Escape-suppressed Spectrometer Research Articles

Production and Study of Neutron-rich Nuclei Using the LICORNE Directional Neutron Source

We have recently successfully demonstrated a new technique for production and study of many of the most exotic neutron-rich nuclei at moderate spins. LICORNE, a newly developed directional inverse-kinematic fast neutron source at the IPN Orsay, was coupled to the MINIBALL high resolution -ray spectrometer to study nuclei the furthest from stability using the 238U(n; f) reaction. This reaction and 232Th(n; f) are the most neutron-rich fission production mechanisms achievable and can be used to simultaneously populate hundreds of neutron-rich nuclei up to spins of 16 ~. High selectivity in the experiment was achieved via triple -ray coincidences and the use of a 400 ns period pulsed neutron beam, a technique which is unavailable to other population mechanisms such as 235U(nth; f) and 252Cf(SF). The pulsing allows time correlations to be exploited to separate delayed rays from isomeric states in the hundreds of nuclei produced, which are then used to cleanly select a particular nucleus and its exotic binary partners. In the recent experiment, several physics cases are simultaneously addressed such as shape coexistence, the evolution of shell closures far from stability, and the spectroscopy of nuclei in the r-process path near N = 82. Preliminary physics results on anomalies in the 238U(n; f) fission yields and the structure of the 138Te and 100Sr nuclei will soon be published. A future project, -ball, to couple LICORNE with a hybrid escape-suppressed spectrometer to refine further the technique and achieve a large increase in the observational limit is discussed.

Spectroscopy of Actinide Nuclei – Perspectives with Position Sensitive HPGe Detectors

Recent advances in in-beam gamma-ray spectroscopy of actinide nuclei are based on highly efficient arrays of escape-suppressed spectrometers. The sensitivity of these detector arrays is greatly enhanced by the combination with powerful mass separators or particle detector systems. This technique is demonstrated by an experiment to investigate excited states in 234U after the one-neutron-transfer reaction 235U(d,t). In coincidence with the outgoing tritons, γ-rays were detected with the highly efficient MINIBALL spectrometer. In the near future an even enhanced sensitivity will be achieved by utilizing position sensitive HPGe detectors which will exploit the novel detection method of gamma-ray energy tracking in electrically segmented germanium detectors. An example for this novel approach is the investigation neutron-rich actinide Th and U nuclei after multi nucleon transfer reactions employing the AGATA demonstrator and PRISMA setup at LNL, Italy. A primary 136Xe beam hitting a 238U target was used to produce the nuclei of interest. Beam-like reaction products after neutron transfer were selected by the PRISMA spectrometer. Coincident γ-rays from excited states in beam and target like particles were measured with the position sensitive AGATA HPGe detectors. Improved Doppler correction and quality of the γ-spectra is based on the novel γ-ray tracking technique, which was successfully exploited in this region.

AGATA and GRETA: The Future of Gamma-Ray Spectroscopy

High precision gamma-ray spectroscopy is one of the powerful tools used to study the structure of excited nuclear states and it has contributed to the discovery of a wide range of new phenomena. Each major advance in gamma-ray detection has resulted in significant new insights into the structure of nuclei. To date, these advances have culminated in two state-of-the-art 4π arrays of escape-suppressed spectrometers, Euroball in Europe [1] and Gammasphere in the United States [2]. These arrays, which consist of more than a 100 large volume germanium crystals each surrounded by a Compton-suppression shield, have pushed this particular detector technology to its limit, with an efficiency for a 1 MeV gamma ray of about 10% and a peak-to-total ratio of 0.5.

Compton-suppression and add-back techniques for the highly segmented TIGRESS HPGe clover detector array

Methods to optimize the performance of the TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS), an array of 12 large-volume, 32-fold segmented HPGe clover detectors to be used at the ISAC-II radioactive ion beam facility, have been developed based on GEANT4 Monte Carlo simulations. These methods rely on the segmentation of the outer electrical contacts of the TIGRESS HPGe clovers, and on the 20-fold segmentation of the Compton-suppression shields. The clover segmentation is utilized to make event-by-event decisions as to whether the γ -ray energy depositions in neighbouring crystals and clovers will be summed. The Compton suppressor segmentation is used to veto events selectively, and to reduce false suppression in experiments with high γ -ray multiplicity. Procedures to determine the optimal techniques and configurations of the array for a particular experiment, dependent on the expected γ -ray energies and multiplicities, and the velocity of the recoil ions, are presented.

Optimization of Compton-suppression and summing schemes for the TIGRESS HPGe detector array

Methods of optimizing the performance of an array of Compton-suppressed, segmented HPGe clover detectors have been developed which rely on the physical position sensitivity of both the HPGe crystals and the Compton-suppression shields. These relatively simple analysis procedures promise to improve the precision of experiments with the TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS). Suppression schemes will improve the efficiency and peak-to-total ratio of TIGRESS for high γ-ray multiplicity events by taking advantage of the 20-fold segmentation of the Compton-suppression shields, while the use of different summing schemes will improve results for a wide range of experimental conditions. The benefits of these methods are compared for many γ-ray energies and multiplicities using a GEANT4 simulation, and the optimal physical configuration of the TIGRESS array under each set of conditions is determined.

Measured and simulated performance of Compton-suppressed TIGRESS HPGe clover detectors

Tests of the performance of a 32-fold segmented HPGe clover detector coupled to a 20-fold segmented Compton-suppression shield, which form a prototype element of the TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS), have been made. Peak-to-total ratios and relative efficiencies have been measured for a variety of γ -ray energies. These measurements were used to validate a GEANT4 simulation of the TIGRESS detectors, which was then used to create a simulation of the full 12-detector array. Predictions of the expected performance of TIGRESS are presented. These predictions indicate that TIGRESS will be capable, for single 1 MeV γ rays, of absolute detection efficiencies of 17% and 9.4%, and peak-to-total ratios of 54% and 61% for the “high-efficiency” and “optimized peak-to-total” configurations of the array, respectively.

The AGATA project

Each major technical advance in gamma-ray detection devices has resulted in significant new insights into the structure of atomic nuclei. These advances have culminated in the construction of 4π arrays of escape-suppressed spectrometers that comprise a Ge detector and scintillation detector suppression shield. The next major step in gamma-ray spectroscopy involves achieving the ultimate goal of a 4π ball of just Ge detectors by using the technique of gamma-ray energy tracking in electrically segmented Ge crystals.The resulting spectrometer will have an unparalleled level of detection power for nuclear electromagnetic radiation. Its sensitivity for selecting the weakest signals from exotic nuclear events will be enhanced by a factor of up to 1000 relative to its predecessors. It will have an unprecedented angular resolution making it ideally suited for high-energy resolution even at recoil velocities up to 50% of the velocity of light. Therefore, it is ideally suited to be used in conjunction with the new generation of radioactive beam accelerators or existing stable beam facilities.A European collaboration has been established to construct a 4π tracking spectrometer called AGATA (Advanced Gamma Tracking Array). This collaboration is currently performing the research and development necessary to finalise the technology for gamma-ray tracking and hence fully specify the full 4π spectrometer. The status of this first phase of the AGATA project will be reported.

TIGRESS: TRIUMF-ISAC gamma-ray escape-suppressed spectrometer

The TRIUMF-ISAC gamma-ray escape-suppressed spectrometer (TIGRESS) is a new γ-ray detector array being developed for use at TRIUMF's Isotope Separator and Accelerator (ISAC) radioactive ion beam facility. TIGRESS will comprise 12 32-fold segmented clover-type HPGe detectors coupled with 20-fold segmented modular Compton suppression shields and custom digital signal processing electronics. This paper provides an overview of the TIGRESS project and progress in its development to date.

The AGATA Project

For decades, the study of the gamma-ray decay from quantal states of the atomic nucleus has played a pivotal role in discovering and elucidating a wide range of phenomena. Each major technical advance in gamma-ray detection devices has resulted in significant new insights into the structure of nuclei. To date, these advances have culminated in the construction of 4π arrays of escape-suppressed spectrometers. The global consensus of opinion is that the next major step in γ-ray spectroscopy involves abandoning the concept of a physical suppression shield and achieving the ultimate goal of a 4π Ge ball using the technique of γ-ray energy tracking in electrically segmented Ge crystals. The resulting spectrometer will have an unparalleled level of detection power for nuclear electromagnetic radiation. Its sensitivity for selecting the weakest signals from exotic nuclear events will be enhanced by a factor of up to 1000 relative to its predecessors. It will have an unprecedented angular resolution making it ideally suited for high-energy resolution even at recoil velocities up to 50% of the velocity of light. Therefore, it is ideally suited to be used in conjunction with the new generation of radioactive beam accelerators or existing stable beam facilities. In Europe, a collaboration has been established to construct a 4π tracking spectrometer called AGATA (advanced gamma tracking array). This collaboration has signed a Memorandum of Understanding for the first phase of the project to perform the research and development necessary to finalize the technology for gamma-ray tracking and hence fully specify the full 4π spectrometer. The status of this first phase of the AGATA project will be reported.

TIGRESS highly-segmented high-purity germanium clover detector

The TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS) will consist of twelve units of four high-purity germanium (HPGe) crystals in a common cryostat. The outer contacts of each crystal will be divided into four quadrants and two lateral segments for a total of eight outer contacts. The performance of a prototype HPGe four-crystal unit has been investigated. Integrated noise spectra for all contacts were measured. Energy resolutions, relative efficiencies for both individual crystals and for the entire unit, and peak-to-total ratios were measured with point-like sources. Position-dependent performance was measured by moving a collimated source across the face of the detector.

AGATA: The advanced gamma tracking array

For decades, the study of the gamma-ray decay of the quanta1 states of the atomic nucleus has played a pivotal role in discovering and elucidating the wide range of phenomena manifested in its structure. Each major technical advance in gamma-ray detection devices has resulted in significant new insights into the structure of nuclei. To date, these advances have culminated in two state-of-the-art 4x arrays of escape-suppressed spectrometers, Euroball in Europe and Gammasphere in the USA.

Superdeformation and other phases at very high spin

Recent advances in the physics of nuclei at high angular momentum are reviewed. In particular the new phenomena observed using the latest generation of γ-ray Escape Suppressed Spectrometer (ESS) arrays are discussed. The breath of current high spin nuclear physics is illustrated by an overview of recent data on quasiparticle alignments, band crossings and band terminations; high spin shell structure; lifetime measurements; magnetic properties; decay of high K isomers. The depth of the subject is illustrated by a more detailed discussion of both discrete superdeformed states and also the decline of static pairing effects at high spin. Plans for new γ-ray spectrometers are mentioned and brief speculations made on the future directions of high spin physics.

Rotational band structures and lifetime measurements in130Ce

The deformed nucleus 130Ce has been studied using the techniques of in-beam gamma -ray spectroscopy. Beams of 16O and 18O were used on targets of 116,117,118Sn to obtain excitation energies, mean lifetimes, spins, parities, mixing ratios and branching ratios. The gamma - gamma coincidence data necessary to determine the decay scheme were obtained using an array of five escape-suppressed spectrometers. The yrast band has been extended to Jpi =24+, or possibly Jpi =26+, with mean lifetimes measured for states up to Jpi =22+. Five new bands have been established and a 150 ns isomer has been identified at an excitation energy of 2454 keV. Backbending is observed due to the alignment of two h11/2 protons at the same rotational frequency in both the yrast band and the bands on the isomer. The gamma band based on the 834 keV (Jpi =2+) state can be identified with a triaxial shape corresponding to gamma approximately=-25 degrees . This band backbends at spin 10+ due to the alignment of two h11/2 protons showing softness of the nucleus and the coexistence of different shapes. The data may be interpreted in terms of the cranking model by assuming a prolate deformation with epsilon 2 approximately=0.25. The measured transition strengths in the yrast band generally show a decrease in epsilon 2 or a change in triaxiality from gamma approximately=0 degrees to gamma approximately=30 degrees . The transition rates measured for the 12+ to 10+ and 14+ to 12+ transitions are bigger than expected and are consistent with an increase in deformation to epsilon 2=0.30-0.35.

Rotational alignment in158Er and159Er

High-spin states in 158Er have been populated with the reactions 145Nd(16O,3n) and 146Nd(16O,4n) and high-spin states in 159Er by the reaction 145Nd(18O,4n). Gamma-gamma coincidence and angular correlation data were obtained using an array of five Ge(Li)-NaI(Tl) escape-suppressed spectrometers. Linear polarisation data were obtained using a sectored Ge(Li) detector and internal conversion data were obtained using a mini-orange spectrometer. In 158Er the yrast band has been observed up to Jpi =26+ and three negative-parity side bands have been observed up to spins Jpi =23-, 22- and 15- respectively. The ground-state band has been observed to continue after the first backbend in the yrast sequence to Jpi =20+ and the odd- and even-spin members of the gamma-vibrational band were observed up to 7+ and 10+, respectively. In 159Er the yrast band has been observed up to Jpi =57/2+ and four side bands have been identified to spins Jpi =15/2-, 39/2+, 49/2-, and 51/2- respectively. The bands observed are interpreted as configurations of quasiparticles using the cranked shell model. Residual quasiparticle interactions are calculated as a function of rotational frequency.

A comprehensive study of the γ-ray spectroscopy of43Ca

The reaction 40Ar( alpha ,n gamma )43Ca was used to populate the levels in 43Ca at bombarding energies between 5.5 and 19 MeV, gamma - gamma coincidence measurements at Ealpha =8.5, 15 and 16 MeV being used to establish the decay scheme of 53 levels. Several new levels and new decays of levels with high angular momentum were established. The Doppler-shift attenuation method was used with solid Ar targets to deduce the mean lifetimes for most of these levels. A Ge(Li)-Na(Tl) escape-suppressed spectrometer and a three-Ge(Li) Compton polarimeter were used to measure the angular distribution and linear polarisation of gamma rays at alpha bombarding energies of 7.0, 14.5 and 17 MeV.

Gamma-ray spectroscopy of high-spin levels in67Ga

Reactions 64Zn( alpha ,p gamma )67Ga and 53Cr(16O, pn gamma )67Ga populated the levels in 67Ga. Gamma-gamma coincidence experiments were carried out at Ealpha =13 and 15 MeV with the ( alpha ,p gamma ) reaction and E(16)O=42 MeV with the heavy-ion reaction. The Doppler-shift attenuation method was used with the ( alpha ,p gamma ) reaction to deduce mean lifetimes (or limits) for certain levels, along with the recoil-distance method utilised both in singles and gamma - gamma coincidence with the heavy-ion reaction. A mean lifetime of tau m=60+or-30 ns for the first excited state at 167 keV was measured using the delayed coincidence method. Gamma-ray angular distributions and linear polarisations were measured at Ealpha =13.5 MeV using a Ge(Li)-NaI(Tl) escape-suppressed spectrometer and a three-Ge(Li) Compton polarimeter and analysis resulted in spin and parity assignments. Transition strengths were calculated and the results are discussed in terms of the particle-cluster-vibrational-core coupling model.

Gamma-ray studies of the odd-parity states in Zn isotopes. II. Levels below 2.0 MeV in excitation energy in67Zn

For pt.I see ibid., vol.4, p.755 (1978). The techniques of gamma -ray spectroscopy have been employed to investigate the structure of levels below 2.0 MeV in excitation energy in 67Zn populated in the 64Ni( alpha ,n)67Zn reaction. Gamma-gamma coincidences have been recorded at several alpha -particle beam energies. Gamma-ray angular distributions and linear polarisations have been measured using an escape-suppressed spectrometer and a three-Ge(Li) Compton polarimeter respectively. The gamma -ray decay modes of the levels have been deduced. Spin and parity assignments have been made. Multipole mixing ratios have been obtained for many of the transitions. The lifetimes of 11 levels have been measured using the Doppler-shift attenuation method. The electromagnetic multipole transition strengths deduced show moderate E2 enhancements. The results have been compared with the particle-vibration coupling model, the Alaga model and the shell model with reasonable agreement in each case.

Lifetimes of levels in46Ti (populated in43Ca(α,n) reaction)

The reaction 43Ca( alpha ,n)46Ti was used to populate levels in 46Ti at alpha particle energies of 6.5 and 7.5 MeV. Gamma rays in 46Ti were observed in a Ge(Li)-NaI(Tl) escape-suppressed spectrometer. The Doppler shift attenuation method was used to measure the mean lifetimes of 10 levels in 46Ti. The Jpi =0+ 2.61 MeV level is shown to have an enhanced E2 transition to the first excited 2+ state.

Lifetimes, spins and decay modes of levels below 5.3 MeV in33S

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.

Gamma-ray linear polarization measurements in33S

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.