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Natural plutonium from supernovaeThe rapid neutron capture process (r-process) produces many of the heavy chemical elements, but the astrophysical settings where it occurs remain unclear. Leading candidates are neutron star mergers and some types of supernovae. Wallneret al.analyzed the plutonium content of a deep-sea crust sample, identifying a few dozen atoms of the r-process isotope plutonium- 244 that were delivered to Earth within the past few million years. There was a simultaneous signal of iron-60, which is known to be produced in supernovae. Comparing the ratios of these isotopes constrains the relative contributions of supernovae and neutron star mergers to r-process nucleosynthesis.Science, this issue p.742

Nuclides synthesized in massive stars are ejected into space via stellar winds and supernova explosions. The solar system (SS) moves through the interstellar medium and collects these nucleosynthesis products. One such product is (60)Fe, a radionuclide with a half-life of 2.6 My that is predominantly produced in massive stars and ejected in supernova explosions. Extraterrestrial (60)Fe has been found on Earth, suggesting close-by supernova explosions ∼2 to 3 and ∼6 Ma. Here, we report on the detection of a continuous interstellar (60)Fe influx on Earth over the past ∼33,000 y. This time period coincides with passage of our SS through such interstellar clouds, which have a significantly larger particle density compared to the local average interstellar medium embedding our SS for the past few million years. The interstellar (60)Fe was extracted from five deep-sea sediment samples and accelerator mass spectrometry was used for single-atom counting. The low number of 19 detected atoms indicates a continued but low influx of interstellar (60)Fe. The measured (60)Fe time profile over the 33 ky, obtained with a time resolution of about ±9 ky, does not seem to reflect any large changes in the interstellar particle density during Earth’s passage through local interstellar clouds, which could be expected if the local cloud represented an isolated remnant of the most recent supernova ejecta that traversed the Earth ∼2 to 3 Ma. The identified (60)Fe influx may signal a late echo of some million-year-old supernovae with the (60)Fe-bearing dust particles still permeating the interstellar medium.

The status and capabilities of the ANTARES AMS facility after 10 years are reviewed. The common AMS radioisotopes, 10 Be, 14 C, 26 Al, 36 Cl and 129 I, are routinely analysed. A capability for the detection of 236 U and other actinide isotopes has been developed. The measurement program includes support to Quaternary science projects at Australian universities and to ANSTO projects in global climate change and nuclear safeguards.

In conjunction with an extensive radiocarbon program, ANTARES has also carried out AMS measurement of the radioisotopes 26 Al and 36 Cl with the recent addition of 129 I. Development of an AMS measurement capability for 10 Be and for the actinides is underway. These projects pose a range of technical challenges both in spectrometer performance and in sample preparation. This paper discusses the special aspects of the facility that have been developed to meet these needs, with emphasis on 36 Cl, 129 I and plans for future actinide measurements.

The ANTARES AMS Centre at Lucas Heights Research Laboratories is currently used for measurements of the long-lived radioisotopes 14C, 26Al, 36Cl and 129I, with measurement of 10Be and actinides under development. Improved precision for 14C (to 0.5%) has been achieved with the recent commissioning of a 59 sample ion source coupled with automated data acquisition. We have developed rigorous data evaluation methods which are invaluable in assessing reproducibility and aid in the identification of physical problems with the system. Low backgrounds, in the accelerator and in the sample preparation laboratories, have allowed us to measure very old samples (to 50 000 years BP) and are also vital for high-accuracy dating of small samples.

The ANTARES AMS facility at the Lucas Heights Research Laboratories is operational and AMS measurements of 14C and 26Al are performed routinely. Measurement programs for a variety of other long-lived cosmogenic radioisotopes are being implemented on specialised beamlines. The overall aim of the facility is to establish an AMS centre for advanced studies in global change and Quaternary science. Other projects in biomedicine and nuclear safeguards monitoring are also being developed.

Microsecond isomers in the exotic proton-richN=82, 83 nuclei153Yb,153Lu,154Lu and154Hf have been identified byγ-ray spectroscopy following mass analysis of102Pd+245 MeV54Fe reaction products using the Daresbury Recoil Separator. The decays of the isomers, interpreted asπh 11 2/ andπh 11 2/ vf7/2 states, are characterized. The reduced E2 transition rates indicate that half-filling of theπh11/2 subshell occurs just belowZ=71, and results for theN=83 odd-odd isotones are also consistent with this finding.

The two-quasiproton structure of 164Dy has been investigated in a study of the 165Ho(t, α) 164Dy reaction. The experiment was performed using a 37.3 MeV triton beam delivered by the Daresbury tandem Van de Graaff accelerator. Magnetic analysis of ejectile ions was achieved using a QMG/2 magnetic spectrometer, and ion identification and detection employed a hybrid gas-filled, multielement, focal-plane detector. The two-quasiproton strengths accessed in this reaction are discussed with reference to the detailed spectroscopy of 164Dy. In particular, a rotational band built on a Kπ = 1+ state, previously interpreted as a collective structure, was observed and assigned as a two-quasiproton excitation which at zero deformation is based on the spherical (h112)2 shell-model state. The structure of the octupole-vibrational band and that of other states in the final nucleus have been elucidated.

We have assigned [gamma]-ray transitions to [sup 136]Te by examining the prompt gamma radiation following spontaneous fission of [sup 248]Cm. The transitions were identified in spectra gated on the Ru complementary fragments, and confirmed by studying prompt [gamma] rays following spontaneous fission of [sup 252]Cf. The results are examined in terms of the systematics of other tellurium and [ital N]=84 nuclei.

The two-quasiproton structure of 174Yb has been investigated in a study of the 175Lu(t,α)174Yb reaction. The reaction was performed using a 35.5 MeV triton beam delivered by the Daresbury tandem Van de Graaff accelerator. Magnetic analysis of ejectile ions was achieved using a QMG 2 magnetic spectrometer and ion identification and detection employed a hybrid gas-filled, multielement, focal plane detector. Elements of the detailed nuclear structure of 174Yb are discussed in terms of the two-quasiproton configurations populated. Of particular interest is a low-lying Kπ = 3+ band, observed in this reaction and discussed in terms of hexadecapole collectivity. No Kπ = 1+ bands were observed, and the consequences of this for a microscopic description of the magnetic-dipole-strength distribution in 174Yb are discussed.

A new double-sided silicon strip detector has been developed to study the phenomenon of ground-state proton radioactivity. Highly proton-rich fusion-evaporation reaction products are velocity and mass separated using the Daresbury recoil separator before being implanted into the strip detector located at the separator focal plane. The double-sided strip detector has 48 strips per face with a pitch of 335 μm. Front and back strips are orthogonal providing an effective pixel area of 0.09 mm2, enabling correlations between implanted ions and subsequent decays to be clearly established. Test results obtained using the reactions 58 Ni + 92 Mo → 150 Yb ∗ and 58 Ni + 54 Fe → 112 Xe ∗ are reported. In the latter commisioning experiment the proton decay of 109I was unambiguously established using the correlated decay sequence 109I→p108Te→α104Sn.

A Residue Implantation Detection System has been used in conjunction with the Daresbury Recoil Mass Separator in a search for radioactivity from the unknown nuclides108I and105Te. In the reaction of 260 MeV58Ni ions with a54Fe target two new decay lines at energies of 3.730±0.025 MeV and 3.885±0.025 MeV corresponding to cross sections of approximately 0.3 μb and 0.4 μb respectively were identified in theA=108 region of the separator's focal plane. These decay lines are tentatively assigned to the alpha decays of108I. No evidence for a proton decay branch of this nuclide could be found. In a second reaction in which a50Cr target was bombarded with 230 MeV58Ni ions, no alpha decay peak from105Te could be identified. Cross section dependent half life limits were determined from which correspondingQ-value limits were deduced.

Rotational bands up to spin of ≈10 h have been studied in even-even and odd- A nuclei in the mass 100 region, by observing prompt γ-rays from the spontaneous fission of 248 Cm. Transitions in the neutro-rich nuclei 103 Zr, 104 Zr, 107 Mo and 108 Mo have been observed for the first time, and evidence for a band in 105 Nb is presented. Rotational bands are also reported in 98–100 Sr, 99,101 Y, 100–102 Zr, 101,103 Nb and 102–106 Mo. In most cases these bands have been extended to much higher spins than previously known. The data on the even-even nuclei provide information on deformation trends in this region. The level structures of the odd- A nuclei indicates which Nilsson orbitals lie close to the Fermi level. Bands based on the 5 2 + [422] and 5 2 − [303] configurations are identified in the odd- A nuclei with Z = 39 and 41. Evidence is presented that certain bands observed in N = 61 and 63 nuclei are built on the 5 2 − [532] configuration, and the role of the neutron h 11 2 orbital in stabilizing the deformation in this region is discussed.

Excited states in the neutron-rich nucleus {sup 142}Xe have been determined by observing prompt {gamma} rays in fragments from {sup 248}Cm spontaneous fission. Estimates of the lowest excited-state energies in {sup 142}Xe based on normal systematics differ considerably from those derived from the {ital N{sub p}N{sub n}} scheme. Experimental values are in good agreement with the {ital N{sub p}N{sub n}} scheme and illustrate its predictive power in this mass region.

Partial decay schemes in the neutron-rich nuclei $^{103}\mathrm{Zr}$ and $^{104}\mathrm{Zr}$ have been measured for the first time and rotational bands in $^{100--102}\mathrm{Zr}$ have been extended to spins of up to 10\ensuremath{\Elzxh} by observing prompt \ensuremath{\gamma} rays from the spontaneous fisson of $^{248}\mathrm{Cm}$. These nuclei are among the most deformed known at low spin and excitation energy. The level structures in the odd-A nuclei show that the ${\mathit{h}}_{11/2}$ intruder orbital plays an important role in stabilizing the deformation in this region.

Experiments using the Daresbury Recoil Mass Separator have identified microsecond isomers in the exotic N = 82, 83 nuclei, 153Yb, 153Lu, 154Hf, and have established their decay schemes. The results for 153Lu and 154Hf, together with those for lighter N = 82 isotones, provide an outstanding illustration of the dependence of E2 transition rates between J n states on the sub-shell occupation, and demonstrate that half-filling of the π h 11/2 subshell in the N = 82 series occurs just below Z = 71 153Lu. The result for the isomer in the even-odd N = 83 nucleus 153Yb shows that the isomerism results from a low-energy E2 transition, rather than an E3 transition as in lighter N = 83 even-odd isotones, which is another consequence of the π h 11/2 subshell being about half-filled. Furthermore, the long-lived isomer observed in the odd-odd N = 83 nucleus 154Lu also reflects that the π h 11/2 subshell is close to being half-filled.

In the last few years, theoretical calculations and experimental results have established that nuclei in the mass 224 and Z = 56, N = 88 regions possess reflection-asymmetric (octupole) shapes. In both regions it is found that even-even nuclei are reflection symmetric in their ground states. However, these nuclei develop octupole deformation with rotation. Recently we have studied such phenomena in neutron-rich Ba and Ce nuclei produced in spontaneous fission. Detailed properties of these nuclei will be discussed.

The fusion reaction74Se+106Cd→180Pb* at a compound nucleus excitation energy Ex ≃ 40 MeV, has been used in a search for the decay of the unknown nucleus177Tl. Evaporation residues were velocity and mass analysed prior to implantation into a position sensitive silicon surface barrier detector. No evidence was discovered for the proton or alpha-decay of177Tl for a cross-section > 10−5+10 nb. This indicates either that177Tl decays too rapidly (t1/2 < 1 μs) by proton emission for the decay to be detected, or that177Tl is produced with a cross-section less than the limit established in the present experiment. The following nuclear decay half-lives were measured with improved accuracy;177Hg (t1/2 = 130±5 ms),178Hg (t1/2=250±25 ms) and177Au (t1/2=1180±70 ms). The experiment also provided the first direct confirmation of the correct mass assignment of the alpha-decay, Eα=6.26 MeV, to the decay of176Au. Two additional halflife measurements of proton-rich Osmium isotopes are also presented from a previous similar experiment;165Os (t1/2=73±8 ms) and166Os (t1/2 = 194±17 ms).

Measurements of γ-rays from mass analyzed nuclear recoils produced in the102Pd +54Fe reaction at 245MeV have allowed us to characterize the decay of a 35 μs isomer in154Lu. The rates of E2 transitions de-exciting isomeric levels in proton-rich N=82, 83 isotones are discussed.

The Q -values for groups observed in the multinucleon transfer reaction 48 Ca( 18 O, 20 N) 46 Sc have been measured using a variety of heavy-ion spectrographic techniques. The group at lowest excitation energy has a Q -value of −25.87 ± 0.02 MeV, corresponding to a mass excess of 22.63 ± 0.06 MeV for 20 N. This is in significant disagreement with earlier determinations, being ∼1 MeV less bound. Possible explanations are discussed.

A detection system has been constructed to measure the alpha and proton decays of short-lived (> μs) nuclear species lying in the proximity of the proton drip line. Proton-rich nuclei produced in fusion-evaporation reactions at angles around 0° are mass separated in flight using the Daresbury Recoil Mass Separator. A two-dimensionally position sensitive silicon surface barrier detector is used to measure evaporation residue implantations and decays. The position information is used to identify the mass of the parent nucleus and to correlate causally related events. The operation and performance of the detection system are described.

The mass excesses of 33Si and 34Si have been measured using the 36S(11B, 14, 13N)33, 34Si; reactions. Values of −20.550 ± 0.030 MeV ( 33 Si) and −20.017 ± 0.025 MeV ( 34 Si) were obtained. In addition, an excited state of 34Si was observed at 5.33 ± 0.05 MeV, and excited levels of 33Si were observed at 1.06 ± 0.02 and 4.32 ± 0.03 MeV. These results are compared with recent shell-model calculations.

The mass of 25 Ne has been determined from the Q -value of the reaction 26 Mg( 13 C, 14 O) 25 Ne at a 13 C energy of 96 MeV. The 14 O ions were momentum-analysed using an Enge split-pole spectrometer and detected at the focal plane in a multi-element, gas-filled detector. The resulting mass excess for 25 Ne is −2.03±0.05 MeV. Excited states of 25 Ne were observed at energies of 1.74±0.03, 3.33±0.03, 4.07±0.03 and 6.28±0.05 MeV. The widths of the states at 1.74 and 3.33 MeV suggest that they may be unresolved doublets, the levels being separated by approximately 110 and 200 keV, respectively. A qualitative discussion of the possible reaction mechanisms and of the nuclear structure of the observed states is presented and a comparison made with recent shell-model calculations.

The ${T}_{z}$=-(3/2), A=4n+1 nuclide $^{61}\mathrm{Ge}$ has been produced via the $^{40}\mathrm{Ca}$${(}^{24}$Mg,3n) reaction and identified by its beta-delayed proton emission. A single proton group at 3.11\ifmmode\pm\else\textpm\fi{}0.03 MeV with ${t}_{1/2}$=40\ifmmode\pm\else\textpm\fi{}15 ms has been observed, corresponding to decay of the T=(3/2) isobaric analog state in $^{61}\mathrm{Ga}$ to the ground state of $^{60}\mathrm{Zn}$. Searches for the beta-delayed proton decay of the next member of this series, $^{65}\mathrm{Se}$, to be produced by the $^{40}\mathrm{Ca}$${(}^{28}$Si,3n) reaction, have proven unsuccessful.

A fast in-beam recoil catcher wheel has been constructed to perform particle spectroscopy on very proton-rich nuclei. The wheel speed can be varied to study nuclides whose half-lives range from 100 μs to ∼ 200 ms. Known beta-delayed proton emissions from the A = 4n + 1, T z = −3 2 nuclides 17 Ne, 21 Mg, and 25 Si as well as the beta-delayed two-proton decay from 22 Al( t 1 2 = 70 ms ) were measured to test the wheel. Subsequently, a beta-delayed two-proton branch from 31 Ar ( t 1 2 = 15 ms ) was observed.

The microscopic structure of the M1 mode in 164 Dy (in which the M1 strength is fragmented over several 1 + states) has been investigated using the 165 Ho(t, α) 164 Dy reaction. The 3 + −8 + members of a proposed K π =1 + rotational band with a band head energy of 2.557±0.015 MeV were observed to be strongly populated in l =5 proton transfer. These results are not consistent with a collective interpretation of this state.

The neutron-rich nuclei 15B and 19N have been produced in reactions between 18O and 48Ca. Mass excesses of 28.968 ± 0.025 MeV and 15.872 ± 0.020 MeV respectively were deduced from the reaction Q-values. These results are compared with previous measurements and theoretical predictions. Updated predictions using two different mass formulae are presented.

The mass excess of the T z = 3 nucleus 40 Cl has been determined using the 40 Ar( 7 Li, 7 Be) 40 Cl reaction. A value −27.527 ± 0.035 MeV was obtained. A number of excited states of 40 Cl were a1so observed. In addition, the 40 Ar( 11 B, 11 C) 40 Cl and 40 Ar( 11 B, 13 N) 38 S reactions were investigated. Excited states of 38 S were observed at 1.28 ± 0.04 and 3.38 ± 0.10 MeV.

The ground-state mass excess of 39 S has been determined from a measurement of the Q - value of the 40 Ar( 13 C, 14 O) 39 S reaction using a gas target. A value of −23.164±0.050 MeV was obtained in good agreement with the predictions of standard mass equations. In addition, a single excited state was observed at 1.469 ±0.025 MeV. The reaction mechanism of the ( 13 C, 140 O) reaction is discussed in terms of a two-step reaction model based on the second-order Born approximation. The degree of angular-momentum matching appears to be an important factor in determining the degree of participation of a possible reaction channel. Calculations indicate that in the case of the 40 Ar( 13 C, 14 O) 39 S reaction, the reaction is dominated by a single path because of poor matching in competing channels.

A system to measure the time of flight for heavy ions around an Enge split-pole magnetic spectrometer has been developed. The system consists of a parallel plate avalanche counter at the focal plane and either a microchannel plate detector or a silicon recoil detector in the target chamber. In-beam tests under experimental conditions have demonstrated a typical resolution of 1.0 to 1.5 ns, compared to a total flight time of ∼ 100 ns. Resolution figures approaching these have been recovered, when using the 4.5° angular acceptance of the spectrometer, by applying calculated corrections to the raw data according to the angle and focal plane position. A description of the detectors is followed by the results of the in-beam tests. An example of an application to the study of a heavy ion reaction is also described.

The 26 Mg( 18 O, 17 F) 27 Na and 26 Mg( 18 O, 15 O) 29 Mg reactions have been used to determine the ground-state masses and partial level schemes of 27 Na and 29 Mg. Values of −5514 ± 60 keV and −10600 ± 45 keV were determined for the mass excesses of 27 Na and 29 Mg, respectively. The probable structure of some of the levels observed was deduced from a comparison with shell-model calculations and with previous experimental information. A number of other 18 O- and 16 O-induced reactions on magnesium targets were also studied in order to obtain information on the reaction mechanisms of the ( 18 O, 17 F) and ( 18 O, 15 O) reactions.

The elastic scattering of $^{7}\mathrm{Li}$+$^{16}\mathrm{O}$ has been measured at ${E}_{\mathrm{Li}}=50$ MeV and fitted with an optical model potential. The $^{16}\mathrm{O}$($^{7}\mathrm{Li}$,$^{7}\mathrm{Be}$)$^{16}\mathrm{N}$ reaction has been investigated at ${E}_{\mathrm{Li}}=50$ MeV. States and groups of states were observed up to 6.3 MeV excitation in the $^{16}\mathrm{N}$+$^{7}\mathrm{Be}$ system. Angular distributions for $^{16}\mathrm{N}$ in ${2}^{\ensuremath{-}}$ 0.0 MeV, ${3}^{\ensuremath{-}}$ 0.297 MeV, and ${4}^{\ensuremath{-}}$ 6.17 MeV states with $^{7}\mathrm{Be}$ in its ground state ($^{7}\mathrm{Be}_{0}$), and for $^{16}\mathrm{N}$ in the ${3}^{\ensuremath{-}}$ 0.297 MeV state with $^{7}\mathrm{Be}$ in its first excited state ($^{7}\mathrm{Be}_{1}$), have been compared with distorted-wave Born approximation calculations. The distorted-wave Born approximation form factors were calculated microscopically using central and tensor forces and realistic transition densities. The calculations fail to reproduce the cross sections.

Microsecond isomers in the very proton-rich N=82 nuclei $^{153}\mathrm{Lu}$ and $^{154}\mathrm{Hf}$ have been identified by \ensuremath{\gamma}-ray spectroscopy following mass analysis of $^{102}\mathrm{Pd}$ +245 MeV $^{54}\mathrm{Fe}$ reaction products using the Daresbury Recoil Separator. The decays of the isomers, interpreted as (\ensuremath{\pi}${h}_{11/2}$${)}^{n}$ states, are characterized. Examination of reduced E2 transition rates between (\ensuremath{\pi}${h}_{11/2}$${)}^{n}$ states in the N=82 isotones indicates that the half-filling of the \ensuremath{\pi}${h}_{11/2}$ subshell occurs just below Z=71.

Recent work in light proton-rich nuclei is reviewed. Evidence for the first T/sub z/ = -5/2 nuclide, /sup 35/Ca, is presented. The mechanisms of two-proton emission following beta-decay is investigated. Future directions in this field are discussed. 23 refs., 5 figs. (WRF)

Gamma-ray measurements following mass analysis of102Pd+245 MeV54Fe reaction products have characterized a 15 μs isomeric decay in153Yb and elucidated the systematics of proton-rich N=83 nuclei.