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Progress Toward a New Beam Measurement of the Neutron Lifetime
M. Scott Dewey Physical Measurement Laboratory National Institute of Standards and Technology Physics of Fundamental Symmetries and Interactions β PSI2016
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Motivation CKM Unitarity Big Bang Nucleosynthesis
π πΆπΎπ = π π’π π π’π π π’π π ππ π ππ π ππ π π‘π π π‘π π π‘π π π’π π π’π π π’π 2 =1 π π’π from measuring π π and Ξ²-decay correlation coefficients Big Bang Nucleosynthesis π π important in prediction of primordial mass fraction of 4He Neutron decay can be used to determine CKM matrix element |Vud| with high precision and fewer theoretical uncertainties. This in turn can test the validity of portions of the standard model. Neutron decay dictates the time scale for big bang nucleosynthesis and lifetime is important for cosmological models that predict cosmic 4He abundance 18 October 2016 PSI2016
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Why Carry Out Another Cold Beam Neutron Lifetime Experiment?
The neutron lifetime is important for the BBN prediction of helium abundance in the universe and for tests of the Standard Model of Particle Physics. It is essential that we know the lifetime to better than the existing uncertainty and eliminate the PDG scaling. At present, confined UCN and cold neutron beam experiments represent the two systematically distinct methods available for carrying out this measurement at a 1 s level of precision. Because each method has such unique sources of systematic uncertainty, consistency among the methods is a critical indicator of the accuracy of the neutron lifetime. The limiting uncertainty in our 2003 measurement (β 2.7 s) arose from uncertainties in the absolute counting of cold neutrons. Since then we have succeeded in using our Alpha-Gamma device to (re)calibrate the lifetime neutron monitor with a relative uncertainty of 0.06 % (β 0.5 s). Another experiment could be carried out in a timely and cost-effective manner. 18 October 2016 PSI2016
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Status of the Neutron Lifetime
See βNeutron lifetime measurement at J-PARC/MLF/BL05β at 12:20 today! 18 October 2016 PSI2016
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The Beam Method B = 4.6 T 1/v neutron monitor Proton trap
central proton trap 6LiF deposit a, t detector precision aperture n p detector B = 4.6 T +800 V mirror door J. Byrne, P.G. Dawber, R.D. Scott, J.M. Robson, and G.L. Greene, NBS SP 711, 48 (1986) 18 October 2016 PSI2016
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NIST Center for Neutron Research (NCNR)
Komives, Thursday, 12:40 pm Slide courtesy K. Grammer 18 October 2016 PSI2016
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NIST Center for Neutron Research (NCNR)
BL2 Slide courtesy K. Grammer 18 October 2016 PSI2016
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2005 Measurement Uncertainty Budget
18 October 2016 PSI2016
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Monochromatic neutron beam
Alpha-Gamma Device 1/v neutron monitor HPGe detector Totally absorbing 10B target foil Monochromatic neutron beam PIPS detector with aperture Alpha-Gamma device HPGe detector Measures neutron flux; calibrates the 1/v neutron monitor Reduces neutron counting efficiency uncertainty 2.7 s β 0.5 s Update the 2005 measurement retroactively (Yue, et.al., PRL (2013)) Operate simultaneously with 1/v neutron monitor & lifetime measurement 18 October 2016 PSI2016
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Uncertainty Budget Projection
0.5 s 18 October 2016 PSI2016
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Absorption of Neutrons by 6Li
Perform wavelength measurement of NG-C beamline Test measurement already performed on NG-6 Operate with multiple, thinner 6Li deposits in neutron monitor 20, 30, and 40 ΞΌg/cm2 nominal Li deposits already characterized Operate with B deposit(s) in neutron monitor Multiple deposits available but not yet characterized Reduce correction and uncertainty by factor of β 2 18 October 2016 PSI2016
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Uncertainty Budget Projection
0.5 s 0.4 s 18 October 2016 PSI2016
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Beam Halo & Trap Non-linearity
1 s uncertainty in 2005 measurement New dysprosium beam images with precision cadmium masks suggest beam halo might have been overestimated Two sizes of proton detector will be used to minimize this uncertainty Trap Non-linearity 5.3 Β± 0.8 s correction in 2005 due to large magnetic field gradient at longest trap length (10 electrodes) Run with shorter traps to reduce correction and minimize uncertainty 18 October 2016 PSI2016
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Uncertainty Budget Projection
0.5 s 0.4 s 0.1 s 0.2 s 18 October 2016 PSI2016
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Proton Counting Improvements
Extensive modeling of the apparatus (MCNP and GEANT) NCNR Cold source upgrade -> 50% more neutrons New low-noise pre-amp Allows operation at lower proton acceleration voltages, reducing backscatter uncertainties Two parallel data acquisition systems Digitization of all proton waveforms, enabling detailed study of multiple-proton events and background events Consistency check Extensive off-line testing of the proton trap and detector Trap instability was a major issue during the previous run of the experiment New version of the proton trap 18 October 2016 PSI2016
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Two Versions of the Proton Trap
Mark II trap: Used in 2005 measurement Well characterized Recent testing shows stable operation under a wide range of conditions Mark III trap: Better pumping of trap volume Better metrology of relevant electrode edges Recent testing shows stable operation under a wide range of conditions Two traps will allow for a wider range of systematic tests 18 October 2016 PSI2016
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Off-line Testing Setup
18 October 2016 PSI2016
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Results from off-line testing
The detector and traps tested over a wide range of parameters Trap timing: 3β30 ms Detector HV: 15β32.5 kV Trap length: longest and shortest have been tested Excess noise and ion discharge correlated with increased pressure maintain good vacuum (< 5Γ 10 β9 Torr) Detector surface quality matters Periodic preventative cleaning step minimizes detector loss and associated down-time 18 October 2016 PSI2016
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Stable Background Scans
25 kV, 10 electrodes, 10 ms trap timing 25 kV, 3 electrodes, 5 ms trap timing Mirror up Ramp off Door closed Mirror up Ramp off Door closed Door open Ramp on Door open Ramp on Mirror down Mirror down These are ~24 hour runs with background counts in each time bin. Expected proton rate is 7/s so signal to background should be great ( counts spread over several time bins) Proton peak S/N: 100/1 Proton peak S/N: 100/1 18 October 2016 PSI2016
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Excess Vacuum Protons Correlated with Pressure
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Detector Surface Details
Compromised detector after running New detector after running Correlation between dusty/damaged surface and compromised behavior. Institute a cleaning step every few weeks/every cycle? Detector damage accumulated after HV sparks 18 October 2016 PSI2016
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Uncertainty Budget Projection
0.5 s 0.4 s 0.1 s 0.2 s 0.4 s 1.0 s 18 October 2016 PSI2016
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Summary and Outlook We have already demonstrated the ability to reduce the neutron counting uncertainty to β 0.5 s with our Alpha-Gamma device Commissioning and testing of the magnet, traps and proton detector nearing completion November 2016βBegin installation on NG-C beamline with a 2-year long run anticipated for data collection and systematic checks Projected uncertainty of β€ 1 s aCORN has completed data taking, disassembly scheduled for Oct 31. We are prepping to go on as we speak. 1 s/day statistics at longest trap length, 1-2 weeks to get 1s statistics for a full complement of trap lengths. 2 years is for all the systematic checks. 18 October 2016 PSI2016
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Collaboration E. S. Anderson1, M. J. Bales2, B. Crawford3, C. DeAngelis4, M. S. Dewey5, N. Fomin6, D. M. Gilliam5, K. Grammer6, G. L. Greene6,7, S. F. Hoogerheide5, H. P. Mumm5, J. S. Nico5, W. M. Snow1, and F. E. Wietfeldt4 1. Indiana University 2. Technische UniversitΓ€t MΓnchen 3. Gettysburg College 4. Tulane University 5. National Institute of Standards and Technology 6. University of Tennessee 7. Oak Ridge National Laboratory Thanks to collaboration for some slides, images, and figures 18 October 2016 PSI2016
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Extra Slides 18 October 2016 PSI2016
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2005 Measurement Uncertainty Budget
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Cold Neutron Beams Available for Fundamental Physics at ILL, SNS, and NIST
18 October 2016 PSI2016
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