ISSN 0021-3640, JETP Letters, 2012, Vol. 96, No. 10, pp. 613–615. © Pleiades Publishing, Inc., 2012. Original Russian Text © V.V. Voronin, Yu.V. Borisov, A.V. Ivanyuta, I.A. Kuznetsov, S.Yu. Semenikhin, V.V. Fedorov, 2012, published in Pis’ma v Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2012, Vol. 96, No. 10, pp. 685–687. 613 1. INTRODUCTION Interest in works associated with the control of the energy of a neutron beam is stimulated by the wide application of neutrons in various scientific fields from materials science to the physics of elementary particles and cosmology. In particular, the possibility of accel- erating neutrons scattered by isomeric nuclei [1–4], in an inversely populated medium [5], and by vibra- tionally excited nitrogen molecules [6] was widely dis- cussed. The foundations of the acceleration of neu- trons in a laser radiation field were considered in [7]. The acceleration of neutrons in an alternating magnetic field was discovered in [8]. To this end, the alternating magnetic field with an amplitude of 0.4 T was used. Furthermore, the acceleration of neutrons at the spin flip by a radiofrequency flipper in a uniform magnetic field is well known and successfully used in physical experiments (see, e.g., [9]). A method for measuring small changes in the energy of a neutron is proposed in this work on the basis of anomalous dispersion in a crystal. Owing to a high sensitivity of the method, the acceleration of the neutron in a fairly weak alternating magnetic field is detected. We describe acceleration in the alternating magnetic field. Let a time-alternating uniform mag- netic field perpendicular to the velocity of the neutron be induced in the spatial region x = [–l/2, +l/2]: (1) (2) The neutron entering the magnetic field at the time instant t leaves the field at the time instant t + l/v, where v is the velocity of the neutron. Thus, the Bt () 0 , l / 2 – x l / 2 , > > = Bt () B 0 ωt ( ) , l / 2 – sin x l / 2 . < < = energy of the neutron after the passage of the system changes by (3) where the signs + and – correspond to two directions of the neutron spin with respect to the magnetic field. 2. EXPERIMENT The experiment was performed using the second horizontal beam of the VVR-M reactor (Petersburg Nuclear Physics Institute). The length of a coil with the alternating field was l = 5 cm, the wavelength of neutrons was λ = 4.9 Å, the initial polarization of the beam was P 0  85%, and the frequency of the magnetic field was ω = 8 kHz; i.e., the time of flight of the neutron through the coil was a quarter of the period of oscillations of the magnetic field. The magnetic field strength was varied in the range of B 0 = (1–10) G. It can be easily verified that the energy difference ΔE ± (t) = 2ΔU(t) between two spin states of the neutron (parallel and antiparallel to the field) is (4) where the latter equality is written for the conditions of our experiment, when the time of flight through the coil is a quarter of the period, i.e., ωt B = π/2. Thus, the maximum ΔE ± (t) value for B 0 = 10 G is approximately 2 × 10 –10 eV. To measure such small energy changes, we used the effect of the anomalous time dispersion of neutrons Δ E ± t () μ B 0 ω t l / v + ( ) [ ] sin ωt ( ) sin – { } , ± = Δ E ± t () 4 μ B 0 ωt ωt B 2 ------ sin sin 2 2 μ B 0 ωt , sin = = Observation of Small Changes in the Energy of a Neutron in an Alternating Magnetic Field V. V. Voronin*, Yu. V. Borisov, A. V. Ivanyuta, I. A. Kuznetsov, S. Yu. Semenikhin, and V. V. Fedorov Petersburg Nuclear Physics Institute, National Research Centre Kurchatov Institute, Gatchina, Leningrad region, 188300 Russia *e-mail: vvv@pnpi.spb.ru Received October 10, 2012 A method for measuring small changes in the energy of a neutron has been proposed on the basis of the anom- alous behavior of the dispersion of the neutron in the crystal near Bragg “resonance.” A high sensitivity of the method allows the observation of the acceleration of the neutron in the alternating magnetic field. It has been found that the small difference between the energies of two spin states of the neutron (parallel and antiparallel to the magnetic field) leads to significant spatial splitting of wave packets and, correspondingly, to the depo- larization of the neutron beam. DOI: 10.1134/S0021364012220158