John S. Risley

Case study of the physics component of an integrated curriculum

Over a four-year time span, several departments at North Carolina State University offered experimental sections of courses taken by freshman engineering students. The acronym IMPEC (Integrated Math, Physics, Engineering, and Chemistry curriculum) describes which classes were involved. This paper discusses the physics component of the curriculum and describes the impact of the highly collaborative, technology-rich, activity-based learning environment on a variety of conceptual and problem-solving assessments and attitude measures. Qualitative and quantitative research results indicate that students in the experimental courses outperformed their cohorts in demographically matched traditional classes, often by a wide margin. Student satisfaction and confidence rates were remarkably high. We also noted substantial increases in retention and success rates for groups underrepresented in science, math, and engineering. Placing students in the same teams across multiple courses appears to have been the most beneficial aspect of the learning environment.

Calibration of a VUV spectrometer–detector system using synchrotron radiation

A new technique and apparatus have been developed for the measurement of absolute electron impact photoemission cross sections in the 30–150-nm wavelength range. Synchrotron light is used as the primary intensity standard for the calibration of the detection efficiency of a vacuum ultraviolet (VUV) spectrometer–detector system. A multiadjustable manipulator was used to position precisely a Seya-Namioka-type spectrometer–detector system with respect to a narrow ray of synchrotron radiation. By scanning the beam of synchrotron radiation across the surface of the grating in the spectrometer, precise simulation of the geometry of the light source encountered in the electron impact photoemission measurement was realized. Analysis of the results underscores the fact that for spectrometer calibrations in the VUV, the calibration procedure depends on the geometry of the experimental source. The simultaneous determination of the absolute apparatus response function of the spectrometer–detector system and the geometrical factors pertaining to the electron impact photoemission source allows photoemission cross sections in the VUV to be determined with unparalleled precision.

Using physlets to teach electrostatics

One of the great difficulties in teaching electrostatics is the abstract nature of the subject. We describe six exercises in electrostatics, how they were used at NCSU, and give suggestions how they might be used in other contexts.

Electron-atom source as a primary radiometric standard for the EUV spectral region

A compact, portable radiometric standard for the EUV wavelength region utilizing single photon counting is described. An energetic beam of electrons is passed through a thin atomic or molecular gas target inducing radiative transitions. The absolute flux of radiation per unit length of beam emitted in a spectral line can be calculated from the current of electrons and the number density of the target gas if the relevant electronimpact photoemission cross section is known at the electron-impact energy used. In typical operating conditions and for a photoemission cross section of 10(-22) m(2), the emitted flux of photons can be 10(10) photons/s/cm of beam length resulting in a count rate of 500 counts/s into an f/6 spectrometer-detector system. Assessment of previously measured electron-impact photoemission cross sections indicates that, once precision benchmark cross-sectional measurements are available, ultimate uncertainties as low as 3% could be obtained for this standard.

Synchrotron radiation intensity for 50-MeV to 50-GeV electrons

Abstract Tables for practical calculations of the intensity of synchrotron radiation are presented for 50-MeV to 50-GeV electrons. The tables, accurate to five significant figures, list the angular distribution of the intensity and the flux of radiation passing through a rectangular aperture. Two extended versions are given in a form suitable for scaling to any electron energy.

A new method for measuring the retardation of a photoelastic modulator using single photon counting techniques

We present a new method to measure in situ the amplitude of the retardation for a photoelastic modulator polarimeter using single photon counting techniques. The method allows the simultaneous measurement of linear and circular polarization of light without the need for a separate calibration procedure. Comparison with measurements made with a λ/4‐retarder polarimeter show excellent agreement, confirming the method used.

Comparison of methods for determining the second-order detection efficiency of a VUV spectrometer

Four independent procedures were developed and tested to measure the apparatus response function of a VUV spectrometer-detector system for unpolarized 46-nm radiation dispersed in second order. These measurements were made to allow the use of continuum synchrotron radiation for the calibration of the response of the spectrometer-detector system for dispersion of 92-nm radiation in first order with full correction for the effects of synchrotron radiation dispersed in second order. In the first method, synchrotron radiation was used in combination with a thin Al foil to block out synchrotron radiation at 92 nm while allowing 46-nm radiation to enter the spectrometer. In the second as well as the third method NeII 46-nm line radiation was used to measure the response function in first and second order. The line radiation was produced by (1) an electron beam exciting a Ne gas target for which the resulting VUV light illuminated the entire grating and (2) a duoplasmatron VUV light source operating with Ne gas producing a small spot of radiation that was scanned across the surface of the spectrometer grating. In the fourth method the difference in the spectral distributions of synchrotron radiation produced by electrons with different kinetic energies was employed to deduce the second-order detection efficiency. The ratio of the second- to firstorder response function for 46-nm radiation could be determined to a precision of 6% using the bandpass filter and electron-beam methods, 10% using the duoplasmatron method, and 250% using the multiple electron energy method.

Charge Transfer in Negative Ion Collisions

Electron detachment is an important inelastic process for negative ion collisions. Because of the large number of electrons produced by electron loss, charge transfer to the target atom or molecule becomes a likely event, whereas in positive ion collisions it is virtually nonexistent. This mechanism is manifested in the formation of temporary negative ions of the target, usually in the form of autodetaching excited states. Electron spectroscopy of the ejected electrons produced during the collision allows one to observe lines which can be identified to specific negative ions. The position, width and relative intensity of these lines can be related to the structure of the negative ion. Examples of collisions involving H- with heavy rare gas atoms and molecular nitrogen will be discussed.

On the formation of H(n=3)dipole moments in collisions of protons on rare gas atoms

We have investigated proton-rare gas atom charge transfer collisions in the energy range from 20 to 100 keV and have extracted the electrostatic dipole moments (EDMs) of the collisionally produced H(n=3) atoms from the optical data. The results show that the EDM decreases with increasing atomic number of the rare gas atoms. Our data provide experimental evidence that post-collisional electrostatic forces do not influence the formation of the EDMs.

VUV Spectrometer-Detector System Calibration Using Synchrotron Radiation

A method of absolute calibration of a VUV spectrometer-detector system is presented in which continuum synchrotron radiation is used. Calibration of the optical system is performed for the purpose of absolute measurements of electron impact photoemission cross sections for discrete atomic (molecular) transitions in the VUV wavelength region. A large computer-controlled multiadjustable manipulator is used for positioning the spectrometer-detector system with respect to a beam of synchrotron radiation. The manipulator allows rotation with respect to two perpendicular axes, one collinear with the electron beam, and the other collinear with the spectrometer entrance slit. Using these rotations, a beam of synchrotron radiation is scanned across the grating surface resulting in a precise simulation of the source geometry encountered in the electron-atom collision source. In this way the absolute response of the spectrometer-detector system and the geometrical factors encountered in the electron-atom source are determined in an integral fashion allowing absolute photoemission cross sections to be determined with unparalleled precision.