辐射致折射率变化用于MeV级脉冲辐射探测的初步研究∗

摘要

High time resolution detecting systems for MeV pulsed radiation are essential for inertial confinement fusion di-agnostics. Traditional detection of system time resolution is restricted by cable bandwidth. Based on recording excess carrier dynamics in semiconductors, a new detecting mechanism, called “RadOptic”, was developed by Lawrence Liv-ermore National Laboratory (LLNL). The variation of intensity of pulsed radiation with time was converted into the variation of intensity of infrared laser probe by using this mechanism. The sensing material was InGaAsP quantum wells with severalmicrometer thickness. Picosecond time resolution for several keV pulsed radiation has been demonstrated. The reported system is not suitable for MeV pulses due to its low efficiency to MeV photons. Multiple cascaded struc-ture for MeV photon to electron transformation was proposed by LLNL. Applying bulk material with several-hundred-micrometer thickness is an alternative. n Based on transient free carrier absorption, a system recording bulk materials’ instantaneous refractive index change is established. The system consists of a probe laser, an interferometer module, a signal transmission module and a signal recording module. The probe is a tunable infrared continuous wave laser whose wavelength is ∼1453 nm, guided by single mode fiber to the interferometer. The interferometer consists of a single mode fiber head coupled directly with the polished face of a bulk semiconductor. The interference pattern forms by multiple beams reflected from the front face and the back face of the bulk. Part of interference light is coupled to the single mode fiber and forms the output signal. Pulsed radiation will deposit energy and generate excess carriers in the bulk material. The refractive index of the bulk material changes therewith according to the Drude model. The interference pattern and the light coupled to the single mode fiber also change therewith. The signal is transmitted by a long single mode fiber. The signal recording module consists of photoelectric detectors and a digital oscilloscope. The signal generation process and the time resolution of the system are analyzed. Intrinsic GaAs refractive index change is exploited under electron pulses and X ray pulses. n The analysis of signal generation process shows that when the excess carriers recombine much faster/much slower than the pulse width, the output signal/output signal differential can be viewed as a measure of intensity variation with time of the incident pulse. For this prototype system, the time resolution is restricted by the digital oscilloscope to 1 GHz. Bulk intrinsic GaAs demonstrates ∼30 ns refractive index response time, which is longer than the incident pulse width. The differential signal can be viewed as a measure of incident pulse intensity when GaAs is exposed to 1 ns∼0.2 MeV electrons pulses. The differential signal width is shorter than the pulse width when GaAs is exposed to 5 ns∼0.2 MeV electrons pulses. Auger recombination process may occur in the pulse duration under this situation. The differential signal width is longer than the pulse width when GaAs is exposed to 1 ns∼0.2 MeV X ray pulses. The poor signal-to-noise ratio affects the signal. The excess carrier generation process may be longer than theoretically estimated one under X ray pulse incident situation. n The generation process and recombination process of excess carriers in GaAs show very different characteristics compared with optical excitation. The relationship between the system output signal and the incident pulsed radiation depends on the type of the incident radiation. With carefully considering the effects from incident pulse type and tran-sient carriers density, the system can be used to detect ∼MeV pulsed radiation. With an upgraded recording module, the system would demonstrate much higher time resolution.%MeV级脉冲辐射的高时间分辨测量是惯性约束核聚变诊断领域迫切需要解决的难题,国际上尚无成熟的解决方案.利用脉冲辐射对半导体折射率的超快调制效应,有望建立新的解决方案.为研究体材料半导体折射率对MeV级脉冲辐射的响应规律,分析了系统输出与入射辐射强度的对应关系,分析了基于半导体折射率变化测量MeV级脉冲辐射系统的时间分辨的影响因素.基于自由载流子折射率调制原理,建立了半导体材料在MeV级脉冲辐射作用下折射率调制测量系统,整个系统的时间分辨<1 ns.在最大能量为0.2 MeV的电子束和X射线束轰击下,本征GaAs折射率恢复时间约30 ns,比可见光激发下要长,分析其原因是高能激发下GaAs内部陷阱参与了载流子复合过程. X射线光子束轰击下,折射率建立时间比电子束轰击下长,光子沉积能量产生过剩载流子的时间过程可达到ns量级.基于建立的系统和分析方法,可对其他半导体在伽马脉冲辐射或电子束辐射作用下折射率变化开展系统的研究,为建立实际的可用于MeV级脉冲辐射测量的快响应探测系统奠定了基础.