Detector thickness

The most important parameter of the detection system is its response function. We have studied this extensively in Monte Carlo and other calculations. The calculated time-spectrum response to monoenergetic neutrons is composed of a Gaussian timing curve (2.97-ns FWHM), a trapezoidal contribution from detector thickness and non-axial paths, and an exponential tail, calculated by Monte Carlo, from multiple scattering in the neutron scintillator. (Spectrum distortion due to neutrons multiply scattered by structural and other parts of the apparatus and arriving at the neutron... 

Mesas in the mirror image of a desired array pattern are formed on the surface of a bar of HgCdTe by etching to a depth greater than the desired final detector array thickness. A lead pattern 14 is formed on a sapphire substrate 12 with contact pads 16 plated to a thickness greater than the final detector thickness. The bar is turned over and the etched surface bonded to the substrate by an epoxy resin 18 to form an unit. The unit is lapped to final detector thickness to form a coplanar surface of the lead pattern contact pads, detector contact areas,... 

Figure 7.21 The Blankenship and Borkowski nomogram that relates resistivity, detector thickness, and detector bias. The detector capacitance as a function of detector thickness is also given.

Consider a true coaxial detector, shown in Fig. 12.39a (see also Fig. 7.26). Since the electric field is radial, electrons and holes will follow a trajectory perpendicular to the axis of the detector. The maximum time required for collection of the charge corresponds to electron-holes being produced either at A or C. That time t is equal to I AC)/v, where 4C is the detector thickness and V is the speed of electrons or holes. For a detector bias of about 2000 V and the size shown in Fig. 12.39a, v 0.1 mm/ns = 10 m/s, which gives a maximum collection time of 120 ns. The best risetime corresponds to electron-holes generated at point B (Fig. 12.39a) and is equal to about 60 ns. 

Absorbers between the sample and detector (thickness and composition)... 

It was clearly demonstrated that the composite BN semiconductor polycrystalline bulk detectors with BN grains embedded in a polymer matrix operate as an effective detector of thermal neutrons even if they contain natural boron only (Uher et al. 2007). A reasonable signal-to-noise ratio was achieved with detector thickness of about 1 mm. A Monte Carlo simulation of neutron thermal reactions in the BN detector was done to estimate the detection efficiency and compare with widely used He-based detectors to prove advantages of BN detectors. They are found to be promising for neutron imaging and for large area sensors. 

The charge collection time is quite short— typically lying in the range of 25 to 100 ns, depending on detector bias voltage, detector thickness, and the position of the photon interaction within the detector. The detector is operated at 77 K to lower the lithium mobility in the crystal and to reduce the noise that would be caused by excessive diode reverse leakage current at higher temperatures. 

Figure 4.32 The calculated detection efficiency for a Si(Li) detector. Only the effects of detector thickness and beryllium window thickness have been incorporated. (Reprinted by courtesy of EG G ORTEC.)... 

As indicated earlier, condition 4 is not easily satisfied by these detectors. Let us require that f=2.5a so that if >0.9, use maximum value for the shallow dopant of this example at which the ionization energy nearly equals the low doping level value then f 2.5 x 10 cm is the required detector thickness. We had used this thickness in the above evaluation of condition 2. Thus even in this "best case" estimate, the Si extrinsic photoconductor must be about 50 times thicker than the comparable Hg<, 8Cdo.2Te intrinsic photoconductor. 

The use of an extrinsic photoconductor with direct injection has the advantage that dc gain can enhance I and therefore g, but gain saturation due to sweepout will limit ac gain to 1/2 at frequencies near f [6.30]. The capacitance C of an extrinsic photoconductor can be an order of magnitude lower than for a photodiode, which will lead to a higher /. With an extrinsic photodetector, crosstalk problems must be considered for the detector thicknesses necessary to provide reasonable quantum efficiency, and even for the 3-5 pm window operating temperatures will tend to be below 50 K. 

Weight (kg) kVp mAs Slice thickness (mm) Pitch Detector thickness - (mm) Increment (mm)... 

Schematic shapes of planar and closed-ended types of high-purity germanium (HPGe) detectors. Thick lines show electrode connections...

Both and increase with temperature approximately as Qxp - EJk T), so that (4.66) places an upper limit on the detector operating temperature. Using values of and from Appendix D and assuming a detector thickness t = 5 X lO cm as in Subsection 4.2.2 to give rj >0.9, we obtain the curve of the left side of (4.66) vs plotted in Fig. 4.10 for an operating temperature of 77 K. 

If the absorption coefficient times detector thickness product is sufficiently larger than 1, quantum efficiency becomes approximately 1. 

The total g-r noise current will be given as an integral of di over the whole detector thickness... 

The detector thickness should be as small as possible. This requirement is opposed to the previous one. Thin detector structures enable a shorter response time (because of the shorter transit time) and lower noise levels (because detector volume is decreased). 

Fig. 2.58 Radiative lifetime versus front-side reflection coefficient for a Hgi- Cd Te detector, thickness 3, 5, 10, and 20 im at 220 K, composition x = 0.185...

There are two distinct zone in the concentration profile, denoted in Fig. 3.46 as A and B. Each bias or detector thickness increase in the zone A results in a dramatic depletion, while in the B zone there is no significant further concentration decrease. The boundary line (dotted line) corresponds to the critical thickness of depleted region L nt, approximately given by (3.151). 

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