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Magnetic trapping of neutrons


Accurate measurement of the lifetime of the neutron (which is unstable to beta decay) is important for understanding the weak nuclear force1 and the creation of matter during the Big Bang2. Previous measurements of the neutron lifetime have mainly been limited by certain systematic errors3; however, these could in principle be avoided by performing measurements on neutrons stored in a magnetic trap. Neutral-particle and charged-particle traps are widely used for studying both composite and elementary particles, because they allow long interaction times and isolation of particles from perturbing environments4. Here we report 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 in the trap have a lifetime of 750+330-200 seconds, mainly limited by their beta decay rather than trap losses. Our experiment verifies theoretical predictions regarding the loading process and magnetic trapping of neutrons. Further refinement of this method should lead to improved precision in the neutron lifetime measurement.

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Figure 1: Half-section view of the neutron trapping apparatus.
Figure 2: Counting rate as a function of time after the neutron beam is turned off (pooled background-subtracted data).


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We thank J. M. Rowe, D. M. Gilliam, G. L. Jones, J. S. Nico, N. Clarkson, G. P. Lamaze, C. Chin, C. Davis, D. Barkin, A. Black, V. Dinu, J. Higbie, H. Park, R. Ramakrishnan, I. Siddiqi and G. Brandenburg for their help with this project. We thank P. McClintock, D. Meredith and P. Hendry for supplying the isotopically pure helium. We acknowledge the support of the NIST, US DOC, in providing the neutron facilities used in this work. This work is supported in part by the US NSF. The NIST authors acknowledge the support of the US DOE.

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Huffman, P., Brome, C., Butterworth, J. et al. Magnetic trapping of neutrons. Nature 403, 62–64 (2000).

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