The basic scientific activity

1. X- and g-ray diffraction in elastically deformed crystals. Precision g-spectroscopy

Fig. 3. X-ray lines K1 of Sn. They are obtained at two-meter focusing crystal-diffraction Cauchois g-spectrometer, using the quartz plates with the same reflecting planes (130), but with the different cut orientation relative to an axis, perpendicular to these planes (k = 0 and k = 2·10^-4 cm^-1). The k value characterizes the curvature of the reflecting planes caused by the bending of plate. It essentially depends on the plate cutoff orientation. The case k = 0 corresponds to flat planes in the bent crystal plate that is used to get a highest resolution. An abscissa axis (l) shows the displacement of the meter lever proportional to a crystal turning angle

In 1961 O.I.Sumbaev and A.I.Smirnov created a new crystal-diffraction focusing gamma-spectrometer (GSK-2) with the 4-meter focal length (Fig. 4) for studies of the g-ray spectra from the (n, g) reaction in the energy range 40-1600 keV [10,11]. Soon after the reactor WWR-M launch it was put into operation. The spectrometer is a unique device operating until now at the WWR-M reactor for so long time and, moreover, after several modernizations conducted by V.L.Alexeev and V.L.Rumiantsev it has now the highest resolu-tion among the focusing gamma-spectrometers in the world.

Fig. 4. General view of the g-spectrometer GSK-2M

One of modernizations was concerned with the appropriate choice of the reflecting crystallographic planes and crystal cut orientation, where the results of works [3-9] performed by O.I.Sumbaev, A.V.Tyunis, A.S.Ryl'nikov and V.M.Samsonov were used. The use of these results of the theory of elastic quasimosaicity and application of modern PNPI (B.G.Turukhano Laboratory) achievements in creation of the interferometeric holographic gratings for angle scale reading allowed V.L.Alexeev, V.L.Rumiantsev et al. [12-14] to get the best resolution in comparison with all the existing focusing gamma-spectrometers. For the first time in the world the spectrometer was equipped with interferometer on high-frequency holographic arrays in order to improve the accuracy of relative measurements of gamma rays diffraction angles [14].

With this spectrometer the angular resolution of 0.34" for the full aperture (40x30 mm²) and 0.2" for the central part of the crystal aperture (11x5 mm²) has been reached in the energy range 95-250 keV. Now this spectrometer allows the measurements of the g-ray energies with accuracy up to several tenths of eV.

One of the first experiments (O.I.Sumbaev, V.L.Alexeev, D.M.Kaminker, A.I.Smirnov, V.A.Shaburov [15,16]) carried out at this spectrometer has shown the results, which still remain a mystery and call for explanation. This experiment was concerned with the study of a rhodium-104 gamma spectrum, in which the energy distribution of g-lines was first discovered to be nonstatistical (see Fig. 5).

Fig. 5. The g-line energy distribution in spectrum of the ¹⁰³Rh (n, g) ¹⁰⁴Rh reaction (a), and the histogram of relative distances x/D between the g-lines (b). Here x is the distance between the neighboring g-lines, D is the mean distance, the open points correspond to the Poisson (random) distribution for rela-tive distances x/D. There is an evident excess of the close spaced lines over the randomly distributed ones

Instead of a well-known random distribution of g-lines their "attraction", i.e. grouping was discovered (one can see at Fig. 5b the essential excess of small distances between the lines over expected from the ran-dom distribution). This corresponds to a tendency of the equidistance for nuclear levels or "attraction" (grouping) of them instead of their "repulsion" or well-known random distribution. Moreover, the distances between the groups of lines (levels) occurred to be divisible. Later O.I.Sumbaev has paid attention that the phenomenon is very like to the well known synchronization effect for a system of oscillators with nonlinear binding, which is displayed as so-called "devil staircase", expressing the preference of some multiple frequencies for oscillations of such a system. His calculations have shown a striking coincidence of the frequency distributions for such oscillators and the energy distributions for nuclear levels, although the nature and even the fact of the existence of phenomenon itself are still argued.

In 1967 the spectrometer GSK-2 was supplied with the interchangeable head, containing two flat crystals [17]. As a result the focusing spectrometer could be converted into the two-crystal one with the angle resolution of 1". Using this head the g-lines from the (n,g) reactions with the energies up to 6 MeV were detected by diffraction for the first time. The specially constructed two-crystal spectrometer of J. Knowles (Can. J. Phys., 1959, 37, 203; 1962, 40, 237), which was considered as the best one at that time, had angle resolution of 3" and energy range up to 4.5 MeV only.

After that for a long time the comprehensive investigations in g-spectroscopy of odd-odd nuclei were carrying out with the use of the spectrometer assembly (GSK-2, the magnetic spectrometer BETSI, Ge(Li)-detectors and a Ge(Li)-Ge(Li) arrangement). This collaboration of a few laboratories (called "Kolkhoz" that means a collective farm in the USSR) has existed up to nineties. The experimental findings obtained from the study of nuclear reactions had given the unique information on the residual proton-neutron interaction. The reliable schemes of excited nuclear levels were created, including hundreds of g-transitions for each nu-cleus [16-22].

Fig.6. The doublet of 204 keV. Quartz (220). N_g is number of counts for 500 s

Fig.7. The parts of a spectrum of a reactor core with gamma lines, accom-panying the decay of the following nuclides are shown: a) ¹³⁸Cs (Eg = 462.821(33) keV), b) ⁹¹Sr (Eg = 555.612(20) keV)

Таблица №1. Specific activity of fission products of ²³⁵U (Gusev N.G. et al. Radiative characteristics of fission products. Atomizdat, Moscow, 1974.)

Recently, for the first time in the world the direct measurements of g-spectrum of the reactor core was carried out, using the unique resolution of the device (V.L.Alexeev, V.L.Rumiantsev, 1998 [23]), and thus the new data were obtained about g-decay of uranium fission nuclides (about hundred of well resolved g-lines in the energy range of 95 - 250 keV was measured). Such a high resolution of the spectrometer allowed, in particular, to discover a new phenomenon. The main feature of the phenomenon is that many intense g-peaks are close doublets (triplets) well resolved in these measurements (see Fig. 6, for example).

It was noticed (V.V.Fedorov) and demonstrated also (V.V.Fedorov, V.L.Alexeev, V.L.Rumiantsev, 2000, [24]) that the crystal-diffraction method of studying the g-activity of the uranium (plutonium) fission fragments can be useful in the transmutation problem solution, namely, for the control of the isotope and element composition in the process of nuclear waste burning.

The residual radioactivity of a reactor fuel on the basis of enriched ²³⁵U with operating period (Т) 3 years and subsequent storage (t) for 10 years are characterized by specific radioactivities Q (T, t) of products of fission that are shown in the Table.

From the Table it is clear, that more than 90% of a radioactivity is due to twolong-living nuclides ⁹⁰Sr, ¹³⁷Cs and their daughter nuclides ⁹⁰Y, ¹³⁷mBa only. The transmutation (burning out in a high neutron flux) of long-living radioactive nuclides produces short-living ⁹¹Sr and ¹³⁸Cs nuclides. In Fig.7 parts of a reactor core spectrum with gamma lines accompanying the decay of nuclides ¹³⁸Cs (Fig. 7a, Eg = 462.821 (33) keV) and ⁹¹Sr (Fig. 7b, Eg = 555.612 (20) keV) are shown. Near these lines the decay gamma lines of the neutron-rich ⁹⁹Zr (Fig. 7a, Eg = 461.035 (28), 461.813 (33) and 459.168 (34) keV; Fig. 7b, Eg = 546.144 (20) keV) are identified.

Because of the short lifetime of ⁹⁹Zr(T_1/2 = 2.35s) the intensities of its lines at the operating reactor are saturated and, hence, can be used as referent ones to control the accumulation and burning out of nuclides under consideration in nuclear fuel.