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Semiconductor detectors are devices that operate essentially like ionizations chambers. The charge carriers in semiconductor are electrons and holes. At present the most successful semiconductor detectors are made of silicon and germanium EMBED Equation.3 .
Both Ge and Si form solid crystals in which the valence-4 atoms form four covalent bonds with neighboring atoms. All valence electrons thus participate in covalent bonds, and the band structure shows a filled valence band and an empty conduction band. In semiconductors, the size of the energy gap is something around 1 eV. At room temperature a small number of electrons is thermally excited across the gap and into the conduction band, leaving a valence-band vacancy known as a hole. To control the electrical conduction in semiconductors, small number of atoms (called dopants) with valence 3 or 5 are introduced into the lattice. In the case of valence-5 atoms the material is called n-type semiconductor, where the charge carriers are mainly electrons. Alternatively, in the case of valence-3 atoms, the primary charge carriers are the holes and the material is called p-type semiconductor.
When p-type and n-type materials are brought into contact, the electrons from the n-type material can diffuse across the junction into the p-type material and combine with the holes creating a region called the depletion region. Eventually an electric field is created across the depletion region.
If radiation enters the depletion region and creates electron-hole pairs, the result is similar to that of an ionization chamber. The charge carriers (electrons and holes) flow in opposite directions and the total number of electrons collected can form an electronic pulse whose amplitude is proportional to the energy of the radiation.
The most important advantage of the semiconductor detectors, compared to other types of radiation detectors can be summarized as follows EMBED Equation.3 :
Higher energy resolution.
Linear response (pulse height versus particle energy) over a wide energy range.
Higher efficiency for a given size, because of the high density of the solid material used for construction.
Fast pulse rise time (relative to gas detectors).
2.1.1: Silicon lithium-drifted [Si(Li)] detectors.
In this type, a crystal of silicon (or germanium for [Ge(Li)] detectors) of p-type material is prepared. A concentration of Li atoms, which tend to form donor states is diffused into the crystal surface and create a this n-type region. Under reverse bias, the Li drifts into the p-type region, making a large depletion region. Following the Li drift, the detector must be kept cold, usually at liquid nitrogen temperature of 77 K, otherwise the Li will migrate out of its lattice sites in the depletion region and destroy the effectiveness of the detector.
2.1.2: The surface-barrier detector.
The n-type layer created in producing Si(Li) or Ge(Li) detectors is of order 1 mm thick, which is easily penetrable by medium energy EMBED Equation.3 -rays. For charged particles, the range is much smaller, for example an EMBED Equation.3 particle of 5 MeV energy has a range of 0.02 mm in Si and hence does not reach the depletion region.
The surface-barrier detector, in which an extremely thin p-type layer is deposited on the surface of n-type Si. The Si crystal is etched with an acid and exposed to air. The oxidation layer thus formed on the surface has the characteristics of a very thin p-type film. A thin layer of gold is then evaporated on the front surface to serve as electrical contact. The total thickness that the particles must penetrate to reach the depletion region is thus made to be about 0.1 EMBED Equation.3 m.
2.1.3: Hyperpure germanium (HPGe) detectors.
Recently, large-volume, high-purity Ge detectors have become available, owing to advances in the techniques of refining Ge crystals. An impurity concentration of EMBED Equation.3 atoms/cm EMBED Equation.3 was achieved and has made possible the construction of detectors without lithium drifting. These detectors do not need to be kept at 77 K, but are operated at that temperature to keep the noise level low. The detector is simply operated by applying reverse bias across bulk germanium. The sensitive depth of the detector (depletion region) depends on the impurity concentration and the applied voltage.
2.1.4: Electronic devices.
In addition to the detector, the counting system is composed of bias supply for the detector, preamplifier, research amplifier, and multichannel analyzer (MCA).
The detector bias supply provides a positive or negative voltage necessary for the operation of the detector. The AC power supply is preconnected by a voltage stabilizer to insure stability in the electronic system and to avoid voltage fluctuation.
The primary purpose of the preamplifier is to provide an optimized coupling between the output of the detector and the rest of counting system. It amplifies the signal from the detector by a factor of 3 orders of magnitude with minimum shaping in a way that preserves the maximum signal to noise ratio. To reduce the loss in cables, the preamplifier is placed as close to the detector as possible.
The research amplifier should be of extremely low noise and wide range gain. To achieve high resolution spectroscopy, the most important features of research amplifier are stability and linearity.
The multichannel analyzer (MCA) records and stores pulses according to their height. Each storage unit is called a channel. The height of the pulse is usually proportional to the energy of the incident particle. Each pulse is in turn stored in a particular channel corresponding to certain energy. However, modern spectroscopy systems are computerized. Instead of the MCA, a special computer card, which consists basically of an analog to digital converter, is plugged into a computer. And by using the proper software, many features can be available.
In radiation measurements the oscilloscope is used to check the quality of the signal as well as the level and type of the electronic noise.
2.2: Gamma Ray Spectrometer.
The study of gamma radiation emitted by a source is the most important mean to learn about the structure of excited nuclear states. Gamma ray detection at high resolution and with high precision is easy to accomplish. Knowledge of the locations and properties of the excited states is essential for the evaluation of calculations based on any nuclear model. An experiment of EMBED Equation.3 -ray spectroscopy provide us with the following information about nuclear excited states EMBED Equation.3 :
A spectrum of the gamma rays shows us the energies and intensities of the transitions.
Coincidence measurements give us clues about how these transitions might be arranged among the excited states.
Measuring internal conversion coefficients can give clues about the character of the radiation and the relative spins and parities of the initial and final states.
Absolute transition probabilities can be found by determining the half-lives of the levels.
2.2.1: Energy calibration.
Energy calibration is simply to assign the correct energy value to the corresponding channel number. The pulse height is assumed to be proportional to the energy of the incident particle. This enables the experimentalist to find the energies of gamma lines emitted by unkown source.
In EMBED Equation.3 -ray spectroscopy with germanium detectors, the pulse height scale must be calibrated in terms of absolute EMBED Equation.3 -ray energy if various peaks in the spectrum are to be properly identified. In many applications, the EMBED Equation.3 -rays expected to appear in the spectrum are well known in advance and the corresponding peaks can be identified by inspection. In other applications, unknown EMBED Equation.3 -ray spectra may occur and hence a separate calibration EMBED Equation.3 -ray source is used to supply peaks of known energy in the spectrum. Accurate calibration should involve a standard source with EMBED Equation.3 -ray energies that are not widely different from those to be measured in the unknown spectrum. Because even the best spectrometer systems often show nonlinearities of a channel or two over a full range of several thousand channels, it is also useful to have multiple calibration peaks at various points along the measured energy range of interest.
The selection of standards to be used for germanium spectrometer calibration depends on the energy range of interest. The annihilation radiation (near 511 KeV) is used sometimes EMBED Equation.3 as a primary standard. Another method of extrapolation to higher energies can be carried out if EMBED Equation.3 -rays from a cascade transition are involved.
2.2.2: Detection efficiency.
Gamma rays must undergo a significant interaction in the detector before detection is possible. Because gamma photons can travel large distances between interactions, detectors are often less than 100% efficient. It then becomes necessary to have a precise figure for the detector efficiency in order to relate the number of pulses counted to the number of photons incident on the detector.
Absolute efficiency is defined as
EMBED Equation.3
and is dependent not only on detector properties but also on the details of the counting geometry such as the distance from the source to the detector.
The intrinsic efficiency is defined as
EMBED Equation.3
and the solid angle subtended by the detector is no longer included as an implicit factor. The intrinsic efficiency of a detector depends on the detector material, the radiation energy, and the physical thickness of the detector in the direction of the incident radiation. A slight dependence on distance between the source and the detector does remain, however, because the average path length of the radiation through the detector will change with this spacing.
The relation between the absolute and the intrinsic efficiencies is given by
EMBED Equation.3
where EMBED Equation.3 is the solid angle of the detector seen from the actual source position. It is convenient to tabulate values of intrinsic rather than absolute efficiencies because the geometric dependence is much easier for the former.
Counting efficiencies are also categorized by the nature of the event recorded. The total efficiency is used when the entire area under the spectrum is a measure of the number of all pulses that are recorded, regardless of amplitude. The peak efficiency assumes that only those interactions that deposit the full energy of the incident radiation are counted.
In germanium EMBED Equation.3 -ray spectroscopy, an efficiency based on the area under the single or double escape peak is sometimes used in place of that based on the full energy peak. Although efficiencies of germanium detectors can be estimated from published measurements or calculations for detectors of similar size, the results based on these values are not accurate.
The relative photopeak efficiency of the detector can be obtained by using standard calibration sources in such a way that it covers the range of interest. The relative efficiency for a given source line is given by
EMBED Equation.3
where EMBED Equation.3 are the area and the published relative intensity of the gamma line under consideration. EMBED Equation.3 is the net photopeak area corresponding to the normalization gamma line (most intense line), EMBED Equation.3 is the intensity of this line and considered to be unity.
By using many calibration sources with known activities, the absolute efficiency of the detector as a function of energy can be determined. The absolute efficiency of the detector for each EMBED Equation.3 -ray line can be calculated from the formula
EMBED Equation.3
where EMBED Equation.3 is the absolute efficiency, EMBED Equation.3 is the area under the photopeak (net counts), EMBED Equation.3 is the absolute intensity of EMBED Equation.3 -transition, and EMBED Equation.3 and EMBED Equation.3 are the activity of the source and the accumulation period respectively.
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