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中子测量的一个重要内容是测定接近于天然中子本底剂量水平的中子数。在宇宙物理研究、辐射剂量防护、加速器和反应堆设计、运行、监察等方面离不开中子本底数据。目前,测定低水平中子数的探测器多数还是采用外包慢化体的BF_3正比计数管。由于是接近本底水平的中子测量,要求探测器:(1)灵敏度高。BF_3计数管的灵敏度同BF_3气体压力p、计数管直径b~2和长度成正比。故要求制高气压大直径BF_3计数管,如管直径50mm、长1100mm,BF_3气体压力为600mmHg的大计数管,其灵敏度约150cps/n·cm~(-2)·s~(-1)。(2)有良好的谱特性。要求计数管由中子产生的脉冲幅度分布谱和γ分布谱间有宽的分布区分界限。在长期连续(昼夜24小时不间断)测量中,电路状态(工作电压、放大倍数、甄别阈值)缓慢漂移,中子计数不损失,噪声、γ计数不混入。大都要求微分谱分辨率<10％。(3)能满足大面积测量。计数管一致性要好,几个计数管有相同的分辨率和幅度(峰位),便于多管并联,排成计数管阵列。另外,各种类型BF_3计数管最佳(或者说极限)分辨率的寻求,对于深入研究(n,a)核反应事件、正比计数管机理和计算、以及谱形状与气体纯化工艺有关的热力学问题等是必不可缺的。研制高灵敏、好分辨率BF_3计数管的意义不限于中子测量本身。
An important part of neutron measurement is the determination of neutron numbers close to the natural neutron background dose levels. In the universe of physical research, radiation dose protection, accelerator and reactor design, operation, monitoring, etc. can not do without neutron background data. Currently, the majority of detectors that measure low-level neutrons use the BF_3 proportional counter for outsourced moderators. Due to the neutron measurement close to the background level, detectors are required: (1) high sensitivity. The sensitivity of BF_3 counter tube is proportional to BF_3 gas pressure p and counter tube diameter b ~ 2 and length. Therefore, the high pressure large diameter BF_3 counter tube is required. For example, the tube with a diameter of 50 mm, a length of 1100 mm and a BF_3 gas pressure of 600 mmHg has a sensitivity of about 150 cps / n · cm -2 s -1. (2) have good spectral characteristics. It is required that the pulse width distribution spectrum of the counting tube generated by the neutron and the wide distribution separation boundary between the gamma distribution spectra. In the long-term continuous (24 hours a day, day and night) measurements, the circuit states (operating voltage, amplification factor, discrimination threshold) drift slowly, neutron counts are not lost, and noise and gamma counts are not mixed. Most require differential spectral resolution <10%. (3) to meet the large area measurement. Counting tube consistency is better, several counting tubes have the same resolution and amplitude (peak position), to facilitate multiple tubes in parallel, arranged in a counting tube array. In addition, the search for optimal (or extreme) resolution for various types of BF_3 counter tubes is of great interest in further studies of (n, a) nuclear reaction events, proportional counter mechanisms and calculations, and thermodynamic issues with spectral shapes and gas purification processes Is essential. Development of highly sensitive, good resolution BF_3 counter meaning is not limited to the neutron measurement itself.