The 75% of Rayleigh scattering actions is concentrated at angles smaller than the so-called "characteristic" angle (eq. 20) (0 < 16 °) (Manninen and Koikkalainen 1984; Duvauchelle, Peix et al. 1999). More prospects for the registration of RSMR seem to have the Scintillation Resonance Detectors – SRD (Mitrofanov, Gor’kov et al. 1978; Athanassiadis 1994; Semenov, Irkaev et al. 1995), which allow us to register relatively very small changes in energy during the Doppler modulation of energy.
Therefore, the application of the RSMR method allows us to ignore the necessity of resonance nucleus existence in the samples such as to have the ability to investigate materials, free from Mossbauer resonance nucleus, with precision that is achieved with methods of NGR (~ 10-8 – 10-9 eV) (Zolotoyabko and Iolin 1986; Athanassiadis 1994).
The methods that are based on the registration of RSMR are characterized by the relatively low intensity of the beam of secondary radiation that carries the information. Therefore, use of radiation sources of big energy or bigger duration of measurement time, is required (Semenov, Irkaev et al. 1995). These restrictions can be overridden with the creation of new types of high sensitivity detectors, among which the Combined Scintillation Resonance Detectors are distinguished, that unify the positives of various types of scintillation detectors and allow us to avoid the difficulties of multi-detector systems, difficulties that are related to their problem of inter-stabilization (Athanassiadis, Chudakov et al. 2008).
The interest for the detection techniques of secondary radiation led to new methods and detection techniques of Mossbauer radiation with the construction of new types of detectors which have the ability of registering relatively minor changes in the energy of y-beams.
The construction of the Mossbauer spectrometer for RSMR measurement in small angles requires the study of its basic metrological characteristics such as the determination of the discrimination velocity, the peak asymmetry and the area under peak.
For SRD with radioactive isotope 119Sn, dioxide of tin (SnC2) is used as converter. This converter satisfies the basic requirements for converters (to have good solubility in the scintillator, in order not to decrease transparency of scintillator and so that there would be no isomeric shift in the converters spectrum relative to the Mossbauer source spectrum). The registration efficiency of Mossbauer radiation from the source 119m SnC2 is about 30% until 40%, while the registration efficiency of non-resonance radiation – is less than 1 %.
The spectrometer is based on the Combined Resonance Detector (Athanassiadis, Anshakov et al. 1993; Athanassiadis, Chudakov et al. 2008), which is constituted by the coaxial organic scintillation detector of 31 mm diameter and the inorganic one of CsI(Tl) of 10 mm diameter. The resonant scintillation detector consists of a thin organic film in the form of a ring, where the converter material is enriched by up to 90 % by dissolution of 119mSn Mossbauer isotope. The spectrometer includes the movement system with constant acceleration of Mossbauer source 119mSn, the stabilization system of Combined Resonance Detector energy scale and the interface for the connection to computer PC. The system ensures the simultaneous accumulation and treatment of two spectra, which correspond in the registration of resonance radiation from the inorganic and organic scintillation detector. The signals division in the detectors’ output from the two scintillators (organic and inorganic) is realized by the difference of scintillation time in the two scintillators (Athanassiadis, Anshakov et al. 1993).
For the study of CM which does not contain Mossbauer nucleus, was created an experimental system, constituted of the SRD and the automated Mossbauer spectrometer, which is given in fig. 5. The spectrometer’s technical characteristics are given in table I.
The SRD measures the intensity of resonance у – radiation which is scattered from the controlled sample. The scattered beam after its transmission from the sample was registered from resonance SRD (5). The SRD is sensitive to register only the coherent scattering of y – radiation. The biggest of resonance absorption is observed with null velocity of y-source movement.
The registration of Rayleigh scattering of Mossbauer Radiation (RSMR) allows a researcher to study the physical and chemical attributes along with the processes taking place inside the CM, which is free from Mossbauer resonance nucleus. This is accomplished with a remarkably high precision, by using methods of Nuclear Gamma Resonance (~ 10-8 – 10-9 eV).
The Combined Scintillation Resonance Detector CSRD is proposed for the determination of the density p, the effective atomic number Zeff and other parameters that characterize the technological attributes of materials. These types of detectors are ideal for registering events that occur in adjacent points in space. The two scintillators are connected through the optical window with a photomultiplier.
Devices of this type are commonly known as "Phoswich detectors". The identification of the registration events in devices of this type depends on the difference between the conversion efficiency or/and the scintillations decay time constants of the scintillators that constitute the combined detector.
A phoswich detector is composed of several independent scintillation crystals coupled to a single photomultiplier tube. Generally, phoswich detectors are designed either for simultaneous detection of different radiation types or for minimizing the background radiation in a radiation field of interest. In both cases scintillation layers are considered to be a unique choice because of their relative sensitivity to the particular radiation type.
 |
 |
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Fig. 5. Mossbauer spectrometer with Scintillation Resonant Detector
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Time Stability for 24 hours of continuous work: |
< 1 % |
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Scintillation time of NaI(Tl) scintillator: |
340-9 sec |
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Scintillation time of Resonance scintillator: |
0.640-6 sec |
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Amplitude ratio of Organic Resonance Scintillator to the inorganic NaI(Tl) |
1 : 0.75 |
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Discrimination Coefficient of Separation circuit: |
~ 10-3 |
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Peak amplitude resolution of the inner conversion electrons: |
47 % |
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Activity of 119mSnO2, Bq: |
8-108 |
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Modulation Frequency, of у-source motion system: |
145 Hz |
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Table 1. Technical Characteristics of Small Angle Spectrometer of Rayleigh scattering of Mossbauer Radiation |
Among other things, the scintillators are chosen to have different decay times so that the shape of the output pulse from the photomultiplier tube is dependent on the relative contribution of scintillation light from the two scintillators. The basic characteristics of the combined system are determined by the signals’ separation efficiency.
2. Conclusion
By using y – resonance and non-resonance methods for the radioactivity control of Composite Materials, we can determinate the effective atomic number Zf and density p. These parameters play fundamental role in the behavior of CM.
Making use of y-densitometry methods along with the use of a single beam’s radiation technology, the researcher is allowed to check the surface density’s change in the address of light’s combustion of Composite Materials (CM). This is due to y-densitometry methods’ great simplicity. It holds true that the CM suffers high temperature effects. The development of a system providing the possibility to dynamically control the measurement, allows the researcher to acquire data concerning the speed of mass’ loss in the unit of his heated surface. Furthermore, it offers the change to registrate the internal sample’s change of density and to construct its’ "behavioral" profile.
These results prove the ability of the use of Mossbauer Effect, via measurements of intensity of Rayleigh scattering, for the study of dynamic physical processes in the condensed matter, which does not contain resonance nucleus, the determination of effective atomic number and the measurement of density in condensed matter(Chudakov and Anshakov 1982).
