Measurement of the delayed neutron fraction and correction of small angle scattering data from a pulsed spallation source
Abstract The fraction of delayed neutrons in a moderated beam from the enriched uranium, pulsed spallation source at the intense pulsed neutron source at Argonne National Laboratory has been measured and found to be 0.0283 for 450-MeV protons. Delayed neutrons produce a sample-dependent background in time-of-flight instruments. Because of the distribution of neutron wavelengths in the prompt pulse, the significance of the delayed neutron contribution to the observed scattering profile is strongly dependent on the wavelength and therefore on the magnitude of the momentum transfer. This effect is demonstrated, and a general correction procedure in current use for small angle neutron scattering data is described.
The Performance of the Small-Angle Diffractometer, SAND at IJ?NS#
The time-of-flight small-angle diffractometer SAND has been serving the scientific user community since 1996. One notable feature of SAND is its capability to measure the scattered intensity in a wide Q (4zsinfl/k, where 26 is the scattering angle and k is the wavelength of the neutrons) range of 0.0035 to 0.5 ~-’ in a single measurement. The optical alignment system makes it easy to set up the instrument and the sample. The cryogenically cooled MgO filter reduces the fast neutrons over two orders of magnitude, while still transmitting over 70% of the cold neutrons. A drum chopper running at 15 Hz suppresses the delayed neutron background. SAND has a variety of ancillary equipment to control the sample environment. In this paper we describe the features of the SAND instrument, compare its data on a few standard samples with those measured at well established centers in the world, and display two scientific examples which take advantage of measuring data in a wide Q-range in a single measurement. With a new set of tight collimators the Qfi. can be lowered to 0.002 ~-’ and the presently installed high-angle bank of detectors will extend the Q~,Xto 2 ~-’. a wide Q-range all at once. At reactor-based instruments the range is spanned by making repetitive measurements in different instrument configurations and wavelengths.
The Intuitive Physics of the Equilibrium of the Lever and of the Hydraulic Pressures: Implications for the Teaching of Elementary Physics.
The research field of intuitive physics focuses on discrepancies between 
theoretical and intuitive physical knowledge. Consideration of these 
discrepancies can help in the teaching of elementary physics. However, 
evidence shows that theoretical and intuitive physical knowledge may also 
be congruent. Physics teaching could further benefit from understanding the 
reasons for this congruence. The present study explored these reasons by 
investigating the intuitive physics of the equilibrium of the lever and of the 
hydraulic pressures. It was found that the intuitive-physics law of the lever 
was multiplicative for all participants while the intuitive-physics law of the 
hydraulic pressures differed among participants. Since these laws are 
equally simple and the layman probably has had extensive experience with 
the lever and scarce or no experience with the hydraulic lift, these findings 
support the general hypothesis that physical laws and corresponding 
intuitive-physics laws are congruent when people have had experience with 
the respective phenomena. The results and theoretical considerations suggest 
two strategic principles for teaching elementary physics.