Measurement of photo-neutron yield from thick lead target bombarded by 1.2 GeV electrons

Abstract

The measurement of a thick target neutron yield has been studied by using intermediate or higher energy electrons than 150 to 270 MeV electrons which was previously reported by Eyss et al. The final goal was to extend the data of Eyss to higher energy range without using a TOF (Time Of Flight) facility. The experiments to measure photo neutrons have been carried out for a Pb target with 20 mm thickness under the conditions of 1.2 GeV energy electron, 10 ps of pulse width with repetition rate of 6.25 Hz, and 3 x 10(exp 8) e/pulse of beam current. The time analyzer of 0.5 ns time resolution was used in the measurements. It was found that the lead collimator covering the detector affects the neutron efficiency by injection of scattered neutrons or secondary gamma rays from the collimator to the detector, and that the collimator contributions to the detector efficiency were dominant at lower pulse height. The method and the conditions of the calculation of neutron spectrum were described. The calculated data were found to underestimate the photo neutron production than the measured data by 30 to 100 percent at 150 MeV, and to agree with the measured data better for 266 MeV. The attenuation curves of neutrons have been measured by varying the thickness of the lead shield, and it was found that the attenuation rate was much smaller than that estimated with the lead total cross sections.

前にEyssが報告した150?270MeVエネルギー電子よりも中速または高速の電子を使って肉厚標的中性子収率の測定を研究した。最終目標は、TOF(飛行時間)設備を使用せずに高いエネルギー領域までEyssのデータを拡張することにある。厚さ20mmのPb標的に対する光中性子測定実験を、エネルギー電子が1.2Gev、反復速度が6.25Hz、パルス幅が10ps、およびビーム電流が3×10(exp 8)の条件下で行った。測定には時間分解能が0.5nsの時間分析器を用いた。検出器を被覆している鉛のコリメータが、コリメータからの散乱中性子や2次ガンマ線を検出器へ入射させて中性子効率に影響を与えること、および検出器効率へのコリメータの影響はパルス波高が低い領域では支配的であることが分かった。中性子スペクトル計算手法と条件を記述した。計算値は、150MeVでは光中性子生成量を測定データよりも30?100%低く見積もること、および266MeVでは測定値と一致することが分かった。中性子の減衰曲線を鉛遮蔽の厚みを変えて測定した結果、減衰率は鉛の全断面積を用いて推定した減衰率よりも小さいことが分かった。