Detector closed aperture

The excitation lamp is enclosed within a compartment which is designed to shield the detector and its associated electronic circuits from the heat generated by the lamp. A small fan flushes cool air into the upper compartment that houses the detection devices. The trigger mounted at the instrument s handle is used to operate a dual shutter that opens and closes the excitation and emission apertures simultaneously. The hand-held instrument is low cost (a/ 2,000), simple to operate, and weighs only a little over 1 kg without the power supply ( 5 kg). A photograph of the prototype instrument is shown in Figure 2. 

The 3-to-2 photon technique, simple counting setups and, possibly mean lifetime measurements could fulfill these criteria. A simple setup, shown schematically in Figure 7.28, is suitable for the first two applications. Positrons are implanted into the sample. Focusing into micron-sized areas is possible. Positronium forms, traps in pores and annihilates in closed pores or escapes through open porosity. Two detectors, one behind the sample and a second with an aperture on the side, observe all positronium (and positron) annihilations and only those from within the sample, respectively. The former detector is also set up to provide 3-to-2 photon ratios. 

To optimize the geometry factor, a source is placed as close to the detector as feasible without causing damage or contamination. A second requirement is preparing the sample so that its diameter is appreciably less than the diameter of the detector. For the simple case of a point source and a detector with a circular aperture, the solid angle ( 2) can be calculated as follows ... 

The only true solid-state imager flown to date is the small demonstration instrument of G. Skinner and colleagues at U. Birmingham. It has a 3x3 array of close-packed Ge detectors (1 cm x 1 cm x 6 cm each) and a passive coded-aperture mask (Skinner et al. 1994). The instrument was flown as a piggyback on the GRIS balloon payload in 1993 and produced images of the Crab and Cyg X-1 (Rideout et al. 1994 Sldnner etal. 1994). 

The aperture is about 50% lai-ger than the detector, to ensure a large FOV within the spatial resources. A one-dimensional coded aperture is defined by four geometrical parameters the element size, the number of elements, the fraction of open elements and the pattern of open and closed elements. We consider the optimum configuration of these four parameters for detecting GRBs with the WXM. 

Coded arrays were originally conceived for applications in X-ray imaging (Mertz and Young 1961 and Dicke 1968). A coded array is defined to be a pattern on a periodic two-dimensional lattice which associates with each lattice point a 0 or a 1 indicating whether the lattice point is open or closed . In coded-aperture imaging, the open and closed lattice points become open and closed cells in an opaque mask which casts a shadow of the photon source on a position-sensitive detector. For a review see Caroli et al. (1987). A uniformly redundant array, or URA (Fenimore and Cannon 1978), is a particular form of coded array. For a URA, each possible vector displacement between pairs of inequivalent open lattice points occurs a uniform number of times. (Equivalent lattice points are separated by a period of the array.)... 

The constant, which will be greater than 1, depends on r ical aperture sizes, in particular the size of the detector pinhole it decrea.ses as the detector pinhole is increased to collect more light. The detection optics should be arranged so that the constant is as close to I as possible, consistent with having, say, 10 or mote photon detections in the mean decay time of the correlation function. In practice. 

The entire microprobe setup is positioned on a movable granite table. Compound refractive lenses are used for focusing to a routinely achievable spot size of 1-2 pm vertically and 12-15 pm horizontally. The intensity of the incoming, the focused, and the transmitted beam is monitored by ionization chambers and photodiodes. A miniature ionization chamber with an aperture of 50 pm diameter as an entrance window was developed at the ESRF for measuring the intensity of the focused beam close to the sample (Somogyi et al. 2003). The characteristic X-ray line intensities are detected with a Si(Li) detector of 30 mm active area, 3.5 mm active thickness, and 8 pm thick Be window placed at 90° to the incoming linearly polarized X-ray beam. Fast scanning XRF measurements (>0.1 s live time/spectrum) are possible. 

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