1990 Fiscal Year Final Research Report Summary
Development of a New Pulsed Polarized Neutron Spectroscopy Using Crosscorrelation Method
| Project/Area Number |
63840007
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| Research Category |
Grant-in-Aid for Developmental Scientific Research (B).
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| Allocation Type | Single-year Grants |
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| Research Field |
固体物性
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| Research Institution | Tohoku University |
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Principal Investigator |
KOHGI Masahumi Tohoku Univ., Dept. Phys., Associate Prof., 理学部, 助教授 (30004451)
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| Co-Investigator(Kenkyū-buntansha) |
YAMADA Kazuyoshi Tohoku Univ., Dept. Phys., Assistant, 理学部, 助手 (70133923)
MOTOYA Kiyoichiro Saitama Univ., Dept. Phys. Associate Prof., 理学部, 助教授 (60114683)
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| Project Period (FY) |
1988 – 1990
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| Keywords | Pulsed Neutron Beam / Polarized Neutrons / Crosscorrelation Method / Neutron Scattering / Spin Waves |
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| Research Abstract |
We developed a new type of polarized neutron spectroscopy utilizing a pulsed polarized white neutron beam. The principle of the present method is as follows ; The polarization of the pulsed incident neutron beam is modulated by using a spin flipper coil whose driving current is modulated according to a pseudorandom binary sequence (PRBS). The polarization modulation is controlled so that the PRBS pattern shifts cyclically at each sequential counting frame triggered by the neutron burst. The procedure continues until one cycle shift of the PRBS is finished, and then it repeats a new sequence of the counting of scattered neutrons. In this way, the scattered neutron events are counted in a two-dimensional time analyzer, one time label of which is the total TOF, and the other corresponding to the counting frame number which represents the shift number of the PRBS. The crosscorrelation of theacount rate along the frame number axis at each total TOF channel with the appropriate PRBS pattern
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recovers the two dimensional TOF spectrum which is characterized by the incident time at the sample and the total TOF. Since only the polarization of the incident neutrons is modulated, the recovered TOF spectrum consists of a spectrum which is proportional to the difference between the spin flipper-on and -off spectra and an otherwise flat background.
A neutron spin flipper system and a two dimensional time analyzer were constructed for the present work. The driving current intensity for the flipper coil is controlled to be proportional to the 1/t relation, where is the time from each neutron burst, so that a 180 degree flip of the neutron spin is ensured for any incident neutron energy. The current is then switched on and off according to the PRBS. The timing of the switching is given by a module which generates the PRBS pattern which shifts by one step at each frame of the neutron burst. The scattered neutron events are counted in the appropriate memories in the two dimensional time analyzer according to their total flight time and frame number.
The measurement of spin wave scattering were carried out on the PEN spectrometer at the spallation neutron source KENS at KEK. The sample was a single crystal of ferromagnetic Heusler alloy Cu2MnAl. We could observed spin wave scattering in a wide range of (Q, omega) space in a single measurement. The results of the present experiment show that the combination of the pulsed polarized white neutron beam with the crosscorrelation method with polarization modulation is quite promising for the study of magnetic excitations or fluctuations in various magnetic materials. In order to obtain results with good resolution an intense pulsed neutron source is required of course. Less
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