A narrow thermal neutron detector includes a slidably receivable ionization thermal neutron detector module within an overall housing body. An active sheet layer of the ionization thermal neutron detector module can be tensioned across its width. The ionization thermal neutron detector module can include module upper major surface extents and module lower surface extents such that, when installed within the housing body, the module upper major surface extents are in a first spaced apart confronting relationship with housing upper major surface extents to define a first clearance and module lower major surface extents are in a second spaced apart confronting relationship with housing lower major surface extents to define a second clearance to accommodate housing flexing due to ambient pressure change. The housing body can be formed with a single opening for receiving the ionization thermal neutron detection module or with opposing first and second opposing end openings.