I. Procházka

Low temperature nuclear orientation of nuclei far from beta stability Spin — Facility at JINR

The experimental facility for the study of the decay of oriented nuclei in a broad range of atomic and mass number and half-lives based on hyperfine interactions at ultralow temperatures — the SPIN facility at JINR — is described. High cooling power top loading 3He-4He dilution refrigerator of the system is capable of fast cooling of the sample inserted into the mixing chamber to the base temperature 10 mK for about 1–2 h. Detection system is designed for the study of angular distribution and temperature dependence of the anisotropy of gamma rays, linear polarisation of gamma rays and NMR/ON studies. Some results of nuclear orientation studies in off-line mode of radioactive nuclei with the half-live down to ∼1 h produced by spallation reaction with 660 MeV protons are presented. The project of nuclear orientation system on-line to the mass-separator and the new 600 MeV Dubna proton phasotron is described.

Decay of oriented149Nd and levels in theN=88149Pm nucleus

Levels in the149Pm nucleus populated via149Nd decay were studied using the low temperature nuclear orientation technique. The parent149Nd nuclei were oriented in a gadolinium host. The normalized gamma-ray intensities were determined for twenty five transitions in the daughter149Pm nucleus. The spins of the149Pm levels at 288.2 and 425.3 keV were determined as 9/2 and 7/2, respectively, and for several other149Pm levels the previous spin-assignments could be unambiguously confirmed. Substantial new information on multipole mixing ratios of gamma-transitions in149Pm is presented. Low-energy level spacing in the149Pm nucleus is found to be in reasonable agreement with recent particle-rotor model calculations including the variable moment of inertia.

Low temperature nuclear orientation of206Bi in gadolinium and iron hosts

Gamma-ray anisotropies were measured for the206Bi radioisotope oriented at low temperature in iron and gadolinium hosts. Multipole mixing ratios for 20 gamma transitions in206Pb nucleus have been deduced, 12 of them for the first time. Several of these data could be compared with predictions of the shell model including the residual interaction of P2-type. Such comparison indicated that interpretation of the present experimental results needed a more developed theoretical description of the206Pb nucleus. A first evidence of orientation of the206Bi nuclei in gadolinium was obtained. Considerable attenuation of several gamma-ray anisotropies due to the 2200 keV isomeric state in206Pb has been found and corresponding attenuation factors G2= =0·71(6) and 0·63(6) have been deduced for iron and gadolinium hosts, respectively.

Nuclear orientation study of the decay of208At (T1/2=1.63 h) and209At (T1/2=5.41 h) implanted in iron host

We have measured the angular distribution of gamma-rays in 208,209Po following the decay of the mass-separated isotopes 208,209At polarized at low temperatures in an iron host. Multipole mixing ratios were deduced for 14 and 9 gamma-ray transitions in 208Po and 209Po, respectively, and unambiguous spin-assignments were made for several levels in both nuclei.

Spin echo and nuclear orientation study of metallic glasses

Hyperfine fields on Co nuclei in amorphous as-quenched and heat-treated Co75Fe5B20 samples were studied by conventional NMR and by very low temperature nuclear orientation techniques. The Co spin echo measurement at 1.4 K yielded broad spectra between 130 – 260 MHz, with narrow maxima at 145.5 MHz and 155.1 MHz for as-quenched sample and with a broad maximum at 227 MHz for heat-treated sample well below the recrystallization point. The60Co nuclear orientation measurements gave the mean value of hyperfine field Bhf∼15 T nearly independent of the sample heat-treatment. The spin-lattice relaxation was studied by pulse NMR and also by nuclear orientation thermal cycling technique.

Nuclear orientation study of the decay of204BiFe

The decay of204Bi nuclei (Iπ=6+, T1/2=11·22 h) oriented in an iron host was investigated on the JINR low-temperature nuclear orientation facility SPIN. The orientation parameterB2=1·17 (6) was obtained from the analysis of six prominent E1 gamma-transitions. From the measured normalized intensities of the gamma-rays observed some 70 values of multipole mixing ratios δ for the gamma-transitions in204Pb nucleus were determined for the first time. The spins 6, 6, 5 and 4 could be uniquely assigned to the204Pb negative parity levels at 3891·5 keV, 3768·4 keV, 3301·5 keV and 2338·2 keV, respectively. The spin-parity assignments of the levels at 4183·8 keV, 4094·2 keV, 3782·0 keV, 2506·9 keV and 2065·1 keV were confirmed as 6−, 6−, 5−, 5− and 5+, respectively. For the level at 3105·1 keV spin-parity 5− was suggested and spinparity 7− of the level at 2696·4 keV was called in question. The possible placements of the gammatransitions 3 1351·7 keV and 1353·4 keV in the decay scheme is discussed. The reorientation parameters for the long-living levels at 2264·2 keV (T1/2=0·45 μs) and 1273·9 keV (T1/2= =265 ns) were determined asG2=0·41 (14) andG2=0·60 (17), respectively. For the isomeric level at 2185·7 keV (T1/2=67·2 min) the value ofG2=0·88 (49) was proposed.