Gamma-ray Linear Polarization Research Articles

Analyzing power of AGATA triple clusters for gamma-ray linear polarization

We have investigated the ability of AGATA triple clusters to measure the linear polarization of gamma rays, exploiting the azimuthal-angle dependence of the Compton scattering differential cross section. To this aim, partially polarized gamma rays have been produced by Coulomb excitation of the first excited state of 104Pd and 108Pd, which decay to the ground state by emission of gamma rays of 555.8 keV and 433.9 keV, respectively. Pulse-shape analysis and gamma-ray tracking techniques have been used to determine the position and time sequence of the interaction points inside the germanium crystals. Anisotropies in the detection efficiency have been taken into account using 661.6 keV gammas from a 137Cs radioactive source. We obtain an average analyzing power of 0.451(34) at 433.9 keV and 0.484(24) at 555.8 keV.

Investigation of 166Er in the respective (n, n′γ) reaction

The spectrum, angular distribution, and linear polarization of gamma rays from the (n, n′γ) reaction on 166Er were measured in a beam of reactor fast neutrons. The known scheme of levels and gamma transitions for this nucleus was supplemented. Previously unknown gamma transitions and levels were found along with their features. The completeness of the scheme of levels whose quantum numbers J π range from 0+ to 4+ and whose excitation energy extends up to 1.9 MeV was established. The absence of rotational levels for the nonrotational states at 1703.11 keV (2+), 1713.42 keV (0+), and 1760.51 keV (4+) was confirmed.

AGATA modules as Compton polarimeters for the measurement of gamma-ray linear polarisation

The ability of a cluster of three segmented Ge crystals (“AGATA module”) acting as a Compton polarimeter to measure the linear polarization of gamma rays has been investigated at an energy close to 511 keV. Partially polarized gamma rays have been produced by Coulomb excitation of the first excited state of 104 Pd a and 108 Pd.

Development of the multi-purpose gamma-ray detection system consisting of a double-sided silicon strip detector and a 25-segmented germanium detector

We developed a position-sensitive gamma-ray detection system consisting of a double-sided silicon strip detector (DSSD) and a 25-segmented germanium detector (25-SEGD). Two major applications of the system are gamma-ray imaging (Compton camera) and linear polarization measurement for gamma rays emitted from oriented nuclei. Customized electronics were developed in order to handle multi-channel signals of both the DSSD and the 25-SEGD. Images for a 133 Ba-based compound source in a square shape with areal dimensions of 1.5 × 1.5 mm 2 are presented. Comparison between experimental images and a Monte Carlo simulation yielded the overall imaging resolution within 1 cm for the present system.

Planar and aplanar tilted bands in the odd–odd nucleus 132Cs

High-spin states in the doubly odd N = 77 nucleus 132Cs have been studied. Gamma-ray linear polarization and angular correlation measurements have been performed to determine the spin and parity of the states. A rotational sequence of enhanced dipole transitions was established. This band can be described within the framework of the the tilted axis cranking (TAC) model as a band based on a weakly deformed structure in which the total angular momentum vector does not coincide with any of the principal axes of the triaxial nucleus but still is in the plane defined by the short and long axes. The theoretical description of the previously proposed chiral bands in 132Cs was extended in the framework of the core–particle coupling model (CPCM) confirming that the total angular momentum for the πh11/2 ⊗ νh11/2 configuration is well outside any planes defined by the principal axes of the nucleus and revealing the important role of the residual proton–neutron interaction between the valence particles.

Polarization sensitivity and efficiency for a planar-type segmented germanium detector as a Compton polarimeter

We have investigated performance of a planar-type germanium detector with 25 segments as a Compton polarimeter to measure the linear polarization of gamma rays. We considered three different configurations in sampling horizontal and vertical Compton-scattering events for linearly polarized gamma rays incident on the detector. The experimental polarization sensitivity Q, coincidence efficiency ϵ coin, and figure of merit F for each configuration have been extracted for gamma rays of interest below 600 keV found in our previous in-beam spectroscopy on 155 Gd . We also obtained theoretical values for these three quantities through a Monte Carlo simulation. Experimental values were in good accord with simulation results for all the three configurations. The third configuration, sampling pairs between the segment of incidence and any off-diagonal segments in coincidence, turned out to have the largest Q as well as ϵ coin value of all the three configurations, thereby resulting in the best figure of merit F.

Coherent proton–neutron contribution to octupole correlations in the neutron-deficient 114Xe nucleus

Gamma ray linear polarization and picosecond lifetimes have been measured for levels in the neutron deficient nucleus 114Xe using the EUROBALL IV spectrometer and the Cologne plunger device. The EUCLIDES Si-ball was used to improve the reaction channel selectivity. The linear polarization results have, for the first time, unambiguously determined the electromagnetic character of the dipole transitions de-exciting the negative parity level sequence, providing clear evidence for enhanced octupole collectivity. The discovery of two E3 transitions and the measurement of the lifetimes of the states depopulated by these transitions have allowed a quantitative determination of the octupole collectivity in the A≈112 mass region. The large measured B(E3) values, close to ≈70 W.u., are among the strongest observed hitherto and indicate a coherent proton–neutron contribution to the octupole moment.

Linear polarization measurement of interband transitions in superdeformed 190hg: model-independent evidence for octupole vibrational structures.

The linear polarization of gamma rays between excited and yrast superdeformed (SD) states in 190Hg was measured using the four-element CLOVER detectors of the EUROBALL IV gamma-ray spectrometer. This measurement shows in a model-independent way that the interband transitions which compete with the highly collective in-band quadrupole transitions are largely enhanced electric dipoles. Not only do these results represent the first measurement of the multipolarity of transitions between different SD states, but they also provide strong evidence for the interpretation of the structures in the SD minimum of the A approximately 190 region in terms of octupole excitations.

Test of a symmetrical four-detector compton polarimeter using low temperature nuclear orientation

A symmetrical four-detector Compton polarimeter was used to measure the linear polarization of gamma-rays emitted from nuclei oriented by the hyperfine interaction at low temperature. Its polarization detection efficiency was determined in the range 0.3–1.4 MeV and compared with some other polarimeters.

Methods of measurement of gamma-ray polarization

The paper discusses the methods for measuring the linear polarization of gamma-rays with energies above 0.1 MeV. Special attention is given to Compton scattering and e−, e+ pair production. The effects of the selection of gamma-events and multiple scattering in the converters on the derived azimuthal distribution are considered. The changes of measuring polarization with gamma-telescopes in a near future are considered.

96Tc levels populated in the (α,n) reaction

The levels of odd–odd 96Tc have been investigated using the 93Nb(α,nγ)96Tc reaction at an alpha-beam energy of 14 MeV. The experiments involved singles, gamma–gamma coincidences, detailed excitation functions, gamma-ray angular distributions, measurements of the gamma-ray linear polarization coefficients, and electron internal-conversion coefficients. Spin and parity assignments are given with a high degree of confidence to most of the 105 energy levels, which are interconnected by 251 gamma-ray transitions. In particular, a new energy level at 36.24 keV completes a sextet of lowest lying levels ranging in spin from 2+ to 7+ formed by a coupling of an odd g9/2 proton to an odd d5/2 neutron. The structure of 96Tc shows some vibrational features coexisting with a significant configuration mixing. Thus, a quartet of nuclei at the onset of a deformation region at A ~ 100, 94,95Mo and 95,96Tc, provides a challenging ground for testing of the microscopic models and may give further insight into the nature of the neutron–proton interaction in complex nuclei.

On line nuclear orientation: A rapidly developing technique

Nuclear orientation facilities are operating fully on-line to acclerators/isotope separators capable of producing wide ranges of radioisotopes. The lowest on-line base temperatures are close to 7 mK and the shortest half-life yet studied is 0·9 s. Experimental results yield magnetic dipole interaction strengths, nuclear spin-lattice relaxation times, directional distributions of alpha, beta and gamma emissions and gamma ray linear polarization. Nuclear magnetic resonance has been observed on-line.

Gamma-ray linear polarization measurement on oriented nuclei

Measurement of linear polarization of gamma-rays from nuclei oriented by hyperfine interaction at low temperatures using the3He/4He dilution refrigerator at the SERC Daresbury Laboratory is reported. Three different types of a Compton polarimeter were assembled using coaxial Ge(Li) detectors. The method was applied to192IrFe and60CoFe and a polarization detection efficiency curve and a figure of merit for each polarimeter were produced. As a result of combined directional distribution and linear polarization measurements multipole mixing ratios of several transitions in192Pt have been obtained and are compared with theoretical predictions.

Gamma-ray linear polarization and nuclear orientation experiments on oriented192, 194IrFe

The combined technique of gamma-ray linear polarization and directional distribution measurement from oriented nuclei (LPγON) has been applied to192IrFe. New polarization data are used to deduce values of mixing ratios in Β and γ transitions.

Linear polarization and directional distribution of gamma-rays from oriented172Lu

The gamma-ray linear polarization data in combination with the results of the directional distribution measurements were used to determine precise values of the multipole mixing ratios of the gamma-transitions in the172Yb nucleus. A detailed analysis of the deduced mixing ratios, supplemented by previous data, lends support to the following description of the two-quasiparticle excitations of192Yb (see also tab. 6): 2073 keVKπ = (4+) {p 4041 ↓ + p 411 ↓} + {n 514↓ − n 521↓} 1663 keVKπ = (3+) {n 514↓ − n 521↓} + {n 512↑ − n 521↓} 1172 keVKπ = 3+ {n 512↑ + n 521↓} + {p404↓ − P 411↓} A supporting evidence has also been given to the spin-parity assignments of the 1640 keV (Iπ = 4−) and 1706 keV (Iπ = 5−) members of theKπ = 4− band.

Gamma-ray polarization and nuclear orientation measurements on182TaFe

The results of combined measurements of the directional distribution and the linear polarization of gamma-rays from oriented182TaFe are reported. Construction and calibration of a Compton polarimeter based on a single planar Ge(Li) detector are described. The degree of the linear polarization and multipole mixing ratios for many gamma-transitions in182W have been deduced. A triple multipole mixingE1+M2+E3 of the 1,274 keV transition has been obtained withδ1(M2/E1)=+0.36(10) andδ2(E3/E1)=−0.28(12). Results are discussed in the framework of the rotational model. Strong evidence of the octupole vibrational nature of the 1,289 keV level in182W has been found.

Symmetrical four-crystal Compton polarimeter for gamma rays: Design and application

The mechanical and electronic configuration of a four-detector system which has been developed for gamma ray linear polarization measurements is described. Outstanding characteristics of the system are (a) a close geometry which gives high efficiency at an only moderate sacrifice in polarization sensitivity, and (b) a high degree of instrumental symmetry which greatly contributes to the speed and precision of polarization measurements. The results of extensive test measurements of the instrument are presented. In addition, typical applications to nuclear spectroscopy and the relevant methods of data analysis are discussed.

Gamma-Ray Angular Distribution, Angular Correlation, Linear Polarization, and Doppler-Shift Attenuation Measurements in the 46Ti(α, nγ) 49Cr Reaction

A detailed investigation of the electromagnetic properties of the 49 Cr levels below 3 MeV has been made. The levels were populated by the 46 Ti (α, nγ) 49 Cr reaction at E α =8 MeV. Analysis of measured gamma-ray angular distributions, linear polarizations, and gamma-gamma angular correlations has given the following unique spin and parity assignments: 9/2 - to the 1083-keV level; 1/2 - to the 1703-keV level; 5/2 + to the 2432-keV level; and 7/2 - to the 2503-keV level. In addition, a unique 5/2 assignment has been given to the 2168-keV level. Lifetimes of some levels have been obtained from Doppler-shift attenuation measurements. New or revised values of branching ratios and mixing ratios have been determined for many transitions. The unique 5/2 + assignment to the 2432-keV level and a large B (E2) value obtained for the transition to the 1981-keV 3/2 + level suggest that the 2432-keV level is a member of the K π =3/2 + band built on the 1981-keV level.

High-spin states in 40K investigated with the 26MG + 16O reaction

Yrast levels in 40K and 40Ar have been investigated with the 26Mg( 16O, pnγ) 40K and 26Mg( 16O, 2pγ) 40Ar reactions at a beam energy of 34 MeV. Gamma-ray angular distribution and γ-γ coincidence measurements have been performed with a high-resolution large volume Ge(Li)-NaI(Tl) Compton-suppression spectrometer. Gamma-ray linear polarizations have been measured with a three-crystal Ge(Li) Compton polarimeter. The 40K decay scheme involves new high-spin levels at E tx = 4365.6±0.3, 4875.6±0.4 and 6227.0±0.5 keV with lifetime limits of < 1, < 1 and < 2ps, respectively. Unambiguous spin-parity assignments of J π = 5 −, 6 +, 8 +, 9 + and (8, 10) − to the 40K levels at E x = 0.89, 2.88, 4.37, 4.88 and 6.23 and of J π = 4 + and 6 + to the 40 Ar levels at E x = 2.89 and 3.46 MeV, respectively, have been obtained. Branching ratios and multipole mixing ratios are reported.

Angular correlation and gamma ray linear polarization measurements inAl29

The decay scheme of $^{29}\mathrm{Al}$ has been investigated with the $^{26}\mathrm{Mg}(\ensuremath{\alpha},p)^{29}\mathrm{Al}$ reaction at ${E}_{\ensuremath{\alpha}}=14.2$ MeV. Simultaneous measurements of $p\ensuremath{-}\ensuremath{\gamma}$ angular correlations and $p\ensuremath{-}\ensuremath{\gamma}$ linear polarizations, utilizing a three-detector Ge(Li) Compton polarimeter, have been accomplished. Spin-parity assignments of ${7/2}^{+}$, ${3/2}^{+}$, ${5/2}^{+}$, ${9/2}^{+}$, ${5/2}^{+}$, and ${7/2}^{+}$ are set forth, respectively, for the 1.75, 2.22, 3.18, 3.58, 4.22, and 5.99 MeV levels. The results suggest ${J}^{\ensuremath{\pi}}={\frac{11}{2}}^{+}$ for the 5.85 MeV level.NUCLEAR REACTIONS $^{26}\mathrm{Mg}(\ensuremath{\alpha},p)$, $E=14.2$ MeV; measured $p\ensuremath{\gamma}$ coincidence, ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}({\ensuremath{\theta}}_{\ensuremath{\gamma}})$, $\ensuremath{\gamma}$ linear polarizations. $^{29}\mathrm{Al}$ deduced levels, ${E}_{x}$, $J$, $\ensuremath{\pi}$, $\ensuremath{\delta}$, branching ratios. Enriched target. Ge(Li) Compton polarimeter.