A new Bonner-sphere set for high energy neutron measurements: Monte Carlo simulation

Abstract

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.

Characterization of extended range Bonner Sphere Spectrometers in the CERF high-energy broad neutron field at CERN

“…Again, the lower height of the high-energy peak as provided by both FRUIT modes is more respectful of the experimental BSS counts, as evidenced in Fig. 5 and especially from the data of the extended range spheres (12 in.þPb,7 in.þPb, 7 in.þCu). Fig.…”

“…Refs. [6][7][8][9][10][11][12]. These systems are conventionally called Extended Range Bonner Sphere Spectrometers (ERBSS).…”

Responses of conventional and extended-range neutron detectors in mixed radiation fields around a 150-MeV electron LINAC

“…High-energy neutrons contribute substantially to the dose fraction, but they result in only a small or negligible response to most conventional moderated-type neutron detectors. Using heavy metals (e.g., lead or tungsten) embedded in neutron moderators [4][5][6][7] can extend the effective detector response up to the GeV range because of the compensation of neutron multiplication effects, such as (n,2n) or spallation reactions induced by high-energy neut rons with heavy nuclei. These extended-range neutron detectors can reflect reasonable dose rates in workplaces with high-energy neutrons, and are therefore typically adopted as neutron dose measuring devices in accelerator facilities for radiation protection purposes.…”

Response of several neutron measuring devices under the fractional change of thermal and fast neutrons

“…High atomic number materials of Pb, W, or Cu also can be inserted into a polyethylene moderator to improve the reaction rate in a sensing body especially for much higher-energetic neutrons over about 10 MeV. (Birattari et al, 1998;Vylet et al, 1997;Hsu et al, 1994;Wiegel and Alevra, 2002) Basically, thermal neutron sensitive detectors can not help influencing the measurements in neutron fields of fast and thermal neutron due to their high sensitivities to thermal neutrons compared those to fast neutrons, even though a thermal neutron does not contribute in the total dose or dose equivalent rate at the normal area monitoring of workplaces composed of thermal and fast neutrons. The Albedo type TLDs used for personal neutron monitoring have a severe limitation in measuring a dose from fast neutrons because it can not use any kinds of moderator with those sensors to improve sensitivity to it.…”