A simple aluminium-walled proportional counter has been developed to measure low fast neutron flux by irradiation of the counter itself and subsequent counting of the β-rays from the 9.8 min Mg27 activity produced by the (n,p) reaction. The operational characteristics are described and the sensitivity discussed. The counter is capable of detecting a fast neutron flux ≈ 103 n·cm−2·sec−1.
In the Van de Graaff electrostatic generator the electric charge at a high voltage is stored on a metallic collecting electrode. In 1933 Van de Graaff et al. stated that the spherical collecting electrode is similar to a Faraday's icepail. A new explanation of the generation of the high voltage in the above generator was presented in 1954 by Simon. It is based on a different fundamental principle, namely, the Volta's condensing electroscope principle. Simon concluded that if the spraying system is maintained at a constant potential, the collecting electrode will also attain a constant potential, the magnitude of which will be proportional to that of the spraying system. The factor of proportionality does not depend on the capacity of the collecting electrode but depends on the height of the machine. The results of the experiments performed on a self-exciting generator of Becker type indicate that the voltage of the generator does not depend on the metallic sphere and the voltage is increased as the height is increased even though the same collecting system is used. This is qualitatively in accordance with the Simon's theory.
A fast diode coincidence unit which can be driven by photomultiplier pulses is described. The coincidence circuit can be changed from a double to a triple or quadruple by a switch. A resolving time, τ, of about 3 mμsec with 100% efficiency can be achieved. The dead time following non-coincident and coincident input pulses are about 16 and 70 mμsec respectively.
A technique is described for producing neutron pulses of 0.25 μsec duration at 400 p.p.s. The electron gun of the Harwell 15 MeV linear accelerator is pulsed from an external modulator during the period of the normal radio frequency pulse of 2 μsec duration. Using timing channels of 0.2 μsec duration and a flight path of 31.8 m a nominal resolution of 8 mμsec m−1 is obtained. The neutron output during the 0.25 μsec pulse is approximately 60% of the output obtained when using the normal 2 μsec electron pulse. Total cross section measurements using a bismuth sample demonstrate the improved resolution in the energy region of 10 to 20 keV.
Storage type pulse-height analyzers for use with low duty cycle pulsed accelerators are discussed and an analyzer described that accomodates up to nine pulses in a single burst with a resolution of two microseconds. The pulses to be analyzed are temporarily stored as lines of charge on the face of a cathode ray tube and are analyzed by means of an electron beam that detects the charge discontinuity at the peak of the pulse. The pulse-height information is then permanently stored in a magnetic core memory.
During shielding measurements in water on LIDO a large discrepancy was observed between fast neutron flux measurements with fission chambers and those using activation techniques. This effect is shown to be due to photofission of the fissionable material in the chambers.
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