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Determination of the neutron lifetime using magnetically trapped neutrons.

We report progress on an experiment to measure the neutron lifetime using magnetically trapped neutrons. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering 0.89 nm neutrons in isotopically pure superfluid su·per·flu·id  
A fluid, such as a liquid form of helium, exhibiting a frictionless flow at temperatures close to absolute zero.

 [.sup.4]He. Neutron decays are detected in real time using the scintillation scintillation /scin·til·la·tion/ (sin?ti-la´shun)
1. an emission of sparks.

2. a subjective visual sensation, as of seeing sparks.

 light produced in the helium by the beta-decay electrons. The measured trap lifetime at a helium temperature of 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron A free neutron is a neutron that exists outside of an atomic nucleus. While neutrons can be stable when bound inside nuclei, free neutrons are unstable and decay with a lifetime of just under 15 minutes (885.7 ± 0.8 s).  lifetime. This is attributed to the presence of neutrons with energies higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an [833.sub.-63.sup.+74] s trap lifetime, consistent with the currently accepted value of the free neutron lifetime.

Key words: magnetic trapping; neutron lifetime; superthermal neutron production; ultracold neutrons.

1. Introduction

We present a progress report on an experimental program to improve the measurement of the neutron lifetime, [[tau].sub.n], using a technique with completely different systematic effects than previous measurements [1]. Ultracold neutrons (UCN UCN Universidad Católica del Norte (Chile)
UCN University College of the North (The Pas, Manitoba, Candad)
UCN Ultra Cold Neutron
UCN Unión del Centro Nacional
) are produced by inelastic scattering inelastic scattering
The scattering of particles resulting from inelastic collision.
 of cold (0.89 nm) neutrons in a reservoir of superfluid [.sup.4]He (the "superthermal" process). These neutrons are then confined by a three-dimensional magnetic trap Magnetic trap refers to one of three types of traps used for atoms or charged particles:
  • Magnetic trap (atoms), used to trap neutral atoms in a magnetic field gradient
. As the trapped neutrons beta decay beta decay

Any of three processes of radioactive disintegration in which a beta particle is spontaneously emitted by an unstable atomic nucleus in order to dissipate excess energy. Beta particles are either electrons or positrons.
, the resulting energetic electrons generate scintillations in the liquid He. Each decay is detectable with high effciency. Thus, [[tau].sub.n] can be directly determined by measuring the scintillation rate as a function of time.

For detailed information on the experiment, the reader is directed to the graduate thesis of S. N. Dzhosyuk [2] and Refs. [3] and [4]. This paper summarizes some highlights of the neutron trapping/lifetime data collected at the National nstitute of Standards and Technology (NIST (National Institute of Standards & Technology, Washington, DC, The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration ( ) from the fall of 2002 until the summer of 2003.

2. Experimental Procedure

In order to correct our data for both time-dependent and time-independent backgrounds, data is collected in what we refer to as "trapping" and "non-trapping" runs. In a trapping run, the magnet is energized while neutrons are loaded into the trap. After the beam has been turned off, the neutron decay events are recorded. In non-trapping (or background) runs, the magnet is deenergized while the beam is on, then raised to the full value as the neutron beam is turned off. In this non-trapping case, the background events arising from neutron activation Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when nuclei capture free neutrons, becoming heavier and entering excited states. , neutron-induced luminescence luminescence, general term applied to all forms of cool light, i.e., light emitted by sources other than a hot, incandescent body, such as a black body radiator. , etc. should be the same. A difference in the count rate versus time between trapping (trapped UCN + backgrounds) and non-trapping (backgrounds only) runs should arise solely due to magnetically trapped UCN. If for some reason the backgrounds are not identical in the trapping and non-trapping runs, then the subtraction subtraction, fundamental operation of arithmetic; the inverse of addition. If a and b are real numbers (see number), then the number ab is that number (called the difference) which when added to b (the subtractor) equals  process will leave a residual difference that could mimic a trapping signal. Measurements made with natural abundance In chemistry, natural abundance (NA) refers to the prevalence of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average) of these isotopes is the atomic weight listed for the element in the periodic table.  helium are used to conclusively determine that a putative trapping signal is, in fact, due to trapped neutrons.


Both trapping and non-trapping data was collected in a number of configurations that will be described below. Analysis of each data set is performed by integrating the pulse area of each digitized photomultiplier tube A vacuum tube that converts light into electrical energy and amplifies it. Photomultiplier tubes are used in high-end drum scanners, because they are more sensitive to light than the CCD elements used in lower-cost devices.  (PMT See photomultiplier tube. ) signal and applying appropriate lower level threshold cuts on the area of the pulses. Since neutron-induced luminescence (occurring as single uncorrelated photons) is known to be present in the coincidence data with thresholds at single photoelectron pho·to·e·lec·tron  
An electron released or ejected from a substance by photoelectric effect.

 levels, thresholds are set to require an area in each pulse equivalent to at least three photoelectrons.

A representative set of data (from approximately 8 weeks of data collection) is shown as the upper curve in Fig. 1. The curve is obtained by taking the difference between the trapping and non-trapping runs and then fit to the function y = [y.sub.0] + A exp(-t/[tau]) with parameter estimates [y.sub.0] = 0.04 [+ or -] 0.01 [s.sup.-1], A = (1.94 [+ or -] 0.03) [s.sup.-1] and [tau] = ([621.sub.-17.sup.+18]) s with [chi square chi square (kī),
n a nonparametric statistic used with discrete data in the form of frequency count (nominal data) or percentages or proportions that can be reduced to frequencies.
] = 0.96, where [chi square] is the reduced chi-squared value. As one can see, the lifetime obtained from this data is substantially shorter than the presently accepted value of the neutron lifetime (885.7 [+ or -] 0.8) s (1 [sigma] uncertainty) [1]. Subsequent runs have led us to identify this systematic effect in our system as arising from marginally trapped neutrons.

3. Study of Systematics systematics: see classification.  

A wide range of experimental configurations has been explored to both measure the lifetime of UCN in our magnetic trap and to understand the observed shift in the measured trap lifetime due to systematic effects. The origin of the systematic shift is not completely understood, but as will be shown below, it appears to arise from a combination of marginally trapped neutrons and material bottling. Lifetime measurements under different experimental conditions yield values for the lifetime in the range of 600 s to 900 s, a spread larger than the statistical uncertainty.

Data was taken using natural helium (with a fractional [.sup.3]He content of approximately [10.sup.-7]) to help understand systematic effects and is shown as the lower curve in Fig. 1. The lifetime of UCN in the trap in the "[.sup.3]He" configuration is less than one second. Fits to this data set are consistent with zero or no trapped neutrons. There is also no evidence of neutron activation or neutron-induced luminescence in the data.

Low temperature (< 300 mK) trapping data has been collected over a number of different experimental conditions chosen to probe potential origins of the systematic shift. These conditions include artificially increasing the fluence Flu´ence

n. 1. Fluency.
 of neutrons (while keeping the 0.89 nm fluence fixed), varying the length of time we observe neutron decays, ramping the magnetic field to remove marginally trapped neutrons, and numerous tests of the electronics and data acquisition systems (DAQ See data acquisition. ). In brief, all of the non-magnet-ramping runs yield results that are consistent with one another. The magnet ramping data gives a considerably longer lifetime, [tau] = ([833.sub.-63.sup.+74]) s (see Fig. 1), consistent with the known neutron beta-decay lifetime. Thus we believe marginal trapping (probably in combination with the interaction between the walls and the marginally trapped neutrons) to be the primary source of our systematic shift in the data.

We expect that relatively few marginally trapped neutrons will be present in the trap and one can remove these trapped neutrons by lowering the magnetic field to 0.3 of its original value and then raising it again to the original value [3]. This process takes approximately 200 s and is performed immediately after the 0.89 nm neutron beam is turned off. The lowering of the field sweeps out practically all marginally trapped neutrons, while throwing away about 50% of the "good" (truly trapped) neutrons.

Our hypothesis for the unusually large number and long lifetime for these marginally trapped neutrons is material bottling of these essentially untrapped UCN within the solid walls of the trap cell. Estimates of the UCN potential of the hydrogenous surfaces surrounding the trapping region (tetraphenyl butadiene, GoreTex (1), etc.) give values of approximately 50 neV. Although the absorption probability per wall interaction is high, we expect that these marginally trapped neutrons (with energies just above the trap depth and likely incident upon the walls at glancing angles) will undergo considerably fewer interactions with the walls than in a simple ballistic model. In fact, these neutrons may interact with the walls only a few times per second, thus having a long lifetime in the cell. We are currently implementing a series of experiments and numerical simulations to further investigate this hypothesis. In any case, if this is the source of the shift, ramping should eliminate it.

3.1 Other Investigations

Data has also been taken at different temperatures in an attempt to verify the theoretical [T.sup.7] dependence of the phonon phonon (fō`nŏn), quantum of vibrational energy. The atoms of any crystal are in a state of vibration, their average kinetic energy being measured by the absolute temperature of the crystal.  upscattering and, perhaps, aide in the diagnosis of the observed lifetime shifts. Non-ramping data are taken at T = 500 mK, T = 700 mK and T = 850 mK. The extracted lifetimes are shown in Fig. 2. There are a couple of things to note. First, as the temperature is raised, the lifetime in the trap gets shorter as expected. Second, the shape of the temperature dependence appears to track the [T.sup.7] dependence if a somewhat shorter intrinsic trap lifetime is assumed. This data is consistent with marginally trapped neutrons causing the shorter lifetime at low temperatures.


Background counts arising from external (to our apparatus) sources continue to be a difficulty; as a point-of-reference, the background count rate drops a factor of five when the reactor is turned off. We significantly reduced background count rates with the addition of an 0.89 nm monochromator A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input.  and a lead "house" surrounding the apparatus. Nevertheless, the changing experimental conditions from neighboring instruments cause substantial time variation in the backgrounds. We have investigated adding additional shielding of high-density polyethylene high-density polyethylene
n. Abbr. HDPE
A strong, relatively opaque form of polyethylene having a dense structure with few side branches off the main carbon backbone.
, Li-loaded polyethylene, and/or doubling the existing 10 cm of lead. We find that the additional thickness of lead does not significantly change the backgrounds, whereas the addition of 5 cm of both high-density polyethylene and 5 cm of Li-loaded plastic (10 cm total) reduces the background count rates by approximately 20% to 30%. Thus we believe that a large fraction of the current backgrounds arises from fast neutron fast neutron

A neutron that is not in thermal equilibrium with the surrounding medium, especially one produced by fission. Compare slow neutron. See also fast-breeder reactor.
 interactions that can be further reduced with additional amounts of hydrogenous materials.

We also investigated placing an external gamma-ray detector just outside the dewar next to the photomultiplier tube (PMT)s to both monitor external rates and hopefully use the data from this detector to normalize normalize

to convert a set of data by, for example, converting them to logarithms or reciprocals so that their previous non-normal distribution is converted to a normal one.
 the trapping data to minimize these systematic shifts in the backgrounds. The gamma-ray data is somewhat correlated (not as much as we would have hoped) and is presently being used to reject data where the background count rate shows significant changes during the course of a run. Work is proceeding on using this technique to help minimize changes in the backgrounds on a run-to-run basis.

The calibration of the detection system is performed using the 113 Sn emission of a 364 keV conversion electron A conversion electron is an electron which results from interactions with metastable atomic nuclei, which results from radioactive decay processes. A metastable nucleus can transfer its energy to an electron that has a certain probability of being in the nucleus. . The source was mounted on a linear track along the central axis of the the diameter of the sphere which is perpendicular to the plane of the circle.

See also: Axis
 cell and the detection effciency was measured at various positions in the trapping region. Factoring in both the positional dependence of the effciency and the beta-decay spectrum of the electrons, we obtain a detection effciency of (48 [+ or -] 6) % when photomultiplier tube (PMT) threshold levels are set to three or more photoelectrons (see Ref. [2]).

4. Future Directions

In order to fully understand the systematic effects that are present in our recent data, an additional improvement to the apparatus is needed to increase the number of trapped neutrons. At present, each lifetime measurement at a given configuration takes approximately 10 days to obtain, so varying the parameters to study the systematics at high accuracy would be both beam-intensive and time-prohibitive. We are proposing to substantially increase the number of neutrons trapped by replacing the present magnetic trap with a significantly deeper trap.

We have on loan from the High Energy Accelerator Research Organization (KEK See CEC. ) in Japan, a high-current superconducting quadrupole magnet Quadrupole magnets are designed to create a magnetic field whose magnitude grows linearly with the radial distance from its longitudinal axis, which is usually centered on and parallel to the main motion of the charged particles.  that we are in the process of turning into an Ioffe trap. This modification entails adding a superconducting solenoid solenoid (sō`lənoid'), device made of a long wire that has been wound many times into a tightly packed coil; it has the shape of a long cylinder.  assembly outside of the existing quadrupole A quadrupole is one of a sequence of configurations of electric charge or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.  assembly. The trap depth from the new magnet will be a factor of three higher than the present magnet.

Once the KEK trap has been incorporated into a dewar, we plan to take neutron trapping data with the new apparatus at NIST. NIST is presently the best source in the United States for this experiment because of the existence of the 0.89 nm monochromatic monochromatic /mono·chro·mat·ic/ (-kro-mat´ik)
1. existing in or having only one color.

2. pertaining to or affected by monochromatic vision.

3. staining with only one dye at a time.
 beam, the high fluence of neutrons available, and the availability of beamtime. The primary issue in returning to NIST is backgrounds from neighboring instruments.

The increased signal-to-noise resulting from the KEK trap will substantially decrease our susceptibility to backgrounds. Nevertheless, we plan to investigate additional ways to lower our sensitivity to backgrounds. About 50% of the background events originate in the helium itself and the other half are from high-energy particles scattering in the acrylic in the lightguides.

Once the systematics have been identified and are under control at NIST, we expect to be able to make a measurement of the neutron lifetime with a statistical accuracy of [approximately equal to]3 s in one reactor cycle (39 d). The experiment would then be poised to move to either the new Spallation Neutron Source The Spallation Neutron Source (SNS) is an accelerator-based neutron source being built in Oak Ridge, Tennessee, USA, by the U.S. Department of Energy (DOE). SNS is being designed and constructed by a unique partnership of six DOE national laboratories: Argonne, Lawrence Berkeley,  (SNS SNS sympathetic nervous system. ) fundamental neutron physics facility or possibly a new external source of UCN. The advantages of these facilities will be the expected lower background rates and the better coupling of the neutron beam into the apparatus. This should allow a measurement with a statistical accuracy of 0.1 s for the same period of running.


This work was supported in part by the National Science Foundation under grant numbers PHY-0354263 and PHY-0354264.

5. References

[1] K. Hagiwara et al., Review of particle physics, Phys. Rev. D 66, 010001 (2002). Available online at

[2] S. N. Dzhosyuk, Magnetic Trapping of Neutrons for Measurement of the Neutron Lifetime. PhD thesis, Harvard University, 2004. Available at

[3] J. M. Doyle and S. K. Lamoreaux, On measuring the neutron beta-decay lifetime using ultracold neutrons produced and stored in a superfluid-[.sup.4]He-filled magnetic trap, Europhys. Lett. 26, 253-258 (1994).

[4] P. R. Huffman, C. R. Brome, J. S. Butterworth, K. J. Coakley, M. S. Dewey, S. N. Dzhosyuk, R. Golub, G. L. Greene, K. Habicht, S. K. Lamoreaux, C. E. H. Mattoni, D. N. McKinsey, F. E. Wietfeldt, and J. M. Doyle, Magnetic trapping of neutrons, Nature 403, 62-64 (2000).

S. N. Dzhosyuk, A. Copete, J. M. Doyle, and L. Yang

Harvard University, Cambridge, MA 02138 USA

K. J. Coakley

National Institute of Standards and Technology National Institute of Standards and Technology, governmental agency within the U.S. Dept. of Commerce with the mission of "working with industry to develop and apply technology, measurements, and standards" in the national interest. , Boulder, CO 80303, USA

R. Golub

North Carolina State University History

Main article: History of North Carolina State University
The North Carolina General Assembly founded NC State on March 7, 1887 as a land-grant college under the name North Carolina College of Agriculture and Mechanic Arts.
, Raleigh, NC 27695, USA


Hahn-Meitner-Institut, Berlin, Germany

E. Korobkina

Hahn-Meitner-Institut, Berlin, Germany

T. Kreft

Tulane University, New Orleans, LA 70118, USA

S. K. Lamoreaux

Los Alamos National Laborataory, Los Alamos, NM 87545, USA

A. K. Thompson and G. L. Yang

National Institute of Standards and Technology, Gaithersburg, MD 20899, USA


P. R. Huffman

North Carolina State University, Raleigh, NC 27695, USA


National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Accepted: August 11, 2004

Available online:

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Author:Huffman, P.R.
Publication:Journal of Research of the National Institute of Standards and Technology
Geographic Code:1USA
Date:Jul 1, 2005
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