NEM433 Radiation Detection and Measurement Laboratory Fall 2013 Osman Şahin ÇELİKTEN

Outline Purpose Equipments Theory Procedure

Purpose To determine thermal neutron flux distribution for an Am- Be source, moderated with water, with BF 3 counter. To calculate source strength for the same source.

Equipments BF 3 Detector Canberra Model 241 Spectroscopy Amplifier Ortec Model 109PC Preamplifier Canberra Model 3002D Power Supply Multichannel Analyzer (Canberra Multiport II and Genie 2000 Software) Oscilloscope

Equipments

Theory Neutron Sources Gas-Filled Detector Slow Neutron Detection Methods Neutron Diffusion And Moderation

Theory Neutron Sources Spontaneous Fission ( 252 Cf) Radioisotope ( , n) Source ( 239 Pu, 210 Po, 238 Pu, 241 Am, 244 Cm, 242 Cm, 226 Ra, and 227 Ac) Photoneutron Source ( 9 Be, 2 D ) Reactions From Accelerated Charged Particles (D-D reaction and D-T reaction)

Theory Am-Be Neutron Source

Theory Gas-Filled Detector ( BF 3 Neutron Detection System) The tube usually filled with gas under atmospheric pressure. BF 3 gas is highly enriched the content of 10 B approximately 96%. 10 B have 3840 b thermal absorption cross section however 11 B absorption cross section is the order of mb(milibarns). BF 3 detectors consist of a gas filled tube which is made of Al(Aluminum). Al has low thermal absorption cross section(σ a =0.230 b). The effective operation voltage is V approximately.

Theory BF 3 detector is proportional gas filled detector which is include in BF 3 (BoronTriFloride) gas. Gas-Filled Detector (BF 3 Neutron Detection System)

Theory Gas-Filled Detector (BF 3 Neutron Detection System)

Theory Theoretically there’re three expected picks from 10 B( , n) 7 Li reaction. From left to right the first expected pick is low-amplitude events peak such as gamma ray interactions, electronic noise, etc. The second expected peak is reaction product full energy peak from exited state and the last one is from ground state. Gas-Filled Detector (BF 3 Neutron Detection System) But in practice the result is quite difference from theoretical. The Figure II-a shows the theoretical result and (b) shows the experimental result. This difference caused from the reactions, which occurred near the surface.

Theory Alpha and Li ions move the opposite direction when the incoming neutron energy assumed as zero If a reaction occurs in the volume near the surface Gas-Filled Detector (BF 3 Neutron Detection System)

Theory N: number of 10 B atoms per unit volume V: volume of the counter σ(E): cross-section of the (n,a) reaction for neutron energy E Φ(E)dE = ν(E) n(E) dE: neutron flux consisting of neutrons with kinetic energy between and E+dE n(E) : number of neutrons per m 3 with kinetic energy between E and E+dE ν(E) : neutron speed (m/s) for energy E E m : upper limit of neutron energy considered Slow Neutron Detection Methods

Theory The 10 B cross-section has a 1/υ dependence over a wide range of neutron energies; Slow Neutron Detection Methods Where σ 0 is the cross-section at some known speed and M is the neutron mass. Then, where n is the total number of neutrons per unit volume, or

Theory Slow Neutron Detection Methods and a total flux Combining these equations gives:

Theory Slow Neutron Detection Methods This equation will be used for determining neutron flux in the experiment. In this equation, V, σ 0 and υ 0 are known. N can be calculated from the ideal gas model, P=NRT where N=n/V. so, if average speed of neutrons is known, rate is proportional to the neutron flux and average neutron flux., Here υ p is the most probable speed and can be obtained from most probable energy E p

Theory Two Group Diffusion Equation, Source strength, one can use the solution of two group diffusion equation for thermal flux. Assumptions : no fission, no upscattering, in the system