Detector Conceptual Design

The aerogel blocks are housed in light-tight, gas-tight containers made of a low-density material, such as aluminized mylar. The inner walls are covered with a highly efficient diffuse reflector, such as millipore paper, perhaps coated with wavelength-shifter. 

Detection of Cerenkov photons is done with a new hybrid or vacuum APD (HAPD) with single-photoelectron capability from Advanced Photonix Inc. The HAPD combines an avalanche photodiode APD with a conventional photocathode. It is insensitive to magnetic fields at least up to 1.5 T [17] [18]. 

The number of Cerenkov photons is giv(ui by N — N sin 6cL, where 6c is the Cerenkov angle, L is the length of the aerogel cell and N = 27ra X number of 

Cerenkov photons produced in the wavelength interval A, aa ]- For the [250-800 nm] interval appropriate for a biakali photocathode we expect N = 1260 Cerenkov photons per centimeter. This can be improved to N = 1.72 x 10 photons/cm by using a Ga-As photocathode in the HAPD. 

The photoelectron yield estimated for the HAPD readout is given by  

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Fig 12 is a conceptual sketch of what a CT expls detection system might look like. The objects to be examined are placed on a conveyor belt, 2 to 3 meters long, which passes between an X-ray source on one side and an array of column detectors on the other. The design speed for the conveyor is 30cm/sec. The column detectors are 60cm hi with vertical slit collimators focused on the extended-anode X-ray source. Source and detector collimation will vary depending on the distance from the source to the detector, so that the spatial dispersion of the X-ray pencil beams thru the examined olqect will be the same for ail projection angles... 

Differential neutron spectra were measured In center of a fast-spectrum molybdenum-reflected critical assembly using spherical proton-recoil detectors. The critical assembly was buiU to investigate geometrical variables and reactivity effects of-materials for a small, fast-spectrum conceptual reactor (lithium-cooled, U nitride-fueled). Measurements were made at Atomics Intemational where the critical asseimbly was designed, built, and operated. Calculations were made, at the NASA Lewis Research Center. 

The precise way in which the detector signal is processed electronically depends on the kind of VCD spectrometer used. In the following sections, three different VCD spectrometer designs are discussed. The simplest of these is the dispersive VCD spectrometer, and we will use this design to illustrate the basic concepts associated with the electronic processing of VCD spectra. The two subsequent cases involving Fourier-transform VCD spectrometers are more complex, but share the same underlying conceptual basis as the dispersive VCD spectrometer. 

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