Gamma-ray spectrometer

Effectiveness and resolution are fundamental characteristics G.- s.. Effectiveness is determined by the probability of forming the second particle and by the probability of its registration. Resolution G.- s. characterizes the possibility of the separation of two gamma peaks, close ones in the energy. As the measure of resolution usually serves the relative width of the line, obtained with the measurement of monokhromatich. - emission; is quantitatively it determined by relation - energy of second particle, the width of line on half of its height (in the energ. units) (see spectral line width).

In magnetic G.- s. second particles appear with the absorption - in- quanta in Vol. n. radiator; their energy is measured just as in the magnetic beta-ray spectrometer (Fig. 1).

Fig. 1. the diagrammatic representation of magnetic gamma-ray spectrometer. In the magnetic field N, directed perpendicularly to the plane of figure, the secondary electrons dvizhutsya in the circles, whose radii are determined by energy of electrons and by field n. with field change detector it records the electrons of different energies. Shading showed protection from lead.

Magnetic field strength N in the spectrometer and radius of curvature electron path determine energy of the electrons, recorded by detector. If radiator is prepared from the substance with the small atomic number, then secondary electrons are formed in essence as a result of the Compton effect; if radiator is prepared from the heavy substance (lead, uranium), and energy - quanta is small, then secondary electrons will appear mainly as a result of the photo effect. With the energies MeV the formation by the gamma-quanta of electron-positron pairs becomes possible. Fig. 2 depicts magnetic paired G.- s. pair formation it occurs in the thin radiator, located in vacuum chamber. The measurement of summary energy of electron and positron makes it possible to determine energy - quantum. Magnetic G.- s. possess the high resolution (usually order 1 % or by portion %); however, effectiveness such G.- s. is small, what leads to the need for using sources - the emission of high activity.

In scintillation G.- s. the secondary electrons appear with interaction - quanta with the scintillator (by substance, in which secondary electrons they excite fluorescence). Light flash is converted into the electric pulse with the aid of the photomultiplier (FEU, Fig. 3); moreover the value of the signal, created BY FEU, is proportional to energy of electron and, therefore, it is connected with the energy - quantum. For measuring the distributions of signals in the amplitude they are used specialist. electronic devices - pulse-height analyzers (see nuclear electronics).

Fig. 2. the diagrammatic representation of paired gamma-ray spectrometer. In the uniform magnetic field N, directed perpendicularly to the plane of drawing, the electrons and positrons dvizhutsya in the circles in opposite directions.

Effectiveness scintillation G.- s. depends on the dimensions of scintillator and with the not very high energy it can be close to 100%. However, its resolution is low. For the gamma-quanta with the energy 662 keV it decreases with an increase in the energy approximately as (for greater detail, see scintillation spectrometer).

Action semiconductor G.- s. is based on the formation - by emission in the volume of the semiconductor crystal (usually Ge with admixture Li) of electron hole pairs. Charge appearing in this case is assembled with the electrodes and is recorded in the form of the electrical signal, whose value is determined by energy - quanta (Fig. 4). Semiconductor G.- s. possess the very high resolution, which is caused low energy, expended to the formation of one electron hole pair. For ~0,5%. effectiveness semiconductor G.- s. are usually lower than scintillation G.- s., since gamma-radiation in Ge is absorbed more weakly than, for example, in the scintillation crystal NaJ. Furthermore, the sizes of the utilized semiconductor detectors are thus far still small. To the deficiencies semiconductor G.- s. should be carried also the need for their cooling down to temperatures, close to the temperature of liquid nitrogen (for greater detail, see semiconductor spectrometer).

The highest accuracy of the measurement of energy of gamma-quanta ensure crystal- diffraction G.- s., in which directly is measured the wavelength of gamma-radiation. Such a G.- s. is analogous to instruments for observing the diffraction of X-rays. Emission, penetrating the crystal of quartz or calcite, is reflected by crystal planes in the dependence on its wavelength under by other one or angle or another and is recorded by photoemulsion or photon counter. Deficiency such G.- s. - low effectiveness.

For spectral measurement - the emissions of low energies (to 100 keV) frequently adapt the proportional counters, whose resolution in the region of low energies considerably higher than in scintillation G.- s. with hv > of 100 keV proportional counters are not used because of too low efficiency. The measurement of spectrum- the emission of very high energies is accomplished with the aid of the shower detectors, which measure the summary energy of the particles of the electron-positron shower, caused by the gamma-quantum of high energy. The formation of shower usually occurs in the radiator of the very large sizes (which ensure the total absorption of all second particles). The flashes of fluorescence (or cherepkovskogo emission) are recorded with pomosh'yu FEU (see Cherepkovskiy counter).

In the some cases for measuring the energy of gamma-quanta the process of the photodisintegration of deuteron is used. If energy - quantum exceeds energy of binding of deuteron (~ 2,23 MeV), then splitting deuteron into the proton and the neutron can occur. Measuring kinetich. of energy of these particles, it is possible to determine energy of the falling in- quanta.

Lit.: Alpha -, is beta- and gamma-spectroscopy, translated from English, edited by K. Siegbahn, in. 1, M., 1969; Methods of measuring the basic values of nuclear physics, translated from English, M., 1964; Kalashnikov V. i., Kozodayev M. S., the detectors of elementary particles, To m.u 1966 (experimental methods of nuclear physics, h. 1).

V. p. parfenova, N. n. delyagin.