Design and Determination of Optimised Dimensions, Efficency and Angular Response of a Long-Countor; and Introducing the Construction Method of a Neutron Counter with a Wide-Range Response

One of the most requirement for the neutron flux measurement is to use detectors with the response independent of neutron energy. Also, detectors should be directional and insensitive to the gamma radiation. Long-Counter detectors are considered to be in this catagory. In this paper, this tye of detector has been designed by the MCNPX code. The counter used in this detector is a gas proportional counter which is filled with BF3 of 400 Torr gas pressure. The effective length and diameter of the counter are 31.1cm and 2.4cm, respectively. When the counter is placed in a moderator, its efficiency grows for fast neutron counting. The moderators optimum dimensions (internal and external) and the depth of track in the internal moderator, according to the calculation, are determined to be 8cm, 8cm, and 10cm, respectively. We found a good literature agreement between the calculated response profile up to 20MeV and the curves appeared in the literature, where all are showing the detector response up to 20MeV. In this paper, however, it is calculated up to 100MeV for the first time and in the energy interval of 40MeV to 100MeV with the detector response profile smoother then the other reports. In addition, to avoid entering the scattered neutron from the surroundings a 1mm cadmium absorber is placed between the external and the internal polyethylen moderator. The angular responses for the energies of 2.5MeV, 5MeV and 19MeV and also their effect on the detector efficiency have been investigated. The maximum value of the counter response has been obtained when the neutron source is placed along the detector axis.