Neutron M. Scott Dewey Neutron Physics Group Radiation Physics - - PowerPoint PPT Presentation

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

International Workshop on Particle Physics at Neutron Sources 2018

“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,

• r we can imagine that its charge is suddenly

turned on the same short time interval.” J.D. Jackson, Classical Electrodynamics, Sec. 15.6

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

MOTIVATIONS for RDK II experiment

Radiative Spectrum and Branching Ratio

International Workshop on Particle Physics at Neutron Sources 2018

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

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”

Experimental Approach

Modeled Detection Efficiencies

International Workshop on Particle Physics at Neutron Sources 2018

• 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.

End of NG-6 Biological shield Proton Detector Electronics Neutron Monitor Beam Dump Guide tubes Collimators Superconducting Solenoid w/ Photon Detectors

RDK II on NG-6

International Workshop on Particle Physics at Neutron Sources 2018

Photon Detectors

Event Waveforms and Timing

International Workshop on Particle Physics at Neutron Sources 2018

• From the digitized waveforms,

determine the energy and timing of each event.

• Make cuts based on energies

and time differences.

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

International Workshop on Particle Physics at Neutron Sources 2018

e-p no g e-p with g(BGO) e-p with g(APD)

Proton Energy Deposit

International Workshop on Particle Physics at Neutron Sources 2018

e-p no g e-p with g(BGO) e-p with g(APD)

Proton Time-of-Flight

International Workshop on Particle Physics at Neutron Sources 2018

e-p no g e-p with g(BGO) e-p with g(APD)

Radiative Decay Spectra

International Workshop on Particle Physics at Neutron Sources 2018

Branching Ratios

International Workshop on Particle Physics at Neutron Sources 2018

R = repg/rep Bexp = Btheory (Rexp/Rsim)

Determination of the branching ratio and shape requires a well understood simulation.

Bales et al, PRL 116 (2016)

Final Uncertainties

International Workshop on Particle Physics at Neutron Sources 2018

Bales, M. J. et al. Precision Measurement of the Radiative β Decay of the Free Neutron. Phys. Rev. Lett. 116, 242501 (2016).

Summary

International Workshop on Particle Physics at Neutron Sources 2018

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

Future Work

International Workshop on Particle Physics at Neutron Sources 2018

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.

RDK II Collaboration

International Workshop on Particle Physics at Neutron Sources 2018

M.J. Bales1,2, R. Alarcon3, C.D. Bass4, E.J. Beise5, H. Breuer5, J. Byrne6, T.E. Chupp1, K.J. Coakley7, R.L. Cooper8, M.S. Dewey4, S. Gardner9, T.R. Gentile4, D. He9, H.P. Mumm4, J.S. Nico4, B. O’Neill3, A.K. Thompson4, and F.E. Wiefeldt10

1) University of Michigan 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 (Ph.D. theses) Thanks to J. Nico and T. Gentile for plots and photos.