泡沫金属表面改性对催化氢分子离子束离解为氢原子离子束的研究

Neutron tubes are compact neutron generators, which can be used in the fields such as detection of explosives, fissile material in cargo or oil well logging while drilling. They are designed to be small, long-lasting and inexpensive to construct. Several types of neutron generators have been developed using a variety of ion sources including vacuum-arc sources, cold-cathod 'Penning'sources, hot-cathode sources etc. The one of most important parameters for these ion sources is the fraction of atomic versus molecular ions which are produced during operation because the energy dependence of the D-T cross-section significantly affects the performace of the neutron generators.. The 'Penning' sources are commonly selected as the ions source for neutron generaters due to it's good stability, uncomplicated design. However, the the fraction of atomic versus molecular ions generated by 'Penning' sources are very low. Recently, the field emission ion sources with a fraction of atomic versus molecular ions of 4:1 were used in neutron generaters, but field electron emission from the gate electrodes causes emitter failure and damage the array as it initiates electrical breakdown in the array structures.. In 2011, a Japanese group found that when molecular hydrogen positive ions produced by a dc arc discharge are irradiated to the porous Ni catalyst, the pair ions plasma with majority of H+ and minority of H- are produced from the back of the irradiation plane.The number of pair ions produced increases proportionally with the irradiation flux. They refer to the production mechanism as catalytic ionization.. Three important processes should be considered during the molecular hydrogen ions pass the porous metal to form atomic hydrogen. The first step is molecular hydrogen being dissociated to atomic hydrogen ions. The second step is covalently bounded hydrogen migrating on the metal surface. The third step is electronic transition between the surface metal atoms and hydrogen atoms during desorption. In this proposal, we suggest to do a numerical analysis into the first and third steps by the molecular dynamic theory and the density functional theory, respectively. The irradiation surface and back surface of porous metal will be modified by coating a series of thin alloy films. By comparing the compositions and flux densities of irradiation beam with emitting beam, the influnce of different composition and stucture of alloys on the first step and the third step will be studied experimentally. With the results of our studies, a modified porous metal which is suitable for convert molecular hydrogen ion beam to atomic hydogen ion beam should be designed. This work will be helpful for improving 'Penning' source for neutron tubes.

中子管是一种安全便携的中子源，可用于爆炸物探测、勘探等领域。提高其离子源发射单原子氘离子比例是在不提高离子源束流和加速电压下增加其中子产额的关键。中子管的离子源一般采用潘宁源，它产生的离子束中单原子离子比分子离子数很低约1：8。目前改进的方法是换用其他离子源如场发射离子源，我们拟从裂解氢分子离子束为氢原子离子束的角度加以改进。2011年日本发现氢分子离子束通过泡沫Ni后可以被催化电离为氢原子吸附在Ni表面，然后氢原子迁移到Ni背面以正负氢原子对等离子体发射。本项目拟对潘宁源产生的氢分子离子在金属表面催化电离、迁移、氢原子离子从金属表面发射的过程和机理进行分子动力学及密度泛函理论模拟研究。实验上重点研究改性的泡沫金属板的面向入射离子束面（正面）和氢原子离子束发射面（背面）对氢分子离子束转化为氢原子离子束的影响和规律。本项目的系统研究工作有望为改进用于便携式中子管的离子源提供一种新的方法。

本项目研究了氢分子离子束在几种金属的表面裂解的机理及氢分子束“透过”多孔金属材料的过程。一定能量的氢分子和多孔材料表面碰撞后，氢分子裂解为氢原子。氢分子裂解的机理是材料表面对氢原子的排斥力场作用直接导致H-H键断裂。在各种情况氢与材料作用过程中，材料与氢的引力场范围内并没有发现氢的断裂。氢原子在多孔金属内依靠表面反射的机理输运，反射过程中因为氢原子注入基体或者吸附到基体表面，仅有少部分可以透过多孔材料。本机理的认识和日本W. Oohara等提出的氢分子表面吸附-迁移-发射机理不同。他们所提出的机理无法解释束流强度在孔道内是如何衰减的，以及如何改进输出束流强度。通过以上研究，本项目研究组认为，要提高氢原子束束流的关键问题是裂解后的氢原子束在多孔材料表面的注入或吸附，因此设计合理的催化材料结构及制备氢原子高反射率材料是实现这一目标的研究方向。