Neutron detection with mercuric iodide

Mercuric iodide is a high-density high-Z semiconducting material useful for gamma ray detection. This makes it convertible to a thermal neutron detector by covering it with a boron-rich material and detecting the 478-keV gamma rays resulting from the ¹⁰B(n,α)⁷Li^* reaction. However, the 374 barn thermal capture cross section of ^natHg, makes the detector itself an attractive absorber, and this has been exploited previously. Since previous work indicates that there are no low-energy gamma rays emitted in coincidence with the 368-keV capture gamma from the dominant ¹⁹⁹Hg(n,γ)²⁰⁰Hg reaction, only the 368-keV capture gamma is seen with any efficiency with a relatively thin (few millimeter) detector. In this paper, we report measurements of neutrons via capture reactions in a bare mercuric iodide crystal and a crystal covered in ¹⁰B-loaded epoxy. The covered detector is an improvement over the bare detector because the presence of both the 478and 368-keV gamma rays removes the ambiguity associated with the observation of only one of them. Pulse height spectra, obtained with and without lead and cadmium absorbers, showed the expected gamma rays and demonstrated that they were caused by neutrons.