关键词: (n,α)反应, 屏栅电离室, 共振截面, 气体样品

Abstract: A method for measuring (n,α) reaction cross sections of the gas sample induced by fast neutrons using a gridded ionization chamber (GIC) was established. There are several advantages using the GIC to measure gas samples. With this method, the (n,α) reaction cross sections of inert gases which are hard to exist as a solid can be measured. And the (n,α) reaction cross section for α particle emitting in all directions can be directly measured by using the gas sample. Furthermore, compared with using solid samples, using gas samples have far more nuclei in the measurement samples. Therefore, it is feasible to use neutron source with a better energy resolution to measure the resonance cross sections with gas sample. However, there are many problems to be solved when measuring (n,α) reaction cross section of gas sample by using the GIC. On the one hand, the nuclear reaction event will be affected by the wall effect, when the nuclear reaction is generated near the electrodes of the GIC or at the edge of the electric field. The wall effect will cause the measured event to locate outside the event area which cannot be counted. Not only is the number of nuclei of the gas samples involved in the (n,α) reaction difficult to determine, but the number of events, which are affected by the background or located below the threshold, is also difficult to count. On the other hand, the neutron fluence involved in the measurement of (n,α) reaction cross section of gas sample is difficult to measure. Aiming at solving the problems of the determination of the sample nuclei number, the measurement of neutron fluence, and the selection of events, which are all difficult for the gas sample, this method included three steps: First, simulation calculation was performed to select a suitable effective volume of the gas samples, so that the event is free of background and easy to be counted. Second, the neutron collimator and the timing information of cathode and anode signals were used to select and count the events generated in the effective volume of the gas sample. Third, combining the fission fragment measurement of the 238U(n,f) reaction and the simulation based on SuperMC, the neutron fluence through the effective volume was obtained. Measurements of relative cross sections of the 14N(n,α0) reaction using gas sample were performed at En=471, 487, 500, 512, 529, and 545 MeV. The results are in good agreement with those of ENDF/BⅧ0 which verified the present measurement method.

Key words: (n,α) reaction, gridded ionization chamber, resonance cross section, gas sample