Magnetic Trapping of Ultracold Neutrons

K.J. Coakley¹, J.M. Doyle², S.N. Dzhosyuk², R. Golub³, E. Korobkina³, P.R. Huffman^2,4, S.K. Lamoreaux⁵, A.K. Thompson⁴, L. Yang², G.L. Yang⁴

¹ National Institute of Standards and Technology, Boulder, CO 80303 USA
² Harvard University, Cambridge, MA 02138, USA
³ Hahn-Meitner-Institut, Berlin, Germany
⁴ National Institute of Standards and Technology, Gaithersburg, MD 20899 USA
⁵ Los Alamos National Laboratory, Los Alamos, NM 87545 USA

Figure 1. Overview of the neutron trapping apparatus.

Measurement of the neutron lifetime expands our knowledge of the weak nuclear force and our understanding of the creation of matter during the Big Bang. Magnetic trapping offers the possibility of a new technique to measure the neutron lifetime which is free from the systematic effects that have limited previous measurements. We have successufully demonstrated the magnetic trapping of neutrons and are in the process of performing a measurement of the neutron lifetime.

Overview

In order to magnetically trap a neutron, its total energy must be reduced to less than the trapping potential while it is located inside the confinement region. The superthermal technique for UCN production satisfies this requirement. As shown in Figure 2, the neutron dispersion curve (Q2/2 m) intersects the Landau-Feynman dispersion curve for elementary excitations in superfluid 4He at an energy of 0.95 meV (11 K or a neutron wavelength of 0.89 nm). Neutrons close to this energy can scatter to near rest by emission of a single phonon. The rate for the inverse process, upscattering by absorption of a phonon, is suppressed by the Boltzmann factor e-11K/T, where T is the temperature of the superfluid helium bath. This allows neutrons with energies less than the trap depth (~1 mK) to remain out of thermal equilibrium with the warmer liquid helium for times much longer than the neutron lifetime. When a trapped neutron decays, the resulting electron recoils through the liquid helium, producing ionization tracks. Ionized helium recombines to form He2* molecules which quickly decay emitting ultraviolet (EUV) photons in a broad peak centered around 80 nm. We frequency downconvert this prompt pulse of EUV light to the visible before transporting it out of the cryostat and into photomultiplier tubes (PMTs) at room temperature.

Figure 2. The superthermal process.

Neutron Trapping Data

Figure 3. Neutron trapping data.

Ultracold neutrons have been magnetically trapped for the first time.

Neutron trapping data from four weeks of running are shown at right in figures 3a and 3c. The neutron beam passes through the cell for 22 minutes following which scintillation events are recorded for one hour. Data are collected either with the magnetic trapping field on for the entire run ("trapping runs") or with the magnetic field off during the loading phase but on during observation ("non-trapping runs"). The results shown are obtained by pooling the data and subtracting the trapping runs from the non-trapping runs. This technique minimizes contributions from time dependent backgrounds, such as luminescence and activation.

While taking trapping data, the trapping region is filled with isotopically pure ⁴He (less than 1 part in 10^{15 3}He). In order to confirm that the observed signal is due to trapped neutrons, additional data were taken with a small amount of ³He (1 part in 10⁷) doped into the isotopically pure ⁴He. This amount of ³He absorbs less than 1 % of the neutron beam but results in a trap lifetime of less than 1 s. The difference of trapping and non-trapping runs with ³He doped into the bath is shown at right in figures 3b and 3d. The exponential decay from the trapped neutron events is absent.

Two sets of background subtracted trapping data were collected: set I with a trap depth of 0.76 mK (3a) and set II with a lower trap depth of 0.50 mK (3c). (The lower trap depth was used due to problems with the magnet.) Most of the run-to-run variation in background rate is eliminated by excluding the first two pairs of runs in which the background rate is changing quickly due to activation of materials with lifetimes > 12 hours. The remaining 23 pairs in set I (from about five days of running) and 120 pairs in set II (from about three weeks of running) are pooled and modeled as:

W_I = a_I e^-t/ + C_I    ,
 W_II = a_II e^-t/ + C_II  .

The subscripts refer to set I and set II, a_i = E N_i/t, N_i is the initial number of trapped neutrons, E is the detection efficiency, and is the lifetime of neutrons in the trap. The constant C_i is present to account for the possible remaining effect of the changing background rate due to long-lived activation. However, in all of our fits the value of C_i is consistent with zero. The fit is performed simultaneously on the two data sets, minimizing the total ² while varying five parameters: a_I, a_II, C_I, C_II and . The only parameter connecting the two data sets is . The best fit values indicate N_I = 560 ± 160 and N_II = 240 ± 65. Calculations using the known beam flux, trap geometry and the theory of the superthermal process predict N_I = 480 ± 100 and N_II = 255 ± 50, in good agreement with the measured values. The best fit value for the lifetime,  = 750 +330/-200 s is consistent with the presently accepted value of the neutron beta-decay lifetime of 886.7 ± 1.9 s. All of the uncertainties quoted correspond to a 68 % confidence interval.

This work demonstrates the loading, trapping, and detection techniques necessary for performing a neutron lifetime measurement using magnetically trapped UCN. Another important result is the direct confirmation of the theoretical prediction of the UCN density in the trap. Our value for the number of trapped neutrons at a 0.76 mK trap depth corresponds to a density of 2 UCN/cm³, compared to the density of 1 UCN/cm³ obtained in UCN material bottle experiments with a comparable UCN cut-off energy and higher flux reactor. Our measured density is consistent with previous measurements of the UCN production rate based on observation of upscattered UCN at higher temperatures.

Additional information reguarding this work can be found at http://www.doylegroup.harvard.edu/neutron/neutron.html.