Precision Measurement of the Radiative Decay of the Free Neutron M. Scott Dewey Neutron Physics Group Radiation Physics Division National Institute of Standards and Technology, Gaithersburg, MD, USA International Workshop on Particle Physics at Neutron Sources 2018
Classical Description of Radiative Beta-decay “Even without knowledge of why or how beta decay tales place, we can anticipate that the sudden creation of a rapidly moving charged particle will be accompanied by the emission of radiation.” “… either we can think of the electron initially at rest and being accelerated violently during a short interval to its final velocity, or we can imagine that its charge is suddenly turned on the same short time interval.” J.D. Jackson, Classical Electrodynamics, Sec. 15.6 International Workshop on Particle Physics at Neutron Sources 2018
MOTIVATIONS for RDK II experiment Precision measurement of the spectrum of neutron radiative decay has never been done. - only existing result is from RDK I: R.L. Cooper et al , Phys. Rev. C 81, 035503 (2010) Extension of RDK I to lower photon energies. Test of comparison of chiral perturbation theory and QED calculations, and corrections. Preamble to possible future more difficult experiments, eg. V. Bernard et al , Phys. Lett. B 593, 105 - 114 (2004). “ Radiative neutron beta- decay in effective field theory” S. Gardner and D. He, Phys. Rev. D 86, 016003 (2012). "T-odd momentum correlation in radiative beta decay” S. Gardner and D. He, Phys. Rev. D 87, 116012 (2013). "Radiative beta decay for studies of CP violation” Side benefits: Electron spectrum can be used to determine b coefficient; photon tag possible Greater understanding of apparatus for neutron lifetime experiment
Radiative Spectrum and Branching Ratio Difficulties: ● Small branching ratio (~10 -3 ) ● Long lifetime (~15 minutes) ● Large gamma backgrounds ● Isolate from external bremsstrahlung International Workshop on Particle Physics at Neutron Sources 2018
Experimental Approach R.L. Cooper et al, Nucl. Instrum. Meth . A 691 , 64 – 71 (2012 ). “A gamma - and X- ray detector for cryogenic, high magnetic field applications” International Workshop on Particle Physics at Neutron Sources 2018
Modeled Detection Efficiencies ● 4.6 T magnetic field traps charged decay products to tight orbits giving large solid angle coverage. ● Delayed electron-proton coincidence provides large suppression of uncorrelated background. ● Electrostatic mirror turns around “wrong - way” protons. ● Waveform-based DAQ. International Workshop on Particle Physics at Neutron Sources 2018
RDK II on NG-6 End of NG-6 Guide tubes Biological shield Collimators Proton Detector Electronics Superconducting Solenoid w/ Photon Detectors Neutron Monitor Beam Dump
Photon Detectors International Workshop on Particle Physics at Neutron Sources 2018
From the digitized waveforms, ● determine the energy and Event Waveforms and Timing timing of each event. Make cuts based on energies ● and time differences. International Workshop on Particle Physics at Neutron Sources 2018
BGO NONPROPORTIONALITY T.R. Gentile et al , Nucl, Instrum. Meth. 784 , 88 - 92 (2015). “ Nonproportionality in the scintillation light yield of bismuth germanate ”
Electron Energy Deposit e-p with g (BGO) e-p no g e-p with g (APD) International Workshop on Particle Physics at Neutron Sources 2018
Proton Energy Deposit e-p no g e-p with g (BGO) e-p with g (APD) International Workshop on Particle Physics at Neutron Sources 2018
Proton Time-of-Flight e-p with g (BGO) e-p with g (APD) e-p no g International Workshop on Particle Physics at Neutron Sources 2018
Radiative Decay Spectra International Workshop on Particle Physics at Neutron Sources 2018
Branching Ratios Determination of the branching ratio and shape requires a well R = r ep g /r ep understood simulation. B exp = B theory (R exp /R sim ) Bales et al , PRL 116 (2016) International Workshop on Particle Physics at Neutron Sources 2018
Final Uncertainties Bales, M. J. et al. Precision Measurement of the Radiative β Decay of the Free Neutron. Phys. Rev. Lett. 116, 242501 (2016). International Workshop on Particle Physics at Neutron Sources 2018
Summary Results: ● Measured radiative spectrum over 3 decades of energy using two detectors (400 eV to 782 keV). ● Improved measurement of the branching ratio. Results consistent with theory. ● First measurement of the shape of the photon energy spectrum International Workshop on Particle Physics at Neutron Sources 2018
Future Work Future Possibilities: ● Refine existing analysis. ● Improve precision to test chiral perturbation theory calculation and recoil order corrections. ● Preamble to more difficult experiments: photon polarization ( i.e. , non V-A currents) - examine new class of angular correlations ( e.g. , ) - ● Determination of other correlations using photon as a tag. International Workshop on Particle Physics at Neutron Sources 2018
RDK II Collaboration M.J. Bales 1,2 , R. Alarcon 3 , C.D. Bass 4 , E.J. Beise 5 , H. Breuer 5 , J. Byrne 6 , T.E. Chupp 1 , K.J. Coakley 7 , R.L. Cooper 8 , M.S. Dewey 4 , S. Gardner 9 , T.R. Gentile 4 , D. He 9 , H.P. Mumm 4 , J.S. Nico 4 , B. O’Neill 3 , A.K. Thompson 4 , and F.E. Wiefeldt 10 1) University of Michigan (Ph.D. theses) 2) Technische Universität München 3) Arizona State University 4) National Institute of Standard and Technology - Gaithersburg 5) University of Maryland – College Park 6) University of Sussex 7) National Institute of Standard and Technology - Boulder 8) Indiana University 9) University of Kentucky 10) Tulane University Thanks to J. Nico and T. Gentile for plots and photos. International Workshop on Particle Physics at Neutron Sources 2018
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