The Model 42-49, 42-49B, and 42-49B-1 Neutron Detectors are designed for detection of thermal and fast neutrons (0.025 eV to approximately 12 MeV). Neutrons are detected indirectly through nuclear reactions which result in energetically charged particles, such as alpha particles. In many instances, intense fields of gamma rays are also found with neutrons. Therefore, it is important to choose a method of neutron detection with the ability to discriminate against these gamma rays in the detection process.
These detectors are designed to be used with portable counting instruments and have a top bracket that allows for convenient mounting of a portable instrument, such as a Ludlum Model 30. Each detector consists of a 2 atm 3He detector (1.6 cm diameter x 2.5 cm thick), surrounded by a high density polyethylene sphere, 19.6 cm (7.7 inches) in diameter. This outer sphere moderates the incoming neutrons and helps provide a near-rem response.
The Model 42-49 and 42-49B differ by the boron concentration in the internal borated layer. The Model 42-49 has a lower boron concentration and offers greater sensitivity, typically 10 cpm per μSv/h (100 cpm per mrem/hr), but tends to overrespond in the 5 keV range. The Model 42-49B has a higher boron concentration and a lower sensitivity, typically 4.5 cpm per μSv/h (45 cpm per mrem/hr), but does not have the same overresponse issue as the Model 42-49.
The Model 42-49B-1 is a variant of the Model 42-49B with a 10 atm detector (HAZMAT) that provides increased sensitivity, typically 7.6 cpm per μSv/h (76 cpm per mrem/hr).
