M. De Poli

A 4π light-charged particle Si detector as a trigger device for in-beam γ-ray spectroscopy

A 4π Si-ball detector system has been constructed as a trigger device for the γ-ray spectrometer GASP. The construction and operation of the detector is reported, together with a brief description of the electronics and of the data acquisition system used. Results are presented for the 32S + 40Ca reaction at 140 MeV of32S beam energy.

Rotational bands in the doubly odd nucleus 134Pr

Abstract The band structures of the doubly odd 134 Pr nucleus has been investigated via the 119 Sn( 19 F,4n) 134 Pr and 110 Pd( 28 Si,p3n) 134 Pr reactions at beam energies of 87 and 130 MeV, respectively. The three previously known rotational bands based on the π[541] 3 2 − (α=± 1 2 )⊗ν[514] 9 2 − and π[423] 5 2 + ⊗ν[514] 9 2 − configurations have been extended to higher spins. The difference of ≈2ħ in the experimental alignment of the bands based on the signature partners of the [541] 3 2 − h 11 2 proton orbital is discussed in terms of shape coexistence and coupling with the γ-phonon but no consistent interpretation can be found. A new band consisting of quadrupole transitions has also been identified and linked unambiguously to the low-lying levels of the yrast band. Total routhian surface and cranked shell model calculations suggest a (π h 11 2 ) 3 ⊗ν[530] 1 2 − configuration for this band. The experimental ratios of reduced transition probabilities B( M 1) B( E 2) of the various rotational structures are compared to the theoretical values obtained from a semiclassical model and from the two-particle plus triaxial-rotor model.

Precise DSAM lifetime measurements in 48Cr and 50Cr as a test of large scale shell model calculations

Precise DSAM lifetime values have been obtained for the positive parity yrast bands of 48Cr and 50Cr, populated with the reaction 28Si+28Si at 115 MeV bombarding energy using a Au backed target. The obtained precision relies on the well-known stopping power at high velocity and on the use of DSAM analysis procedures which avoid the sidefeeding contribution. The deduced B(E2) and B(M1) values are in very good agreement with those predicted by full fp shell model (SM) calculations. A new procedure named narrow gate on transition below has been applied for the analysis of the lifetimes. Clear evidence was found in both nuclei for a low-lying negative parity band which has been successfully described by SM in the fp shell extended to include a hole in the 1d32 orbital. Also in this case the derived B(E2) and B(M1) values are in good agreement with the SM calculations.

Delayed g9/2 Alignment in the N=Z Nucleus 72Kr

The structure of Kr-72 has been investigated at GASP through the Ca-40(Ca-40,2 alpha) reaction at a beam energy of 160 MeV using the 4 pi ISIS Si-ball for reaction channel selection. The level scheme has been extended up to an excitation energy of approximate to 8.5 MeV. The spins and parities of the observed levels are assigned tentatively. The observed band shows the predicted change from oblate to prolate shape. The four quasi-particle g(9/2) alignment is found to be significantly delayed in rotational frequency with respect to the heavier Kr isotopes. Such a delay contradicts the predictions of standard mean-field calculations and may reflect either additional correlations in the T = 0 pairing channel or coupling to vibrational degrees of freedom or both

Application of the pulse-shape technique to proton-alpha discrimination in Si-detector arrays

Abstract The capability of the pulse-shape technique with reversed n-type Si detectors for discrimination of protons and alphas produced in fusion-evaporation reactions was tested at the VICKSI cyclotron in Berlin. We applied a zero-crossing technique which does not need any external time reference, and which can therefore be exploited at DC accelerators. Excellent proton-alpha discrimination in the full energy range of the evaporation spectra, but also charge and even isotope resolution for heavier ions produced in projectile fragmentation, was obtained with detectors of an existing Si ball. There is no doubt that the pulse-shape discrimination works well with detectors from serial production and under experimental conditions which are typical for nuclear structure studies. An application of this technique in Si detector arrays is obvious, but some special features must be considered in the design of the electronics. The particle discrimination depends strongly on the electric field distribution inside the detector. Stabilization of the bias voltage at the detector is therefore recommended. A consequence of the rear-side injection mode is a strong variation of the charge-collection time with energy, charge, and mass number of the detected ion. To obtain a precise energy signal it is indispensable to correct for the ballistic deficit.

Linking transitions between the highly deformed states and the yrast states of normal deformation in 133Nd

Abstract Transitions linking the highly deformed band to low deformation states have been unambiguously established in 133 Nd in the very first experiment performed with the GASP array. the highly deformed band becomes yrast at 29 2 + , where it mixes with the positive parity band built upon the [404] 7 2 Nilsson state. The experimentally deduced interaction amounts to 13 keV. The measurement of the spins in the highly deformed intruder band sheds some light upon the reported backbend anomaly in this mass region.

Identification of light charged particles and heavy ions in silicon detectors by means of pulse-shape discrimination

Pulse-shaped discrimination with totally depleted Si-detectors in reverse mount has been investigated and shown to be an excellent method of charged-particle identification in the energy range of /spl ap/2 to 20 AMeV. In test experiments with heavy-ion beams we obtained element identification up to Ti and isotope resolution even for elements heavier than carbon. The promising results and the simplicity of the electronics recommend this technique for applications in multidetector arrays. In particular, small and compact 4/spl pi/ Si balls with relatively low thresholds for charged-particle identification to be combined with 4/spl pi/ neutron detectors or /spl gamma/ arrays can be constructed.

Band termination in the N = Z odd-odd nucleus

High spin states in the odd-odd N=Z nucleus 46V have been identified. At low spin, the T=1 isobaric analogue states of 46Ti are established up to I = 6+. Other high spin states, including the band terminating state, are tentatively assigned to the same T=1 band. The T=0 band built on the low-lying 3+ isomer is observed up to the 1f7/2-shell termination at I=15. Both signatures of a negative parity T=0 band are observed up to the terminating states at I = 16- and I = 17-, respectively. The structure of this band is interpreted as a particle-hole excitation from the 1d3/2 shell. Spherical shell model calculations are found to be in excellent agreement with the experimental results.

^{46}

Abstract Lifetimes have been measured in 139Sm with the DSAM method, using the reaction 62Ni(81Br,p3n)139Sm at 350 MeV. In the negative-parity regular dipole band based on the 25 2 − state at 3325 keV values of 0.60(21), 0.40(14) and 0.25(8) ps were found for the 337, 409 and 473 keV transitions, respectively. The deduced B(M1) and B(E2) are in agreement with the predictions of the tilted axis cranking model, allowing a triaxial equilibrium shape for the assumed νh 11 2 ⊗ πh 11 2 2 configuration.

V.

Excited states in {sup 52}Fe have been studied up to spin 10{h_bar} in the reaction {sup 28}Si+{sup 28}Si at 115 MeV beam energy by using in-beam {gamma}-ray spectroscopy methods at the GASP array. The excitation energy of the yrast 10{sup +} state is 7.381 MeV, almost 0.5 MeV above the well known {beta}{sup +}-decaying yrast 12{sup +} state. Experimental upper limits for the B(E4) transition probabilities from the 12{sup +} isomer to the 8{sub 1}{sup +} and 8{sub 2}{sup +} states have been determined. The mean lifetimes of five excited states have been measured by using the Doppler shift attenuation method. Complete diagonalizations in the pf major shell lead to very good agreement with the experimental level scheme and transition probabilities. The lifetime, log ft value, branching ratios, and B(E4) values are calculated for the 12{sup +} isomer. The positive parity states are also interpreted in terms of a Nilsson projected method. The structure of the yrast levels of {sup 52}Fe is compared with those of its cross conjugate {sup 44}Ti. {copyright} {ital 1998} {ital The American Physical Society}