FAST NEUTRON TRANSMISSION IMAGING OF OBJECTS NEAR WALLS

摘要

Transmission imaging relies on comparing the transmission with the object present (Ⅰ) to thatwhen the object is absent (Ⅰ_0), using the relationship Ⅰ/Ⅰ_0 = exp[-μx], where μ is the attenuationcoefficient and x is the path length through the object. Transmission imaging is difficult when astationary object is adjacent to a wall since the imaging detectors cannot be positioned behind theobject for a transmission measurement. One possible solution is to locate the imaging detectorsbehind the wall. Measurement of the wall’s characteristics can be accomplished by two differentmethods. In the first method, the imaging system is raised above the object and Ⅰ_0 is measuredthrough the wall (assumes the wall composition does not vary with height). The Ⅰ measurementis then taken through both the object and the wall. Since the wall was included in the Ⅰ_0measurement, Ⅰ/Ⅰ_0 will have the wall effects removed from the transmission image. The secondmethod involves performing a traditional Ⅰ_0 measurement (without the wall). In this method, Ⅰ/Ⅰ_0= exp[-μ_wx_w]exp[-μx], where μ_w is the attenuation coefficient of the wall material and x_w is thepath length through the wall. The wall characteristics are then estimated with a parametric fittingalgorithm. This paper presents a fast neutron imaging measurement of a depleted uraniumcasting that was performed using concrete blocks to simulate a wall. A Thermo Fisher ScientificAPI-120 DT neutron generator was located in front of the casting, and the imaging detectorswere located behind the concrete block wall. The wall effects are estimated using the twomethods previously described. Using the first method, the object’s thickness and attenuationcoefficient are determined from measured data within 12% and 14%, respectively. The secondmethod (not including the wall in the Ⅰ_0) performed much better when the thickness of the wallwas known and included as an input to the parametric fitting model, correctly yielding theobject’s thickness and attenuation coefficient within 4% and 1%, respectively. This experimentdemonstrates how a fast neutron image can be obtained for a stationary object near a wall.