Presently there are several projects to build new UCN sources in order to provide the desired increase in intensity. The essential issue is the efficient use of the UCN converter. At the UCN source at PSI, due to the use of sD2 as UCN converter, the UCN density will be increased by about two orders of magnitude compared to the strongest source currently available (at Institut Laue-Langevin (ILL)). The UCN converter here is to be understood as a medium which reduces the velocity of cold neutrons (CN, velocity of about 600 m/s) to the velocity of UCN (velocity of about 6 m/s). Its performance depends on the interaction of CN and UCN with the converter ma- terial. We can distinguish three aspects: (i) the transmission of CN through the material, (ii) the efficiency of the conversion of CN to UCN (so-called UCN production) and (iii) the UCN transmission through the material. The first two issues are covered in this work, the last topic has been investigated, for sD2, in our previous experiments [1]. The experimental research has been done at the FUNSPIN beamline of the Swiss Spal- lation Neutron Source (SINQ) at PSI. We have measured the production of UCN from a CN beam in D2 [2], O2 and CD4 and the CN transmission through all three materials [3]. In order to understand the underlying processes of the UCN production in gaseous and solid D2 the CN energy dependent UCN production was measured [4]. The polarization of UCN produced from polarized CN in sD2 and various methods of crystal preparation have been tested. The obtained results have been interpreted in terms of the neutron scattering theory. Contents 1 Introduction 1 1.1TheCPTTheoremandTimeReversalSymmetryViolation......... 2 1.2EDMandUltracoldNeutrons.......................... 3 2 PSI Ultracold Neutron Source and Solid Deuterium 5 2.1OverviewofthePSIUCNSource........................ 5 2.2SolidDeuteriumModerator........................... 6 3 The Moderation of Neutrons 16 3.1ElasticScatteringandModeration....................... 17 4 Experiment 22 4.1Setup....................................... 22 4.1.1 CNBeamwithoutVelocitySelector.................. 25 4.1.2 VelocitySelector............................. 25 4.1.3 TheTargetCell.............................. 26 4.1.4 TheRamanSpectroscopy........................ 27 4.1.5 UCNDetectionSystem......................... 27 4.1.6 CNDetectionSystem.......................... 31 4.2PreparationoftheSamples-FreezingMethods................ 33 4.2.1 FreezingfromLiquid........................... 33 4.2.2 FreezingfromGas............................ 33 4.3TheMeasurements................................ 41 4.3.1 UCNProduction............................. 41 4.3.2 CNTransmission............................. 42 5 The Detailed Treatement of Neutron Scattering 43 5.1TheMolecule................................... 43 5.2 Neutron Scattering by Gas D2 .......................... 45 5.3TheCrystalStructure.............................. 54 5.4ElasticScatteringofaCrystalLattice..................... 57 5.5TheDynamicsoftheCrystalLattice...................... 69 5.6InelasticScattering................................ 72 5.7Scatteringbyliquids............................... 76 CONTENTS iii 6 The Production of Ultracold Neutrons - Results of The Measurements 79 6.1UCNProductioninDeuterium......................... 79 6.1.1 UCNStorageMode........................... 80 6.1.2 UCNProductionintheFlow-ThroughMode............. 80 6.2EnergyDependentUCNProductioninDeuterium.............. 99 6.2.1 TheoreticalModels............................ 99 6.2.2 DataAnalysis............................... 103 6.3UCNProductioninDifferentConverters.................... 107 7 Conclusions 114 7.1Overview..................................... 114 7.2 Absolute Production Cross Sections in D2 ................... 115 7.3 Energy Dependent UCN Production in D2 ................... 115 7.4 UCN Production in D2,O2,andCD4 ..................... 116 7.5ColdNeutronTotalCrossSections....................... 116 A UCN polarization 118 B Systematic Effects and Detector Calibration 121 B.1ColdNeutronFluxMeasurements....................... 121 B.2CountRatesintheCNDetectorforanEmptyTargetCell.......... 124 B.3DeterminationofHomogeneityoftheTargetCellIllumination........ 124 B.4DetectorEfficiency-Summary......................... 130 B.5SystematicEffects................................ 131 B.5.1BeamlineShutter............................. 131 B.5.2NeutronWindows............................ 131 List of Figures 2.1 Layout of the UCN source at PSI. The proton beam hits the spallation target from the left. Spallation neutrons will be thermalized in the ambient temperature D2O moderator, further cooled and downscattered into the UCN regime in the cold sD2 moderator. Through a vertical neutron guide, the UCN reach the storage volume where they can be trapped and distributed totheexperiments................................. 8 2.2 Layout of the PSI proton accelerator and the site of the UCN source. 9 2.3SchemeoftheUCNprotonbeamline...................... 10 2.4Layoutoftheprotonline,UCNsourceandtheUCNexperiments...... 10 2.5 Drawing of the D2O moderator tank (grey). One can see the proton beam pipe (horizontal pipe coloured in pink and blue) and the vertical neutron guide (yellow). The D2O tank, made of aluminium alloy, has a diameter of 1.6 m and volume of about 3330 liters. Heavy water will be used to moderate the spallation neutrons and also to cool the spallation target. The sD2 tank (coloured in green) is inserted into the system through the vertical guide. 11 2.6 A model of the UCN storage tank together with one of the UCN guides, throughwhichtheUCNaretransportedtotheexperiment.......... 12 2.7 Drawing of the UCN shutter placed between the sD2 converter and the UCN storage volume (at the end of the vertical neutron guide (see Figure 2.5)). 13 2.8 PSI UCN source operating scheme using a 1% duty cycle. UCN intensities in the storage tank and in the experiment and expected background are plotted inarbitraryunitsovertime(figurebyJ.Sromicki). ............. 14 2.9 A cut through the sD2 container......................... 15 2.10 The toroidal shaped sD2 converterlidmadefromAlMg3........... 15 3.1 Kinematics in the laboratory (L) system (left) and center of mass (CM) system(right)................................... 17 3.2 The differential scattering cross section of a gas of D2 molecules, simplified model of scattering of thermal (red, dashed curve) and cold (black, dotted curve) neutrons on a hypothetic gas of D2 molecules at T = 8 K, taking into accountonlythethermalmotionofthemolecules............... 21 LIST OF FIGURES v 4.1 A 3D view of the experimental setup.1-CN “flight tube”, 2- cryostat with the target cell, 3 & 4-UCN reflecting mirrors, 5-entrance UCN shutter, 6- storage tube, 7-exit UCN shutter, 8-UCN detector, 9-CN chopper, 10-CN TOF tube, 11-CN detector. For the CN transmission and UCN production measurements we have used the full CN beam. For the CN energy dependent UCN production measurements, a velocity selector was mounted upstream ofthecryostat................................... 23 4.2 A photograph of the experimental setup. Top view. The numbers which de- scribe the parts of the setup are the same as above, in Figure 4.1, additionally: 12-velocity selector mounted in front of the cryostat (see also Figure 4.5), 13- gas system (see also Figures 4.8 and 4.9), 14-He dewar connected via transfer linetothecryostat................................. 23 4.3 A photograph of part of the experimental setup, side view. The cold neutron beam from the SINQ source comes from the left and is collimated (40 mm) over a length of about 1.5 m before hitting the target cell, which sits behind a 38 mm aperture. The cell with the UCN converter under investigation ismountedonthecryostatandallowsittobecooleddownto8K.The Helmholtz coils around the cryostat, the CN coil and coil surrounding the neutron guide generate the magnetic field of about 10 Gauss strength for guidingtheneutronspin(seeAppendixA).................. 24 4.4 The UCN reflecting mirror, the Fe coated sillicon wafer mounted on the Al support....................................... 25 4.5 The picture shows velocity selector (1) mounted in front of the cryostat (2) 26 4.6 A photograph of the target cell mounted on the cryostat and a schema of the cell with a copper support. Numbers denote the following parts: 1 - the capillaries, through which D2,O2 and CD4 were transported to the cell; the cell has two capillaries, the shorter one, used during freezing from the liquid phase, and the longer one (the end