Research On Microstructure Semiconductor Neutron Detector Technology

The 3He-based proportional counters are widely used in scientific research, homeland security and environment, nuclear reactor safety, neutron radiation protection as well as other fields. Today’s 3He supply comes from the β decay of tritium, which is a key component used to enhance a nuclear weapon’s power. With the end of the Cold War, the decrease of 3He supply due to the reduction of tritium stockpile, coupled with the worldwide increased demand, makes the problem of 3He shortage more severe. The microstructured semiconductor neutron detectors (MSNDs) solve the problem of low detection efficiency existed in planar semiconductor neutron detectors, and their quick response time, small size, low power requirement, high n/γ ratio etc., make the MSNDs have great advantages in terms of 3He replacement. In this paper, a systematic research on physical design of MSNDs, neutron converter material backfilling methods and experimental studies is conducted.To study how the structures, parameters, neutron converter material backfilling densities and the values of lower-level discriminator (LLD) influence the intrinsic thermal neutron detection efficiency, we simulate three type of microstructures with different parameters, neutron converter material backfilling densities and LLD settings. It is found that there is a general trend for efficiency to increase as the trench depth, hole depth and pillar height increases. In the small size, the trench design offers good intrinsic efficiency and excellent stability for various LLD settings, the hole design offers the lowest efficiency but best stability and the pillar design yields the worst stability for various LLD settings. In the large size, the trench design has the highest efficiency, the hole design has the lowest efficiency, and the efficiency of the pillar design is between those of the hole and trench designs. The higher is the converter density, the smaller is the range of the charged reaction products, but at the same time there are a larger number of 6Li target atoms per unit volume, resulting in a larger number of neutron captures. Thus for 6LiF converters with a higher packing density, the efficiency raises faster than for those with a lower density and, once peaked, it drops more rapidly due to the reaction product energy self-absorption problem.In the detector fabrication processes, an initial exploration focused on the neutron converter material backfilling methods is developed. For the three backfilling methods which has been investigated, the ultrasonic vibration method doesn’t backfill densely, and the backfilled material may drop from the microstructures. The electron beam evaporation method is more suitable for backfilling small depth microstructures, otherwise it will take too much time. The centrifugal backfilling process is simple and good if the particle size of the material is small, so it is an ideal material backfiling method.For the MSND prototype’s electrical performance, the Ⅰ-Ⅴ test result indicates that the prototype device presents a leakage current of 16.5 μA at an applied bias of -80 V and it doesn’t suffer a breakdown, so it has ideal reverse current characteristics. Then a simple alpha particle measurement is arranged. An 241Am source, which emits 5.486 MeV alpha particles, is used to test the prototype device. Experiment shows that the MSND prototype device displays good response characteristics to alpha particles and it satisfies the basic condition for neutron detection.As to the neutron detection efficiency of the dry-etched, ultrasonic vibration-backfilled MSND prototype device, the prototype device and a silicon-based positive-intrinsic-negative (Si-PIN) detector are used respectively to measure the moderated neutron emitted from 252Cf spontaneous fission neutron source. These two detectors both provide evidence of clear neutron signals and their neutron detection efficiencies are calculated. The results demonstrate that the thermal neutron detection efficiency of the MSND prototype device is about 3.77%, 2.4 times of Si-PIN planar detector.By studying the above problems, we develop a new type of neutron detection technology based on the microstructured semiconductor and provide a very important basis on the new generation MSNDs’ fabrication and experimentation.