943-60

The Model 943-60 Neutron detector, when used with a Model 942A Universal Digital Ratemeter (UDR) or Model 960 Digital Radiation Processor Controller, provides a neutron radiation monitoring system for applications where the direct measurement of thermal and fast neutrons in the vicinity of the detector is required. The Model 943-60 is a locally mounted, single-channel detector with moderator, suitable for the monitoring of neutron activity in the area surrounding the detector. The detector consists of a BF3 proportional counter tube, a boron impregnated neutron attenuator and a two (2) piece polyethylene moderator. Since the BF3 proportional counter tube will only respond to thermal neutrons, a polyethylene moderator, which slows the incident fast neutrons to thermal energies, surrounds the neutron sensitive tube. The outer moderator is also used as a support surface for mounting the detector to a wall. The moderator provides a response to neutrons over a thermal energy range of 0.025 eV to 15 MeV. The boron attenuator is used to reduce the number of thermal neutrons that reach the detector. The attenuator is made from silicone rubber, impregnated with boron. The attenuator is designed with 10 mm holes over 22% of its surface to provide a response to neutrons over a range of 1.0 x 10-1 to 1.0 x 104 mR/h, with a neutron sensitivity of 1.26 x 10-2 mR/h/CPM. The overall accuracy is within ± 20% of the theoretical CRP dose rate. The sensitive element of the detector is the BF3 proportional counter tube. The neutron counter is a gas filled tube containing two (2) electrodes. The BF3 tube is filled with boron tri-fluoride gas, enriched to 96% with 10B, at a pressure of20 cm Hg. The incident neutrons produce alpha particles and lithium nuclei that ionize the gas in the tube. A high voltage potential is applied between the2 electrodes. The detector high voltage is generated by the associated ratemeter or readout device. The high-voltage gradient near the anode results in a multiplication of the electrons in the tube, producing a pulse of electrons at the anode. The pulse height is proportional to the energy of the initial ionizing event, and is a value of approximately one milli-volt. Incident gamma rays will also react with the fill gas. The gamma rays, however, will produce fewer ions than the neutrons, resulting in a smaller pulse. By properly adjusting a discriminator in the preamplifier, gamma pulses may easily be rejected. The preamplifier consists of a Darlington input stage, a two-stage feedback amplifier, a Darlington output driver, and a constant current generator. The milli-volt pulses from the BF3 tube are capacitance coupled to the input amplifier. A gain of approximately 200 is applied to the pulse, and- 5 VDC pulse is output to the associated ratemeter for display and alarm processing. A junction box is mounted on the top of the shield. The junction box provides a waterproof enclosure for connecting the detector to the control room readout cable. The junction box also contains a solenoid operated check source mechanism. The check source, provided separately, provides a method to verify detector operation from the control room readout device. The preamplifier is located in a junction box mounted on the polyethylene moderator.