Coaxial detectors

Unfortunately, Ge detectors have poor response functions. For a 5 x 5 cm coaxial detector (nominal 20%), only about 3/4 of impinging 1.33 MeV y rays do interact, and, of these, only 15-20% give useful full-energy peaks. 

It is a coaxial detector with a relative efficiency of 105% and was produced by CANBERRA Semiconductor n.v. in 2000. It is a so called extended range detector with a p-type crystal and a submicron deadlayer (0.5 Lim). It has also a high purity aluminium endcap. The entrance window thickness is 1.5 mm. The active volume of the crystal is 400 cm and the crystal diameter is 80 mm. The detector was first installed with a copper endcap with a carbon epoxy window glued to it. This paper describes the background reduction that was achieved by changing the endcap. 

Figure 12.29 (a) REGe coaxial detector, (ft) Absolute efficiency as a function of energy for a... 

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. 

Figure 12J9 (a) In a true coaxial detector, electrons and holes travel along the direction ABC. (b) In wrap-around coaxial detectors, the carriers may travel along ABC but also along the longer path A B C. 

A standard method is described in IEEE standard test procedures (IEEE 1996) to define the relative detection efficiency for a coaxial detector. The relative efficiency 6r for a coaxial detector is defined as follows ... 

A novel gamma-ray detector consisting of a high-purity 3x3 germanium matrix housed in a single cylindrical aluminum cryostat was used for these experiments. Each of the single Ge bars is an n-type coaxial detector with dimensions of 1.5 cm x 1.5 cm x 4 cm and an internal electrode hole of 4-mm diameter. The distance between the front surface and the electrode hole is 1 cm. Two adjacent elements are separated by 2 mm with an indium surface of 0.5 cm. ... 

If any detector could be described as having a standard configuration , it would be the closed-end p-type coaxial detector mounted in an aluminium outer cap. This type of detector has an outer 700fxmn- - contact and cannot be used much below 40keV regardless of the type of detector cap used. It is possible to buy modem p-type... 

Since the detector capacitance reduces as the ratio r2/ri increases, there is every reason to keep the central contact hole as small as possible. Again, as an example to illustrate the capacitance expected of a detector we can consider a coaxial detector, 64 mm long and 50 mm diameter, with a core diameter of 8 mm (dimensions typical of a detector of 38 % relative efficiency). Equation (3.7b) provides an estimate of 31 pF, not accounting for the closed end of the detector. 

Figure 3.10 Calculated shapes of the rising edges of an n-type germanium detector for interactions at different points within a coaxial detector (a) near to core (b) mid-way (c) near to outside...

Figure 3.11 The electric field strength across different types of detector (a) planar detector (b) true coaxial detector...

Figure 3.14 Carrier migration in (a) p-type (normal) and (b) n-type (reverse electrode) coaxial detectors...

Pehl, R.H. Goulding, F.S., Landis, D.A. andLenzUnger, M. (1968). Accurate determination of the ionization energy in semiconductor detectors, Nucl. Instr. Meth. Phys. Res., 59,45-55. Raudorf, T.W. and Pehl, R.H. (1987). Effect of charge carrier trapping on germanium coaxial detector Une shapes, Nucl. Instr. Meth. Phys. Res., A, 255, 538-551. 

Figure 7.7 Efficiency curve for a p-type closed coaxial detector. The point lying below the line is that representing the 511 keV annihilation peak...

Such a specification would refer to a standard coaxial detector. Specification sheets for other types of detector will contain other infomiation and not all the parameters noted in Table 11.3 will be warranted. For example, for a well detector geometric details of the well and the active volume will be quoted. It is likely that only the detector resolution will be warranted. Peak-to-Compton ratio may be measured, but not warranted. For low-energy detectors, only the resolution wiU be warranted. 

This figure-of-merit relative efficiency is defined as the ratio of the counts per second in the 1332.5 keV peak of 00 when the source is counted at 250 mm source-to-detector distance on the axis of the detector, to the counts per second in the 1332.5 keV peak when the same source is measured at the same distance by a 3 x 3 Nal(Tl) detector. It is strictly only relevant to wide range coaxial detectors. Planar and other low-energy detectors are not designed to cover the energy range up to 1332.5 keV. 

Do you need a general purpose detector, and have no particular interest in lower-energy X-rays If so, choose a p-type coaxial detector. 

Figure 13.4 Relative counting time as a function of relative efficiency for coaxial p-type coaxial detectors, counting to a fixed MDA. Case where Compton background is dominant > Bg (recalculated from data in Keyser et at. (1990))...

Canberra has reported measurements where the efficiency of a well detector is five times better than an 80% coaxial detector. However, the capacitance of a well detector will be higher than for an equivalent coaxial detector, resulting in worse resolution - perhaps 2.3 keV FWHM rather than 2.0 keV. As we saw above in Section 13.2.2, worse resolution means worse (i.e. higher) MDA, meaning that the improvement in MDA expected based on efficiency ratio would not quite be achieved. 

Figure 14.1 The dimensions of a bulletized square cylindrical coaxial detector...

Figure 14.2 Calculated maximum charge collection times for coaxial detectors as a function of relative efficiency at 1332.5 keV...

COAXIAL detector A cylindrical detector with one contact within a hole drilled into its base. (See Chapter 3, Section 3.4.4)... 

FIGURE 9.1 Schematic of the ion source and linear regions of a PSD time-of-flight mass spectrometer showing the electrostatic gating system and coaxial detector. (Reprinted with permission from reference I). 

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