1. Improved Determination of the Neutron Lifetime M. Scott Dewey PSI2013 Physics of Fundamental Symmetries and Interactions 8-12 September 2013PsI20131

2. Cold neutron beam  n measurement ABSOLUTE MEASUREMENT of beam fluence and decay rate Requires known proton trapping/detection efficiency and known neutron detection efficiency Decay proton counting volume ( ) Neutron beam Beam fluence measurement ( ) 8-12 September 2013PsI20132

3. The NIST beam lifetime experiment Proton trap electrostatically traps decay protons and directs them to detector via B field Neutron monitor measures incident neutron rate by counting n + 6 Li reaction products (  + t) Proton trap Neutron monitor 6 LiF deposit ,t detector Precision aperture n p detector B = 4.6 T +800 V 8-12 September 2013PsI20133

4. Alpha-Gamma Determining  n Proton rate measured for varying trap lengths Proton detection efficiency n + 6 Li reaction product counting Neutron flux monitor efficiency for 8-12 September 2013PsI20134

5. Definition of fluence monitor (FM) , t detection probability Neutron absorption probability Absorbed neutrons Detected  + t ( ) Neutron beam ( ) wavelength 6 Li deposit 8-12 September 2013PsI20135

6. Figure courtesy J. Nico et al. Neutron detection efficiency 8-12 September 2013PsI20136

7. Neutron monitor The Alpha-Gamma device Monochromatic neutron beam HPGe detector Alpha-Gamma device PIPS detector with aperture Totally absorbing 10 B target foil HPGe detector 8-12 September 2013PsI20137

8. Alpha-Gamma device Neutron monitor measurement device 8-12 September 2013PsI20138

9. Thick target 1/ “  /FM” Thin target 1/ “  /  ” 239 Pu counting “R Pu ” Wavelength “R ” 8-12 September 2013PsI20139

10. Uncertainty budget 8-12 September 2013PsI201310

11. Final result To update lifetime, temporal stability of neutron monitor (measurements of and ) must be assessed. - new metrology of lifetime aperture fixture - Bayesian analysis of past and present measurements of neutron-induced activity from three 6 LiF foils 8-12 September 2013PsI201311

12. Our measurements show that both quantities are zero, and thus: Can also update the based lifetime result with ENDF-B/VII and corrected : Independent methods for determining the neutron monitor efficiency give the same lifetime. 8-12 September 2013PsI201312

13. To improve lifetime fluence determination, we must check the temporal stability of neutron monitor. (measurements of and ) - new metrology of lifetime aperture fixture - Bayesian analysis of past and present measurements of neutron-induced activity from three 6 LiF foils 8-12 September 2013PsI201313

14. Determining - comparison of metrology Origin to aperture distance (mm) Aperture diameter (mm) Aperture tilt (deg) Fractional difference in distance squared Fractional difference in aperture area Discrepancy driven by aperture diameter 8-12 September 2013PsI201314

15. Determining - IRMM measurements of “radiometer” foil a.k.a. “30”  n foil a.k.a. “40” ~20  g/cm 2 foil a.k.a. “20” 8-12 September 2013PsI201315

16. The data: Modeled with 3 “true” charged particle rates ( ), a conversion constant ( ), and a mass loss term ( ) that is forced to be negative No loss Lifetime foil loss “handling” All loss “evaporation” 8-12 September 2013PsI201316

17. OpenBUGS - Bayesian analysis shows a strong preference for the no- loss model. In those instances where a loss model is preferred, the loss is on average ~0.1% of the lifetime deposit areal density. Simple  2 minimization routine - strong preference for no-loss model  2 nearly the same for each model, the extra degree of freedom makes the no-loss the best fit Need to decide the appropriate to use Use based on strong preference for no-loss model and ~0.1% uncertainty from OpenBUGS result results 8-12 September 2013PsI201317

18. Updated (2013) Including this change… 8-12 September 2013PsI201318

19. 8-12 September 2013PsI201319

20. Thank you for your attention! A preprint is available on the archive: http://arxiv.org/abs/1309.2623 http://arxiv.org/abs/1309.2623 Thanks to: – Andrew Yue, David Gilliam, Geoff Greene, Sasha Laptev, Jeff Nico, Mike Snow, and Fred Wietfeldt 8-12 September 2013PsI201320