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NIST Neutron Lifetime Experiment

NIST Beam Lifetime Collaboration

Collaborators

M.S. Dewey,1 D.M. Gilliam,1 J.S. Nico,1 M.S. Snow,2 and F.E. Wietfeldt3

1National Institute of Standards and Technology, Gaithersburg, MD 20899
2Indiana University Cyclotron Facility, Bloomington, IN 47405
3Tulane University, New Orleans, LA 70118

Free neutrons are unstable and decay with a lifetime of about 15 minutes into a proton, an electron, and an antineutrino. An accurate determination of this natural lifetime is important for tests of the Standard Model of Fundamental Particles and Interactions, as well for understanding certain aspects of cosmology and astrophysics. The neutron lifetime influences the predictions of the theory of Big Bang nucleosynthesis for the primordial helium abundance in the universe and the number of different types of light neutrinos. Since the dominant uncertainty for the primordial helium abundance comes from the accuracy of the neutron decay rate [1], improved neutron decay rate measurements are needed for sharpening this prediction.

The measurement of the lifetime of free neutrons in a cold neutron beam was one of the first experiments operating in the NCNR Guide Hall. The experiment occupied the NG-6 End Station alternatively with other fundamental physics experiments through March 2001. A statistical precision of one part per thousand was achieved in the final result. An overall standard uncertainty of about four parts per thousand has been obtained, and that uncertainty may be eventually reduced by as much as a factor of two by ongoing developments in characterization of the neutron counting for this experiment.

Method

Figure 1 shows a schematic diagram of the apparatus. An axial 5 Tesla magnetic field provides radial containment of the decay protons, while axial containment is achieved by the positive electrostatic charge on two end electrodes, called the mirror and the gate. This combination of magnetic and electrostatic fields is called a "Penning Trap." The neutron detector is comprised of a well-characterized isotopic target of 6Li, viewed by a set of four charged particle detectors. The triton and alpha particles from neutron reactions in the isotopic target are counted with an accuracy of better than one part per thousand. The reaction cross sections for these standard cross sections are known to about 0.14 %.

Diagram 1

Figure 1: Schematic drawing of the NIST Penning trap neutron lifetime experiment.

When a neutron in the beam decays inside the trapping region, the recoil proton is trapped. Periodically, the gate electrode is lowered to ground potential to allow the accumulated trapped proton(s) to exit the trap and be counted by the proton detector. A bend in the magnetic field makes it possible to locate the proton detector outside of the incoming neutron beam. The trapped protons have a maximum energy of 751 eV and are undetectable at those low energies, but by holding the proton detector at a negative high voltage of the order of -30 kV, the protons may be detected with a signal well above noise levels. The signal-to-noise ratio is further enhanced by only counting for less than 100 microseconds after the trap is opened. Typical trapping times are of the order of 10 milliseconds, and typical decay event rates are of the order of a few protons per second.

Figure 2

Figure 2. A plot of the proton count rate (normalized to the neutron monitor count rate) versus trap length. The slope of the line is inversely proportional to the measured neutron lifetime.

Determination of the lifetime requires either an accurately known trap length or a variable trap length with accurately known differences in length. The latter is more easily realized physically and is achieved in this apparatus by the precision machining of the segmented trap structure. The hard-to-define end effects do not vary significantly as the trap length is varied by applying the mirror voltage to different segments. Figure 2 shows a plot of the measured proton rate (after normalization to the neutron rate) as a function of trap length; the slope of this line is inversely proportional to the neutron lifetime. Specifically,

$\tau_n = \frac{N}{v_{0}}\;\frac{\epsilon_{p}}{\epsilon}\; \frac{\Delta L}{\Delta N_{p}}$ (Eq. 1)

where taun is the neutron lifetime, N is the number of triton and alpha particles recorded in an arbitrary counting period, Delta Np is the number of protons counted during the same period of time and coming from a section of the trap of length DeltaL, epsilonp is the efficiency of the proton detector, and epsilon is the probability of detecting a triton or alpha particle when a neutron of velocity vo = 2200 ms-1 strikes the 6Li foil.

The largest uncertainties arise from counting statistics on Np (0.1 %) and ascertaining the value of epsilon (0.4 %).

Refinements

Since the time when some of the present staff of this experiment participated in a closely related experiment at the Institut Laue-Langevin [2], a large number of improvements have been made to improve the accuracy. Perhaps the most important factor was simply having much more beam time for evaluation of significant systematic effects and accumulation of better statistical precision. Other factors include Monte Carlo corrections due to magnetic field inhomogeneities, improved beam profiles and proton alignment, better trap voltage stability and monitoring, improved definition of the areal mass density of the isotopic targets, use of a new trap designed to minimize instabilities, and improved analysis methods.

Result and Uncertainty

Figure 3 shows the extrapolation of the measured lifetime as a function of the backscattering fraction for protons from the detector surface to zero backscattering. The preliminary lifetime value obtained in this experiment is 885.8 ± 3.4 s [3].
Figure 3.

Figure 3. A linear fit of the measured neutron lifetime at varying values of the detector backscattering fraction. The extrapolation to zero backscattering gives the free neutron lifetime.

Continuing efforts to measure the neutron count rate are underway by both calorimetric and coincidence techniques, which should reduce the final uncertainty by half. They will not only improve the results of this experiment but also provide a more accurate and direct calibration of the NIST Standard Neutron Source.

Figure 4

Figure 4. NIST Penning trap neutron lifetime apparatus installed on NG-6.


Figure 5

Figure 5. The proton trap is constructed from 16 segments, each of which is fabricated from fused quartz to micron tolerances, coated with gold, and electrically connected to its own high voltage switch.

References

1.    J. Burles, K.M. Nollett, J.W. Truran, and M.S. Turner, Phys. Rev. Lett. 82, 4176 (1999).
2. J. Byrne et al., Phys. Rev. Lett. 65, 289 (1990); also
J. Byrne et al., Europhys. Lett. 33, 187 (1996).
3 M.S. Dewey et al., manuscript in preparation.

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Online: October 2003