FAST NEUTRON RECORDER

Recent estimates of the harmful action of fast neutrons indicate that fast-neutron detectors are necessary for occupational-safety purposes.

The authors*) have constructed a fairly simple and reliable instrument that makes it possible to continuously monitor a flux of fast neutrons. The instrument uses two photomultipliers operating from a scintillator by means of recoil protons. To reduce the background, the photomultipliers are connected in a coincidence circuit.

*) W. Moulton and C. Sherwin, Rev. Sci. Instr. 20, 766 (1949).

A “sandwich” was placed between the windows of the photomultipliers; it was made by applying a small amount of activated zinc-sulfide powder between sheets of lucite. The lucite sheets measured \(1/16 \times 1/2 \times 2\) inches. The sandwich was compressed to make it transparent and was secured in a holder. The photomultiplier windows and the sandwich were placed as close together as possible. Photomultipliers were selected with the same pulse height; however, inequality in pulse height could be reduced by operating the photomultipliers at different voltages. The pulses from the photomultipliers were amplified, and a discrimination device was used in the circuit. The circuit had a resolving time of \(50\ \mu\text{s}\).

Labels in the figure: ordinate—“count rate per hour”; abscissa—“bias on discriminator,” “volts”; curves/annotations—“Ra–Be neutrons,” “operating point,” \(\gamma\), “background.”

According to calculation, the background should have amounted to 1 pulse in 280 hours. The actual background was 1.5 pulses per hour. This discrepancy is evidently explained by the \(\alpha\)-activity of the sandwich due to radioactive contamination, since without the sandwich not a single pulse was counted in 14 hours. In the experiments a fast-neutron flux of \(30\ \text{neutrons}/\text{cm}^2\text{s}\) from a (Ra–Be) source containing \(25\ \text{mg}\) Ra was used. Gamma radiation with an intensity of 150 milliroentgens per hour was obtained from a calibrated Ra source. The figure shows the results of testing the apparatus. It follows from the figure that the effect of \(\gamma\)-radiation at the operating point of the instrument is completely eliminated. The efficiency of the instrument for fast neutrons at \(-25\) volts on the discriminator was equal to \((1.5 \pm 50\%) \cdot 10^{-4}\). (The instrument gave 34 counts per hour at a fast-neutron flux of \(30\ \text{neutrons}/\text{cm}^2\text{s}\); the effective area of the sandwich was \(2\ \text{cm}^2\).) It is noted that the efficiency can be considerably increased by using a sandwich of larger area and by employing optical focusing of the scintillations onto the photosensitive surface of the multiplier. The instrument makes it possible to register a flux of \(20\ \text{neutrons}/\text{cm}^2\text{s}\) with a counting intensity ten times greater than the background.

K. D. Tolstov