Short-lived Nuclei Research Articles

Fast-switching NMR system for measurements of ground-state quadrupole moments of short-lived nuclei

A β-ray detecting nuclear quadrupole resonance system has been developed at NSCL/MSU to measure ground-state electric quadrupole moments of short-lived nuclei produced as fast rare isotope beams. This system enables quick and sequential application of multiple transition frequencies over a wide range. Fast switching between variable capacitors in resonance circuits ensures sufficient power delivery to the coil in the β-ray detecting nuclear magnetic resonance technique. The fast-switching technique enhances detection efficiency of resonance signals and is especially useful when the polarization and/or production rate of the nucleus of interest are small and when the nuclear spin is large.

Activation cross-section measurements for producing short-lived nuclei with 14 MeV neutrons—Ge, Pd, Yb

Activation cross sections for 70 Ge ( n , p ) 70 Ga , 74 Ge ( n , p ) 74 Ga , 108 Pd ( n , p ) 108 Rh , 110 Pd ( n , α ) 107 Ru , and 174 Yb ( n , p ) 174 Tm reactions producing short-lived nuclei with half-lives of several minutes were measured in the energy between 13.5 and 14.8 MeV using activation technique in this work. All cross-section values were relatively obtained on the basis of the standard cross section of 93 Nb ( n , 2 n ) 92 m Nb or 27 Al ( n , α ) 24 Na , and the neutron energies were measured by the method of cross-sectional ratios for 90 Zr ( n , 2 n ) 89 m + g Zr to 93 Nb ( n , 2 n ) 92 m Nb reactions. Careful attention on corrections was paid to neutron irradiation and induced activities measurement. The measured results were discussed and compared with the previous works.

Thermal-Hydraulic Experiment on Beam Window for Developing the Accelerator-Driven Transmutation System

The accelerator-driven nuclear transmutation system aims at transmuting minor actinides (MAs) and long-lived fission products to stable or short-lived nuclei. A feasibility design study of the proton beam window, which is an interface component between an accelerator and a nuclear reactor, needs a method to be able to evaluate heat transfer characteristics of the beam window under flowing lead bismuth. However, the validity of the numerical simulation model of the beam window has not been proven yet and a database on heat transfer characteristics has also not been established. Two experiments were conducted: one was particle image velocimetry measurement around the beam window in flowing water and the other was temperature measurement at the beam window under flowing lead bismuth. Numerical simulation was also done to validate the beam window model for design work. Results show that heat transfer characteristics of the beam window averaged in space and time under flowing lead bismuth was formulized by the experimental equation. The numerical simulation model can estimate the mean heat transfer coefficient. However, the local heat transfer coefficient was not stable: it fluctuates with time and even in space, especially around the stagnation point. The reason is discussed in detail.

Cross-section measurement for the reactions producing short-lived nuclei induced by neutrons around 14 MeV

The cross-sections of 180W(n,2n) 179mW, 186W(n,2n) 185mW, 165Ho(n,2n) 164mHo, 64Ni(n, α) 61Fe, 165Ho(n, α) 162Tb and 51V(n,p) 51Ti reactions induced by neutrons around 14 MeV were measured using activation technique and calculated by a previously developed formula in this work. The neutron flux was determined using the monitor reactions 93Nb(n,2n) 92mNb and 27Al(n, α) 24Na, the neutron energies were measured with the method of cross-section ratios for 90Zr(n,2n) 89Zr to 93Nb(n,2n) 92mNb reactions. The results of this work are compared with data published previously.

Supernova Mixtures Reproducing Isotopic Ratios of Presolar Grains

Most of supernova-originating presolar grains, such as silicon carbide type X (SiC X) and low-density graphite, show excesses of 28Si. Some of them also indicate evidence for the original presence of short-lived nuclei 44Ti. In order to reproduce isotopic and elemental signatures of these grains, large-scale heterogeneous mixing in supernova ejecta is required. I investigate supernova mixtures that reproduce as many isotopic ratios as possible of 18 individual SiC X and 26 individual low-density graphite grains. The supernova ejecta are divided into seven layers, i.e., the Ni, Si/S, O/Si, O/Ne, C/O or O/C, He/C, and He/N layers. Then, mixtures indicating isotopic ratios with small differences from those of individual single grains are sought. Mixtures reproducing five isotopic ratios in 12C/13C, 14N/15N, 26Al/ 27Al, 29Si/28Si, 30Si/ 28Si, and 44Ti/48Ti are obtained for three SiC grains. For 10 SiC X grains for which five, four, or three isotopic ratios have been measured, the mixtures reproducing all measured isotopic ratios are also found. For 20 low-density graphite grains, the mixtures reproducing six isotopic ratios in 12C/13C, 14N/ 15N, 16O/17O, 16O/18O, 26Al/27Al, 29Si/28Si, 30Si/28Si, and 44Ti/48Ti are obtained. The mixing ratios of each mixture strongly depend on reproduced isotopic ratios. The main component of most mixtures is one of the Ni, He/C, and He/N layers. The carbon-to-oxygen ratio of the mixtures is larger than unity for most cases. The ratios of Fe/C and Fe/Si in the mixtures have large varieties. Most of the mixtures for low-density graphite indicate Fe/Si larger than unity. The elemental signatures of the mixtures will constrain grain formation in supernovae.

A solenoidal spectrometer for reactions in inverse kinematics

A new type of spectrometer for the study of reactions in inverse kinematics is proposed, in which the target and detector are placed inside a uniform magnetic field. This device has significant advantages over more conventional approaches to measurements of key reactions, and should prove useful in the study of short-lived nuclei. The technique is directly applicable to existing research programs at present accelerators with secondary beams, and the technique is likely to be important at future radioactive beam facilities.

Highly segmented detector arrays for studying the resonant decay of unstable nuclei

Resonant spectroscopy of short-lived nuclei produced in nucleus–nucleus collisions can provide information on the nature of the nuclear surface and the formation of clusters. Detection of these resonant decays requires detector arrays with good energy and angular resolution. This newest generation of detector array is typically comprised of silicon strip detectors with several hundred to several thousand independent segments. As a key issue of such arrays is the processing of their signals, we report on the development of a non-ASIC system designed to simplify the analog processing and readout from a highly segmented silicon detector array. The non-ASIC system called MASE (multiplexed analog shaper electronics) focuses on providing good energy resolution and adequate timing information for up to 4096 channels. It consists of 16-channel boards which can be either used independently or as part of a larger system. The analog portion of each channel has low and high gain shapers with associated leading edge discriminators and peak hold circuits. The logic for readout of the analog signals is performed by two FPGA chips located on each board. Readout of MASE channels is multiplexed. Logical signals are transferred via LVDS while the analog signals are sequenced into a multisampling ADC. Signals are also multiplexed for inspection purposes. Shaper gains and discriminator thresholds are adjustable through DACs via a USB interface.

Future Facility: FAIR at GSI

The Facility for Antiproton and Ion Research, FAIR, is a new particle accelerator facility to be built at the GSI site in Germany. The research at FAIR will cover a wide range of topics in nuclear and hadron physics, high density plasma and atomic physics, and applications in condensed matter physics and biology. A 1.1 km circumference double ring of rapidly cycling 100 and 300 Tm synchrotrons, will be FAIR's central accelerator system. It will be used to produce, inter alia , high intensity secondary beams of antiprotons and short-lived radioactive nuclei. A subsequent suite of cooler and storage rings will deliver heavy ion and antiproton beams of unprecedented quality. Large experiments are presently being designed by the NUSTAR, PANDA, PAX, CBM, SPARC, FLAIR, HEDgeHOB and BIOMAT collaborations.

Shell structure of exotic nuclei

Theoretical predictions and experimental discoveries for neutron-rich, short-lived nuclei far from stability indicate that the familiar concept of nucleonic shell structure should be considered as less robust than previously thought. The notion of single-particle motion in exotic nuclei is reviewed with a particular focus on three aspects: (i) variations of nuclear mean field with neutron excess due to tensor interactions; (ii) importance of many-body correlations; and (iii) influence of open channels on properties of weakly bound and unbound nuclear states.

A cooler ion trap for the TITAN on-line trapping facility at TRIUMF

We present simulations of electron and proton cooling of highly charged ions in a Penning trap, including the potentially detrimental effects of radiative, dielectronic, and three-body recombination in electron cooling. We show a preliminary design for a cooler trap accommodating both electron and proton cooling, which will be a component of the TITAN ion-trap facility under construction at TRIUMF for precision mass measurements of short-lived radioactive nuclei.

A new experimental approach for isochronous mass measurements of short-lived exotic nuclei with the FRS-ESR facility

Projectile fragments separated in flight with the FRS have been injected into the storage ring ESR. Operating the ESR in the isochronous mode has enabled high-resolution mass measurements of short-lived isotopes. Already in our pilot experiments the Isochronous Mass Spectrometry (IMS) has provided accurate results for a restricted isotope range in the neighborhood of a selected isotope characterized by the best isochronous condition. In the present experiment this restriction has been overcome by precise external magnetic rigidity determination (1.5·10–4) at the dispersive midplane of the FRS. In this way the mass resolving power for neutron-rich fission fragments covering a large mass-over-charge ratio in one spectrum has been improved by up to one order of magnitude and the accuracy by more than a factor of two.

A low initial abundance of 247Cm in the early solar system and implications for r-process nucleosynthesis

The short-lived nuclide 247Cm is produced by r-process nucleosynthesis. When the presolar nebula formed, 247Cm became isolated from r-process production and its abundance diminished as a result of radioactive decay. Given its short half-life of only ∼ 16 million years, 247Cm is presently extinct, but its former presence should be detectable as small variations in 235U/ 238U in primitive meteoritic material, provided Cm was chemically fractionated from U at the time these solid objects formed. The magnitude of U isotopic anomalies in meteorites can thus be used to elucidate the timing and character of the last r-process nucleosynthetic event for input into models describing the formation and evolution of the early solar system. Here we report coupled U isotopic determinations and Nd/U proxy measurements for Cm/U in a series of acid-etched leachates and mineral assemblages extracted from meteorites containing primitive phases expected to show strong Cm–U fractionations. Using multiple-collector ICPMS, we are able to determine 235U/ 238U with 2 σ analytical uncertainties of ± 1 epsilon (1 epsilon = 1 part in 10,000) on sample sizes consisting of < 3 ng of 238U and < 20 pg of 235U. A double-spiking procedure using a mixed 236U– 233U spike was employed to allow instrumental mass fractionation to be reliably corrected internally and at high precision. Uranium isotopic results for almost 40 different phases show no resolvable deviations in 235U/ 238U from the chondritic value, at the ∼ 1–2 epsilon level. These data supplement our previous observations for a suite of bulk meteorite samples [C.H. Stirling, A.N. Halliday, D. Porcelli, In search of live 247Cm in the early solar system, Geochim. Cosmochim. Acta 69 (2005) 1059–1071] and provide evidence for a solar system initial 247Cm/ 235U of < 8 × 10 − 5 . Such a low value is difficult to explain without a long time-scale of ∼ 2.3 × 10 8 years between the last actinide producing r-process event and the formation of the solar system. As such it is difficult to reconcile with a model of actinide production in the same r-process forming event as 182Hf with a half-life of 8.9 My [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actinides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109–L113]. The alternative models of 182Hf production via a neutron-rich fast s-process, occurring, for example, in the helium burning shell in a 25 solar mass star during explosive nucleosynthesis [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actinides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109–L113], or via a distinct r-process event that is separate from actinide production [G.J. Wasserburg, M. Busso, R. Gallino, K.M. Nollet, Short-lived nuclei in the early solar system: Possible AGB sources, Nucl. Phys. A (in press)], may provide a viable explanation. However, further studies are also required to assess the veracity of Cm–U systematics, which are critically dependent on the suitability of using Nd and the light rare earth elements (LREEs) as a chemical proxy for Cm.

Investigation of the low-lying isomer in 229Th by collinear laser spectroscopy

A new ion beam of 229Th is available at the Jyvasklya IGISOL facility, produced from the α decay of 233U. A small branching ratio (≈ 2%) is believed to populate the inferred low-lying (5.5 eV) isomeric state in 229Th. A laser ionization scheme is currently being developed to improve the yield of 229Th from the source. The ion source uses a novel electric field configuration for fast and efficient extraction of α-recoils and is able to provide beams of short lived (τ≥ 30 ms) radioactive nuclei. Identification of the isomeric state by collinear laser spectroscopy will reduce the lower lifetime limit of the state and provide the first direct evidence for its existence.

Stellar nucleosynthetic contribution of extinct short-lived nuclei in the early solar system and the associated isotopic effects

Abstract— A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short-lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short-lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf-Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low-mass AGB stars imply large isotopic anomalies in Ca-Al-rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short-lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high-injection mass cut is a possible scenario. There is a possibility of the contribution of short-lived nuclides by a 1.5–3 M⊙ AGB star as it implies the smallest shift in stable isotopes. A low-mass AGB star of relatively low metallicity would be even a better source of short-lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf-Rayet stars that evolved to supernovae Ib/c were involved.

A Late Episode of Irradiation in the Early Solar System: Evidence from Extinct36Cl and26Al in Meteorites

Late-formed halogen-rich phases in a refractory inclusion and a chondrule from the Allende meteorite exhibit large S-36 excesses that linearly correlate with the chlorine concentration, providing strong evidence in support of the existence of the short-lived nuclide Cl-36 (mean life of 0.43 Myr) in the early solar system. The inferred Cl-36/Cl-35 ratios at the time when these phases formed are very high (similar to 4 x 10(-6)) and essentially the same for the inclusion and the chondrule and confirm the earlier report of S-36 excess in another meteorite. In addition, the Cl-36 is decoupled from Al-26. The observed and any possible higher levels of Cl-36 cannot be the result of a supernova or AGB stellar source but require a late episode of energetic particle bombardment by the early Sun, in support of the arguments based on the previous discovery of Be-10. It is now clear that a blend of several sources is required to explain the short-lived nuclei when the solar system formed.

Precision experiments on exotic nuclei at IGISOL

Cooling and trapping techniques of low-energy radioactive ion beams of refractory elements employed at the IGISOL facility are presented with emphasis on high-precision measurements of the ground state properties of exotic nuclei. The impact of the new generation Paul and Penning traps on mass measurements of short-lived nuclei is discussed with examples on precision measurements of masses of super-allowed beta emitters and neutron-rich nuclei. As a new concept the trap-assisted spectroscopy of radioactive ions is presented with applications in collinear laser spectroscopy, decay spectroscopy of isobarically purified sources and in nuclear cross-section measurements by ion counting.

Hadron Physics at FAIR

To deepen our understanding of the properties and structure of matter, new large-scale facilities are under construction or in preparation worldwide that provide for novel research capabilities in many areas of science. The GSI-Darmstadt laboratory together with the international user community, has developed the concept for such a facility to broadly address frontiers in strong-interaction physics and in the general research with intense ion beams. The new facility builds, and substantially expands, on the present accelerator system. Primary beams of high-energy ions, and secondary beams including antiprotons and beams of short-lived nuclei, together with an intricate system of beam-storage and experimental rings, are its key features. This report provides a short overview of the science motivation of the hadron community for the facility, as well as of its technical layout and performance characteristics.

Measuring reaction probability ratios to simulate neutron-induced cross-sections of short-lived nuclei

Measuring the neutron-induced fission cross-sections of short-lived nuclei represents an experimental challenge due to target activity and the low intensity of neutron beams. One way to alleviate the problems inherent in the direct measurement is to use the surrogate method, where one measures the decay probability of the same compound nucleus formed using a charged beam and a stable target. The decay probability of the compound nucleus is then used to estimate the neutron-induced cross-section. As an extension to the surrogate method, we introduce a new method of reporting the fission probabilities of two compound nuclei as a ratio, which has the advantage of removing most of the systematic uncertainties. The ratio method was checked in a known case, the 236U(n, f)/238U(n, f) cross-section ratio, which turned out to be the same as the probability ratio of P(236U(d, pf))/P(238U(d, pf)). As an application, the 237U(n, f)/235U(n, f) cross-section ratio was inferred, on the basis of the measured P(238U(d, d′f))/P(236U(d, d′f)) probability ratio.

A multi-reflection time-of-flight mass spectrometer for mass measurements of short-lived nuclei

In order to determine binding energies of short-lived nuclei we have developed a time-of-flight mass spectrometer of high mass resolving power m/Δm. This spectrometer achieves a very long ion flight path by repeatedly reflecting ions between two electrostatic ion mirrors. The nuclei to be investigated are produced in heavy ion fragmentations and separated in-flight by a fragment separator. These energetic ions are thermalized in a catcher gas cell injected into an RF ion-guide and then into an ion trap to be cooled, bunched and entered into the time-of-flight spectrometer. This technique should allow to determine the masses of short-lived nuclei with high efficiency. Using stable ions, the so far achieved mass resolving power m/Δm exceeded 65,000.

Short-lived nuclei in the early Solar System: Possible AGB sources

The abundances of short-lived radionuclides in the early Solar System (ESS) are reviewed, as well as the methodology used in determining them. These results are compared with the inventory estimated for a uniform galactic production model. It is shown that, to within a factor of two, the observed abundances of 238U, 235U, 232Th, 244Pu, 182Hf, 146Sm, and 53Mn are roughly compatible with long-term galactic nucleosynthesis. 129I is an exception, with an ESS inventory much lower than expected from uniform production. The isotopes 107Pd, 60Fe, 41Ca, 36Cl, 26Al, and 10Be require late addition to the protosolar nebula. 10Be is the product of energetic particle irradiation of the Solar System as most probably is 36Cl. Both of these nuclei appear to be present when 26Al is absent. A late injection by a supernova (SN) cannot be responsible for most of the short-lived nuclei without excessively producing 53Mn; it can however be the source of 53Mn itself and possibly of 60Fe. If a late SN injection is responsible for these two nuclei, then there remains the problem of the origin of 107Pd and several other isotopes. Emphasis is given to an AGB star as a source of many of the nuclei, including 60Fe; this possibility is explored with a new generation of stellar models. It is shown that if the dilution factor (i.e. the ratio of the contaminating mass to the solar parental cloud mass) is f 0 ∼ 4 × 10 −3 , a reasonable representation for many nuclei is obtained; this requires that ( 60Fe/ 56Fe) ESS ∼ 10 −7 to 2 × 10 −6 . The nuclei produced by an AGB source do not include 53Mn, 10Be or 36Cl if it is very abundant. The role of irradiation is discussed with regard to 26Al, 36Cl and 41Ca, and the estimates of bulk solar abundances of these isotopes are commented on. The conflict between various scenarios is emphasized as well as the current absence of an astrophysically plausible global interpretation for all the existing data. Examination of abundances for the actinides indicates that a quiescent interval of ∼ 10 8 yr is required for actinide group production. This is needed in order to explain the data on 244Pu and the new bounds on 247Cm. Because this quiescent interval is not compatible with the 182Hf data, a separate type of r-process event is needed for at least the actinides, distinct from the two types that have previously been identified. The apparent coincidence of the 129I and trans-actinide time scales suggests that the last heavy r contribution was from an r-process that produced very heavy nuclei but without fission recycling so that the yields at Ba and below (including I) were governed by fission.