Correlation time-of-flight spectrometry of ultracold neutrons

Abstract

The features of the correlation method used in time-of-flight spectrometry of ultracold neutrons are analyzed. The time-of-flight spectrometer for the energy range of ultracold neutrons is described, and results of its testing by measuring spectra of neutrons passing through interference filters are presented. However, at available low intensities, the statistical accuracy of measurements is improved using special methods, e.g., the correlation method discussed in this paper. CORRELATION METHOD IN TIME-OF-FLIGHT SPECTROMETRYThe significant drawback of the standard time-of-flight (TOF) method is its low luminosity, which is defined by the ratio of the duration of the neutron pulse produced by the chopper to the modulation period of the particle beam. Numerous available neutron TOF spectrometers have a duty cycle of a few percent or under. In 1964, the authors of [8,9] independently proposed using the correlation method, which was formerly employed in various fields of experimental physics, in TOF spectrometry. An impetus for further development of the correlation method was received in designing neutron and molecular beam spectrometers (see reviews in [10][11][12] At the initial stages of investigations with UCNs, the neutron fluxes were extremely low, and, sometimes, the neutron background under the experimental conditions was high. This required that the correlation method be used in UCN TOF spectrometry, since it offers a chance to overcome these difficulties. Let us briefly consider the basic principles of the correlation TOF spectrometry.This method implies that a chopper opens n times during beam modulation period T rather than once, as is the case of the classical TOF method. As a result, the effective particle flux incident on the detector increases n times, whereas the background level remains constant. The chopper modulates the particle beam according to the lawwhere g(t) describes beam modulation by a single-slit chopper, and N is the number of elementary intervals with duration ∆T in the period of sequence T = N ∆T . For the standard chopper,and, for the correlation method, a i is the binary sequence of zeros and ones. If channel width of the time analyzer δτ is equal to elementary interval ∆T , the number of neutrons detected in the k-th channel iswhere f i is the TOF spectrum corresponding to the i-th channel of the analyzer, and u k is the mean background level in the channel. In the matrix form, this equation is