The usual Bonner sphere set consists of six to eight high density polyethylene spheres with diameters varying from 3 to 12 inches. Either a BF, counter, LiI scintillator, or ,He detector is located at the center of each sphere to detect the moderated neutrons. The responses of these spheres for high energy neutrons are very low even for the largest 12 inch sphere. To increase the response for high energy neutrons, high Z material such as Pb, W, TI, or Au can be added to the sphere to utilize the (n,xn) reaction of these materials. Monte Carlo Simulations for the Pb-added case with a 3He detector as an example will be presented. The response at high energy for this case is enhanced by a factor of 5 to 8 depending upon the sphere diameter.The use of the Bonner sphere sets for neutron spectra measurements above 20 MeV has been limited because of small neutron interaction cross sections in the sphere materials. Recent calculations on the use of 3He detectors in Bonner spheres for neutrons up to 20 MeV are given in [3]. The addition of fissile materials such as 235U would be ideal because of their production of induced-fission neutrons by the higher energy neutrons.However, such materials were not used because of their inherent radioactivity. Enhanced response can be obtained through the use of other high Z materials, such as W, Au, T1, or Pb which have large (n,xn') cross sections. Lead was chosen for these calculations.This paper describes calculations done to investigate the use of lead to enhance the high energy response.
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