Andy Buffler

High-energy neutron reference fields for the calibration of detectors used in neutron spectrometry

Quasi-monoenergetic reference neutron beams in the energy range between 20 and 100 MeV have been produced and characterized with a proton recoil telescope, a scintillation spectrometer, a U-238 fission chamber and a Bonner sphere spectrometer. The beams are well suited for the calibration of detectors used in neutron spectrometry. A new method is described which reduces the correction for the contribution from low-energy neutrons present in the beams

Material classification by fast neutron scattering

Abstract The scattering of a beam of fast monoenergetic neutrons is used to determine elemental compositions of bulk samples (0.2–0.8 kg) of materials composed from one or more of the elements H, C, N, O, Al, S, Fe and Pb. Scattered neutrons are detected by liquid scintillators placed at forward and at backward angles. Different elements are identified by their characteristic scattering signatures derived either from a combination of time-of-flight and pulse height measurements, or from pulse height measurements alone. Scattering signatures measured for multi-element samples are analysed to determine atom fractions for H, C, N, O and other elements in the sample. Atom fractions determined from scattering signatures are insensitive to neutron interactions in material surrounding the scattering sample, provided the amount of material is not excessive. The atom fraction data are used to classify scattering material into categories including “explosives”, “illicit drugs” and “other materials” for the purpose of contraband detection.

Teaching Measurement in the Introductory Physics Laboratory

Traditionally physics laboratory courses at the freshman level have aimed to demonstrate various principles of physics introduced in lectures. Experiments tend to be quantitative in nature with experimental and data analysis techniques interwoven as distinct strands of the laboratory course.1 It is often assumed that, in this way, students will end up with an understanding of the nature of measurement and experimentation. Recent research studies have, however, questioned this assumption.2,3 They have pointed to the fact that freshmen who have completed physics laboratory courses are often able to demonstrate mastery of the mechanistic techniques (e.g., calculating means and standard deviations, fitting straight lines, etc.) but lack an appreciation of the nature of scientific evidence, in particular the central role of uncertainty in experimental measurement. We believe that the probabilistic approach to data analysis, as advocated by the International Organization for Standardization (ISO), will result i...

Cross sections for neutron-induced fission of U-235, U-238, Bi-209, and Pb-nat in the energy range from 33 to 200 MeV measured relative to n-p scattering

Abstract The cross sections for neutron-induced fission of 235U, 238U, 209Bi, and natPb in the intermediate-energy region were measured using parallel plate fission ionization chambers. The experiments were carried out relative to the differential n-p scattering cross section using quasi-monoenergetic neutron beams with peak energies ranging from 33 to 200 MeV. The experimental cross sections were compared to International Nuclear Data Committee reference fission cross sections, to results of nuclear model calculations, and to cross sections calculated with the nuclear models implemented in the radiation transport code MCNPX. The experimental results for 235U and 209Bi are consistent with the available reference cross sections and theoretical data while the 238U(n,f) cross section exceeds the reference cross section systematically by ˜7% between 30 and 60 MeV. The experimental results for natPb agree with a parameterization of other experimental data for natPb(n,f).

Neutron fluence and kerma spectra of a p(66)/Be(40) clinical source

High-resolution neutron fluence spectra have been measured in the National Accelerator Centres p(66)/Be(40) neutron therapy beam by the pulsed-beam time-of-flight method. ICRU muscle kerma spectra have been derived from the fluence spectra. Spectral changes resulting from different irradiation conditions have been quantified in terms of the average neutron energy and the fractional low-energy (< 16 MeV) contribution. The changes observed with different thicknesses of polyethylene filtration are consistent with changes in quality parameters determined in biological and microdosimetric experiments. The dosimetry parameters (KtissueA150) N and (Wgas) N calculated for the measured spectra agree with the values recommended in the neutron dosimetry protocol. The shapes of the present fluence spectra differ from previous measurements of p(> 40)/Be spectra. In particular, they differ significantly from the spectrum measured by recoil techniques in an identical neutron therapy unit at the Clatterbridge Hospital, UK. The reasons for the difference are not known.

The relationship between students' views of the nature of science and their views of the nature of scientific measurement

The present study explores the relationship between students’ views of the nature of science (NOS) and their views of the nature of scientific measurement. A questionnaire with two‐tier diagnostic multiple‐choice items on both the NOS and measurement was administered to 179 first‐year physics students with diverse school experiences. Students’ views on the NOS were classified into four NOS ‘profiles’, and views on measurement were classified according to either the point or set paradigms. The findings show that students with a NOS profile dominated by a belief that the laws of nature are to be discovered by scientists are more likely to have a view of the nature of scientific measurement characterised by a belief in ‘true’ values. On the other hand, students who believe that scientific theories are inventions of scientists, constructed from observations that are then validated through further experimentation, are more likely to have a view of the nature of scientific measurement that is underpinned by the uncertain nature of scientific evidence. The implications for teaching scientific measurement at tertiary level are discussed.

A course in tools and procedures for Physics I

A one-semester course covering the tools, skills, and procedures that are required to engage meaningfully with first-year university physics is described. The course forms part of the Science Foundation Programme at the University of Cape Town which was set up to provide access to a science degree for students who have been educationally disadvantaged, part of the legacy of racial discrimination in South Africa. The course comprises three basic elements: a theoretical component, a laboratory-based experimental component, and a communication skills component. The theory component consists of the various mathematical techniques used in a calculus-based Physics I course, grouped into cognate areas so that each technique is presented immediately in the full range of contexts that will be encountered later on. Part of the theory component involves written explanations of the mathematical formalism. The focus of the communication skills component is on report writing which follows as a natural consequence of th...

Teaching Measurement and Uncertainty the GUM Way

This paper describes a course aimed at developing understanding of measurement and uncertainty in the introductory physics laboratory. The course materials, in the form of a student workbook, are based on the probabilistic framework for measurement as recommended by the International Organization for Standardization in their publication Guide to the Expression of Uncertainty in Measurement (GUM).

Point and Set Paradigms in Students' Handling of Experimental Measurements

The procedural understanding of first year university students before and after instruction has been investigated in the context of experimental work in physics. A written instrument was used to probe the students’ ideas about data collection, processing and comparison. The responses of the students are analysed in terms of “point” and “set” paradigms which are proposed as a framework for evaluating the effectiveness of laboratory curricula.

Element analysis by fast neutron scattering

A method for element analysis is proposed which is based on measurements of energy changes in fast neutron scattering. Liquid scintillators (NE213 and NE230) are considered as neutron spectrometers for these energy measurements. Lineshapes of NE213 and NE230 detectors for monoenergetic 7.5 MeV neutrons have been determined and the response functions of the NE230 detector to neutrons have been determined and the response functions of the NE230 detector to neutrons scattered at 150 degrees from samples containing the elements C, N, O, Si, Al and Fe have been measured. A method for estimating C:N:O atomic ratios from scattering measurements on compounds containing these elements is described and discussed.