BC-702 is a highly efficient scintillation detector of thermal neutrons, and provides excellent discrimination against gamma background. The detector is a disc 0.25" (6.35mm) thick available in several diameters which can be mounted directly to photomultiplier tubes or light guides and surrounded by an appropriate moderator.

The detector incorporates a matrix of a lithium compound enriched to 95% ⁶Li dispersed in a fine ZnS(Ag) phosphor powder. The detection process employs the nuclear reaction ⁶Li (n, α) ³H in which the resulting alpha particle and triton produce scintillations upon interacting with the ZnS(Ag).

The neutron detection efficiency varies with neutron energy as the following approximate values indicate.

In most applications, the BC-702 will give counting efficiencies up to twice those possible with similar
detectors based on ¹⁰B. The neutron detection efficiency attainable in a specific application will depend on the
level of the gamma ray background. Gamma discrimination can be achieved to enable efficient detection of thermal neutron fluxes in gamma fields as high as 10R/hr (10⁷ gamma rays per neutron). In lower gamma fields (<100mR/hr) discrimination is easily achieved by setting a lower threshold. In fields above 1R/hr, several gamma interactions occurring within the resolving time of the detector could produce a composite pulse as large as that from a neutron. In this case, pulse shape discrimination may be effectively used.

BC-720 is a scintillator designed specifically for detecting fast neutrons while being insensitive to gamma radiation. It consists of ZnS(Ag) phosphor embedded in a clear hydrogenous plastic and functions by means of the proton recoil interaction in the plastic, the proton being detected by the ZnS.

The detector is a plastic disc 0.625" (15.9mm) thick, which can be mounted directly to photomultiplier tubes or to light guides using a variety of optical greases or epoxies. It is best used with PMTs with high blue sensitivities such as those having bialkali, S-20 or S-11 type photocathodes.

Performance Characteristics
The gamma discrimination capability provided by the BC-720 is of particular value. The gamma pulse height is usually less than the neutron pulse height, and in gamma fields below 1R/hr, the gamma rays can be easily rejected by pulse height discrimination. However, due to the random generation of recoil protons throughout the detector, the neutron spectrum is quite broad. In high gamma fields, the simultaneous detection of two or more gammas could produce pulse heights falling within the lower energy portion of the neutron spectrum, and the use of a lower discriminator for gamma rejection would reduce the neutron detection efficiency. Gamma rejection may also be achieved by the use of time constants of a few microseconds.