D A Viggars

Construction and use of a three Ge(Li) Compton polarimeter

Abstract A three Ge(Li) Compton polarimeter has been constructed, and its sensitivity to γ-ray linear polarisation determined in the energy range 0.4 to 4.4 MeV. An example is given describing its application to resolve a spin ambiguity for the 4868 keV level in 33 S populated by the reaction 30 Si(α,n) 33 S at an α-particle bombarding energy of 9.8 MeV.

Properties of 37Ar II. Levels above 3.8 MeV excitation energy (populated in 34S(α,n γ)37Ar)

For pt. I, see abstr. A12157 of 1974. Excitation energies, mean lifetimes, gamma ray branching ratios, angular distributions and linear polarizations have been measured for levels in 37Ar above 3.8 MeV excitation energy. The states were populated using the 34S( alpha , n gamma )37Ar reaction at beam energies between 10.5 MeV and 16.0 MeV. Gamma-gamma coincidence measurements at beam energies of 11.5 MeV and 16.0 MeV were used to determine the decay scheme. The results of an intermediate coupling vibrational model calculation for the negative parity states are presented.

The properties of low-lying levels in 37Cl (populated in 34S(α,pγ) reaction)

Excitation energies, mean lifetimes, gamma-ray branching ratios, angular correlations and linear polarizations have been measured for levels below 5.3 MeV in 37Cl. The states were populated using the 34S( alpha ,p gamma )37Cl reaction at bombarding energies between 9.0 MeV and 12.2 MeV. A gamma-gamma coincidence experiment was performed at 11.5 MeV to obtain the decay scheme. New measurements were made for the following levels: Ex( tau m)Jpi ; 3087 keV (<40 fs) 5/2+; 3105 keV (48+or-5 ps) 7/2-; 3626 keV (40+or-20 fs) 3/2, 5/2, 7/2+; 3708 keV (<25 fs) 3/2+, 5/2-; 3740 keV (<20 fs) 5/2-; 4011 keV (31+or-3 ps) 9/2-; 4547 keV (2.7+or-0.6 ps) 11/2- and 5271 keV (2.7+or-0.4 ps) 13/2-. The results are compared with the properties of high-spin negative-parity states in neighbouring nuclei and are discussed in terms of recent shell-model calculations.

Lifetime measurements in 36Ar and 36Cl using the recoil distance method.

The recoil distance method has been used to measure the mean lifetimes of the 788 keV, 1164 keV, 2519 keV and 2810 keV levels in 36Cl and of the 4178 keV and 5171 keV levels in 36Ar. The measured lifetimes are 30+or-1 ps, 7.1+or-0.5 ps, 2.36+or-0.16 ns, 9.0+or-0.6 ps, less than 8 ps and 127+or-5 ps respectively. The data support a correspondence between the 2519 keV and 2810 keV levels of 36Cl with 5- and 4- states given by the shell model calculations of Erne.

Mean lifetimes of levels in 32P and 32S (populated in 29Si(α, p) and 29Si(α, n) reactions)

Lifetimes of low-lying levels in 32P and 32S have been measured using the Doppler shift attenuation method. The levels were populated via the 29Si( alpha , p)32P and 29Si( alpha , n)32S reactions at alpha particle energies of 8.0 and 9.5 MeV. Seven new lifetimes and two new lifetime limits have been measured in 32P and one new lifetime has been measured in 32S. The principal decay modes of some levels have been measured.

Properties of 37Ar. I. Levels below 3.8 MeV (populated in 34S(α,nγ)37Ar)

Excitation energies, mean lifetimes, gamma -ray branching ratios, angular correlations, mixing ratios and linear polarizations have been measured for the first twelve levels of 37Ar using the 34S( alpha ,n gamma )37Ar reaction. Parity assignments were made for the following levels: 3173 keV, 5/2+, 3274 keV, 5/2-, 3527 keV, 7/2-, and 3602 keV, 3/2+ or 5/2-. The spin and parity of the 3707 keV state ( tau m=370+or-95 fs) was determined, Jpi =11/2-. Three new low energy gamma -ray transitions were observed and six new mixing ratios were determined. The authors found that the negative parity levels which decay to the 1611 keV 7/2- state are represented qualitatively by the core excitation model.

Lifetimes, spins and decay modes of levels below 5.3 MeV in 33S

The reaction 30Si( alpha , n)33S was used to populate levels in 33S at alpha particle energies between 7.00 and 10.00 MeV. Gamma rays in 33S were observed in a Ge(Li)-NaI(Tl) escape-suppressed and pair-escape spectrometer. The Doppler shift attenuation method was used to measure the mean lifetimes of 20 levels and to set limits on the lifetimes of 3 levels in 33S. The analysis of angular distributions of gamma rays together with the measured lifetimes and branching ratios gave gamma ray multipole mixing ratios and spin and parity assignments of 7/2+, (5/2, 9/2)+, (3/2+, 5/2) and (7/2, 11/2)- for the levels at 2971, 4050, 4145 and 4868 keV respectively.

Angular correlation measurements on levels in 29Si above 5 MeV excitation (populated in 26Mg(α,n)29Si reaction)

Angular correlation measurements have been made in 29Si to determine spins and mixing ratios using the reaction 26Mg( alpha ,n)29Si. Previously undetermined mixing ratios have been measured for the following transitions: (5.653 MeV-4.742 MeV), delta =-0.02+or-0.20; (5.813 MeV-3.068 MeV), delta =-0.06+or-0.03. The levels at 7.069, 7.522, 8.331 and 8.476 MeV were found to have mean lifetimes pi <15 fs, those at 8.641 and 8.861 MeV have mean lifetimes tau <40 fs.

Gamma-ray linear polarization measurements in 33S

The reaction 30Si( alpha , n)33S was used to populate levels in 33S at alpha particle energies of 7.7 MeV and 9.8 MeV. Gamma rays in 33S were detected in a Ge(Li)-NaI(Tl) escape-suppressed spectrometer and a three Ge(Li) Compton polarimeter. The analysis of angular distributions and experimental linear polarizations of gamma rays gave spin and parity assignments of 5/2+, 9/2+, 7/2+ and 11/2- for the levels at 2868, 4050, 4096 and 4868 keV respectively.

High spin negative parity states in 29Si associated with oblate deformation (population using 26Mg(α,n) reaction)

Spin and parity assignments of 11/2- and 13/2- have been made for levels at 6.781 MeV and 8.761 MeV respectively in 29Si using angular correlation, gamma ray linear polarization and lifetime measurements. The properties of the levels are consistent with their being high spin members of a Kpi =7/2- rotational band based on the Nilsson orbit (303) with oblate deformation.