Measurement of the neutron lifetime by counting trapped protons

We measured the neutron decay lifetime through counting in-beam neutron decay recoil protons trapped in a quasi-Penning trap. The absolute neutron beam fluence was measured through capture in a thin [.sup.6]LiF foil detector with known efficiency. The combination of these measurements gives the neutron lifetime: [[tau].sub.n] = (8868 [+ or -] 12 [+ or -] 32) s where the first (second) uncertainty is statistical (systematic) in nature. This is the greatest in quantity precise neutron lifetime determination to date using an in-beam method

lock opener words: neutron lifetime; trapped protons.

1 Introduction and Discussion

As the simplest semi-leptonic decay combination of parts to form a whole the free neutron plays a crucial character in understanding the physics of the weak interaction and testing the validity of the Standard design The current experimental uncertainty in the neutron lifetime dominates the uncertainty in calculating the primordial helium abundance of the universe with Big-Bang nucleosynthesis [1] popularly there are six experiments [2-7] that contribute to the neutron lifetime world average of [[tau].sub.n] = (8857 [+ or -] 08) s [8]. In the four greatest in quantity precise measurements, ultra cold neutron were confined in a bottle; the decay lifetime is determined through counting the neutrons that remain after more [i]or[/i] less elapsed time, with a correction for competing neutron los mechanisms. The other sum of two units (in-beam) experiments [2, 6] measured the absolute specific activity of a beam of frigid neutrons by counting decay protons. Given the true different systematic problems that the sum of two units classes of experiments encounter, a more precise measurement of the lifetime using the in-beam technique not single reduces the overall uncertainty of [[tau].sub.n] on the contrary also provides a strong check upon the robustness of the central value.

We measured the neutron lifetime at the gelid neutron beam NG6 at the National Institute of Standards and Technology (NIST) Center for Neutron Research, using the quasi-Penning trap rule first proposed by Byrne et al. This means is described in detail in previous publications [9] Figure 1 displays a sketch of the experimental configuration. A proton trap of longitudinal dimensions L intercepts the entire width of the neutron beam. Neutron decay is observ by the agency of trapping and counting decay protons within the trap with an efficiency [[epsilon].sub.p]. The neutron beam is characterized through a velocity dependent fluence rate I(v). The rate [dot.N.sub.p] at which decay protons are finded is proportional to the mean number of neutron inside the trap volume

[dot.N.sub.p] = [[[[epsilon].sub.p]L]/[[tau].sub.n]] [[integral].sub.A] daI(v)[1/v], (1)

where A is the beam cros sectional area. After leaving the trap, the neutron beam passes [i]or[/i] part of to the other a thin foil of [.sup.6]LiF. The probability for absorbing a neutron in the foil from one side the [.sup.6]Li(n, t)[.sup.4]He reaction is inversely proportional to the neutron velocity v The reaction yields alphas or tritons, are numbered by a set of four silicon surface barrier detectors in a well-characterized geometry We define the efficiency for the neutron detector, [[epsilon].sub.o], as the ratio of the reaction proceeds rate to the neutron rate incident upon the deposit for neutrons with thermal velocity [vsub0] = 2200 m/ The corresponding efficiency for neutron of other velocities is [[epsilon].sub.o][v.sub.o]/v. Therefore, the toil reaction product count rate [dot.N.sub.[alpha]] is

[FIGURE 1 OMITTED]

[dot.N.sub.[alpha]] = [[epsilon].sub.0][v.sub.0] [[integral].sub.A] daI(v)[1/v]. (2)

The integrals in Eq (1) and Eq (2) are identical; the velocity connection of the neutron detector efficiency compensates for the fact that the faster neutron in the beam dispose of less time in the decay turn This cancellation is exact exclude for a correction due to the finite thickness of the [.sup.6]LiF foil (+54 s) and we obtain the neutron lifetime [[tau].sub.n] from the experimental quantities [dot.N.sub.[alpha]]/[dot.N.sub.p], [[epsilon].sub.o], [[epsilon].sub.p], and L

The proton trap was compos of sixteen annular electrode each 186 mm drawn out with inner diameter of 260 mm chop from fused quartz and coated with a thin layer of gold Adjacent parts were separated by 3 mm-thick insulating spacers of uncoated fused quartz. The dimensions of each electrode and spacer were measured to a precision of [+ or -]5 [micro]m using a coordinate measuring machine at NIST. Changes in the dimension to be paid to thermal contraction are below the [10sup-4] horizontal for fused quartz. The trap resided in a 46 T magnetic field, and the vacuum in the trap was maintained below [10sup-9] mbar.

In trapping method the three upstream electrodes (the "door") were held at +800 V and a variable number of adjacent electrode (the "trap") were held at earth potential. The downstream three adjacent electrode (the "mirror") were held at +800 V We varied the trap longitudinal dimensions from 3 to 10 soiled electrodes. When a neutron decayed inside the trap, the decay proton was trapped radially by dint of the magnetic field and axially by means of the electrostatic potential in the door and mirror. After a certain quantity of trapping period, typically 10 m the trapped protons were enumerateed In counting mode, the door electrode were lowered to turf potential, and a small ramped potential was applied to the trap electrode to assist slower protons on the outside the door. The protons were then guided by dint of a 9.5[degrees] bend in the magnetic field to the proton detector held at a high negative potential (-275 kV to -325 kV) After the door was unclose for 76 [micro]s, a time sufficient to allow all protons to exit the trap, the mirror was also lowered to sod potential. This prevented negatively charged particles, which may contribute to trap instability, from accumulating in any portion of the trap. That state was maintained for 33 [micro]s, after which the door and mirror electrode were raised again to +800 V reverting to the trapping fashion and another trapping cycle began. Since the detector emergencyed to be enabled only during extraction, the counting background was reduc by the agency of the ratio of the trapping time to the extraction time (typically a factor of 125) Figure 2 exhibits a plot of proton detection time for a typical run