C.P. Sargent

Focal plane instrumentation: A very high resolution MWPC system for inclined tracks

Abstract A focal plane system has been developed for the MIT energy-loss magnetic spectrometer. The arrival time information from adjacent wires of one MWPC (s = 2 mm), is used to trace particle trajectories with a position resolution of 120 μm (2σ) and an angular resolution of less than 17 mrad (2σ). The tracks are inclined to the MWPC at about 45°. The readout uses 3 delay lines connected to successive sence wires in a cyclical pattern. Coarse wire positions are determined by differences and drift times by sums of signal arrival times at the ends of the delay lines. A Cherenkov counter provides a fiducial signal. Interpolation is independent of drift velocity since the drift is normal to the sense plane. A similar readout with a second chamber provides position information perpendicular to momentum plane. This information is used to correct on-line for focal plane curvatures and other spectrometer aberrations. Final momentum resolution is about 10−4.

Photoprotons from lithium and carbon with 100 MeV bremsstrahlung

Abstract Measurements have been carried out of the distribution in energy and angle of the high-energy protons emitted in the photodisintegration of 6 Li, with some limited studies also of 7 Li and 12 C. The experiments were performed at the University of Saskatchewan Accelerator Laboratory using a 100 MeV bremsstrahlung beam. Protons of energies between 40 MeV and 95 MeV were detected with about 4% resolution, at eight angles from 45° to 126° by an array of plastic scintillation counter telescopes. Proton spectra for “monochromatic” photons were obtained by forming a properly normalized difference between spectra measured with 102 and 95 MeV bremsstrahlung beams. The 6 Li(γ, p) data are compared with the prediction of a phenomenological quasi-deuteron model, and this model is found to reproduce satisfactorily the shape and scale of the data over the entire range of the measurement. The experimental results are also discussed in terms of a single-particle model. A Born approximation calculation using shell-model wave functions does not provide sufficient amplitudes at high momenta to account for the 6 Li(γ, p) data.

High resolution spectrometers for electron scattering

Abstract In this article we discuss high-precision, high-resolution magnetic spectrometry as applied to electron scattering at intermediate energies (⩽1 GeV). Beginning with a brief summary of some aspects of the electron-scattering process, we develop the specifications for systems capable of achieving precision of about 1% in cross-section and 10 −4 in momentum resolution. Following a brief outline of the general formalism of magnetic optics, we describe in considerable detail the MIT DD electron energy-loss spectrometer including: (1) general optical specifications, (2) mechanical construction methods and special features, (3) focal plane instrumentation, and (4) examples of performance to date. In a concluding section we make some comparisons with more recent systems and indicate where further improvements in the MIT system may possibly be achieved.

The p-shell proton momentum distribution in 16O above the Fermi momentum

An effective momentum distribution in the region from about 300 to 900 MeV/c has been obtained from recent high-energy 16O(γ, p0) data. The result is compared with two different interpretations of other experimental data and with theoretical momentum distributions.

The 6−T=1 resonance in 28si via high-resolution inelastic electron scattering

Abstract High-resolution (e,e′) was used to measure the form factor of the 6 − , T =1 resonance in 28 Si. The results disagree with previous experimental results and with theoretical calculations. The role of meson-exchange currents in producing the observed quenching of magnetic strength, and the relevance of (e,e′) to other reactions are briefly discussed.

Precise measurement of the charge-distribution differences of the oxygen isotopes

Abstract The charge form factors of 17 O and 18 O have been measured by electron scattering, and the charge-distribution differences between these nuclei and 16 O have been extracted. The spherical part of the 17 O distribution is very close to that of 16 O while the 18 O distribution is substantially different.

Electroexcitation and the determination of the K-band structure in 24Mg

Abstract The high-resolution electron scattering facility at the MIT-Bates accelarator was used to resolve the 4 + 1 4.12 MeV and 2 + 2 4.24 MeV levels in 24 Mg. The respective E2 and E4 Coulomb form factors were measured and compared to form factors calculated theoretically; the 4 + 1 form factor exhibits a momentum-transfer dependence which strongly suggests that K is a good quantum number in 24 Mg.

A test of the interacting boson approximation using electron scattering

Abstract New electron scattering data are presented for levels of 154 Gd. The study includes the three lowest-lying 2 + levels at 0.123, 0.816, and 0.996 MeV excitation energies. These data provide a test of the Interacting Boson Approximation (IBA) prediction that three electric quadrupole transition charge densities are clearly dependent. Deviation of the experimental densities from linear dependence is presented and discussed.

Evidence for fragmentation of stretched 6− strength in Si (e, e′)

Abstract From natSi (e, e′) spectra taken at 90° and 160°, we present evidence for fragmented 6− T=1 strength in 28Si. A narrow peak at 17.35±0.10 MeV excitation is shown to be consistent with 6− T=1 strength.

Electroexcitation of 2+ and 4+ states in 28Si: A test of the renormalized shell model☆

Abstract The electromagnetic form factors of the lowest five 2 + and the lowest two 4 + states in 28 Si are measured via high-resolution electron scattering, some for the first time. Comparison with the renormalized shell model calculations of Chung and Wildenthal reveals deficiencies in the shell model description of these states.