Crystal, lithium drifted

The energy-dispersive (EDX) solid state detector (SSD, Figs 4.6, 4.7) is made of lithium-drifted Si crystal (Si(Li)). Between a thin p-type and an n-type layer lies a high-resistivity Si crystal of centimeter dimensions. The front and end planes of the crystal are coated with Au and serve as electrodes. The crystal, cooled to 77 K by liquid nitrogen, represents a p-i-n diode (Fig. 4.7). An incident X-ray photon with... 

The introduction of high-resolution, high-efficiency /-ray detectors composed of lithium-drifted germanium crystals has revolutionised /-measurement techniques. Thus, /-spectrometry allows the rapid measurement of relatively low-activity samples without complex analytical preparations. A technique described by Michel et al. [25] uses Ge(Li) /-ray detectors for the simultaneous measurements of 228radium and 226radium in natural waters. This method simplifies the analytical procedures and reduces the labour while improving the precision, accuracy, and detection limits. 

The alternative approach to detection and analysis incorporates a solid state detector and a multichannel pulse height analysis system. The crystals used are of silicon (of the highly pure intrinsic type), or the lithium drift principle (p. 463 etseq.) is utilized. All emitted radiations are presented to the detector simultaneously and a spectrum is generated from an electronic analysis of the mixture of voltage pulses produced. Chapter 10 contains a more detailed account of pulse height analysis and solid state detectors. Production of an X-ray spectrum in this way is sometimes known as energy dispersive analysis ofX-rays (EDAX) and where an electron microscope is employed as SEM-EDAX. 

As discussed above, the measurement of characteristic y rays is very similar to the methods used in EDXRF. Early studies used a scintillation counter, typically a crystal of sodium iodide containing a small amount of thallium (Tite 1972). y ray absorption by these counters produces visible light, which is converted into an electrical pulse using a photosensitive detector. More recently semiconductor detectors have been used, either a lithium drifted germanium crystal, or, more typically, a pure ( intrinsic )... 

Lithium-Drifted Germanium Detectors. Two kinds of planar Ge(Li) diodes were fabricated in our Laboratory for the spectrometer (I). The chief consideration was maximum cross section with a resolution of less than 3 k.e.v. at 60Co FWHM (full width half maximum). The available single-crystal germanium material dictated the shape of the detector. The first detector was fabricated from a 15-cm. long Sylvania ingot with a trapezoidal cross section its finished dimensions were 6 cm. X 3 cm. X 1 cm. Our second detector was fabricated from circular Hoboken stock (NPC Metal and Chemical Co., Los Angeles) its finished dimensions were 2.8 cm. diameter X 1.2 cm. thick. 

Germanium metal is also used in specially prepared Ge single crystals for y-ray detectors (54). Both the older lithium-drifted detectors and the newer intrinsic detectors, which do not have to be stored in liquid nitrogen, do an excellent job of spectral analysis of y-radiation and are important analytical tools. Even more sensitive Ge detectors have been made using isotopically enriched Ge crystals. Most of these have been made from enriched 7<5Ge and have been used in neutrino studies (55—57). 

Lithium drifted detectors are produced by first doping an intrinsic crystal (Ge or Si) with p-type impurities such as B, Al, Ga or In. Lithium, a strongly n-type element, is then drifted into one end of the crystal. On refrigeration of... 

Analysis. The analytical system used for gamma-ray measurements consisted of a lithium drifted germanium (GeLi) crystal detector, a 4096 multi-channel analyzer, a PDP 11 computer, and a cassette magnetic tape storage. The germanium detector crystal has a volume of 55 cm with FWHM resolution of 2.3 keV at 1.33 MeV. The computer was used to analyze the gamma ray spectra, to identify the radio isotopes, and to calculate the concentration (Table III). 

Most of the time, the silicon crystal with little leakage current is obtained by the lithium drifting process. It should be noted that this process in the case of silicon results in a very stable product. At room temperature (without high voltage connected), a... 

The result is the lithium-drifted silicon counter sketched in Fig. 7-20. The crystal is virtually all intrinsic, with the p and n portions confined to thin surface layers, which are exaggerated in the drawing. The very small pulses from the counter are amplified to the millivolt level by a field-effect transistor, abbreviated FET. (There is no charge amplification, such as occurs in a gas counter. The pulse from the counter contains only the charge liberated by the absorbed x-rays.)... 

Figure 12-12 illustrates one form ofa lithium-drifted detector, which is fashioned from a wafer of crystalline silicon, There are three layers in the crystal a p-type semiconducting layer that faces the X-ray source, a central intrintic zone, and an -type layer. T he outer surface of the p-iype layer is coaled with a thin layer of gold for electrical contact often, it is also covered with a thin beryllium window that is transparent to X-rays. The signal output is taken from an aluminum layer that coats the n-type silicon this output is fed into a preampliftcr vc ith a gain of about 10. The preamplifier is frequently a field-effect transistor that is fabricated as an integral part of the detector. 

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... 

The most common semiconductor detector for laboratory EDXRF systems is the lithium-drifted silicon diode, represented as Si(Li). (It is called a silly detector for short). A schematic diagram of a silicon lithium-drifted detector is shown in Fig. 8.30. A cylindrical piece of pure, single crystal silicon is used. The size of this piece is... 

We conclude that the best equipment is pulsed-neutron instrumentation and hl -resolution gamnm Spectrometers (lithium-drifted germahiuin crystals). Lacking ttiese, much can be done with less expensive instrumehts. 

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