Site location and requirements for the installation of a nuclear magnetic resonance scanning unit

The installation of a nuclear magnetic resonance unit in any diagnostic facility involves certain location and environmental requirements. Based on our experience with the FONAR QED 80 system, we have delineated the major factors which must be considered when planning for such an installation. The major requirements are location and space. Suggested layouts for the suite and the control room are included. Certain environmental specifications will vary depending on the location of the facility and the type of unit installed.     

Gallium arsenide for laser window applications

The choice of a window material suitable for high power CO₂ lasers emitting at 10.6 μm is discussed. Gallium arsenide was chosen and its preparation is discussed at length. The physical, electrical and optical properties of GaAs are also discussed with particular reference to the high-resistivity form which is necessary to reduce free-carrier absorption at 10.6 μm to an acceptable level. The method used to measure the optical absorption coefficient is a calorimetric one making use of a low power CO₂ laser. The variation of the optical absorption coefficient with impurities, different dopants, resistivity, laser beam polarization and various growth parameters has been examined. It was found that GaAs could be routinely prepared with an absorption coefficient below 0.02 cm^-1 at 10.6 μm, a minimum value of 0.005 cm^-1 being the best observed. The prime cause of this absorption has not yet been established, but the need for further theoretical work is clear. The quality of the GaAs prepared makes it a prime candidate for a window material in high power CO₂ lasers and systems. This work was partially supported by the Office of Naval Research, Physics Branch, under Contract N00014-70-C-0132.     

A measurement of the 50Ti(γ, n) and 50Ti(γ, n0) cross sections

Measurements of the ⁵⁰Ti(γ, n) and ⁵⁰Ti(γ, n₀) cross sections have been made in the energy range of the giant dipole resonance (GDR). Assuming the GDR is split into two isospin components, approximated as Lorentzians, a calculation based on statistical decay of the GDR states is consistent with the experimental results.     

A sequential sampling rule for selecting the most probable multinomial event

Summary A sequential sampling rule is given for selecting the most probable event from a multinomial distribution withk cells. A random number of observations is taken from the given multinomial distribution at each stage of sampling, where the number is distributed according to a Poisson distribution with mean λ. The sampling is stopped when the count in any cell is greater than or equal to a given positive integerN. The cell with the highest count is selected for the most probable event. The mathematical analysis of the problem is simplified as a result of the statistical independence of the cell frequencies due to the randomization of the sample number. The expected value of the stage when the sampling terminates is decreasing in λ. The sequential sampling scheme in which one observation is taken at a time until the highest cell count is equal toN, corresponds to λ→0. A table is given showing some properties of the given selection procedure.     

Backward photoproduction of ϱ° and f between 2.8 GeV and 4.8 GeV

A measurement of backward photoproduction of charged pion pairs on protons is reported. The pion pair mass spectrum shows strong ?° and f production. Data are presented on the u and s dependence for ?^O and f, together with the decay polarisation for the ?^O.     

The nonexistence of a certain Steiner system S(3, 12, 112)

Although the automorphism group of a projective plane of order 10, if one exists, must be very small, such a plane could be the derived design of a Steiner system S(3, 12, 112) with a larger group. There are several reasons why the Frobenius group of order 56 is a promising candidate for the latter group. However, in this paper it is shown that there is no S(3, 12, 112) which is fixed by this Frobenius group.     

The evaluation of total absorption cross-section for photons in high density electron gas

Zeitschrift für Physik B Condensed Matter - The total absorption cross-section for a photon by an electron gas is calculated in the high density limit. The calculation uses the Green’s...