Simulation And Design On 4π Spherical ~3He Neutron Detector Using Monte Carlo Method

Based on the measurement of (n,2n) reaction cross section, the basic scheme using 4Ï€spherical 3He neutron detector to solve this problem was proposed.The detector efficiency of 3He proportional counter changing with different thickness of polyethylene moderator, different incident neutron energy and different detection distance was simulated respectively by MCNP program. The result of the detection efficiency vs. the polyethylene moderator thickness showed that the polyethylene moderator thickness more than 15cm was appropriate for the (n,2n) reaction neutron detector design using the multiple 3He proportional counters; that of detection efficiency vs. incident neutron energy showed that the detection efficiency was relatively stable for the neutrons in 10keV-6MeV energy region in the conditions of the polyethylene moderator thickness of 15cm and detection distance of 10cm; and that of detection efficiency vs. detection distance showed that the detection distance should be as short as possible in the spherical 4Ï€neutron detector design, if other conditions permitted. But in the overall design, all aspects should be weighed. If the moderator thickness outside 3He proportional counter is ensured, the distance from the source to each counter will increase; on the contrary, making the detection distance of less small, the thickness will slowly become thinner, which is disadvantage to the improvement of detection efficiency.To verify the reliability of simulation data, the experimental detection efficiency was measured using the standard 252Cf neutron source. At the same time, the calculated data was given by MCNP program in the same conditions as the experiment, and the data was compared with the experimental result, which showed that the relative deviation was 9.504%.Measured the pulse height spectra of 90 3He proportional counters, the peak positions and the count rates were statistically analyzed. The testing results showed that the standard deviations of the peak positions and count rates statistical distributions were less than 3% and 2% respectively, which expressed that the performance of most counters had a good uniformity, can well meet the requirements for the design of 4% spherical 3He neutron detector. The overall design scheme of 4Ï€spherical 3He neutron detector was given and the performance parameters were studied preliminarily by MCNP simulation. Besides, the neutron detection efficiency and the neutron energy response of 4Ï€neutron detector were calculated. By the analysis of neutron detection efficiency, the optimal outside and inside diameter size of the detector system were 80cm and 21cm. Comparing the 4Ï€neutron detector energy response with a single 3He proportional counter showed the former response was 50-80 times than that of the latter for the same incident neutron energy, and the former peak was about in 1MeV, having a higher response for 0.1-4.0MeV neutrons.Using the model to further calculate the Am-Be neutron source moderated spectrum, the main peaks were in 3.05,4.70,7.70 and 9.63MeV, which had less bias with other measured and calculated results.