Depletion depth

The difference in efficiency between the large planar 18-cm.3 Ge(Li) detector and the small round 8-cm.3 Ge(Li) detector depends essentially on their cross-sections since they have approximately the same depletion depths. This is apparent in comparing the peaks from 10 pCi of 137Cs generated by the large and small detectors (Figure 9). The geometry of the aluminum can was determined to be less efficient by approximately a factor of 10 than a disc-mounted point source. 

The silicon box comprises ten silicon detectors surrounding the target with almost 4tt steradian, as shown in fig.l two of them are of an annular, square type and the others are plain, rectangular either of them has no marginal insensitive area. A half of the box subtends the front 2 with respect to the target and the other half the rear 2ir. Each silicon detector works as a partially depleted aE counter and discriminates between protons and alpha-particles. Its depletion depth is chosen such that the energies deposited by protons do not exceed the minimum energy deposited by alpha-particles. A typical depletion depth employed was 0.4 mm. 

The CCDs for the direct measurement of the antiprotonic X-rays were installed in the second bore hole of the cyclotron trap close to the stop volume. Thus, a few per mille of the full solid angle were covered. The relative efficiency and the in-beam resolution function were obtained in a separate measurement from the saturated X-ray transitions in pN. Two different types of CCDs were used (i) MOS CCDs with a typical depletion depth of about 30 pm [22,23] and (ii) the prototype of a high-rate X-ray detector based on a fully depleted (290 pm) pn-CCD [24],... 

At the junction there is a repulsion of majority carrier (electrons in the n-type and holes in p-type) so that a depleted region exists. An applied reverse bias widens this depleted region which is the sensitive detector volume, and can be extended to the limit of breakdown voltage. Detectors are generally available with depletion depths of 100 and 1000 pm, with the cost approximately proportional to the depletion depth. 

Since the depletion depth is inversely proportional to net electrical impurity concentration, and since counting efficiency is also dependent on the purity of the material, large volumes of very pure material are needed to ensure high counting efficiency for high energy photons. 

The operating mechanism of PLEDs is quite different from conventional p-n junction LEDs. In a PLED, a pure undoped film of luminescent semiconducting polymer is sandwiched between a high work function metal anode and a low work function metal cathode. The charge carrier concentration in such pure semiconducting films is sufficiently low (>=1014-1015 cm-3) that any residual carriers introduced by impurities etc. are swept out by the built-in field that arises from the difference in work functions of the two electrodes. The depletion depth of pristine poly(phenylene vinylene) (PPV) is approximately 250 qm, which is much larger than the thickness of the polymer layer in an LED (typically < 100 nm). Consequently, the electronic structure of the LED can be approximated by the rigid band... 

The lithium drifting process, developed by Pell, consists of two major steps (1) formation of an n-p junction by lithium diffusion, and (2) increase of the depletion depth by ion drifting. 

Figure 7.23 Depletion depth as a funetion of impurity coneentration and applied voltage for planar diodes of high-purity germanium (from Ref. 21).

The radiation sensitive depleted layer is available in various thicknesses, < 5 mm, enough to stop electrons of 2.2 MeV, p of 32 MeV, and a of 120 MeV. A typical silicon surface barrier detector for a-spectroscopy has a sensitive area of 300 mm, 300 fim depletion depth, 20 keV FWHM (full width at half maximum) and operates at 100 V reverse bias. The resolving time is about 10 s. Special "rugged" detectors are available which have an acid resistant Si02 surface layer permitting cleaning and contact with liquids. Detailed information for detector selection is available from various detector manufacturers. 

The probability of y-interaction is so small in the small depletion depth of the surface barrier detectors that they are not very useful for y-spectroscopy. Large depleted volumes can be created by drifting lithium atoms into a silicon or germanium crystal. Lithium does not occupy a crystal site in the crystal, but is small enough to go into interstitial sites. The ease of ionization of Li to Li makes it a donor impurity. The lithium is drifted from one side of the crystal using an electric field. Its concentration at the "entrance" side becomes high and then decreases towards the other end of the crystal. The amount of lithium in the... 

Surface-barrier and diffused p-n junction detectors are the best detectors available for low-energy and heavy-charged particles. Typical detector energy resolutions are in the order of 10-20 keV with 100% detector efficiency. Practical limitations in the construction of these detectors restrict the depletion depths to less than 2 mm. The cost of these detectors is low. 

This equation, due to Higbie, was originally derived to describe mass transfer between rising gas bubbles and a surrounding liquid Tran. AIChE, 31,368 [1935]). It applies quite generally to situations where the contact time between the phases is short and the penetration (or depletion) depth is so small that transfer may be viewed as taking place from a plant to a semiinfinite domain. In Section 4.1.2.3 we will provide a quantitative criterion for this approach, which is also referred to as the Penetration Theory. It also describes both the short- and long-term behavior in diffusion between a plane and a semi-infinite space, and we used this property in Chapter 1, Table 1.4, to help us set upper and lower bounds to mass transfer coefficients and "film" thickness Zp j. 

Energy range 10 keV to MeV. Upper limit depends on depletion depth 1 keV-50 keV. Lower limit depends on window upper limit depends on crystal thickness 30 keV-10 MeV. Lower limit depends on window and electronic noise upper limit depends on crystal dimensions... 

When an applied voltage is imposed on both sides of the jimction, the system becomes asymmetrical. The depleted depths are the solutiorts of the equatiorrs ... 

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