Doppler Correction Research Articles

A detector for filtering γ-ray spectra from weak fusion–evaporation reactions out of strong background and for Doppler correction: The recoil filter detector, RFD

A detector has been designed and built to assist in-beam γ-ray spectroscopy with fusion–evaporation reactions. It measures with high efficiency the evaporation residues that recoil out of a thin target into the angular interval from 1.8° to 9.0° at an adjustable distance of 1000–1350 mm from a target, in coincidence with γ-rays detected in a Ge-detector array. This permits filtering of such γ-rays out of a much stronger background of other reaction products and scattered beam. Evaporation residues are identified by their time-of-flight and the pulse height using a pulsed beam. The velocity vector of the γ-emitting recoil is also measured in the event-by-event mode, facilitating to correct the registered γ-ray energy for the Doppler shift, with the resulting significant improvement of the energy resolution. The heavy-ion detection scheme uses emission of secondary electrons caused by the recoiling ions when hitting a thin foil. These electrons are then electrostatically accelerated and focused onto a small scintillator that measures the summed electron energy, which is proportional to the number of electrons. The detector is able to operate at high frequency of the order of 1 MHz and detect very heavy nuclei with as low kinetic energy as 5 MeV. The paper describes the properties of the detector and gives examples of measurements with the OSIRIS, GAREL+ and EUROBALL IV γ-ray spectrometers. The usefulness of the technique for spectroscopic investigations of nuclei with a continuous beam is also discussed.

Numerical prediction of coherent integration time at 75 Hz, 0.03 s temporal resolution at 3250 km

Coherence time of sound is modeled for a 3250 km section in the Pacific at 75 Hz, 0.03 s resolution. The model is based on the temporal evolution of a standard spectrum of internal gravity waves and an accurate approximation for the acoustic wave equation. Modeled coherence time extends up to about 40 min. The probability is 0.6 that the signal-to-noise ratio first decreases at times of 13 min or less. Afterward, the s/n usually increases, reaching a peak near 40 min. The 40 min time is longer than experimentally reported coherence times of 12.7 min [J. Acoust. Soc. Am., 105, 3185–3201 (1999)]. The experimental value is similar to the modeled time of 13 min or less at which the probability is 0.6 for finding the s/n ratio to decrease for the first time. The experimental value of 12.7 min was found using a single Doppler correction speed yielding maximum output of the impulse response. Analysis of accelerations of the source and receiver indicates a variable Doppler correction scheme should be used to estimate coherence time. The experimental procedure for assuming a constant Doppler speed correction appears to be invalid. [Work supported by the ONR.]

The effect of Doppler correction on measures of storm track intensity

In climate research, the intensity of a storm track is often measured by the band pass filtered eddy kinetic energy, geopotential variance or related variables. The interpretation of such fields disregards the influence the advection speed might have on these measures. In this paper the need for a Doppler corrected storm track measure is discussed. A Doppler corrected measure is defined and applied to 10 years of ECMWF reanalysis data, correcting the storm track measure for spatial and temporal variability of the advection speed. The storm track intensity is also calculated correcting only for the temporal variability of the advection speed. It is also related to fluctuations of the NAO teleconnection pattern. The Doppler correction suggests that (1) maximum and minimum baroclinic activity is found somewhat downstream of the locations indicated by non-corrected measures, (2) the storm track activity estimated by conventional measures is much too low in the areas of the eastern ends of the storm tracks, (3) the monthly mean time series of the strength of a storm track, as estimated by conventional measures, is strongly influenced by the variability of the advection speed at times, (4) the strength of the storm track seems to be less strongly connected with teleconnection patterns such as NAO or with the background mean flow speed than usually thought on the basis of conventional Eulerian statistics.

Doppler effects in heterogeneous media with applications to ocean acoustic modeling

Doppler shift corrections to ocean acoustic signals are complicated by the multi-spatial-scale structure of the ocean medium, resulting in a multi-time-scale structure of the acoustic Green function. Repeated reflections and refractions lead in general to an infinite number of acoustic paths or modes, with different times of flight, connecting source and receiver. The rate of change of these flight times with source or receiver motion gives rise to Doppler shift corrections, and each acoustic path or mode has a different correction. A clean Doppler correction (in the sense of an observable coherent motion-induced frequency shift for each path or mode) is shown to emerge only when the medium is homogeneous along the direction of source or receiver motion, even when it is highly inhomogeneous in directions orthogonal to the motion. A very general quantitative theory for this correction is developed, encompassing earlier results in the literature, and presented in a form amenable to efficient numerical implementation in data processing.

Future Perspectives in Nuclear Structure: From high intensity stable to radioactive nuclear beams

Future perspectives in nuclear structure rely on radioactive nuclear beams as well as on high intensity beams of stable ions. Especially for neutron rich nuclei, deep-inelastic and multi-nucleon transfer reactions can be used to populate yrast and non yrast states. Particularly powerful is here the combination of large acceptance spectrometers with highly segmented γ-detector arrays. Such devices, eventually complemented by large coverage particle detectors, can provide the necessary channel selectivity to identify very rare signals. An example is the CLARA γ-ray detector array coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL). The physics aims achievable with this setup will complement studies performed with current radioactive beam (RIB) facilities. With such set-up we have recently investigated the stability of the N=50 shell closure when moving towards more exotic systems. Here the comparison of the experimental data with shell model calculations seems to indicate a persistence of the N=50 shell gap down to Z=32. Future perspectives at LNL are based on an increase in intensity as well as on the availability of heavy ion species. Beams like 136 Xe or 208 Pb, which will be provided by the new PIAVE injector, can be used to drive the multinucleon flux toward the more exotic regions. Moreover a new ISOL facility (SPES) dedicated to the production and acceleration of radioactive neutron rich species is now under development at LNL. It will be based on an high intensity proton and deuteron LINAC. Induced fission fragments will be ionized and then accelerated using the presently existing superconductive LINAC (ALPI). Among the new instrumentation the concept of γ-ray tracking has been recently introduced in nuclear spectroscopy. Detectors based on γ-ray tracking have position resolution capabilities with excellent performances both in efficiency and in achievable Doppler correction. A new γ-ray detector array (AGATA) based on such technique is now under study in a wide european collaboration. 1. Probing Nuclear Structure with Radioactive and Stable ion beams

The Advanced Gamma Ray Tracking Array AGATA

New accelerator facilities for radioactive–ion beams will enter into operation in the next few years, providing the opportunity to explore unknown territories of the nuclear landscape. The foreseen harsh experimental conditions require the construction of a new generation of γ–ray detector arrays based on the emerging technique of γ–ray tracking. The “Advanced GAmma Tracking Array” (AGATA), proposed in Europe, will be built out of 120 or 180 highly segmented Ge crystals operated in position sensitive mode by means of digital data techniques and pulse shape analysis of the segment signals. AGATA will be capable of measuring γ radiation in a large energy range (from ∼ 10 keV to ∼ 10 MeV), with the largest possible photopeak efficiency (25% at M γ = 30 ) and with good spectral response. The very good Doppler correction and background rejection capability of this γ–ray tracking array will allow to perform γ–ray spectroscopy experiments using fragmentation beams with sources moving at velocities up to β ∼ 0.5 .

The advanced gamma ray tracking array AGATA

New accelerator facilities for radioactive-ion beams and high-intensity stable beams will start operation in a few years. Although these beams will provide interesting opportunities for exploring unknown territories of the nuclear landscape, the experimental conditions will be very challenging and, indeed, the nuclear structure community has realized that a new generation of powerful arrays for gamma-ray spectroscopy has to be built in order to cope with them. As a result of years of experience with Compton suppressed germanium arrays and of intensive R&D work targeted to extend their limits, it is now clear that the next 4<FONT FACE=Symbol>p g</FONT>-ray spectrometers will be built fully from germanium detectors and will be based on the technique of gamma-ray tracking. The "Advanced GAmma Tracking Array" (AGATA), proposed in Europe, will be an instrument of major importance for nuclear structure studies at the very limits of nuclear stability. It will be built out of 120/180 highly segmented Ge crystals operated in position sensitive mode by means of digital data techniques and pulse shape analysis of the segment signals. AGATA will be able to measure gamma radiation in a large energy range (from ~ 10 keV to ~ 10 MeV), with the largest possible photopeak effi ciency (25 % at Mgamma = 30) and with a good spectral response. In particular, its very good Doppler correction and background rejection capabilities will allow to perform "standard" gamma-ray spectroscopy experiments using fragmentation beams with sources moving at velocities up to beta ~ 0.5.

Gamma-ray tracking with segmented HPGe detectors

This paper gives a brief overview of the technical progress that can be achieved with the newly available segmented HPGe detectors. Gamma-ray tracking detectors are a new generation of HPGe detectors which are currently being developed to improve significantly the efficiency and resolving power of the 4pi germanium detectors arrays for high-precision gamma-ray spectroscopy. They consist of highly segmented HPGe detectors associated with fast digital front-end electronics. Through the pulse-shape analysis of the signals it is possible to extract the energy, timing and spatial information on the few interactions a gamma-ray undergoes in the HPGe detector. The tracks of the gamma-rays in the HPGe detector can then be reconstructed in three dimensions based on the Compton scattering formula. Such a detector has been used for the first time during an in-beam experiment. The gamma-decay of the Coulomb excitation of a 56Fe nucleus was measured with the highly segmented MARS prototype positioned at 135 degree. The energy resolution has been improved by a factor of 3 as compared to standard HPGe detectors due to very precise Doppler correction based on knowledge of the gamma-ray track.

Gamma-ray tracking with the MARS detector

The feasibility of the entire process of $\gamma$ -ray tracking is demonstrated experimentally for the first time. The accuracy of the results is tested by the capability for Doppler correction of $\gamma$ -rays emitted in flight. The resolution of the 847.8 keV ( $2^ + \to 0^ + $ ) transition detected with the MARS detector after Coulomb excitation of a 56Fe beam could be improved from 15 keV to below 5 keV (FWHM).

Estimating the velocity of a moving object submerged in an ocean waveguide with active sonar

Standard active sonar and radar systems are often used to estimate the velocity of a moving target in free space by resolving the Doppler shift of the scattered waveform. In an ocean waveguide, multimodal propagation and dispersion make the Doppler effects far more complicated than in free space. In a waveguide, multiple frequency components are typically present in the field scattered from a moving object even if the active source of radiation is harmonic. Applying a free space Doppler correction to the field scattered from a moving object in a waveguide is insufficient to account for the Doppler distortion of the signal in typical ocean waveguides and active sonar scenarios. Spectral and modal formulations for the Doppler-shifted field scattered from a horizontally moving target in a horizontally stratified medium have been developed by Lai and Makris [J. Acoust. Soc. Am. 110, 2724 (2001)]. These formulations are used with the maximum likelihood method to estimate the velocity of the moving target by modeling the multiply Doppler distorted scattered signal measured with noise in a shallow water waveguide. The performance of the maximum likelihood estimate is evaluated by asymptotic analysis of its bias and mean-square error as a function of signal-to-noise ratio.

Pulse distributions and tracking in segmented detectors

A study of the performance of a cylindrical HPGe detector segmented in 25 segments is presented. It is based on simulations made with the computer code GEANT and focuses on the reconstruction of a γ-ray path. The effects of the segmentation are initially discussed in terms of Doppler correction. The role of the pulse shape analysis and its effects on tracking algorithms are discussed as a function of the capability to reconstruct a γ-ray path when multiple signals (direct and induced) are present in a single segment. It is found that it is critical to identify correctly at least two signals in a segment in order to have a reasonable efficiency and Compton suppression in the spectra and to make a good use of this type of detectors.

A position-sensitive particle detector for Coulomb excitation experiment

For Coulomb excitation experiments using a γ-ray detector array, a position-sensitive particle detector system that has a good angular resolution, capability of managing a high-counting rate, robustness for a radiation damage, compactness (under 110 mm φ) and easiness of handling is necessary. For this purpose we developed a new device utilizing plastic and yttrium aluminum perovskite activated by cerium (formula YAlO3:Ce abbreviated YAP Ce) scintillators and a position-sensitive photomultiplier tube. The detector system could operate at a high-counting rate (about 105), and with a good position resolution of 0.5–1.2 mm, which was enough for Doppler correction of the γ rays. It could sustain the radiation damage caused by Coulomb excitation experiments with heavy ions for more than 50 days. The detector chamber was well fitted in the 110 mm φ diameter of the internal space of a compact γ-ray detector array, GEMINI. This was much easier to handle than the position-sensitive gas counter. In Coulomb excitation experiment utilizing this device, the nucleus Ge74 has been Coulomb excited using a Pb target. The transitional E2 matrix elements have been determined and the device has turned out to be useful in Coulomb excitation experiments with heavy-ion beams.

The Doppler Effect and Spectral Energy Distribution of Blazars

The relativistic beaming model is adopted to discuss quantitatively the observational differences between radio-selected BL Lac objects (RBLs) and X-ray-selected BL Lac objects (XBLs), and between BL Lac objects and flat spectrum radio quasars (FSRQs). The main results are the following: (1) In the Doppler corrected color-color (alpha (in)(ru) - alpha (in)(ox)) diagram, XBLs and FSRQs occupy separated regions, while RBLs bridge the gap between them. These properties suggest that similar intrinsic physical processes operate in all the objects under a range of intrinsic physical conditions. (2) Our results are consistent with the results of Sambruna, Maraschi and Urry (1996) from other methods. We show the alpha (xox) introduced by Sambruna to be a good index for describing the energy distribution because it represents the intrinsic energy distribution and includes the Doppler correction. (3) The Doppler effect of relativistic beaming is the main mechanism, and the physical differences (such as magnetic fields, electron energies) are also important complementary factors for understanding the relation between XBLs and RBLs.

MINIBALL A Ge detector array for radioactive ion beam facilities

Dedicated Ge-detector arrays are being developed for the investigation of rare γ decays with low γ-ray multiplicity at the upcoming radioactive ion beam facilities. These arrays are optimized for the high full-energy peak efficiency and angular resolution of the γ-ray detection needed for a proper Doppler correction of the γ-rays emitted by fast recoiling nuclei. MINIBALL will consist of 40 six-fold segmented, encapsulated Ge detectors which are clustered in eight cryostats with three detectors each and four cryostats with four detectors, respectively. The individual components - the six-fold segmented Ge detector, the cryostats, the fast preamplifier, the digital pulse-processing electronics and the mechanical frame - and their properties are described. The results of test measurements with the first MINIBALL cluster detector using a 137Cs source and the in-beam reaction D( 37Cl, n) 38Ar are presented. It is shown that from pulse-shape analysis of the events within a detector segment the effective granularity of the MINIBALL array can be enhanced from 240 to ∼ 4000. The specifications of MINIBALL are compiled on the basis of experimental data. First results with a 12-fold segmented, encapsulated detector are discussed with respect to the feasibility of future γ-ray tracking arrays.

Interaction of photons with plasmas and liquid metals - photoabsorption and scattering

Formulae for describing the photoabsorption and the photon scattering by a plasma or a liquid metal are derived in a unified manner. It is shown how the nuclear motion, the free-electron motion and the core-electron behaviour of each ion in the system determine the structure of the photoabsorption and scattering in an electron-ion mixture. The absorption cross section in the dipole approximation consists of three terms which represent the absorption caused by the nuclear motion, the absorption owing to the free-electron motion producing optical conductivity or inverse Bremsstrahlung and the absorption ascribed to the core-electron behaviour of each ion with the Doppler correction. Also, the photon scattering formula provides an analysis method for experiments observing the ion-ion dynamical structure factor (DSF), the electron-electron DSF giving plasma oscillations and the core-electron DSF yielding the x-ray Raman (Compton) scattering with a clear definition of the background scattering for each experiment, in a unified manner. A formula for anomalous x-ray scattering is also derived for a liquid metal. At the same time, Thomson scattering in plasma physics is discussed from this general point of view.

First results with DIAMANT and EUROGAM

The coupling of efficient particle and γ detection arrays is a powerful tool to study the deexcitation of both compound and residual nuclei in fusion evaporation reactions.We show first results obtained coupling the DIAMANT (4π light charged particle array) and EUROGAM (4π γ array) detectors demonstrating the improvement in the resolving power by this combination. For γ rays of ≃800keV improvements in the Doppler correction up to 50% have been reached for the 2α exit channel following the fusion of 32S+58Ni at 120 MeV bombarding energy (β ≃ 3%).

A FREQUENCY OFFSET ESTIMATION AND COMPENSATION ALGORITHM FOR K/Ka-BAND COMMUNICATIONS

This paper presents a new open-loop technique for estimating and correcting Doppler frequency shift in K/Ka-band communication systems with special reference to the advanced communications technology satellite (ACTS) mobile terminal (AMT) modem, which utilizes square-wave pulse-shaped, binary differential phase shift-keyed (DPSK) modulation. The novelty of this estimation scheme is that it exploits the Doppler-induced phase shift over a fraction of a symbol interval to provide an estimate of the Doppler offset, without requiring symbol synchronization. Furthermore, by utilizing time-differential detection (delay-and-multiply), the proposed technique can tolerate much larger frequency offsets than existing open- or closed-loop techniques. Analytical results are provided for the variance of the above estimator and the error probability performance of the AMT is evaluated in the presence of the Doppler correction. Practical design considerations are also discussed, including a method for modifying the front end, digital bandlimiting filter in such a way that Doppler bias effects in the new estimator are eliminated. Simulation results reveal that, in general, performance improves with increasing data rates, i.e., the new frequency offset estimation/compensation algorithm induces a degradation from ideal of approximately 1 dB at a 6 dB energy per data symbol (bit) and a 2.4 kbps data rate. However, there is no appreciable degradation when the data rate is increased to 9.6 or 19.2 kbps.

A FREQUENCY OFFSET ESTIMATION AND COMPENSATION ALGORITHM FOR K/Ka‐BAND COMMUNICATIONS

This paper presents a new open-loop technique for estimating and correcting Doppler frequency shift in K/Ka-band communication systems with special reference to the advanced communications technology satellite (ACTS) mobile terminal (AMT) modem, which utilizes square-wave pulse-shaped, binary differential phase shift-keyed (DPSK) modulation. The novelty of this estimation scheme is that it exploits the Doppler-induced phase shift over a fraction of a symbol interval to provide an estimate of the Doppler offset, without requiring symbol synchronization. Furthermore, by utilizing time-differential detection (delay-and-multiply), the proposed technique can tolerate much larger frequency offsets than existing open- or closed-loop techniques. Analytical results are provided for the variance of the above estimator and the error probability performance of the AMT is evaluated in the presence of the Doppler correction. Practical design considerations are also discussed, including a method for modifying the front end, digital bandlimiting filter in such a way that Doppler bias effects in the new estimator are eliminated. Simulation results reveal that, in general, performance improves with increasing data rates, i.e., the new frequency offset estimation/compensation algorithm induces a degradation from ideal of approximately 1 dB at a 6 dB energy per data symbol (bit) and a 2⋅4 kbps data rate. However, there is no appreciable degradation when the data rate is increased to 9⋅6 or 19⋅2 kbps.

Ground state transitions in one- and two-electron Bi projectiles

By using segmented solid state X-ray detectors and applying X-ray/charge-state selective particle coincidences, the ionic structures of 83Bi 82+ and 83Bi 81+ have been studied separately at 82 MeV/u under single collision conditions. The high granularity of the Ge(i) X-ray detectors used allowed a partial Doppler correction for the X-ray events while maintaining a large total solid angle. An absoluute precision of K-transition energies of 10 −3 is feasible; the relative accuracy is better than 30 eV. The experimental values compare very well with the theoretical transition energies in H- and He-like Bi ions.

HIFE signals and general results.

HIFE transmitted for 28 full hours and 8 half hours, repeating the same signal continuously for each transmission. HIFE had a repertoire of cw, Pentaline, and four different phase-coded time-resolving signals based on m sequences. The several purposes of each signal will be explained. Receptions were processed on line at 14 sites at ranges of 200 to 18 000 km; to date, the digital tapes from 8 sites have been processed at CSPL. The feasibility questions concerning power, stability, time resolution, and dispersion were answered, and a thorough study of the receptions begun. Examples of the on-line processing and selected frequency domain and travel-time-domain time series will be presented and discussed. An extraordinary look at the phase stability of the propagation was revealed through comparison of (1) the diagonal ship surge deviation from a uniform path measured at the transmitter, and (2) the integrated autocorrelation phase (IAP1) after constant Doppler correction measured at a 9000-km distant receiver.