H. Crannell

The bremsstrahlung tagged photon beam in Hall B at JLab

We describe the design and commissioning of the photon tagging beamline installed in experimental Hall B at the Thomas Jefferson National Accelerator Facility (JLab). This system can tag photon energies over a range from 20% to 95% of the incident electron energy, and is capable of operation with beam energies up to 6.1 GeV. A single dipole magnet is combined with a hodoscope containing two planar arrays of plastic scintillators to detect energy-degraded electrons from a thin bremsstrahlung radiator. The first layer of 384 partially overlapping small scintillators provides photon energy resolution, while the second layer of 61 larger scintillators provides the timing resolution necessary to form a coincidence with the corresponding nuclear interaction triggered by the tagged photon. The definitions of overlap channels in the first counter plane and of geometric correlation between the two planes are determined using digitized time information from the individual counters. Auxiliary beamline devices are briefly described, and performance results to date under real operating conditions are presented. The entire photon-tagging system has met or exceeded its design goals.

Energy calibration of a cosmic ray ionization spectrometer

Abstract The NASA/GSFC High Energy Cosmic Ray Experiment was calibrated at the AGS at Brookhaven National Laboratory during the summer of 1970 using protons and pions with energies from 9.3 GeV to 17.6 GeV. The best measure found for the energy E of an incoming primary particle is ΣI , the total number of ionizing particles observed in the instrument, summed over the various iron modules. The resolution in the calibration energy range is about ±30% (s.d.) over a wide range of incident angles and positions. The calibration function may be parameterized as E = ∑ I / K , where K is predominantly a function of the location of the first interaction and the trajectory of the incoming particle. To a fair approximation the geometrical dependence of K can be encompassed by writing K as a function of d , the distance from the first interaction along the primary ray to the edge of the instrument. Empirically, K =5.83 [1−exp(− d / λ )] cos θ , where λ is a characteristic length which is a slowly varying function of energy. The value of K , and thus the average energy values calculated from the experimental data are accurate to about ±10% under calibration conditions.

Photofission of Heavy Nuclei at Energies up to 4 GeV

Total photofission cross sections for 238U, 235U, 233U, 237Np, 232Th, and natPb have been measured simultaneously, using tagged photons in the energy range Egamma=0.17-3.84 GeV. This was the first experiment performed using the Photon Tagging Facility in Hall B at Jefferson Lab. Our results show that the photofission cross section for 238U relative to that for 237Np is about 80% over the entire energy range, implying the presence of important processes which compete with fission. If we assume that for 237Np the photofission probability is equal to unity, we observe a significant shadowing effect, starting below 1.5 GeV.

Elastic electron scatterring from 14C

Abstract The ground state charge distribution of the nucleus 14 C has been studied using elastic electron scattering. Data were accumulated and analysed for an incident electron energy of 374.6 MeV and for laboratory scattering angles between 32° and 70°. Cross section data for 12 C were also obtained, for purposes of normalization, at the same incident energy and angles. The data were fit, by phase shift analysis, to a two-parameter Fermi model and to the modified harmonic oscillator model. Both models give reasonable fits to the 14 C data except in the region of the first diffraction minimum. The results for the modified harmonic oscillator model indicate that the rms radius of 14 C is approximately 0.10 fm larger than that of 14 C.

Low-multipolarity magnetic transitions in Si-30, S-32, and S-34 studied by 180-degrees electron scattering

Low-multipolarity magnetic transitions of [sup 30]Si, [sup 32]S, and [sup 34]S were studied by 180[degree] inelastic electron scattering at low momentum transfers ([ital q][similar to]0.3--0.5 fm[sup [minus]1]). These measurements, made in the excitation energy region from 9 to 14 MeV, revealed several previous unreported levels. These are the first low-[ital q] results obtained by electron scattering for [sup 30]Si and [sup 34]S. Multipolarities and transition strengths for all the observed transitions were determined in a model-independent analysis. A large fragmentation of [ital M]1 strength is observed in the 4[ital N]+2 nuclei [sup 30]Si and [sup 34]S, while more strength is concentrated into fewer transitions in the self-conjugate nucleus [sup 32]S. The experimental [ital M]1 strength distributions are compared with configuration-mixing shell-model calculations. The sums of the transition strengths are in good agreement with recent shell-model calculations using an effective [ital M]1 operator.

Inelastic electron scattering at 180° from 14C☆

Abstract Inelastic scattering of 37, 50, and 60 MeV electrons at 180° from 14 C has been studied. Cross sections for the excitation of eight states in 14 C with excitation energy less than 16 MeV have been observed. Most of the strength is observed to be concentrated in one transition at 11.31 MeV which is assigned a spin and parity of 1 + . The total width of this state is observed to be 207 ± 13 keV, while the electromagnetic transition width is determined to be 6.8 ± 1.4 eV.

Elastic electron scattering from the isotopes 36Ar and 40Ar

Abstract The charge structure of 36 Ar and 40 Ar has been investigated using the technique of elastic electron scattering. Data were collected at the National Bureau of Standards Linear Accelerator with incident electron energies ranging from 65 to 115 MeV and a scattering angle of 110°. The data span a range of q 2 between 0.29 and 0.92 fm −2 . Phase shift fits to a two-parameter Fermi distribution yield model dependent rms charge-radius values of 3.33 ±0.02 fm for 36 Ar and 3.39 ± 0.02 fm for 40 Ar, where the quoted uncertainties are statistical only. These values are compared with the results of previous experiments on these nuclei.

Inelastic electron scattering from low-lying states in the nuclei 36Ar and 40Ar☆

Abstract The low-lying level structure of 36 Ar and 40 Ar has been investigated using the technique of inelastic electron scattering. Data were collected at the National Bureau of Standards Linear Accelerator with incident electron energies between 65 and 115 MeV and scattering angles of 92.5° and 110°. The data span a range of momentum transfer squared between 0.29 and 0.92 fm −2 . Tassie model and Helm model analyses have been applied to data for levels at 1.97 and 4.18 MeV in 36 Ar and at 1.46, 2.52, 3.21 and 3.68 MeV in 40 Ar. A 2 + assignment to the 3.21 MeV state in 40 Ar is suggested. Transition strengths, transition radii, and mean lifetimes for these states are computed and compared with results of previous experiments.

Particle Track Enhancement in Cellulose Nitrate by Application of an Electric Field

The number and length of etchable tracks, created by alpha particles in a cellulose nitrate sheet, are significantly enhanced by the application of an electric field across the cellulose nitrate.

Elastic electron scattering from the isotopes 35Cl and 37Cl

Abstract The charge structure of 35Cl and 37Cl has been investigated using the technique of elastic electron scattering. Data were collected at the University of Saskatchewan Linear Accelerator Laboratory with incident electron energies of 116 and 194 MeV and at scattering angles ranging from 45 to 140°. The data span a range of q between 0.60 and 1.71 fm−1. Phase-shift fits to a threeparameter Fermi distribution yield model-dependent rms charge-radius values of 3.388 ±0.015 fm for 35Cl and 3.384±0.015 fm for 37Cl.