A charged-particle spectrometer for ρR measurements (abstract)

We are developing an advanced electronic charged-particle spectrometer for ICF ρR measurements. Knock-on and other charged fusion products are bent by a magnetic field in order to avoid line-of-sight x rays from directly impinging on the detector. The detector itself is a stack of four charge coupled devices sandwiched between ''ranging filters''. To avoid blooming effects in the CCDs, we plan to use only ''thinned'' CCDs, for which each element is ∼20 μm thick. (The CCD is supported by a silicon substrate of thickness ∼200 μm.) The CCDs are sandwiched between four aluminum filters which have two functions. The first filter reduces the background by completely ranging out select charged particles, such as 3.5 MeV α's (range congruent 13 μm). Also, the first filter, with a thickness ∼50 μm, severely attenuates scattered soft x rays and visible light. The following three filters are each ∼300 μm. The total thickness of these filters and CCDs (with support substrate) are about 2000 μm, a thickness which completely stops energetic protons up to congruent 15 MeV. Equally important, the next three ranging filters downshift the particle energy, and this is reflected in increased energy deposition in the CCD (since dE/dx∼1/E, where E is the kinetic energy). With this spectrometer, a charged particle experiences four separate interactions with the CCDs and ranging filters: this allows the incident particle energy to be overdetermined and the particle to be uniquely identified. Furthermore, because the CCD has small picture elements (∼ 20 μm), pulse pileup can be avoided. A crucial tool in the development of this spectrometer is the MIT Cockcroft--Walton fusion product generator. This generator will be used to test and validate design concepts, study energetic particle trajectories through the magnetic field and through the detector stack