Magnetic Trapping of Ultracold Neutrons

C.R.Brome*, J.S.Butterworth*, K.J.Coakley§, M.S.Dewey†, S.N.Dzhosyuk*, R.Golub‡, G.L.Greene¶, K.Habicht‡, P.R.Huffman*†, S.K.Lamoreaux¶, C.E.H.Mattoni*, D.N.McKinsey*, F.E.Wietfeldt*†, J.M.Doyle*

* Harvard University, Cambridge, MA 02138, USA.
§ National Institute of Standards and Technology, Boulder, CO 80303, USA.
† National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
‡ Hahn-Meitner-Institut, Berlin, Germany.
¶ Los Alamos National Laboratory, Los Alamos, NM 87545, USA.




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 for a new technique to measure the neutron lifetime which is free from the systematic errors which have limited previous measurements. We have successufully demonstrated the magnetic trapping of neutrons. The trapping region is filled with superfluid 4He, which is used to load neutrons into the trap and as a scintillator to detect their decay. Neutrons have a lifetime in the trap of 750+330/-200 seconds, limited primarily by their beta-decay. This work verifies the theoretical predictions of the loading process and the technique of magnetic trapping of neutrons. Further refinement of this method should lead to improved precision in the measurement of the beta-decay lifetime of the neutron.


Our method of trapping neutrons employs the interaction of the magnetic moment of the neutron (|µn| = 1.9µN = 0.7mK/T) with a static but spatially varying magnetic field. Low field seeking states (ms = +1/2) are trapped in a potential well surrounding a magnetic field minimum in free space. Axial confinement is provided by two solenoids with the same current sense. Radial confinement is provided by four current bars parallel to the beam axis which together form a quadrupolar field. This defines a cylindrical confinement region centered around the beam axis.

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, as proposed by Golub and Pendlebury, satisfies this requirement. The neutron dispersion curve (Q2/2m) intersects the Landau-Feynman dispersion curve for elementary excitations in superfluid 4He at an energy of 0.95meV (11K or a neutron wavelength of 0.89nm). 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 (~1mK) 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 less than 1.5cm long. Ionized helium recombines to form He2* molecules, in both singlet and triplet states. Molecules in the singlet states decay promptly, emitting extreme ultraviolet (EUV) photons in a broad peak centered around 80nm. The combined process of ionization leading to EUV radiation is very efficient; roughly 15 prompt EUV photons are emitted per keV of beta energy.

Although in principle it is possible to detect the EUV light di"RECT"ly, such an approach is impractical due to experimental difficulties in using EUV detectors at low temperatures and in the presence of neutrons. Furthermore, the absence of materials for transmitting and reflecting such radiation precludes the use of room temperature EUV detection. Therefore we frequency downconvert the EUV light to the visible before transporting it out of the cryostat and into a photomultiplier tube (PMT) at room temperature.


A picture of our apparatus is shown above. The key requirement for the apparatus is to allow a cold neutron beam to pass through the center of the trapping region while this region is filled with <250mK superfluid helium. In addition, neutron-induced activation and luminescence of surrounding materials must be kept to a minimum.


The entire cryogenic apparatus resides within a dewar which is shaped like an inverted-"T". The dewar has two sections: a vertical section which houses a 400µW (cooling power at 100mK) dilution refrigerator and a horizontal section attached below which holds the magnetic trap and detection system. The two sections are assembled independently and joined using an arrangement of sliding seals. Neutrons enter the apparatus through a series of teflon windows, which form vacuum seals and transmit neutrons with low scatter. Beryllium foils at 77K and 4K prevent heating from blackbody radiation.

The innermost teflon window makes a superfluid helium seal at the end of a cupronickel tube. The tube is thermally anchored to the mixing chamber of the dilution refrigerator and contains one liter of ultra-pure 4He (less than 1 part in 1015 3He). This tube passes through the 5cm diameter bore of the magnet assembly. The magnet assembly (shown at right) consists of four racetrack-shaped coils which form the magnetic quadrupole, and two solenoids which provide axial confinement. The trapping region produced is 34cm long, 3.0cm in diameter and 1.0T deep. UCN in the low-field seeking state and with energies less than 0.7mK are magnetically confined within the trapping region. Two-dimensional contour plots of the magnetic field and one-dimensional plots of the field magnitude along the beam and radial axes are shown below.


The incident neutron beam is collimated by a ring of neutron-absorbing material immediately preceding the trapping region. A beam stop is placed at the end of the trapping region and the inside of the cupronickel tube is shielded to prevent activation. Neutrons scattered to energies below the trapping potential remain confined in the trap, and are detected upon decay.


The scintillation process and light detection system are depicted to the right and below. A thin layer of polystyrene which is doped with the organic fluor tetraphenyl butadiene (TPB) is coated on the inside surface of an acrylic tube surrounding the trapping region. The TPB converts the EUV into blue light which travels down the tube. The tube is optically connected to an acrylic light guide which transports the light to the end of the cupronickel tube. The light exits the superfluid helium region, passes through a window at 4K, and then into a second light guide which exits the dewar and is coupled to the PMTs. In order to employ coincidence detection, which is necessary to reduce backgrounds, the light in the light guide is split equally into two PMTs using a "Y"-shaped aluminized acrylic coupler.


The light collection system has been characterized using radioactive sources placed in the center of the trapping region. Using a beta source and a single PMT, we have found that approximately 20photoelectrons per MeV of beta energy can be detected in a pulse 20ns wide. For neutron decays, the recoiling electron can have energies up to 782keV, with an average decay energy of 250keV. We have also investigated the single photon background due to triplet molecule decay. These molecules are created in large numbers by ionizing radiation in the helium (along with the singlet molecules that decay promptly), and decay with a roughly 13s lifetime.

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 a and c. The neutron beam passes through the cell for 22minutes following which scintillation events are recorded for one hour. Data are collected either with the magnetic 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 1015 3He). 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 1second. The difference of trapping and non-trapping runs with 3He doped into the bath is shown at right in figures b and d. 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 0.76mK (a) and set II with a lower trap depth of 0.50mK (c). (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 >12hours. 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:

W1 = a1 e-t/Tau+C1, W2 = a2 e-t/Tau+C2.

The subscripts refer to sets I and II, ai=ENi/t, Ni is the initial number of trapped neutrons, E is the detection efficiency, and Tau is the lifetime of neutrons in the trap. The constant Ci 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 Ci is consistent with zero. The fit is performed simultaneously on the two data sets, minimizing the total Chi2 while varying five parameters: a1, a2, C1, C2 and Tau. The only parameter connecting the two data sets is Tau. The best fit values indicate NI=560±160 and NII=240±65. Calculations using the known beam flux, trap geometry and the theory of the superthermal process predict NI=480±100 and NII=255±50, in good agreement with the measured values. The best fit value for the lifetime, Tau=750+330/-200s is consistent with the presently accepted value of the neutron beta-decay lifetime of 886.7±1.9s. All of the errors 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 di"RECT" confirmation of the theoretical prediction of the UCN density in the trap. Our value for the number of trapped neutrons at a 0.76mK trap depth corresponds to a density of 2UCN/cm3, compared to the density of 1UCN/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.

Future Di"RECT"ions

We are currently limited by statistics and are in the process of building a larger and deeper trap in addition to implementing minor upgrades to the neutron collimation and detection system. We expect these improvements to ulitimately yield a factor of 200 increase in the number trapped neutrons. With this new apparatus, we expect to make a lifetime measurement with significantly higher precision than the present world average.

There are four principal sources of systematic error due to neutron loss, all of which are controllable to our desired precision. First, 3He dissolved in the superfluid bath can absorb neutrons. We find that ultra-pure 4He with a fractional 3He content of 5x10-16 gives an absorption loss time of greater than 2.8years. (This ultra-pure helium is obtainable by employing the heat flush technique developed by Hendry and McClintock.) Second is inelastic upscattering of the UCN. The dominant process at temperatures less than 1K is two-phonon upscattering. The one-phonon upscattering rate is negligible. The two-phonon rate depends on temperature as 102T-7s-1, where T is measured in Kelvin. A negligible upscattering lifetime requires T<250mK. The third possibility is that neutrons with total energy greater than the trap depth may be temporarily contained within the trap in semi-stable orbits. We have shown, both analytically and through simulations, that it is possible to remove these neutrons by lowering the depth of the trap for a short time and then raising it again. A fourth possibility is loss by Majorana or spin-flip transitions in low-field regions. The use of a fixed-bias Joffe trap configuration with no zero-field regions greatly reduces such losses. An axial field of Bz>0.2T will suppress the rate of Majorana transitions below our expected statistical accuracy. The reduction of trap phase space due to this field is insignificant. Several systematic effects associated with previous lifetime experiments, such as wall losses, betatron oscillations, and pulse pile-up losses are eliminated by three-dimensional magnetic trapping and continuous monitoring of the neutron decay. In addition, our technique removes dependence on detector efficiencies, requiring only that the detection threshold remain stable throughout a trapping cycle (about two hours).

Based on our current understanding of the potential systematic effects, a precision of 10-5 in the neutron lifetime could ultimately be possible with this technique. For the foreseeable future, however, this experiment will remain statistics limited.

For more information, please see our latest publication P. R. Huffman et al., Nature, 403, 62 (2000). A complete list of publications related to this work can be found at