An instrument for measuring thermal neutron flux

AN INSTRUMENT has been developed to measure thermal neutron flux directly and automatically in the range from 10
¹³
to 10
¹⁴
neutrons per square centimeter per second at ambient temperatures up to 350 C. When two nearly identical elements, one containing enriched, and the other natural uranium are placed together in a neutron flux, a temperature difference will exist between them caused by fissioning in the fueled element. Since both are heated equally by gamma radiation, this temperature difference can be related to neutron flux. If the element containing natural uranium is heated electrically to the temperature of the fueled element, the electric power required will be proportional to the difference in heat generated between the two elements and to the neutron flux. The system, shown in Fig. 1, functions as follows: When a temperature differential exists because of fissioning in the sensing element, the electric potential in the differential thermocouple actuates a proportioning controller, which regulates the power output of a magnetic amplifier supplying electric heating power to the balancing element. This automatic adjustment of power to the balancing element continues until the temperature difference between the sensing and balancing element is zero. The equivalent of the net fission power is read directly from a wattmeter.