Resonance detector

In 1963 Mitrofanov K. P. suggested a resonant detector which is sensitive only for recoilless gamma-quanta (Mitrofanov, Gor’kov et al. 1978). A resonant detector is a special instrument for Mossbauer spectroscopy whose principle of operation is also based on the Mossbauer Effect. The Scintillation Resonant Detector – SRD that are used in Mossbauer spectroscopy, are intended for the selective registration of resonance radiation and allows us to increase resolution and sensitivity and thus to expand the information potential of this method. The operation of resonant detectors is based on the registration of secondary radiation, primarily
secondary electrons (Belyaev, Volodin et al. 2010). In this type of detector, a resonant substance (a converter) is placed inside the working volume of the detector.

The principle of operation of SRD is illustrated in fig. 4 (Maltsev, Mehner et al. 2002). The resonant у-quanta excite the nuclei of the grains of the resonant substance. In the case that the nucleus deexcites by emission of a conversion electron, this electron will excite along its path a number of atoms of the scintillator, which surrounds the resonant substance grain. The excited atoms of the scintillator produce photons. The photons produce photoelectrons in the photocathode of the photoelectronic multiplier tube.

Scintillator

Resonance detector

Fig. 4. Principle of operation of the Scintillation Resonance Detector – SRD

The recoilless resonant absorption of у-rays is followed by a variety of deexcitation processes involving reemission of у-ray, internal conversion electrons, Auger electrons and X-rays (Sawicki and Tyliszczak 1983). As a matter of fact, only the high-energy part of these deexcitation processes is utilized in Mossbauer scattering experiments, see (Bonchev, Jordanov et al. 1969; Liljequist 1981; Sawicki, Sawicka et al. 1991), whereas low-energy events represented by UV – and optical photons (Lichtenwalner, Guggenheim et al. 1976) or secondary electrons have hardly been investigated.(Sawicki and Tyliszczak 1983).

The method of resonance detection is based on the energy modulation, with moving absorber of Mossbauer radiation placed between the radioactive source (which is in resonance) and the resonance detector.

In y – resonance spectroscopy with the use of SRD the source and the resonance detector create a resonance pair.

Basic characteristics of these pairs constitute:

• the selectivity,

• the efficiency of registration of recoilless у-quanta,

• the permissible loading ability.

The selectivity is determined as the ratio of output of registration of у-beams when E = E0 and E =», in case we have resonance and not, respectively. The SRD with plastic Scintillator ensure possibility of loading more than 106 cm-1. The value of the resonant effect can be as high as several hundred percent, when investigating the aftereffects of nuclear transformations in the emission mode (Belyaev, Volodin et al. 2010).

The substance inside the resonant detector must have (Belyaev, Volodin et al. 2010):

• a spectral absorption line width that is close to the natural width;

• a large value of the Lamb-Mossbauer factor (the f-factor);

• a large number of resonant atoms;

• a spectral line coinciding in the energy scale with the position of the source’s spectral line. Even a small isomeric shift leads to distortion of the shape of the experimental spectrum with a loss of sensitivity and large isomeric shifts that considerably exceed the line width, leading to the disappearance of the effect.

Mossbauer spectroscopy represents one of the most informative methods for an investigation of a structure of the matter. It is well known that, an essential disadvantage of Mossbauer spectroscopy is a long time of the spectrum recording, usually of about 10 h in transmission geometry, and few tens of hours in the scattering geometry. The use of resonance detectors in Mossbauer spectroscopy allows a reliable determination of the non­resonant background level, which should be taken into account when the absolute value of the Mossbauer fraction (f is needed. Simple formulae for finding the factors f from the areas of the experimental spectra have been derived in (Mitrofanov, Gor’kov et al. 1978). Also, the use of resonance detectors is promising way to increase a productivity of Mossbauer measurements (Mashlan, Kholmetskii et al. 2006).