Shielding for thermal neutrons.

The problem of calculating the neutron capture gamma-ray dose rate due to thermal neutron capture in a boron or cadmium rectangular shield is considered. An example is given for shielding for a door at the exit of medical accelerator room maze in order to determine the optimum location of lead relative to the borated polyethylene.

Improvement of linear accelerator depth-dose curves.

A semiempirical analytic description of the accelerator depth-dose curve is described along with its physical explanation. The results of Monte Carlo calculations are presented and compared with experimental data to test this model. Calculations were made for different atomic number (Z) materials used as x-ray targets and flatteners, with the results showing that medium-Z materials are the logical choice. It is demonstrated empirically that Dmax is a simple function of the average energy (E) of the x-ray spectrum. The variation of E with Z of the target and flattener is demonstrated. As a practical example, Monte Carlo calculations and experimental data for old and new Clinac 35 accelerators are presented.

Photon and electron response of silicon-diode neutron detectors.

The photon response of silicon-diode neutron detectors is analyzed theoretically and measured in the 15-25-MeV region. The main mechanism for producing a response in the diode is shown to be the displacement of silicon atoms by scattering of electrons. If the photon source is an electron accelerator target, the response is mostly due to electrons originating in the target with a smaller contribution from electrons produced in the diode by photons generated at small angles to the beam.

Room scattered neutrons.

A method of evaluating the photoneutron fluence in the maze of accelerator facilities due to room scattered neutrons is presented. Measurements were made to demonstrate that the room scattered fluence is reduced by a factor of 2pi in going from the treatment room to the inner maze entrance.

The Stanford Linear Accelerator Center pulsed x-ray facility.

The Stanford Linear Accelerator Center (SLAC) operates a high-energy (up to 33 GeV) linear accelerator delivering pulses up to a few microseconds wide. The pulsed nature of the electron beam creates problems in the detection and measurement of radiation both from the accelerator beam and the klystrons that provide the radio-frequency power for the accelerator. Hence, a pulsed x-ray facility has been built at SLAC mainly for testing the response of different radiation detection instruments to pulsed radiation fields. The x-ray tube consists of an electron gun with a control grid. This provides a stream of pulsed electrons that can be accelerated towards a confined target window. The window consists of Al 0.051 cm (20 mils) thick, plated on the vacuum side with a layer of Au 0.0006 cm (1/4 mil) thick. The frequency of electron pulses can be varied by an internal pulser from 60 to 360 pulses per second with pulse widths of 360 ns to 5 microseconds. The pulse amplitude can be varied over a wide range of currents. An external pulser can be used to obtain other frequencies or special pulse shapes. The voltage across the gun can be varied from 0 to 100 kV. The maximum absorbed dose rate obtained at 6.35 cm below the target window as measured by an ionization chamber is 258 Gy/h. The major part of the x-ray tube is enclosed in a large walk-in cabinet made of 1.9-cm-thick (3/4-inch-thick) plywood and lined with 0.32-cm-thick (1/8-inch-thick) Pb to make a very versatile facility.

The effect of the static magnetic field on the response of radiation survey instruments.

The effects on the response of eight radiation survey instruments in static magnetic fields up to 0.03 T (300 G) have been investigated. The instruments studied are the Xetex 303B Pacer, the Bicron Micro Rem survey meter, the Victoreen 450p survey meter, the Victoreen 440 survey meter, the SLAC orange meter, the Keithley 36150 survey meter, the Anderson-Braun neutron remmeter, and the Victoreen 488 neutron survey meter. The results show that the effect may depend on several factors such as instrument design, alignment of the instrument axis with the magnetic flux lines, whether the instrument is stationary or moving relative to the magnetic field, the direction of the movement relative to the magnetic flux lines, and the magnetic field intensity. Also presented are results of work to enhance magnetic shielding of some of the instruments.