Gamma-gamma Coincidence Measurements Research Articles

Application of multiple γ-ray detection to long-lived radioactive nuclide determination in environmental samples

ABSTRACT Gamma-gamma coincidence measurement utilized in γ-ray spectroscopy experiments is well known to be effective for the improvement of signal-to-noise ratio in a γ-ray spectrum. We study its applicability to the determination of long-lived radioactive nuclides in environmental samples. The γ-ray simulation code Geant 4.10.2 was used. A conventional and effective detector system comprising five Ge detectors was assumed. We took up 38 nuclides which need to be determined for the evaluation of fission product leakage at the nuclear accident in the Fukushima nuclear power plants in Japan. Among them 12 nuclides emit γ-rays and five nuclides of 60Co, 94Nb, 134Cs, 152Eu, and 154Eu can be the objectives of the multiple γ-ray detection methods. The simulation results indicate that the signal-to-noise ratio can be improved by a factor between 9.84 and 283, and the detection limit by a factor between 2.71 and 8.53 relative to the singles measurement, implying that the method can be well applied to the determination of the long-lived radioactive nuclides and will provide a quick and non-destructive analysis method.

Gamma-gamma coincidence performance of LaBr3:Ce scintillation detectors vs HPGe detectors in high count-rate scenarios

A radiation detection system consisting of two cerium doped lanthanum bromide (LaBr3:Ce) scintillation detectors in a gamma-gamma coincidence configuration has been used to demonstrate the advantages that coincident detection provides relative to a single detector, and the advantages that LaBr3:Ce detectors provide relative to high purity germanium (HPGe) detectors. Signal to noise ratios of select photopeak pairs for these detectors have been compared to high-purity germanium (HPGe) detectors in both single and coincident detector configurations in order to quantify the performance of each detector configuration. The efficiency and energy resolution of LaBr3:Ce detectors have been determined and compared to HPGe detectors. Coincident gamma-ray pairs from the radionuclides 152Eu and 133Ba have been identified in a sample that is dominated by 137Cs. Gamma-gamma coincidence successfully reduced the Compton continuum from the large 137Cs peak, revealed several coincident gamma energies characteristic of these nuclides, and improved the signal-to-noise ratio relative to single detector measurements. LaBr3:Ce detectors performed at count rates multiple times higher than can be achieved with HPGe detectors. The standard background spectrum consisting of peaks associated with transitions within the LaBr3:Ce crystal has also been significantly reduced. It is shown that LaBr3:Ce detectors have the unique capability to perform gamma-gamma coincidence measurements in very high count rate scenarios, which can potentially benefit nuclear safeguards in situ measurements of spent nuclear fuel.

Charged paricle activation analysis for characterizing parameters of laser-accelerated protons

The protons accelerated by ultra-high intensity laser have been extensively studied. The most commonly used detectors for measuring laser-driven proton are Tomspon parabola ion energy analyser (TP) and filtered nuclear track detectors, such as radiochromic films (RCF). The TP uses a parallel magneto-electric field to distinguish ions. This conventional technique can precisely identify the species and energy spectra of ions. However, the strong electromagnetic field produced by the laser-plasma interaction has an effect on TP, which results in no spatial resolution of TP. The RCF can give the spatial integration spectrum of proton, but it is easy to be saturated and cannot be reused anymore. In this paper, we present a method based on the traditional charged particle activation analysis and the gamma-gamma coincidence measurement to measure the spectrum of protons accelerated by ultra intense lasers. In this method, a copper plate stack is placed in the proton emission direction. Colliding with MeV proton converts 63Cu in the copper plates into radionuclide 63Zn whose decay can be easily observed and measured. Proton spectrum is then recovered from 63Zn decay counts from layers in the copper stack. The layout of diagnostics and the method to solve proton spectrum are discussed in detail and a self-consistent test is given. This spectrum analysis method is used in a laser-driven proton acceleration experiment carried out on XG-Ⅲ laser facility. The results show that protons up to 18 MeV are obtained, and the spatial integrated spectrum and a laser-proton conversion efficiency of 1.07% are achieved. In conclusion, our method has some advantages as a laser-driven ion diagnostic tool. It has no saturation problem and is not affected by strong electromagnetic fields. The basic principle of charged particle activation analysis is based on nuclear reaction, and can be extended to the measuring of other charged particle beams besides protons, such as deuterons, helium ions produced by ultra-high intensity laser.

Shape coexistence inHo153

The high-spin states in 153Ho, have been studied by 139 57 La(20Ne, 6n) reaction at a projectile energy of 139 MeV at Variable Energy Cyclotron Centre (VECC), Kolkata, India, utilizing an earlier campaign of Indian National Gamma Array (INGA) setup. Data from gamma-gamma coincidence, directional correlation and polarization measurements have been analyzed to assign and confirm the spins and parities of the levels. We have suggested a few additions and revisions of the reported level scheme of 153Ho. The RF-gamma time difference spectra have been useful to confirm the half-life of an isomer in this nucleus. From the comparison of experimental and theoretical results, it is found that there are definite indications of shape coexistence in this nucleus. The experimental and calculated lifetimes of several isomers have been compared to follow the coexistence and evolution of shape with increasing spin.

Electron capture decay of 58-minU92229and levels inPa91229

Electron capture decay of $^{229}\mathrm{U}$ is investigated by measuring the $\ensuremath{\gamma}$-ray and conversion electron spectra of mass-separated and unseparated $^{229}\mathrm{U}$ sources with high-resolution germanium and silicon detectors, respectively. Gamma-gamma coincidence measurements are also performed using germanium detectors. These studies provide level energies and level ordering in $^{229}\mathrm{Pa}$. Single-particle assignments are given to these levels which are in agreement with the systematics in this region and also with theory. In a previous study, we report the observation of a 5/${2}^{\ifmmode\pm\else\textpm\fi{}}$ parity doublet in the $^{229}\mathrm{Pa}$ ground state, which is a signature of octupole deformation. The present analysis of the data still shows a splitting of $60\ifmmode\pm\else\textpm\fi{}50$ eV, but with this large uncertainty the existence of the doublet is not certain.

Nuclear structure of30S and its implications for nucleosynthesis in classical novae

The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations.

Evolution of nuclear spectroscopy at Saha Institute of Nuclear Physics

Experimental studies of nuclear excitations have been an important subject from the earliest days when the institute was established. The construction of 4 MeV proton cyclotron was mainly aimed to achieve this goal. Early experiments in nuclear spectroscopy were done with radioactive nuclei with the help of beta and gamma ray spectrometers. Small NaI(Tl) detectors were used for gamma-gamma coincidence, angular correlation and life time measurements. The excited states nuclear magnetic moments were measured in perturbed gamma-gamma angular correlation experiments. A high transmission magnetic beta ray spectrometer was used to measure internal conversion coefficients and beta-gamma coincidence studies. A large number of significant contributions were made during 1950–59 using these facilities. Proton beam in the cyclotron was made available in the late 1950’s and together with 14 MeV neutrons obtained from a C-W generator a large number of short-lived nuclei were investigated during 1960’s and 1970’s. The introduction of high resolution Ge gamma detectors and the improved electronics helped to extend the spectroscopic work which include on-line (p 7 p′γ) and (p 7 nγ) reaction studies. Nuclear spectroscopic studies entered a new phase in the 1980’s with the availability of 40–80 MeV alpha beam from the variable energy cyclotron at VECC, Calcutta. A number of experimental groups were formed in the institute to study nuclear level schemes with (α 7 xnγ) reactions. Initially only two unsuppressed Ge detectors were used for coincidence studies. Later in 1989 five Ge detectors with a large six segmented NaI(Tl) multiplicitysum detector system were successfully used to select various channels in (α 7 xnγ) reactions. From 1990 to date a variety of medium energy heavy ions were made available from the BARC-TIFR Pelletron and the Nuclear Science Centre Pelletron. The state of the art gamma detector arrays in these centres enabled the Saha Institute groups to undertake more sophisticated experiments. Front line nuclear spectroscopy works are now being done and new informations are obtained for a large number of nuclei over a wide mass range. Currently Saha Institute is building a multi-element gamma heavy ion neutron array detector (MEGHNAD), which will have six high efficiency clover Ge detector together with charged particle ball and other accessories. The system is expected to be usable in 2002 and will be used in experiments using high energy heavy ions from VECC.

The new prompt gamma-activation analysis facility at Budapest

Prompt gamma-activation analysis (PGAA) is an important complementary technique to conventional instrumental activation analysis that can be successfully used in a number of cases when INAA is not applicable. Therefore, a PGAA facility has been constructed at the recently refurbished and upgraded Budapest Research Reactor. It occupies the end position of a new curved themal guide of 30 m length and 2.5×10 cm2 cross section which provides a clean beam of low energy neutrons. The sophisticated HPGe-BGO γ-ray spectrometer system can be operated in Compton-suppression and pair-spectrometer modes simultaneously. The octal splitting of the main BGO improves efficient pair mode operation when coincidences between pairs of opposite segments and the HPGe detector are required separately. Gamma-gamma coincidence measurements will also be possible when the new multiparameter data acquisition system is completed. One of the main tasks at the new facility will be the accumulation of new spectroscopic data for detector calibration and standardisation, as well as for the construction of a more accurate prompt γ-ray library for the chemical elements. Various applications are planned, such as the determination of hydrogen in fullerenes and of toxic trace elements in environmental samples.

Spectroscopy of high spin states in 92Mo and 90Mo.

High spin states in $^{92}\mathrm{Mo}$ and $^{90}\mathrm{Mo}$ have been studied using the reactions $^{59}\mathrm{Co}$${(}^{37}$Cl,2p2n${)}^{92}$Mo, $^{66}\mathrm{Zn}$${(}^{30}$Si,2p2n${)}^{92}$Mo, and $^{66}\mathrm{Zn}$${(}^{28}$Si,2p2n${)}^{90}$Mo at beam energies 90--120 MeV. Level energies, spins, and parities have been established up to spin 17\ensuremath{\Elzxh} (${\mathit{E}}_{\mathit{x}}$=8--9 MeV) on the basis of gamma-gamma coincidence and angular distribution measurements. All the states in these two nuclei could be described by spherical shell-model calculations.

Search for a light neutral boson associated with beta decay

A gamma-gamma coincidence measurement was performed in order to search for a neutral scalar particle associated with beta decay with a mass near 17 keV. The measurement used a 35S source and two LEPS-type Ge detectors. A coincidence signal above the background generated by random bremsstrahlung coincidences would be a signature of the two-photon decay of a light neutral scalar particle. No evidence was found for such a particle with a mass near 17 keV with a lifetime shorter than 1 nanosecond.

High-spin studies of 172,173Os: Complex alignment mechanism.

High-spin states of $^{172}\mathrm{Os}$ have been investigated by \ensuremath{\gamma}-\ensuremath{\gamma} coincidence studies. These states were populated by the $^{144}\mathrm{Nd}$${(}^{32}$S,4n) reaction using 162-MeV $^{32}\mathrm{S}$ ions. Gamma-gamma coincidence measurements were made with a Compton-suppression spectrometer system. The yrast band was observed up through the ${30}^{+}$ state, and four sidebands were deduced from \ensuremath{\gamma}-\ensuremath{\gamma} coincidence relationships. A band identified as belonging to $^{173}\mathrm{Os}$ was also observed. Results were interpreted using average nuclear field calculations which incorporated the deformed Woods-Saxon potential with its ``universal'' parametrization, a cranking approximation, and the Hartree-Fock-Bogoliubov method. The calculations indicate the presence of strongly interacting quasiparticle band crossings for 0.2l\ensuremath{\Elzxh}\ensuremath{\omega}l0.3, in semiquantitative agreement with experiment. Both the quasiparticle band structures and decline of neutron pairing are discussed.

Optimum time for gamma-gamma coincidence measurements of decaying sources

Assuming that the true and chance coincidences between the two energy-selected radiations are measured simultaneously, the measurement time is optimized with respect to the initial activity of the decaying source and its half-life as well as with respect to the steady activity of the interfering background radiation.

Decay studies of131Ba

The electron-capture decay of131Ba (T1/2=11.7 d) to excited levels of131Cs has been studied using high resolution semiconductor detectors. The presence of 1046.9 keV gamma-ray in between 1170.6 and 123.8 keV levels, in addition to 1047.6 keV gamma-ray from 1047.6 keV level to ground state of131Cs is confirmed through gamma-gamma coincidence measurements. TheK-capture probabilities to 1047 keV, 696 keV and 620 keV levels are measured usingX-gamma coincidence method. The gamma-gamma directional correlation measurements of the cascades 496-123, 240-133, 486-133, 404-216, 480-216 and 832-216 keV are carried out and spins and multipolarities of the levels and transitions involved are studied.

The level structure of140Ce

The reinvestigation of the level structure of140Ce fed from theβ-decay of140La (T=40.2 h) has been carried out by means of singles and gamma-gamma coincidence measurements using Ge(Li), hyperpure Ge and NaI (Tl) detectors.

Studies on the energy levels of153Eu following the ??-decay of153Sm

The level scheme of153Eu was investigated by observing the gamma-rays following theβ−-decay of the 47 h153Sm using a high purity Ge(HPGe) detector and a Ge(Li)-Nal(Tl) fast-slow coincidence spectrometer. The energies and relative intensities of about fifty six gamma-ray transitions were determined. Five of them at energies of 124.91, 431.65, 443.24, 487.75 and 623.73 keV were observed and confirmed for the first time in the present singles and/or gamma-gamma coincidence measurements. These new transitions could be fitted into a proposed level scheme of153Eu. Theβ−-decay branching ratios as well as the log (ft) values were obtained.

Getting coincidence information from analysis of sum peaks in singles Ge(Li) spectra test, evaluation and improvement of the method

Analysis of sum peaks from singles spectra obtained by a large volume Ge(Li) detector in many cases gives more coincidence information than traditional two-detector gamma-gamma coincidence measurements do. Approximate methods are given to obtain and use coincidence information from sum peaks of a single spectrum. The method gives numbers of coincidence counts several orders of magnitude higher than the traditional two detector methods do, without the use of sophisticated multiparameter analysis equipment and during comparable time. The sum peak analysis method requires much less instrumentation and memory capacity, than the traditional methods, and also multiple coincidences can be measured with rather good efficiency. The possibilities and limitations of the sum peak analysis method are discussed, illustrated by different measurements as examples.

Gamma-ray studies of62Zn

The reaction 58Ni(6Li,pn gamma )62Zn was used to populate levels in 62Zn at bombarding energies between 15 and 24 MeV. Gamma-gamma and neutron-gamma coincidence measurements at beam energies of 20 and 24 MeV were used to establish a decay scheme up to 7.4 MeV in excitation. Spins and parities of the levels together with branching and mixing ratios of the gamma-ray transitions have been deduced from angular distribution and linear polarisation measurements. Mean lifetimes of many levels have been measured using the doppler-shift attenuation method and the recoil-distance method. Comparisons are made with the shell model and the interacting boson approximation model.

Gamma-ray spectroscopy in63Zn and63Cu

Electromagnetic decay transitions in 63Zn and 63Cu have been studied using the 60Ni( alpha ,n gamma )63Zn and 60Ni( alpha ,p gamma )63Cu reactions at incident beam energies of 9.5-19.0 MeV, the 54Fe(11B,np gamma )63Zn and 54Fe(11B,2p gamma )63Cu reactions at EB=30.0 MeV, and the 58Ni(7Li,pn gamma )63Zn reaction at ELi=20.0 MeV. Gamma-gamma coincidence measurements have been used to determine the decay schemes. Spins and parities of the levels together with branching and mixing ratios of the gamma -ray transitions have been deduced from angular distribution and linear polarisation measurements. Mean lifetimes of the 193, 248, 1704 and 2585 keV levels in 63Zn have been obtained using the recoil-distance method (RDM). Lifetimes of many levels in 63Zn and 63Cu have been deduced from the Doppler-shift attenuation method (DSAM). The results are discussed in terms of the shell model and the intermediate coupling model.

High-spin states above 3.5 MeV in 91Nb

High-spin states in 91Nb populated by the 89Y(α, 2nγ) reaction were studied at 24.0 MeV and 35.7 MeV incident α-particle energy. Gamma-gamma coincidence and γ-ray angular distribution measurements were made. Several high-spin states in 91Nb were observed at 35.7 MeV bombarding energy which were not observable at 24.0 MeV. The shell model structure of these states is discussed.

Gamma-ray and particle spectroscopy of 63Zn

The nucleus 63Zn has been studied by the (p, d), (p, nγ), (α, nγ) and ( 12C, 2pnγ) reactions. Gamma-gamma coincidence measurements have allowed a level scheme to be established up to ≈ 5 MeV. Angular distributions of γ-rays and of deuterons were used to assign spins and parities. Qualitative agreement with simple shell-model calculations of level positions and neutron pick-up spectroscopic factors was found.